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Remediation of hexavalent chromium spiked soil by using synthesized iron sulfide particles
Chemosphere 169 (2017) 131e138
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Remediation of hexavalent chromium spiked soil by using synthesized iron sulfide particles Yujie Li a, Wanyu Wang a, Liqiang Zhou b, Yuanyuan Liu a, *, Zakaria A. Mirza c, Xiang Lin d a
Key Laboratory of the Three Gorges Reservoir Region's Eco-environment of Ministry of Education, Chongqing University, Chongqing, 400044, PR China Chongqing Solid Waste Management Center, Chongqing, 401147, PR China c Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing, 400714, PR China d State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing, 400044, PR China b
h i g h l i g h t s CMC stabilized FeS particles are synthesized and characterized. The effects of remediation process factors are discussed. Synthesized FeS particles offer an effective immobilization of Cr(VI) in soil. FeS particles and FeSO4 are compared for Cr(VI) contaminated soil remediation. Synthesized FeS particles cause less pH change and more stable Cr fractions in soil.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 April 2016 Received in revised form 6 November 2016 Accepted 11 November 2016
Carboxymethyl cellulose (CMC) stabilized microscale iron sulfide (FeS) particles were synthesized and applied to remediate hexavalent chromium (Cr(VI)) spiked soil. The effects of parameters including dosage of FeS particles, soil moisture, and natural organic matter (NOM) in soil were investigated with comparison to iron sulfate (FeSO4). The results show that the stabilized FeS particles can reduce Cr(VI) and immobilize Cr in soil quickly and efficiently. The soil moisture ranging from 40% to 70% and NOM in soil had no significant effects on Cr(VI) remediation by FeS particles. When molar ratio of FeS to Cr(VI) was 1.5:1, about 98% of Cr(VI) in soil was reduced by FeS particles in 3 d and Cr(VI) concentration decreased from 1407 mg kg1 to 16 mg kg1. The total Cr and Cr(VI) in Toxicity Characteristic Leaching Procedure (TCLP) leachate were reduced by 98.4% and 99.4%, respectively. In FeS particles-treated soil, the exchangeable Cr fraction was mainly converted to Fe-Mn oxides bound fraction because of the precipitation of Cr(III)-Fe(III) hydroxides. The physiologically based extraction test (PBET) bioaccessibility of Cr was decreased from 58.67% to 6.98%. Compared to FeSO4, the high Cr(VI) removal and Cr immobilization efficiency makes prepared FeS particles a great potential in field application of Cr(VI) contaminated soil remediation. © 2016 Elsevier Ltd. All rights reserved.
Handling Editor: X. Cao Keywords: Hexavalent chromium/Cr(VI) FeS particles Soil remediation Immobilization
1. Introduction Chromium (Cr) is one of the most common heavy metals used in many industrial facilities, such as wood preservation, chrome plating and alloy formation (Fruchter, 2002). The uncontrolled treatment of Chromite ore processing residue and Cr-containing wastewater from chromium producing or consuming industries can lead to the Cr contamination of the surrounding soil (Farmer
* Corresponding author. E-mail address: [email protected] (Y. Liu). http://dx.doi.org/10.1016/j.chemosphere.2016.11.060 0045-6535/© 2016 Elsevier Ltd. All rights reserved.
et al., 2002; Saha and Orvig, 2010). The contamination of Cr causes the accumulation of chromium in plants and has captured a worldwide attention for its extensive scope (Fruchter, 2002; Barrera-Díaz et al., 2012). Via plants, it enters the food chain and presents potential threat to human's health. Cr is considered to be one of the top 20 contaminants on the Superfund priority list of hazardous substances for the past 15 years (Dhal et al., 2013). Hexavalent Cr(VI) and trivalent Cr(III) are the two oxidation states of Cr in natural environment, with distinct bioavailability and toxicity (Choppala et al., 2013). Cr(VI) is highly toxic, mutagenic and 2 teratogenic, and it primarily exists as HCrO 4 at acidic pH and CrO4
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at neutral and alkaline pH (Rai et al., 1989). These anions are hard to be adsorbed to soil colloids with negative charges. Cr(III) is known to have 100-fold lower toxicity than Cr(VI) due to its limited water solubility and mobility. In common environmental conditions (pH 6e9), Cr(III) is normally found in the form of Cr(OH)3, which is also of relatively low solubility (ksp ¼ 6.7 1031) and generally considered amorphous. It binds with ligands such as acetic and tartaric acid, and forms relatively stable complex with low available for plant uptake (Leita et al., 2011; Taghipour and Jalali, 2016). As Cr(III) is the species of Cr with low mobility and toxicity compared to other ones, reduction of Cr(VI) to Cr(III) is used as the main mechanism to remediate Cr(VI) contaminated soil. Chemical reduction is the most commonly used approach for the remediation of Cr(VI) contaminated sites, by which inorganic or organic electron donors reduce Cr(VI) to Cr(III), and insoluble Cr(III) hydroxides are formed. In general, there are three groups of reducing agents for the transformation of Cr(VI) to Cr(III) including organic compounds such as ascorbate, soil organic matter and composts (Bolan et al., 2003; Xu et al., 2004; Zhang et al., 2012; Scaglia et al., 2013), reduced sulfur compounds such as sodium sulfide (Na2S) (Lan et al., 2005) and calcium polysulfide (CaSx) (Chrysochoou et al., 2010; Chrysochoou and Johnston, 2015), and iron-based materials such as zero-valent iron nanoparticles (nZVI) (Xu and Zhao, 2007; Singh et al., 2011; Du et al., 2012; Wang et al., 2014a,b) and dissolved ferrous iron (Dermatas et al., 2006; Moon et al., 2009; Di Palma et al., 2015) and solids containing ferrous iron (Jung et al., 2007; Mullet et al., 2007; Liu et al., 2015). The sulfur compounds reduce Cr(VI) to Cr(III), which usually form Cr(III) hydroxides with lower mobility and toxicity (Zhou et al., 2012). Besides, the iron-based materials may efficiently reduce Cr(VI) to Cr(III) and form a passive layer of CrxFe1-x (OH)3 or CrxFe1-xOOH with ferric iron (Fe(III)) at the surface of solid products (Mullet et al., 2007; Di Palma et al., 2015). The Cr(III)-Fe(III) hydroxides are of extended stability in the pH range from 4.8 to 13.5, more stable than amorphous Cr(III) hydroxides (Papassiopi et al., 2014). nZVI is seen as an effective reductant for the immobilization of Cr(VI), however it is usually prepared by a relatively expensive method, borohydride reduction (Reyhanitabara et al., 2012), and it may have a harmful effect on microorganisms, animal cells, plant cells, and human cells (Stefaniuk et al., 2016). Iron sulfide minerals, such as pyrite (FeS2) (Mullet et al., 2007; Liu et al., 2015) and mackinawite (FeS1-x) (Boursiquot et al., 2002; Mullet et al., 2004), have been widely applied to reduce Cr(VI) in aqueous solution. FeS as a common iron-based material may act to immobilize Cr(VI) in environment with a relatively high reducing capacity. Patterson et al. (1997) indicated that amorphous FeS effectively removed Cr(VI) in solution at initial Cr(VI) concentrations ranging from 50 to 5000 mM at pH from 5 to 8 by the mechanism that Cr(VI) was reduced and precipitated by FeS following the reaction in Equation (1): 3CrO24 þ 2FeS (s) þ 9H2O / 4Cr0.75Fe0.25(OH)3 (s) þ Fe2þ þ S2O23 þ 6OH
(1)
Carboxymethyl cellulose (CMC) is a kind of anionic highpolymer cellulose and frequently applied as stabilizer in application suspensions, because numerous carboxyls and hydroxyls exist in the macromolecular chain of CMC, which can coordinate with multivalent cations, such as Fe3þ and Al3þ (Gillies et al., 2016; Ren et al., 2016). Compared to FeS minerals, the diminutive size and large specific surface area of FeS particles can contribute to their enhanced reactivity (Rivero-Hugue and Marshall, 2009). Compared to soluble reducing agents such as ferrous and sulfide, FeS particles may not only gradually release the ferrous and sulfide ions to reduce Cr(VI), but also sorb Cr(VI) through surface reaction (Han
et al., 2014). FeS has been frequently applied in Cr(VI) removal from aqueous solutions, however limited studies on the utilization of FeS particles in remediation of Cr(VI) contaminated soil were reported. The overall goal of this study was to examine the effectiveness of the remediation process using FeS particles with a micro-level size to immobilize Cr(VI) in soil and explore the effects of the remediation conditions. The specific objectives were as follows: (1) prepare and characterize CMC stabilized FeS particles in lab; (2) examine the effects of FeS particles dosage, moisture and NOM in soil on Cr(VI) removal; (3) better understand the remediation mechanisms of Cr (VI); and (4) assess the efficiency of FeS compared to FeSO4 in the remediation of Cr(VI) spiked soil. 2. Materials and methods 2.1. Materials All reagents except for CMC were of analytical or higher grade. The sodium form of CMC (M.W. ¼ 90,000) was chemical grade and purchased from Qiangshun Chemical (Shanghai, China). All solutions were prepared with deionized water (18.25 MU cm1). The Cr(VI) spiked soil was prepared by adding potassium dichromate to raw soil samples following the procedure of Wang et al. (2014a). The concentrations of Cr(VI) and total Cr in Cr(VI) spiked soil were 1407 mg kg1 and 2089 mg kg1, respectively. The raw soil samples were collected from a farm in Chongqing, China, SP (2913019.0800 N and 106150 52.8500 E, altitude 409 m). The farm is used for agriculture and no industrial use was involved in