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Rapid Cr(VI) reduction and immobilization in contaminated soil by mechanochemical treatment with calcium polysulfide
Chemosphere 227 (2019) 657e661
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Rapid Cr(VI) reduction and immobilization in contaminated soil by mechanochemical treatment with calcium polysulfide Wenyi Yuan a, c, *, Weitong Xu b, Ziwei Zhang b, Xiaoyan Wang b, Qiwu Zhang d, Jianfeng Bai a, c, Jingwei Wang a, c a
Shanghai Collaborative Innovation Centre for WEEE Recycling, Shanghai Polytechnic University, Shanghai, 201209, China School of Environmental and Materials Engineering, Shanghai Polytechnic University, Shanghai, 201209, China Research Center of Resource Recycling Science and Engineering, Shanghai Polytechnic University, Shanghai, 201209, China d School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan, Hubei, 430070, China b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Ball milling was carried out to remediating Cr(Ⅵ) contaminated soil with CPS. Reduction reaction between Cr(Ⅵ) in contaminated soil and CPS happened. After treatment, leachable Cr(Ⅵ) concentration reduced and be lower than 5 mg L 1.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 November 2018 Received in revised form 10 April 2019 Accepted 14 April 2019 Available online 15 April 2019
Mechanochemical treatment with calcium polysulfide (CPS) was applied to remediate the Cr(VI) contaminated soil. The effects of parameters including milling speed, milling time, ball to powder ratio (BPR) and dosage of CPS were investigated. The effectiveness of mechanical treatment with or without CPS is estimated by analyzing the leachable fraction of Cr(VI). The results show that mechanochemical treatment with CPS can decrease and immobilize Cr in soil more quickly and efficiently with comparison to the case without additive. Under a milling speed of 500 rpm, milling time of 2 h, BPR of 9 and CPS dosage of 3%, the Cr(VI) leaching concentration significantly decreased from 115 mg L 1 to 0.51 mg L 1, much lower than the regulatory limit of 5 mg L 1. Additionally, XPS results demonstrated that Cr(VI) can be converted into Cr(III) during ball milling with CPS. The high Cr(VI) removal and Cr immobilization capacity makes mechanochemical treatment a great potential in field remediation. © 2019 Elsevier Ltd. All rights reserved.
Handling Editor: X. Cao Keywords: Cr(VI) contaminated soil Ball milling Calcium polysulfide Redox reaction Soil remediation
* Corresponding author. Shanghai Collaborative Innovation Centre for WEEE Recycling, Shanghai Polytechnic University, Shanghai, 201209, China. E-mail addresses: [email protected] (W. Yuan), [email protected] (W. Xu), [email protected] (Z. Zhang), [email protected] (X. Wang), zhangqiwu@whut. edu.cn (Q. Zhang), [email protected] (J. Bai), [email protected] (J. Wang). https://doi.org/10.1016/j.chemosphere.2019.04.108 0045-6535/© 2019 Elsevier Ltd. All rights reserved.
1. Introduction As an important raw and processed material, chromium has a wide range of applications, such as leather tanning, textile
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production, printing and dyeing, chrome plating, machine manufacturing, paint manufacturing, production of building materials and chemicals production etc. (Apte et al., 2006; Rinklebe et al., 2016). The industrial waste water and solid waste produced in these industries can result in chromium pollution in natural water systems and soils if handled improperly (or due to accidents, leaks, poor storage etc.) (Berardi et al., 2015; Plugaru et al., 2017; Gheju et al., 2017). In fact, chromium pollution in soils has become an urgent environmental issue and the remediation of soil contaminated by chromium is a critical problem in China. Chromium commonly exists in two highly stable oxidation states, Cr(III) and Cr(VI); the toxicity of chromium is closely related to its oxidation states (Zhang et al., 2012; Li et al., 2017). In fact, the environmental and biological behaviors and the mobility of chromium are critically dependent upon its actual chemical forms. Compared with the relatively immobile Cr(III), Cr(VI) species are much more soluble and mobile. Cr(VI) can lead not only to an enlargement of the scope of soil pollution, but can also cause the contamination of both surface and underground waters due to its mobility and solubility (Gochfeld, 1991; Kunhikrishnan et al., 2017; Xue et al., 2018). Therefore, considering the great harm of Cr(VI), reducing Cr(VI) into Cr(III), which is much less mobile and easily adsorbed by soil colloids, is an important topic of current research. It is well known that mechanochemical treatment has been successfully applied to the remediation of soils contaminated by persistent organic pollutants (POPs) such as pentachlorophenol, polyvinylchloride, polychlorinated biphenyls, dibenzo-p-dioxins, hexachlorobenzene, polychlorinated benzofuran etc. (Zhang et al., 2014; Ren et al., 2015; Deng et al., 2017). Mechanochemical technology has become one of the most commercially applicable technologies for treatment of POPs, because of its simplicity, and high processing efficiency. Recently, some papers have been devoted to investigating the effect of mechanochemical treatment on the immobilization capacity of heavy metals in contaminated soils (Concas et al., 2007; Montinaro et al., 2008, 2009; Yuan et al., 2018). In particular, the use of dry ball milling for the treatment of spiked soils of kaolinitic, sandy and bentonite type was investigated by Montinaro et al. (2008, 2009). Similarly, the research conducted by Zhang (2008) has shown that mechanical ball milling has great potential for immobilizing heavy metals in soils, because metal oxides and hydroxides could be sequestered with quartz to form structures that bind the heavy metals even in the presence of boiling aqueous acids. High remediation effectiveness was obtained on synthetic soils (Montinaro et al., 2008, 2009; Concas et al., 2007), so mechanochemical ball milling technology may be considered potentially applicable for real-world remediation of soils contaminated by heavy metals (Graham et al., 2006; Chrysochoou et al., 2010). In this study, the mechanochemical treatment with or without additive was applied to remediate the Cr(VI) contaminated soil. The objective of this study was exam the effectiveness of the remediation process and explore the mechanism of the mechanochemical treatment. This study indicates the great future potential of mechanochemical reduction applied to remediate Cr(VI)contaminated soil.
