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Physical and chemical treatments for removal of perchlorate from water–A review
Accepted Manuscript Title: Physical and Chemical Treatments for Removal of Perchlorate from Water—A Review Authors: Yanhua Xie, Lulu Ren, Xueqian Zhu, Xi Gou, Siyu Chen PII: DOI: Reference:
S0957-5820(18)30041-7 https://doi.org/10.1016/j.psep.2018.02.009 PSEP 1295
To appear in:
Process Safety and Environment Protection
Received date: Revised date: Accepted date:
17-12-2017 3-2-2018 11-2-2018
Please cite this article as: Xie, Yanhua, Ren, Lulu, Zhu, Xueqian, Gou, Xi, Chen, Siyu, Physical and Chemical Treatments for Removal of Perchlorate from Water—A Review.Process Safety and Environment Protection https://doi.org/10.1016/j.psep.2018.02.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Physical and
Chemical Treatments
for Removal of
Perchlorate from Water—A Review
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Yanhua Xiea,b,* , Lulu Rena,b, Xueqian Zhua,b, Xi Goua,b and Siyu Chena,b
a: State Key Laboratory of Geohazard Prevention and Geoenvironment Protection,
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Chengdu University of Technology, Chengdu 610059, China;
b: State Environmental Protection Key Laboratory of Synergetic Control and Joint
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Remediation for Soil & Water Pollution, Chengdu University of Technology,
Corresponding author: Yanhua Xie, College of Environment, Chengdu University of
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Technology, Chengdu, 610059, China
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*
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Chengdu 610059, China.
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Tel: +86 15928561958; Fax: +86 15928561958
Highlights
The key findings of physical and chemical treatments for perchlorate removal are
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E-mail: [email protected].
systematically summarized in tables.
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The key findings, mechanisms, influencing factors, advantages and disadvantages of recent researches are elaborated. The coated nanoscale zero-valent iron is superior for treating high-concentration perchlorate.
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Abstract: Perchlorate, which could not be easily degraded in the environment, is a persistent inorganic pollutant in water with high water-solubility, diffusivity, and stability. Its pollution has become a global environmental problem. Perchlorate in
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surface water and groundwater can keep in food and drinking water by various ways, which may cause a series of healthy problems in human body. Physical and chemical
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treatments are the technologies commonly used to remove perchlorate from water. This review begins with the existing treatments of perchlorate in water, then describes
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the perchlorate pollution status in different countries and its hazard for human being.
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In addition, the key findings of technologies and materials of physical and chemical
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treatments on perchlorate removal from water up to now are systematically
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summarized, and also the following research findings in recent years are elaborated in
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detail, such as the key findings, mechanisms, influencing factors, advantages and disadvantages. In particular, a novel material of nanoscale zero-valent iron (nZVI)
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which is used to remove perchlorate, is also presented. The coated nZVI introduced in
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this paper is superior for treating high-concentration perchlorate. This overall summary of the technologies and materials currently used can guide future studies on
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the removal of perchlorate from water.
Keywords: perchlorate removal; water; physical-chemical treatment; chemical treatment; nanoscale zero-valent iron (nZVI)
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1. Introduction In recent years, perchlorate has been widely used as an oxidizing agent in the production of rocket propellants, missile fuels, explosives, pyrotechnics, automobile airbags, batteries, and other industrial products (Lv et al., 2014). As such, more and
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more perchlorate is being discharged into the environment. Because perchlorate is highly water-soluble, diffusible, and accumulated readily as bioaccumulation
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(Srinivasan and Viraraghavan, 2009), it is difficult to generally entrap and degrade it,
which results in pollution diffusion and other environmental problems. The presence
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of perchlorate has also been detected in drinking water (Ting et al., 2006), which can
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environmental protection agencies.
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directly harm human health and have attracted the attention of governmental
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A number of effective technologies for removal of perchlorate from water have
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been developed in recent years. The latest review of relatively comprehensive summary about perchlorate treatment technologies was published in 2012. These
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technologies could be roughly grouped into four categories: physical-chemical
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treatment, chemical treatment, bioremediation, and combined processes (Ye et al., 2012). Bioremediation includes phytoremediation and microbial remediation. Plants and their rhizosphere microorganisms can degrade perchlorate in soil or water through
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the absorption, accumulation and metabolism of life activities (Seyfferth and Parker, 2007; Yifru and Nzengung, 2008; Bhaskaran et al., 2013). Microbial remediation by microorganisms and their enzymes can gradually reduce ClO4- to Cl- under anaerobic conditions, and the selected microbial species and electronic donor are key factors 3
affecting the degradation result (Wan et al., 2017; Zhang et al., 2016; Wen et al., 2016). The cost of bioremediation technology is relatively low and it is suitable for repairing large-scale or heavily ClO4- polluted areas in soil or water, but the bioremediation rate is relatively slow. Also, the metabolic products of the
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microorganisms may cause secondary pollution and other issues when used in the treatment of drinking water.
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Physical-chemical treatment includes adsorption, membrane filtration, and ion
exchange (IX). These methods are the most commonly used technologies for removal
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of trace-level ClO4-, and have the advantages of low cost, high treatment efficiency,
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and easy operation (Mahmudov and Huang, 2010; Gu et al., 2007; Xie et al., 2016b).
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However, physical-chemical treatment requires the disposal of regenerative brine or
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rejected streams with high concentrations of perchlorate (Srinivasan and Sorial, 2009;
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Fox et al., 2014). Chemical treatment includes chemical, electrochemical, and catalytic reduction, which can completely convert ClO4- into Cl-. However, the typical
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chemical reduction processes are relatively slow due to the high activation energy and
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stable structure of perchlorate, and the use of metal catalysts can lead to secondary pollution (Liu et al., 2014). Combined processes exhibit more flexibility and higher degradation efficiency during removal of ClO4- (Kim and Choi, 2014; Sharbatmaleki
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et al., 2015; Lian et al., 2016; Ren et al., 2015). Nevertheless, these technologies are still in the exploration stage and require further research before their practical application. Since physical-chemical treatment can efficiently remove ClO4- in trace level and 4
chemical treatment can completely convert ClO4- to Cl-, new materials and technologies on perchlorate removal are being rapidly developed. Based on Ye et al.’s (2012) review paper, this review systematically summarizes the key findings of technologies and materials of physical and chemical treatments on perchlorate
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removal from water in tables and also the research findings in recent years are elaborated in detail. In addition, nZVI and its modified materials for removal of
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perchlorate are described, which can thoroughly and efficiently reduce ClO4- to Cl-. The key findings, mechanisms, influencing factors and advantages or disadvantages
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of these recent researches are discussed in detail. This review has important
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environmental significance with respect to perchlorate pollution prevention and
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control.
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2. Pollution status and hazards of perchlorate
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2.1 Pollution status
At the end of the 1990s, due to improvements in separation and detection
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techniques, the detection limit for perchlorate was reduced from 400 μg/L to 4 μg/L,
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and even close to 1 μg/L (Dirtu et al., 2012). Since that time, perchlorate has been found to exist in various environmental media and the associated environmental
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problems caused by perchlorate has attracted increasing attention. Perchlorate has been detected in the environmental media of many countries. In
America, a survey was done using stable isotope ratios to trace perchlorate sources and behavior in the Laurentian Great Lakes, which found perchlorate concentrations in the Great Lakes ranged from 0.05 to 0.13 μg per liter (Poghosyan et al., 2014). In 5
Japan, Kosaka et al. (2007) investigated the perchlorate concentrations in 30 tap water samples and found perchlorate concentrations of more than 1 mg/L in 19 tap water samples and of more than 10 mg/L in 13 samples. In South Korea, Her et al. (2011) detected perchlorate in local bottled water, tap water, and seawater samples, and
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determined that local drinking water sources had also been polluted by perchlorate. In the Tamil Nadu State of South India, the observed concentrations of perchlorate range
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between 0.005–7690 μg/L in groundwater, 0.005–30.2 μg/L in surface water, and
0.063–0.393 μg/L in tap water. Researchers have also found that ClO4- in groundwater
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in the fireworks factory area are significantly higher than other regions, which
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indicates that the fireworks and safety match industries are principal perchlorate
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pollution sources (Isobe et al., 2013; Sijimol and Mohan, 2014). Nadaraja et al. (2017)
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firstly reported the spatio-temporal distribution of ClO4- at various higher levels in a
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5-km area around an ammonium perchlorate production unit in Aluva, Ernakulam, Kerala (India). They found the ClO4- concentration in ground water samples close to
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the production unit increased to > 40,000 μg/L, and detected a ClO4- concentration of
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1740 μg/L in well water 1.6 km from the production unit. The study results also indicate that a public pond in the area showed an increased ClO4- level up to 29,000 μg/L. These data reveal the high risks to humans exposed to environments associated
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with long-term ClO4-. Besides water, perchlorate has been detected in various other environmental media. A study of the concentrations of perchlorate in the dust fall of Malta in Europe suggested wherever intensive burning of fireworks takes place, the 6
environmental impact might be much longer lived than degraded, mainly due to the re-suspension and deposition of contaminated settled dust in the urban environment (Vella et al., 2015). In China, researchers have found perchlorate to be presented in milk, vegetables, saliva, serum, and breast milk (Wu et al., 2010a). Recently, Wan et
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al. (2015) conducted the first multinational survey of occurrence of perchlorate in indoor dust in 12 countries during the 2010–2014 period. The authors found the
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concentrations of perchlorate in dust to range from 0.02–104 μg/g (geometric mean: 0.41 μg/g) and the indoor dust samples from China contained the highest
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concentrations (geometric mean: 5.38 μg/g). Lybrand et al. (2016) found the
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perchlorate concentrations in the Atacama Desert to average 206 mg/kg, which is
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two to three orders of magnitude greater than in Antarctica and other sites. In
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summary, perchlorate commonly detected in various environmental media in
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different countries indicates that perchlorate pollution has become a global
2.2 Hazards
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environmental problem.
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Perchlorate was first identified as a chemical of concern by the U.S. EPA in 1985 following its discovery in wells at hazardous waste sites in California (Rice et al., 2007). And it is difficult to be reduced and easily mobilized in environmental media
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due to its inherent characteristics. There are several possible perchlorate transformation pathways in the environment and Fig. 1 illustrates the ways in which humans may be exposed to this contaminant (Kumarathilaka et al., 2016). The ingestion of food products containing perchlorate, which is mobilized 7
through the water-soil system and accumulates in edible plant species characterized by high human consumption, represents a potential human health risk due to its adverse effects on thyroid, hormone, and neuronal development, mainly in infants and fetuses (Calderón et al., 2017). Maternal placental supplies of thyroxine (T4), which is
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closely linked to iodine nutrition, is the major contributor to fetal serum T4 levels in early pregnancy and plays a crucial role in infants’ neurodevelopment in utero (de
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Escobar et al., 2004). Iodine deficiency may result in developmental delay in infancy,
particularly in language and memory skills, and subclinical disease induced by
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maternal hypothyroidism may lead to impaired neurological development and
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intelligence quotient (IQ) scores in their offspring (Leung et al., 2010).
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Perchlorate has been reported to competitively inhibit the uptake of iodide by a
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sodium iodide symporter (NIS) in the thyroid gland at medicinal levels (e.g., 800 mg)
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(Tonacchera et al., 2004). The EPA is leading efforts to set regulatory standards for perchlorate in drinking water and to derive a target maximum contaminant level for
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perchlorate (Lumen and George, 2017). As such, countries worldwide can be guided
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by U.S. in setting regulatory standards and are striving to develop effective methods for removal of perchlorate.
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3. Physical and chemical treatments for removal of perchlorate The inertia kinetic of perchlorate makes it difficult to be removed, especially
when the perchlorate concentration in water is low. Physical and chemical treatments are the most commonly used technologies for removal of perchlorate from water. The main technologies as well as the relative researches are summarized below. 8
3.1 Physical-chemical treatment 3.1.1 Ion exchange technology Ion exchange (IX) technology is one of the most effective and widely used methods for removal of perchlorate from water. Resins are the most commonly used
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IX material for removal of perchlorate. For drinking water slightly contaminated with ClO4- (10–100 ppb), treatment by selective cross-linked styrene-divinyl benzene
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resins is the most widely used process (Chen et al., 2012). Recently, a novel corn-stalk-based modified-magnetic-biopolymer IX resin (CS-MAB) was prepared
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and used to remove perchlorate from aqueous solution (Song et al., 2017). The
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authors found that the saturated magnetization and average pore diameter of CS-MAB
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were 3.82 emu/g and 4.83 nm, respectively. Adsorption batch experiments revealed
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the optimal adsorbent dosage and pH value for removal of perchlorate to be 1.0–1.5
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g/L and 3.0–10.0, respectively. The adsorption kinetics fitted well with the intraparticle-diffusion model in the first 90 min, and the pseudo-second-order model
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from 90–360 min, which suggested that the potential mechanisms of perchlorate
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removal were physical adsorption and an IX reaction between the ClO4- and Cl-containing groups of CS-MAB. Darracq et al. (2014) examined the IX capacity of five cationic resins (A532E, A520E, A400E, PWA-5, PSR-2) with regard to
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perchlorate via batch kinetics and isotherms. Their isotherm experiments showed that the IX equilibrium for perchlorate could be modelled by the Langmuir relation for all resins. This confirmed that A532E resins and PSR-2 had the highest specificity, which is a major advantage for the removal of trace amounts of perchlorate from water 9
containing relatively high concentrations of other anions. IX technology also uses some inorganic materials or their modified materials, such as montmorillonite, activated carbon (AC) and permselective membranes, to remove perchlorate. Chitrakar et al. (2012) reported anion-exchange properties of modified
with
hexadecylpyridinium
chloride
(HDPyCl-
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Na-montmorillonite
montmorillonite) for removal of perchlorate. The authors found the perchlorate taken
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up by HDPyCl-montmorillonite from 0.01 mmol/dm3 or 0.10 mmol/dm3 solution rapidly attained equilibrium within 4 h. These results revealed that the anion was
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taken up by HDPyCl-montmorillonite accompanied by the release of chloride ion into
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solution through anion-exchange processes. Lin et al. (2013) employed five cationic
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surfactants to modify activated carbon for removal of perchlorate and their results
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revealed that surfactants with smaller micelle structures were more easily loaded than
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those with larger micelles. In addition, they found that the activated carbon-loaded surfactant presented a much more positively charged surface, as manifested by the
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obvious improvement in perchlorate adsorption. This result demonstrated that
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perchlorate was mainly adsorbed via IX with surfactant-loaded AC. Wang et al. (2015) synthesized perchlorate permselective membranes with a thickness of 300 μm prepared with polyvinyl chloride (PVC) and quaternary ammonium salts (QAs) in
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solvent at room temperature. The authors found methyltributylammonium chloride (MTBA) to exhibit superior perchlorate permselectivity due in part to the favorable steric effect of the alkyl chain length, and the functional groups responsible for the IX to have been successfully incorporated in the membrane matrix. 10
The key findings of studies on the use of IX technology to remove perchlorate from water are summarized in Table 1. In general, IX is an effective method for removal of ClO4- in water, but there are also a number of drawbacks in its practical application. For example, resins used are of high prices, the IX regeneration liquid
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contains a high concentration of ClO4-. In addition, the coexisting ions in water affect the removal of ClO4-, and there are regeneration problems with selective IX resins. All
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these issues are not conducive to practical application in IX industry. 3.1.2 Membrane filtration
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Membrane filtration technology can be divided into the following three
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categories: (1) reverse osmosis (RO); (2) nanofiltration (NF) and ultrafiltration (UF);
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and (3) electrodialysis (ED). However, it is important to note that the reverse osmosis
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or electrodialysis has no limit requirement on removal of various irons, so these two
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technologies may remove all the ions in solution. Some studies have removed ClO4- from water with polyelectrolyte-enhanced
as
SO42-
and
NO3-)
on
removal
efficiency
of
ClO4- by
poly
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(such
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ultrafiltration (PEUF). Huq et al. (2007) investigated the effects of coexisting ions
(diallyldimethylammonium) chloride (PDADMAC)-enhanced ultrafiltration. The results showed that the ClO4- removal efficiency reached 90% or more in the absence
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of other coexisting ions when the amount of PDADMAC added ranged from 0.5–1 mmol/L and the initial concentration of ClO4- was 1 mmol/L, whereas the removal rate of ClO4- by PEUF was significantly reduced when coexisting ions were present. Roach and Tush (2008) also investigated the parameters affecting perchlorate 11
filtration (including polyelectrolyte concentration, pH, and ionic strength), and concluded that the perchlorate separations were greater than 95% even in the presence of 10-fold excesses of competing ions containing chloride, sulfate, and carbonate. Based on previous studies, Roach et al. (2011) compared the performances of poly
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(4-vinylpyridine) (P4VP)- and PDADMAC-enhanced ultrafiltration in the removal of perchlorate and found that the retention values of ClO4- by the protonated pyridine
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residues of P4VP were affected by polymer concentration, solution pH, and
competing-ion concentrations. In the absence of added salt, they observed slightly
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higher perchlorate retentions of PDADMAC compared to P4VP, but these retention
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values became more closely aligned as the concentration of added sodium chloride
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increased.
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RO and NF can also effectively remove perchlorate. Sanyal et al. (2015) used
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poly allylamine hydrochloride (PAH) and poly acrylic acid (PAA) to modify commercial NF membranes using layer-by-layer (LbL) assembly to enhance ion
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rejection of these membranes and achieve much higher permeability. The results
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demonstrated that the permeability of the modified membranes was 1.5 times of BW 30 membrane and 6 times of SW 30 membranes, respectively, and also had much superior permselectivity than the commercial membranes. The mechanism of ClO4-
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ion rejection by these modified membranes was a size-based exclusion rather than a charge-based separation. This research result is one of the highest permselectivity values reported thus far for PEM-based RO membrane specifically targeting monovalent ion removal. 12
The key findings on the use of membrane filtration technology to remove perchlorate are shown in Table 2. Inherent drawbacks, such as membrane fouling, a large amount of retention waste containing high concentrations of ClO4-, and high cost, make membrane filtration rarely applicable for removal of large-scale ClO4-.
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Although electrodialysis technology is more effective in removing ClO4- from wastewater than conventional membrane technology, it also results in excessive
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handling costs (Srinivasan and Viraraghavan, 2009). Therefore, few studies have
explored the use of membrane filtration technology for the removal of perchlorate in
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recent years and researchers have tended to combine this technology with other
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technologies to deal with perchlorate pollution in water.
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3.1.3.1 Activated carbon
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3.1.3 Adsorption
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Adsorption can effectively remove many kinds of pollutants in water, so it is the most widely used technology in treating drinking water and wastewater. AC is the
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earliest and most commonly used adsorbent for the removal of perchlorate, but its
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adsorption capacity for ClO4- is limited, so researchers tend to use modified methods to improve its adsorption performance and capacity. Some studies have used quaternary ammonium/epoxide-forming compounds
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(QAE) as modifying agents to remove perchlorate. Hou et al. (2013a) anchored QAE within bituminous and wood-based granular activated carbons (GACs) to enhance perchlorate removal from perchlorate-spiked natural groundwater. Using bituminousbased carbon pre-anchored with QUAB360 (QAE reagent) to process natural 13
groundwater that had been spiked with 30–35 ppb perchlorate, bed volumes (BVs) of 21,000 could be achieved. However, wood-based GAC anchored with this QUAB360 achieved only 3000 BV. The authors surmised that the active quaternary ammonium sites within tailored bituminous-based GAC were more exposed and thus more
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accessible for perchlorate adsorption than those within wood-based GAC. Based on this study, they developed a new method for anchoring QAEs in asphalt-based GACs
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(QAE-anchored GACs) for removal of perchlorate (Hou et al., 2013b). Statistical analysis revealed that the interaction of temperature and pH incurred the greatest
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effect on adsorption capacity. U17 and U2 exhibited 18–20 times greater BV to 6 ppb
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breakthrough than did the pristine GAC (900 BV) when processing groundwater that
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had been spiked with 30–35 ppb perchlorate. The main results of this research are
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shown in Fig. 2.
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Researchers have discovered that nitrogen functional groups and iron salts also can be modifying agents for removal of perchlorate. Byrne et al. (2014) introduced
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positively charged nitrogen functional groups into bituminous GAC to increase
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perchlorate removal efficiency from drinking water. The authors concluded that the functionalized bituminous GAC provided six times more perchlorate removal than pristine bituminous GAC. Moreover, the GAC media with absorbed perchlorate can
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be regenerated by electrochemical reduction, and its capacity for perchlorate adsorption could be mostly restored. The authors reported that this tailored functionality and redox regeneration of a relatively inexpensive media such as activated carbon could offer a novel opportunity for the adsorption industry. Xu et al. 14
(2016) synthesized and tested a new reactive material Fe-GAC by dipping various iron salts, such as NO3-, SO42-, or Cl-, onto GAC to remove perchlorate from water. They found that NO3- on Fe(NO3)3-GAC behaved as a strong competing ion with perchlorate, which resulted in the lowest perchlorate adsorption capacity. These
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results might help to guide the selection of the optimal iron salt to be impregnated on GAC for perchlorate adsorption from contaminated water in a continuous reactor
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(CTR). Fig. 3 shows a schematic of the perchlorate adsorption mechanism of Fe-GAC by impregnating various iron salts on GAC.