history. Before use, the soil samples were air-dried and sieved with a 2-mm standard mesh, and then characterized for physicochemical properties. The pH of raw soil was 6.75 and the cation exchange capacity was 18.8 mmol kg1. The raw soil contains 0.63% of organic carbon and was initially free of Cr(VI) and Hg. The contents of Cr, Pb and Cd in raw soil were 63 mg kg1, 71 mg kg1 and 5.3 mg kg1, respectively. The soil was silty loam with 22.2 wt % sand, 60.5 wt % silt and 17.3 wt % clay. 2.2. Synthesis and characterization of FeS particles FeS particles were prepared following the procedures adapted from the method of Xiong et al. (2009) and Gong et al. (2012). FeSO4 solution (20 mL, 0.852 M) was mixed with deoxygenated CMC solution (110 mL, 0.2%, w/w). The mixture was then purged with nitrogen gas (99.99% pure) for 30 min to complete the formation of Fe2þ-CMC complexes. Then Na2S solution (20 mL, 0.852 M) was titrated into the Fe2þ-CMC solution at a speed of 0.05 mL s1 to yield the FeS particles. The FeS particles-CMC suspension was centrifuged at 1000 rpm, and the supernatant was decanted and CMC solution (1%, w/w) was added to keep a constant volume. The mixture was sonicated for half an hour to restabilize the FeS particles with ultrasonic instrument (KQ-200KDE, Kunshan Ultrasonic Instrument, China). The FeS particle solids were collected by filtering the suspension using a 0.45 mm membrane filter and subsequently freeze-dried under vacuum at 50 C using a freeze dryer (MudolYOD-230, Thermo Fisher, American) for 48 h. The X-ray diffraction (XRD) patterns of FeS particles and CMC were obtained on a Shimadzu XRD-7000 device at a scan speed of 2.0 min1. The XRD patterns were processed using the computer program known as MDI Jade 6.5 loaded with ICDD database (Materials Data Inc., Livermore, CA, U.S.A.). The surface composition and morphology were investigated by scanning electron microscopy (SEM) and energy dispersive Xray (EDX) analysis at 15 kV (JSM-7800F, JEOL, Japan). The mean hydrodynamic diameter of FeS particles were determined by Nano
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Zetasizer ZS90 (ZEN 3690, Malvern Instruments, UK). The FeS particles suspension was diluted to a concentration of 100 mg L1 and sonicated for 30 min before analysis (Singh et al., 2011). 2.3. Kinetics of Cr(VI) reduction tests In kinetics tests of Cr(VI) reduction by FeS particles in soil, 12 mL and 36 mL of FeS particles suspension (molar ratio of FeS to Cr(VI) in soil: 0.5:1 and 1.5:1, respectively) was added to 100 g of Cr(VI) spiked soil. The Cr(VI) spiked soil and FeS particles were mixed by using an electronic blender with a speed of 500 r min1 for 10 min and then sealed and incubated in the dark at room temperature (20 ± 2 C). At predetermined times, the treated soils were sampled, air-dried and passed through a 2-mm mesh. The pH, Cr(VI) concentration in soil, and total Cr and Cr(VI) in TCLP leachate of FeS particles-treated soils were analyzed. The TCLP extraction fluid consisted of 0.1 M glacial acetic acid and 0.0643 M NaOH with a pH of 4.93. The extraction tests were carried out by mixing 2.5 g of an air-dried soil sample with 50 mL TCLP fluid #1 in a Teflon vial. The mixtures were rotated (30 rpm) for 18 h at 23 ± 2 C and centrifuged at 3000 r min1 for 10 min. Then, the supernatant was filtered through 0.45 mm filter membrane. The filtrates were preserved with HNO3 for total Cr and Cr(VI) analysis. Another test was conducted using a solution containing 1% CMC without FeS particles. All tests were conducted in duplicate. 2.4. Effect of remediation conditions: FeS particles dosage, moisture and NOM A series of batch tests were conducted to investigate the effects of remediation conditions: dosage of FeS particles, moisture and NOM in soil samples. The experimental procedures were similar to Section 2.3 except for the fixed treatment period of 7 d and the desired remediation conditions. Total Cr and Cr(VI) in TCLP leachate of treated soils were analyzed to evaluated the effects. In dosage effect experiments, molar ratios of FeS to Cr(VI) were 0.33:1, 0.5:1, 1:1, 1.5:1 and 2:1, respectively, with a fixed soil moisture of 50%. To assess the soil moisture effect, experiments were carried out at the water content levels of 30%, 40%, 50%, 60%, and 70% with a fixed molar ratio of FeS to Cr(VI) ¼ 1.5:1. In NOM effect experiments, humic acid was added in Cr(VI) spiked soil to the levels of 1%, 3%, and 5%, which was then aged for 7 d. FeS particles were subsequently added in the aged soil with a fixed molar ratio of FeS to Cr(VI) ¼ 1.5:1 and soil moisture of 50%. Control tests were conducted at 5% humic acid and without addition of FeS particles to soil. All experiments in this study were conducted in duplicate. 2.5. Comparative remediation tests by FeS particles and FeSO4 Parallel experiments were conducted to compare the effectiveness of FeS particles and FeSO4 solution in remediation of Cr(VI) spiked soil. The Cr(VI) spiked soil was treated by FeS particles at molar ratio of FeS to Cr(VI) 1.5:1 and by 0.4 M Fe2þ solution at molar ratios of Fe2þ to Cr(VI) 1.5:1 and 4.5:1. The following indicators for both FeS particles and FeSO4-treated soils were determined and analyzed: soil pH, Cr(VI) concentration in soil, total Cr and Cr(VI) in TCLP leachate, fractions and in vitro bioaccessibility of Cr. The fractions of Cr were quantified by sequential extraction procedures developed by Tessier et al. (1979). In vitro bioaccessibility of Cr was estimated by PBET method following the procedure of Yang et al. (2002) and Jardine et al. (2013). The same remediation procedures were used in Section 2.3 except for the treatment period of 7 d. All experiments in this study were conducted in duplicate.
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2.6. Analysis method and quality assurance Cr(VI) in soil was extracted following alkaline digestion procedure (EPA, 1996). Cr(VI) in filtrate was measured by diphenyl hydrazine spectrophotometry (Ministry of Environmental Protection of PR China, 1987). Total Cr in filtrate was analyzed by atomic absorption spectrometry (AA-7000, Shimadzu, Japan). Method blanks (solvent) and sample duplicates were routinely analyzed with treated soil samples. The correlation coefficients (R2) of the calibration curve (lower than 10 mg L1) for Cr(VI) analysis in solution were 0.9998. The method detection limit of Cr(VI) in soil was 2 mg kg1. 3. Results and discussions 3.1. Characterization of FeS particles Fig. 1 (a) shows the hydrodynamic diameter distribution of CMC stabilized FeS particles based on dynamic light scattering. The particles size ranged from 60 nm to 2000 nm with a mean size of 614 nm. Fig. 1 (b) shows the XRD diffractograms of the CMC stabilized FeS particles. The peak at 2q ¼ 20.8 was the typical diffraction peak of CMC, and two peaks at 2q ¼ 17.1 and 28.6 could be attributed to the diffraction of FeS crystal. The morphology of stabilized FeS particles was investigated by SEM and EDX analysis. The FeS particles present shuttle-like morphology with an average length of 400 nm and diameter of about 100 nm in the middle part (Fig. 1(c)). The diameter of FeS particles obtained from SEM was smaller than hydrodynamic diameter. Since SEM image excludes the CMC molecules attached on FeS particle surface while hydrodynamic diameter is based on the overall diffusivity of the CMC-anchored FeS particles (Xu, 1998; Xiong et al., 2009). Although the same materials and similar procedures for FeS particles synthesis were used, the shape of FeS particles prepared in our study was very different from the FeS nanoparticles which were microsphere prepared by Xiong et al. (2009) and Gong et al. (2012). The procedures of FeS particles synthesis had an effect on particle shape and crystal. EDX spectra (Fig. 1(d)) for the Spot 1 in Fig. 1(c) was composed of peaks of Fe, O, and S mainly as well as C and Na. O peaks could be due to facile oxidation of the particles or the adsorbed CMC which also induced the C peaks and Na peaks. 3.2. Kinetics of Cr(VI) reduction in soil Patterson et al. (1997) and Mullet et al. (2004) studied the kinetics of Cr(VI) reduction by FeS and indicated that Cr(VI) was completely removed from solution within several hours. Considering the effect of the soil media, a longer treatment period of 15 d was set, which is also practical for a remediation process. The Cr(VI) contents in soil may be an effective indicator to control the leachability of Cr(VI) and the Cr(VI)-caused ecotoxicity in soil. Fig. 2(a) illustrated the change of Cr(VI) concentration in FeS particles-treated soils when the molar ratio of FeS to Cr(VI) were 0.5:1 and 1.5:1 in 15 d. Little decrease of Cr(VI) concentration from 1407 mg kg1 to 1338 mg kg1 was observed in 1% CMC-treated soil, which was consistent with the findings by Wang et al. (2014a) that CMC had little effects on the reduction of Cr(VI) in soil treated by 0.1% CMC solution for 24 h. With the addition of FeS particles, Cr(VI) concentration decreased sharply in 3 d and then reached equilibrium when molar ratio of FeS to Cr(VI) were 0.5:1 and 1.5:1, respectively. Cr(VI) concentration in FeS particles-treated soil was reduced to 164 mg kg1 and 16 mg kg1 in 15 d for the both dosages of FeS particles. And the Cr(VI) removal capacities of FeS particles were
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(a)
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50 FeS Fe (SO ) 20.8
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Fig. 1. Characterization of CMC stabilized FeS particles. (a) Hydrodynamic diameter distribution. (b) XRD spectra. (c) SEM image. (d) EDX spectra.
