Removal of thallium from wastewater by a combination of persulfate oxidation and iron coagulation

Accepted Manuscript Title: Removal of thallium from wastewater by a combination of persulfate oxidation and iron coagulation Authors: Keke Li, Huosheng Li, Tangfu Xiao, Gaosheng Zhang, Jianyou Long, Dinggui Luo, Hongguo Zhang, Jingfang Xiong, Qimin Wang PII: DOI: Reference:

S0957-5820(18)30233-7 https://doi.org/10.1016/j.psep.2018.08.018 PSEP 1488

To appear in:

Process Safety and Environment Protection

Received date: Revised date: Accepted date:

5-6-2018 8-8-2018 15-8-2018

Please cite this article as: Li K, Li H, Xiao T, Zhang G, Long J, Luo D, Zhang H, Xiong J, Wang Q, Removal of thallium from wastewater by a combination of persulfate oxidation and iron coagulation, Process Safety and Environmental Protection (2018), https://doi.org/10.1016/j.psep.2018.08.018 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.

Removal of thallium from wastewater by a combination of persulfate oxidation and iron coagulation

Keke Li a, Huosheng Li a, b, Tangfu Xiao a, Gaosheng Zhang a, b, Jianyou Long a, c*,

Key Laboratory for Water Quality and Conservation of the Pearl River Delta, Ministry of Education;

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a

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Dinggui Luo a, Hongguo Zhang a, d Jingfang Xiong a, Qimin Wang a

School of Environmental Science and Engineering, Guangzhou University, Guangzhou 510006, China

Research Institute of Environmental Studies at Greater Bay, Guangzhou University, Guangzhou

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b

Guangdong Provincial Key Laboratory of radionuclides pollution control and resources,

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c

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510006, China

Linköping University – Guangzhou University Research Center on Urban Sustainable Development,

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d

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Guangzhou 510006, China

∗Corresponding

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Guangzhou University, 510006 Guangzhou, China

author: Dr. Jianyou Long

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Affiliation: Key Laboratory for Water Quality and Conservation of the Pearl River Delta, Ministry of Education; School of Environmental Science and Engineering, Guangzhou University, Guangzhou

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510006, China

E-mail address: [email protected] Tel: 86+ 13570339968

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Highlights 

Fe2+-S2O82- system is effective in thallium removal from both synthetic and real wastewaters



The coagulation pH plays a key role in the thallium removal



Thallium removal mechanisms include oxidation by persulfate and sulfate radicals, surface

Sulfate radicals are more important than hydroxyl radicals for thallium removal

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

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complexation by iron colloids.

Abstract

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Thallium (Tl) removal from wastewater using a ferrous iron-persulfate (Fe2+-S2O82-) Fenton-like

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system was investigated. Factors influencing Tl removal, namely S2O82- dosage, Fe2+/S2O82 molar ratio,

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reaction pH, coagulation pH, co-existing metal ions, co-existing organic matter, and initial Tl

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concentration, were examined. The results show that Tl removal efficiency increased with increasing S2O82- dosage. Effective Tl removal (>96%) was achieved when the Fe2+/S2O82- molar ratio was higher

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than 1:1. The reaction pH had little effect on Tl removal, while the coagulation pH significantly affected it. Coagulation pH exceeded 10 was favorable to Tl removal (>96%). Tl removal efficiency

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was reduced by about 30% when the concentration of co-existing organic matter was higher than 100

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mmol/L. More than 90% of Tl was removed when the initial Tl concentration increased from 20 μM to 150 μM. Based on the SEM-EDS, XPS and FT-IR spectroscopic analyses, it is concluded that the absorption of colloidal ferric hydroxide and the oxidative precipitation of the Fe2+-S2O82- system are

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the main mechanisms for the removal of Tl. Given the excellent Tl removal and stable performance, the Fe2+-S2O82- system could be an effective and promising alternative for Tl removal from wastewater. Keywords: thallium; persulfate; oxidation; coagulation; heavy metals; sulfate radicals

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1. Introduction Thallium (Tl) is a rare heavy metal with high toxicity and is highly accumulative in the human body and other living organisms (Casiot et al., 2011; Vanek et al., 2018; Xiao et al., 2012). It can damage the human nervous system and cause serious health problems (Davis et al., 1981). The lethal dose of Tl is only 10-15 mg/kg for humans, and thus, it is more toxic than lead, cadmium, and arsenic (Xiao

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et al., 2004). In the aquatic environment, Tl exists in two oxidation states, namely Tl(I) and Tl(III) (Mulkey and Oehme, 1993) . Tl(I) is very similar to K+ in terms of chemical properties, and shows

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good mobility and stability in water (Peter and Viraraghavan, 2005). Due to its weak adsorptive ability to the surface of many materials, Tl(I) removal from wastewater is more difficult compared to that of other heavy metals such as Cd, Pb, and Cu (Giovanni Bidoglio et al., 1997; Jacobson et al., 2005; Wan

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et al., 2014). Tl(III) has strong oxidability and its chemical properties resemble those of Al3+. It tends

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to be hydrolysed and is prone to form Tl2O3 precipitates when strong oxidants are present (Li et al.,

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2017a; Peter and Viraraghavan, 2005). The background level of Tl in natural water is very low (Zhang et al., 2018). However, in recent decades, the risk of Tl contamination in waterbodies has increased

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dramatically due to industrial activities such as mining, metallurgy, and coal burning (Zhang et al., 2018). Tl pollution has become increasingly severe in many cities worldwide. Therefore, Tl removal

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technologies have attracted much attention in recent years (Li et al., 2018a).