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Recently, researchers developed a cost-effective method with a high adsorption
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capacity for removal of perchlorate from drinking water using phosphoric-acid-treated
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GAC (GAC-PH) and acetic-acid-treated GAC (GAC-AA) (Krishnan et al., 2017). The
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study results showed that the equilibrium time was 60 min and remained the same for
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varying adsorbent dosages and adsorbate concentrations. The maximum removal efficiency was achieved at a pH of 1, which revealed that an acid pH was more
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favorable for removal of perchlorate. The mechanism of perchlorate adsorption by
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acid-modified GAC was mainly through chemisorption. Table 3 summarizes the key findings of studies investigating the removal of
perchlorate from water using AC and its modified materials. In the actual operation,
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ClO4- is vulnerable to penetration of AC adsorption column, which can reduce the adsorption effect. The modified AC can improve the adsorption capacity of ClO4-, but this process also increases the operational cost. In addition, the modification of AC affects its ability to adsorb other pollutants and the regenerated AC solutions 15
containing high concentrations of ClO4- is impractical. 3.1.3.2 Other adsorbent materials Apart from AC, a variety of other adsorbent materials have been investigated for removal of ClO4- from water, including chitosan and its modified materials, IX resins,
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bentonite/montmorillonite and their modified materials, granular iron hydroxide (GFH) and its modified materials, organoclay and its modified materials, and carbon
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nanotubes (CNTs) and other new materials. These adsorbent materials also exhibit excellent adsorption performance for removal of ClO4-. With respect to their chemical
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properties, these materials can be roughly divided into three categories: organic,
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(1) Organic adsorbent materials
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inorganic, and composite adsorbent materials.
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Chitosan and its derivative have abundant amino (-NH2) and hydroxy (-OH)
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functional groups that exhibit high modification and adsorption activities (AZLAN et al., 2009). Our research group conducted a series of perchlorate removal studies using
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chitosan and its modified materials. Xie et al. (2010) first used proton-crosslinked
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chitosan (PCLC) to remove perchlorate from aqueous solution and reported that the effluent perchlorate could be steadily maintained below 24.5 μg/L up to about 95 BV with a 10-mg/L influent of perchlorate. To balance the protonated degree of the amino
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groups and the effect of ions competing with respect to the adsorption capacity, the optimal pH value was determined to be about 4.0. Based on that study, the authors used cross-linked quaternary ammonium chitosan salt (QCS) to selectively remove perchlorate (Xie et al., 2012) and found the adsorption process to be nearly 16
independent with a pH ranging from 4.0 to 10.3, which was much wider than using PCLC with a pH range of 4.0–6.0. The perchlorate adsorption might be due to the ion-exchange interaction between the chloride ions on the cross-linked QCS and the perchlorate ions in solution. In addition, Xie et al.(2016b) synthesized cross-linked
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magnetic chitosan/poly (vinyl alcohol) beads (CM-CS/PVA Bs) to remove perchlorate from water. The CM-CS/PVA Bs could also efficiently adsorb perchlorate in a wide
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pH range from 4 to 10. The adsorption process was rapid and could reach equilibrium within 30 min. In addition, after adsorption, the CM-CS/PVA Bs could be regenerated
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easily by NaCl. However, as the actual samples were complex, higher sorbent dosages
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were needed in the field tests.
et
al.
(2013) used a newly synthesized 25, 27-bis-(N,
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water. Memon
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Resin is another efficient organic adsorbent for removal of perchlorate from
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N-dimethyl-2-aminoethyl) appended Amberlite XAD-4 resin (resin-5) to remove perchlorate. The authors concluded that the adsorption process followed
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pseudo-second-order kinetics. Fig.4 presents the effect of pH (2.5–7.5) on perchlorate
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adsorption by resin-5. Tang et al. (2013) investigated the adsorption characteristics of perchlorate on magnetic ion exchange (MIEX) resin and the kinetics data indicated that the perchlorate adsorption process reached equilibrium within 30 min. The results
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also revealed that major anions reduced the amount of perchlorate adsorption in the order SO42- > PO43- > CO32- > NO3-. However, humic acid had no obvious effect on perchlorate removal. Zhu et al. (2015) studied the perchlorate sorption rate and sorption capacity of three resins—Purolite A530E，Purolite A532E, and MIEX—and 17
found the Purolite A530E and Purolite A532E resins to exhibit superior perchlorate selectivity to that of the MIEX resin because of their matrix and functional groups. These results suggested that the removal efficiency of perchlorate increased with the increase of resin dosage and temperature of three resins, and that the sorption process
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was nearly independent of the solution pH over a wide range from 4–10. Song et al. (2015) investigated column adsorption of perchlorate by amine-crosslinked
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biopolymer based resin. It observed that columns with bed depths of 1.8, 3.4 and 5.1 cm adsorbed about 185.2, 170.4 and 158.9 mg/g of perchlorate, respectively. And
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breakthrough time and exhaustion time extended as the increase of bed depth.
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Faccini et al. (2016) studied the adsorption and desorption behavior of perchlorate
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onto a strong-base-anion (SBA) exchange resin and found that both the adsorption
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and desorption experimental data were best described by the pseudo-second order
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model, which suggested that the rate-limiting stage for the adsorption and desorption of perchlorate ion into the resin might be chemisorption.
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In addition, some novel organic adsorbent materials have been explored. Xing et
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al. (2015) used amine-impregnated cotton stalks (AICS) as an effective adsorbent for removal of perchlorate. The adsorption capacity in a fixed-bed column was optimum in neutral conditions (pH: 6.0 mg/g, 70.8 mg/g) with a bed depth of 2.7 cm and a flow
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rate of 5 ml/min. The authors found that perchlorate uptake by AICS was mainly based on the IX between loaded Cl- and free perchlorate ions, the possible adsorption mechanism of which is shown in Fig. 5. In addition, chemical regeneration by HCl or NaOH (0.1 mol/ L) achieved more than 95% regeneration efficiency. Li et al. (2015) 18
fabricated a novel cationic metal organic frameworks (MOFs) material ((NH2 SO3-Cu-(4, 4’-bipy)2), ASC) for removal of perchlorate, which can effectively remove perchlorate over a broad range of pH values from 2 to 11 at room temperature. Perchlorate adsorption on ASC could be described well by the pseudo-first-order
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kinetic model and Langmuir adsorption isotherm, which offered a simple and efficient route for perchlorate ion removal by the MOF materials. Recently, Zhang et al. (2017)
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synthesized two different MOFs, including Cu2(4,4’-bipy)2(O3SNH2) (ASC) and
Cu2(4,4’-bipy)2(O3SCH2CH2SO3) (ESC), to remove ClO4- and co-existing anions
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from aqueous solution. The authors found that the sorption rates of the anionic
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pollutants followed a pseudo-first-order reaction. These results demonstrated MOFs
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with a sulfonic acid ligand to be a promising alternative adsorbent for wastewater
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containing ClO4- and PO43-, whereas the tendency of the uptake capacity of PO43- was
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opposite that of ClO4-. The key findings of these studies on the removal of perchlorate
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by organic adsorbent materials are summarized in Table 4.
19
(2) Inorganic adsorbent materials CNTs have been an effective material used in removal of organic pollutants and heavy metal (Lin and Xing, 2008; Peng et al., 2005). Some researchers have also investigated the effect of perchlorate removal by CNTs. Fang and Chen (2012)
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investigated the adsorption of ClO4- with raw and oxidized CNTs to understand the affinity mechanism of CNTs with anion pollutants. The authors found the adsorption
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of ClO4- into different CNTs was progressively increased in the following order: multi-walled CNTs < single-walled CNTs < double-walled CNTs (DWCNTs).
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Maximum ClO4- adsorption occurred at pH = pHIEP (pH = the isoelectric point) rather
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than at pH < pHIEP, which could not be explained by electrostatic interactions alone.
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The results showed that co-existing anions significantly weakened ClO4- adsorption,
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whereas Fe3+ and cetyltrimethylammonium cations increased ClO4- adsorption
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2–3-fold. Hsu et al. (2015) studied the removal of perchlorate from water using single-walled CNTs (SWCNTs). The authors obtained lower perchlorate adsorption
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reaction rates using SWCNTs at higher temperatures and higher humic acid
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concentrations, which could be explained by the high humic acid concentrations inducing the compression of the electric double layer, which consequently reduced the
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surface potential energy and electrostatic repulsion. The calcined products of hydrotalcite compounds also be used as absorbents to
remove perchlorate. Yang et al. (2012a) used the calcination products containing Mg(II), Al(III), and Fe(III) with varying Mg/Al/Fe molar ratios at 550 ℃ as the adsorbent. It showed that the existence of ferric iron in calcined Mg/(Al-Fe) 20
hydrotalcite compound (CHMAF) was favorable to removal of perchlorate from water, and the best ratio of Mg/Al/Fe was 3:0.8:0.2 (CHMAF5%). Then Yang et al. (2012b) synthesized a new calcined iron-based layered double hydroxide material (MgFe-CLDH) to adsorb perchlorate. Results showed that the best synthesis
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conditions of the calcined MgFe-CLDH were the calcination temperature of 550 ℃ and [Mg]/[Fe]=3. At 25 ℃, MgFe-3 CLDH=1.33 g/L, the initial solution pH of 4-10,
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2000μg/L of perchlorate was almost all adsorbed within 720 min under the best
synthesis conditions. Lin et al. (2014a) prepared a series of Mg/Al carbonate layered
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double hydroxides (MgAl-LDHs) with different Mg/Al ratios and their calcined
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LDHs (MgAl-CLDHs) as a promising adsorbent of perchlorate. The results showed
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the adsorption of perchlorate to the parent MgAl-LDHs was very weak and
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independent of the positive charge density of the hydroxide layers, while the
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MgAl-CLDHs exhibited a high adsorption capacity which was mainly driven by the structural memory effect of the MgAl-LDHs with perchlorate as an interlayer anion.
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Fig. 6 presents the scheme of perchlorate uptake mechanism by MgAl-LDHs and
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MgAl-CLDHs. Then Lin et al. (2014b) further prepared MgAl-CLDHs, MgFeCLDHs and ZnAl-CLDHs with different M2+/M3+ ratios and compared their adsorption performance for perchlorate. It found that the adsorption of perchlorate
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with MgAl-CLDHs and MgFe-CLDHs increased as the M2+/M3+ ratio from 2 to 4, which was dominated by the structural memory effect and the hydrogen bonds. For ZnAl-CLDHs, however, the adsorption was hardly affected by the M2+/M3+ ratio and it was controlled by the structural memory effect only. The schematic of perchlorate 21
adsorption by the three kinds of CLDHs is demonstrated in Fig. 7. (Lin et al., 2014a).
Bentonite, as a common absorbent, has been extensively used to remove many types of pollutions, including perchlorate. Yang et al. (2013) synthesized three
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modified inorganic bentonites, including hydroxy-aluminum pillared bentonite (Al-Bent), hydroxy-iron pillared bentonite (Fe-Bent), and mixed hydroxy-iron-
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aluminum pillared bentonite (Fe-Al-Bent), to remove ClO4- from water. Their results indicated that the adsorption capacity of perchlorate followed the order: Fe-Bent > Fe–Al-Bent > Al-Bent, and that pH changes had little impact on the perchlorate
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adsorption by the Fe-Bent, whereas perchlorate removal by Al-Bent and Fe-Al-Bent
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decreased significantly with increases in pH. The key findings of research on the
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removal of perchlorate by inorganic adsorbent materials are summarized in Table 5. (3) Composite adsorbent materials
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A number of researchers have conducted a series of perchlorate removal studies
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using organics to modify montmorillonite, which can improve the removal rate of perchlorate compared with that achieved using raw montmorillonite. Luo et al.
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(2015a) was first to synthesize a series of organic montmorillonite (organic-Mt) using various cationic surfactants and the study results revealed that increasing the
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alkyl-chain length and number significantly enhanced the capacity and selectivity of perchlorate adsorption, but resulted in a decrease in the adsorption rate. In the same year, Luo et al. (2015b) synthesized a modified organo-montmorillonite (organic-MMT) with benzyloctadecyldimethylammonium chloride (BODMA-Cl), which typically has a long alkyl chain and a benzene ring. The results revealed that 22
the high adsorption capacity of perchlorate on organo-MMTs was achieved with a rapid adsorption rate that can be stably maintained despite variations in the temperature, the initial solution pH, and co-existing competitive anions. Fig. 8 illustrates the main results of this research. Then, Luo et al. (2016b) investigated the
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sequential modification of montmorillonite with dimethyl dioctadecyl ammonium chloride (DDAC) and benzyl octadecyl dimethyl ammonium chloride (BODAC) for
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removal of perchlorate and found that the simultaneous-mixing method resulted in
modified Mt with much poorer ClO4- removal performance than that of the sequential
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modification method, due to the negligible contribution of DDAC to the BODAC
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uptake by Mt. Subsequently, Luo et al. (2016a) investigated the adsorption of
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perchlorate onto hexadecylpyridinium -modified montmorillonite (OMt) using both
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in-situ (III-OMt) and ex-situ strategies, and compared innovative wet OMt without
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washing (I-OMt) and conventionally synthesized dried OMt (II-OMt) with washing in terms of their ClO4- adsorption densities. The results demonstrated that I-OMt to be a
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more suitable adsorbent than the other two for practical water treatment, and that the
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re-dispersion of I-OMt paste in solution should be improved in the future. Recently, Luo et al. (2017) evaluated the effects of grinding montmorillonite (Mt) and illite (Ilt) on the modification by DDAC and the adsorption of perchlorate and concluded that
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the swelling or self-exfoliation property and fully hydrated Na+ led to poor efficiency in the mechanical treatment of Mt in terms of the DDAC loading and ClO4- uptake. In contrast, significant improvement was achieved with Ilt because mechanical treatment facilitated the disintegration/exfoliation of the Ilt layers and exposed the unavailable 23
interlayer surface for anchoring the DDAC. Organoclays are a group of surfactant-modified clays with hydrophobic properties, which have been extensively used in the remediation of heavy metals, herbicides and pesticides, organic compounds, and anionic contaminants (Park et al.,
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2011). Seliem et al. (2013) synthesized three organosilicas, including MCM-41, MCM-48, and a mesoporous layered organosilica (MCM-50), as well as a
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commercially available organoclay (Cloisite® 10A) and tested these materials with
the occluded surfactant as adsorbents for removal of perchlorate. The results revealed
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MCM-48 silica to have the highest perchlorate uptake capacities. In addition, the
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authors found the amount of ClO4- uptake to decrease in the presence of SO42- for the
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MCM-41, whereas the uptake increased in the presence of SO42- and decreased in the
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presence of Cl- and CO32- for the MCM-48. As for the MCM-50, the uptake increased
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slightly in the presence of different anions, but was impervious to Cloisite® 10A in the presence of Cl-, SO42-, and CO32-. Bagherifam et al. (2014) prepared a cationic
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surfactant-modified organoclay using montmorillonite and hexadecylpyridinium
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(HDPy) chloride to remove nitrate and perchlorate anions and found the nitrate and perchlorate uptakes to reach equilibrium within 4 h. The authors also found the uptakes of nitrate and perchlorate by HDPy-montmorillonite to be highly selective in
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the presence of Cl-, SO42-, and CO32-. Clark and Keller (2012) studied the removal efficiency of oxyanions in drinking water sources, including perchlorate, nitrate, phosphate, and sulfate, onto magnetic permanently confined micelle arrays (Mag-PCMAs) in both competitive and 24
non-competitive environments. The authors found that perchlorate and nitrate did not compete significantly for binding sites on the Mag-PCMAs and had almost equal sorption efficiencies greater than 90%, which differed from the results obtained using ion exchange resins or activated carbon with cationic surfactants in which these
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anions would compete for sorption sites. Lv et al. (2014) used a cross-linked Fe(III)-Chitosan complex (Fe-CB) as a perchlorate adsorbent and found that a pH
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ranging from 3.0–10.2 exhibited very little effect on the adsorption capability. The
eluent of 2.5% (W=V) NaCl could efficiently regenerate the exhausted adsorbent. Fig.
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9 presents the mechanism of ClO4- sorption by cross-linked Fe-CB and Table 6 shows
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the key findings of studies investigating the removal of perchlorate by composite
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adsorbent materials.
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Compared with the perchlorate adsorption capacities of GAC or modified GAC
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in Table 3, the perchlorate adsorption capacities of other adsorption materials are much higher. It needs to note in particular that perchlorate adsorption capacity of the
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newly synthesized 25, 27-bis-(N, N-dimethyl-2-aminoethyl) appended Amberlite
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XAD-4 resin (resin-5) has reached 11.35 mmol/g, which is approximately 30 times that of the modified GAC. At present, the research on novel adsorbent materials is not very mature, with most studies still in the experimental research stage. The
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temperature, pH, the nature of the adsorbent, the amount of adsorbent, the coexistence of anions, and other factors will affect the removal effect of ClO4-. These influencing factors and adsorption mechanisms require further study. 3.2 Chemical treatment 25
3.2.1 Chemical reduction Chemical reduction technology can completely convert perchlorate into chloride, and with respect to environmental impact, it is an environmentally friendly method for removal of perchlorate. The reaction equation for the removal of ClO4- in water is
ClO4- + 8H+ + 8e- → Cl- + 4H2O E0 = 1.287 V (1-1)
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as follows (Srinivasan and Viraraghavan, 2009):
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Some researchers have used oxidized titanium ions to reduce perchlorate. Park
et al. (2012a) used aqueous Ti(II) produced by the oxidative dissolution of zero-valent
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titanium as a perchlorate reducing agent. The results showed that a low pH was
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needed to produce Ti (II) from Ti (0) and the acid amount increased as the
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concentration of Ti (0) increased. Kinetic data showed that HCl was more effective
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than H2SO4 in promoting perchlorate degradation in lower pH conditions. Next, the
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authors investigated the chemical degradation of perchlorate using Ti(II) and Ti(III) (Park et al., 2012b) and found that the rate of perchlorate degradation was fastest at
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the lowest F/Ti (0) ratio of 0.5 (25 mM KF). Of the catalysts investigated, only
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rhenium enhanced the perchlorate degradation in the presence of Ti(II), whereas no catalyst effect was observed in the presence of Ti(III). The authors also concluded that high ionic strength did not enhance the perchlorate-Ti(III) reaction whereas high acid
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concentration did. The rate constants for perchlorate degradation in Ti(III) were double that in Ti(II) when using 5 N HCl. Increasing the temperature can increase the decomposition efficiency of ClO4-. Hori et al. (2012) studied the effect of pressurized hot water (PHW) on the 26
decomposition of perchlorate and found that ClO4- demonstrated little reactivity in pure PHW up to 300 ℃. At the same time, the addition of zero-valent metals to the reaction system enhanced the decomposition of ClO4- to Cl- with an increasing activity order of (no metal) ≈ Al < Cu < Zn < Ni << Fe. It is worth mentioning that
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this method was successfully used in the decomposition of ClO4- in a water sample contaminated with ClO4- following a fireworks display at Albany, New York, USA.
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The main results of this research are shown in Fig. 10. Vellanki and Batchelor (2013) used sulfite/ultraviolet advanced reduction processes (ARP) to reduce perchlorate and
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their results indicated that the rate of perchlorate degradation by sulfite/UV-L
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accelerated with increases in the pH, temperature, and sulfite concentration.
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Table 7 shows the key findings for the removal of perchlorate by chemical
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reduction. Since the chemical reduction of perchlorate is a relatively slow process, it
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is necessary to adopt a strong reducing agent with high activity or to add a catalyst to reduce the activation energy of the reaction. Also, most of the metal reducing agents
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are toxic and unsuitable for the removal of perchlorate in drinking water.
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3.2.2 Electrochemical reduction Electrochemical reduction can also convert perchlorate into chloride without the
addition of a catalyst to reduce the activation energy of the reaction. Wang et al.
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(2009) investigated the removal of perchlorate and nitrate at the Ti-water interface by an indirect electrochemical reduction process using a single perchlorate or nitrate solution and a coexisting solution. The results indicated that perchlorate and nitrate could be simultaneously and readily reduced at the surface of a Ti anode. The 27
dominant end product of this indirect electrochemical reduction was chloride or nitrite in a synthetic single-anion solution, and the nitrite concentration was negligible when these two anions were coexisting. The removal efficiency of perchlorate by zero-valent metals or soluble metal,
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which have reducing property, is said to be kinetically limited. But Lee et al. (2011) concluded that zero-valent titanium (ZVT) and its soluble states had a high
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thermodynamic potential to reduce perchlorate. These authors measured the pitting
potential of ZVT as 12.77 ± 0.04 V (SHE) for a 100-mM solution of perchlorate. The
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experimental results showed that solution pH and the surface area of the ZVT
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electrodes had negligible effects on the rates of perchlorate reduction. However, this
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process might not be immediately applicable to perchlorate treatment due to its high
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potentials, huge amount of titanium consumed, and inhibition from chloride.