estimated to be 1047.5 mg Cr(VI) per gram FeS and 390.7 mg Cr(VI) per gram FeS, respectively. Though the remediation Cr(VI) contaminated soil by nZVI is faster than FeS particles (Reyhanitabara et al., 2012; Wang et al., 2014a), the Cr(VI) removal capacity of FeS particles was apparently larger than that of nZVI (226.7 mg Cr(VI) per gram nZVI) at initial Cr(VI) in soil of 102 mg kg1 for 24 h reported by Wang et al. (2014a). TCLP has been widely used to evaluate the immobilization of heavy metals in soil to indicate remediation effectiveness (Xu and Zhao, 2007; Du et al., 2012; Wang et al., 2014b). As shown in Fig. 2(b), total Cr and Cr(VI) in TCLP leachates of Cr(VI) spiked soil were 65.91 mg L1 and 59.74 mg L1, respectively. When Cr(VI) spiked soil were treated by FeS particles at a molar ratio of FeS to Cr(VI) ¼ 0.5:1 in 3 d, total Cr and Cr(VI) in TCLP leachate decreased rapidly to 7.86 mg L1 and 6.83 mg L1 respectively. Total Cr and Cr(VI) in TCLP leachate were further reduced by only 0.19 mg L1 and 0.1 mg L1 when the treatment period was lengthened from 3 d to 15 d. When molar ratio of FeS to Cr(VI) was 1.5:1, total Cr and Cr(VI) in TCLP leachate deceased to 1.04 mg L1 and 0.34 mg L1 in 3 d, and total Cr and Cr(VI) in TCLP leachate reached equilibrium and remained constant levels of 0.81 mg L1 and 0.05 mg L1 from
3 d to 15 d, respectively. About 98% of Cr(VI) in soil was removed and 98.4% of total Cr was immobilized by using FeS particles at a molar ratio of FeS to Cr(VI) ¼ 1.5:1 in 3 d. While a month or a longer time is required to remediate Cr(VI) contaminated soil when using FeSO4 (Dermatas et al., 2006; Moon et al., 2009) and CaSx (Chrysochoou et al., 2010). 3.3. Effect of remediation conditions 3.3.1. Effect of FeS particles dosage The total Cr and Cr(VI) in TCLP leachate of FeS particles-treated soils with FeS particles dosage increase were investigated (Fig. SM1). When the molar ratio of FeS to Cr(VI) increased to 1.5:1, total Cr and Cr(VI) in TCLP leachate decreased to a level below 1 mg L1 and 0.5 mg L1 respectively. With the further increase, total Cr and Cr(VI) in the TCLP leachate did not continue to change apparently. Taking both the Cr(VI) immobilization and cost into consideration, a molar ratio of 1.5:1 was chosen as the optimal dosage for the remediation of Cr(VI) spiked soil. As a reaction product of FeS with Cr(VI), Cr(III) may precipitate as Cr(III) hydroxides or Cr(III)-Fe(III) hydroxides with good stability,
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Fig. 2. Cr(VI) concentration and Cr TCLP-leachability in soils treated by stabilized FeS particles. (a) Cr(VI) concentration in soils. (b) Total Cr and Cr(VI) in TCLP leachate of soils. A Cr(VI) in Cr(VI) spiked soil ¼ 1407 mg kg1, molar ratio of FeS to Cr(VI) ¼ 0.5:1 and 1.5:1. Data plotted as mean of duplicates and the error bars (calculated as standard deviation) indicate data reproducibility.
and become immobilized after sorption on soil colloids (Leita et al., 2011; Papassiopi et al., 2014). Besides, several sulfur intermediates are involved when Cr(VI) in soil is reduced by FeS. Patterson et al. (1997) mentioned that the products of FeS reacting with Cr(VI) are Fe(III), sulfite, sulfate, thiosulfate, and Cr(III). Thiosulfate is the dominant sulfur phase under neutral and low pH. Mullet et al. (2004, 2007) also found that elemental sulfur is the oxidation product of sulfide by Cr(VI). 3.3.2. Effect of soil moisture Soil moisture is one of the key parameters in the heavy metal contaminated soil remediation, which may influence the remediation efficiency and cause potential water pollution. The characteristics of soil and the remediation requirements may determine the selection of the soil moisture level. The variation of total Cr and Cr(VI) in TCLP leachate of FeS particles-treated soils with the increase of fixed soil moisture were investigated (Fig. SM-2). The soil pH fluctuated between 6.93 and 7.10 when soil moisture was increased from 30% to 70%. Total Cr and Cr(VI) in TCLP leachate were 3.8 mg L1 and 3.2 mg L1 at fixed soil moisture level of 30%, and decreased sharply to 0.83 mg L1 and 0.37 mg L1 at soil moisture 40%. At soil moisture 50%, total Cr in TCLP leachate was as low as 0.78 mg L1 and Cr(VI) was below the detection limit of 0.004 mg L1. Within the soil moisture range from 40% to 70%, total Cr and Cr(VI) in TCLP leachate were consistently lower than 1 mg L1 and 0.5 mg L1, respectively. When below 40%, the soil moisture had a negative influence on Cr(VI) immobilization, due to the inhibition to the contact of FeS particles and Cr(VI) attached on soil. Nevertheless, the soil moisture above 40% showed little effect on Cr(VI) immobilization, which could be attributed to the enough diffusion of FeS particles for the contact between FeS and Cr(VI) in soil and the characteristics of high silt and clay content in the tested soil. 3.3.3. Effect of NOM NOM in soil generally lies between 0.5% and 5%, and was 0.63% in the tested soil. Humic acid, a typical NOM, was selected to investigate the effect of NOM on the remediation of Cr(VI) spiked soil by using FeS particle (Zhang et al., 2012). In control tests, total
Cr and Cr(VI) in TCLP leachate were 54.9 mg L1 and 50.4 mg L1 at 5% humic acid without the addition of FeS particles, respectively (Fig. SM-3). Within a humic acid content range from 1% to 5%, total Cr and Cr(VI) in TCLP leachate stayed at the levels lower than 1 mg L1 and 0.5 mg L1 at a molar ratio of FeS to Cr(VI) ¼ 1.5:1, respectively. Humic acid had no significant effect on Cr(VI) immobilization by using FeS particles. Scaglia et al. (2013) reported that the rates of Cr(VI) reduction by humic acid were strongly pHdependent and more than 60% Cr(VI) was reduced by using humic acid at pH ¼ 2.5, while no Cr(VI) reduction was observed at pH ¼ 6. In this study, soil pH did not change much and fluctuated around 6 after adding humic acid, which may not influence the effect of humic acid on Cr(VI) reduction by FeS particles. The results were consistent with the findings of no apparent effect of humic acid on the Cr(VI) removal by pyrite (Liu et al., 2015). 3.4. Effectiveness: efficiency, Cr fraction and in vitro bioaccessibility 3.4.1. Efficiency of FeS particles and FeSO4 Fe2þ has been previously used as a chemical reducing agent to reduce Cr(VI) (Dermatas et al., 2006; Moon et al., 2009; Di Palma et al., 2015). The reductive reactions are listed as follows (Mullet et al., 2004; Pan et al., 2014): 2þ þ 8Hþ / Cr3þ þ 3Fe3þ þ 4H2O CrO24 þ 3Fe
(2)
xCr3þ þ (1-x)Fe3þ þ 3H2O / (CrxFe1-x)(OH)3 (s) þ 3Hþ (x < 1) (3) FeS particles was compared to FeSO4 as the reducing agent in terms of the Cr(VI) removal efficiency under similar experimental conditions. Fig. 3 illustrate Cr(VI) concentration in treated soils and total Cr and Cr(VI) in TCLP leachate when various dosages of FeS particles and FeSO4 were used to remediated Cr(VI) soils. Cr(VI) in FeSO4-treated soil was reduced to 923 mg kg1, about 34.4% of Cr(VI) in Cr(VI) spiked soil, and total Cr and Cr(VI) in TCLP leachate decreased by about 50% at molar ratio of FeSO4 to Cr(VI) ¼ 1.5:1. When the molar ratio of FeSO4 to Cr(VI) was raised to 4.5:1, Cr(VI) concentration in treated soil, total Cr and Cr(VI) in TCLP leachate decreased to 18.2 mg kg1, 0.84 mg L1 and 0.05 mg L1, almost
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Fig. 3. Cr(VI) concentration and total Cr and Cr(VI) in TCLP leachate in soils treated by stabilized FeS particles and FeSO4.A Cr(VI) in Cr(VI) spiked soil ¼ 1407 mg kg1, molar ratio of FeS to Cr(VI) ¼ 1.5:1, molar ratios of FeSO4 to Cr(VI) ¼ 1.5:1 and 4.5:1, treatment period ¼ 7 d. Data plotted as mean of duplicates and the error bars (calculated as standard deviation) indicate data reproducibility.