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Currently, commonly used techniques for Tl removal include ion exchange (Horne, 1958), adsorption (Chen, 2018; Li et al., 2018b), solvent extraction (Preston, 1985), and oxidation combined with precipitation (Rosengrant and Craig, 1990). Among these Tl removal techniques, the oxidation-

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precipitation method has some advantages over other techniques such as high efficiency, low cost, and strong operational flexibility (Ku and Jung, 2001). The use of strong oxidants to oxidize Tl(I) to Tl(III) and onward to the precipitates of Tl2O3, combined with coagulants to accelerate sedimentation, is an effective approach for Tl removal from both synthetic and real wastewaters. Kikuchi et al. (1990) reported the successful use of H2O2 and waste iron metal for oxidation and coagulation of Tl, achieving 3

an effluent Tl concentration of 1-20 mg/L when the influent Tl concentration was as high as 100 to 500 mg/L. The combination of strong oxidants and iron coagulation thus appears to be an effective means for Tl removal. Recently, advanced oxidation with sulfate radical (SO4-·) has become popular for wastewater treatment. SO4-· exhibits strong oxidation potential (2.5 to 3.1 V) and a longer stabilization time for a

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wide range of pH from 2.5 to 11 (Neta et al., 1977). SO4-· is typically produced from persulfate when subjected to heat, ultraviolet light, microwaves, ultrasonic waves, or activation by alkali metal-

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promoted transition metals (Matzek and Carter, 2016). The most frequently used transition metal is ferrous ions (Fe2+) due to its low cost, easy availability, and high efficiency (Romero et al., 2010; Yan

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et al., 2017). Moreover, iron salts can remove or reduce the concentration of Tl because of the excellent

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coagulation and adsorption ability of iron hydroxides (Sherman M. Ponder et al., 2000). It is speculated

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that the Fe2+-S2O82- system may show excellent performance with regard to Tl removal from

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wastewater through combined oxidation, precipitation, and coagulation. In addition, persulfate is more

H2O2 (Ahmad et al., 2010a).

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convenient and safe to transport and store, and it is also cheaper compared to other oxidants such as

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Despite the widespread use of the Fe2+-S2O82- system in wastewater treatment, its potential for Tl removal from wastewater has not been demonstrated yet. The influencing factors and related

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mechanisms of Tl removal are still not clear. Therefore, the purpose of this study is to investigate the feasibility of using the Fe2+-S2O82- system to remove Tl from wastewater. The factors affecting Tl

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removal were determined, and the associate mechanism was elucidated. 2. Materials and Methods 2.1 Reagents and solutions All the chemicals and reagents used were of analytical grade and used as received. Deionized water was used to prepare the synthetic wastewater and reagent solutions. FeSO4·7H2O was used as the iron

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source and Na2S2O8 was used for oxidation. The stock solution (1000 mg/L) of Tl(I) was prepared by dissolving TlNO3 (99.9%, Aldrich, USA) in deionized water. Standard working solutions were prepared daily from the stock solution by serial dilution with deionized water. NaNO3, MgSO4, and CaCl2 solutions were also added as the co-existing ions as needed. Humic acid (HA) sodium salt (C9H8Na2O6) was dosed as co-existing organic matter when required. Solutions of HNO3 (0.001, 0.1

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and 1.0 mol/L) and NaOH (0.001, 0.1 and 1.0 mol/L) were used to adjust the pH values. 2.2 Batch tests

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In a typical batch test, 25 mL of Tl-containing solution was placed in 50-mL beakers with a temperature-controlled (298 K) magnetic stirrer at a stirring speed of 300 rpm. Then, FeSO4·7H2O and

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Na2S2O8 were dosed to the Tl solution. After 30 min of oxidation, NaOH solution was used to adjust

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the pH to a designated value to allow coagulation for 10 min. After 30 min of sedimentation, the

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supernatant was filtered with a syringe filter (0.45 μm), acidified with 1% (v/v) HNO3 solution, and Tl

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reported as mean and standard deviation.

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was measured within two days. All experiments were conducted in triplicate, and the results are

The variables used in the factorial experiments for each group of tests are listed below. The

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following factors were considered: S2O82- dosage, Fe2+/S2O82- molar ratio, initial pH, coagulation pH, co-existing metal ions, co-existing organic matter, and initial concentration of Tl.

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The following variables were fixed for the S2O82- dosage factor: FeSO4·7H2O concentration of 3.0 mM, initial pH of 7, coagulation pH of 10, initial Tl concentration of 50 μM, oxidation time of 30 min, and coagulation time of 10 min. There were six Na2S2O8 concentrations: 0, 0.5, 1.0, 1.5, 3.0, and 5.0

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mM.