2. Materials and methods 2.1. Experimental soil and chemicals For this project, the Cr(VI) contaminated soil sample was collected from a planting site in Beijing, China. The soil was airdried and passed through an 80 mesh sieve. Next, the total Cr(VI) content was determined following extraction with alkaline digestion for hexavalent chromium (EPA METHOD 3060A, 1992). After filtration through a 0.45 mm nylon syringe filter, the filtrates were analyzed for Cr(VI) content. The Cr(VI) concentration was measured by the 1,5-diphenylcarbohydrazide spectrophotometric method on a UVeVis spectrophotometer (Evolution 300, Thermo, USA) at a wavelength of 540 nm. The Cr(VI) content was 2360 mg kg 1. All reagents used in this study were of trace metal grade or better, and all solutions were prepared using distilled, deionized water (DDIW) (Milli-Q water, 18.2 MU-cm resistivity, Millipore Corp.,Milford,MA). Calcium polysulfide (CaSx, 25% wt) was used as the reductant and purchased from Lianyungang Yosoo Industrial Technology Co., Ltd. 2.2. Experimental procedures The hexavalent chromium contaminated soils were mixed with CPS in a planetary ball mill (Retsch PM100, Germany), using 50 mL zirconia vials and 90 g of 20.0 mm diameter balls. The vials were then sealed under normal atmospheric conditions and fixed on the mill. Soil samples were mechanochemically treated under air atmosphere with different milling speed, milling time, ball to powder ratio (BPR) and CPS dosage. The operating conditions of the ball milling trials are summarized in Table 1. In addition, 5 g of each treated sample was mixed with 50 mL of an extraction solution, prepared by adding a 2:1 mass ratio mixture of concentrated sulfuric and nitric acids to distilled deionized water (DDIW) (Milli-Q water, 18.2 MU-cm resistivity, Millipore Corp., Milford, MA) until the pH was 3.20 ± 0.05 at a liquid-to-solid ratio of 10 (mL/g) in capped polypropylene wide-mouth bottles on a rotary oscillator at 30 ± 2 rpm for 18 ± 2 h (23 ± 2 C) according to the “Solid wasteextraction procedure for leaching toxicity-sulfuric acid & nitric acid method” (HJ/T299, 2007). After extraction, the final liquid Cr(VI) concentrations of the leachates were measured by the 1,5diphenylcarbohydrazide spectrophotometric method on a UVeVis spectrophotometer (Evolution 300, Thermo, USA) at a wavelength of 540 nm and the Cr(VI) leaching concentration of the untreated soil was 115 mg L 1. X-ray diffraction (XRD, D8; Bruker, Karlsruhe, Germany) was used to identify the mineralogical changes upon treatment. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher, U.S.) was carried out to determine the valence variation of the elements (Cr, S) before and after mechanical treatment. 3. Results and discussion 3.1. Cr(VI) reduction and immobilization The results in Fig. 1(a) show that the leaching concentration of Cr(VI) changes with milling time when the milling speed, ball to
Table 1 Operating conditions for the ball milling trials (single factor experiment). Milling speed (rpm)
Milling time (h)
BPR
Dosage of CPS (%)
200, 300, 400,500,600 Milling time 2 h, BPR 9, CPS dosage of 3%.
0.5, 1.0, 2.0, 4.0, 6.0 Milling speed 500 rpm, BPR 9, CPS dosage of 3%.
6, 9, 12, 15, 18 Milling speed 500 rpm, Milling time 2 h, CPS dosage of 3%.
1, 2, 3, 4, 5 Milling speed 500 rpm, Milling time 2 h, BPR 9.
