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Leachability and Stability of Hexavalent-Chromium-Contaminated Soil Stabilized by Ferrous Sulfate and Calcium Polysulfide

Leachability and Stability of Hexavalent-Chromium-Contaminated Soil Stabilized by Ferrous Sulfate... applied sciences Article Leachability and Stability of Hexavalent-Chromium-Contaminated Soil Stabilized by Ferrous Sulfate and Calcium Polysulfide 1 , 2 1 , 3 , 1 , 3 Ting-Ting Zhang , Qiang Xue * and Ming-Li Wei State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Bayi Road, Wuchang District, Wuhan 430071, China; ztt_cersm@163.com (T.-T.Z.); weimingli830716@sina.com (M.-L.W.) University of Chinese Academy of Sciences, Beijing 100049, China Hubei Key Laboratory of Contaminated Clay Science & Engineering, Wuhan 430071, China * Correspondence: qiangx@whrsm.ac.cn Received: 16 July 2018; Accepted: 19 August 2018; Published: 22 August 2018 Abstract: Ferrous sulfate (FeSO ) and calcium polysulfide (CaS ) stabilization are practical 4 5 approaches to stabilizing hexavalent chromium (Cr(VI))-contaminated soil. The leachability and stability of Cr(VI) and Cr are important factors affecting the effectiveness of stabilized Cr(VI)-contaminated soil. This study compared the leachability and stability of Cr(VI) and Cr in Cr(VI)-contaminated soil stabilized by using FeSO and CaS . The contaminated soil was 4 5 characterized before and after stabilization, and the effectiveness of FeSO and CaS stabilization 4 5 was assessed using leaching, bioaccessibility, alkaline digestion, sequential extraction, and X-ray diffraction tests. Results showed that FeSO and CaS significantly reduced the leachability and 4 5 Cr(VI) content in the contaminated soil. The acid-buffering capacity and stability (leachability, bioaccessibility, speciation distribution, and mineral composition) of the Cr(VI)/Cr and Cr(VI) content of CaS were better than those of FeSO . This study demonstrated that CaS had a better 5 4 5 effect than FeSO on the stabilization of Cr(VI) in Cr(VI)-contaminated soil. The CaS significantly 4 5 enhanced the stabilization and immobilization of Cr(VI) and reduced its leachability and toxicity. Keywords: hexavalent chromium; contaminated soil; leachability; stability; speciation 1. Introduction Soil contamination by chromium (Cr) is a serious problem in China [1–3]. Cr is released into the soil by various industries, including the wood preservation, leather tanning, chromate manufacturing, and electroplating industries [4]. Cr in soil occurs primarily in its Cr(III) and Cr(VI) redox states; Cr(III) is a nutrient for plant growth, whereas Cr(VI) is a dangerous species and human carcinogen [5]. Chemical reduction removes Cr(VI) rapidly and effectively based on the use of reducing agents, such as ferrous sulfate, calcium polysulfide, or sodium bisulfate, followed by precipitation as Cr(OH) [6]. Calcium polysulfide (CaS ) and ferrous sulfate (FeSO ) are promising reagents that have been used at 5 4 many Cr-contaminated sites and for chromite ore-processing residue (COPR). The reduction of Cr(VI) with FeSO and CaS (denoted by its average chemical formula, CaS ) can be written as follows [6,7]: 4 5 5 2+ + 3+ 3+ 3Fe + HCrO + 7H $3Fe + Cr + 4H O (1) 4 2 2 + 2+ 2CrO + 3CaS + 10H $2Cr(OH) + 15S + 3Ca + 2H O. (2) 4 5 3 2 Many studies have been performed on Cr(VI)-contaminated soils stabilized by FeSO and CaS . 4 5 However, most of them focused on the leachability and content of Cr(VI) and Cr. Palma et al. [8] Appl. Sci. 2018, 8, 1431; doi:10.3390/app8091431 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 1431 2 of 11 applied FeSO to reduce Cr(VI) at a contaminated industrial soil site in Italy and found that FeSO 4 4 successfully lowers the amount of Cr(VI) in the soil. An alkaline digestion test showed that Cr(VI) is almost completely reduced when the Fe(II)/Cr(VI) molar ratio is 30. John et al. [9] reported the use of FeSO to treat Cr(VI)-contaminated soil through a column treatment. The Cr(VI) concentrations in the leachate after a toxicity characteristic leaching procedure test (TCLP) range between 0.59 and 0.7 mg/L. Buerge et al. [10] found that FeSO is a useful treatment reagent in the in-situ remediation of Cr(VI)-contaminated soils in Switzerland. Chrysochoou et al. [11] performed a column treatment of Cr(VI)-contaminated soil treated with CaS and found that up to 99% Cr(VI) was reduced with an injection of CaS at 12 times the stoichiometric requirement. Wazne et al. [12] reported that 62% of Cr(VI) is reduced in COPR with CaS addition at twice the stoichiometric ratio. Chrysochoou et al. [13] applied CaS in batch studies of highly contaminated soil from a Cr plating facility in Putnam. Redox potential results showed that CaS maintains a highly reducing environment for a prolonged period of time and that the alkaline digestion and synthetic precipitation leaching procedure concentrations are lower than Environmental Protection Agency (EPA) regulatory levels. Although these applications indicated that FeSO and CaS are effective reductants, insufficient 4 5 information is available about the difference between the FeSO and CaS remediation of Cr(VI) 4 5 in contaminated soil. Moreover, most works only used the TCLP or alkaline digestion test, and systematic investigations about the remediation capacities of FeSO and CaS on Cr(VI), based on 4 5 the bioaccessibility and speciation of Cr, are lacking. The toxicity and mobility of heavy metals in soil are not only related to their total content but are also determined to a greater degree by the distribution of their speciation [14]. Zimmerman et al. [15] demonstrated that the availability and extraction effectiveness of heavy metals in soil decrease in the order of acid soluble forms > reducible forms > oxidizable forms > residual forms. This study compared the leachability and stability of Cr(VI)-contaminated soils stabilized by FeSO and CaS . Toxicity characteristic leaching procedure (TCLP), simplified bioaccessibility 4 5 extraction test (SBET), alkaline digestion, sequential extraction, and X-ray diffraction (XRD) tests were performed on Cr(VI)-contaminated soils. This study can serve as a basis for designing the remediation of Cr(VI)-contaminated soils by using FeSO and CaS . 4 5 2. Materials and Methods 2.1. Cr(VI) Contaminated Soil Preparation The raw soil was collected from a subway excavation site in Wuhan City (China). The soil was dried, ground, and then sieved through a 2-mm screen. The detailed description of the physical characterization of the raw soil and Cr(VI)-contaminated soil are presented in Table 1, which was obtained according to the “Standard for soil test method” of China. The Light Proctor compaction method was used for the compaction test [16,17]. Cr(VI)-contaminated soils were obtained by adding K Cr O solution until the Cr(VI) content in the soil reached 1000 mg/kg, which represents 2 2 7 a universal content for Cr(VI)-contaminated soil in China [18–22]. Deionized water was then added to the contaminated soil until the water content reached 19.5% (optimum moisture content). The contaminated soil was mixed evenly and braised for 180 days under standard curing conditions (20  2 C, 95% humidity) to allow K Cr O and the soil to react adequately. After homogenization, 2 2 7 the contaminated soil was air dried and pulverized to achieve the required particle size (<2 mm). The entire quantity of soil was made to pass through the sieve to avoid any fractionation [23]. All reagents in this study were supplied from Sinopharm Chemical Reagent Co., Ltd. (Ningbo, China) and used as American Chemical Society-certified reagents without any further purification. Appl. Sci. 2018, 8, 1431 3 of 11 Table 1. Physicochemical and mechanical properties of raw soil and chromium-contaminated soil. Items Raw Soil Chromium-Contaminated Soil Water content/% 20.78 — pH 8.53 7.76 Specific gravity 2.72 2.79 Physicochemical properties Liquid limit/% 41.63 40.18 Plastic limit/% 21.84 21.33 Mn (mg/kg) 798.36 797.48 C.E.C (meq/100 g) 9.12 9.87 Optimum moisture content/% 19.53 18.95 Mechanical properties 1.72 1.73 Maximum dry density/(g/cm ) Brunauer-Emmett-Teller specific 30.74 29.62 surface Area (m /g) Clay content (<0.005 mm) 4.62 3.23 Grain-size distribution (%) Silt content (0.005–0.075 mm) 74.29 71.76 Sand content (0.075–2 mm) 21.09 25.01 Al O 22.12 21.67 2 3 SiO 64.2 64.37 K O 2.78 2.85 CaO 1.43 1.42 Chemical composition (%) TiO 0.84 0.86 MnO 0.12 0.13 Fe O 8.51 8.59 2 3 Cr O — 0.11 2 3 2.2. Stabilized of Cr(VI) Contaminated Soil Representative 500 g of air-dried Cr(VI)-contaminated soil were introduced into a 10 L SPAR type mixer. FeSO and CaS were added to the Cr-contaminated soil as reductant to a dry soil ratio 4 5 of 1%, 3%, and 5%. The experimental design is presented in Table 2. The soil was homogenized for 10 min prior to the addition of distilled water. It was ensured that the ratio of addition of water to the reductant and dry soil was 1:2. The mixture was withdrawn from the sealed plastic bottles after being incubated for 7 d at room temperature (20  1 C). All the samples were prepared in triplicate. The reported stability results are the averages of three replicates. Table 2. Experimental design for the stability study. Test No. Reductant Dosage (%) 1 FeSO 0 2 FeSO 1 3 FeSO 3 4 FeSO 5 5 CaS 0 6 CaS 1 7 CaS 3 8 CaS 5 3. Test Methods The soil cation exchange capacity (CEC) and MnO content were determined using standard methods [24]. Soil acid digestion was performed to determine the Cr and Mn content in soil according to EPA Method 3050B [25]. Nitrogen adsorption-desorption measurements were determined by a surface area analyzer (Nova 1000e, Quantachrome Instruments, USA). The chemical composition of the samples was measured by an X-ray fluorescence (XRF) scan. The size distribution of the waste particles was measured using a Malvern MS3000 laser diffraction particle size analyzer. pH values for all soil were measured as per ASTMD4972-01 [26]. The toxicity characteristic leaching procedure (TCLP) of Cr was conducted as per USEPA Method 1311 [27].The bioaccessibility test was performed according to the U.S. EPA (2007) protocol [28] and the British Geological Survey [29]. The Cr(VI) content of contaminated soil was measured using the USEPA Method 3060A alkaline digestion method [30]. Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 11 Table 2. Experimental design for the stability study. Test No. Reductant Dosage (%) 1 FeSO4 0 2 FeSO4 1 Appl. Sci. 2018, 8, 1431 4 of 11 3 FeSO4 3 4 FeSO4 5 5 CaS5 0 The Cr(VI) concentration in the filtrate was measured using U.S. EPA Method 7196A colorimetric 6 CaS5 1 analyses [31]. The modified European Community Bureau of Reference (BCR) sequential extraction 7 CaS5 3 procedure was conducted as per the method recommended by Rauret et al. [32]. The sequential 8 CaS5 5 extraction procedure consisted of four steps, which corresponded to the exchangeable, reducible, oxidizable, and residue fractions. 4. Results and Discussion 4. Results and Discussion 4.1. pH of Stabilized Soils 4.1. pH of Stabilized Soils Figure 1 shows the pH of the stabilized soils with different FeSO4 and CaS5 dosages. The results indicated that CaS5 addition can increase the pH of the stabilized soil, which was contrary to the effect Figure 1 shows the pH of the stabilized soils with different FeSO and CaS dosages. The results 4 5 of FeSO4 addition. For illustration, the pH of the FeSO4 stabilized soil decreased from 7.86 to 2.62 indicated that CaS addition can increase the pH of the stabilized soil, which was contrary to the when the reductant dosage increased from 0% to 5%. However, the pH of the CaS5 stabilized soil effect of FeSO addition. For illustration, the pH of the FeSO stabilized soil decreased from 7.86 to 4 4 increased from 7.86 to 9.57. The changes in the FeSO4 and CaS5 tread soil were attributed to the 2.62 when the reductant dosage increased from 0% to 5%. However, the pH of the CaS stabilized different stabilization mechanisms in the Cr(VI)-contaminated soils. For the FeSO4 stabilized soil, soil increased from 7.86 to 9.57. The changes in the FeSO and CaS tread soil were attributed to the 4 5 Cr(VI) was different stabilization reducted by mechanisms Fe(II) and in the formed Fe Cr(VI)-contaminated (III) hydroxide precipitati soils. For the FeSO on anstabilized d released a soil, Cr(VI) was reducted by Fe(II) and formed Fe(III) hydroxide precipitation and released a considerably considerably greater amount of H [33]. The alkalinity of the CaS5 stabilized soils can be attributed to the f greater aamount ct that Ca of S5H is an [33 a]. lkal The ine alkalinity material [of 13the ]. This CaS result stabilized indicatsoils es that can CaS be5 was attributed more adv to the antageous fact that CaS is an alkaline material [13]. This result indicates that CaS was more advantageous than FeSO in than FeSO4 in which stabilized soil pH can reach values of 9.57 or higher, which increases the acid- 5 5 4 buffe which ring cap stabilized acity. soil pH can reach values of 9.57 or higher, which increases the acid-buffering capacity. Untreated FeSO CaS 4 5 0 1 5 Dosage of reductant(%) Figure 1. Effect of reductant types and dosage on the pH of stabilized soils. Figure 1. Effect of reductant types and dosage on the pH of stabilized soils. 4 4.2. .2. Red Redox ox Potential of Stabilized Potential of Stabilized Soils Soils The redox pot The redox potentials entials of t of the he st stabilized abilized soils are pr soils are presented in Figure esented in Figure 2. The result 2. The results s indicat indicated ed tthat hat addition of FeSO addition of FeSO4 and C and CaS aS5 could decrease t could decrease the he redox pot redox potential ential of of st stabilized abilized soil. T soil. This his phenomenon ca phenomenon can n 4 5 be a be attributed ttributed tto o the f the fact act tha that t Cr( Cr(VI) VI) wa was s redu reduced ced wi with th i incr ncrea easing sing F FeSO eSO4 and C and CaS aS5 dosages. Figure dosages. Figure 2 also 2 also 4 5 shows shows tha that t th the e redo redox x p potential otential of of the the F FeSO eSO4-st -stabilized abilized soil was higher than soil was higher than t that hat of t of the he CaS CaS 5-st -stabilized abilized 4 5 soil with the soil with the same reductant dosage same reductant dosage. . Wh When en tthe he rre eductant ductant d dosage osage in incr creased from eased from 0% 0%tto o 5%, 5%, the redox the redox pot potential ential in in t the he FeSO FeSO4-st -stabilized abilized soil was soil was decreased from decreased from 530.5 m 530.5 mv v to to −96.3 m 96.3 mv v. For t . For the he C CaS aS5-st -stabilized abilized 4 5 soil, the redox potential was decreased from 530.5 mv to 390.6 mv. Compared with FeSO -stabilized soil, CaS could maintain a reducing environment in the stabilized soil. pH of stabilized soil Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 11 soil, the redox potential was decreased from 530.5 mv to −390.6 mv. Compared with FeSO4-stabilized Appl. Sci. 2018, 8, 1431 5 of 11 soil, CaS5 could maintain a reducing environment in the stabilized soil. 0.6 Untreated FeSO CaS 0.5 4 5 0.4 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.5 0 1 3 5 Dosage of reductant(%) Figure 2. Effect of reductant types and dosage on the redox potential of stabilized soils. Figure 2. Effect of reductant types and dosage on the redox potential of stabilized soils. 4.3. Leachability of Cr/Cr(VI) from Contaminated Soil in TCLP Leaching 4.3. Leachability of Cr/Cr(VI) from Contaminated Soil in TCLP Leaching Figure 3 shows the Cr(VI) and Cr concentrations of the TCLP leachate. The Cr(VI) and Cr Figure 3 shows the Cr(VI) and Cr concentrations of the TCLP leachate. The Cr(VI) and Cr concentrations decreased with the increase of reductant addition. For the untreated soil, the Cr(VI) concentrations decreased with the increase of reductant addition. For the untreated soil, the Cr(VI) and total Cr leaching concentrations were approximately 38.8 mg/L and 40.4 mg/L, respectively, and total Cr leaching concentrations were approximately 38.8 mg/L and 40.4 mg/L, respectively, which exceeds the regulatory limit in the standards for hazardous wastes in China [34]. For the FeSO4- which exceeds the regulatory limit in the standards for hazardous wastes in China [34]. For the stabilized soil, the leached Cr(VI) and Cr concentrations decreased from 6.4 mg/L and 12.6 mg/L to FeSO0.-stabilized 62 mg/L and 2. soil, 6 mthe g/L, r leached espectiveCr(VI) ly, when t and he red Cr uconcentrations ctant dosage wasdecr increa eased sed from from 1% t 6.4 o 5%. mg/L For and the CaS5-stabilized soil, the leached Cr(VI) and Cr concentrations decreased from 4.6 mg/L and 7.4 12.6 mg/L to 0.62 mg/L and 2.6 mg/L, respectively, when the reductant dosage was increased mg/L to 0.05 mg/L and 0.86 mg/L, respectively. In addition, with the same reductant dosage, the from 1% to 5%. For the CaS -stabilized soil, the leached Cr(VI) and Cr concentrations decreased leached Cr(VI) and Cr concentrations of the CaS5-stabilized soils decreased more noticeably relative from 4.6 mg/L and 7.4 mg/L to 0.05 mg/L and 0.86 mg/L, respectively. In addition, with the same to those of the FeSO4-stabilized soils. These results clearly demonstrate the lower leachability of Cr reductant dosage, the leached Cr(VI) and Cr concentrations of the CaS -stabilized soils decreased more in the CaS5-stabilized soil than in the FeSO4-stabilized soil under acidic solution conditions. This noticeably relative to those of the FeSO -stabilized soils. These results clearly demonstrate the lower result agrees with most previous research results. [10,13]. leachability of Cr in the CaS -stabilized soil than in the FeSO -stabilized soil under acidic solution 5 4 conditions. This result agrees with most previous research results [10,13]. Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 11 FeSO4 stabilzed CaS5 stabilzed Cr(VI) Cr(T) 15mg/L,China regulatory limit 10 5mg/L,EPA regulatory limit 0.1 0 1 3 5 1 3 Dosage of reductant (%) Figure 3. Effect of reductant types and dosage on Cr/Cr(VI) concentration in the toxicity characteristic Figure 3. Effect of reductant types and dosage on Cr/Cr(VI) concentration in the toxicity characteristic leaching procedure (TCLP) leachate. EPA, Environmental Protection Agency. leaching procedure (TCLP) leachate. EPA, Environmental Protection Agency. 4.4. Bioaccessibility of Cr/Cr(VI) from Contaminated Soil As shown in Figure 4, the leached Cr(VI) and Cr concentrations in the SBET test were higher than those from the TCLP leaching method. In addition, the Cr(VI) concentrations decreased with the increase in FeSO4 and CaS5 dosage. The leached Cr(VI) of the CaS5-stabilized soils decreased more noticeably relative to that of the FeSO4-stabilized soils. For the FeSO4-stabilized soils, the Cr concentrations changed slightly (9.8–10.1 mg/L) regardless of the FeSO4 dosage, suggesting that the bioaccessibility risk of Cr was not reduced. The increased availability of Cr in the FeSO4-stabilized soils can be explained by the pH-dependent characteristics of Cr for the pH of the leachant used in the SBET test, which was 1.5. The leached Cr concentration of the CaS5-stabilized soil decreased from 5.4 mg/L to 0.9 mg/L when the CaS5 dosage was increased from 1% to 5%. This phenomenon indicated that CaS5 could notably reduce the bioaccessibility risk compared to FeSO4-stabilized soil. CaS5 treated FeSO4 treated Cr(VI) Cr(T) 0.1 0.01 0 3 5 13 5 Dosage of reductant (%) Figure 4. Effect of reductant types and dosage on Cr/Cr(VI) concentration in the simplified bioaccessibility extraction test (SBET) leachate. Leaching concentration(mg/L) Leaching concentration(mg/L) Eh(v) Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 11 FeSO4 stabilzed CaS5 stabilzed Cr(VI) Cr(T) 15mg/L,China regulatory limit 10 5mg/L,EPA regulatory limit 0.1 1 3 1 0 5 3 5 Dosage of reductant (%) Figure 3. Effect of reductant types and dosage on Cr/Cr(VI) concentration in the toxicity characteristic Appl. Sci. 2018, 8, 1431 6 of 11 leaching procedure (TCLP) leachate. EPA, Environmental Protection Agency. 