The following variables were fixed for the Fe2+/S2O82- molar ratio factor: Na2S2O8 concentration of

3.0 mM, initial pH of 7, coagulation pH of 10, initial Tl concentration of 50 μM, oxidation time of 30 min, and coagulation time of 10 min. There were six FeSO4·7H2O concentrations: 0, 0.5, 1.0, 1.5, 3.0, and 5.0 mM. 5

The following variables were fixed for the factor of initial pH: FeSO4·7H2O concentration of 3.0 mM, Na2S2O8 concentration of 3.0 mM, coagulation pH of 10, initial Tl concentration of 50 μM, oxidation time of 30 min, and coagulation time of 10 min. Eight initial pH values were considered: 2, 4, 6, 7, 8, 9, 10, and 11. The following variables were fixed for the factor of coagulation pH: FeSO4·7H2O concentration of

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3.0 mM, Na2S2O8 concentration of 3.0 mM, initial pH of 7, initial Tl concentration of 50 μM, oxidation time of 30 min, and coagulation time of 10 min. Six coagulation pH values were considered: 7, 8, 9,

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10, 11, and 12.

The following variables were fixed for the factor of co-existing ions: FeSO4·7H2O concentration of 3.0 mM, Na2S2O8 concentration of 3.0 mM, initial pH of 7, coagulation pH of 10, initial Tl

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concentration of 50 μM, oxidation time of 30 min, and coagulation time of 10 min. In addition,

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different concentrations of NaNO3/MgSO4/CaCl2 solutions (1, 10, 100, 500, and 1000 mM) were

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added as background ions.

The following variables were fixed for the factor of co-existing organic matter: FeSO4·7H2O

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concentration of 3.0 mM, Na2S2O8 concentration of 3.0 mM, initial pH of 7, coagulation pH of 10, initial Tl concentration of 50 μM, oxidation time of 30 min, and coagulation time of 10 min. In addition,

considered.

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different concentrations of organic matter (HA sodium salt) solutions (50, 100, and 500 μM) were also

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The following variables were fixed for the factor of initial Tl concentration: FeSO4·7H2O concentration of 3.0 mM, Na2S2O8 concentration of 3.0 mM, initial pH of 7, coagulation pH of 10,

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oxidation time of 30 min, and coagulation time of 10 min. There were six initial concentrations of Tl: 20, 40, 60, 80, 100, and 150 μM. The Tl removal efficiency (Er) is defined as follows:

E r (%) 

C0  C 100 C0

Eq. (1)

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where Er is the Tl removal efficiency, C0 is the initial concentration of Tl (mg/L), and C is the final concentration of Tl (mg/L). 2.3 Analytical methods The pH value was measured with a portable pH meter (PHB-3, Shanghai Sanxin, China). Metal

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concentrations were determined by an atomic absorption spectrometer (Thermo Scientific, USA). When the Tl concentration was lower than 0.500 mg/L, an inductively coupled plasma-mass spectrometer (ICP-MS, NexION 300, PerkinElmer Inc., USA) with a Tl detection limit of 0.01 µg/L

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was used to measure Tl concentrations. Samples with organic complexes were digested with aqua regia and then diluted in 1% (v/v) HNO3 prior to measurement. The ferric precipitates from the Fe2+-S2O82-

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reaction system were collected after adjusting the coagulation pH to 10 and dried at 50 °C for 48 h

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prior to characterization in order to gain more insights into the mechanism of Tl removal. Three ferric

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precipitate samples, attained with a control solution (no Tl) and Tl solutions of 10 mg/L and 200 mg/L

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were prepared. The characterization techniques included Scanning Electron Microscopy/Energy

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Dispersive X-Ray Spectroscopy (SEM-EDS; JSM-7001F JEOL, Japan), Fourier transform infrared (FT-IR) (Bruker, Germany) and X-ray photoelectron spectroscopy (XPS) (Kratos Axis Ultra, Japan).

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In addition, electron spin resonance (ESR) was used to measure the free radicals generated in the reaction system. The operating parameters of the ESR were set as follows: 9.7499 GHz microwave

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frequency, 22.78 mM power, 3465 G center magnetic field, and 100 G scan width. Scanning was conducted five times.

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3. Results and discussion 3.1 Effects of Fe2+ and S2O82- dosage Persulfate is a strong oxidant and can also be used as a source of sulfate radicals (Huie et al., 1991). Thus, its dosage has an important effect on the oxidability of the Fe2+-S2O82- system. As shown in Fig. 1a, when S2O82- was not added, the removal efficiency of Tl was 87%, indicating that the iron

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hydroxide provides good removal of Tl at the studied conditions. This is consistent with a previous report in which iron hydroxide has strong affinity to Tl under alkaline conditions (pH > 10) (Coup and Swedlund, 2015). With the increase in S2O82- dosage, the removal efficiency of Tl gradually increased. When the concentration of S2O82- increased further to 5 mM, the Tl removal efficiency did not improve; it remained constant at 96%. This phenomenon is similar to that described in a previous report, in

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which the increase in H2O2 dosage to a certain concentration led to improved Tl oxidation and removal, but a further increase in the dosage failed to show any improvement (Zhang et al., 2013). With an

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oxidation potential of 2.01 V, persulfate is a strong oxidant (Govindan et al., 2014). The potential of oxidation from Tl+ to Tl3+ is 1.28 V (Downs, 1993), implying that persulfate itself can oxidize Tl+ to

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Tl3+. Moreover, the Fe2+ in the Fe2+-S2O82- system can activate the persulfate to produce SO4-·, which

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has a higher redox potential, thus leading to a further improvement in oxidability (Neta et al., 2009).