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Fig. 1. The effect of mechanochemical parameters on Cr(VI) leaching concentration: (a) milling time; (b) milling speed; (c) BPR; (d) CPS dosage.
powder ratio (BPR) and CPS dosage were maintained at 500 rpm, 9 and 3%, respectively. Fig. 1 clearly shows that the concentrations of Cr(VI) in the leachates significantly decreased as the milling time prolonged. When the milling time was 0.5 h, the leaching concentration of Cr(VI) in treated soil with or without CPS was 62.91 mg L 1 and 11.20 mg L 1, respectively, suggested mechanical treatment contributed to the decrease of Cr(VI) leaching concentration. As the milling time increased from 0.5 h to 4 h, the leaching concentrations of Cr(VI) decreased from 62.91 mg L 1 to 11.20 mg L 1 to 4.03 mg L 1 and 0.50 mg L 1, respectively. In particular, when milling time is carried out at 6 h, both the case with and without the use of CPS were capable to decrease the leachable fraction of Cr(VI) to levels lower than the regulatory limit. It is attracted that the Cr(VI) concentration in leachate was nearly impossible to detect after mechanically treatment under the conditions of milling time 2 h, milling speed 500 rpm, BPR 9 and CPS dosage of 3%. In fact, milling time is an important mechanochemical parameter and prolonging the milling time can guarantee sufficient contact between CPS and Cr(VI) to further enhance the mechanochemical reaction process. The measured Cr(VI) concentration remained basically unchanged after milling for 4 h, which suggested that the mechanochemical reaction had reached equilibrium. It should be noted that the addition of CPS can almostly lead Cr(VI) leaching concentration decrease to the regulatory limit required only 1 h of milling. This can be ascribed to the redoxreaction between Cr(VI) and CPS under condition of solid phase. However, a more prolonged milling time (2 h) is needed for the case without CPS. In fact, the decrease of Cr(VI) leaching concentration would be explained for both reduction and immobilization. In the case of without CPS, Cr(VI) concentration in leachate could meet the regulatory threshold when milling time is longer than 4 h and
this result which may be explained with aggregation in soil particles may prevail on breakage ones at longer milling time. Particle refinement process and growth of aggregation were enhanced under the action of long milling time and particles with higher surface energies will easily form aggregates under van der Waals adhesion force and electrostatic force, which can be of availing solidification effectiveness. The more compacted aggregates form, the less likely Cr(VI) to release. Additionally, significant difference of Cr(VI) leaching concentration after mechanical treatment with and without CPS, demonstrated that reduction of Cr(VI) is the main reason for the decrease of leaching concentration. The effect of other parameters (milling speed, BPR, CPS dosage) on the Cr(VI) leaching concentration was shown in Fig. 1(bed). For the sake of brevity, the results for these parameters are not reported.
3.2. Mechanism analysis XRD analysis was performed to verify whether mechanical treatment caused significant alternations to the mineralogical changes of the contaminated soil. The XRD spectra of untreated and treated Cr(VI) contaminated soil samples are shown in Fig. 2. Crystalline compounds, SiO2 and NaAlSi3O8, were detected by XRD analyses. After mechanochemical treatment for 6 h, the peak intensities of SiO2 and NaAlSi3O8 decreased and the effect of a partial amorphization in the crystalline phases was confirmed. It is known that such typical phenomenon induced by high energy ball milling treatment which determines the transformation of crystalline materials into amorphous phases, an accumulation of structural defects, vacancies and dislocations. It may be clearly seen that no Cr(III) phase was detected in treated soil samples by XRD, suggesting that the Cr(III) phase may be in an amorphous formation rather than in crystalline materials. Similar results were obtained
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Intensity
SiO2 Na(AlSi3O8)
untreated soil
treatment of 6 h Fig. 4. XPS patterns of untreated and treated soil samples (500 rpm, BPR 9, CPS dosage 3%).
10
20
30
40
2 °
50
60
70
80
Fig. 2. XRD patterns of an untreated soil sample and a soil sample after mechanochemical treatment for 6 h (Milling speed 500 rpm, BPR 9, CPS dosage 3%).