4.4. 4.4. Bioaccessi Bioaccessibility bility of Cr/Cr( of Cr/Cr(VI) VI) from Contaminated Soil from Contaminated Soil As shown in Figure 4, the leached Cr(VI) and Cr concentrations in the SBET test were higher As shown in Figure 4, the leached Cr(VI) and Cr concentrations in the SBET test were higher than than those f those fr rom the TCLP l om the TCLP e leaching aching method. In a method. Indaddition, dition, the Cr( the Cr(VI) VI) concentra concentrations tions decreased with the decreased with increase in FeSO4 and CaS5 dosage. The leached Cr(VI) of the CaS5-stabilized soils decreased more the increase in FeSO and CaS dosage. The leached Cr(VI) of the CaS -stabilized soils decreased 4 5 5 mor noticea e noticeably bly relative to that of relative to that the FeSO of the FeSO 4-stabilized -stabilized soils soils. . FoFor r ththe e Fe FeSO SO4-s-stabilized tabilized so soils, ils, tthe he Cr Cr 4 4 concentrations changed slightly (9.8–10.1 mg/L) regardless of the FeSO4 dosage, suggesting that the concentrations changed slightly (9.8–10.1 mg/L) regardless of the FeSO dosage, suggesting that the bioaccessibility bioaccessibility risk o risk of f Cr Cr was not was not reduced. reduced. The incr The increased ease availability d availabilit ofy o Crfin Cr in the FeSO the FeSO -stabilized 4-stabili soils zed soils can be explained by the pH-dependent characteristics of Cr for the pH of the leachant used in can be explained by the pH-dependent characteristics of Cr for the pH of the leachant used in the SBET the SBET test, which was 1.5. The leached Cr concentration of the CaS5-stabilized soil decreased from test, which was 1.5. The leached Cr concentration of the CaS -stabilized soil decreased from 5.4 mg/L 5.4 mg/L to 0.9 mg/L when the CaS5 dosage was increased from 1% to 5%. This phenomenon indicated to 0.9 mg/L when the CaS dosage was increased from 1% to 5%. This phenomenon indicated that that CaS5 could notably reduce the bioaccessibility risk compared to FeSO4-stabilized soil. CaS could notably reduce the bioaccessibility risk compared to FeSO -stabilized soil. 5 4 CaS5 treated FeSO4 treated Cr(VI) Cr(T) 0.1 0.01 0 1 3 5 13 5 Dosage of reductant (%) Figure 4. Effect of reductant types and dosage on Cr/Cr(VI) concentration in the simplified Figure 4. Effect of reductant types and dosage on Cr/Cr(VI) concentration in the simplified bioaccessibility extraction test (SBET) leachate. bioaccessibility extraction test (SBET) leachate. 4.5. Cr(VI) Content in Soils before and after Stabilization The Cr(VI) contents of the stabilized soils are shown in Figure 5. For the untreated soil, the Cr(VI) content was approximately 971.3 mg/kg. After FeSO and CaS stabilization, the Cr(VI) content in the 4 5 stabilized soils was reduced significantly with the increase of reductant dosage. This phenomenon can be attributed to the fact that Cr(VI) was reduced as the FeSO and CaS dosage increased. 4 5 Figure 5 also shows that the Cr(VI) content of the FeSO -stabilized soil was higher than that of the CaS -stabilized soil with the same reductant dosage. When the reductant dosage increased from 1% to 3%, the Cr(VI) content in the FeSO -stabilized soil was decreased from 168 mg/kg to 58 mg/kg, whereas that in the CaS -stabilized soil was decreased from 94 mg/kg to 22 mg/kg. The Cr(VI) content in the CaS -stabilized soil with 3% dosage was below the threshold allowed by China’s Environmental Regulations for industrial reuse (<30 mg/kg) [35]. Similarly, the Cr(VI) content in the FeSO -stabilized soil with 5% dosage was 21 mg/kg higher than that in the CaS -stabilized soil 4 5 (4.2 mg/kg). The Cr(VI) content in the CaS -stabilized soil with 5% dosage was below the threshold of civil reuse (<5 mg/kg) [35]. CaS presented a better effect than FeSO in the stabilization of Cr(VI). 5 4 Leaching concentration(mg/L) Leaching concentration(mg/L) Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 11 4.5. Cr(VI) Content in Soils before and after Stabilization The Cr(VI) contents of the stabilized soils are shown in Figure 5. For the untreated soil, the Cr(VI) content was approximately 971.3 mg/kg. After FeSO4 and CaS5 stabilization, the Cr(VI) content in the stabilized soils was reduced significantly with the increase of reductant dosage. This phenomenon can be attributed to the fact that Cr(VI) was reduced as the FeSO4 and CaS5 dosage increased. Figure 5 also shows that the Cr(VI) content of the FeSO4-stabilized soil was higher than that of the CaS5-stabilized soil with the same reductant dosage. When the reductant dosage increased from 1% to 3%, the Cr(VI) content in the FeSO4-stabilized soil was decreased from 168 mg/kg to 58 mg/kg, whereas that in the CaS5-stabilized soil was decreased from 94 mg/kg to 22 mg/kg. The Cr(VI) content in the CaS5-stabilized soil with 3% dosage was below the threshold allowed by China’s Environmental Regulations for industrial reuse (<30 mg/kg) [35]. Similarly, the Cr(VI) content in the FeSO4-stabilized soil with 5% dosage was 21 mg/kg higher than that in the CaS5-stabilized soil (4.2 mg/kg). The Cr(VI) content in the CaS5-stabilized soil with 5% dosage was below the threshold of Appl. Sci. 2018, 8, 1431 7 of 11 civil reuse (<5 mg/kg) [35]. CaS5 presented a better effect than FeSO4 in the stabilization of Cr(VI). Untreated FeSO CaS 4 5 30mg/kg,China industrial reuse regulatory limit 5mg/kg,China civilreuse regulatory limit 0 1 3 Dosage of reductant(%) Figure 5. Effect of reductant types and dosage on Cr(VI) content of stabilized soils. Figure 5. Effect of reductant types and dosage on Cr(VI) content of stabilized soils. 4.6. Species Distribution of Cr in Soils before and after Stabilization 4.6. Species Distribution of Cr in Soils before and after Stabilization Figure 6 shows the changes in the Cr speciation distribution of the untreated and stabilized soils. Figure 6 shows the changes in the Cr speciation distribution of the untreated and stabilized The primary Cr species for the untreated soil were mainly distributed in the exchangeable content (0.82 soils. The primary Cr species for the untreated soil were mainly distributed in the exchangeable mg/g), and the reducible, oxidizable, and residual contents were 0.075 mg/g, 0.074 mg/g, and 0.0026 content mg/g, respectively. (0.82 mg/g), andThes thee results reducible, indica oxidizable, ted that and Cr was more residual contents mobile and were to0.075 xic in the untreat mg/g, 0.074ed mg/g, contaminated soil. The Cr speciation of stabilized soil was changed significantly. For the FeSO4- and 0.0026 mg/g, respectively. These results indicated that Cr was more mobile and toxic in the stabilized soil, the exchangeable fraction was mainly converted to reducible fraction, which was untreated contaminated soil. The Cr speciation of stabilized soil was changed significantly. For the increased to 0.76 mg/g, when the FeSO4 dosage was increased from 0% to 5%. For the CaS5-treated soil, FeSO -stabilized soil, the exchangeable fraction was mainly converted to reducible fraction, which was the exchangeable fraction was mainly converted to oxidizable fraction. The oxidizable fraction of Cr in increased to 0.76 mg/g, when the FeSO dosage was increased from 0% to 5%. For the CaS -treated 4 5 the CaS5-stabilized soil with 3% dosage was 0.40 mg/g higher than that in the FeSO4-stabilized soil (see soil, the exchangeable fraction was mainly converted to oxidizable fraction. The oxidizable fraction of Figure 6), Similarly, the oxidizable fraction of Cr in the CaS5-stabilized soil with 5% dosage was 0.61 Cr in the CaS -stabilized soil with 3% dosage was 0.40 mg/g higher than that in the FeSO -stabilized 5 4 mg/g higher than that in the FeSO4-stabilized soil. These results clearly demonstrate the better chemical soil (see Figure 6), Similarly, the oxidizable fraction of Cr in the CaS -stabilized soil with 5% dosage stability of Cr in the CaS5-stabilized soil compared to that in the FeSO4 stabilized soil. This distinctive was 0.61 mg/g higher than that in the FeSO -stabilized soil. These results clearly demonstrate the alteration in the Cr speciation of the CaS5-stabilized soil, especially the substantial increase in the oxidizable fraction, accounted for the reduced leachability and bioaccessibility of Cr. better chemical stability of Cr in the CaS -stabilized soil compared to that in the FeSO stabilized soil. 5 4 This distinctive alteration in the Cr speciation of the CaS -stabilized soil, especially the substantial Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 11 increase in the oxidizable fraction, accounted for the reduced leachability and bioaccessibility of Cr. 1.2 Exchangeable Reducible Oxidisable Redidual FeSO4 stabilized CaS5 stabilized 1.0 0.8 0.6 0.4 0.2 0.0 Untreated 3 5 3 5 Dosage of reductant (%) Figure 6. Effect of reductant types and dosage on Cr speciation distribution. Figure 6. Effect of reductant types and dosage on Cr speciation distribution. 