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The Fe2+/S2O82- molar ratio has a strong influence on the oxidation performance of the Fe2+-S2O82-

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system. Moreover, the coagulation and adsorption ability of colloidal ferric hydroxide produced by the Fe2+ could further improve the Tl removal efficiency. Therefore, it is necessary to study the effect of

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the Fe2+/S2O82- molar ratio on the reaction system. As shown in Fig. 1b, the removal efficiency of Tl is very low when only the persulfate was added. However, when the Fe2+/S2O82- molar ratio was

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increased from 0 to 1, the removal efficiency improved significantly. There are two possible

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interpretations for this observation. The first one is that the increase in Fe2+ dosage led to increased generation of colloidal ferric hydroxide, which harbours the considerable coagulation and adsorption ability to remove Tl (Coup and Swedlund, 2015; Govindan et al., 2014). The other interpretation is

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that the increase in Fe2+ dosage resulted in enhanced decomposition of the persulfate, thus increasing the generation of SO4-· to improve Tl oxidation (Tsai et al., 2009).

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90

1.5 Effluent Tl concentration Tl removal percentage

1.2

80 70

0.9 60 0.6 50

0.3 0.0

40

-0.3

30 1

2

3

4

S2O82- concentration (mM)

10 Effluent Tl concentration Tl removal percentage

8

80

6

60

4

40

2

20

0

0

-0.4

5

100

0.0

0.4

0.8

1.2

1.6

2.0

Molar ratio of Fe2+/S2O82-

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0

120

Tl removal percentage (%)

1.8

Effluent Tl concentration (mg/L)

(b) 12

100

Tl removal percentage (%)

Effluent Tl concentration (mg/L)

(a) 2.1

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Fig. 1 Effects of (a) S2O82- dosage and (b) Fe2+/S2O82- molar ratio on the removal efficiency of Tl (Initial pH 7, coagulation pH 10, initial Tl concentration 50 μM, temperature 25.0 ± 0.4 °C). 3.2 Effect of pH

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The initial pH of the solution is an important parameter affecting the oxidability of the Fe2+-S2O82-

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system (Romero et al., 2010). As shown in Fig. 2a, the initial pH had little effect on Tl removal, which

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differs from the findings of a previous study that utilized the Fe2+-S2O82- system for organic matter

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degradation (Hu et al., 2017). In an acidic solution, the oxygen-oxygen bond of persulfate is

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asymmetrical, which is beneficial for the production of additional SO4-· to degrade organic matter (Liang et al., 2008; Xu and Li, 2010). Whereas theoretical calculation suggests that when in contact

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with strong oxidants (S2O82-, ·OH, and SO4-·), Tl(I) is prone to be oxidized and precipitated as Tl2O3 under alkaline conditions (Vink, 1993). Furthermore, the experiments on initial pH factor were

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conducted at high coagulation pH of 10, under which electrostatic attraction between the Tl ions and negatively charged iron colloids contribute to the effective Tl removal. This could explain the

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phenomenon that initial pH has little effect on Tl removal over a wide initial pH range when the coagulation pH is fixed at 10. The coagulation pH plays an important role in the adsorption ability of iron hydroxide colloids and the precipitation of Tl(III). As shown in Fig. 2b, the coagulation pH has a large influence on Tl removal; the Tl removal efficiency increased from 80% to about 96% when the pH value increased from 7 to

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12. Under alkaline conditions, the colloidal ferric hydroxide, which was produced from the iron salts, can absorb the Tl2O3 precipitates by flocculation, thereby effectively reducing the concentration of Tl in the solution. Previous studies have shown that the iron oxides begin to adsorb Tl+ when the solution pH is higher than 3 (Nriagu, 1998), and iron hydroxide also shows excellent adsorption performance when the pH exceeds 10 (Coup and Swedlund, 2015). This is because the negatively charged iron

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colloids produced in the system can adsorb positively charged Tl ions under alkaline conditions, thereby effectively reducing the concentration of Tl in the solution. This also explains why the addition

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of Fe2+ alone results in high Tl removal at a coagulation pH of 10; the ferrous hydroxide quickly turns into ferric hydroxide at alkaline conditions, and captures Tl via surface complexation and electrostatic

Effluent Tl concentration Tl removal percentage

90

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100

Effluent Tl concentration Tl removal percentage

85 0.3

80

0.0 -0.3

75

2

4

6

10

1.0

40

0.5

20

0

0.0

12

7

8

9

10

11

12

Coagulation pH

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Initial pH

8

80

60

1.5

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0.6

2.0

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0.9

Effluent Tl concentration (mg/L)

95

Tl removal percentage (%)

1.2

(b) 2.5

Tl removal percentage (%)

100

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Effluent Tl concentration (mg/L)

(a) 1.5

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attraction.

Fig. 2 Effects of (a) initial pH and (b) coagulation pH on the removal efficiency of Tl (Fe2+

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concentration of 3 mM, Fe2+/S2O82- molar ratio 1:1, initial Tl concentration 50 μM, temperature 25.0 ± 0.4 °C).

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3.3 Effect of co-existing compounds A variety of cations in wastewater could interfere with Tl removal from wastewater. The effects of coexisting Na+, Mg2+, and Ca2+ ions on Tl removal were also studied, as shown in Fig. 3a. The presence of Na+ and Mg2+ at low strength (<0.1 mol/L in both cases) had little impact on Tl removal. High concentration of Na+ and Mg2+ (>0.5 mol/L) began to inhibit Tl removal, due to the competition of 10

cations for binding sites. Notably, the removal of Tl was significantly reduced when the concentration of Ca2+ increased from 0.5 to 1.0 mol/L. This is because the sulfate and SO4-· tend to react with Ca2+ and produce insoluble calcium sulfate precipitates (Cihacek et al., 2015; Huangfu et al., 2017), thus leading to a lower Tl removal efficiency. Previous studies have shown that CaCl2, a neutral salt and the main component of electrolyte in soils, can be used as an effective extractant for acidic, neutral, or

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calcareous soils through the exchange of Ca2+ with the adsorbed heavy metal ions (Houba et al., 1991). Therefore, the existence of Ca2+ can significantly influence the removal of Tl in the Fe2+-S2O82- system.