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Table 8 summarizes the key findings for the removal of perchlorate by electrochemical reduction. The main disadvantages of electrochemical reduction are
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that the electrode is prone to be corrosive and it consumes a large amount of
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electricity in the removal process. Hence, developing less expensive and better performing electrode materials will facilitate improvements in the removal efficiency of perchlorate by electrochemical reduction technology.
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3.2.3 Catalytic reduction Catalytic reduction technology can effectively reduce the activation energy of the reduction of perchlorate. A previous study demonstrated that Re-Pd/C could effectively transform aqueous perchlorate via chemical reduction using hydrogen as 28
an electron donor at ambient temperature and pressure (Choe et al., 2010). The results of this study revealed that the catalyst activity and stability were heavily dependent on the solution composition and Re content in the catalyst. In a similar experiment, these researchers related these parameters to changes in the speciation and molecular
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structure of Re immobilized on the catalyst and investigated the mechanism of ClO4removal with a Re-Pd/C catalyst (Choe et al., 2014). The authors concluded that the
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identified Re structures supported a revised mechanism for the catalytic reduction of
ClO4- involving oxygen atom transfer reactions between odd-valence oxorhenium
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species and the oxyanion (Re oxidation steps) as well as atomic hydrogen species (Re
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reduction steps) formed by the Pd-catalyzed dissociation of H2. Liu et al. (2013)
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examined the applicability of Re-Pd/C catalyst in a perchlorate reduction process for
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treatment of waste IX brines. Experiments conducted in synthetic NaCl-only brine
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(6-12 wt %) showed higher Re-Pd/C catalyst activity than in comparable freshwater solutions, but the rate constant for ClO4- reduction measured in a real IX waste brine
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was found to be 65 times lower than in the synthetic NaCl brine.
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Photocatalysis and photoelectrocatalysis are emerging technologies that have been successfully applied in the degradation of toxic environmental pollutants (Hamadanian et al., 2011; Gong et al., 2013). Ye et al. (2013) evaluated the
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photocatalytic reduction of aqueous perchlorate using Cu-TiO2/SiO2 catalysts in the presence of a hole scavenger (citric acid, Cit) in a UV/Cu-TiO2/SiO2 system. The results indicated that the catalyst had the best catalytic activity when the nominal mass ratio of Cu2+ to TiO2 was 0.5%. The authors also concluded that Cl- was the end 29
product whereas ClO3- was the main intermediate in the course of ClO4-→Cl- in the presence of Cit. However, the ClO4- concentration increased after Cit had been exhausted. Then, Jia et al. (2016) investigated the photoelectrocatalytic reduction of aqueous perchlorate using Ag-nanoparticle-loaded TiO2 nanotube arrays (Ag-TNTs)
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in the same environmental conditions. The results indicated that Ag-TNTs had the best photoelectrocatalytic activity when the nominal mass ratio of Ag to TiO2 was 0.84%.
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The mechanism deduced by the authors is as follows: ClO4- was reduced by the ClO2generated in the solution or by the e- transferred to the photocathode through the
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external circuit.
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Liu et al. (2015) used a highly active catalyst prepared by the noncovalent
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immobilization of the rhenium complex ReV(O)-(hoz)2Cl (hoz = 2-(2’-hydroxyphenyl)
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-2-oxazoline), together with Pd0 nanoparticles on a porous carbon support, to reduce
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ClO4- to Cl- with 1 atm H2 at 25 ℃. The results showed that the immobilized Re complex served as a single site for oxygen atom transfer from ClO4- and ClOx-
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intermediates, whereas Pd0 nanoparticles provided atomic hydrogen reducing
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equivalents to sustain the redox cycling of the immobilized Re sites. The authors suggested that hybrid catalysts, which combined single-site transition metal complexes with hydrogenactivating metal nanoparticles, could be integrated into
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heterogeneous catalysts for use in environmental remediation and green chemistry applications. The proposed schematic summary of the transformation of Re species during Re(hoz)2-Pd/C catalyst preparation, catalytic reactions with ClO4-, and the potential catalyst decomposition is shown in Fig. 11. 30
Table 9 shows the key findings for the removal of perchlorate by catalytic reduction technology in water. The catalyst can improve the reduction efficiency of ClO4-, but the reaction conditions of catalytic reduction (such as solution pH, temperature) have certain requirements and the use of precious metals also increases
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the treatment cost. In addition, the use of a metal catalyst may bring additional environmental risks, which requires further study.
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3.3 nZVI/modified nZVI for removal of perchlorate
In recent years, nanoscale zero-valent iron (nZVI) particles, which has the
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advantages of large specific surface area, high reaction activity, and strong reductive
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power, has been successfully developed and used for removal of various wastewater
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contaminants (such as nitrite, selenate, organic dyes, aromatic halides, and heavy
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metals contaminants) (Shi et al., 2018). Studies have shown that nZVI and especially
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modified nZVI demonstrate an excellent perchlorate degradation rate. Cao et al . (2005) investigated perchlorate removal by nZVI and found that nZVI could
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effectively reduce ClO4- in water at a relatively high reaction temperature. Their
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results showed that the ClO4- (initial concentration 200 mg/L) removal efficiency could be nearly 90% at 75 °C within 24 hours with a nanoparticle dose of 10 g/L. A related study has shown that after the addition of nZVI to perchlorate solution under
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acidic conditions, the nZVI particles undergo the following reactions (Xie et al., 2016a): 2Fe0（s） + 4H+(aq) + O2(aq) → 2Fe2+(aq) + 2H2O(l) Fe0(s) + 2H2O(l) → Fe2+(aq) + H2(g)+ 2OH-(aq) 31
(1) (2)
ClO4-(aq) + 4Fe0(s) +8H+(aq) → 4Fe2+(aq) + Cl-(aq) + 4H2O(l)
(3)
However, the stability and reactivity of nZVI would be greatly reduced due to the tendency for agglomeration and easy oxidation, which limits its practical use for contaminant removal. To overcome these shortcomings, a series of modification
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methods have been explored, which can be roughly divided into two categories: supported nZVI and stabilized nZVI.
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3.3.1 Supported nZVI materials
The Supported nZVI is obtained by evenly diffusing the nZVI over the carrier
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surface of the with certain material as a carrier, which can effectively prevent nZVI
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particles from agglomeration and oxidation and maintain its high reactivity. Xu et al.
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(2010) synthesized iron compounds supported on granular activated carbon (ICs/GAC)
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for removal of perchlorate and found that ICs/GAC contained large amounts of
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FeOHSO4 and Fe2O3, and a small amount of nZVI, and that the perchlorate adsorption on FeOHSO4 and Fe2O3 played an important role in removal of perchlorate. Then, the
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authors synthesized and characterized a new nanoscale iron hydroxide-doped granular
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activated carbon (Fe-GAC) and tested it for the adsorption of ClO4- in water (Xu et al., 2013). They found that the solution pH and iron content were two key factors controlling the perchlorate removal efficiency, and determined the optimal pH range
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to be 2–3 with an adsorbing capacity of 17.5 mmol/1 g iron. In addition, they found that coexisting anions might slow down the perchlorate adsorption in the order NO3- > SO42- > Cl-. Electrostatic attraction, ion exchange, and surface complexation were the main mechanisms of the perchlorate adsorption. Based on these two studies, the 32
authors then conducted a comparative study of Fe-GAC for removal of bromate and perchlorate from water (Xu et al., 2015). They found that Fe-GAC exhibited much greater removal capacity of bromate than perchlorate. Fe-GAC performed well within the optimal pH range of 6–8 in the reduction of bromate, whereas the removal of
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perchlorate was more dependent on pH . Loading nZVI onto solid materials is cheap and easily performed, and can not
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only overcome the characteristics of agglomeration and oxidation of nZVI, but also
increase the treatment availability of effluents containing perchlorate. At present, the
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carrier material used in supported nZVI for the removal of perchlorate is basically
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limited to activated carbon. However, many other materials, such as pumice, clay
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minerals, and graphite, have demonstrated excellent support performance on nZVI
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particles. Therefore, the removal of perchlorate with supported nZVI materials has a
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wide research scope. 3.3.2 Coated nZVI materials
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Coated nZVI is the nZVI coated on polymer materials or surfactants, which can
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enhance the dispersal performance and antioxidant properties while maintaining the high reactivity of nZVI. Xiong et al. (2007b) tested the feasibility of two different stabilized nZVI materials, including starch- and carboxymethyl cellulose(CMC)
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-stabilized nZVIs, for removal of perchlorate in water or IX brine. The authors showed that about 90% of perchlorate in both fresh water and a simulated IX brine (NaCl = 6% (w/w)) was destroyed at an iron dosage of 1.8 g/L at moderately elevated temperatures (90–95 °C) within 7 h. The addition of a metal catalyst (Al, Cu, Co, Ni, 33
Pd, or Re) showed no significant improvement in perchlorate degradation. Xie et al.(2016a) prepared chitosan-stabilized nZVI (CS-nZVI) to test the degradation of perchlorate in water. Their results showed that CS-nZVI exhibited an excellent removal rate compared with ZVI and nZVI, especially in high-concentration (200
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mg/L) perchlorate-polluted water. Most perchlorate could be reduced to chloride through initial adsorption and subsequent reduction processes. This research provided
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an effective method for perchlorate degradation in contaminated water with high concentrations or high salinity.
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As coated nZVI has a high dispersion property and reactivity, its ability to
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degrade perchlorate is far superior to that of ordinary nZVI. As yet, there have been
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few studies of the removal of perchlorate by stabilized nZVI, but more types of coated
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nZVI materials have been obtained. Therefore, this technology has good prospects in
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the study of perchlorate removal by coated nZVI. The key findings of studies on the removal of perchlorate by coated nZVI materials and supported nZVI materials are
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summarized in Table 10.
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4. Conclusions and future prospects 4.1 Conclusions
The environmental pollution problems caused by perchlorate have brought
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serious harm to human health and threatens human survival as well as development. Its characteristics of high water solubility, high stability, less volatility, and difficulty in being absorbed make it difficult to be removed. This paper summarizes and analyzes the general removal techniques of perchlorate in water by physical-chemical 34
treatments and chemical treatments, including the application of new nZVI materials. The main conclusions are as follows: (1) Physical-chemical treatments are efficient technologies for the removal of ClO4- in trace level that does not cause secondary pollution and could be used to
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remove ClO4- in drinking water. IX seems to be one of the most effective technologies for removal of trace perchlorate in water. However, the resins are of high prices and
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needed to be regenerated, and the high concentration of regenerated liquid containing
perchlorate must to be disposed. Membrane filtration can effectively remove
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perchlorate, but its inherent drawbacks include membrane fouling and high cost,
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rendering it impractical for large-scale applications of ClO4- removal. In recent years,
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there have been only fewer studies to remove perchlorate using membrane filtration
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combined with other methods.
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technology, and it has been more liked to deal with perchlorate pollution in water
(2) Adsorption is another widely used technology that can effectively remove
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perchlorate in water. GAC or modified GAC is the earliest and commonly used
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adsorbent for removal of trace perchlorate in water. However, more and more innovative materials have been explored to adsorb perchlorate, and their adsorption capacities of perchlorate are much higher than GAC or modified GAC. It is
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particularly to note the newly synthesized 25, 27-bis-(N, N-dimethyl-2-aminoethyl) appended Amberlite XAD-4 resin, which has approximately 30 times adsorption capacity than that of modified GAC. Although adsorption technology can effectively remove perchlorate from water, the regenerating solution of the adsorbents contains a 35
high concentration of ClO4-, which must also be disposed. Most of the new adsorbent materials are in the experimental research stage and their potential for practical use requires further study. (3) Chemical treatments can thoroughly degrade perchlorate to chloride, so it
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could be used to treat high-concentration perchlorate wastewater. However, the chemical reduction rate is relatively slow, so it is necessary to use a strong reducing
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agent with high activity or to add a catalyst to reduce the activation energy of the
reaction. Also, most of the metal reducing agents are toxic and not suitable for the
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removal of perchlorate in drinking water. Electrochemical reduction can convert ClO4-
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into Cl- without the addition of a catalyst, but the reduction process is
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electricity-intensive and the electrode is prone to be corrosive. Catalytic reduction can
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improve the reduction efficiency of ClO4-, but the reaction conditions are relatively
further studied.
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harsh, and the use of a metal catalyst may bring environmental risks, which should be
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(4) Modified nZVI, which overcomes the drawbacks of easy agglomeration and
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oxidation, improves the practical potential of materials. Currently, the modified nZVI materials used for treatment of perchlorate mainly include supported nZVI and coated nZVI. These two kinds of materials can rapidly and effectively remove perchlorate by
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the reaction activity of nZVI. It's worth mentioning that coated nZVI is superior for treatment of high-concentration perchlorate and can effectively degrade ClO4- into Cl-, so this material has good prospects for removal of perchlorate. As yet, research on modified nZVI has been relatively scant. Hence, there remains an enormous research 36
scope in this field. 4.2 Future prospects Today, the exploration of new materials and technologies which are more economical and efficient for treatment of perchlorate more has become a hot research
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topic. This review may have an important environmental significance with respect to pollution prevention and control problems related to perchlorate. Based on the
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conclusions drawn in this review, some prospects about perchlorate removal are proposed in the end.
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(1) Single water treatment processes are characterized by drawbacks that have an
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effect on pollutant removal efficiency. Processes that combine the advantages and
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disadvantages of two or more technologies may have more flexibility and higher
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degradation efficiency for removal of pollutant. Therefore, combined processes are a
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promising treatment approach for removal of perchlorate . (2) The regeneration liquid produced in physical-chemical treatments usually
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contains a high concentration of ClO4- and, as such, presents an awkward problem.
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The effective disposal of this liquid could be achieved by chemical or microbiological methods.
(3) Modified nZVI materials have strong application advantages for the removal
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of organic and heavy pollutants in water, but their current applications in perchlorate removal are relatively few. With respect to the modification methods of nZVI and its existing pollutant degradation mechanisms, further improvements and the optimization of perchlorate removal in water would be broadened in its application 37
prospects and engineering value about pollution remediation.
Acknowledgements This work was supported by grants from the Chinese National Natural Science
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Foundations (No.41472230).
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A
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waters, and bottled water from China and its association with other inorganic anions and with
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N
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A
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M
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N
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U
carbon coated with cetyltrimethyl ammonium bromide. J. Colloid Interface Sci. 357,
A
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M
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Ye, L., You, H., Yao, J., Su, H., 2012. Water treatment technologies for perchlorate: A review.
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Yifru, D.D., Nzengung, V.A., 2008. Organic carbon biostimulates rapid rhizodegradation of perchlorate. Environ. Toxicol. Chem. 27, 2419–2426. https://doi.org/10.1897/08-008.1 Yoon, J., Amy, G., Chung, J., Sohn, J., Yoon, Y., 2009. Removal of toxic ions (chromate,
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arsenate, and perchlorate) using reverse osmosis, nanofiltration, and ultrafiltration
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membranes. Chemosphere 77, 228–235. https://doi.org/10.1016/j.chemosphere.2009.07.028
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Yoon, J., Yoon, Y., Amy, G., Cho, J., Foss, D., Kim, T.H., 2003. Use of surfactant modified
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ultrafiltration for perchlorate (ClO4-) removal. Water Res. 37, 2001–2012. https://doi.org/
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Yoon, Y., Amy, G., Cho, J., Her, N., Pellegrino, J., 2002. Transport of perchlorate (ClO4-) through NF and UF membranes. Desalination 147, 11–17. https://doi.org/10.1016/S0011-9164(02) 00564-7
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Yoon, Y., Amy, G., Yoon, J., 2005. Effect of pH and conductivity on hindered diffusion of perchlorate ions during transport through negatively charged nanofiltration and ultrafiltration
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Zhang, Y., Chen, J., Wen, L., Tang, Y., Zhao, H., 2016. Effects of salinity on simultaneous reduction of perchlorate and nitrate in a methane-based membrane biofilm reactor. Environ. Sci. Pollut. Res. 23, 24248–24255. https://doi.org/10.1007/s11356-016-7678-x Zhang, Y., Hurley, K.D., Shapley, J.R., 2011. Heterogeneous Catalytic Reduction of Perchlorate 52
in Water with Re-Pd/C Catalysts Derived from an Oxorhenium (V) Molecular Precursor. Inorg. Chem. 50, 1534–1543. https://doi.org/10.1021/ic102158a Zhang, Y., Mu, S., Deng, B., Zheng, J., 2010. Electrochemical removal and release of perchlorate using poly(aniline-co-o-aminophenol). J. Electroanal. Chem. 641, 1–6. https://doi.org/10. 1016/j.jelechem.2010.01.021 Zhu, Y., Gao, N., Wang, Q., Wei, X., 2015. Adsorption of perchlorate from aqueous solutions by
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anion exchange resins: Effects of resin properties and solution chemistry. Colloids Surf. A
A
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PT
ED
M
A
N
U
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Physicochem Eng. Asp. 468, 114–121. https://doi.org/10.1016/j.colsurfa.2014.11.062
53
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A
N
U
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Fig. 1. Flow, transport, and transformation pathways of perchlorate in the environment (Kumarathilaka et al., 2016).
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ED
M
Fig. 2. Schematic showing main results of perchlorate removal by QAE-anchored GACs (Hou et al., 2013b).
A
Fig. 3. Perchlorate adsorption mechanism of Fe-GAC by impregnating various iron salts on GAC (Xu et al., 2016).
54
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M
A
N
U
SC R
Fig. 4. Effect of pH on the percent adsorption of perchlorate (Memon et al., 2013).
A
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Fig. 5. Adsorption mechanism of AICS for perchlorate (Xing et al., 2015).
Fig. 6. Proposed scheme of perchlorate uptake mechanism by MgAl-LDHs and MgAl-CLDHs
55
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M
A
N
U
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Fig. 7. Schematic of perchlorate adsorption by the three kinds of CLDHs (Lin et al., 2014b).
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Fig. 8. Schematic of the main perchlorate removal results by organic MMTs (Luo et al., 2015b).
A
Fig. 9. Mechanism of ClO4- sorption by cross-linked Fe-CB (Lv et al., 2014).
56
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Fig. 10. Schematic of main perchlorate removal results by PHW (Hori et al., 2012).
A
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M
A
N
U
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Fig. 11. Proposed scheme for immobilization, reaction, and decomposition of hoz-coordinated Re species (Liu et al., 2015).
57
Table 1 – Application of ion exchange for perchlorate treatment. Ion-exchange medias
Key
Findings
A530E, A500, A520E, and A850), Bio-Rad (Dowex 1-X8), and Wandong Chemical
references
The presence of SO42- had little impact on the sorption of U(VI) but significantly affected the sorption of ClO4-, particularly on monofunctional
(Gu et al., 2005)
SBA resins.
Plant(WBR109)
Bifunctional resin (Purolite
groundwater before a significant breakthrough of ClO4- occurred at an average -
A-530E)
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Purolite A-530E treated about 37000 empty bed volumes (BVs) of
flow rate of 150 gpm (or 1 BV/min) and a feed ClO4 concentration of about
SC R
860 µg/L. SBA, WBA exchangers,
IXF offered a comparable perchlorate capacity to that of styrenic resins as
A-530E, PLEs, and IXF
well as unparalleled kinetics (with a sorption equilibrium time < 1.5 h).
Mono- or bifunctionalized
bifunctionalized mesoporous
The ammonium bifunctionalized mesoporous medium showed a 1.14–1.39
U
SBA-15, IRA-900 and QA
times higher adsorption capacity than those of mono-functionalized SBA-15.
N
molecular sieves
(Gu et al., 2007)
(Xiong et al., 2007a)
(Kim et al., 2008)
The HIX prepared via the in-situ precipitation of Fe(III) (M treatment) had higher separation factors for ClO4- over Cl- than the HIX obtained by the
(Hristovski et
media
KMnO4/Fe(II) treatments, suggesting that permanganate may adversely impact
al., 2008)
Copolymer
(PANOA) film Polyaniline (PANI) films
The copolymer PANOA had fairly good redox properties and affinity to perchlorate over chloride in a wide pH range of 1 to pH 9.0.
ED
poly(aniline-co-o-aminophenol)
M
the IX base media.
A
Hybrid ion-exchange (HIX)
PANI-H2SO4 film represented higher selectivity for ClO4- than PANI-HCl and PANI-HClO4.
PT
The
HDPyCl-montmorillonite
ClO4-
(Zhang et al., 2010) (Gao et al., 2011)
uptake on HDPyCl-montmorillonite followed the Langmuir
isotherm model with capacity of 1.02 mmol/g.
(Chitrakar et al., 2012)
The Qmax of perchlorate was increased nearly 3 times with surfactant loading,
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CPC, HDTMA, CTAC, MTAB, DTAB
A532E, A520E, A400E,
A
PWA-5, PSR-2
MTBA membrane
which was mainly attributed to the extra adsorption sites contributed by the quaternary ammonium groups from surfactants. The maximal exchange capacities for perchlorate were of the same order of magnitude for all resins (from 183 mg/g for A400E to 115 mg/g for A532E).
2013) (Darracq et al., 2014)
About 60% of ClO4- and 9% of other anions, specifically, SO42-, NO3- and HCO3-, passed through the membrane under otherwise identical operation conditions while in the presence of an electric field.