equal to the results of FeS particles (16 mg kg1, 0.81 mg L1 and 0.05 mg L1) at molar ratio of FeS to Cr(VI) ¼ 1.5:1. Although FeSO4 is a widely applied reducing agent for Cr(VI) reduction, soils treated by FeSO4 was acidified with an apparent pH decrease from 6.75 to 3.05 at M ratio of FeSO4 to Cr(VI) ¼ 4.5:1 (Fig. SM-4). The pH decrease was attributed to protons production in the formation of Cr(III)-Fe(III) hydroxides, which causes soil acidification and maintains the system at low pH (Singh et al., 2011; Pan et al., 2014). During the remediation, the pH of soil treated by FeS particles fluctuated slightly between from 6.76 to 7.13 at a molar ratio of FeS to Cr(VI) ¼ 1.5:1. Comparing to FeSO4, the addition of FeS particles not only had a higher Cr(VI) removal efficiency, but also did not change pH of the soil apparently. 3.4.2. Soil bound Cr SEM elemental mapping analysis was used to reveal the spatial associations between FeS and Cr on the surface of FeS particlestreated soil as shown in Fig. 4. The distribution of Cr and Fe, S on
treated soil surface shows the combination of Cr and Fe, S after the addition of FeS particles. Especially the binding between Cr (labeled in blue) and S (labeled in green) indicated the sorption of Cr by FeS particles and attachment on the treated soil. The sequential extraction procedures have been often applied to identify the relative availability and to reveal the immobilization of metals in the solid phase (Jung et al., 2007; Wang et al., 2014b; Di Palma et al., 2015). According to the method by Tessier et al. (1979), the five Cr fractions are defined as exchangeable (EX), carbonate bound (CB), Fe-Mn oxides bound (OX), organic bound (OM), and residual phases (RS). The relative availability of a metal follows the sequence: EX > CB > OX > OM > RS, and OX, OM and RS are relatively stable fractions. Cr fraction distribution of the Cr(VI) spiked soil and soils treated by FeS particles and FeSO4 was shown in Fig. SM-5. EX was the most predominant Cr fraction, occupying 74.8% in the Cr(VI) spiked soil, and the other one were CB (7.3%), OX (6.8%), OM (1.9%), and RS (16.3%), respectively. The distribution of Cr fractions in the
Fig. 4. SEM image and Fe, S and Cr mapping in SEM image of stabilized FeS particles-treated soil. (a) SEM image of stabilized FeS particles-treated soil. (b) Fe, S and Cr mapping of the scanned part of FeS particles-treated soil. A Cr(VI) in Cr(VI) spiked soil ¼ 1407 mg kg1, molar ratio of FeS to Cr(VI) ¼ 1.5:1, treatment period ¼ 7 d.
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soil was notably changed after the addition of FeS particles and FeSO4. In FeS particles-treated soil, the fraction of EX decreased to 0.3%, and the fractions of OX and OM increased by 54% and 17%, respectively. The major Cr fractions in FeS particles-treated soils were OX (60.3%), OM (19%) and RS (17%). The distinctive alteration of Cr fractions, especially the substantial increase in OX and OM fractions, accounted for the reduced relative availability of Cr (Jung et al., 2007). The OX fraction increase might be largely attributed to the precipitation of Cr(III)-Fe(III) hydroxides during the FeS particles treatment (Leita et al., 2011; Papassiopi et al., 2014). In FeSO4-treated soil, the major Cr fractions were OX (52.3%) and RS (30.9%), but the EX and CB fractions still accounted for 12.9%, apparently higher than that in FeS particles-treated soils (3.7%). The more fractions of stable species of Cr in soils treated by FeS particles may contribute to the better immobilization of Cr(VI) than by FeSO4. 3.4.3. In vitro bioaccessibility Compared to TCLP, the PBET method uses an extreme condition, a more aggressive extracting agent (pH 1.5) and a higher liquid-tosoil ratio (100:1), to simulate the gastrointestinal environment (Yang et al., 2002; Jardine et al., 2013). It was designed to assess the in vitro bioaccessibility of Cr in FeS particles-treated soil for humans. The bioaccessibility of Cr in FeS particles and FeSO4 -treated soils for 7 d was investigated. Under the PBET conditions, the bioaccessibility of Cr in Cr(VI) spiked soil was 58.67%, and that in 1% CMC treated soil was 53.45% (Fig. SM-6). The bioaccessibility of Cr in FeS particles-treated soil decreased to 6.98% at molar ratio of FeS to Cr(VI) ¼ 1.5:1, which was apparently lower than that of soil treated by FeSO4 (18.05%) at a higher molar ratio of FeSO4 to Cr(VI) ¼ 4.5:1. Although total Cr and Cr(VI) in TCLP leachate of treated by FeS particles (molar ratio of FeS to Cr(VI) ¼ 1.5:1) and FeSO4 were comparable, a much lower bioaccessibility of Cr in soils indicated the higher efficiency of FeS particles in remediation of Cr(VI) spiked soil. 4. Conclusions CMC stabilized FeS particle suspension with a concentration as high as 10000 mg L1 was successfully prepared. The freshly prepared FeS particles showed shuttle-like morphology with an average length of 400 nm and diameter of about 100 nm. Stabilized FeS particles may effectively remediate Cr(VI) spiked soil. About 98% of Cr(VI) in soil was reduced and immobilized by FeS particles in 3 d at a molar ratio of FeS to Cr(VI) ¼ 1.5:1. The soil moisture ranging from 40% to 70% and NOM in soil had no significant effects on Cr(VI) remediation by FeS partices. The exchangeable Cr fraction was converted to Fe-Mn oxides bound and organic bound fractions, and PBET-based bioaccessibility of Cr was decreased from 58.67% to 6.98%. FeS particles with a dosage of FeS to Cr(VI) molar ratio ¼ 1.5:1 had a high Cr(VI) removal and Cr immobilization efficiency compared to FeSO4 at a molar ratio of FeSO4 to Cr(VI) ¼ 4.5:1, meanwhile the pH of treated soil fluctuated around 7. The PBET-based bioaccessibility of Cr in FeS particlestreated soil was apparently lower than that in FeSO4-treated soil. The prepared FeS particles hold a great remediation effectiveness in Cr(VI) spiked soil with higher Cr(VI) removal and Cr immobilization efficiency, a lower PBET-based bioaccessibility of Cr, and a slighter pH influence. This study provides an effective technology for the remediation of Cr(VI) contaminated soil, which can efficiently reduce Cr(VI) and immobilize Cr in soil without changing the properties of raw soil apparently. FeS particles shows a promising potential in field application in the remediation of Cr(VI) contaminated soil.