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Real wastewater rarely contains a high Ca2+ concentration, and therefore, this negative effect inflicted by Ca2+ is unlikely to be a concern.

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Real wastewater often contains a high concentration of organic matter, which may affect the

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efficiency of the reaction system for removing Tl. As shown in Fig. 3b, as the concentration of organic

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HA increased from 50 μM to 500 μM, the Tl removal efficiency decreased from 96% to 78%. This

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reveals that the existence of organic matter has a negative effect on Tl removal. Huangfu et al. (2015) reported that the co-existing HA molecules can cause passivation of the adsorbent surface and compete

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for the binding sites with Tl, since they can be strongly adsorbed onto adsorption sites used to adsorb Tl through strong specific interactions (i.e., surface complexation) besides a few weakly bonded

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interactions (i.e., electrostatics interaction) (Guo et al., 2009). Second, the HAs dissolved in water tend

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to form Tl complexes with Tl(III) (Li et al., 2017b). Similarly, any type of Fe(III)–HA complex formed in the presence of HAs will probably reduce the yield of amorphous ferric hydroxide precipitates

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during the reaction, and consequently decrease Tl removal efficiency (Guo et al., 2009).

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(b) 5

120

Mg

2+

2+

Ca

80 60 40 20 0 0.001

0.01

0.1

0.5

Effluent Tl concentration (mg/L)

Tl removal percentage (%)

Na

100

4

100

3

80

2

60

1

40

0

1.0

Tl removal percentage (%)

Effluent Tl concentration Tl removal percentage

+

20 100

50

500

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(a) 120

Humic acid sodium salt (μmol/L)

Co-existing metal ions (mol/L)

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Fig. 3 Effects of (a) co-existing metal ions and (b) organic complexes on the removal efficiency of Tl (Fe2+ concentration of 3 mM, Fe2+/S2O82- molar ratio 1:1, initial Tl concentration 50 μM, initial pH 7, coagulation pH 10, temperature 25.0 ± 0.4 °C).

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3.4 Effect of initial concentration of Tl

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To evaluate the performance of this Fe2+-S2O82- system for different types of wastewater, the effect of

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different initial Tl concentrations on Tl removal was investigated. When the initial Tl concentration

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increased from 20 μM to 150 μM, the removal efficiency still reached 90%, indicating the high

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effectiveness of the Fe2+-S2O82- system (Fig. 4). The Tl removal efficiency decreased slightly with an increase in the Tl concentration. A possible explanation for this might be the fact that the higher the

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initial Tl concentration, the greater the final Tl concentration, provided the Fe2+ and S2O82- dosages and other conditions remain constant. Similar results have been reported by previous studies (Dang et

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al., 2009; Gupta and Bhattacharyya, 2006). The finding also shows that the coagulation and adsorption ability of the colloidal ferric hydroxide, coupled with the oxidability of persulfate, was slightly inhibited in a solution with a higher initial Tl concentration. Therefore, to achieve better removal

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efficiency, it may be necessary to increase the dosages of Fe2+ and S2O82-.

12

105

4

90

Effluent Tl concentration Tl removal percentage

75

3 60 2 45 1

30

0

15

Tl removal percentage (%)

0

-1 0

25

50 75 100 125 Initial Tl concentration (μmol/L)

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Effluent Tl concentration (mg/L)

5

150

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Fig. 4 Effect of initial Tl concentration on the removal efficiency of Tl (Fe2+ concentration of 3 mM, Fe2+/S2O82- molar ratio 1:1, initial pH 7, coagulation pH 10, temperature 25.0 ± 0.4 °C).

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3.5 Characterization

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The surface morphologies of the iron hydroxide colloids with and without Tl are shown in Fig. 5. The

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precipitates after reaction have irregular shapes and exhibit a large number of small particles on the

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surface. In addition, the results of the EDS analysis show that the concentration of Tl in the precipitates

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colloidal ferric hydroxide.

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increased significantly after reaction with Tl (Table 1), indicating the effective capture of Tl by

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Fig. 5 SEM images for the precipitates obtained with diverse initial Tl concentrations: (a) 0 mg/L

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of Tl concentration at magnification of 100000 and (b) of 20000; (c) 10 mg/L of Tl concentration at magnification of 100000 and (d) of 20000; (e) 100 mg/L of Tl concentration at magnification of

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100000 and (f) of 20000.

Table 1 The EDS analyse of the precipitates before and after reaction.