4.7. Probable Immobilization Mechanism of Cr The XRD results of the Cr(VI)-contaminated soil, FeSO4-stabilized soil, and CaS5-stabilized soil are shown in Figure 7. In Cr(VI)-contaminated soil, illite, quartz, albite, and calcite were identified as the major phases. In the FeSO4-stabilized soil, ferric hydroxide (Fe(OH)3) and chromium hydroxide (Cr(OH)3) were identified. For the CaS5-stabilized soil, sulfur (S) and ettringite (Ca6Al2(OH)12(SO4)3.26H2O) were identified. The differences in the leachability, bioaccessibility, and speciation distribution of Cr in the FeSO4- and CaS5-stabilized soils can be attributed to (1) the different hydration products in the FeSO4- and CaS5-stabilized soils, and (2) the different pH conditions and Eh in the FeSO4- and CaS5-stabilized soils. Kostarelos et al. [36] suggested that Cr(VI) was reduced to Cr(III) by FeSO4 and formed Cr(III)-Fe(III) hydroxide precipitation (Cr(OH)3 and CrxFe1−x(OH)3). Wang et al. [37] found that Cr(OH)3 and CrxFe1−x(OH)3 were disintegrated and desorbed Cr(III) under strong acid conditions. Therefore, the Cr(OH)3 and CrxFe1−x(OH)3 were disintegrated and desorbed Cr(III) in the SBET test. For the CaS5-stabilized soil, the elemental sulfur precipitated and ettringite formed by the reaction of CaS5 with Cr(VI) [10]. Sulfur and ettringite were the key compounds responsible for Cr(III) immobilization [38]. Zhou et al. [38] suggested that the ettringite increases Cr(III) uptake into the matrix, making it difficult for Cr(III) to be released from the soils during the leaching test. Chrysochoou et al. [10] and Graham et al. [39] found that Cr(III) was bound to sulfides or adsorbed. Jacobs et al. [40] found that the leachability of Cr(III) was strongly dependent on solution pH, soil pH, and redox potential. The Cr solubility increased at low pH values, specifically at pH values below 6, because amorphous Cr(OH)3 dissolves to form the soluble + 2+ chromium hydroxide cations Cr(OH)3, Cr(OH)2 , and Cr(OH) . The lowest leachability of Cr was observed between pH 6 and 11. Insoluble and amorphous Cr(OH)3 forms were observed at a pH of approximately 8.0 and under reducing (-Eh) conditions. Compared with TCLP, the SBET method uses extreme conditions, a more aggressive extracting agent (pH 1.5), and a higher liquid-to-soil ratio (100:1) to simulate the gastrointestinal environment [41,42]. Cr(VI)of the treatment soil (mg/kg) Cont ent(mg/g) Appl. Sci. 2018, 8, 1431 8 of 11 4.7. Probable Immobilization Mechanism of Cr The XRD results of the Cr(VI)-contaminated soil, FeSO -stabilized soil, and CaS -stabilized 4 5 soil are shown in Figure 7. In Cr(VI)-contaminated soil, illite, quartz, albite, and calcite were identified as the major phases. In the FeSO -stabilized soil, ferric hydroxide (Fe(OH) ) and 4 3 chromium hydroxide (Cr(OH) ) were identified. For the CaS -stabilized soil, sulfur (S) and ettringite 3 5 (Ca Al (OH) (SO ) .26H O) were identified. The differences in the leachability, bioaccessibility, and 6 2 12 4 3 2 speciation distribution of Cr in the FeSO - and CaS -stabilized soils can be attributed to (1) the different 4 5 hydration products in the FeSO - and CaS -stabilized soils, and (2) the different pH conditions and 4 5 Eh in the FeSO - and CaS -stabilized soils. Kostarelos et al. [36] suggested that Cr(VI) was reduced 4 5 to Cr(III) by FeSO and formed Cr(III)-Fe(III) hydroxide precipitation (Cr(OH) and Cr Fe (OH) ). 4 3 x 1x 3 Wang et al. [37] found that Cr(OH) and Cr Fe (OH) were disintegrated and desorbed Cr(III) under 3 x 1x 3 strong acid conditions. Therefore, the Cr(OH) and Cr Fe (OH) were disintegrated and desorbed 3 x 1x 3 Cr(III) in the SBET test. For the CaS -stabilized soil, the elemental sulfur precipitated and ettringite formed by the reaction of CaS with Cr(VI) [10]. Sulfur and ettringite were the key compounds responsible for Cr(III) immobilization [38]. Zhou et al. [38] suggested that the ettringite increases Cr(III) uptake into the matrix, making it difficult for Cr(III) to be released from the soils during the leaching test. Chrysochoou et al. [10] and Graham et al. [39] found that Cr(III) was bound to sulfides or adsorbed. Jacobs et al. [40] found that the leachability of Cr(III) was strongly dependent on solution pH, soil pH, and redox potential. The Cr solubility increased at low pH values, specifically at pH values below 6, because amorphous Cr(OH) dissolves to form the soluble chromium hydroxide + 2+ cations Cr(OH) , Cr(OH) , and Cr(OH) . The lowest leachability of Cr was observed between 3 2 pH 6 and 11. Insoluble and amorphous Cr(OH) forms were observed at a pH of approximately 8.0 and under reducing (-Eh) conditions. Compared with TCLP, the SBET method uses extreme conditions, a more aggressive extracting agent (pH 1.5), and a higher liquid-to-soil ratio (100:1) to simulate the gastrointestinal environment [41,42]. Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 11 10,000 1:Illite 2:Quartz 3:Albite 4:Calcite 5:Chromium hydroxide 6:Ferric hydroxide 8,000 7:Ettringite 8: Sulfur 6,000 4,000 CaS5 stabilized soils 2,000 FeSO4 stabilized soils Chromium contaminated soil 5 1015 2025 30 3540 45 50 2theta Figure 7. The XRD patterns of the Cr(VI)-contaminated soil, FeSO4-stabilized soil, and CaS5-stabilized Figure 7. The XRD patterns of the Cr(VI)-contaminated soil, FeSO -stabilized soil, and CaS -stabilized soil. 4 5 soil. 4.8. Conclusions 4.8. Conclusions This study compared the leachability and stability of FeSO - and CaS -stabilized Cr(VI)- 4 5 This study compared the leachability and stability of FeSO4- and CaS5-stabilized Cr(VI)- contaminated soils. A series of toxicity characteristic leaching procedure (TCLP), simplified contaminated soils. A series of toxicity characteristic leaching procedure (TCLP), simplified bioaccessibility extraction (SBET), alkaline digestion, sequential extraction, and X-ray diffraction bioaccessibility extraction (SBET), alkaline digestion, sequential extraction, and X-ray diffraction (XRD) tests were performed on Cr(VI)-contaminated soil. The influence of reductant dosage on the leachability and stability was assessed. The following conclusions can be drawn: 1. The pH and redox potential of the CaS5-stabilized soils were better than those of the FeSO4- stabilized soils regardless of CaS5 dosage. The concentrations of Cr(VI) and Cr leached from the stabilized soils with FeSO4 were larger than those of CaS5 at the same dosage. The Cr(VI) content in the stabilized soils was decreased with the increase in FeSO4 and CaS5 dosages, and that in the CaS5-stabilized soils decreased more noticeably compared with that in the FeSO4-stabilized soils at the same dosage. This finding reflected that CaS5 presented a better effect than FeSO4 in the stabilization of Cr(VI) and Cr. 2. The leached Cr(VI)/Cr from the SBET leaching test was considerably larger than that from TCLP leaching due to the difference in the stabilization mechanism and the pH of the leaching solutions. The bioaccessibility risk of Cr in the FeSO4-stabilized soils was higher than that in the CaS5-stabilized soils due to the difference in stabilization mechanisms of Cr(VI) between FeSO4 and CaS5. 3. The differences in the leachability, bioaccessibility, and toxicity of Cr(VI) and Cr in the FeSO4- and CaS5-stabilized soils were attributed to the changes in mineral composition. For the FeSO4- stabilized soil, the Cr(VI) was mainly converted to Cr(OH)3 and CrxFe1−x(OH)3. For the CaS5- stabilized soil, the Cr(VI) was reducted to Cr(III) and formed ettringite and sulfur. The Cr(III) was retained in the crystal structures of ettringite and sulfur through anion exchange. Author Contributions: T.-T.Z. and Q.X. conceived, designed and performed the experients and wrote the paper; M.-L.W. analyzed the data. Funding: This research was funded by [National Science Foundation for Distinguished Young Scholars of China] grant number [51625903] and [Chinese National Natural Science Foundation] grant number [51479194,41702349] And The APC was funded by [41702349]. Conflicts of Interest: The authors declare no conflicts of interest. Intensity Appl. Sci. 2018, 8, 1431 9 of 11 (XRD) tests were performed on Cr(VI)-contaminated soil. The influence of reductant dosage on the leachability and stability was assessed. The following conclusions can be drawn: 1. The pH and redox potential of the CaS -stabilized soils were better than those of the FeSO - 5 4 stabilized soils regardless of CaS dosage. The concentrations of Cr(VI) and Cr leached from the stabilized soils with FeSO were larger than those of CaS at the same dosage. The Cr(VI) content 4 5 in the stabilized soils was decreased with the increase in FeSO and CaS dosages, and that in the 4 5 CaS -stabilized soils decreased more noticeably compared with that in the FeSO -stabilized soils 5 4 at the same dosage. This finding reflected that CaS presented a better effect than FeSO in the 5 4 stabilization of Cr(VI) and Cr. 2. The leached Cr(VI)/Cr from the SBET leaching test was considerably larger than that from TCLP leaching due to the difference in the stabilization mechanism and the pH of the leaching solutions. The bioaccessibility risk of Cr in the FeSO -stabilized soils was higher than that in the CaS -stabilized soils due to the difference in stabilization mechanisms of Cr(VI) between FeSO 5 4 and CaS . 3. The differences in the leachability, bioaccessibility, and toxicity of Cr(VI) and Cr in the FeSO - and CaS -stabilized soils were attributed to the changes in mineral composition. For the FeSO -stabilized soil, the Cr(VI) was mainly converted to Cr(OH) and Cr Fe (OH) . For the 4 3 1x 3 CaS -stabilized soil, the Cr(VI) was reducted to Cr(III) and formed ettringite and sulfur. The Cr(III) was retained in the crystal structures of ettringite and sulfur through anion exchange. Author Contributions: T.-T.Z. and Q.X. conceived, designed and performed the experients and wrote the paper; M.-L.W. analyzed the data. Funding: This research was funded by [National Science Foundation for Distinguished Young Scholars of China] grant number [51625903] and [Chinese National Natural Science Foundation] grant number [51479194,41702349] And The APC was funded by [41702349]. Conflicts of Interest: The authors declare no conflicts of interest. References 1. Yin, W.; Li, Y.; Wu, J.; Chen, G.; Jiang, G.; Li, P.; Gu, J.; Liang, H.; Liu, C. Enhanced Cr (VI) removal from groundwater by Fe -H O system with bio-amended iron corrosion. J. Hazard. Mater. 2017, 332, 42–50. 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Immobilization and phytotoxicity of chromium in contaminated soil remediated by CMC-stabilized nZVI. J. Hazard. Mater. 2014, 275, 230–237. [CrossRef] [PubMed] 21. Du, J.; Lu, J.; Wu, Q.; Jing, C. Reduction and immobilization of chromate in chromite ore processing residue with nanoscale zero-valent iron. J. Hazard. Mater. 2012, 215, 152–158. [CrossRef] [PubMed] 22. Xu, W.; Li, X.; Zhou, Q.; Peng, Z.; Liu, G.; Qi, T. Remediation of chromite ore processing residue by hydrothermal process with starch. Process Saf. Environ. Prot. 2011, 89, 179–185. [CrossRef] 23. Jagupilla, S.C.; Moon, D.H.; Wazne, M.; Christodoulatos, C.; Kim, M.G. Effects of particle size and acid addition on the remediation of chromite ore processing residue using ferrous sulfate. J. Hazard. Mater. 2009, 168, 121–128. [CrossRef] [PubMed] 24. Liu, C.; Evett, J.B. Soil Properties, Testing, Measurement, and Evaluation, 5th ed.; Prentice-Hall: New York, NY, USA, 2002. 25. 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Influence of soil geochemical and physical properties on chromium (VI) sorption and bioaccessibility. Environ. Sci. Technol. 2013, 47, 11241–11248. [CrossRef] [PubMed] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Applied Sciences Multidisciplinary Digital Publishing Institute