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Acknowledgments This research was supported by the Fundamental Research Funds for the Central Universities, Project No. CDJZR12210007 and Chongqing Natural Science Foundation, Project No. cstc2014jcyjA20016. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.chemosphere.2016.11.060. References Barrera-Díaz, C.E., Lugo-Lugo, V., Bilyeu, B., 2012. A review of chemical, electrochemical and biological methods for aqueous Cr(VI) reduction. J. Hazard. Mater 223e224, 1e12. Bolan, N.S., Adriano, D.C., Natesan, R., Koo, B.J., 2003. Effect of organic amendments on the reduction and phytoavailability of chromate in mineral soil. J. Environ. Qual. 32, 120e128. Boursiquot, S., Mullet, M., Ehrhardt, J.J., 2002. XPS study of the reaction of chromium (VI) with mackinawite (FeS). Surf. Interface Anal. 34, 293e297. Choppala, G., Bolan, N., Seshadri, B., 2013. Chemodynamics of chromium reduction in soils: implications to bioavailability. J. Hazard. Mater 261, 718e724. Chrysochoou, M., Ferreira, D.R., Johnston, C.P., 2010. Calcium polysulfide treatment of Cr(VI)-contaminated soil. J. Hazard. Mater 179, 650e657. Chrysochoou, M., Johnston, C.P., 2015. Polysulfide speciation and reactivity in chromate-contaminated soil. J. Hazard. Mater 281, 87e94. Dermatas, D., Chrysochoou, M., Moon, D.H., Grubb, D.G., Wazne, M., Christodoulatos, C., 2006. Ettringite-induced heave in chromite ore processing residue (COPR) upon ferrous sulfate treatment. Environ. Sci. Technol. 40, 5786e5792. Dhal, B., Thatoi, H.N., Das, N.N., Pandey, B.D., 2013. Chemical and microbial remediation of hexavalent chromium from contaminated soil and mining/metallurgical solid waste: a review. J. Hazard. Mater 250e251, 272e291. Di Palma, L., Gueye, M.T., Petrucci, E., 2015. Hexavalent chromium reduction in contaminated soil: a comparison between ferrous sulphate and nanoscale zerovalent iron. J. Hazard. Mater 281, 70e76. Du, J., Lu, J., Wu, Q., Jing, C., 2012. Reduction and immobilization of chromate in chromite ore processing residue with nanoscale zero-valent iron. J. Hazard. Mater 215e216, 152e158. EPA, 1996. Method 3060A: Alkaline Digestion for Hexavalent Chromium. Farmer, J.G., Thomas, R.P., Graham, M.C., Geelhoed, J.S., Lumsdon, D.G., Paterson, E., 2002. Chromium speciation and fractionation in ground and surface waters in the vicinity of chromite ore processing residue disposal sites. J. Environ. Monit. 4, 235e243. Fruchter, J., 2002. In situ treatment of chromium-contaminated groundwater. Environ. Sci. Technol. 36, 464e472. Gillies, G., Mackenzie, K., Kopinke, F.D., Georgi, A., 2016. Fluorescence labelling as tool for zeolite particle tracking in nanoremediation approaches. Sci. Total Environ. 550, 820e826. Gong, Y., Liu, Y., Xiong, Z., Kaback, D., Zhao, D., 2012. Immobilization of mercury in field soil and sediment using carboxymethyl cellulose stabilized iron sulfide nanoparticles. Nanotechnology 23, 294007. Han, D.S., Orillano, M., Khodary, A., Duan, Y., Batchelor, B., Abdel-Wahab, A., 2014. Reactive iron sulfide (FeS)-supported ultrafiltration for removal of mercury (Hg(II)) from water. Water Res. 53, 310e321. Jardine, P.M., Stewart, M.A., Barnett, M.O., Basta, N.T., Brooks, S.C., 2013. Influence of soil geochemical and physical properties on chromium(VI) sorption and bioaccessibility. Environ. Sci. Technol. 47, 11241e11248. Jung, Y., Choi, J., Lee, W., 2007. Spectroscopic investigation of magnetite surface for the reduction of hexavalent chromium. Chemosphere 68, 1968e1975. Lan, Y., Deng, B., Kim, C., Thornton, E.C., Xu, H., 2005. Catalysis of elemental sulfur nanoparticles on chromium (VI) reduction by sulfide under anaerobic conditions. Environ. Sci. Technol. 39, 2087e2094. Leita, L., Margon, A., Sinicco, T., Mondini, C., 2011. Glucose promotes the reduction of hexavalent chromium in soil. Geoderma 164, 122e127. Liu, Y., Mou, H., Chen, L., Mirza, Z.A., Liu, L., 2015. Cr(VI)-contaminated groundwater remediation with simulated permeable reactive barrier (PRB) filled with natural pyrite as reactive material: environmental factors and effectiveness. J. Hazard. Mater 298, 83e90. Ministry of Environmental Protection of the People's Republic of China, 1987. Water Quality-Determination of Chromium(VI) -1, 5 Diphenylcarbohydrazide Spectrophotometric Method. GB 7464e1987. Moon, D.H., Wazne, M., Koutsospyros, A., Christodoulatos, C., Gevgilili, H., Malik, M., Kalyon, D.M., 2009. Evaluation of the treatment of chromite ore processing residue by ferrous sulfate and asphalt. J. Hazard. Mater 166, 27e32. Mullet, M., Boursiquot, S., Ehrhardt, J.J., 2004. Removal of hexavalent chromium from solutions by mackinawite tetragonal FeS. Colloids Surf. A Physicochem. Eng. Asp. 244, 77e85.
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Remediation of hexavalent chromium spiked soil by using synthesized iron sulfide particles Yujie Li a, Wanyu Wang a, Liqiang Zhou b, Yuanyuan Liu a, *, Zakaria A. Mirza c, Xiang Lin d a
Key Laboratory of the Three Gorges Reservoir Region's Eco-environment of Ministry of Education, Chongqing University, Chongqing, 400044, PR China Chongqing Solid Waste Management Center, Chongqing, 401147, PR China c Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing, 400714, PR China d State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing, 400044, PR China b
h i g h l i g h t s CMC stabilized FeS particles are synthesized and characterized. The effects of remediation process factors are discussed. Synthesized FeS particles offer an effective immobilization of Cr(VI) in soil. FeS particles and FeSO4 are compared for Cr(VI) contaminated soil remediation. Synthesized FeS particles cause less pH change and more stable Cr fractions in soil.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 April 2016 Received in revised form 6 November 2016 Accepted 11 November 2016
Carboxymethyl cellulose (CMC) stabilized microscale iron sulfide (FeS) particles were synthesized and applied to remediate hexavalent chromium (Cr(VI)) spiked soil. The effects of parameters including dosage of FeS particles, soil moisture, and natural organic matter (NOM) in soil were investigated with comparison to iron sulfate (FeSO4). The results show that the stabilized FeS particles can reduce Cr(VI) and immobilize Cr in soil quickly and efficiently. The soil moisture ranging from 40% to 70% and NOM in soil had no significant effects on Cr(VI) remediation by FeS particles. When molar ratio of FeS to Cr(VI) was 1.5:1, about 98% of Cr(VI) in soil was reduced by FeS particles in 3 d and Cr(VI) concentration decreased from 1407 mg kg1 to 16 mg kg1. The total Cr and Cr(VI) in Toxicity Characteristic Leaching Procedure (TCLP) leachate were reduced by 98.4% and 99.4%, respectively. In FeS particles-treated soil, the exchangeable Cr fraction was mainly converted to Fe-Mn oxides bound fraction because of the precipitation of Cr(III)-Fe(III) hydroxides. The physiologically based extraction test (PBET) bioaccessibility of Cr was decreased from 58.67% to 6.98%. Compared to FeSO4, the high Cr(VI) removal and Cr immobilization efficiency makes prepared FeS particles a great potential in field application of Cr(VI) contaminated soil remediation. © 2016 Elsevier Ltd. All rights reserved.
Handling Editor: X. Cao Keywords: Hexavalent chromium/Cr(VI) FeS particles Soil remediation Immobilization
1. Introduction Chromium (Cr) is one of the most common heavy metals used in many industrial facilities, such as wood preservation, chrome plating and alloy formation (Fruchter, 2002). The uncontrolled treatment of Chromite ore processing residue and Cr-containing wastewater from chromium producing or consuming industries can lead to the Cr contamination of the surrounding soil (Farmer
* Corresponding author. E-mail address: [email protected] (Y. Liu). http://dx.doi.org/10.1016/j.chemosphere.2016.11.060 0045-6535/© 2016 Elsevier Ltd. All rights reserved.
et al., 2002; Saha and Orvig, 2010). The contamination of Cr causes the accumulation of chromium in plants and has captured a worldwide attention for its extensive scope (Fruchter, 2002; Barrera-Díaz et al., 2012). Via plants, it enters the food chain and presents potential threat to human's health. Cr is considered to be one of the top 20 contaminants on the Superfund priority list of hazardous substances for the past 15 years (Dhal et al., 2013). Hexavalent Cr(VI) and trivalent Cr(III) are the two oxidation states of Cr in natural environment, with distinct bioavailability and toxicity (Choppala et al., 2013). Cr(VI) is highly toxic, mutagenic and 2 teratogenic, and it primarily exists as HCrO 4 at acidic pH and CrO4