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Wt%

At%

Elements

0

10 mg/L

200 mg/L

0

10 mg/L

200 mg/L

C

12.01

12.98

14.14

20.28

26.07

30.22

N

2.55

1.94

0.64

3.69

3.34

1.17

O

49.58

32.16

33.21

62.87

48.50

53.27

14

S

0.57

0.50

0.22

0.36

0.38

0.18

Fe

35.21

49.45

25.91

12.79

21.36

11.91

Tl

0.08

2.97

25.88

0.01

0.35

3.25

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The FT-IR peaks at 3400 cm−1 and 1625 cm−1 were attributed to the stretching and bending vibrations of the surface hydroxyl group (Fig. 6), respectively (Rana et al., 2010). The asymmetric broad peak near 3150 cm-1 was assigned to the hydroxyl groups in the metal hydroxide (Zhang et al.,

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2018). The absorption peak at 1500 cm-1 is related to the H–O–H bending vibration (Ahmad et al., 2010b). As the Tl concentration increased, the intensity of the absorption peak near 1500 cm-1

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gradually increased as well, which may be related to the formation of more adsorbed water after the

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precipitation and adsorption of Tl oxide. The peak at 1340 cm-1 was mainly attributed to the flexural

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vibration of the hydroxyl group of the iron oxides (Zhang et al., 2007; Zhang et al., 2011). When the

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Tl concentration increased, the adsorption peak near 1340 cm-1 also gradually increased, which may be related to the presence of precipitates of thallic oxides. The electroneutral attraction of Tl ions by

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the negatively charged ferric hydroxide surface may lead to the capture of Tl by the ferric precipitates. The increase in the intensity of adsorption by iron hydroxide colloids may be due to the surface

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complexation with Tl. The more the amount of adsorbed Tl, the higher the observed peak. The

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adsorption peak near 1111 cm-1 was attributed to the bending vibration of hydroxyl groups on the iron oxide surface (Nakamoto and Nakamoto, 2008). The peak located at 615 cm-1 was probably related to

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the Fe–O bending vibration (Mousavi et al., 2018).

15

Before reaction Tl: 0 mg/L

1625

1111

615

1500 1340

Transmittance(%)

3400 3150

After reaction Tl: 10 mg/L

4000

3500

3000

2500 2000 1500 -1 Wavenumber (cm )

1000

500

IP T

After reaction Tl: 200 mg/L

SC R

Fig. 6 FT-IR spectra of the iron hydroxides before and after reaction (Fe2+ concentration of 3.0 mM, S2O82- concentration of 3.0 mM, initial Tl concentration of 10 and 200 mg/L).

U

The XPS spectra obtained are shown in Fig. 7. Fe 2p1/2 (724.5 eV) and Fe 2p3/2 (710.9 eV) were

N

the characteristic XPS peaks of Fe 2p (Fig. 7a). Moreover, the energy difference between the Fe 2p3/2

A

and Fe 2p1/2 spin orbital energy level was approximately 13.6 eV (Fig. 7a), which is consistent with

M

the findings of previous reports (Zhao et al., 2014). In addition to these two peaks, the occurrence of a satellite peak at about 718.3 eV can be attributed to the presence of Fe2+ (Xia et al., 2012). Fig. 7b

ED

shows the XPS broad spectra of the iron hydroxide precipitate. The precipitate contained the elements Fe, O, C, and Tl. Moreover, the peaks of Tl 5d and Tl 4f appeared after the reaction, indicating that Tl

PT

was effectively adsorbed onto the iron hydroxides. Fig. 7c shows the fitted spectra of Tl 4f fine spectra.

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Most of the Tl(I) was oxidized to Tl(III). Based on the calculated area of the spectra, Tl(III) and Tl(I) accounted for approximately 59% and 41% (Fig. 7c), respectively. Given that E(Tl3+/ Tl+) = +1.28 V (Shaw, 1952), Tl(III) is more chemically reactive than Tl(I), and therefore, the Tl(III) formed may be

A

unstable and tends to be slowly reduced to Tl(I) (Peter and Viraraghavan, 2005). Therefore, the proportion of Tl(III) formed after the reaction was likely higher than the outcomes revealed by the XPS analyses. Fig. 7d shows the O1s peak in the range of 528-534 eV. The O 1s spectra were composed of overlapped peaks of oxide oxygen (O2-), hydroxyl (OH-), and sorbed water (H2O). The O 1s peak at 529.48 eV is typical for oxygen bonded to transition metal (Fe-O) (Glisenti, 2000). The

16

second peak at 531.29 eV can be assigned to surface-adsorbed oxygen in the form of -OH group (Nesbitt and Banerjee, 1998). The additional band (533.2 eV) belongs most likely to adsorbed oxygen and is referred to as chemisorbed oxygen (Wu et al., 2005). After the capture of Tl(I), the O2− content and chemisorbed water species increased from 37% to 44.52% and 9.71% to 7%, respectively, while

Fe-OH groups and Tl ions might also play an important role in Tl uptake. (a)

(b)

Fe2p Scan

Before reaction Tl: 0 mg/L

After reaction Tl: 10 mg/L

Intensity

Intensity

Fe2p

3/2 O1s

C1s

1/2 Fe2p3/2

C1s

Fe2p

1/2 Fe2p

Tl4f

3/2 C1s

Tl4f

740

730 720 710 Binding Energy (eV)

700

690

1400

M

750

A

N

U

Before reaction (Tl: 0 mg/L) After reaction (Tl: 10 mg/L) After reaction (Tl: 200 mg/L)

After reaction Tl: 200 mg/L

Fe2p 1/2

SC R

Fe2p

IP T

the OH- content decreased from 45.77% to 56%, indicating that the surface complexation between the

Raw Fitted 4f5/2 Tl(I)

1000 800 600 400 Binding Energy (eV)

O1s

531.1 eV

4f7/2

4f7/2 Tl(I) 4f5/2 Tl(III)