by Graham et al. (2006), who used CPS as the reductant for COPR treatment (Wazne et al., 2007). In order to further verify the reaction between Cr(VI) in soil and CPS, pure K2CrO4 and CPS were utilized at 1:1 M ratio as starting materials for mechanochemical process. Fig. 3 shows the images of a mixture of K2CrO4 and CPS, with a sample mechanochemically treated for 60 min at a milling speed of 500 rpm. The starting materials were initially yellow and brown, while the ground sample looks green, implying that a mechanochemical reaction occurred between K2CrO4 and CPS during ball milling. The green ground sample may be ascribed to the existence of Cr2O3. However，no Cr(III) crystalline phase was detected in the mechanochemically treated soil samples by XRD analysis, which could be ascribed to the amorphization of Cr2O3 induced by mechanical ball milling. To further elucidate the mechanism involved in the removal of Cr(VI), untreated and treated soil samples were analyzed by X-ray photoelectron spectroscopy (XPS). Detailed XPS surveys of the Cr2p and S2p regions of the soil before and after treatment are shown in Fig. 4. As shown in Fig. 4 (a), one Cr(VI) photo-electron peak was observed in the untreated soil sample; the 2p3/2 peak was centered at 579.3 eV and is consistent with reported values for Cr(VI) which range from 579.0 to 579.8 eV (Boursiquot et al., 2010). For the treated soil sample, one 2p3/2 peak was centered at 577.2 eV and one 2p1/2 peak was centered at 586.9 eV; both peaks are comparable to reported binding energy ranges of 576.2e577.5 eV and 586.7e587.0 eV for 2p3/2 and 2p1/2, respectively (Dupont et al., 2003; Legrand et al., 2004; Noubactep et al., 2008). It is worth noting that no Cr(VI) photo-electron peak was detected after 6 h of
mechanical ball milling with CPS. In fact, the leachable fraction of Cr(VI) was reduced to less than 0.05 mg L 1 and can be ascribed to the reduction of Cr(VI). Moreover, the formation of Cr(III) further strengthens the remediation effectiveness. Two S photo-electron peaks were observed for the treated soil sample after 6 h of mechanical treatment as shown in Fig. 4 (b). The binding energy peak values at 163.3 eV and 168.7 eV are consistent with sulphidic and sulphonic forms, respectively (Marinov et al., 2004). The existence of the sulphidic form may be due to excess CPS. In addition, the existence of a sulphonic form further suggests that a redox reaction occurs between Cr(VI) and CPS. The XPS results revealed that mechanochemical reduction with CPS could result in a nearly complete reduction of Cr(VI). The reduction of Cr(VI) to Cr(III) is responsible for the decease of Cr(VI) in the leachate. At the same time, the results of XPS agree well with the results of single factor experiments (shown in section 3.1). 3.3. The proposed conceptual processes of the mechanochemical treatment A conceptual model for the solidification and stabilization of Cr(VI) in contaminated soil by mechanical ball milling with calcium polysulfide is illustrated in Fig. 5. Generally, it is likely to assume that Cr(VI) can be adsorbed onto the surface of soil collides through a surface coordination process and be entrapped within the surface of two overlapping soil particles (Concas et al., 2007). Cr(VI), adsorbed onto the surface of soil collides, easily leaches and available for plants in an aqueous environment. However, the latter is relatively stable and hard to released. Under the action of high energy ball milling, the mixtures, soil particles and calcium polysulfide, were mixed fully. As the mechanical action prolonged, particle refinement process was also
Fig. 3. Images of K2CrO4 (yellow) and CPS (brown) before and after milling (milling time 1 h, milling speed 500 rpm).
W. Yuan et al. / Chemosphere 227 (2019) 657e661
Fig. 5. A conceptual model for the reduction and immobilization of Cr(VI) in contaminated soil.
enhanced, which is conducive to the sufficient contact between Cr(VI) and calcium polysulfide. Meanwhile, Cr(VI) incorporated in the solid matrix was gradually exposed due to the breakage of soil particles induced by mechanical treatment and this process improves the stabilization efficiency (Montinaro et al., 2008, 2009). After intensive mixing between soil particles and CPS, a redox reaction occurred between the Cr(VI) in the soil and the sulfur in CPS at the surfaces of these particles, and Cr(VI) was reduced to Cr(III). Additionally, particle aggregation is the typical phenomena in mechanical treatment and stable aggregates with compacted structures might be formed and Cr was immobilized by “cover layer”, which was formed by soil particles (Montinaro et al., 2008, 2009; Mallampati et al., 2013). 4. Conclusions This work demonstrated that mechanical ball milling with CPS could efficiently remediate Cr(VI) contaminated soil. Cr(VI) concentration in leachate significantly decreased from 115 mg L 1 to 0.51 mg L 1 under a milling speed of 500 rpm, milling time of 2 h, BPR of 9 and CPS dosage of 3%. This research found that Cr(VI) can be directly concerted into Cr(III) under the action of mechanical ball milling and the reduction of Cr(VI) should be responsible for the decrease of Cr(VI) in leachates. Although the Cr(VI) leaching concentration also can decrease to the regulatory limit in the case without CPS, but required harsh conditions. Additionally, Cr(VI) might be release again only by mechanical treatment. Therefore, considering the long-term stability of remediation, additives should be added to enhance the remediation effectiveness. Mechanochemical technique holds significant promise and potential for real-world contamination remediation. Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21876106), Gaoyuan Discipline of Shanghai-Environmental Science and Engineering (Resource Recycling Science and Engineering), and SSPU Foundation (A01GY18EX04, A01GY18F022-d02). References Apte, A.D., Tare, V., Bose, P., 2006. Extent of oxidation of Cr(III) to Cr(VI) under various conditions pertaining to natural environment. J. Hazard Mater. 128, 164e174. Berardi, R., Pellei, C., Valeri, G., Pistelli, M., Onofri, A., Morgese, F., Caramantic, M.,