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at neutral and alkaline pH (Rai et al., 1989). These anions are hard to be adsorbed to soil colloids with negative charges. Cr(III) is known to have 100-fold lower toxicity than Cr(VI) due to its limited water solubility and mobility. In common environmental conditions (pH 6e9), Cr(III) is normally found in the form of Cr(OH)3, which is also of relatively low solubility (ksp ¼ 6.7 1031) and generally considered amorphous. It binds with ligands such as acetic and tartaric acid, and forms relatively stable complex with low available for plant uptake (Leita et al., 2011; Taghipour and Jalali, 2016). As Cr(III) is the species of Cr with low mobility and toxicity compared to other ones, reduction of Cr(VI) to Cr(III) is used as the main mechanism to remediate Cr(VI) contaminated soil. Chemical reduction is the most commonly used approach for the remediation of Cr(VI) contaminated sites, by which inorganic or organic electron donors reduce Cr(VI) to Cr(III), and insoluble Cr(III) hydroxides are formed. In general, there are three groups of reducing agents for the transformation of Cr(VI) to Cr(III) including organic compounds such as ascorbate, soil organic matter and composts (Bolan et al., 2003; Xu et al., 2004; Zhang et al., 2012; Scaglia et al., 2013), reduced sulfur compounds such as sodium sulfide (Na2S) (Lan et al., 2005) and calcium polysulfide (CaSx) (Chrysochoou et al., 2010; Chrysochoou and Johnston, 2015), and iron-based materials such as zero-valent iron nanoparticles (nZVI) (Xu and Zhao, 2007; Singh et al., 2011; Du et al., 2012; Wang et al., 2014a,b) and dissolved ferrous iron (Dermatas et al., 2006; Moon et al., 2009; Di Palma et al., 2015) and solids containing ferrous iron (Jung et al., 2007; Mullet et al., 2007; Liu et al., 2015). The sulfur compounds reduce Cr(VI) to Cr(III), which usually form Cr(III) hydroxides with lower mobility and toxicity (Zhou et al., 2012). Besides, the iron-based materials may efficiently reduce Cr(VI) to Cr(III) and form a passive layer of CrxFe1-x (OH)3 or CrxFe1-xOOH with ferric iron (Fe(III)) at the surface of solid products (Mullet et al., 2007; Di Palma et al., 2015). The Cr(III)-Fe(III) hydroxides are of extended stability in the pH range from 4.8 to 13.5, more stable than amorphous Cr(III) hydroxides (Papassiopi et al., 2014). nZVI is seen as an effective reductant for the immobilization of Cr(VI), however it is usually prepared by a relatively expensive method, borohydride reduction (Reyhanitabara et al., 2012), and it may have a harmful effect on microorganisms, animal cells, plant cells, and human cells (Stefaniuk et al., 2016). Iron sulfide minerals, such as pyrite (FeS2) (Mullet et al., 2007; Liu et al., 2015) and mackinawite (FeS1-x) (Boursiquot et al., 2002; Mullet et al., 2004), have been widely applied to reduce Cr(VI) in aqueous solution. FeS as a common iron-based material may act to immobilize Cr(VI) in environment with a relatively high reducing capacity. Patterson et al. (1997) indicated that amorphous FeS effectively removed Cr(VI) in solution at initial Cr(VI) concentrations ranging from 50 to 5000 mM at pH from 5 to 8 by the mechanism that Cr(VI) was reduced and precipitated by FeS following the reaction in Equation (1): 3CrO24 þ 2FeS (s) þ 9H2O / 4Cr0.75Fe0.25(OH)3 (s) þ Fe2þ þ S2O23 þ 6OH
(1)
Carboxymethyl cellulose (CMC) is a kind of anionic highpolymer cellulose and frequently applied as stabilizer in application suspensions, because numerous carboxyls and hydroxyls exist in the macromolecular chain of CMC, which can coordinate with multivalent cations, such as Fe3þ and Al3þ (Gillies et al., 2016; Ren et al., 2016). Compared to FeS minerals, the diminutive size and large specific surface area of FeS particles can contribute to their enhanced reactivity (Rivero-Hugue and Marshall, 2009). Compared to soluble reducing agents such as ferrous and sulfide, FeS particles may not only gradually release the ferrous and sulfide ions to reduce Cr(VI), but also sorb Cr(VI) through surface reaction (Han
et al., 2014). FeS has been frequently applied in Cr(VI) removal from aqueous solutions, however limited studies on the utilization of FeS particles in remediation of Cr(VI) contaminated soil were reported. The overall goal of this study was to examine the effectiveness of the remediation process using FeS particles with a micro-level size to immobilize Cr(VI) in soil and explore the effects of the remediation conditions. The specific objectives were as follows: (1) prepare and characterize CMC stabilized FeS particles in lab; (2) examine the effects of FeS particles dosage, moisture and NOM in soil on Cr(VI) removal; (3) better understand the remediation mechanisms of Cr (VI); and (4) assess the efficiency of FeS compared to FeSO4 in the remediation of Cr(VI) spiked soil. 2. Materials and methods 2.1. Materials All reagents except for CMC were of analytical or higher grade. The sodium form of CMC (M.W. ¼ 90,000) was chemical grade and purchased from Qiangshun Chemical (Shanghai, China). All solutions were prepared with deionized water (18.25 MU cm1). The Cr(VI) spiked soil was prepared by adding potassium dichromate to raw soil samples following the procedure of Wang et al. (2014a). The concentrations of Cr(VI) and total Cr in Cr(VI) spiked soil were 1407 mg kg1 and 2089 mg kg1, respectively. The raw soil samples were collected from a farm in Chongqing, China, SP (2913019.0800 N and 106150 52.8500 E, altitude 409 m). The farm is used for agriculture and no industrial use was involved in history. Before use, the soil samples were air-dried and sieved with a 2-mm standard mesh, and then characterized for physicochemical properties. The pH of raw soil was 6.75 and the cation exchange capacity was 18.8 mmol kg1. The raw soil contains 0.63% of organic carbon and was initially free of Cr(VI) and Hg. The contents of Cr, Pb and Cd in raw soil were 63 mg kg1, 71 mg kg1 and 5.3 mg kg1, respectively. The soil was silty loam with 22.2 wt % sand, 60.5 wt % silt and 17.3 wt % clay. 2.2. Synthesis and characterization of FeS particles FeS particles were prepared following the procedures adapted from the method of Xiong et al. (2009) and Gong et al. (2012). FeSO4 solution (20 mL, 0.852 M) was mixed with deoxygenated CMC solution (110 mL, 0.2%, w/w). The mixture was then purged with nitrogen gas (99.99% pure) for 30 min to complete the formation of Fe2þ-CMC complexes. Then Na2S solution (20 mL, 0.852 M) was titrated into the Fe2þ-CMC solution at a speed of 0.05 mL s1 to yield the FeS particles. The FeS particles-CMC suspension was centrifuged at 1000 rpm, and the supernatant was decanted and CMC solution (1%, w/w) was added to keep a constant volume. The mixture was sonicated for half an hour to restabilize the FeS particles with ultrasonic instrument (KQ-200KDE, Kunshan Ultrasonic Instrument, China). The FeS particle solids were collected by filtering the suspension using a 0.45 mm membrane filter and subsequently freeze-dried under vacuum at 50 C using a freeze dryer (MudolYOD-230, Thermo Fisher, American) for 48 h. The X-ray diffraction (XRD) patterns of FeS particles and CMC were obtained on a Shimadzu XRD-7000 device at a scan speed of 2.0 min1. The XRD patterns were processed using the computer program known as MDI Jade 6.5 loaded with ICDD database (Materials Data Inc., Livermore, CA, U.S.A.). The surface composition and morphology were investigated by scanning electron microscopy (SEM) and energy dispersive Xray (EDX) analysis at 15 kV (JSM-7800F, JEOL, Japan). The mean hydrodynamic diameter of FeS particles were determined by Nano
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Zetasizer ZS90 (ZEN 3690, Malvern Instruments, UK). The FeS particles suspension was diluted to a concentration of 100 mg L1 and sonicated for 30 min before analysis (Singh et al., 2011). 2.3. Kinetics of Cr(VI) reduction tests In kinetics tests of Cr(VI) reduction by FeS particles in soil, 12 mL and 36 mL of FeS particles suspension (molar ratio of FeS to Cr(VI) in soil: 0.5:1 and 1.5:1, respectively) was added to 100 g of Cr(VI) spiked soil. The Cr(VI) spiked soil and FeS particles were mixed by using an electronic blender with a speed of 500 r min1 for 10 min and then sealed and incubated in the dark at room temperature (20 ± 2 C). At predetermined times, the treated soils were sampled, air-dried and passed through a 2-mm mesh. The pH, Cr(VI) concentration in soil, and total Cr and Cr(VI) in TCLP leachate of FeS particles-treated soils were analyzed. The TCLP extraction fluid consisted of 0.1 M glacial acetic acid and 0.0643 M NaOH with a pH of 4.93. The extraction tests were carried out by mixing 2.5 g of an air-dried soil sample with 50 mL TCLP fluid #1 in a Teflon vial. The mixtures were rotated (30 rpm) for 18 h at 23 ± 2 C and centrifuged at 3000 r min1 for 10 min. Then, the supernatant was filtered through 0.45 mm filter membrane. The filtrates were preserved with HNO3 for total Cr and Cr(VI) analysis. Another test was conducted using a solution containing 1% CMC without FeS particles. All tests were conducted in duplicate. 2.4. Effect of remediation conditions: FeS particles dosage, moisture and NOM A series of batch tests were conducted to investigate the effects of remediation conditions: dosage of FeS particles, moisture and NOM in soil samples. The experimental procedures were similar to Section 2.3 except for the fixed treatment period of 7 d and the desired remediation conditions. Total Cr and Cr(VI) in TCLP leachate of treated soils were analyzed to evaluated the effects. In dosage effect experiments, molar ratios of FeS to Cr(VI) were 0.33:1, 0.5:1, 1:1, 1.5:1 and 2:1, respectively, with a fixed soil moisture of 50%. To assess the soil moisture effect, experiments were carried out at the water content levels of 30%, 40%, 50%, 60%, and 70% with a fixed molar ratio of FeS to Cr(VI) ¼ 1.5:1. In NOM effect experiments, humic acid was added in Cr(VI) spiked soil to the levels of 1%, 3%, and 5%, which was then aged for 7 d. FeS particles were subsequently added in the aged soil with a fixed molar ratio of FeS to Cr(VI) ¼ 1.5:1 and soil moisture of 50%. Control tests were conducted at 5% humic acid and without addition of FeS particles to soil. All experiments in this study were conducted in duplicate. 2.5. Comparative remediation tests by FeS particles and FeSO4 Parallel experiments were conducted to compare the effectiveness of FeS particles and FeSO4 solution in remediation of Cr(VI) spiked soil. The Cr(VI) spiked soil was treated by FeS particles at molar ratio of FeS to Cr(VI) 1.5:1 and by 0.4 M Fe2þ solution at molar ratios of Fe2þ to Cr(VI) 1.5:1 and 4.5:1. The following indicators for both FeS particles and FeSO4-treated soils were determined and analyzed: soil pH, Cr(VI) concentration in soil, total Cr and Cr(VI) in TCLP leachate, fractions and in vitro bioaccessibility of Cr. The fractions of Cr were quantified by sequential extraction procedures developed by Tessier et al. (1979). In vitro bioaccessibility of Cr was estimated by PBET method following the procedure of Yang et al. (2002) and Jardine et al. (2013). The same remediation procedures were used in Section 2.3 except for the treatment period of 7 d. All experiments in this study were conducted in duplicate.