56%

Intensity

4f5/2

4f7/2 Tl(III)

Tl(III)

Background

PT

Intensity

ED

(d)

(c)

1200

Tl5d

200

0

529.59 eV Before reaction (Tl: 0 mg/L)

37%

533.05 eV 7%

135

A

140

130 125 120 115 Binding Energy (eV)

O1s

531.29 eV 529.48 eV

Intensity

CC E

Tl(I)

110

105

533.22 eV

45.8%

44.5%

After reaction (Tl: 200 mg/L)

9.7%

534

532

530

528

526

Binding Energy (eV)

Fig. 7 XPS spectra of the iron precipitates obtained before and after reaction: (a) Fe 2p core level; (b) raw wide spectra and (c) Tl 4f core level after reaction; (d) O 1s core level (Fe2+ concentration of 3.0 mM, S2O82- concentration of 3.0 mM, initial Tl concentration of 10 and 200 mg/L).

17

To study the oxidation and coagulation mechanisms of the Fe2+-S2O82- system in more detail, the ESR measurements of the ·OH and SO4·- produced by the Fe2+-S2O82- system were carried out, as shown in Fig. 8. When Fe2+ was absent, characteristic peaks of the adducts formed by DMPO and the free radicals were negligible. This means that it is difficult to decompose persulfate into the sulfate radical without activators. After the addition of Fe2+ ions, clear signals of the adduct formed by DMPO

IP T

with free radicals were observed, because Fe2+ ions are effective activators that catalyze the generation of free radicals by peroxides (Zhang et al., 2017). Among them, four strong lines with an amplitude

SC R

ratio of 1:2:2:1 represent the formation of the DMPO-·OH adduct (Hebels et al., 2010). The six relatively weak lines suggest the formation of the DMPO- SO4·- adduct, which is activated by the multivalent iron (Zhang et al., 2017). This shows that the Fe2+-S2O82- system can produce two strong

U

oxidative radicals, ·OH and SO4-·, which is consistent with the literature (Zhang et al., 2017). With the

N

increase in the Fe2+ dosage from 0.772 g/L to 1.544 g/L, the intensity and amplitude of the peaks

A

associated with ·OH and SO4-· increased to some extent, indicating that more ·OH and SO4-· were

M

produced at higher Fe2+ dosage, which could well explain the improved Tl removal at higher Fe2+

ED

dosages, as demonstrated in Fig. 1. Under alkaline conditions (coagulation pH of 10), high concentrations of both ·OH and SO4-· can be produced, which is consistent with previous reports

PT

carried out by Chen et al. (2018); Jiang et al. (2018), leading to better Tl removal performance. When the reaction system included Tl ions, the amounts of ·OH and SO4-· appeared to increase significantly,

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indicating that Tl ions can enhance the production of ·OH and SO4-·. This phenomenon has not been previously reported in the literature. However, there were no identifiable signals of oxygenated radicals

A

in the iron-free Tl+-S2O82- system. Therefore, Tl itself is unlikely to effectively activate S2O82- to form ·OH and SO4-·. The possible interpretation is that Tl(I) can react with S2O82-, ·OH, and SO4-· during oxidation, leading to a slight higher concentration of Fe2+, which activates more ·OH and SO4-·. The interaction among Tl, Fe, and S2O82- is complex, the findings of this study may stimulate more research on the advanced oxidation process involving Tl.

18

+

Na2S2O8(5 mM) + Tl (10 mg/L)(coagulation pH 10) Na2S2O8(5 mM)

2+

2+

Na2S2O8(5 mM) + Fe (5 mM)

2+

Na2S2O8(5 mM)+Fe (5 mM)(coagulation pH 10)

IP T

Intensity (a.u.)

Na2S2O8(5 mM) + Fe (2.5 mM)

+

2+

Na2S2O8(5 mM)+Fe (5 mM) + Tl

3475

3500 Magnetic Field(G)

3525

3550

U

3450

SC R

(coagulation pH 10)

N

Fig. 8 The ESR spectra of the oxygenated free radicals captured by DMPO (O: ·OH， ：SO4-·).

A

To identify the types of radicals generated and the major reactive oxygenated radicals dominating

M

the degradation reaction, 1, 4-benzoquinone and TBA were added to the reaction solution as radical quenching agents. Radical quenching agents with and without α-hydrogen show different reactivity

ED

with radical species (Anipsitakis and Dionysiou, 2017; Li et al., 2013). 1, 4-benzoquinone (with αhydrogen) reacts with ·OH or SO4-·at comparable rates, while TBA (without α-hydrogen) has a 1000-

PT

fold higher rate constant with ·OH (3.8×108 to 7.6×108 L/(mol·s)) compared to SO4-·(4.0×105 to

CC E

9.1×105 L/(mol·s)) (Muller et al., 2015; Stemmler and Burrows, 2001). Therefore, 1, 4-benzoquinone was used to scavenge both radicals, and TBA was used to selectively quench ·OH (Qi et al., 2016). As shown in Fig. 9, the Tl removal efficiency was 96.6% when no quenching agent was added. In

A

the presence of 20 mM TBA, the Tl removal efficiency decreased only by 1.5% during the reaction, owing to the quenching of ·OH. However, when 1, 4-benzoquinone was added, the Tl removal was decreased by 23%. The much higher decrease in Tl removal by 1,4-benzoquinone than by TBA suggested that the primary radical species generated during the reaction was SO4-·, even though ·OH was also involved in this process. Furthermore, it reveals that SO4-· played a more important role in Tl 19

120

4

100

3

80

Effleunt Tl concentration Tl removal

2

60

1

40

0

IP T

5

Tl removal (%)

20

Blank

TBA

1,4-Benzoquinone

SC R

Effluent Tl concentration (mg/L)

removal than ·OH.