CS-MAB
(Lin et al.,
The maximum adsorption capacities of CS-MAB were 119.05 mg/g, 126.58 mg/g and 178.57 mg/g at 20 °C, 30 °C, and 40 °C, respectively.
58
(Wang et al., 2015) (Song et al., 2017)
Table 2 – Application of membrane technologies for perchlorate treatment. Membrane
Findings
Key references
ClO4- could be excluded by NF and UF membranes in a pure component system, but this
(Yoon et al.,
rejection capability was quickly lost in the presence of a sufficient amount of other ions.
2002)
Although the ClO4- rejection capability was reduced in the presence of a cationic
(Yoon et al.,
technologies NF and UF Surfactant modified
-
surfactant, a desired amount of the ClO4 was excluded by steric exclusion.
2003)
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UF Negatively-charged
Hindered (effective) diffusion associated with partitioning of ClO4- was affected by
porous NF and UF
charge repulsion for negatively charged porous membranes.
Cr(VI), As(V), and ClO4- rejection followed the order: LFC-1 (>90%) > MX07 (25–95%) > ESNA (30–90%) > GM (3–47%) at all pH conditions.
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RO, NF, and UF
About 60–80% of ClO4- was removed with 30-mM PDADMAC in the presence of both
PDADMAC
-
1-mM to 10-mM NO3 and SO4
whereas greater than 95% was removed even in the
presence of 10-fold excesses of competing ions such as
Cl-,
SO4
2-,
2-.
and CO3
U
enhanced PEUF
2-,
P4VP and
provided up to 95.8% retention of ClO4- under the solution conditions investigated.
A
enhanced PEUF
N
The greater affinity of ClO4- over chloride for the protonated pyridine residues of P4VP
PDADMAC
PEM-based RO and
of NF by PAH and
About 93% perchlorate rejection was achieved when both PAH and PAA were deposited
M
surface modification
on a NF 90 membrane at a pH of 6.5 and cross-linked with glutaraldehyde.
A
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PAA
59
(Yoon et al., 2005)
(Yoon et al., 2009)
(Huq et al., 2007; Roach and Tush, 2008)
(Roach et al., 2011)
(Sanyal et al., 2015)
Table 3 – Application of GAC or modified-GAC adsorption for perchlorate treatment. Findings
materials
-
wood, lignite coal, and bituminous coal Modified GAC with iron-oxalic acid
Maximum adsorption
Adsorption
Key
capacity
model
references
The major mechanism for ClO4 adsorption on
0.32 mmol/g for
ACs was specific chemical interactions between
Filtrasorb 400 and
-
ClO4
and
surface
functional
groups
in
0.19 mmol/g for
combination with electrostatic forces. The adsorption capacity of preloaded GAC with iron-oxalic acid could be improved up to 42%.
Not obtainable
with QAs
removed for 27,000–35,000 bed volumes before
Not mentioned
-
the effluent ClO4 rose above 1 ppb. GAC preloaded with CTAC shown to be an
Not
(Na et al.,
obtainable
2002)
Not
and
mentioned
Cannon,
Not mentioned
2005)
Not
(Parette et
mentioned
al., 2005)
Not
(Chen et
mentioned
al., 2005)
Langmuir and
(Jang et
Freundlich
al., 2009)
N
groundwaters containing ClO4- concentrations
U
effective adsorbent for removing perchlorate from three low conductivity (50–66 mS/cm)
Huang, 2010)
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THAB, CTAC or CPC, 75 ppb ClO4 was
ov and
(Parette
-
GAC tailored
with CTAC
Langmuir
Nuchar SA
By pre-loading bituminous AC with DTAB,
GAC preloaded
(Mahmud
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Adsorption
A
of 0.85 ppb, 1.0 ppb, and 5.6 ppb, respectively.
Modified GAC could increase the nitrogen
with ammonia
content of the carbon, thereby significantly
M
GAC tailored
increasing the positive charge of the carbon
Not mentioned
surface, which was the most important surface
with iron or QA GAC tailored
4500 bed volumes before 6 mg/L perchlorate
2C-75
Bituminous-based GAC that had been preloaded with 0.24g/g arquad 2C-75 exhibited a 6-μg/L
A
Not obtainable
-
ClO4 breakthrough after 33,000 BVs.
et al., 2010)
pH 2.5
(Xu et al.,
The primary mechanisms for the adsorption of
and 0.094 mmol/g at
while
2011a)
perchlorate on GAC-CTAC or GAC-CTAB
pH 5.6
Freundlich at
surface complexation, and ion exchange.
with CTAB
Fe-GAC
obtainable
(Patterson
0.36 mmol/g at pH 2.5
pH 5.6
were associated with electrostatic interaction, GAC coated
Not
Langmuir at
GAC coated with CTAC
Not mentioned
breakthrough.
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with arquad
GAC impregnated with iron and QA could treat
PT
GAC tailored
ED
property influencing adsorption.
0.13 mmol/g
The type of iron salt impregnated on GAC
FeCl2-GAC
played an important role in the perchlorate
(0.121 mmol/g) ≈
adsorption, and electrostatic attraction was the
FeCl3-GAC(0.117
main mechanism.
mmol/g) 60
Freundlich
Langmuir
(Xu et al., 2011b)
(Xu et al., 2016)
more positive surface charge at pH 7.5 than did QAE-anchored GACs
U17 and U2 exhibited 18–20 times longer bed
pristine GAC (900 BV).
ted GACs
times more perchlorate removal than pristine
32.48 mg/g for U2
mentioned
Not mentioned
processes
were
rapid
and
obeyed
the
20.1, 37.5 and 44.0 mg/g for GAC, GAC-AA and
pseudo-second order model.
GAC-PH at 303 K,
A
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M
A
N
U
respectively
61
(Hou et al., 2013a) (Hou et al., 2013b)
Not
(Byrne et
mentioned
al., 2014)
(Krishnan
Langmuir
SC R
GAC-AA
Not
bituminous GAC.
GAC-AA, and GAC-PH showed that the uptake
Langmuir
30.86 mg/g for U17,
Functionalized bituminous GAC provided 6
The adsorption kinetics of perchlorate on GAC, GAC-PH and
at pH 14
the wood-based QAE-GAC.
volume (BV) to 6 ppb breakthrough than did the
Pyridinium-graf
33.2 mg/g for U360H
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The bituminous-GAC-based QAE-GAC hosted
et al.,
2017)
Table 4 – Application of organic adsorbent materials for perchlorate treatment.
Findings
materials
adsorption capacity
by quaternary amine
Modified Wheat straw(MWS)
Electrostatic attraction between perchlorate anion and positively charged quaternary amine groups on the MR was the primary mechanism. The adsorption mechanism of ClO4- by MWS was IX and the maximum adsorption capacities of ClO4-
were electrostatic attraction and physical force.
Cross-linked
QCS
showed
high
adsorption
capacity, which was 2.5 times higher than PCLC.
45.455 mg/g
N
The CM-CS/PVA Bs could adsorb perchlorate efficiently in a wide pH range from 4 to 10.
references
Langmuir-
(Baidas et
Freundlich
al., 2011)
Langmuir
and
Freundlich Langmuir
119.0 mg/g
28.352 mg/g at 318K
A
CM-CS/PVA Bs
model
Langmuir
U
Cross-linked QCS
119.4 mg/g
decreased with increase in temperature. The main driving forces for perchlorate adsorption
PCLC
169.34 mg/g
Key
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Reed modified(MR)
Adsorption
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Maximum
Adsorption
and
Tempkin Langmuir and Freundlich
Resin-5
M
The adsorption process was endothermic and spontaneous at higher temperature, the maximum -
ED
Perchlorate removal was above 98% at pH values ranging from 4.0 to 9.0.
PT resins
followed
the
11.35 mmol/g
50.00 mg/mL
order
of
Purolite
Langmuir
reached the equilibrium much faster than Purolite
Purolite A532E
CC E
MIEX
A530E and Purolite A532E.
A
2012)
(Xie et al., 2016b)
et al.,
(Tang et al., 2013)
Freundlich
(Zhu et al., 2015)
and MIEX, respectively
Perchlorate adsorption capacity was optimum at neutral condition (pH: 6.0, 170.4 mg/g), and decreased at acidic (pH: 3.0, 96.4 mg/g) or alkalic
Not mentioned
Thomas
(Song et al., 2015)
(pH: 12.0, 72.8 mg/g)
A strong base anion
The rate-limiting stage for the adsorption of
(SBA) exchange
perchlorate into the ion exchange resin could be
resin
Freundich
Purolite A530E，
resin
(Xie et al.,
90.91 mg/g on
A530E≈MIEX > Purolite A532E, and MIEX
biopolymer based
2010)
2013)
Purolite A532E and
Amine-crosslinked
(Xie et al.,
90.09, 104.2,
The equilibrium capacity of the three investigated Purolite A530E，
2012)
(Memon
adsorption of ClO4 could be achieved at pH 4.5. MIEX resin
(Tan et al.,
Not mentioned
explained as a chemisorption process
Cationic MOFs
The proposed adsorption mechanism was the
material
co-effect of the electrostatic force and IX between 62
133.5 mg/g
Not
(Faccini et
mentioned
al., 2016)
Langmuir
(Li et al., 2015)
((NH2SO3-Cu-(4,4’
the ClO4- and MOFs material.
-bipy)2), ASC) The adsorption capacity of ClO4- by AICS
AICS
decreased with increases in temperature.
Two types of cationic MOFs
-
3-.
MOFs followed the order ClO4 > PO4
Langmuir
143.48 mg/g on ASC and 119.97
Langmuir
mg/g on ESC
(Xing et al., 2015) (Zhang et al., 2017)
A
CC E
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ED
M
A
N
U
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(ASC and ESC)
The competitive ClO4- and PO43- ions on the two
89.1 mg/g
63
Table 5 – Application of inorganic adsorbent materials for perchlorate treatment. Adsorption
Findings
materials
Key
capacity
model
references
Perchlorate was adsorbed on GFH through electrostatic attraction between perchlorate and
Approximately 90–95% of perchlorate was
zero-valent
removed within 24 h in the presence of 35 g/L
aluminum
aluminum at acidic pH (4.5 ± 0.2).
double hydroxide (CLDH)
Not mentioned
Not mentioned
The rapid step of the uptake process was controlled by diffusion, and the slow step was
285.9 mg/g
controlled by the reaction of perchlorate with the CLDH rather than by diffusion. The existence of CHMAF was favorable for the
Not mentioned
U
removal of ClO4 from water, and the best ratio of Mg/Al/Fe was 3:0.8:0.2 (CHMAF5%).
Freundlich
Linear
-
CHMAF
Langmuir
positively charged surface sites.
Acid-washed
Calcined layered
20.0 mg/g
adsorption isotherm
N
At 25 °C, MgFe-3 CLDH = 1.33 g/L, the initial
Three modified inorganic-
M
all adsorbed within 720 min.
A
pH range of 4-10, 2000 μg/L of ClO4- was almost
Mg/Fe-CLDH
The ClO4- uptake on these three modified bentonites from 0.1 mmol/L solution rapidly
bentonites
ED
attained equilibrium within 100 min.
Not mentioned
(Lien et
al., 2010)
(Wu et al., 2010b)
(Yang et al., 2012a) (Yang et
Freundlich
al.,
0.117, 0.107 and 0.102 mmol/g on Fe-Bent, Fe-Al-Bent and
Langmuir
(Yang et al., 2013)
Al-Bent, respectively Freundlich for
PT
MgAl-LDHs and
enhanced as the positive layer charge density
280 mg/g for
MgAl-CLDHs
decreased, which can be explained by anion
MgAl-CLDHs
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exchange mechanism and H-bonding interactions.
MgAl-LDHs and Langmuir for
(Lin et al., 2014a)
MgAl-CLDHs MgAl-CLDH-4
MgAl-CLDHs,
The binding mechanisms of ClO4- by CLDHs are
MgFe-CLDHs
much more dependent on the unique M2+
A
al., 2010)
2012b)
The ClO4- uptake by the MgAl-CLDHs was
and ZnAl-CLDHs
(Kumar et
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hydroxide (GFH)
Adsorption
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Granular ferric
Maximum adsorption
compared to the type of M3+.
(279.6 mg/g) > MgFe-CLDH-4 (120.8 mg/g) > ZnAl-CLDH-4(1 0.81 mg/g)
64
Langmuir
(Lin et al., 2014b)
1.499, 2.627 and Hydrogen bonding was proposed to be the CNTs
dominant mechanism at neutral pH for the
3.554 mg/g for raw DWCNTs, 2
Freundlich and
h-DWCNT,8
Langmuir
interaction of ClO4- with CNTs.
h-DWCNT,
(Fang and Chen, 2012)
respectively 6 mg/g for
facilitated the adsorption of perchlorate onto
SWCNTs and 3
SWCNTs at a fixed pH.
mg/g for GFH
Modified
(Hsu et
Freundlich
al., 2015)
A
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ED
M
A
N
U
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GFH
Low temperatures and low humic acid contents
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SWCNTs and
65
Table 6 – Application of composite adsorbent materials for perchlorate treatment. Maximum Adsorption materials
Findings
adsorption capacity
Adsorption
Key
model
references
Mag-PCMA removed over 98% of the Mag-PCMAs
aqueous
perchlorate
anions
across
(Clark and
a
Not mentioned
Freundlich
2012) 0.421, 0.400, 0.280,
organosilicas and organoclay
absorbents MCM-48
uptake have silica
capacities the
>
by
following MCM-50
0.246 meq/g on
the
MCM-48 silica,
order:
silica
>
Cloisite®. 10A > MCM-41 silica.
MCM-50 silica,
Langmuir
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Perchlorate
Synthetic
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concentration range of 60–500 mg/L.
Keller,
Cloisite® 10A and
(Seliem et al., 2013)
MCM-41 silica, respectively
Exchange of anions from the entrapped neutral surfactant appeared to be the main
modified organoclay
mechanism of nitrate and perchlorate uptakes
U
Cationic surfactant
N
by HDPy-montmorillonite.
A
Electrostatic attraction and chelation of the Fe-CB
1.11 mmol/g
Fe(III) center in the complex were the main
M
driving forces for perchlorate adsorption.
Langmuir
(Bagherifa m et al., 2014)
29.851 mg/g at 298K
Langmuir
(Lv et al., 2014)
Adsorption performance was synergistically influenced by the alkyl-chain length, head
ED
Organo-Mt
group, and alkyl-chain number of the
0.95 mmol/g on C18-BM/Mt
Langmuir
(Luo et al., 2015a)
surfactants used for modification. High
adsorption
capacity
was
stably
PT
Organo-MMTs modified by
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BODMA-Cl
Modified OMt with
DDAC and BODAC
A
sequentially
maintained with high selectivity against variations in temperatures of 25 °C to 45 °C
(Luo et 0.90 mmol/g
Langmuir
al., 2015b)
and initial pH range of 2.5–12.5. The highest adsorption capacity for ClO4- was obtained
using
sequential
modification,
whereby
the
addition
of
DDAC
corresponding to 0.05 times the cation exchange
capacity
of
Mt
(Luo et 1.08 mmol/g
Langmuir
al., 2016b)
significantly
increased BODAC uptake. Innovative
wet
OMt
without
Freundlich
washing
Hexadecylpyridinium
(I-OMt) showed relatively high adsorption
-modified OMt
capacity of ClO4- and negligible release of
and 0.0926 mmol/g
dushkevich
HDPy. Mt and Ilt modified
Dubinin–Ra
(Luo et al., 2016a)
(D–R)
Wet grinding of Ilt significantly influenced 66
0.58534
mmol/g
Not
(Luo et
by DDAC
the
DDAC
loading
ClO4-
and
uptake
and 0.5187 mmol/g
compared to Mt, and grinding for 30 min
on OMt and OIlt-30,
showed the best performance for both
respectively
mentioned
al., 2017)
minerals.
Table 7 – Application of chemical reduction for perchlorate treatment. Reducing agent
Findings
Fisher iron filings,
Perchlorate removal was proportional to the iron dosage in the batch reactors,
Elemental iron
with up to 66% removal in 336 h in the highest dosage system (1.25 g/mL).
(Moore et al., 2003)
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electrolytic iron
Key references
Up to 98% of aqueous ClO4- (initial concentration about 1.0 mM) was removed in 1 h at 200 °C using a microwave digester.
(Oh et al., 2006)
More than 99.9% removal of perchlorate was obtained in a solution
containing [ClO4-] = 1.0 mM, [Ti(III)] = 40 mM, and [β-alanine] = 120 mM after 2.5 h of reaction at 50 °C.
Ti2+
Higher concentrations of Ti(0) produced higher concentrations of Ti(II), which resulted in more rapid perchlorate destruction.
(Wang et al., 2010)
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Ti3+
(Park et al., 2012a)
U
This agent could enhance the production of Ti(II) or a mixture of Ti(II) and Ti2+/Ti3+
Ti(III) at the lowest F/Ti (0) ratio of 0.5 (25 mM KF), which resulted in more
N
effective reduction of perchlorate.
The ultimate product of ClO4- degradation by the sulfite/UV-L ARP was chloride, but chlorate was detected as an intermediate.
A
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PT
ED
Sulfite/UV-L
Cl- was formed with a yield of 85% after 6 h.
M
Fe
A
Al、Cu、Zn、Ni、 At 150 °C, iron powder degraded ClO4- from 104 Μm to 0.58 Μm in 1 h, and
67
(Park et al., 2012b)
(Hori et al., 2012) (Vellanki and Batchelor, 2013)
Table 8 – Application of electrochemical reduction for perchlorate treatment. Electrode material
Ni/Pt
Ti
references
A number of chemical and electrochemical reactions resulted in perchlorate reduction
(Rusanova
and oxidative dissolution of the working Ni electrode.
et al., 2006)
Perchlorate could be electrochemically reduced on rhodium and the reduction rate of
(Láng et al.,
chlorate was much higher than that of perchlorate in the same system.
2008)
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Polycrystalline Rh
Key
Findings
Perchlorate and nitrate at initial concentration of 200 ppm and 1000 ppm individually
(Wang et
were reduced to < 20 and < 200 ppb, respectively, over a short reaction time of 6–8 h.
al., 2009)
Perchlorate reduction using electrochemically induced pitting corrosion of Ti was independent of the imposed potential as long as the potential was maintained above the
SC R
ZVT
A
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PT
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M
A
N
U
pitting potential, whereas it was proportional to the applied current.
68
(Lee et al., 2011)
Table 9 – Application of catalytic reduction for perchlorate treatment. Catalyst
Findings
Key references -
Re-Pd/C
Ti-TiO2
Under standard batch conditions at room temperature, 1 bar of hydrogen, 200 ppm ClO4 was reduced to less than 1 ppm in 5 h.
Shapley, 2007)
Up to 90% of ClO4- could be reduced to Cl- by molecular hydrogen gas and metallic -
catalysts in 2 weeks at an initial ClO4 concentration of 2 ppm.
Cu–TiO2/
25 °C over a range of initial concentrations (2–200 ppm) at 1 atm of H2 and a pH range of 2.7–3.7.
2010)
(Zhang et al., 2011)
The efficiency of ClO4- reduction in the presence of Cit (0.15 mM) could reach 56.0% after 140 min of irradiation (368 ± 0.5 K) when the initial concentration of ClO4- was 0.001 mM.
U
SiO2
The catalysts were efficient for the complete reduction of ClO4- to Cl- within a few hours at
SC R
Cl-Pd/C
(Choe et al.,
IP T
ClO4-, and that reduced Re surface species (41.5 eV) to a greater extent were more resistant to reoxidation by short-term exposure to oxygen.
ReO(hoz)2
(Wang et al., 2008)
The two reduced Re (Re(V)/Re(VII)) surface species that both played a role in reducing Re/Pd-AC
(Hurley and
(Ye et al., 2013)
The identified Re structures supported a revised mechanism for the catalytic reduction of ClO4- involving oxygen atom transfer reactions between odd-valence oxorhenium species
(Choe et al.,
and the oxyanion (Re oxidation steps) and atomic hydrogen species (Re reduction steps)
2014)
Cl-Pd/C
enabled easy ClO4 reduction by the Re-Pd/C catalyst. Redox cycling between hoz-coordinated ReV and ReVII species served as the main catalytic cycle for ClO4- reduction.
(Liu et al., 2013) (Liu et al., 2015)
The ClO4- reduction efficiency could reach 62% with Cit (0.15 mM) after 6 h reaction (368 ± 0.5 K) at applied voltage of 1.5 V With an initial ClO4- concentration of 0.001 mM.
A
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PT
Ag-TNTs
-
M
ReO(hoz)2
Pre-treatment of NO3- using In-Pd/Al2O3 improved selectivity for N2 over NH4+ and
ED
Re-Pd/C
A
formed by the Pd-catalyzed dissociation of H2.
N
Re-Pd/C
69
(Jia et al., 2016)
Table10 – Application of coated nZVI for perchlorate treatment. Material
Findings
Key references
-
About 97% of ClO4 was removed within 10 hr at 90 °C and 86% of ClO4 was removed
GAC
within 12 hr at 25 °C at ICs/GAC dosage of 20 g/L.