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2.6. Analysis method and quality assurance Cr(VI) in soil was extracted following alkaline digestion procedure (EPA, 1996). Cr(VI) in filtrate was measured by diphenyl hydrazine spectrophotometry (Ministry of Environmental Protection of PR China, 1987). Total Cr in filtrate was analyzed by atomic absorption spectrometry (AA-7000, Shimadzu, Japan). Method blanks (solvent) and sample duplicates were routinely analyzed with treated soil samples. The correlation coefficients (R2) of the calibration curve (lower than 10 mg L1) for Cr(VI) analysis in solution were 0.9998. The method detection limit of Cr(VI) in soil was 2 mg kg1. 3. Results and discussions 3.1. Characterization of FeS particles Fig. 1 (a) shows the hydrodynamic diameter distribution of CMC stabilized FeS particles based on dynamic light scattering. The particles size ranged from 60 nm to 2000 nm with a mean size of 614 nm. Fig. 1 (b) shows the XRD diffractograms of the CMC stabilized FeS particles. The peak at 2q ¼ 20.8 was the typical diffraction peak of CMC, and two peaks at 2q ¼ 17.1 and 28.6 could be attributed to the diffraction of FeS crystal. The morphology of stabilized FeS particles was investigated by SEM and EDX analysis. The FeS particles present shuttle-like morphology with an average length of 400 nm and diameter of about 100 nm in the middle part (Fig. 1(c)). The diameter of FeS particles obtained from SEM was smaller than hydrodynamic diameter. Since SEM image excludes the CMC molecules attached on FeS particle surface while hydrodynamic diameter is based on the overall diffusivity of the CMC-anchored FeS particles (Xu, 1998; Xiong et al., 2009). Although the same materials and similar procedures for FeS particles synthesis were used, the shape of FeS particles prepared in our study was very different from the FeS nanoparticles which were microsphere prepared by Xiong et al. (2009) and Gong et al. (2012). The procedures of FeS particles synthesis had an effect on particle shape and crystal. EDX spectra (Fig. 1(d)) for the Spot 1 in Fig. 1(c) was composed of peaks of Fe, O, and S mainly as well as C and Na. O peaks could be due to facile oxidation of the particles or the adsorbed CMC which also induced the C peaks and Na peaks. 3.2. Kinetics of Cr(VI) reduction in soil Patterson et al. (1997) and Mullet et al. (2004) studied the kinetics of Cr(VI) reduction by FeS and indicated that Cr(VI) was completely removed from solution within several hours. Considering the effect of the soil media, a longer treatment period of 15 d was set, which is also practical for a remediation process. The Cr(VI) contents in soil may be an effective indicator to control the leachability of Cr(VI) and the Cr(VI)-caused ecotoxicity in soil. Fig. 2(a) illustrated the change of Cr(VI) concentration in FeS particles-treated soils when the molar ratio of FeS to Cr(VI) were 0.5:1 and 1.5:1 in 15 d. Little decrease of Cr(VI) concentration from 1407 mg kg1 to 1338 mg kg1 was observed in 1% CMC-treated soil, which was consistent with the findings by Wang et al. (2014a) that CMC had little effects on the reduction of Cr(VI) in soil treated by 0.1% CMC solution for 24 h. With the addition of FeS particles, Cr(VI) concentration decreased sharply in 3 d and then reached equilibrium when molar ratio of FeS to Cr(VI) were 0.5:1 and 1.5:1, respectively. Cr(VI) concentration in FeS particles-treated soil was reduced to 164 mg kg1 and 16 mg kg1 in 15 d for the both dosages of FeS particles. And the Cr(VI) removal capacities of FeS particles were
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estimated to be 1047.5 mg Cr(VI) per gram FeS and 390.7 mg Cr(VI) per gram FeS, respectively. Though the remediation Cr(VI) contaminated soil by nZVI is faster than FeS particles (Reyhanitabara et al., 2012; Wang et al., 2014a), the Cr(VI) removal capacity of FeS particles was apparently larger than that of nZVI (226.7 mg Cr(VI) per gram nZVI) at initial Cr(VI) in soil of 102 mg kg1 for 24 h reported by Wang et al. (2014a). TCLP has been widely used to evaluate the immobilization of heavy metals in soil to indicate remediation effectiveness (Xu and Zhao, 2007; Du et al., 2012; Wang et al., 2014b). As shown in Fig. 2(b), total Cr and Cr(VI) in TCLP leachates of Cr(VI) spiked soil were 65.91 mg L1 and 59.74 mg L1, respectively. When Cr(VI) spiked soil were treated by FeS particles at a molar ratio of FeS to Cr(VI) ¼ 0.5:1 in 3 d, total Cr and Cr(VI) in TCLP leachate decreased rapidly to 7.86 mg L1 and 6.83 mg L1 respectively. Total Cr and Cr(VI) in TCLP leachate were further reduced by only 0.19 mg L1 and 0.1 mg L1 when the treatment period was lengthened from 3 d to 15 d. When molar ratio of FeS to Cr(VI) was 1.5:1, total Cr and Cr(VI) in TCLP leachate deceased to 1.04 mg L1 and 0.34 mg L1 in 3 d, and total Cr and Cr(VI) in TCLP leachate reached equilibrium and remained constant levels of 0.81 mg L1 and 0.05 mg L1 from
3 d to 15 d, respectively. About 98% of Cr(VI) in soil was removed and 98.4% of total Cr was immobilized by using FeS particles at a molar ratio of FeS to Cr(VI) ¼ 1.5:1 in 3 d. While a month or a longer time is required to remediate Cr(VI) contaminated soil when using FeSO4 (Dermatas et al., 2006; Moon et al., 2009) and CaSx (Chrysochoou et al., 2010). 3.3. Effect of remediation conditions 3.3.1. Effect of FeS particles dosage The total Cr and Cr(VI) in TCLP leachate of FeS particles-treated soils with FeS particles dosage increase were investigated (Fig. SM1). When the molar ratio of FeS to Cr(VI) increased to 1.5:1, total Cr and Cr(VI) in TCLP leachate decreased to a level below 1 mg L1 and 0.5 mg L1 respectively. With the further increase, total Cr and Cr(VI) in the TCLP leachate did not continue to change apparently. Taking both the Cr(VI) immobilization and cost into consideration, a molar ratio of 1.5:1 was chosen as the optimal dosage for the remediation of Cr(VI) spiked soil. As a reaction product of FeS with Cr(VI), Cr(III) may precipitate as Cr(III) hydroxides or Cr(III)-Fe(III) hydroxides with good stability,
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Fig. 2. Cr(VI) concentration and Cr TCLP-leachability in soils treated by stabilized FeS particles. (a) Cr(VI) concentration in soils. (b) Total Cr and Cr(VI) in TCLP leachate of soils. A Cr(VI) in Cr(VI) spiked soil ¼ 1407 mg kg1, molar ratio of FeS to Cr(VI) ¼ 0.5:1 and 1.5:1. Data plotted as mean of duplicates and the error bars (calculated as standard deviation) indicate data reproducibility.