Scavenging agents

Fig. 9 Removal of Tl with or without scavengers of TBA or 1,4-benzoquinone (Fe2+:3 mM, S2O82-:

N

U

3 mM, TBA: 20 mM, 1,4-benzoquinone: 20 mM).

A

3.6 Treatment of real industrial wastewater

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The Fe2+-S2O82- system was used to treat real industrial wastewater generated by a zinc oxide production plant (Table 2). In addition to the presence of Tl (451 ± 86 µg/L), high concentrations of

ED

Na (1110 ± 89 mg/L), Cd (23.2 ± 1.7 mg/L), and Zn (171 ± 21 mg/L) were found in the wastewater. Effective removal of Tl by the Fe2+-S2O82- system was observed, and an increase in Fe2+ dosage

PT

resulted in an enhanced Tl removal, the lowest Tl concentration in the effluent being 1.6 ± 0.082 µg/L

CC E

at the highest Fe2+ dosage of 6 mM. The treatment by the Fe2+-S2O82- system at the dosages of 3 mM Fe2+ and 5 mM S2O82- enabled the Tl concentration in the effluent to comply with the local discharge limit (5 µg/L) for Tl (MEP, 2015). The total cost of the reagents used is estimated to be approximately

A

1.77 $/m3 (Table 3). Toxic Cd was also efficiently removed to a trace level of 12 ± 1.1 µg/L at the highest Fe2+ dosage. The concentrations of other metals (Ca and Zn) also reduced gradually with the increase in Fe2+ dosage. Thus, the Fe2+-S2O82- system effectively removed both Tl and other heavy metals, and showed excellent application potential for the treatment of real wastewater.

20

Table 2 The treatment of real wastewater by the Fe2+-S2O82- system with varied Fe2+/S2O82- molar ratio (Fe2+ dosage 3 mM, coagulation pH 10, reaction time 30 min, reaction temperature 298 K, S2O82-

K (mg/L)

Na (mg/L)

Cd (mg/L)

Ca (mg/L)

Tl (µg/L)

Zn (mg/L)

Raw

425±36

1110±89

23.2±1.7

335±24

451±86

171±21

Group 1

2520±95

6120±486

0.21±0.023

11±1.43

44±9.5

27.6±2.63

Group 2

2550±82

5360±397

0.14±0.017

11±0.95

25±3.2

26.4±1.96

Group 3

2570±71

5380±375

0.040±0.10

8.8±0.76

12±1.1

19.8±1.59

Group 4

2600±93

6180±508

0.022±0.0026

4.9±0.26

5.6±0.84

17.1±1.75

Group 5

2540±68

5300±434

0.018±0.0019

5.6±0.41

3.7±0.39

15.8±1.37

Group 6

2500±73

5420±412

0.012±0.0011

4.3±0.45

1.6±0.082

13.3±1.13

SC R

N

ED

M

A

IP T

Elements

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dosage for Group 1~ 6: 1, 2, 3, 4, 5, 6 mM).

Table 3 The reagents’ cost of the Fe2+-S2O82- system for the treatment of real wastewater from a zinc

Items

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oxide production plant.

FeSO4·7H2O

Na2S2O8

NaOH

73.3

923.9

879.9

Dosage (kg/m3)

0.834

1.19

0.69

Reagents’ cost ($/m3)

0.061

1.099

0.607

A

CC E

Price ($/t)

21

Total cost

1.77

4. Conclusions Efficient Tl removal (> 96%) can be achieved over a wide initial pH range of 2 to 11, indicating that the reaction system is applicable to both acid and alkali environments. In contrast, the coagulation pH has a significant influence on the removal of Tl and is required to be higher than 11 to achieve good Tl removal. Overall, the Tl removal efficiency can reach up to 96% when the S2O82- dosage is 3 mM,

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the Fe2+/S2O82- molar ratio is 1:1, and the coagulation pH is 10. The increase in the co-existing metal ions of Na+, Mg2+, and Ca2+ from 0.001 to 1 M can reduce the Tl removal by about 30%, 20%, and

SC R

80%, respectively. Tl removal is inhibited when the concentration of the co-existing HA sodium salt exceeds 100 μM. Tl is mainly removed by iron hydroxides via coagulation, surface complexation and electrostatic attraction under alkaline conditions. Tl(I) is oxidized to Tl(III) with S2O82-, ·OH, and

N

U

SO4-·to form Tl2O3 precipitates, which also contributes to the Tl removal. ESR analyses show that Tl can enhance the production of free oxygenated radicals, and the quenching experiments indicate that

Acknowledgements

ED

M

A

SO4-· has a higher impact on Tl removal than ·OH in the Fe2+-S2O82- system.

PT

The research was supported by the Foundation for Fostering the Scientific and Technical Innovation of Guangzhou University, the Guangzhou Education Bureau (1201630390), the Science and

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Technology Program of Guangzhou (201804010281), and the National Natural Science Foundation of

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China (51678562, 41673110, 41673138, U1612442).

Conflicts of interest: None.

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