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Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Rapid Cr(VI) reduction and immobilization in contaminated soil by mechanochemical treatment with calcium polysulfide Wenyi Yuan a, c, *, Weitong Xu b, Ziwei Zhang b, Xiaoyan Wang b, Qiwu Zhang d, Jianfeng Bai a, c, Jingwei Wang a, c a
Shanghai Collaborative Innovation Centre for WEEE Recycling, Shanghai Polytechnic University, Shanghai, 201209, China School of Environmental and Materials Engineering, Shanghai Polytechnic University, Shanghai, 201209, China Research Center of Resource Recycling Science and Engineering, Shanghai Polytechnic University, Shanghai, 201209, China d School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan, Hubei, 430070, China b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Ball milling was carried out to remediating Cr(Ⅵ) contaminated soil with CPS. Reduction reaction between Cr(Ⅵ) in contaminated soil and CPS happened. After treatment, leachable Cr(Ⅵ) concentration reduced and be lower than 5 mg L 1.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 November 2018 Received in revised form 10 April 2019 Accepted 14 April 2019 Available online 15 April 2019
Mechanochemical treatment with calcium polysulfide (CPS) was applied to remediate the Cr(VI) contaminated soil. The effects of parameters including milling speed, milling time, ball to powder ratio (BPR) and dosage of CPS were investigated. The effectiveness of mechanical treatment with or without CPS is estimated by analyzing the leachable fraction of Cr(VI). The results show that mechanochemical treatment with CPS can decrease and immobilize Cr in soil more quickly and efficiently with comparison to the case without additive. Under a milling speed of 500 rpm, milling time of 2 h, BPR of 9 and CPS dosage of 3%, the Cr(VI) leaching concentration significantly decreased from 115 mg L 1 to 0.51 mg L 1, much lower than the regulatory limit of 5 mg L 1. Additionally, XPS results demonstrated that Cr(VI) can be converted into Cr(III) during ball milling with CPS. The high Cr(VI) removal and Cr immobilization capacity makes mechanochemical treatment a great potential in field remediation. © 2019 Elsevier Ltd. All rights reserved.
Handling Editor: X. Cao Keywords: Cr(VI) contaminated soil Ball milling Calcium polysulfide Redox reaction Soil remediation
* Corresponding author. Shanghai Collaborative Innovation Centre for WEEE Recycling, Shanghai Polytechnic University, Shanghai, 201209, China. E-mail addresses: [email protected] (W. Yuan), [email protected] (W. Xu), [email protected] (Z. Zhang), [email protected] (X. Wang), zhangqiwu@whut. edu.cn (Q. Zhang), [email protected] (J. Bai), [email protected] (J. Wang). https://doi.org/10.1016/j.chemosphere.2019.04.108 0045-6535/© 2019 Elsevier Ltd. All rights reserved.
1. Introduction As an important raw and processed material, chromium has a wide range of applications, such as leather tanning, textile
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production, printing and dyeing, chrome plating, machine manufacturing, paint manufacturing, production of building materials and chemicals production etc. (Apte et al., 2006; Rinklebe et al., 2016). The industrial waste water and solid waste produced in these industries can result in chromium pollution in natural water systems and soils if handled improperly (or due to accidents, leaks, poor storage etc.) (Berardi et al., 2015; Plugaru et al., 2017; Gheju et al., 2017). In fact, chromium pollution in soils has become an urgent environmental issue and the remediation of soil contaminated by chromium is a critical problem in China. Chromium commonly exists in two highly stable oxidation states, Cr(III) and Cr(VI); the toxicity of chromium is closely related to its oxidation states (Zhang et al., 2012; Li et al., 2017). In fact, the environmental and biological behaviors and the mobility of chromium are critically dependent upon its actual chemical forms. Compared with the relatively immobile Cr(III), Cr(VI) species are much more soluble and mobile. Cr(VI) can lead not only to an enlargement of the scope of soil pollution, but can also cause the contamination of both surface and underground waters due to its mobility and solubility (Gochfeld, 1991; Kunhikrishnan et al., 2017; Xue et al., 2018). Therefore, considering the great harm of Cr(VI), reducing Cr(VI) into Cr(III), which is much less mobile and easily adsorbed by soil colloids, is an important topic of current research. It is well known that mechanochemical treatment has been successfully applied to the remediation of soils contaminated by persistent organic pollutants (POPs) such as pentachlorophenol, polyvinylchloride, polychlorinated biphenyls, dibenzo-p-dioxins, hexachlorobenzene, polychlorinated benzofuran etc. (Zhang et al., 2014; Ren et al., 2015; Deng et al., 2017). Mechanochemical technology has become one of the most commercially applicable technologies for treatment of POPs, because of its simplicity, and high processing efficiency. Recently, some papers have been devoted to investigating the effect of mechanochemical treatment on the immobilization capacity of heavy metals in contaminated soils (Concas et al., 2007; Montinaro et al., 2008, 2009; Yuan et al., 2018). In particular, the use of dry ball milling for the treatment of spiked soils of kaolinitic, sandy and bentonite type was investigated by Montinaro et al. (2008, 2009). Similarly, the research conducted by Zhang (2008) has shown that mechanical ball milling has great potential for immobilizing heavy metals in soils, because metal oxides and hydroxides could be sequestered with quartz to form structures that bind the heavy metals even in the presence of boiling aqueous acids. High remediation effectiveness was obtained on synthetic soils (Montinaro et al., 2008, 2009; Concas et al., 2007), so mechanochemical ball milling technology may be considered potentially applicable for real-world remediation of soils contaminated by heavy metals (Graham et al., 2006; Chrysochoou et al., 2010). In this study, the mechanochemical treatment with or without additive was applied to remediate the Cr(VI) contaminated soil. The objective of this study was exam the effectiveness of the remediation process and explore the mechanism of the mechanochemical treatment. This study indicates the great future potential of mechanochemical reduction applied to remediate Cr(VI)contaminated soil.