Hydroxide-doped GAC
The highest ClO4- adsorption capacity was achieved by Fe (0.97)-GAC (0.169 mmol/g) with an iron content of 0.97% wt. of GAC.
impregnated GAC Starch and CMC
(Xu et al., 2010)
(Xu et al., 2013)
(Xu et al., 2015)
IP T
Less FeOOH of Fe–GAC was favorable for removing perchlorate, whereas more FeOOH of Fe–GAC favored the removal of bromate.
Starch and CMC-stabilized nZVI could degrade perchlorate 1.8 and 3.3 times faster than non-stabilized nZVI, respectively.
(Xiong
et
2007b)
SC R
Hydroxide
Increasing the temperature could enhance the removal efficiency of perchlorate, and the -
removal rate of ClO4 reached to 99.9% at 90 °C with the initial perchlorate of 200 mg/L.
A
CC E
PT
ED
M
A
N
U
Chitosan
-
70
(Xie et al., 2016a)
al.,
S0957-5820(18)30041-7 https://doi.org/10.1016/j.psep.2018.02.009 PSEP 1295
To appear in:
Process Safety and Environment Protection
Received date: Revised date: Accepted date:
17-12-2017 3-2-2018 11-2-2018
Please cite this article as: Xie, Yanhua, Ren, Lulu, Zhu, Xueqian, Gou, Xi, Chen, Siyu, Physical and Chemical Treatments for Removal of Perchlorate from Water—A Review.Process Safety and Environment Protection https://doi.org/10.1016/j.psep.2018.02.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Physical and
Chemical Treatments
for Removal of
Perchlorate from Water—A Review
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Yanhua Xiea,b,* , Lulu Rena,b, Xueqian Zhua,b, Xi Goua,b and Siyu Chena,b
a: State Key Laboratory of Geohazard Prevention and Geoenvironment Protection,
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Chengdu University of Technology, Chengdu 610059, China;
b: State Environmental Protection Key Laboratory of Synergetic Control and Joint
U
Remediation for Soil & Water Pollution, Chengdu University of Technology,
Corresponding author: Yanhua Xie, College of Environment, Chengdu University of
M
Technology, Chengdu, 610059, China
A
*
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Chengdu 610059, China.
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Tel: +86 15928561958; Fax: +86 15928561958
Highlights
The key findings of physical and chemical treatments for perchlorate removal are
CC E
PT
E-mail: [email protected].
systematically summarized in tables.
A
The key findings, mechanisms, influencing factors, advantages and disadvantages of recent researches are elaborated. The coated nanoscale zero-valent iron is superior for treating high-concentration perchlorate.
1
Abstract: Perchlorate, which could not be easily degraded in the environment, is a persistent inorganic pollutant in water with high water-solubility, diffusivity, and stability. Its pollution has become a global environmental problem. Perchlorate in
IP T
surface water and groundwater can keep in food and drinking water by various ways, which may cause a series of healthy problems in human body. Physical and chemical
SC R
treatments are the technologies commonly used to remove perchlorate from water. This review begins with the existing treatments of perchlorate in water, then describes
U
the perchlorate pollution status in different countries and its hazard for human being.
N
In addition, the key findings of technologies and materials of physical and chemical
A
treatments on perchlorate removal from water up to now are systematically
M
summarized, and also the following research findings in recent years are elaborated in
ED
detail, such as the key findings, mechanisms, influencing factors, advantages and disadvantages. In particular, a novel material of nanoscale zero-valent iron (nZVI)
PT
which is used to remove perchlorate, is also presented. The coated nZVI introduced in
CC E
this paper is superior for treating high-concentration perchlorate. This overall summary of the technologies and materials currently used can guide future studies on
A
the removal of perchlorate from water.
Keywords: perchlorate removal; water; physical-chemical treatment; chemical treatment; nanoscale zero-valent iron (nZVI)
2
1. Introduction In recent years, perchlorate has been widely used as an oxidizing agent in the production of rocket propellants, missile fuels, explosives, pyrotechnics, automobile airbags, batteries, and other industrial products (Lv et al., 2014). As such, more and
IP T
more perchlorate is being discharged into the environment. Because perchlorate is highly water-soluble, diffusible, and accumulated readily as bioaccumulation
SC R
(Srinivasan and Viraraghavan, 2009), it is difficult to generally entrap and degrade it,
which results in pollution diffusion and other environmental problems. The presence
U
of perchlorate has also been detected in drinking water (Ting et al., 2006), which can
A
environmental protection agencies.
N
directly harm human health and have attracted the attention of governmental
M
A number of effective technologies for removal of perchlorate from water have
ED
been developed in recent years. The latest review of relatively comprehensive summary about perchlorate treatment technologies was published in 2012. These
PT
technologies could be roughly grouped into four categories: physical-chemical
CC E
treatment, chemical treatment, bioremediation, and combined processes (Ye et al., 2012). Bioremediation includes phytoremediation and microbial remediation. Plants and their rhizosphere microorganisms can degrade perchlorate in soil or water through
A
the absorption, accumulation and metabolism of life activities (Seyfferth and Parker, 2007; Yifru and Nzengung, 2008; Bhaskaran et al., 2013). Microbial remediation by microorganisms and their enzymes can gradually reduce ClO4- to Cl- under anaerobic conditions, and the selected microbial species and electronic donor are key factors 3
affecting the degradation result (Wan et al., 2017; Zhang et al., 2016; Wen et al., 2016). The cost of bioremediation technology is relatively low and it is suitable for repairing large-scale or heavily ClO4- polluted areas in soil or water, but the bioremediation rate is relatively slow. Also, the metabolic products of the
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microorganisms may cause secondary pollution and other issues when used in the treatment of drinking water.
SC R
Physical-chemical treatment includes adsorption, membrane filtration, and ion
exchange (IX). These methods are the most commonly used technologies for removal
U
of trace-level ClO4-, and have the advantages of low cost, high treatment efficiency,
N
and easy operation (Mahmudov and Huang, 2010; Gu et al., 2007; Xie et al., 2016b).
A
However, physical-chemical treatment requires the disposal of regenerative brine or
M
rejected streams with high concentrations of perchlorate (Srinivasan and Sorial, 2009;
ED
Fox et al., 2014). Chemical treatment includes chemical, electrochemical, and catalytic reduction, which can completely convert ClO4- into Cl-. However, the typical
PT
chemical reduction processes are relatively slow due to the high activation energy and
CC E
stable structure of perchlorate, and the use of metal catalysts can lead to secondary pollution (Liu et al., 2014). Combined processes exhibit more flexibility and higher degradation efficiency during removal of ClO4- (Kim and Choi, 2014; Sharbatmaleki
A
et al., 2015; Lian et al., 2016; Ren et al., 2015). Nevertheless, these technologies are still in the exploration stage and require further research before their practical application. Since physical-chemical treatment can efficiently remove ClO4- in trace level and 4
chemical treatment can completely convert ClO4- to Cl-, new materials and technologies on perchlorate removal are being rapidly developed. Based on Ye et al.’s (2012) review paper, this review systematically summarizes the key findings of technologies and materials of physical and chemical treatments on perchlorate
IP T
removal from water in tables and also the research findings in recent years are elaborated in detail. In addition, nZVI and its modified materials for removal of
SC R
perchlorate are described, which can thoroughly and efficiently reduce ClO4- to Cl-. The key findings, mechanisms, influencing factors and advantages or disadvantages
U
of these recent researches are discussed in detail. This review has important
N
environmental significance with respect to perchlorate pollution prevention and
A
control.
M
2. Pollution status and hazards of perchlorate
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2.1 Pollution status
At the end of the 1990s, due to improvements in separation and detection
PT
techniques, the detection limit for perchlorate was reduced from 400 μg/L to 4 μg/L,
CC E
and even close to 1 μg/L (Dirtu et al., 2012). Since that time, perchlorate has been found to exist in various environmental media and the associated environmental
A
problems caused by perchlorate has attracted increasing attention. Perchlorate has been detected in the environmental media of many countries. In
America, a survey was done using stable isotope ratios to trace perchlorate sources and behavior in the Laurentian Great Lakes, which found perchlorate concentrations in the Great Lakes ranged from 0.05 to 0.13 μg per liter (Poghosyan et al., 2014). In 5
Japan, Kosaka et al. (2007) investigated the perchlorate concentrations in 30 tap water samples and found perchlorate concentrations of more than 1 mg/L in 19 tap water samples and of more than 10 mg/L in 13 samples. In South Korea, Her et al. (2011) detected perchlorate in local bottled water, tap water, and seawater samples, and
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determined that local drinking water sources had also been polluted by perchlorate. In the Tamil Nadu State of South India, the observed concentrations of perchlorate range
SC R
between 0.005–7690 μg/L in groundwater, 0.005–30.2 μg/L in surface water, and
0.063–0.393 μg/L in tap water. Researchers have also found that ClO4- in groundwater
U
in the fireworks factory area are significantly higher than other regions, which
N
indicates that the fireworks and safety match industries are principal perchlorate
A
pollution sources (Isobe et al., 2013; Sijimol and Mohan, 2014). Nadaraja et al. (2017)
M
firstly reported the spatio-temporal distribution of ClO4- at various higher levels in a
ED
5-km area around an ammonium perchlorate production unit in Aluva, Ernakulam, Kerala (India). They found the ClO4- concentration in ground water samples close to
PT
the production unit increased to > 40,000 μg/L, and detected a ClO4- concentration of
CC E
1740 μg/L in well water 1.6 km from the production unit. The study results also indicate that a public pond in the area showed an increased ClO4- level up to 29,000 μg/L. These data reveal the high risks to humans exposed to environments associated
A
with long-term ClO4-. Besides water, perchlorate has been detected in various other environmental media. A study of the concentrations of perchlorate in the dust fall of Malta in Europe suggested wherever intensive burning of fireworks takes place, the 6
environmental impact might be much longer lived than degraded, mainly due to the re-suspension and deposition of contaminated settled dust in the urban environment (Vella et al., 2015). In China, researchers have found perchlorate to be presented in milk, vegetables, saliva, serum, and breast milk (Wu et al., 2010a). Recently, Wan et
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al. (2015) conducted the first multinational survey of occurrence of perchlorate in indoor dust in 12 countries during the 2010–2014 period. The authors found the
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concentrations of perchlorate in dust to range from 0.02–104 μg/g (geometric mean: 0.41 μg/g) and the indoor dust samples from China contained the highest
U
concentrations (geometric mean: 5.38 μg/g). Lybrand et al. (2016) found the
N
perchlorate concentrations in the Atacama Desert to average 206 mg/kg, which is
A
two to three orders of magnitude greater than in Antarctica and other sites. In
M
summary, perchlorate commonly detected in various environmental media in
ED
different countries indicates that perchlorate pollution has become a global
2.2 Hazards
PT
environmental problem.
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Perchlorate was first identified as a chemical of concern by the U.S. EPA in 1985 following its discovery in wells at hazardous waste sites in California (Rice et al., 2007). And it is difficult to be reduced and easily mobilized in environmental media
A
due to its inherent characteristics. There are several possible perchlorate transformation pathways in the environment and Fig. 1 illustrates the ways in which humans may be exposed to this contaminant (Kumarathilaka et al., 2016). The ingestion of food products containing perchlorate, which is mobilized 7
through the water-soil system and accumulates in edible plant species characterized by high human consumption, represents a potential human health risk due to its adverse effects on thyroid, hormone, and neuronal development, mainly in infants and fetuses (Calderón et al., 2017). Maternal placental supplies of thyroxine (T4), which is
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closely linked to iodine nutrition, is the major contributor to fetal serum T4 levels in early pregnancy and plays a crucial role in infants’ neurodevelopment in utero (de
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Escobar et al., 2004). Iodine deficiency may result in developmental delay in infancy,
particularly in language and memory skills, and subclinical disease induced by
U
maternal hypothyroidism may lead to impaired neurological development and
N
intelligence quotient (IQ) scores in their offspring (Leung et al., 2010).
A
Perchlorate has been reported to competitively inhibit the uptake of iodide by a
M
sodium iodide symporter (NIS) in the thyroid gland at medicinal levels (e.g., 800 mg)
ED
(Tonacchera et al., 2004). The EPA is leading efforts to set regulatory standards for perchlorate in drinking water and to derive a target maximum contaminant level for
PT
perchlorate (Lumen and George, 2017). As such, countries worldwide can be guided
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by U.S. in setting regulatory standards and are striving to develop effective methods for removal of perchlorate.
A
3. Physical and chemical treatments for removal of perchlorate The inertia kinetic of perchlorate makes it difficult to be removed, especially
when the perchlorate concentration in water is low. Physical and chemical treatments are the most commonly used technologies for removal of perchlorate from water. The main technologies as well as the relative researches are summarized below. 8
3.1 Physical-chemical treatment 3.1.1 Ion exchange technology Ion exchange (IX) technology is one of the most effective and widely used methods for removal of perchlorate from water. Resins are the most commonly used
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IX material for removal of perchlorate. For drinking water slightly contaminated with ClO4- (10–100 ppb), treatment by selective cross-linked styrene-divinyl benzene
SC R
resins is the most widely used process (Chen et al., 2012). Recently, a novel corn-stalk-based modified-magnetic-biopolymer IX resin (CS-MAB) was prepared
U
and used to remove perchlorate from aqueous solution (Song et al., 2017). The
N
authors found that the saturated magnetization and average pore diameter of CS-MAB
A
were 3.82 emu/g and 4.83 nm, respectively. Adsorption batch experiments revealed
M
the optimal adsorbent dosage and pH value for removal of perchlorate to be 1.0–1.5
ED
g/L and 3.0–10.0, respectively. The adsorption kinetics fitted well with the intraparticle-diffusion model in the first 90 min, and the pseudo-second-order model
PT
from 90–360 min, which suggested that the potential mechanisms of perchlorate
CC E
removal were physical adsorption and an IX reaction between the ClO4- and Cl-containing groups of CS-MAB. Darracq et al. (2014) examined the IX capacity of five cationic resins (A532E, A520E, A400E, PWA-5, PSR-2) with regard to
A
perchlorate via batch kinetics and isotherms. Their isotherm experiments showed that the IX equilibrium for perchlorate could be modelled by the Langmuir relation for all resins. This confirmed that A532E resins and PSR-2 had the highest specificity, which is a major advantage for the removal of trace amounts of perchlorate from water 9
containing relatively high concentrations of other anions. IX technology also uses some inorganic materials or their modified materials, such as montmorillonite, activated carbon (AC) and permselective membranes, to remove perchlorate. Chitrakar et al. (2012) reported anion-exchange properties of modified
with
hexadecylpyridinium
chloride
(HDPyCl-
IP T
Na-montmorillonite
montmorillonite) for removal of perchlorate. The authors found the perchlorate taken
SC R
up by HDPyCl-montmorillonite from 0.01 mmol/dm3 or 0.10 mmol/dm3 solution rapidly attained equilibrium within 4 h. These results revealed that the anion was
U
taken up by HDPyCl-montmorillonite accompanied by the release of chloride ion into
N
solution through anion-exchange processes. Lin et al. (2013) employed five cationic
A
surfactants to modify activated carbon for removal of perchlorate and their results
M
revealed that surfactants with smaller micelle structures were more easily loaded than
ED
those with larger micelles. In addition, they found that the activated carbon-loaded surfactant presented a much more positively charged surface, as manifested by the
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obvious improvement in perchlorate adsorption. This result demonstrated that
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perchlorate was mainly adsorbed via IX with surfactant-loaded AC. Wang et al. (2015) synthesized perchlorate permselective membranes with a thickness of 300 μm prepared with polyvinyl chloride (PVC) and quaternary ammonium salts (QAs) in
A
solvent at room temperature. The authors found methyltributylammonium chloride (MTBA) to exhibit superior perchlorate permselectivity due in part to the favorable steric effect of the alkyl chain length, and the functional groups responsible for the IX to have been successfully incorporated in the membrane matrix. 10
The key findings of studies on the use of IX technology to remove perchlorate from water are summarized in Table 1. In general, IX is an effective method for removal of ClO4- in water, but there are also a number of drawbacks in its practical application. For example, resins used are of high prices, the IX regeneration liquid
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contains a high concentration of ClO4-. In addition, the coexisting ions in water affect the removal of ClO4-, and there are regeneration problems with selective IX resins. All
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these issues are not conducive to practical application in IX industry. 3.1.2 Membrane filtration
U
Membrane filtration technology can be divided into the following three
N
categories: (1) reverse osmosis (RO); (2) nanofiltration (NF) and ultrafiltration (UF);
A
and (3) electrodialysis (ED). However, it is important to note that the reverse osmosis
M
or electrodialysis has no limit requirement on removal of various irons, so these two
ED
technologies may remove all the ions in solution. Some studies have removed ClO4- from water with polyelectrolyte-enhanced
as
SO42-
and
NO3-)
on
removal
efficiency
of
ClO4- by
poly
CC E
(such
PT
ultrafiltration (PEUF). Huq et al. (2007) investigated the effects of coexisting ions
(diallyldimethylammonium) chloride (PDADMAC)-enhanced ultrafiltration. The results showed that the ClO4- removal efficiency reached 90% or more in the absence
A
of other coexisting ions when the amount of PDADMAC added ranged from 0.5–1 mmol/L and the initial concentration of ClO4- was 1 mmol/L, whereas the removal rate of ClO4- by PEUF was significantly reduced when coexisting ions were present. Roach and Tush (2008) also investigated the parameters affecting perchlorate 11
filtration (including polyelectrolyte concentration, pH, and ionic strength), and concluded that the perchlorate separations were greater than 95% even in the presence of 10-fold excesses of competing ions containing chloride, sulfate, and carbonate. Based on previous studies, Roach et al. (2011) compared the performances of poly
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(4-vinylpyridine) (P4VP)- and PDADMAC-enhanced ultrafiltration in the removal of perchlorate and found that the retention values of ClO4- by the protonated pyridine
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residues of P4VP were affected by polymer concentration, solution pH, and
competing-ion concentrations. In the absence of added salt, they observed slightly
U
higher perchlorate retentions of PDADMAC compared to P4VP, but these retention
N
values became more closely aligned as the concentration of added sodium chloride
A
increased.
M
RO and NF can also effectively remove perchlorate. Sanyal et al. (2015) used
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poly allylamine hydrochloride (PAH) and poly acrylic acid (PAA) to modify commercial NF membranes using layer-by-layer (LbL) assembly to enhance ion
PT
rejection of these membranes and achieve much higher permeability. The results
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demonstrated that the permeability of the modified membranes was 1.5 times of BW 30 membrane and 6 times of SW 30 membranes, respectively, and also had much superior permselectivity than the commercial membranes. The mechanism of ClO4-
A
ion rejection by these modified membranes was a size-based exclusion rather than a charge-based separation. This research result is one of the highest permselectivity values reported thus far for PEM-based RO membrane specifically targeting monovalent ion removal. 12
The key findings on the use of membrane filtration technology to remove perchlorate are shown in Table 2. Inherent drawbacks, such as membrane fouling, a large amount of retention waste containing high concentrations of ClO4-, and high cost, make membrane filtration rarely applicable for removal of large-scale ClO4-.
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Although electrodialysis technology is more effective in removing ClO4- from wastewater than conventional membrane technology, it also results in excessive
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handling costs (Srinivasan and Viraraghavan, 2009). Therefore, few studies have
explored the use of membrane filtration technology for the removal of perchlorate in
U
recent years and researchers have tended to combine this technology with other
N
technologies to deal with perchlorate pollution in water.
M
3.1.3.1 Activated carbon
A
3.1.3 Adsorption
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Adsorption can effectively remove many kinds of pollutants in water, so it is the most widely used technology in treating drinking water and wastewater. AC is the
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earliest and most commonly used adsorbent for the removal of perchlorate, but its
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adsorption capacity for ClO4- is limited, so researchers tend to use modified methods to improve its adsorption performance and capacity. Some studies have used quaternary ammonium/epoxide-forming compounds
A
(QAE) as modifying agents to remove perchlorate. Hou et al. (2013a) anchored QAE within bituminous and wood-based granular activated carbons (GACs) to enhance perchlorate removal from perchlorate-spiked natural groundwater. Using bituminousbased carbon pre-anchored with QUAB360 (QAE reagent) to process natural 13
groundwater that had been spiked with 30–35 ppb perchlorate, bed volumes (BVs) of 21,000 could be achieved. However, wood-based GAC anchored with this QUAB360 achieved only 3000 BV. The authors surmised that the active quaternary ammonium sites within tailored bituminous-based GAC were more exposed and thus more
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accessible for perchlorate adsorption than those within wood-based GAC. Based on this study, they developed a new method for anchoring QAEs in asphalt-based GACs
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(QAE-anchored GACs) for removal of perchlorate (Hou et al., 2013b). Statistical analysis revealed that the interaction of temperature and pH incurred the greatest
U
effect on adsorption capacity. U17 and U2 exhibited 18–20 times greater BV to 6 ppb
N
breakthrough than did the pristine GAC (900 BV) when processing groundwater that
A
had been spiked with 30–35 ppb perchlorate. The main results of this research are
M
shown in Fig. 2.