Leachability and Stability of Hexavalent-Chromium-Contaminated Soil Stabilized by Ferrous Sulfate and Calcium Polysulfide

Zhang, Ting-Ting; Xue, Qiang; Wei, Ming-Li
Applied Sciences , Volume 8 (9) – Aug 22, 2018
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applied sciences Article Leachability and Stability of Hexavalent-Chromium-Contaminated Soil Stabilized by Ferrous Sulfate and Calcium Polysulfide 1 , 2 1 , 3 , 1 , 3 Ting-Ting Zhang , Qiang Xue * and Ming-Li Wei State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Bayi Road, Wuchang District, Wuhan 430071, China; ztt_cersm@163.com (T.-T.Z.); weimingli830716@sina.com (M.-L.W.) University of Chinese Academy of Sciences, Beijing 100049, China Hubei Key Laboratory of Contaminated Clay Science & Engineering, Wuhan 430071, China * Correspondence: qiangx@whrsm.ac.cn Received: 16 July 2018; Accepted: 19 August 2018; Published: 22 August 2018 Abstract: Ferrous sulfate (FeSO ) and calcium polysulfide (CaS ) stabilization are practical 4 5 approaches to stabilizing hexavalent chromium (Cr(VI))-contaminated soil. The leachability and stability of Cr(VI) and Cr are important factors affecting the effectiveness of stabilized Cr(VI)-contaminated soil. This study compared the leachability and stability of Cr(VI) and Cr in Cr(VI)-contaminated soil stabilized by using FeSO and CaS . The contaminated soil was 4 5 characterized before and after stabilization, and the effectiveness of FeSO and CaS stabilization 4 5 was assessed using leaching, bioaccessibility, alkaline digestion, sequential extraction, and X-ray diffraction tests. Results showed that FeSO and CaS significantly reduced the leachability and 4 5 Cr(VI) content in the contaminated soil. The acid-buffering capacity and stability (leachability, bioaccessibility, speciation distribution, and mineral composition) of the Cr(VI)/Cr and Cr(VI) content of CaS were better than those of FeSO . This study demonstrated that CaS had a better 5 4 5 effect than FeSO on the stabilization of Cr(VI) in Cr(VI)-contaminated soil. The CaS significantly 4 5 enhanced the stabilization and immobilization of Cr(VI) and reduced its leachability and toxicity. Keywords: hexavalent chromium; contaminated soil; leachability; stability; speciation 1. Introduction Soil contamination by chromium (Cr) is a serious problem in China [1–3]. Cr is released into the soil by various industries, including the wood preservation, leather tanning, chromate manufacturing, and electroplating industries [4]. Cr in soil occurs primarily in its Cr(III) and Cr(VI) redox states; Cr(III) is a nutrient for plant growth, whereas Cr(VI) is a dangerous species and human carcinogen [5]. Chemical reduction removes Cr(VI) rapidly and effectively based on the use of reducing agents, such as ferrous sulfate, calcium polysulfide, or sodium bisulfate, followed by precipitation as Cr(OH) [6]. Calcium polysulfide (CaS ) and ferrous sulfate (FeSO ) are promising reagents that have been used at 5 4 many Cr-contaminated sites and for chromite ore-processing residue (COPR). The reduction of Cr(VI) with FeSO and CaS (denoted by its average chemical formula, CaS ) can be written as follows [6,7]: 4 5 5 2+ + 3+ 3+ 3Fe + HCrO + 7H $3Fe + Cr + 4H O (1) 4 2 2 + 2+ 2CrO + 3CaS + 10H $2Cr(OH) + 15S + 3Ca + 2H O. (2) 4 5 3 2 Many studies have been performed on Cr(VI)-contaminated soils stabilized by FeSO and CaS . 4 5 However, most of them focused on the leachability and content of Cr(VI) and Cr. Palma et al. [8] Appl. Sci. 2018, 8, 1431; doi:10.3390/app8091431 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 1431 2 of 11 applied FeSO to reduce Cr(VI) at a contaminated industrial soil site in Italy and found that FeSO 4 4 successfully lowers the amount of Cr(VI) in the soil. An alkaline digestion test showed that Cr(VI) is almost completely reduced when the Fe(II)/Cr(VI) molar ratio is 30. John et al. [9] reported the use of FeSO to treat Cr(VI)-contaminated soil through a column treatment. The Cr(VI) concentrations in the leachate after a toxicity characteristic leaching procedure test (TCLP) range between 0.59 and 0.7 mg/L. Buerge et al. [10] found that FeSO is a useful treatment reagent in the in-situ remediation of Cr(VI)-contaminated soils in Switzerland. Chrysochoou et al. [11] performed a column treatment of Cr(VI)-contaminated soil treated with CaS and found that up to 99% Cr(VI) was reduced with an injection of CaS at 12 times the stoichiometric requirement. Wazne et al. [12] reported that 62% of Cr(VI) is reduced in COPR with CaS addition at twice the stoichiometric ratio. Chrysochoou et al. [13] applied CaS in batch studies of highly contaminated soil from a Cr plating facility in Putnam. Redox potential results showed that CaS maintains a highly reducing environment for a prolonged period of time and that the alkaline digestion and synthetic precipitation leaching procedure concentrations are lower than Environmental Protection Agency (EPA) regulatory levels. Although these applications indicated that FeSO and CaS are effective reductants, insufficient 4 5 information is available about the difference between the FeSO and CaS remediation of Cr(VI) 4 5 in contaminated soil. Moreover, most works only used the TCLP or alkaline digestion test, and systematic investigations about the remediation capacities of FeSO and CaS on Cr(VI), based on 4 5 the bioaccessibility and speciation of Cr, are lacking. The toxicity and mobility of heavy metals in soil are not only related to their total content but are also determined to a greater degree by the distribution of their speciation [14]. Zimmerman et al. [15] demonstrated that the availability and extraction effectiveness of heavy metals in soil decrease in the order of acid soluble forms > reducible forms > oxidizable forms > residual forms. This study compared the leachability and stability of Cr(VI)-contaminated soils stabilized by FeSO and CaS . Toxicity characteristic leaching procedure (TCLP), simplified bioaccessibility 4 5 extraction test (SBET), alkaline digestion, sequential extraction, and X-ray diffraction (XRD) tests were performed on Cr(VI)-contaminated soils. This study can serve as a basis for designing the remediation of Cr(VI)-contaminated soils by using FeSO and CaS . 4 5 2. Materials and Methods 2.1. Cr(VI) Contaminated Soil Preparation The raw soil was collected from a subway excavation site in Wuhan City (China). The soil was dried, ground, and then sieved through a 2-mm screen. The detailed description of the physical characterization of the raw soil and Cr(VI)-contaminated soil are presented in Table 1, which was obtained according to the “Standard for soil test method” of China. The Light Proctor compaction method was used for the compaction test [16,17]. Cr(VI)-contaminated soils were obtained by adding K Cr O solution until the Cr(VI) content in the soil reached 1000 mg/kg, which represents 2 2 7 a universal content for Cr(VI)-contaminated soil in China [18–22]. Deionized water was then added to the contaminated soil until the water content reached 19.5% (optimum moisture content). The contaminated soil was mixed evenly and braised for 180 days under standard curing conditions (20  2 C, 95% humidity) to allow K Cr O and the soil to react adequately. After homogenization, 2 2 7 the contaminated soil was air dried and pulverized to achieve the required particle size (<2 mm). The entire quantity of soil was made to pass through the sieve to avoid any fractionation [23]. All reagents in this study were supplied from Sinopharm Chemical Reagent Co., Ltd. (Ningbo, China) and used as American Chemical Society-certified reagents without any further purification. Appl. Sci. 2018, 8, 1431 3 of 11 Table 1. Physicochemical and mechanical properties of raw soil and chromium-contaminated soil. Items Raw Soil Chromium-Contaminated Soil Water content/% 20.78 — pH 8.53 7.76 Specific gravity 2.72 2.79 Physicochemical properties Liquid limit/% 41.63 40.18 Plastic limit/% 21.84 21.33 Mn (mg/kg) 798.36 797.48 C.E.C (meq/100 g) 9.12 9.87 Optimum moisture content/% 19.53 18.95 Mechanical properties 1.72 1.73 Maximum dry density/(g/cm ) Brunauer-Emmett-Teller specific 30.74 29.62 surface Area (m /g) Clay content (<0.005 mm) 4.62 3.23 Grain-size distribution (%) Silt content (0.005–0.075 mm) 74.29 71.76 Sand content (0.075–2 mm) 21.09 25.01 Al O 22.12 21.67 2 3 SiO 64.2 64.37 K O 2.78 2.85 CaO 1.43 1.42 Chemical composition (%) TiO 0.84 0.86 MnO 0.12 0.13 Fe O 8.51 8.59 2 3 Cr O — 0.11 2 3 2.2. Stabilized of Cr(VI) Contaminated Soil Representative 500 g of air-dried Cr(VI)-contaminated soil were introduced into a 10 L SPAR type mixer. FeSO and CaS were added to the Cr-contaminated soil as reductant to a dry soil ratio 4 5 of 1%, 3%, and 5%. The experimental design is presented in Table 2. The soil was homogenized for 10 min prior to the addition of distilled water. It was ensured that the ratio of addition of water to the reductant and dry soil was 1:2. The mixture was withdrawn from the sealed plastic bottles after being incubated for 7 d at room temperature (20  1 C). All the samples were prepared in triplicate. The reported stability results are the averages of three replicates. Table 2. Experimental design for the stability study. Test No. Reductant Dosage (%) 1 FeSO 0 2 FeSO 1 3 FeSO 3 4 FeSO 5 5 CaS 0 6 CaS 1 7 CaS 3 8 CaS 5 3. Test Methods The soil cation exchange capacity (CEC) and MnO content were determined using standard methods [24]. Soil acid digestion was performed to determine the Cr and Mn content in soil according to EPA Method 3050B [25]. Nitrogen adsorption-desorption measurements were determined by a surface area analyzer (Nova 1000e, Quantachrome Instruments, USA). The chemical composition of the samples was measured by an X-ray fluorescence (XRF) scan. The size distribution of the waste particles was measured using a Malvern MS3000 laser diffraction particle size analyzer. pH values for all soil were measured as per ASTMD4972-01 [26]. The toxicity characteristic leaching procedure (TCLP) of Cr was conducted as per USEPA Method 1311 [27].The bioaccessibility test was performed according to the U.S. EPA (2007) protocol [28] and the British Geological Survey [29]. The Cr(VI) content of contaminated soil was measured using the USEPA Method 3060A alkaline digestion method [30]. Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 11 Table 2. Experimental design for the stability study. Test No. Reductant Dosage (%) 1 FeSO4 0 2 FeSO4 1 Appl. Sci. 2018, 8, 1431 4 of 11 3 FeSO4 3 4 FeSO4 5 5 CaS5 0 The Cr(VI) concentration in the filtrate was measured using U.S. EPA Method 7196A colorimetric 6 CaS5 1 analyses [31]. The modified European Community Bureau of Reference (BCR) sequential extraction 7 CaS5 3 procedure was conducted as per the method recommended by Rauret et al. [32]. The sequential 8 CaS5 5 extraction procedure consisted of four steps, which corresponded to the exchangeable, reducible, oxidizable, and residue fractions. 4. Results and Discussion 4. Results and Discussion 4.1. pH of Stabilized Soils 4.1. pH of Stabilized Soils Figure 1 shows the pH of the stabilized soils with different FeSO4 and CaS5 dosages. The results indicated that CaS5 addition can increase the pH of the stabilized soil, which was contrary to the effect Figure 1 shows the pH of the stabilized soils with different FeSO and CaS dosages. The results 4 5 of FeSO4 addition. For illustration, the pH of the FeSO4 stabilized soil decreased from 7.86 to 2.62 indicated that CaS addition can increase the pH of the stabilized soil, which was contrary to the when the reductant dosage increased from 0% to 5%. However, the pH of the CaS5 stabilized soil effect of FeSO addition. For illustration, the pH of the FeSO stabilized soil decreased from 7.86 to 4 4 increased from 7.86 to 9.57. The changes in the FeSO4 and CaS5 tread soil were attributed to the 2.62 when the reductant dosage increased from 0% to 5%. However, the pH of the CaS stabilized different stabilization mechanisms in the Cr(VI)-contaminated soils. For the FeSO4 stabilized soil, soil increased from 7.86 to 9.57. The changes in the FeSO and CaS tread soil were attributed to the 4 5 Cr(VI) was different stabilization reducted by mechanisms Fe(II) and in the formed Fe Cr(VI)-contaminated (III) hydroxide precipitati soils. For the FeSO on anstabilized d released a soil, Cr(VI) was reducted by Fe(II) and formed Fe(III) hydroxide precipitation and released a considerably considerably greater amount of H [33]. The alkalinity of the CaS5 stabilized soils can be attributed to the f greater aamount ct that Ca of S5H is an [33 a]. lkal The ine alkalinity material [of 13the ]. This CaS result stabilized indicatsoils es that can CaS be5 was attributed more adv to the antageous fact that CaS is an alkaline material [13]. This result indicates that CaS was more advantageous than FeSO in than FeSO4 in which stabilized soil pH can reach values of 9.57 or higher, which increases the acid- 5 5 4 buffe which ring cap stabilized acity. soil pH can reach values of 9.57 or higher, which increases the acid-buffering capacity. Untreated FeSO CaS 4 5 0 1 5 Dosage of reductant(%) Figure 1. Effect of reductant types and dosage on the pH of stabilized soils. Figure 1. Effect of reductant types and dosage on the pH of stabilized soils. 4 4.2. .2. Red Redox ox Potential of Stabilized Potential of Stabilized Soils Soils The redox pot The redox potentials entials of t of the he st stabilized abilized soils are pr soils are presented in Figure esented in Figure 2. The result 2. The results s indicat indicated ed tthat hat addition of FeSO addition of FeSO4 and C and CaS aS5 could decrease t could decrease the he redox pot redox potential ential of of st stabilized abilized soil. T soil. This his phenomenon ca phenomenon can n 4 5 be a be attributed ttributed tto o the f the fact act tha that t Cr( Cr(VI) VI) wa was s redu reduced ced wi with th i incr ncrea easing sing F FeSO eSO4 and C and CaS aS5 dosages. Figure dosages. Figure 2 also 2 also 4 5 shows shows tha that t th the e redo redox x p potential otential of of the the F FeSO eSO4-st -stabilized abilized soil was higher than soil was higher than t that hat of t of the he CaS CaS 5-st -stabilized abilized 4 5 soil with the soil with the same reductant dosage same reductant dosage. . Wh When en tthe he rre eductant ductant d dosage osage in incr creased from eased from 0% 0%tto o 5%, 5%, the redox the redox pot potential ential in in t the he FeSO FeSO4-st -stabilized abilized soil was soil was decreased from decreased from 530.5 m 530.5 mv v to to −96.3 m 96.3 mv v. For t . For the he C CaS aS5-st -stabilized abilized 4 5 soil, the redox potential was decreased from 530.5 mv to 390.6 mv. Compared with FeSO -stabilized soil, CaS could maintain a reducing environment in the stabilized soil. pH of stabilized soil Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 11 soil, the redox potential was decreased from 530.5 mv to −390.6 mv. Compared with FeSO4-stabilized Appl. Sci. 2018, 8, 1431 5 of 11 soil, CaS5 could maintain a reducing environment in the stabilized soil. 0.6 Untreated FeSO CaS 0.5 4 5 0.4 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.5 0 1 3 5 Dosage of reductant(%) Figure 2. Effect of reductant types and dosage on the redox potential of stabilized soils. Figure 2. Effect of reductant types and dosage on the redox potential of stabilized soils. 4.3. Leachability of Cr/Cr(VI) from Contaminated Soil in TCLP Leaching 4.3. Leachability of Cr/Cr(VI) from Contaminated Soil in TCLP Leaching Figure 3 shows the Cr(VI) and Cr concentrations of the TCLP leachate. The Cr(VI) and Cr Figure 3 shows the Cr(VI) and Cr concentrations of the TCLP leachate. The Cr(VI) and Cr concentrations decreased with the increase of reductant addition. For the untreated soil, the Cr(VI) concentrations decreased with the increase of reductant addition. For the untreated soil, the Cr(VI) and total Cr leaching concentrations were approximately 38.8 mg/L and 40.4 mg/L, respectively, and total Cr leaching concentrations were approximately 38.8 mg/L and 40.4 mg/L, respectively, which exceeds the regulatory limit in the standards for hazardous wastes in China [34]. For the FeSO4- which exceeds the regulatory limit in the standards for hazardous wastes in China [34]. For the stabilized soil, the leached Cr(VI) and Cr concentrations decreased from 6.4 mg/L and 12.6 mg/L to FeSO0.-stabilized 62 mg/L and 2. soil, 6 mthe g/L, r leached espectiveCr(VI) ly, when t and he red Cr uconcentrations ctant dosage wasdecr increa eased sed from from 1% t 6.4 o 5%. mg/L For and the CaS5-stabilized soil, the leached Cr(VI) and Cr concentrations decreased from 4.6 mg/L and 7.4 12.6 mg/L to 0.62 mg/L and 2.6 mg/L, respectively, when the reductant dosage was increased mg/L to 0.05 mg/L and 0.86 mg/L, respectively. In addition, with the same reductant dosage, the from 1% to 5%. For the CaS -stabilized soil, the leached Cr(VI) and Cr concentrations decreased leached Cr(VI) and Cr concentrations of the CaS5-stabilized soils decreased more noticeably relative from 4.6 mg/L and 7.4 mg/L to 0.05 mg/L and 0.86 mg/L, respectively. In addition, with the same to those of the FeSO4-stabilized soils. These results clearly demonstrate the lower leachability of Cr reductant dosage, the leached Cr(VI) and Cr concentrations of the CaS -stabilized soils decreased more in the CaS5-stabilized soil than in the FeSO4-stabilized soil under acidic solution conditions. This noticeably relative to those of the FeSO -stabilized soils. These results clearly demonstrate the lower result agrees with most previous research results. [10,13]. leachability of Cr in the CaS -stabilized soil than in the FeSO -stabilized soil under acidic solution 5 4 conditions. This result agrees with most previous research results [10,13]. Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 11 FeSO4 stabilzed CaS5 stabilzed Cr(VI) Cr(T) 15mg/L,China regulatory limit 10 5mg/L,EPA regulatory limit 0.1 0 1 3 5 1 3 Dosage of reductant (%) Figure 3. Effect of reductant types and dosage on Cr/Cr(VI) concentration in the toxicity characteristic Figure 3. Effect of reductant types and dosage on Cr/Cr(VI) concentration in the toxicity characteristic leaching procedure (TCLP) leachate. EPA, Environmental Protection Agency. leaching procedure (TCLP) leachate. EPA, Environmental Protection Agency. 