2. Materials and methods 2.1. Experimental soil and chemicals For this project, the Cr(VI) contaminated soil sample was collected from a planting site in Beijing, China. The soil was airdried and passed through an 80 mesh sieve. Next, the total Cr(VI) content was determined following extraction with alkaline digestion for hexavalent chromium (EPA METHOD 3060A, 1992). After filtration through a 0.45 mm nylon syringe filter, the filtrates were analyzed for Cr(VI) content. The Cr(VI) concentration was measured by the 1,5-diphenylcarbohydrazide spectrophotometric method on a UVeVis spectrophotometer (Evolution 300, Thermo, USA) at a wavelength of 540 nm. The Cr(VI) content was 2360 mg kg 1. All reagents used in this study were of trace metal grade or better, and all solutions were prepared using distilled, deionized water (DDIW) (Milli-Q water, 18.2 MU-cm resistivity, Millipore Corp.,Milford,MA). Calcium polysulfide (CaSx, 25% wt) was used as the reductant and purchased from Lianyungang Yosoo Industrial Technology Co., Ltd. 2.2. Experimental procedures The hexavalent chromium contaminated soils were mixed with CPS in a planetary ball mill (Retsch PM100, Germany), using 50 mL zirconia vials and 90 g of 20.0 mm diameter balls. The vials were then sealed under normal atmospheric conditions and fixed on the mill. Soil samples were mechanochemically treated under air atmosphere with different milling speed, milling time, ball to powder ratio (BPR) and CPS dosage. The operating conditions of the ball milling trials are summarized in Table 1. In addition, 5 g of each treated sample was mixed with 50 mL of an extraction solution, prepared by adding a 2:1 mass ratio mixture of concentrated sulfuric and nitric acids to distilled deionized water (DDIW) (Milli-Q water, 18.2 MU-cm resistivity, Millipore Corp., Milford, MA) until the pH was 3.20 ± 0.05 at a liquid-to-solid ratio of 10 (mL/g) in capped polypropylene wide-mouth bottles on a rotary oscillator at 30 ± 2 rpm for 18 ± 2 h (23 ± 2 C) according to the “Solid wasteextraction procedure for leaching toxicity-sulfuric acid & nitric acid method” (HJ/T299, 2007). After extraction, the final liquid Cr(VI) concentrations of the leachates were measured by the 1,5diphenylcarbohydrazide spectrophotometric method on a UVeVis spectrophotometer (Evolution 300, Thermo, USA) at a wavelength of 540 nm and the Cr(VI) leaching concentration of the untreated soil was 115 mg L 1. X-ray diffraction (XRD, D8; Bruker, Karlsruhe, Germany) was used to identify the mineralogical changes upon treatment. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher, U.S.) was carried out to determine the valence variation of the elements (Cr, S) before and after mechanical treatment. 3. Results and discussion 3.1. Cr(VI) reduction and immobilization The results in Fig. 1(a) show that the leaching concentration of Cr(VI) changes with milling time when the milling speed, ball to
Table 1 Operating conditions for the ball milling trials (single factor experiment). Milling speed (rpm)
Milling time (h)
BPR
Dosage of CPS (%)
200, 300, 400,500,600 Milling time 2 h, BPR 9, CPS dosage of 3%.
0.5, 1.0, 2.0, 4.0, 6.0 Milling speed 500 rpm, BPR 9, CPS dosage of 3%.
6, 9, 12, 15, 18 Milling speed 500 rpm, Milling time 2 h, CPS dosage of 3%.
1, 2, 3, 4, 5 Milling speed 500 rpm, Milling time 2 h, BPR 9.