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Researchers have discovered that nitrogen functional groups and iron salts also can be modifying agents for removal of perchlorate. Byrne et al. (2014) introduced
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positively charged nitrogen functional groups into bituminous GAC to increase
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perchlorate removal efficiency from drinking water. The authors concluded that the functionalized bituminous GAC provided six times more perchlorate removal than pristine bituminous GAC. Moreover, the GAC media with absorbed perchlorate can
A
be regenerated by electrochemical reduction, and its capacity for perchlorate adsorption could be mostly restored. The authors reported that this tailored functionality and redox regeneration of a relatively inexpensive media such as activated carbon could offer a novel opportunity for the adsorption industry. Xu et al. 14
(2016) synthesized and tested a new reactive material Fe-GAC by dipping various iron salts, such as NO3-, SO42-, or Cl-, onto GAC to remove perchlorate from water. They found that NO3- on Fe(NO3)3-GAC behaved as a strong competing ion with perchlorate, which resulted in the lowest perchlorate adsorption capacity. These
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results might help to guide the selection of the optimal iron salt to be impregnated on GAC for perchlorate adsorption from contaminated water in a continuous reactor
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(CTR). Fig. 3 shows a schematic of the perchlorate adsorption mechanism of Fe-GAC by impregnating various iron salts on GAC.
U
Recently, researchers developed a cost-effective method with a high adsorption
N
capacity for removal of perchlorate from drinking water using phosphoric-acid-treated
A
GAC (GAC-PH) and acetic-acid-treated GAC (GAC-AA) (Krishnan et al., 2017). The
M
study results showed that the equilibrium time was 60 min and remained the same for
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varying adsorbent dosages and adsorbate concentrations. The maximum removal efficiency was achieved at a pH of 1, which revealed that an acid pH was more
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favorable for removal of perchlorate. The mechanism of perchlorate adsorption by
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acid-modified GAC was mainly through chemisorption. Table 3 summarizes the key findings of studies investigating the removal of
perchlorate from water using AC and its modified materials. In the actual operation,
A
ClO4- is vulnerable to penetration of AC adsorption column, which can reduce the adsorption effect. The modified AC can improve the adsorption capacity of ClO4-, but this process also increases the operational cost. In addition, the modification of AC affects its ability to adsorb other pollutants and the regenerated AC solutions 15
containing high concentrations of ClO4- is impractical. 3.1.3.2 Other adsorbent materials Apart from AC, a variety of other adsorbent materials have been investigated for removal of ClO4- from water, including chitosan and its modified materials, IX resins,
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bentonite/montmorillonite and their modified materials, granular iron hydroxide (GFH) and its modified materials, organoclay and its modified materials, and carbon
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nanotubes (CNTs) and other new materials. These adsorbent materials also exhibit excellent adsorption performance for removal of ClO4-. With respect to their chemical
U
properties, these materials can be roughly divided into three categories: organic,
A
(1) Organic adsorbent materials
N
inorganic, and composite adsorbent materials.
M
Chitosan and its derivative have abundant amino (-NH2) and hydroxy (-OH)
ED
functional groups that exhibit high modification and adsorption activities (AZLAN et al., 2009). Our research group conducted a series of perchlorate removal studies using
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chitosan and its modified materials. Xie et al. (2010) first used proton-crosslinked
CC E
chitosan (PCLC) to remove perchlorate from aqueous solution and reported that the effluent perchlorate could be steadily maintained below 24.5 μg/L up to about 95 BV with a 10-mg/L influent of perchlorate. To balance the protonated degree of the amino
A
groups and the effect of ions competing with respect to the adsorption capacity, the optimal pH value was determined to be about 4.0. Based on that study, the authors used cross-linked quaternary ammonium chitosan salt (QCS) to selectively remove perchlorate (Xie et al., 2012) and found the adsorption process to be nearly 16
independent with a pH ranging from 4.0 to 10.3, which was much wider than using PCLC with a pH range of 4.0–6.0. The perchlorate adsorption might be due to the ion-exchange interaction between the chloride ions on the cross-linked QCS and the perchlorate ions in solution. In addition, Xie et al.(2016b) synthesized cross-linked
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magnetic chitosan/poly (vinyl alcohol) beads (CM-CS/PVA Bs) to remove perchlorate from water. The CM-CS/PVA Bs could also efficiently adsorb perchlorate in a wide
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pH range from 4 to 10. The adsorption process was rapid and could reach equilibrium within 30 min. In addition, after adsorption, the CM-CS/PVA Bs could be regenerated
U
easily by NaCl. However, as the actual samples were complex, higher sorbent dosages
N
were needed in the field tests.
et
al.
(2013) used a newly synthesized 25, 27-bis-(N,
M
water. Memon
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Resin is another efficient organic adsorbent for removal of perchlorate from
ED
N-dimethyl-2-aminoethyl) appended Amberlite XAD-4 resin (resin-5) to remove perchlorate. The authors concluded that the adsorption process followed
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pseudo-second-order kinetics. Fig.4 presents the effect of pH (2.5–7.5) on perchlorate
CC E
adsorption by resin-5. Tang et al. (2013) investigated the adsorption characteristics of perchlorate on magnetic ion exchange (MIEX) resin and the kinetics data indicated that the perchlorate adsorption process reached equilibrium within 30 min. The results
A
also revealed that major anions reduced the amount of perchlorate adsorption in the order SO42- > PO43- > CO32- > NO3-. However, humic acid had no obvious effect on perchlorate removal. Zhu et al. (2015) studied the perchlorate sorption rate and sorption capacity of three resins—Purolite A530E，Purolite A532E, and MIEX—and 17
found the Purolite A530E and Purolite A532E resins to exhibit superior perchlorate selectivity to that of the MIEX resin because of their matrix and functional groups. These results suggested that the removal efficiency of perchlorate increased with the increase of resin dosage and temperature of three resins, and that the sorption process
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was nearly independent of the solution pH over a wide range from 4–10. Song et al. (2015) investigated column adsorption of perchlorate by amine-crosslinked
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biopolymer based resin. It observed that columns with bed depths of 1.8, 3.4 and 5.1 cm adsorbed about 185.2, 170.4 and 158.9 mg/g of perchlorate, respectively. And
U
breakthrough time and exhaustion time extended as the increase of bed depth.
N
Faccini et al. (2016) studied the adsorption and desorption behavior of perchlorate
A
onto a strong-base-anion (SBA) exchange resin and found that both the adsorption
M
and desorption experimental data were best described by the pseudo-second order
ED
model, which suggested that the rate-limiting stage for the adsorption and desorption of perchlorate ion into the resin might be chemisorption.
PT
In addition, some novel organic adsorbent materials have been explored. Xing et
CC E
al. (2015) used amine-impregnated cotton stalks (AICS) as an effective adsorbent for removal of perchlorate. The adsorption capacity in a fixed-bed column was optimum in neutral conditions (pH: 6.0 mg/g, 70.8 mg/g) with a bed depth of 2.7 cm and a flow
A
rate of 5 ml/min. The authors found that perchlorate uptake by AICS was mainly based on the IX between loaded Cl- and free perchlorate ions, the possible adsorption mechanism of which is shown in Fig. 5. In addition, chemical regeneration by HCl or NaOH (0.1 mol/ L) achieved more than 95% regeneration efficiency. Li et al. (2015) 18
fabricated a novel cationic metal organic frameworks (MOFs) material ((NH2 SO3-Cu-(4, 4’-bipy)2), ASC) for removal of perchlorate, which can effectively remove perchlorate over a broad range of pH values from 2 to 11 at room temperature. Perchlorate adsorption on ASC could be described well by the pseudo-first-order
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kinetic model and Langmuir adsorption isotherm, which offered a simple and efficient route for perchlorate ion removal by the MOF materials. Recently, Zhang et al. (2017)
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synthesized two different MOFs, including Cu2(4,4’-bipy)2(O3SNH2) (ASC) and
Cu2(4,4’-bipy)2(O3SCH2CH2SO3) (ESC), to remove ClO4- and co-existing anions
U
from aqueous solution. The authors found that the sorption rates of the anionic
N
pollutants followed a pseudo-first-order reaction. These results demonstrated MOFs
A
with a sulfonic acid ligand to be a promising alternative adsorbent for wastewater
M
containing ClO4- and PO43-, whereas the tendency of the uptake capacity of PO43- was
ED
opposite that of ClO4-. The key findings of these studies on the removal of perchlorate
A
CC E
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by organic adsorbent materials are summarized in Table 4.
19
(2) Inorganic adsorbent materials CNTs have been an effective material used in removal of organic pollutants and heavy metal (Lin and Xing, 2008; Peng et al., 2005). Some researchers have also investigated the effect of perchlorate removal by CNTs. Fang and Chen (2012)
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investigated the adsorption of ClO4- with raw and oxidized CNTs to understand the affinity mechanism of CNTs with anion pollutants. The authors found the adsorption
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of ClO4- into different CNTs was progressively increased in the following order: multi-walled CNTs < single-walled CNTs < double-walled CNTs (DWCNTs).
U
Maximum ClO4- adsorption occurred at pH = pHIEP (pH = the isoelectric point) rather
N
than at pH < pHIEP, which could not be explained by electrostatic interactions alone.
A
The results showed that co-existing anions significantly weakened ClO4- adsorption,
M
whereas Fe3+ and cetyltrimethylammonium cations increased ClO4- adsorption
ED
2–3-fold. Hsu et al. (2015) studied the removal of perchlorate from water using single-walled CNTs (SWCNTs). The authors obtained lower perchlorate adsorption
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reaction rates using SWCNTs at higher temperatures and higher humic acid
CC E
concentrations, which could be explained by the high humic acid concentrations inducing the compression of the electric double layer, which consequently reduced the
A
surface potential energy and electrostatic repulsion. The calcined products of hydrotalcite compounds also be used as absorbents to
remove perchlorate. Yang et al. (2012a) used the calcination products containing Mg(II), Al(III), and Fe(III) with varying Mg/Al/Fe molar ratios at 550 ℃ as the adsorbent. It showed that the existence of ferric iron in calcined Mg/(Al-Fe) 20
hydrotalcite compound (CHMAF) was favorable to removal of perchlorate from water, and the best ratio of Mg/Al/Fe was 3:0.8:0.2 (CHMAF5%). Then Yang et al. (2012b) synthesized a new calcined iron-based layered double hydroxide material (MgFe-CLDH) to adsorb perchlorate. Results showed that the best synthesis
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conditions of the calcined MgFe-CLDH were the calcination temperature of 550 ℃ and [Mg]/[Fe]=3. At 25 ℃, MgFe-3 CLDH=1.33 g/L, the initial solution pH of 4-10,
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2000μg/L of perchlorate was almost all adsorbed within 720 min under the best
synthesis conditions. Lin et al. (2014a) prepared a series of Mg/Al carbonate layered
U
double hydroxides (MgAl-LDHs) with different Mg/Al ratios and their calcined
N
LDHs (MgAl-CLDHs) as a promising adsorbent of perchlorate. The results showed
A
the adsorption of perchlorate to the parent MgAl-LDHs was very weak and
M
independent of the positive charge density of the hydroxide layers, while the
ED
MgAl-CLDHs exhibited a high adsorption capacity which was mainly driven by the structural memory effect of the MgAl-LDHs with perchlorate as an interlayer anion.
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Fig. 6 presents the scheme of perchlorate uptake mechanism by MgAl-LDHs and
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MgAl-CLDHs. Then Lin et al. (2014b) further prepared MgAl-CLDHs, MgFeCLDHs and ZnAl-CLDHs with different M2+/M3+ ratios and compared their adsorption performance for perchlorate. It found that the adsorption of perchlorate
A
with MgAl-CLDHs and MgFe-CLDHs increased as the M2+/M3+ ratio from 2 to 4, which was dominated by the structural memory effect and the hydrogen bonds. For ZnAl-CLDHs, however, the adsorption was hardly affected by the M2+/M3+ ratio and it was controlled by the structural memory effect only. The schematic of perchlorate 21
adsorption by the three kinds of CLDHs is demonstrated in Fig. 7. (Lin et al., 2014a).
Bentonite, as a common absorbent, has been extensively used to remove many types of pollutions, including perchlorate. Yang et al. (2013) synthesized three
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modified inorganic bentonites, including hydroxy-aluminum pillared bentonite (Al-Bent), hydroxy-iron pillared bentonite (Fe-Bent), and mixed hydroxy-iron-
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aluminum pillared bentonite (Fe-Al-Bent), to remove ClO4- from water. Their results indicated that the adsorption capacity of perchlorate followed the order: Fe-Bent > Fe–Al-Bent > Al-Bent, and that pH changes had little impact on the perchlorate
N
U
adsorption by the Fe-Bent, whereas perchlorate removal by Al-Bent and Fe-Al-Bent
A
decreased significantly with increases in pH. The key findings of research on the
M
removal of perchlorate by inorganic adsorbent materials are summarized in Table 5. (3) Composite adsorbent materials
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A number of researchers have conducted a series of perchlorate removal studies
PT
using organics to modify montmorillonite, which can improve the removal rate of perchlorate compared with that achieved using raw montmorillonite. Luo et al.
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(2015a) was first to synthesize a series of organic montmorillonite (organic-Mt) using various cationic surfactants and the study results revealed that increasing the
A
alkyl-chain length and number significantly enhanced the capacity and selectivity of perchlorate adsorption, but resulted in a decrease in the adsorption rate. In the same year, Luo et al. (2015b) synthesized a modified organo-montmorillonite (organic-MMT) with benzyloctadecyldimethylammonium chloride (BODMA-Cl), which typically has a long alkyl chain and a benzene ring. The results revealed that 22
the high adsorption capacity of perchlorate on organo-MMTs was achieved with a rapid adsorption rate that can be stably maintained despite variations in the temperature, the initial solution pH, and co-existing competitive anions. Fig. 8 illustrates the main results of this research. Then, Luo et al. (2016b) investigated the
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sequential modification of montmorillonite with dimethyl dioctadecyl ammonium chloride (DDAC) and benzyl octadecyl dimethyl ammonium chloride (BODAC) for
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removal of perchlorate and found that the simultaneous-mixing method resulted in
modified Mt with much poorer ClO4- removal performance than that of the sequential
U
modification method, due to the negligible contribution of DDAC to the BODAC
N
uptake by Mt. Subsequently, Luo et al. (2016a) investigated the adsorption of
A
perchlorate onto hexadecylpyridinium -modified montmorillonite (OMt) using both
M
in-situ (III-OMt) and ex-situ strategies, and compared innovative wet OMt without
ED
washing (I-OMt) and conventionally synthesized dried OMt (II-OMt) with washing in terms of their ClO4- adsorption densities. The results demonstrated that I-OMt to be a
PT
more suitable adsorbent than the other two for practical water treatment, and that the
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re-dispersion of I-OMt paste in solution should be improved in the future. Recently, Luo et al. (2017) evaluated the effects of grinding montmorillonite (Mt) and illite (Ilt) on the modification by DDAC and the adsorption of perchlorate and concluded that
A
the swelling or self-exfoliation property and fully hydrated Na+ led to poor efficiency in the mechanical treatment of Mt in terms of the DDAC loading and ClO4- uptake. In contrast, significant improvement was achieved with Ilt because mechanical treatment facilitated the disintegration/exfoliation of the Ilt layers and exposed the unavailable 23
interlayer surface for anchoring the DDAC. Organoclays are a group of surfactant-modified clays with hydrophobic properties, which have been extensively used in the remediation of heavy metals, herbicides and pesticides, organic compounds, and anionic contaminants (Park et al.,
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2011). Seliem et al. (2013) synthesized three organosilicas, including MCM-41, MCM-48, and a mesoporous layered organosilica (MCM-50), as well as a
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commercially available organoclay (Cloisite® 10A) and tested these materials with
the occluded surfactant as adsorbents for removal of perchlorate. The results revealed
U
MCM-48 silica to have the highest perchlorate uptake capacities. In addition, the
N
authors found the amount of ClO4- uptake to decrease in the presence of SO42- for the
A
MCM-41, whereas the uptake increased in the presence of SO42- and decreased in the
M
presence of Cl- and CO32- for the MCM-48. As for the MCM-50, the uptake increased
ED
slightly in the presence of different anions, but was impervious to Cloisite® 10A in the presence of Cl-, SO42-, and CO32-. Bagherifam et al. (2014) prepared a cationic
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surfactant-modified organoclay using montmorillonite and hexadecylpyridinium
CC E
(HDPy) chloride to remove nitrate and perchlorate anions and found the nitrate and perchlorate uptakes to reach equilibrium within 4 h. The authors also found the uptakes of nitrate and perchlorate by HDPy-montmorillonite to be highly selective in
A
the presence of Cl-, SO42-, and CO32-. Clark and Keller (2012) studied the removal efficiency of oxyanions in drinking water sources, including perchlorate, nitrate, phosphate, and sulfate, onto magnetic permanently confined micelle arrays (Mag-PCMAs) in both competitive and 24
non-competitive environments. The authors found that perchlorate and nitrate did not compete significantly for binding sites on the Mag-PCMAs and had almost equal sorption efficiencies greater than 90%, which differed from the results obtained using ion exchange resins or activated carbon with cationic surfactants in which these
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anions would compete for sorption sites. Lv et al. (2014) used a cross-linked Fe(III)-Chitosan complex (Fe-CB) as a perchlorate adsorbent and found that a pH
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ranging from 3.0–10.2 exhibited very little effect on the adsorption capability. The
eluent of 2.5% (W=V) NaCl could efficiently regenerate the exhausted adsorbent. Fig.
U
9 presents the mechanism of ClO4- sorption by cross-linked Fe-CB and Table 6 shows
N
the key findings of studies investigating the removal of perchlorate by composite
A
adsorbent materials.
M
Compared with the perchlorate adsorption capacities of GAC or modified GAC
ED
in Table 3, the perchlorate adsorption capacities of other adsorption materials are much higher. It needs to note in particular that perchlorate adsorption capacity of the
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newly synthesized 25, 27-bis-(N, N-dimethyl-2-aminoethyl) appended Amberlite
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XAD-4 resin (resin-5) has reached 11.35 mmol/g, which is approximately 30 times that of the modified GAC. At present, the research on novel adsorbent materials is not very mature, with most studies still in the experimental research stage. The
A
temperature, pH, the nature of the adsorbent, the amount of adsorbent, the coexistence of anions, and other factors will affect the removal effect of ClO4-. These influencing factors and adsorption mechanisms require further study. 3.2 Chemical treatment 25
3.2.1 Chemical reduction Chemical reduction technology can completely convert perchlorate into chloride, and with respect to environmental impact, it is an environmentally friendly method for removal of perchlorate. The reaction equation for the removal of ClO4- in water is
ClO4- + 8H+ + 8e- → Cl- + 4H2O E0 = 1.287 V (1-1)
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as follows (Srinivasan and Viraraghavan, 2009):
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Some researchers have used oxidized titanium ions to reduce perchlorate. Park
et al. (2012a) used aqueous Ti(II) produced by the oxidative dissolution of zero-valent
U
titanium as a perchlorate reducing agent. The results showed that a low pH was
N
needed to produce Ti (II) from Ti (0) and the acid amount increased as the
A
concentration of Ti (0) increased. Kinetic data showed that HCl was more effective
M
than H2SO4 in promoting perchlorate degradation in lower pH conditions. Next, the
ED
authors investigated the chemical degradation of perchlorate using Ti(II) and Ti(III) (Park et al., 2012b) and found that the rate of perchlorate degradation was fastest at
PT
the lowest F/Ti (0) ratio of 0.5 (25 mM KF). Of the catalysts investigated, only
CC E
rhenium enhanced the perchlorate degradation in the presence of Ti(II), whereas no catalyst effect was observed in the presence of Ti(III). The authors also concluded that high ionic strength did not enhance the perchlorate-Ti(III) reaction whereas high acid
A
concentration did. The rate constants for perchlorate degradation in Ti(III) were double that in Ti(II) when using 5 N HCl. Increasing the temperature can increase the decomposition efficiency of ClO4-. Hori et al. (2012) studied the effect of pressurized hot water (PHW) on the 26
decomposition of perchlorate and found that ClO4- demonstrated little reactivity in pure PHW up to 300 ℃. At the same time, the addition of zero-valent metals to the reaction system enhanced the decomposition of ClO4- to Cl- with an increasing activity order of (no metal) ≈ Al < Cu < Zn < Ni << Fe. It is worth mentioning that
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this method was successfully used in the decomposition of ClO4- in a water sample contaminated with ClO4- following a fireworks display at Albany, New York, USA.
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The main results of this research are shown in Fig. 10. Vellanki and Batchelor (2013) used sulfite/ultraviolet advanced reduction processes (ARP) to reduce perchlorate and
U
their results indicated that the rate of perchlorate degradation by sulfite/UV-L
N
accelerated with increases in the pH, temperature, and sulfite concentration.
A
Table 7 shows the key findings for the removal of perchlorate by chemical
M
reduction. Since the chemical reduction of perchlorate is a relatively slow process, it
ED
is necessary to adopt a strong reducing agent with high activity or to add a catalyst to reduce the activation energy of the reaction. Also, most of the metal reducing agents
PT
are toxic and unsuitable for the removal of perchlorate in drinking water.