4.4. Bioaccessibility of Cr/Cr(VI) from Contaminated Soil As shown in Figure 4, the leached Cr(VI) and Cr concentrations in the SBET test were higher than those from the TCLP leaching method. In addition, the Cr(VI) concentrations decreased with the increase in FeSO4 and CaS5 dosage. The leached Cr(VI) of the CaS5-stabilized soils decreased more noticeably relative to that of the FeSO4-stabilized soils. For the FeSO4-stabilized soils, the Cr concentrations changed slightly (9.8–10.1 mg/L) regardless of the FeSO4 dosage, suggesting that the bioaccessibility risk of Cr was not reduced. The increased availability of Cr in the FeSO4-stabilized soils can be explained by the pH-dependent characteristics of Cr for the pH of the leachant used in the SBET test, which was 1.5. The leached Cr concentration of the CaS5-stabilized soil decreased from 5.4 mg/L to 0.9 mg/L when the CaS5 dosage was increased from 1% to 5%. This phenomenon indicated that CaS5 could notably reduce the bioaccessibility risk compared to FeSO4-stabilized soil. CaS5 treated FeSO4 treated Cr(VI) Cr(T) 0.1 0.01 0 3 5 13 5 Dosage of reductant (%) Figure 4. Effect of reductant types and dosage on Cr/Cr(VI) concentration in the simplified bioaccessibility extraction test (SBET) leachate. Leaching concentration(mg/L) Leaching concentration(mg/L) Eh(v) Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 11 FeSO4 stabilzed CaS5 stabilzed Cr(VI) Cr(T) 15mg/L,China regulatory limit 10 5mg/L,EPA regulatory limit 0.1 1 3 1 0 5 3 5 Dosage of reductant (%) Figure 3. Effect of reductant types and dosage on Cr/Cr(VI) concentration in the toxicity characteristic Appl. Sci. 2018, 8, 1431 6 of 11 leaching procedure (TCLP) leachate. EPA, Environmental Protection Agency. 4.4. 4.4. Bioaccessi Bioaccessibility bility of Cr/Cr( of Cr/Cr(VI) VI) from Contaminated Soil from Contaminated Soil As shown in Figure 4, the leached Cr(VI) and Cr concentrations in the SBET test were higher As shown in Figure 4, the leached Cr(VI) and Cr concentrations in the SBET test were higher than than those f those fr rom the TCLP l om the TCLP e leaching aching method. In a method. Indaddition, dition, the Cr( the Cr(VI) VI) concentra concentrations tions decreased with the decreased with increase in FeSO4 and CaS5 dosage. The leached Cr(VI) of the CaS5-stabilized soils decreased more the increase in FeSO and CaS dosage. The leached Cr(VI) of the CaS -stabilized soils decreased 4 5 5 mor noticea e noticeably bly relative to that of relative to that the FeSO of the FeSO 4-stabilized -stabilized soils soils. . FoFor r ththe e Fe FeSO SO4-s-stabilized tabilized so soils, ils, tthe he Cr Cr 4 4 concentrations changed slightly (9.8–10.1 mg/L) regardless of the FeSO4 dosage, suggesting that the concentrations changed slightly (9.8–10.1 mg/L) regardless of the FeSO dosage, suggesting that the bioaccessibility bioaccessibility risk o risk of f Cr Cr was not was not reduced. reduced. The incr The increased ease availability d availabilit ofy o Crfin Cr in the FeSO the FeSO -stabilized 4-stabili soils zed soils can be explained by the pH-dependent characteristics of Cr for the pH of the leachant used in can be explained by the pH-dependent characteristics of Cr for the pH of the leachant used in the SBET the SBET test, which was 1.5. The leached Cr concentration of the CaS5-stabilized soil decreased from test, which was 1.5. The leached Cr concentration of the CaS -stabilized soil decreased from 5.4 mg/L 5.4 mg/L to 0.9 mg/L when the CaS5 dosage was increased from 1% to 5%. This phenomenon indicated to 0.9 mg/L when the CaS dosage was increased from 1% to 5%. This phenomenon indicated that that CaS5 could notably reduce the bioaccessibility risk compared to FeSO4-stabilized soil. CaS could notably reduce the bioaccessibility risk compared to FeSO -stabilized soil. 5 4 CaS5 treated FeSO4 treated Cr(VI) Cr(T) 0.1 0.01 0 1 3 5 13 5 Dosage of reductant (%) Figure 4. Effect of reductant types and dosage on Cr/Cr(VI) concentration in the simplified Figure 4. Effect of reductant types and dosage on Cr/Cr(VI) concentration in the simplified bioaccessibility extraction test (SBET) leachate. bioaccessibility extraction test (SBET) leachate. 4.5. Cr(VI) Content in Soils before and after Stabilization The Cr(VI) contents of the stabilized soils are shown in Figure 5. For the untreated soil, the Cr(VI) content was approximately 971.3 mg/kg. After FeSO and CaS stabilization, the Cr(VI) content in the 4 5 stabilized soils was reduced significantly with the increase of reductant dosage. This phenomenon can be attributed to the fact that Cr(VI) was reduced as the FeSO and CaS dosage increased. 4 5 Figure 5 also shows that the Cr(VI) content of the FeSO -stabilized soil was higher than that of the CaS -stabilized soil with the same reductant dosage. When the reductant dosage increased from 1% to 3%, the Cr(VI) content in the FeSO -stabilized soil was decreased from 168 mg/kg to 58 mg/kg, whereas that in the CaS -stabilized soil was decreased from 94 mg/kg to 22 mg/kg. The Cr(VI) content in the CaS -stabilized soil with 3% dosage was below the threshold allowed by China’s Environmental Regulations for industrial reuse (<30 mg/kg) [35]. Similarly, the Cr(VI) content in the FeSO -stabilized soil with 5% dosage was 21 mg/kg higher than that in the CaS -stabilized soil 4 5 (4.2 mg/kg). The Cr(VI) content in the CaS -stabilized soil with 5% dosage was below the threshold of civil reuse (<5 mg/kg) [35]. CaS presented a better effect than FeSO in the stabilization of Cr(VI). 5 4 Leaching concentration(mg/L) Leaching concentration(mg/L) Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 11 4.5. Cr(VI) Content in Soils before and after Stabilization The Cr(VI) contents of the stabilized soils are shown in Figure 5. For the untreated soil, the Cr(VI) content was approximately 971.3 mg/kg. After FeSO4 and CaS5 stabilization, the Cr(VI) content in the stabilized soils was reduced significantly with the increase of reductant dosage. This phenomenon can be attributed to the fact that Cr(VI) was reduced as the FeSO4 and CaS5 dosage increased. Figure 5 also shows that the Cr(VI) content of the FeSO4-stabilized soil was higher than that of the CaS5-stabilized soil with the same reductant dosage. When the reductant dosage increased from 1% to 3%, the Cr(VI) content in the FeSO4-stabilized soil was decreased from 168 mg/kg to 58 mg/kg, whereas that in the CaS5-stabilized soil was decreased from 94 mg/kg to 22 mg/kg. The Cr(VI) content in the CaS5-stabilized soil with 3% dosage was below the threshold allowed by China’s Environmental Regulations for industrial reuse (<30 mg/kg) [35]. Similarly, the Cr(VI) content in the FeSO4-stabilized soil with 5% dosage was 21 mg/kg higher than that in the CaS5-stabilized soil (4.2 mg/kg). The Cr(VI) content in the CaS5-stabilized soil with 5% dosage was below the threshold of Appl. Sci. 2018, 8, 1431 7 of 11 civil reuse (<5 mg/kg) [35]. CaS5 presented a better effect than FeSO4 in the stabilization of Cr(VI). Untreated FeSO CaS 4 5 30mg/kg,China industrial reuse regulatory limit 5mg/kg,China civilreuse regulatory limit 0 1 3 Dosage of reductant(%) Figure 5. Effect of reductant types and dosage on Cr(VI) content of stabilized soils. Figure 5. Effect of reductant types and dosage on Cr(VI) content of stabilized soils. 4.6. Species Distribution of Cr in Soils before and after Stabilization 4.6. Species Distribution of Cr in Soils before and after Stabilization Figure 6 shows the changes in the Cr speciation distribution of the untreated and stabilized soils. Figure 6 shows the changes in the Cr speciation distribution of the untreated and stabilized The primary Cr species for the untreated soil were mainly distributed in the exchangeable content (0.82 soils. The primary Cr species for the untreated soil were mainly distributed in the exchangeable mg/g), and the reducible, oxidizable, and residual contents were 0.075 mg/g, 0.074 mg/g, and 0.0026 content mg/g, respectively. (0.82 mg/g), andThes thee results reducible, indica oxidizable, ted that and Cr was more residual contents mobile and were to0.075 xic in the untreat mg/g, 0.074ed mg/g, contaminated soil. The Cr speciation of stabilized soil was changed significantly. For the FeSO4- and 0.0026 mg/g, respectively. These results indicated that Cr was more mobile and toxic in the stabilized soil, the exchangeable fraction was mainly converted to reducible fraction, which was untreated contaminated soil. The Cr speciation of stabilized soil was changed significantly. For the increased to 0.76 mg/g, when the FeSO4 dosage was increased from 0% to 5%. For the CaS5-treated soil, FeSO -stabilized soil, the exchangeable fraction was mainly converted to reducible fraction, which was the exchangeable fraction was mainly converted to oxidizable fraction. The oxidizable fraction of Cr in increased to 0.76 mg/g, when the FeSO dosage was increased from 0% to 5%. For the CaS -treated 4 5 the CaS5-stabilized soil with 3% dosage was 0.40 mg/g higher than that in the FeSO4-stabilized soil (see soil, the exchangeable fraction was mainly converted to oxidizable fraction. The oxidizable fraction of Figure 6), Similarly, the oxidizable fraction of Cr in the CaS5-stabilized soil with 5% dosage was 0.61 Cr in the CaS -stabilized soil with 3% dosage was 0.40 mg/g higher than that in the FeSO -stabilized 5 4 mg/g higher than that in the FeSO4-stabilized soil. These results clearly demonstrate the better chemical soil (see Figure 6), Similarly, the oxidizable fraction of Cr in the CaS -stabilized soil with 5% dosage stability of Cr in the CaS5-stabilized soil compared to that in the FeSO4 stabilized soil. This distinctive was 0.61 mg/g higher than that in the FeSO -stabilized soil. These results clearly demonstrate the alteration in the Cr speciation of the CaS5-stabilized soil, especially the substantial increase in the oxidizable fraction, accounted for the reduced leachability and bioaccessibility of Cr. better chemical stability of Cr in the CaS -stabilized soil compared to that in the FeSO stabilized soil. 5 4 This distinctive alteration in the Cr speciation of the CaS -stabilized soil, especially the substantial Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 11 increase in the oxidizable fraction, accounted for the reduced leachability and bioaccessibility of Cr. 1.2 Exchangeable Reducible Oxidisable Redidual FeSO4 stabilized CaS5 stabilized 1.0 0.8 0.6 0.4 0.2 0.0 Untreated 3 5 3 5 Dosage of reductant (%) Figure 6. Effect of reductant types and dosage on Cr speciation distribution. Figure 6. Effect of reductant types and dosage on Cr speciation distribution. 