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Fig. 1. The effect of mechanochemical parameters on Cr(VI) leaching concentration: (a) milling time; (b) milling speed; (c) BPR; (d) CPS dosage.
powder ratio (BPR) and CPS dosage were maintained at 500 rpm, 9 and 3%, respectively. Fig. 1 clearly shows that the concentrations of Cr(VI) in the leachates significantly decreased as the milling time prolonged. When the milling time was 0.5 h, the leaching concentration of Cr(VI) in treated soil with or without CPS was 62.91 mg L 1 and 11.20 mg L 1, respectively, suggested mechanical treatment contributed to the decrease of Cr(VI) leaching concentration. As the milling time increased from 0.5 h to 4 h, the leaching concentrations of Cr(VI) decreased from 62.91 mg L 1 to 11.20 mg L 1 to 4.03 mg L 1 and 0.50 mg L 1, respectively. In particular, when milling time is carried out at 6 h, both the case with and without the use of CPS were capable to decrease the leachable fraction of Cr(VI) to levels lower than the regulatory limit. It is attracted that the Cr(VI) concentration in leachate was nearly impossible to detect after mechanically treatment under the conditions of milling time 2 h, milling speed 500 rpm, BPR 9 and CPS dosage of 3%. In fact, milling time is an important mechanochemical parameter and prolonging the milling time can guarantee sufficient contact between CPS and Cr(VI) to further enhance the mechanochemical reaction process. The measured Cr(VI) concentration remained basically unchanged after milling for 4 h, which suggested that the mechanochemical reaction had reached equilibrium. It should be noted that the addition of CPS can almostly lead Cr(VI) leaching concentration decrease to the regulatory limit required only 1 h of milling. This can be ascribed to the redoxreaction between Cr(VI) and CPS under condition of solid phase. However, a more prolonged milling time (2 h) is needed for the case without CPS. In fact, the decrease of Cr(VI) leaching concentration would be explained for both reduction and immobilization. In the case of without CPS, Cr(VI) concentration in leachate could meet the regulatory threshold when milling time is longer than 4 h and
this result which may be explained with aggregation in soil particles may prevail on breakage ones at longer milling time. Particle refinement process and growth of aggregation were enhanced under the action of long milling time and particles with higher surface energies will easily form aggregates under van der Waals adhesion force and electrostatic force, which can be of availing solidification effectiveness. The more compacted aggregates form, the less likely Cr(VI) to release. Additionally, significant difference of Cr(VI) leaching concentration after mechanical treatment with and without CPS, demonstrated that reduction of Cr(VI) is the main reason for the decrease of leaching concentration. The effect of other parameters (milling speed, BPR, CPS dosage) on the Cr(VI) leaching concentration was shown in Fig. 1(bed). For the sake of brevity, the results for these parameters are not reported.
3.2. Mechanism analysis XRD analysis was performed to verify whether mechanical treatment caused significant alternations to the mineralogical changes of the contaminated soil. The XRD spectra of untreated and treated Cr(VI) contaminated soil samples are shown in Fig. 2. Crystalline compounds, SiO2 and NaAlSi3O8, were detected by XRD analyses. After mechanochemical treatment for 6 h, the peak intensities of SiO2 and NaAlSi3O8 decreased and the effect of a partial amorphization in the crystalline phases was confirmed. It is known that such typical phenomenon induced by high energy ball milling treatment which determines the transformation of crystalline materials into amorphous phases, an accumulation of structural defects, vacancies and dislocations. It may be clearly seen that no Cr(III) phase was detected in treated soil samples by XRD, suggesting that the Cr(III) phase may be in an amorphous formation rather than in crystalline materials. Similar results were obtained
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Intensity
SiO2 Na(AlSi3O8)
untreated soil
treatment of 6 h Fig. 4. XPS patterns of untreated and treated soil samples (500 rpm, BPR 9, CPS dosage 3%).
10
20
30
40
2 °
50
60
70
80
Fig. 2. XRD patterns of an untreated soil sample and a soil sample after mechanochemical treatment for 6 h (Milling speed 500 rpm, BPR 9, CPS dosage 3%).