and become immobilized after sorption on soil colloids (Leita et al., 2011; Papassiopi et al., 2014). Besides, several sulfur intermediates are involved when Cr(VI) in soil is reduced by FeS. Patterson et al. (1997) mentioned that the products of FeS reacting with Cr(VI) are Fe(III), sulfite, sulfate, thiosulfate, and Cr(III). Thiosulfate is the dominant sulfur phase under neutral and low pH. Mullet et al. (2004, 2007) also found that elemental sulfur is the oxidation product of sulfide by Cr(VI). 3.3.2. Effect of soil moisture Soil moisture is one of the key parameters in the heavy metal contaminated soil remediation, which may influence the remediation efficiency and cause potential water pollution. The characteristics of soil and the remediation requirements may determine the selection of the soil moisture level. The variation of total Cr and Cr(VI) in TCLP leachate of FeS particles-treated soils with the increase of fixed soil moisture were investigated (Fig. SM-2). The soil pH fluctuated between 6.93 and 7.10 when soil moisture was increased from 30% to 70%. Total Cr and Cr(VI) in TCLP leachate were 3.8 mg L1 and 3.2 mg L1 at fixed soil moisture level of 30%, and decreased sharply to 0.83 mg L1 and 0.37 mg L1 at soil moisture 40%. At soil moisture 50%, total Cr in TCLP leachate was as low as 0.78 mg L1 and Cr(VI) was below the detection limit of 0.004 mg L1. Within the soil moisture range from 40% to 70%, total Cr and Cr(VI) in TCLP leachate were consistently lower than 1 mg L1 and 0.5 mg L1, respectively. When below 40%, the soil moisture had a negative influence on Cr(VI) immobilization, due to the inhibition to the contact of FeS particles and Cr(VI) attached on soil. Nevertheless, the soil moisture above 40% showed little effect on Cr(VI) immobilization, which could be attributed to the enough diffusion of FeS particles for the contact between FeS and Cr(VI) in soil and the characteristics of high silt and clay content in the tested soil. 3.3.3. Effect of NOM NOM in soil generally lies between 0.5% and 5%, and was 0.63% in the tested soil. Humic acid, a typical NOM, was selected to investigate the effect of NOM on the remediation of Cr(VI) spiked soil by using FeS particle (Zhang et al., 2012). In control tests, total
Cr and Cr(VI) in TCLP leachate were 54.9 mg L1 and 50.4 mg L1 at 5% humic acid without the addition of FeS particles, respectively (Fig. SM-3). Within a humic acid content range from 1% to 5%, total Cr and Cr(VI) in TCLP leachate stayed at the levels lower than 1 mg L1 and 0.5 mg L1 at a molar ratio of FeS to Cr(VI) ¼ 1.5:1, respectively. Humic acid had no significant effect on Cr(VI) immobilization by using FeS particles. Scaglia et al. (2013) reported that the rates of Cr(VI) reduction by humic acid were strongly pHdependent and more than 60% Cr(VI) was reduced by using humic acid at pH ¼ 2.5, while no Cr(VI) reduction was observed at pH ¼ 6. In this study, soil pH did not change much and fluctuated around 6 after adding humic acid, which may not influence the effect of humic acid on Cr(VI) reduction by FeS particles. The results were consistent with the findings of no apparent effect of humic acid on the Cr(VI) removal by pyrite (Liu et al., 2015). 3.4. Effectiveness: efficiency, Cr fraction and in vitro bioaccessibility 3.4.1. Efficiency of FeS particles and FeSO4 Fe2þ has been previously used as a chemical reducing agent to reduce Cr(VI) (Dermatas et al., 2006; Moon et al., 2009; Di Palma et al., 2015). The reductive reactions are listed as follows (Mullet et al., 2004; Pan et al., 2014): 2þ þ 8Hþ / Cr3þ þ 3Fe3þ þ 4H2O CrO24 þ 3Fe
(2)
xCr3þ þ (1-x)Fe3þ þ 3H2O / (CrxFe1-x)(OH)3 (s) þ 3Hþ (x < 1) (3) FeS particles was compared to FeSO4 as the reducing agent in terms of the Cr(VI) removal efficiency under similar experimental conditions. Fig. 3 illustrate Cr(VI) concentration in treated soils and total Cr and Cr(VI) in TCLP leachate when various dosages of FeS particles and FeSO4 were used to remediated Cr(VI) soils. Cr(VI) in FeSO4-treated soil was reduced to 923 mg kg1, about 34.4% of Cr(VI) in Cr(VI) spiked soil, and total Cr and Cr(VI) in TCLP leachate decreased by about 50% at molar ratio of FeSO4 to Cr(VI) ¼ 1.5:1. When the molar ratio of FeSO4 to Cr(VI) was raised to 4.5:1, Cr(VI) concentration in treated soil, total Cr and Cr(VI) in TCLP leachate decreased to 18.2 mg kg1, 0.84 mg L1 and 0.05 mg L1, almost
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Fig. 3. Cr(VI) concentration and total Cr and Cr(VI) in TCLP leachate in soils treated by stabilized FeS particles and FeSO4.A Cr(VI) in Cr(VI) spiked soil ¼ 1407 mg kg1, molar ratio of FeS to Cr(VI) ¼ 1.5:1, molar ratios of FeSO4 to Cr(VI) ¼ 1.5:1 and 4.5:1, treatment period ¼ 7 d. Data plotted as mean of duplicates and the error bars (calculated as standard deviation) indicate data reproducibility.
equal to the results of FeS particles (16 mg kg1, 0.81 mg L1 and 0.05 mg L1) at molar ratio of FeS to Cr(VI) ¼ 1.5:1. Although FeSO4 is a widely applied reducing agent for Cr(VI) reduction, soils treated by FeSO4 was acidified with an apparent pH decrease from 6.75 to 3.05 at M ratio of FeSO4 to Cr(VI) ¼ 4.5:1 (Fig. SM-4). The pH decrease was attributed to protons production in the formation of Cr(III)-Fe(III) hydroxides, which causes soil acidification and maintains the system at low pH (Singh et al., 2011; Pan et al., 2014). During the remediation, the pH of soil treated by FeS particles fluctuated slightly between from 6.76 to 7.13 at a molar ratio of FeS to Cr(VI) ¼ 1.5:1. Comparing to FeSO4, the addition of FeS particles not only had a higher Cr(VI) removal efficiency, but also did not change pH of the soil apparently. 3.4.2. Soil bound Cr SEM elemental mapping analysis was used to reveal the spatial associations between FeS and Cr on the surface of FeS particlestreated soil as shown in Fig. 4. The distribution of Cr and Fe, S on
treated soil surface shows the combination of Cr and Fe, S after the addition of FeS particles. Especially the binding between Cr (labeled in blue) and S (labeled in green) indicated the sorption of Cr by FeS particles and attachment on the treated soil. The sequential extraction procedures have been often applied to identify the relative availability and to reveal the immobilization of metals in the solid phase (Jung et al., 2007; Wang et al., 2014b; Di Palma et al., 2015). According to the method by Tessier et al. (1979), the five Cr fractions are defined as exchangeable (EX), carbonate bound (CB), Fe-Mn oxides bound (OX), organic bound (OM), and residual phases (RS). The relative availability of a metal follows the sequence: EX > CB > OX > OM > RS, and OX, OM and RS are relatively stable fractions. Cr fraction distribution of the Cr(VI) spiked soil and soils treated by FeS particles and FeSO4 was shown in Fig. SM-5. EX was the most predominant Cr fraction, occupying 74.8% in the Cr(VI) spiked soil, and the other one were CB (7.3%), OX (6.8%), OM (1.9%), and RS (16.3%), respectively. The distribution of Cr fractions in the
Fig. 4. SEM image and Fe, S and Cr mapping in SEM image of stabilized FeS particles-treated soil. (a) SEM image of stabilized FeS particles-treated soil. (b) Fe, S and Cr mapping of the scanned part of FeS particles-treated soil. A Cr(VI) in Cr(VI) spiked soil ¼ 1407 mg kg1, molar ratio of FeS to Cr(VI) ¼ 1.5:1, treatment period ¼ 7 d.
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soil was notably changed after the addition of FeS particles and FeSO4. In FeS particles-treated soil, the fraction of EX decreased to 0.3%, and the fractions of OX and OM increased by 54% and 17%, respectively. The major Cr fractions in FeS particles-treated soils were OX (60.3%), OM (19%) and RS (17%). The distinctive alteration of Cr fractions, especially the substantial increase in OX and OM fractions, accounted for the reduced relative availability of Cr (Jung et al., 2007). The OX fraction increase might be largely attributed to the precipitation of Cr(III)-Fe(III) hydroxides during the FeS particles treatment (Leita et al., 2011; Papassiopi et al., 2014). In FeSO4-treated soil, the major Cr fractions were OX (52.3%) and RS (30.9%), but the EX and CB fractions still accounted for 12.9%, apparently higher than that in FeS particles-treated soils (3.7%). The more fractions of stable species of Cr in soils treated by FeS particles may contribute to the better immobilization of Cr(VI) than by FeSO4. 3.4.3. In vitro bioaccessibility Compared to TCLP, the PBET method uses an extreme condition, a more aggressive extracting agent (pH 1.5) and a higher liquid-tosoil ratio (100:1), to simulate the gastrointestinal environment (Yang et al., 2002; Jardine et al., 2013). It was designed to assess the in vitro bioaccessibility of Cr in FeS particles-treated soil for humans. The bioaccessibility of Cr in FeS particles and FeSO4 -treated soils for 7 d was investigated. Under the PBET conditions, the bioaccessibility of Cr in Cr(VI) spiked soil was 58.67%, and that in 1% CMC treated soil was 53.45% (Fig. SM-6). The bioaccessibility of Cr in FeS particles-treated soil decreased to 6.98% at molar ratio of FeS to Cr(VI) ¼ 1.5:1, which was apparently lower than that of soil treated by FeSO4 (18.05%) at a higher molar ratio of FeSO4 to Cr(VI) ¼ 4.5:1. Although total Cr and Cr(VI) in TCLP leachate of treated by FeS particles (molar ratio of FeS to Cr(VI) ¼ 1.5:1) and FeSO4 were comparable, a much lower bioaccessibility of Cr in soils indicated the higher efficiency of FeS particles in remediation of Cr(VI) spiked soil. 4. Conclusions CMC stabilized FeS particle suspension with a concentration as high as 10000 mg L1 was successfully prepared. The freshly prepared FeS particles showed shuttle-like morphology with an average length of 400 nm and diameter of about 100 nm. Stabilized FeS particles may effectively remediate Cr(VI) spiked soil. About 98% of Cr(VI) in soil was reduced and immobilized by FeS particles in 3 d at a molar ratio of FeS to Cr(VI) ¼ 1.5:1. The soil moisture ranging from 40% to 70% and NOM in soil had no significant effects on Cr(VI) remediation by FeS partices. The exchangeable Cr fraction was converted to Fe-Mn oxides bound and organic bound fractions, and PBET-based bioaccessibility of Cr was decreased from 58.67% to 6.98%. FeS particles with a dosage of FeS to Cr(VI) molar ratio ¼ 1.5:1 had a high Cr(VI) removal and Cr immobilization efficiency compared to FeSO4 at a molar ratio of FeSO4 to Cr(VI) ¼ 4.5:1, meanwhile the pH of treated soil fluctuated around 7. The PBET-based bioaccessibility of Cr in FeS particlestreated soil was apparently lower than that in FeSO4-treated soil. The prepared FeS particles hold a great remediation effectiveness in Cr(VI) spiked soil with higher Cr(VI) removal and Cr immobilization efficiency, a lower PBET-based bioaccessibility of Cr, and a slighter pH influence. This study provides an effective technology for the remediation of Cr(VI) contaminated soil, which can efficiently reduce Cr(VI) and immobilize Cr in soil without changing the properties of raw soil apparently. FeS particles shows a promising potential in field application in the remediation of Cr(VI) contaminated soil.
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