4.7. Probable Immobilization Mechanism of Cr The XRD results of the Cr(VI)-contaminated soil, FeSO4-stabilized soil, and CaS5-stabilized soil are shown in Figure 7. In Cr(VI)-contaminated soil, illite, quartz, albite, and calcite were identified as the major phases. In the FeSO4-stabilized soil, ferric hydroxide (Fe(OH)3) and chromium hydroxide (Cr(OH)3) were identified. For the CaS5-stabilized soil, sulfur (S) and ettringite (Ca6Al2(OH)12(SO4)3.26H2O) were identified. The differences in the leachability, bioaccessibility, and speciation distribution of Cr in the FeSO4- and CaS5-stabilized soils can be attributed to (1) the different hydration products in the FeSO4- and CaS5-stabilized soils, and (2) the different pH conditions and Eh in the FeSO4- and CaS5-stabilized soils. Kostarelos et al. [36] suggested that Cr(VI) was reduced to Cr(III) by FeSO4 and formed Cr(III)-Fe(III) hydroxide precipitation (Cr(OH)3 and CrxFe1−x(OH)3). Wang et al. [37] found that Cr(OH)3 and CrxFe1−x(OH)3 were disintegrated and desorbed Cr(III) under strong acid conditions. Therefore, the Cr(OH)3 and CrxFe1−x(OH)3 were disintegrated and desorbed Cr(III) in the SBET test. For the CaS5-stabilized soil, the elemental sulfur precipitated and ettringite formed by the reaction of CaS5 with Cr(VI) [10]. Sulfur and ettringite were the key compounds responsible for Cr(III) immobilization [38]. Zhou et al. [38] suggested that the ettringite increases Cr(III) uptake into the matrix, making it difficult for Cr(III) to be released from the soils during the leaching test. Chrysochoou et al. [10] and Graham et al. [39] found that Cr(III) was bound to sulfides or adsorbed. Jacobs et al. [40] found that the leachability of Cr(III) was strongly dependent on solution pH, soil pH, and redox potential. The Cr solubility increased at low pH values, specifically at pH values below 6, because amorphous Cr(OH)3 dissolves to form the soluble + 2+ chromium hydroxide cations Cr(OH)3, Cr(OH)2 , and Cr(OH) . The lowest leachability of Cr was observed between pH 6 and 11. Insoluble and amorphous Cr(OH)3 forms were observed at a pH of approximately 8.0 and under reducing (-Eh) conditions. Compared with TCLP, the SBET method uses extreme conditions, a more aggressive extracting agent (pH 1.5), and a higher liquid-to-soil ratio (100:1) to simulate the gastrointestinal environment [41,42]. Cr(VI)of the treatment soil (mg/kg) Cont ent(mg/g) Appl. Sci. 2018, 8, 1431 8 of 11 4.7. Probable Immobilization Mechanism of Cr The XRD results of the Cr(VI)-contaminated soil, FeSO -stabilized soil, and CaS -stabilized 4 5 soil are shown in Figure 7. In Cr(VI)-contaminated soil, illite, quartz, albite, and calcite were identified as the major phases. In the FeSO -stabilized soil, ferric hydroxide (Fe(OH) ) and 4 3 chromium hydroxide (Cr(OH) ) were identified. For the CaS -stabilized soil, sulfur (S) and ettringite 3 5 (Ca Al (OH) (SO ) .26H O) were identified. The differences in the leachability, bioaccessibility, and 6 2 12 4 3 2 speciation distribution of Cr in the FeSO - and CaS -stabilized soils can be attributed to (1) the different 4 5 hydration products in the FeSO - and CaS -stabilized soils, and (2) the different pH conditions and 4 5 Eh in the FeSO - and CaS -stabilized soils. Kostarelos et al. [36] suggested that Cr(VI) was reduced 4 5 to Cr(III) by FeSO and formed Cr(III)-Fe(III) hydroxide precipitation (Cr(OH) and Cr Fe (OH) ). 4 3 x 1x 3 Wang et al. [37] found that Cr(OH) and Cr Fe (OH) were disintegrated and desorbed Cr(III) under 3 x 1x 3 strong acid conditions. Therefore, the Cr(OH) and Cr Fe (OH) were disintegrated and desorbed 3 x 1x 3 Cr(III) in the SBET test. For the CaS -stabilized soil, the elemental sulfur precipitated and ettringite formed by the reaction of CaS with Cr(VI) [10]. Sulfur and ettringite were the key compounds responsible for Cr(III) immobilization [38]. Zhou et al. [38] suggested that the ettringite increases Cr(III) uptake into the matrix, making it difficult for Cr(III) to be released from the soils during the leaching test. Chrysochoou et al. [10] and Graham et al. [39] found that Cr(III) was bound to sulfides or adsorbed. Jacobs et al. [40] found that the leachability of Cr(III) was strongly dependent on solution pH, soil pH, and redox potential. The Cr solubility increased at low pH values, specifically at pH values below 6, because amorphous Cr(OH) dissolves to form the soluble chromium hydroxide + 2+ cations Cr(OH) , Cr(OH) , and Cr(OH) . The lowest leachability of Cr was observed between 3 2 pH 6 and 11. Insoluble and amorphous Cr(OH) forms were observed at a pH of approximately 8.0 and under reducing (-Eh) conditions. Compared with TCLP, the SBET method uses extreme conditions, a more aggressive extracting agent (pH 1.5), and a higher liquid-to-soil ratio (100:1) to simulate the gastrointestinal environment [41,42]. Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 11 10,000 1:Illite 2:Quartz 3:Albite 4:Calcite 5:Chromium hydroxide 6:Ferric hydroxide 8,000 7:Ettringite 8: Sulfur 6,000 4,000 CaS5 stabilized soils 2,000 FeSO4 stabilized soils Chromium contaminated soil 5 1015 2025 30 3540 45 50 2theta Figure 7. The XRD patterns of the Cr(VI)-contaminated soil, FeSO4-stabilized soil, and CaS5-stabilized Figure 7. The XRD patterns of the Cr(VI)-contaminated soil, FeSO -stabilized soil, and CaS -stabilized soil. 4 5 soil. 4.8. Conclusions 4.8. Conclusions This study compared the leachability and stability of FeSO - and CaS -stabilized Cr(VI)- 4 5 This study compared the leachability and stability of FeSO4- and CaS5-stabilized Cr(VI)- contaminated soils. A series of toxicity characteristic leaching procedure (TCLP), simplified contaminated soils. A series of toxicity characteristic leaching procedure (TCLP), simplified bioaccessibility extraction (SBET), alkaline digestion, sequential extraction, and X-ray diffraction bioaccessibility extraction (SBET), alkaline digestion, sequential extraction, and X-ray diffraction (XRD) tests were performed on Cr(VI)-contaminated soil. The influence of reductant dosage on the leachability and stability was assessed. The following conclusions can be drawn: 1. The pH and redox potential of the CaS5-stabilized soils were better than those of the FeSO4- stabilized soils regardless of CaS5 dosage. The concentrations of Cr(VI) and Cr leached from the stabilized soils with FeSO4 were larger than those of CaS5 at the same dosage. The Cr(VI) content in the stabilized soils was decreased with the increase in FeSO4 and CaS5 dosages, and that in the CaS5-stabilized soils decreased more noticeably compared with that in the FeSO4-stabilized soils at the same dosage. This finding reflected that CaS5 presented a better effect than FeSO4 in the stabilization of Cr(VI) and Cr. 2. The leached Cr(VI)/Cr from the SBET leaching test was considerably larger than that from TCLP leaching due to the difference in the stabilization mechanism and the pH of the leaching solutions. The bioaccessibility risk of Cr in the FeSO4-stabilized soils was higher than that in the CaS5-stabilized soils due to the difference in stabilization mechanisms of Cr(VI) between FeSO4 and CaS5. 3. The differences in the leachability, bioaccessibility, and toxicity of Cr(VI) and Cr in the FeSO4- and CaS5-stabilized soils were attributed to the changes in mineral composition. For the FeSO4- stabilized soil, the Cr(VI) was mainly converted to Cr(OH)3 and CrxFe1−x(OH)3. For the CaS5- stabilized soil, the Cr(VI) was reducted to Cr(III) and formed ettringite and sulfur. The Cr(III) was retained in the crystal structures of ettringite and sulfur through anion exchange. Author Contributions: T.-T.Z. and Q.X. conceived, designed and performed the experients and wrote the paper; M.-L.W. analyzed the data. Funding: This research was funded by [National Science Foundation for Distinguished Young Scholars of China] grant number [51625903] and [Chinese National Natural Science Foundation] grant number [51479194,41702349] And The APC was funded by [41702349]. Conflicts of Interest: The authors declare no conflicts of interest. Intensity Appl. Sci. 2018, 8, 1431 9 of 11 (XRD) tests were performed on Cr(VI)-contaminated soil. The influence of reductant dosage on the leachability and stability was assessed. The following conclusions can be drawn: 1. The pH and redox potential of the CaS -stabilized soils were better than those of the FeSO - 5 4 stabilized soils regardless of CaS dosage. The concentrations of Cr(VI) and Cr leached from the stabilized soils with FeSO were larger than those of CaS at the same dosage. The Cr(VI) content 4 5 in the stabilized soils was decreased with the increase in FeSO and CaS dosages, and that in the 4 5 CaS -stabilized soils decreased more noticeably compared with that in the FeSO -stabilized soils 5 4 at the same dosage. This finding reflected that CaS presented a better effect than FeSO in the 5 4 stabilization of Cr(VI) and Cr. 2. The leached Cr(VI)/Cr from the SBET leaching test was considerably larger than that from TCLP leaching due to the difference in the stabilization mechanism and the pH of the leaching solutions. The bioaccessibility risk of Cr in the FeSO -stabilized soils was higher than that in the CaS -stabilized soils due to the difference in stabilization mechanisms of Cr(VI) between FeSO 5 4 and CaS . 3. The differences in the leachability, bioaccessibility, and toxicity of Cr(VI) and Cr in the FeSO - and CaS -stabilized soils were attributed to the changes in mineral composition. 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Influence of soil geochemical and physical properties on chromium (VI) sorption and bioaccessibility. Environ. Sci. Technol. 2013, 47, 11241–11248. [CrossRef] [PubMed] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Applied Sciences – Multidisciplinary Digital Publishing Institute

Published: Aug 22, 2018

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