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3.2.2 Electrochemical reduction Electrochemical reduction can also convert perchlorate into chloride without the
addition of a catalyst to reduce the activation energy of the reaction. Wang et al.
A
(2009) investigated the removal of perchlorate and nitrate at the Ti-water interface by an indirect electrochemical reduction process using a single perchlorate or nitrate solution and a coexisting solution. The results indicated that perchlorate and nitrate could be simultaneously and readily reduced at the surface of a Ti anode. The 27
dominant end product of this indirect electrochemical reduction was chloride or nitrite in a synthetic single-anion solution, and the nitrite concentration was negligible when these two anions were coexisting. The removal efficiency of perchlorate by zero-valent metals or soluble metal,
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which have reducing property, is said to be kinetically limited. But Lee et al. (2011) concluded that zero-valent titanium (ZVT) and its soluble states had a high
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thermodynamic potential to reduce perchlorate. These authors measured the pitting
potential of ZVT as 12.77 ± 0.04 V (SHE) for a 100-mM solution of perchlorate. The
U
experimental results showed that solution pH and the surface area of the ZVT
N
electrodes had negligible effects on the rates of perchlorate reduction. However, this
A
process might not be immediately applicable to perchlorate treatment due to its high
M
potentials, huge amount of titanium consumed, and inhibition from chloride.
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Table 8 summarizes the key findings for the removal of perchlorate by electrochemical reduction. The main disadvantages of electrochemical reduction are
PT
that the electrode is prone to be corrosive and it consumes a large amount of
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electricity in the removal process. Hence, developing less expensive and better performing electrode materials will facilitate improvements in the removal efficiency of perchlorate by electrochemical reduction technology.
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3.2.3 Catalytic reduction Catalytic reduction technology can effectively reduce the activation energy of the reduction of perchlorate. A previous study demonstrated that Re-Pd/C could effectively transform aqueous perchlorate via chemical reduction using hydrogen as 28
an electron donor at ambient temperature and pressure (Choe et al., 2010). The results of this study revealed that the catalyst activity and stability were heavily dependent on the solution composition and Re content in the catalyst. In a similar experiment, these researchers related these parameters to changes in the speciation and molecular
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structure of Re immobilized on the catalyst and investigated the mechanism of ClO4removal with a Re-Pd/C catalyst (Choe et al., 2014). The authors concluded that the
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identified Re structures supported a revised mechanism for the catalytic reduction of
ClO4- involving oxygen atom transfer reactions between odd-valence oxorhenium
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species and the oxyanion (Re oxidation steps) as well as atomic hydrogen species (Re
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reduction steps) formed by the Pd-catalyzed dissociation of H2. Liu et al. (2013)
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examined the applicability of Re-Pd/C catalyst in a perchlorate reduction process for
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treatment of waste IX brines. Experiments conducted in synthetic NaCl-only brine
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(6-12 wt %) showed higher Re-Pd/C catalyst activity than in comparable freshwater solutions, but the rate constant for ClO4- reduction measured in a real IX waste brine
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was found to be 65 times lower than in the synthetic NaCl brine.
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Photocatalysis and photoelectrocatalysis are emerging technologies that have been successfully applied in the degradation of toxic environmental pollutants (Hamadanian et al., 2011; Gong et al., 2013). Ye et al. (2013) evaluated the
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photocatalytic reduction of aqueous perchlorate using Cu-TiO2/SiO2 catalysts in the presence of a hole scavenger (citric acid, Cit) in a UV/Cu-TiO2/SiO2 system. The results indicated that the catalyst had the best catalytic activity when the nominal mass ratio of Cu2+ to TiO2 was 0.5%. The authors also concluded that Cl- was the end 29
product whereas ClO3- was the main intermediate in the course of ClO4-→Cl- in the presence of Cit. However, the ClO4- concentration increased after Cit had been exhausted. Then, Jia et al. (2016) investigated the photoelectrocatalytic reduction of aqueous perchlorate using Ag-nanoparticle-loaded TiO2 nanotube arrays (Ag-TNTs)
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in the same environmental conditions. The results indicated that Ag-TNTs had the best photoelectrocatalytic activity when the nominal mass ratio of Ag to TiO2 was 0.84%.
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The mechanism deduced by the authors is as follows: ClO4- was reduced by the ClO2generated in the solution or by the e- transferred to the photocathode through the
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external circuit.
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Liu et al. (2015) used a highly active catalyst prepared by the noncovalent
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immobilization of the rhenium complex ReV(O)-(hoz)2Cl (hoz = 2-(2’-hydroxyphenyl)
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-2-oxazoline), together with Pd0 nanoparticles on a porous carbon support, to reduce
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ClO4- to Cl- with 1 atm H2 at 25 ℃. The results showed that the immobilized Re complex served as a single site for oxygen atom transfer from ClO4- and ClOx-
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intermediates, whereas Pd0 nanoparticles provided atomic hydrogen reducing
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equivalents to sustain the redox cycling of the immobilized Re sites. The authors suggested that hybrid catalysts, which combined single-site transition metal complexes with hydrogenactivating metal nanoparticles, could be integrated into
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heterogeneous catalysts for use in environmental remediation and green chemistry applications. The proposed schematic summary of the transformation of Re species during Re(hoz)2-Pd/C catalyst preparation, catalytic reactions with ClO4-, and the potential catalyst decomposition is shown in Fig. 11. 30
Table 9 shows the key findings for the removal of perchlorate by catalytic reduction technology in water. The catalyst can improve the reduction efficiency of ClO4-, but the reaction conditions of catalytic reduction (such as solution pH, temperature) have certain requirements and the use of precious metals also increases
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the treatment cost. In addition, the use of a metal catalyst may bring additional environmental risks, which requires further study.
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3.3 nZVI/modified nZVI for removal of perchlorate
In recent years, nanoscale zero-valent iron (nZVI) particles, which has the
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advantages of large specific surface area, high reaction activity, and strong reductive
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power, has been successfully developed and used for removal of various wastewater
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contaminants (such as nitrite, selenate, organic dyes, aromatic halides, and heavy
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metals contaminants) (Shi et al., 2018). Studies have shown that nZVI and especially
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modified nZVI demonstrate an excellent perchlorate degradation rate. Cao et al . (2005) investigated perchlorate removal by nZVI and found that nZVI could
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effectively reduce ClO4- in water at a relatively high reaction temperature. Their
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results showed that the ClO4- (initial concentration 200 mg/L) removal efficiency could be nearly 90% at 75 °C within 24 hours with a nanoparticle dose of 10 g/L. A related study has shown that after the addition of nZVI to perchlorate solution under
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acidic conditions, the nZVI particles undergo the following reactions (Xie et al., 2016a): 2Fe0（s） + 4H+(aq) + O2(aq) → 2Fe2+(aq) + 2H2O(l) Fe0(s) + 2H2O(l) → Fe2+(aq) + H2(g)+ 2OH-(aq) 31
(1) (2)
ClO4-(aq) + 4Fe0(s) +8H+(aq) → 4Fe2+(aq) + Cl-(aq) + 4H2O(l)
(3)
However, the stability and reactivity of nZVI would be greatly reduced due to the tendency for agglomeration and easy oxidation, which limits its practical use for contaminant removal. To overcome these shortcomings, a series of modification
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methods have been explored, which can be roughly divided into two categories: supported nZVI and stabilized nZVI.
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3.3.1 Supported nZVI materials
The Supported nZVI is obtained by evenly diffusing the nZVI over the carrier
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surface of the with certain material as a carrier, which can effectively prevent nZVI
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particles from agglomeration and oxidation and maintain its high reactivity. Xu et al.
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(2010) synthesized iron compounds supported on granular activated carbon (ICs/GAC)
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for removal of perchlorate and found that ICs/GAC contained large amounts of
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FeOHSO4 and Fe2O3, and a small amount of nZVI, and that the perchlorate adsorption on FeOHSO4 and Fe2O3 played an important role in removal of perchlorate. Then, the
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authors synthesized and characterized a new nanoscale iron hydroxide-doped granular
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activated carbon (Fe-GAC) and tested it for the adsorption of ClO4- in water (Xu et al., 2013). They found that the solution pH and iron content were two key factors controlling the perchlorate removal efficiency, and determined the optimal pH range
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to be 2–3 with an adsorbing capacity of 17.5 mmol/1 g iron. In addition, they found that coexisting anions might slow down the perchlorate adsorption in the order NO3- > SO42- > Cl-. Electrostatic attraction, ion exchange, and surface complexation were the main mechanisms of the perchlorate adsorption. Based on these two studies, the 32
authors then conducted a comparative study of Fe-GAC for removal of bromate and perchlorate from water (Xu et al., 2015). They found that Fe-GAC exhibited much greater removal capacity of bromate than perchlorate. Fe-GAC performed well within the optimal pH range of 6–8 in the reduction of bromate, whereas the removal of
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perchlorate was more dependent on pH . Loading nZVI onto solid materials is cheap and easily performed, and can not
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only overcome the characteristics of agglomeration and oxidation of nZVI, but also
increase the treatment availability of effluents containing perchlorate. At present, the
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carrier material used in supported nZVI for the removal of perchlorate is basically
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limited to activated carbon. However, many other materials, such as pumice, clay
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minerals, and graphite, have demonstrated excellent support performance on nZVI
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particles. Therefore, the removal of perchlorate with supported nZVI materials has a
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wide research scope. 3.3.2 Coated nZVI materials
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Coated nZVI is the nZVI coated on polymer materials or surfactants, which can
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enhance the dispersal performance and antioxidant properties while maintaining the high reactivity of nZVI. Xiong et al. (2007b) tested the feasibility of two different stabilized nZVI materials, including starch- and carboxymethyl cellulose(CMC)
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-stabilized nZVIs, for removal of perchlorate in water or IX brine. The authors showed that about 90% of perchlorate in both fresh water and a simulated IX brine (NaCl = 6% (w/w)) was destroyed at an iron dosage of 1.8 g/L at moderately elevated temperatures (90–95 °C) within 7 h. The addition of a metal catalyst (Al, Cu, Co, Ni, 33
Pd, or Re) showed no significant improvement in perchlorate degradation. Xie et al.(2016a) prepared chitosan-stabilized nZVI (CS-nZVI) to test the degradation of perchlorate in water. Their results showed that CS-nZVI exhibited an excellent removal rate compared with ZVI and nZVI, especially in high-concentration (200
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mg/L) perchlorate-polluted water. Most perchlorate could be reduced to chloride through initial adsorption and subsequent reduction processes. This research provided
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an effective method for perchlorate degradation in contaminated water with high concentrations or high salinity.
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As coated nZVI has a high dispersion property and reactivity, its ability to
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degrade perchlorate is far superior to that of ordinary nZVI. As yet, there have been
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few studies of the removal of perchlorate by stabilized nZVI, but more types of coated
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nZVI materials have been obtained. Therefore, this technology has good prospects in
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the study of perchlorate removal by coated nZVI. The key findings of studies on the removal of perchlorate by coated nZVI materials and supported nZVI materials are
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summarized in Table 10.
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4. Conclusions and future prospects 4.1 Conclusions
The environmental pollution problems caused by perchlorate have brought
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serious harm to human health and threatens human survival as well as development. Its characteristics of high water solubility, high stability, less volatility, and difficulty in being absorbed make it difficult to be removed. This paper summarizes and analyzes the general removal techniques of perchlorate in water by physical-chemical 34
treatments and chemical treatments, including the application of new nZVI materials. The main conclusions are as follows: (1) Physical-chemical treatments are efficient technologies for the removal of ClO4- in trace level that does not cause secondary pollution and could be used to
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remove ClO4- in drinking water. IX seems to be one of the most effective technologies for removal of trace perchlorate in water. However, the resins are of high prices and
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needed to be regenerated, and the high concentration of regenerated liquid containing
perchlorate must to be disposed. Membrane filtration can effectively remove
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perchlorate, but its inherent drawbacks include membrane fouling and high cost,
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rendering it impractical for large-scale applications of ClO4- removal. In recent years,
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there have been only fewer studies to remove perchlorate using membrane filtration
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combined with other methods.
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technology, and it has been more liked to deal with perchlorate pollution in water
(2) Adsorption is another widely used technology that can effectively remove
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perchlorate in water. GAC or modified GAC is the earliest and commonly used
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adsorbent for removal of trace perchlorate in water. However, more and more innovative materials have been explored to adsorb perchlorate, and their adsorption capacities of perchlorate are much higher than GAC or modified GAC. It is
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particularly to note the newly synthesized 25, 27-bis-(N, N-dimethyl-2-aminoethyl) appended Amberlite XAD-4 resin, which has approximately 30 times adsorption capacity than that of modified GAC. Although adsorption technology can effectively remove perchlorate from water, the regenerating solution of the adsorbents contains a 35
high concentration of ClO4-, which must also be disposed. Most of the new adsorbent materials are in the experimental research stage and their potential for practical use requires further study. (3) Chemical treatments can thoroughly degrade perchlorate to chloride, so it
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could be used to treat high-concentration perchlorate wastewater. However, the chemical reduction rate is relatively slow, so it is necessary to use a strong reducing
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agent with high activity or to add a catalyst to reduce the activation energy of the
reaction. Also, most of the metal reducing agents are toxic and not suitable for the
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removal of perchlorate in drinking water. Electrochemical reduction can convert ClO4-
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into Cl- without the addition of a catalyst, but the reduction process is
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electricity-intensive and the electrode is prone to be corrosive. Catalytic reduction can
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improve the reduction efficiency of ClO4-, but the reaction conditions are relatively
further studied.
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harsh, and the use of a metal catalyst may bring environmental risks, which should be
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(4) Modified nZVI, which overcomes the drawbacks of easy agglomeration and
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oxidation, improves the practical potential of materials. Currently, the modified nZVI materials used for treatment of perchlorate mainly include supported nZVI and coated nZVI. These two kinds of materials can rapidly and effectively remove perchlorate by
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the reaction activity of nZVI. It's worth mentioning that coated nZVI is superior for treatment of high-concentration perchlorate and can effectively degrade ClO4- into Cl-, so this material has good prospects for removal of perchlorate. As yet, research on modified nZVI has been relatively scant. Hence, there remains an enormous research 36
scope in this field. 4.2 Future prospects Today, the exploration of new materials and technologies which are more economical and efficient for treatment of perchlorate more has become a hot research
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topic. This review may have an important environmental significance with respect to pollution prevention and control problems related to perchlorate. Based on the
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conclusions drawn in this review, some prospects about perchlorate removal are proposed in the end.
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(1) Single water treatment processes are characterized by drawbacks that have an
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effect on pollutant removal efficiency. Processes that combine the advantages and
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disadvantages of two or more technologies may have more flexibility and higher
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degradation efficiency for removal of pollutant. Therefore, combined processes are a
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promising treatment approach for removal of perchlorate . (2) The regeneration liquid produced in physical-chemical treatments usually
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contains a high concentration of ClO4- and, as such, presents an awkward problem.
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The effective disposal of this liquid could be achieved by chemical or microbiological methods.
(3) Modified nZVI materials have strong application advantages for the removal
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of organic and heavy pollutants in water, but their current applications in perchlorate removal are relatively few. With respect to the modification methods of nZVI and its existing pollutant degradation mechanisms, further improvements and the optimization of perchlorate removal in water would be broadened in its application 37
prospects and engineering value about pollution remediation.
Acknowledgements This work was supported by grants from the Chinese National Natural Science
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Foundations (No.41472230).
38
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U
SC R
Fig. 1. Flow, transport, and transformation pathways of perchlorate in the environment (Kumarathilaka et al., 2016).
CC E
PT
ED
M
Fig. 2. Schematic showing main results of perchlorate removal by QAE-anchored GACs (Hou et al., 2013b).
A
Fig. 3. Perchlorate adsorption mechanism of Fe-GAC by impregnating various iron salts on GAC (Xu et al., 2016).
54
IP T
M
A
N
U
SC R
Fig. 4. Effect of pH on the percent adsorption of perchlorate (Memon et al., 2013).
A
CC E
PT
ED
Fig. 5. Adsorption mechanism of AICS for perchlorate (Xing et al., 2015).
Fig. 6. Proposed scheme of perchlorate uptake mechanism by MgAl-LDHs and MgAl-CLDHs
55
IP T
M
A
N
U
SC R
Fig. 7. Schematic of perchlorate adsorption by the three kinds of CLDHs (Lin et al., 2014b).
CC E
PT
ED
Fig. 8. Schematic of the main perchlorate removal results by organic MMTs (Luo et al., 2015b).
A
Fig. 9. Mechanism of ClO4- sorption by cross-linked Fe-CB (Lv et al., 2014).
56
IP T
Fig. 10. Schematic of main perchlorate removal results by PHW (Hori et al., 2012).
A
CC E
PT
ED
M
A
N
U
SC R
Fig. 11. Proposed scheme for immobilization, reaction, and decomposition of hoz-coordinated Re species (Liu et al., 2015).
57
Table 1 – Application of ion exchange for perchlorate treatment. Ion-exchange medias
Key
Findings
A530E, A500, A520E, and A850), Bio-Rad (Dowex 1-X8), and Wandong Chemical
references
The presence of SO42- had little impact on the sorption of U(VI) but significantly affected the sorption of ClO4-, particularly on monofunctional
(Gu et al., 2005)
SBA resins.
Plant(WBR109)
Bifunctional resin (Purolite
groundwater before a significant breakthrough of ClO4- occurred at an average -
A-530E)
IP T
Purolite A-530E treated about 37000 empty bed volumes (BVs) of
flow rate of 150 gpm (or 1 BV/min) and a feed ClO4 concentration of about
SC R
860 µg/L. SBA, WBA exchangers,
IXF offered a comparable perchlorate capacity to that of styrenic resins as
A-530E, PLEs, and IXF
well as unparalleled kinetics (with a sorption equilibrium time < 1.5 h).
Mono- or bifunctionalized
bifunctionalized mesoporous
The ammonium bifunctionalized mesoporous medium showed a 1.14–1.39
U
SBA-15, IRA-900 and QA
times higher adsorption capacity than those of mono-functionalized SBA-15.
N
molecular sieves
(Gu et al., 2007)
(Xiong et al., 2007a)
(Kim et al., 2008)
The HIX prepared via the in-situ precipitation of Fe(III) (M treatment) had higher separation factors for ClO4- over Cl- than the HIX obtained by the
(Hristovski et
media
KMnO4/Fe(II) treatments, suggesting that permanganate may adversely impact
al., 2008)
Copolymer
(PANOA) film Polyaniline (PANI) films
The copolymer PANOA had fairly good redox properties and affinity to perchlorate over chloride in a wide pH range of 1 to pH 9.0.
ED
poly(aniline-co-o-aminophenol)
M
the IX base media.
A
Hybrid ion-exchange (HIX)
PANI-H2SO4 film represented higher selectivity for ClO4- than PANI-HCl and PANI-HClO4.
PT
The
HDPyCl-montmorillonite
ClO4-
(Zhang et al., 2010) (Gao et al., 2011)
uptake on HDPyCl-montmorillonite followed the Langmuir
isotherm model with capacity of 1.02 mmol/g.
(Chitrakar et al., 2012)
The Qmax of perchlorate was increased nearly 3 times with surfactant loading,
CC E
CPC, HDTMA, CTAC, MTAB, DTAB
A532E, A520E, A400E,
A
PWA-5, PSR-2
MTBA membrane
which was mainly attributed to the extra adsorption sites contributed by the quaternary ammonium groups from surfactants. The maximal exchange capacities for perchlorate were of the same order of magnitude for all resins (from 183 mg/g for A400E to 115 mg/g for A532E).
2013) (Darracq et al., 2014)
About 60% of ClO4- and 9% of other anions, specifically, SO42-, NO3- and HCO3-, passed through the membrane under otherwise identical operation conditions while in the presence of an electric field.
CS-MAB
(Lin et al.,
The maximum adsorption capacities of CS-MAB were 119.05 mg/g, 126.58 mg/g and 178.57 mg/g at 20 °C, 30 °C, and 40 °C, respectively.
58
(Wang et al., 2015) (Song et al., 2017)
Table 2 – Application of membrane technologies for perchlorate treatment. Membrane
Findings
Key references
ClO4- could be excluded by NF and UF membranes in a pure component system, but this
(Yoon et al.,
rejection capability was quickly lost in the presence of a sufficient amount of other ions.
2002)
Although the ClO4- rejection capability was reduced in the presence of a cationic
(Yoon et al.,
technologies NF and UF Surfactant modified
-
surfactant, a desired amount of the ClO4 was excluded by steric exclusion.
2003)
IP T
UF Negatively-charged
Hindered (effective) diffusion associated with partitioning of ClO4- was affected by
porous NF and UF
charge repulsion for negatively charged porous membranes.
Cr(VI), As(V), and ClO4- rejection followed the order: LFC-1 (>90%) > MX07 (25–95%) > ESNA (30–90%) > GM (3–47%) at all pH conditions.