by Graham et al. (2006), who used CPS as the reductant for COPR treatment (Wazne et al., 2007). In order to further verify the reaction between Cr(VI) in soil and CPS, pure K2CrO4 and CPS were utilized at 1:1 M ratio as starting materials for mechanochemical process. Fig. 3 shows the images of a mixture of K2CrO4 and CPS, with a sample mechanochemically treated for 60 min at a milling speed of 500 rpm. The starting materials were initially yellow and brown, while the ground sample looks green, implying that a mechanochemical reaction occurred between K2CrO4 and CPS during ball milling. The green ground sample may be ascribed to the existence of Cr2O3. However，no Cr(III) crystalline phase was detected in the mechanochemically treated soil samples by XRD analysis, which could be ascribed to the amorphization of Cr2O3 induced by mechanical ball milling. To further elucidate the mechanism involved in the removal of Cr(VI), untreated and treated soil samples were analyzed by X-ray photoelectron spectroscopy (XPS). Detailed XPS surveys of the Cr2p and S2p regions of the soil before and after treatment are shown in Fig. 4. As shown in Fig. 4 (a), one Cr(VI) photo-electron peak was observed in the untreated soil sample; the 2p3/2 peak was centered at 579.3 eV and is consistent with reported values for Cr(VI) which range from 579.0 to 579.8 eV (Boursiquot et al., 2010). For the treated soil sample, one 2p3/2 peak was centered at 577.2 eV and one 2p1/2 peak was centered at 586.9 eV; both peaks are comparable to reported binding energy ranges of 576.2e577.5 eV and 586.7e587.0 eV for 2p3/2 and 2p1/2, respectively (Dupont et al., 2003; Legrand et al., 2004; Noubactep et al., 2008). It is worth noting that no Cr(VI) photo-electron peak was detected after 6 h of
mechanical ball milling with CPS. In fact, the leachable fraction of Cr(VI) was reduced to less than 0.05 mg L 1 and can be ascribed to the reduction of Cr(VI). Moreover, the formation of Cr(III) further strengthens the remediation effectiveness. Two S photo-electron peaks were observed for the treated soil sample after 6 h of mechanical treatment as shown in Fig. 4 (b). The binding energy peak values at 163.3 eV and 168.7 eV are consistent with sulphidic and sulphonic forms, respectively (Marinov et al., 2004). The existence of the sulphidic form may be due to excess CPS. In addition, the existence of a sulphonic form further suggests that a redox reaction occurs between Cr(VI) and CPS. The XPS results revealed that mechanochemical reduction with CPS could result in a nearly complete reduction of Cr(VI). The reduction of Cr(VI) to Cr(III) is responsible for the decease of Cr(VI) in the leachate. At the same time, the results of XPS agree well with the results of single factor experiments (shown in section 3.1). 3.3. The proposed conceptual processes of the mechanochemical treatment A conceptual model for the solidification and stabilization of Cr(VI) in contaminated soil by mechanical ball milling with calcium polysulfide is illustrated in Fig. 5. Generally, it is likely to assume that Cr(VI) can be adsorbed onto the surface of soil collides through a surface coordination process and be entrapped within the surface of two overlapping soil particles (Concas et al., 2007). Cr(VI), adsorbed onto the surface of soil collides, easily leaches and available for plants in an aqueous environment. However, the latter is relatively stable and hard to released. Under the action of high energy ball milling, the mixtures, soil particles and calcium polysulfide, were mixed fully. As the mechanical action prolonged, particle refinement process was also
Fig. 3. Images of K2CrO4 (yellow) and CPS (brown) before and after milling (milling time 1 h, milling speed 500 rpm).
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Fig. 5. A conceptual model for the reduction and immobilization of Cr(VI) in contaminated soil.
enhanced, which is conducive to the sufficient contact between Cr(VI) and calcium polysulfide. Meanwhile, Cr(VI) incorporated in the solid matrix was gradually exposed due to the breakage of soil particles induced by mechanical treatment and this process improves the stabilization efficiency (Montinaro et al., 2008, 2009). After intensive mixing between soil particles and CPS, a redox reaction occurred between the Cr(VI) in the soil and the sulfur in CPS at the surfaces of these particles, and Cr(VI) was reduced to Cr(III). Additionally, particle aggregation is the typical phenomena in mechanical treatment and stable aggregates with compacted structures might be formed and Cr was immobilized by “cover layer”, which was formed by soil particles (Montinaro et al., 2008, 2009; Mallampati et al., 2013). 4. Conclusions This work demonstrated that mechanical ball milling with CPS could efficiently remediate Cr(VI) contaminated soil. Cr(VI) concentration in leachate significantly decreased from 115 mg L 1 to 0.51 mg L 1 under a milling speed of 500 rpm, milling time of 2 h, BPR of 9 and CPS dosage of 3%. This research found that Cr(VI) can be directly concerted into Cr(III) under the action of mechanical ball milling and the reduction of Cr(VI) should be responsible for the decrease of Cr(VI) in leachates. Although the Cr(VI) leaching concentration also can decrease to the regulatory limit in the case without CPS, but required harsh conditions. Additionally, Cr(VI) might be release again only by mechanical treatment. Therefore, considering the long-term stability of remediation, additives should be added to enhance the remediation effectiveness. Mechanochemical technique holds significant promise and potential for real-world contamination remediation. Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21876106), Gaoyuan Discipline of Shanghai-Environmental Science and Engineering (Resource Recycling Science and Engineering), and SSPU Foundation (A01GY18EX04, A01GY18F022-d02). References Apte, A.D., Tare, V., Bose, P., 2006. Extent of oxidation of Cr(III) to Cr(VI) under various conditions pertaining to natural environment. J. Hazard Mater. 128, 164e174. Berardi, R., Pellei, C., Valeri, G., Pistelli, M., Onofri, A., Morgese, F., Caramantic, M.,
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