SC R
RO, NF, and UF
About 60–80% of ClO4- was removed with 30-mM PDADMAC in the presence of both
PDADMAC
-
1-mM to 10-mM NO3 and SO4
whereas greater than 95% was removed even in the
presence of 10-fold excesses of competing ions such as
Cl-,
SO4
2-,
2-.
and CO3
U
enhanced PEUF
2-,
P4VP and
provided up to 95.8% retention of ClO4- under the solution conditions investigated.
A
enhanced PEUF
N
The greater affinity of ClO4- over chloride for the protonated pyridine residues of P4VP
PDADMAC
PEM-based RO and
of NF by PAH and
About 93% perchlorate rejection was achieved when both PAH and PAA were deposited
M
surface modification
on a NF 90 membrane at a pH of 6.5 and cross-linked with glutaraldehyde.
A
CC E
PT
ED
PAA
59
(Yoon et al., 2005)
(Yoon et al., 2009)
(Huq et al., 2007; Roach and Tush, 2008)
(Roach et al., 2011)
(Sanyal et al., 2015)
Table 3 – Application of GAC or modified-GAC adsorption for perchlorate treatment. Findings
materials
-
wood, lignite coal, and bituminous coal Modified GAC with iron-oxalic acid
Maximum adsorption
Adsorption
Key
capacity
model
references
The major mechanism for ClO4 adsorption on
0.32 mmol/g for
ACs was specific chemical interactions between
Filtrasorb 400 and
-
ClO4
and
surface
functional
groups
in
0.19 mmol/g for
combination with electrostatic forces. The adsorption capacity of preloaded GAC with iron-oxalic acid could be improved up to 42%.
Not obtainable
with QAs
removed for 27,000–35,000 bed volumes before
Not mentioned
-
the effluent ClO4 rose above 1 ppb. GAC preloaded with CTAC shown to be an
Not
(Na et al.,
obtainable
2002)
Not
and
mentioned
Cannon,
Not mentioned
2005)
Not
(Parette et
mentioned
al., 2005)
Not
(Chen et
mentioned
al., 2005)
Langmuir and
(Jang et
Freundlich
al., 2009)
N
groundwaters containing ClO4- concentrations
U
effective adsorbent for removing perchlorate from three low conductivity (50–66 mS/cm)
Huang, 2010)
SC R
THAB, CTAC or CPC, 75 ppb ClO4 was
ov and
(Parette
-
GAC tailored
with CTAC
Langmuir
Nuchar SA
By pre-loading bituminous AC with DTAB,
GAC preloaded
(Mahmud
IP T
Adsorption
A
of 0.85 ppb, 1.0 ppb, and 5.6 ppb, respectively.
Modified GAC could increase the nitrogen
with ammonia
content of the carbon, thereby significantly
M
GAC tailored
increasing the positive charge of the carbon
Not mentioned
surface, which was the most important surface
with iron or QA GAC tailored
4500 bed volumes before 6 mg/L perchlorate
2C-75
Bituminous-based GAC that had been preloaded with 0.24g/g arquad 2C-75 exhibited a 6-μg/L
A
Not obtainable
-
ClO4 breakthrough after 33,000 BVs.
et al., 2010)
pH 2.5
(Xu et al.,
The primary mechanisms for the adsorption of
and 0.094 mmol/g at
while
2011a)
perchlorate on GAC-CTAC or GAC-CTAB
pH 5.6
Freundlich at
surface complexation, and ion exchange.
with CTAB
Fe-GAC
obtainable
(Patterson
0.36 mmol/g at pH 2.5
pH 5.6
were associated with electrostatic interaction, GAC coated
Not
Langmuir at
GAC coated with CTAC
Not mentioned
breakthrough.
CC E
with arquad
GAC impregnated with iron and QA could treat
PT
GAC tailored
ED
property influencing adsorption.
0.13 mmol/g
The type of iron salt impregnated on GAC
FeCl2-GAC
played an important role in the perchlorate
(0.121 mmol/g) ≈
adsorption, and electrostatic attraction was the
FeCl3-GAC(0.117
main mechanism.
mmol/g) 60
Freundlich
Langmuir
(Xu et al., 2011b)
(Xu et al., 2016)
more positive surface charge at pH 7.5 than did QAE-anchored GACs
U17 and U2 exhibited 18–20 times longer bed
pristine GAC (900 BV).
ted GACs
times more perchlorate removal than pristine
32.48 mg/g for U2
mentioned
Not mentioned
processes
were
rapid
and
obeyed
the
20.1, 37.5 and 44.0 mg/g for GAC, GAC-AA and
pseudo-second order model.
GAC-PH at 303 K,
A
CC E
PT
ED
M
A
N
U
respectively
61
(Hou et al., 2013a) (Hou et al., 2013b)
Not
(Byrne et
mentioned
al., 2014)
(Krishnan
Langmuir
SC R
GAC-AA
Not
bituminous GAC.
GAC-AA, and GAC-PH showed that the uptake
Langmuir
30.86 mg/g for U17,
Functionalized bituminous GAC provided 6
The adsorption kinetics of perchlorate on GAC, GAC-PH and
at pH 14
the wood-based QAE-GAC.
volume (BV) to 6 ppb breakthrough than did the
Pyridinium-graf
33.2 mg/g for U360H
IP T
The bituminous-GAC-based QAE-GAC hosted
et al.,
2017)
Table 4 – Application of organic adsorbent materials for perchlorate treatment.
Findings
materials
adsorption capacity
by quaternary amine
Modified Wheat straw(MWS)
Electrostatic attraction between perchlorate anion and positively charged quaternary amine groups on the MR was the primary mechanism. The adsorption mechanism of ClO4- by MWS was IX and the maximum adsorption capacities of ClO4-
were electrostatic attraction and physical force.
Cross-linked
QCS
showed
high
adsorption
capacity, which was 2.5 times higher than PCLC.
45.455 mg/g
N
The CM-CS/PVA Bs could adsorb perchlorate efficiently in a wide pH range from 4 to 10.
references
Langmuir-
(Baidas et
Freundlich
al., 2011)
Langmuir
and
Freundlich Langmuir
119.0 mg/g
28.352 mg/g at 318K
A
CM-CS/PVA Bs
model
Langmuir
U
Cross-linked QCS
119.4 mg/g
decreased with increase in temperature. The main driving forces for perchlorate adsorption
PCLC
169.34 mg/g
Key
SC R
Reed modified(MR)
Adsorption
IP T
Maximum
Adsorption
and
Tempkin Langmuir and Freundlich
Resin-5
M
The adsorption process was endothermic and spontaneous at higher temperature, the maximum -
ED
Perchlorate removal was above 98% at pH values ranging from 4.0 to 9.0.
PT resins
followed
the
11.35 mmol/g
50.00 mg/mL
order
of
Purolite
Langmuir
reached the equilibrium much faster than Purolite
Purolite A532E
CC E
MIEX
A530E and Purolite A532E.
A
2012)
(Xie et al., 2016b)
et al.,
(Tang et al., 2013)
Freundlich
(Zhu et al., 2015)
and MIEX, respectively
Perchlorate adsorption capacity was optimum at neutral condition (pH: 6.0, 170.4 mg/g), and decreased at acidic (pH: 3.0, 96.4 mg/g) or alkalic
Not mentioned
Thomas
(Song et al., 2015)
(pH: 12.0, 72.8 mg/g)
A strong base anion
The rate-limiting stage for the adsorption of
(SBA) exchange
perchlorate into the ion exchange resin could be
resin
Freundich
Purolite A530E，
resin
(Xie et al.,
90.91 mg/g on
A530E≈MIEX > Purolite A532E, and MIEX
biopolymer based
2010)
2013)
Purolite A532E and
Amine-crosslinked
(Xie et al.,
90.09, 104.2,
The equilibrium capacity of the three investigated Purolite A530E，
2012)
(Memon
adsorption of ClO4 could be achieved at pH 4.5. MIEX resin
(Tan et al.,
Not mentioned
explained as a chemisorption process
Cationic MOFs
The proposed adsorption mechanism was the
material
co-effect of the electrostatic force and IX between 62
133.5 mg/g
Not
(Faccini et
mentioned
al., 2016)
Langmuir
(Li et al., 2015)
((NH2SO3-Cu-(4,4’
the ClO4- and MOFs material.
-bipy)2), ASC) The adsorption capacity of ClO4- by AICS
AICS
decreased with increases in temperature.
Two types of cationic MOFs
-
3-.
MOFs followed the order ClO4 > PO4
Langmuir
143.48 mg/g on ASC and 119.97
Langmuir
mg/g on ESC
(Xing et al., 2015) (Zhang et al., 2017)
A
CC E
PT
ED
M
A
N
U
SC R
IP T
(ASC and ESC)
The competitive ClO4- and PO43- ions on the two
89.1 mg/g
63
Table 5 – Application of inorganic adsorbent materials for perchlorate treatment. Adsorption
Findings
materials
Key
capacity
model
references
Perchlorate was adsorbed on GFH through electrostatic attraction between perchlorate and
Approximately 90–95% of perchlorate was
zero-valent
removed within 24 h in the presence of 35 g/L
aluminum
aluminum at acidic pH (4.5 ± 0.2).
double hydroxide (CLDH)
Not mentioned
Not mentioned
The rapid step of the uptake process was controlled by diffusion, and the slow step was
285.9 mg/g
controlled by the reaction of perchlorate with the CLDH rather than by diffusion. The existence of CHMAF was favorable for the
Not mentioned
U
removal of ClO4 from water, and the best ratio of Mg/Al/Fe was 3:0.8:0.2 (CHMAF5%).
Freundlich
Linear
-
CHMAF
Langmuir
positively charged surface sites.
Acid-washed
Calcined layered
20.0 mg/g
adsorption isotherm
N
At 25 °C, MgFe-3 CLDH = 1.33 g/L, the initial
Three modified inorganic-
M
all adsorbed within 720 min.
A
pH range of 4-10, 2000 μg/L of ClO4- was almost
Mg/Fe-CLDH
The ClO4- uptake on these three modified bentonites from 0.1 mmol/L solution rapidly
bentonites
ED
attained equilibrium within 100 min.
Not mentioned
(Lien et
al., 2010)
(Wu et al., 2010b)
(Yang et al., 2012a) (Yang et
Freundlich
al.,
0.117, 0.107 and 0.102 mmol/g on Fe-Bent, Fe-Al-Bent and
Langmuir
(Yang et al., 2013)
Al-Bent, respectively Freundlich for
PT
MgAl-LDHs and
enhanced as the positive layer charge density
280 mg/g for
MgAl-CLDHs
decreased, which can be explained by anion
MgAl-CLDHs
CC E
exchange mechanism and H-bonding interactions.
MgAl-LDHs and Langmuir for
(Lin et al., 2014a)
MgAl-CLDHs MgAl-CLDH-4
MgAl-CLDHs,
The binding mechanisms of ClO4- by CLDHs are
MgFe-CLDHs
much more dependent on the unique M2+
A
al., 2010)
2012b)
The ClO4- uptake by the MgAl-CLDHs was
and ZnAl-CLDHs
(Kumar et
IP T
hydroxide (GFH)
Adsorption
SC R
Granular ferric
Maximum adsorption
compared to the type of M3+.
(279.6 mg/g) > MgFe-CLDH-4 (120.8 mg/g) > ZnAl-CLDH-4(1 0.81 mg/g)
64
Langmuir
(Lin et al., 2014b)
1.499, 2.627 and Hydrogen bonding was proposed to be the CNTs
dominant mechanism at neutral pH for the
3.554 mg/g for raw DWCNTs, 2
Freundlich and
h-DWCNT,8
Langmuir
interaction of ClO4- with CNTs.
h-DWCNT,
(Fang and Chen, 2012)
respectively 6 mg/g for
facilitated the adsorption of perchlorate onto
SWCNTs and 3
SWCNTs at a fixed pH.
mg/g for GFH
Modified
(Hsu et
Freundlich
al., 2015)
A
CC E
PT
ED
M
A
N
U
SC R
GFH
Low temperatures and low humic acid contents
IP T
SWCNTs and
65
Table 6 – Application of composite adsorbent materials for perchlorate treatment. Maximum Adsorption materials
Findings
adsorption capacity
Adsorption
Key
model
references
Mag-PCMA removed over 98% of the Mag-PCMAs
aqueous
perchlorate
anions
across
(Clark and
a
Not mentioned
Freundlich
2012) 0.421, 0.400, 0.280,
organosilicas and organoclay
absorbents MCM-48
uptake have silica
capacities the
>
by
following MCM-50
0.246 meq/g on
the
MCM-48 silica,
order:
silica
>
Cloisite®. 10A > MCM-41 silica.
MCM-50 silica,
Langmuir
SC R
Perchlorate
Synthetic
IP T
concentration range of 60–500 mg/L.
Keller,
Cloisite® 10A and
(Seliem et al., 2013)
MCM-41 silica, respectively
Exchange of anions from the entrapped neutral surfactant appeared to be the main
modified organoclay
mechanism of nitrate and perchlorate uptakes
U
Cationic surfactant
N
by HDPy-montmorillonite.
A
Electrostatic attraction and chelation of the Fe-CB
1.11 mmol/g
Fe(III) center in the complex were the main
M
driving forces for perchlorate adsorption.
Langmuir
(Bagherifa m et al., 2014)
29.851 mg/g at 298K
Langmuir
(Lv et al., 2014)
Adsorption performance was synergistically influenced by the alkyl-chain length, head
ED
Organo-Mt
group, and alkyl-chain number of the
0.95 mmol/g on C18-BM/Mt
Langmuir
(Luo et al., 2015a)
surfactants used for modification. High
adsorption
capacity
was
stably
PT
Organo-MMTs modified by
CC E
BODMA-Cl
Modified OMt with
DDAC and BODAC
A
sequentially
maintained with high selectivity against variations in temperatures of 25 °C to 45 °C
(Luo et 0.90 mmol/g
Langmuir
al., 2015b)
and initial pH range of 2.5–12.5. The highest adsorption capacity for ClO4- was obtained
using
sequential
modification,
whereby
the
addition
of
DDAC
corresponding to 0.05 times the cation exchange
capacity
of
Mt
(Luo et 1.08 mmol/g
Langmuir
al., 2016b)
significantly
increased BODAC uptake. Innovative
wet
OMt
without
Freundlich
washing
Hexadecylpyridinium
(I-OMt) showed relatively high adsorption
-modified OMt
capacity of ClO4- and negligible release of
and 0.0926 mmol/g
dushkevich
HDPy. Mt and Ilt modified
Dubinin–Ra
(Luo et al., 2016a)
(D–R)
Wet grinding of Ilt significantly influenced 66
0.58534
mmol/g
Not
(Luo et
by DDAC
the
DDAC
loading
ClO4-
and
uptake
and 0.5187 mmol/g
compared to Mt, and grinding for 30 min
on OMt and OIlt-30,
showed the best performance for both
respectively
mentioned
al., 2017)
minerals.
Table 7 – Application of chemical reduction for perchlorate treatment. Reducing agent
Findings
Fisher iron filings,
Perchlorate removal was proportional to the iron dosage in the batch reactors,
Elemental iron
with up to 66% removal in 336 h in the highest dosage system (1.25 g/mL).
(Moore et al., 2003)
IP T
electrolytic iron
Key references
Up to 98% of aqueous ClO4- (initial concentration about 1.0 mM) was removed in 1 h at 200 °C using a microwave digester.
(Oh et al., 2006)
More than 99.9% removal of perchlorate was obtained in a solution
containing [ClO4-] = 1.0 mM, [Ti(III)] = 40 mM, and [β-alanine] = 120 mM after 2.5 h of reaction at 50 °C.
Ti2+
Higher concentrations of Ti(0) produced higher concentrations of Ti(II), which resulted in more rapid perchlorate destruction.
(Wang et al., 2010)
SC R
Ti3+
(Park et al., 2012a)
U
This agent could enhance the production of Ti(II) or a mixture of Ti(II) and Ti2+/Ti3+
Ti(III) at the lowest F/Ti (0) ratio of 0.5 (25 mM KF), which resulted in more
N
effective reduction of perchlorate.
The ultimate product of ClO4- degradation by the sulfite/UV-L ARP was chloride, but chlorate was detected as an intermediate.
A
CC E
PT
ED
Sulfite/UV-L
Cl- was formed with a yield of 85% after 6 h.
M
Fe
A
Al、Cu、Zn、Ni、 At 150 °C, iron powder degraded ClO4- from 104 Μm to 0.58 Μm in 1 h, and
67
(Park et al., 2012b)
(Hori et al., 2012) (Vellanki and Batchelor, 2013)
Table 8 – Application of electrochemical reduction for perchlorate treatment. Electrode material
Ni/Pt
Ti
references
A number of chemical and electrochemical reactions resulted in perchlorate reduction
(Rusanova
and oxidative dissolution of the working Ni electrode.
et al., 2006)
Perchlorate could be electrochemically reduced on rhodium and the reduction rate of
(Láng et al.,
chlorate was much higher than that of perchlorate in the same system.
2008)
IP T
Polycrystalline Rh
Key
Findings
Perchlorate and nitrate at initial concentration of 200 ppm and 1000 ppm individually
(Wang et
were reduced to < 20 and < 200 ppb, respectively, over a short reaction time of 6–8 h.
al., 2009)
Perchlorate reduction using electrochemically induced pitting corrosion of Ti was independent of the imposed potential as long as the potential was maintained above the
SC R
ZVT
A
CC E
PT
ED
M
A
N
U
pitting potential, whereas it was proportional to the applied current.
68
(Lee et al., 2011)
Table 9 – Application of catalytic reduction for perchlorate treatment. Catalyst
Findings
Key references -
Re-Pd/C
Ti-TiO2
Under standard batch conditions at room temperature, 1 bar of hydrogen, 200 ppm ClO4 was reduced to less than 1 ppm in 5 h.
Shapley, 2007)
Up to 90% of ClO4- could be reduced to Cl- by molecular hydrogen gas and metallic -
catalysts in 2 weeks at an initial ClO4 concentration of 2 ppm.
Cu–TiO2/
25 °C over a range of initial concentrations (2–200 ppm) at 1 atm of H2 and a pH range of 2.7–3.7.
2010)
(Zhang et al., 2011)
The efficiency of ClO4- reduction in the presence of Cit (0.15 mM) could reach 56.0% after 140 min of irradiation (368 ± 0.5 K) when the initial concentration of ClO4- was 0.001 mM.
U
SiO2
The catalysts were efficient for the complete reduction of ClO4- to Cl- within a few hours at
SC R
Cl-Pd/C
(Choe et al.,
IP T
ClO4-, and that reduced Re surface species (41.5 eV) to a greater extent were more resistant to reoxidation by short-term exposure to oxygen.
ReO(hoz)2
(Wang et al., 2008)
The two reduced Re (Re(V)/Re(VII)) surface species that both played a role in reducing Re/Pd-AC
(Hurley and
(Ye et al., 2013)
The identified Re structures supported a revised mechanism for the catalytic reduction of ClO4- involving oxygen atom transfer reactions between odd-valence oxorhenium species
(Choe et al.,
and the oxyanion (Re oxidation steps) and atomic hydrogen species (Re reduction steps)
2014)
Cl-Pd/C
enabled easy ClO4 reduction by the Re-Pd/C catalyst. Redox cycling between hoz-coordinated ReV and ReVII species served as the main catalytic cycle for ClO4- reduction.
(Liu et al., 2013) (Liu et al., 2015)
The ClO4- reduction efficiency could reach 62% with Cit (0.15 mM) after 6 h reaction (368 ± 0.5 K) at applied voltage of 1.5 V With an initial ClO4- concentration of 0.001 mM.
A
CC E
PT
Ag-TNTs
-
M
ReO(hoz)2
Pre-treatment of NO3- using In-Pd/Al2O3 improved selectivity for N2 over NH4+ and
ED
Re-Pd/C
A
formed by the Pd-catalyzed dissociation of H2.
N
Re-Pd/C
69
(Jia et al., 2016)
Table10 – Application of coated nZVI for perchlorate treatment. Material
Findings
Key references
-
About 97% of ClO4 was removed within 10 hr at 90 °C and 86% of ClO4 was removed
GAC
within 12 hr at 25 °C at ICs/GAC dosage of 20 g/L.
Hydroxide-doped GAC
The highest ClO4- adsorption capacity was achieved by Fe (0.97)-GAC (0.169 mmol/g) with an iron content of 0.97% wt. of GAC.
impregnated GAC Starch and CMC
(Xu et al., 2010)
(Xu et al., 2013)
(Xu et al., 2015)
IP T
Less FeOOH of Fe–GAC was favorable for removing perchlorate, whereas more FeOOH of Fe–GAC favored the removal of bromate.
Starch and CMC-stabilized nZVI could degrade perchlorate 1.8 and 3.3 times faster than non-stabilized nZVI, respectively.
(Xiong
et
2007b)
SC R
Hydroxide
Increasing the temperature could enhance the removal efficiency of perchlorate, and the -
removal rate of ClO4 reached to 99.9% at 90 °C with the initial perchlorate of 200 mg/L.
A
CC E
PT
ED
M
A
N
U
Chitosan
-
70
(Xie et al., 2016a)
al.,