Mechanochemical treatment of Cr(VI) contaminated soil using a sodium sulfide coupled solidification/stabilization process

doi:10.1016/j.chemosphere.2018.08.121

Chemosphere

Volume 212, December 2018, Pages 540-547

https://doi.org/10.1016/j.chemosphere.2018.08.121 Get rights and content

Highlights

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Mechanochemical reduction was applied for the remediation of Cr (Ⅵ) contaminated soil with Na₂S.
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Mechanochemical reaction between Cr (Ⅵ) in contaminated soil and Na₂S occurred.
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After treatment, leachable Cr (Ⅵ) concentrations reduced and be lower than 5 mg L⁻¹.
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Mechanochemical reduction technique is promising for Cr (Ⅵ) contaminated soil remediation.

Abstract

In this research, mechanochemical reduction was carried out to remediate Cr(VI) contaminated soil, and the reduction effectiveness was evaluated by analyzing the corresponding leachable fraction obtained through the toxicity characteristic leaching procedure (TCLP) proposed by the EPA. The results indicated that mechanochemical reduction can efficiently reduce the Cr(VI) concentration in the leachate. Under a milling time of 2 h, milling speed of 500 rpm, ball-to-powder weight ratio of 14 and Na₂S dosage of 5%, the Cr(VI) leaching concentration significantly decreased from 663.98 mg L⁻¹ to 0.84 mg L⁻¹, much lower than the regulatory limit of 5 mg L⁻¹. In addition, the significant decrease in Cr(VI) was mainly due to the reduction of Cr(VI) to Cr(III), as supported by X-ray photoelectron spectroscopy (XPS). The mechanochemical reduction with mechanism proposed in this experiment may involve two major processes: solidification and reduction (stabilization).

Graphical abstract

Introduction

Currently, the country of China is one of the leading producers of chromate with an annual production of greater than 160,000 tons (Deng, 2016). With increasing urbanization, many chromium contaminated sites have been created (Cao et al., 2016, Sinha et al., 2017, Wang et al., 2017). Soil surrounding a chromate plant was considerably contaminated due to most of the waste chromium residue accumulating in the air and chromium-containing waste water discharging into the environment. Considering the great hazards of hexavalent chromium pollution to both the environment and human health, chromium contamination has attracted widespread attention from the public and government (Gao and Xia, 2011, Homa et al., 2016, Wu et al., 2017a, Wu et al., 2017b). In China, chromium pollution in soil has become an urgent environmental issue, and the remediation of chromium contaminated soil is a critical issue (Costa and Klein, 2006, Plugaru et al., 2009, Rumpa et al., 2011).

Typical remediation techniques of chromium contaminated soil include solidification/stabilization (S/S), chemical reduction, ex-situ chemical washing, electrokinetic remediation and bioremediation (Graham et al., 2006, Franco et al., 2010, Zhang et al., 2010, Achal et al., 2011, Yang et al., 2011, Lyu et al., 2018). Recently, bioremediation has attracted particular attention and observed rapid development; however, due to its long remediation cycle, the use of this technique has been confined to laboratory (Chao et al., 2016, Qu et al., 2016). S/S treatment of Cr(VI) contaminated soil can reduce the mobility and bioavailability of Cr(VI). Commonly used S/S agents in remediating Cr(VI) contaminated soil include cement, lime, asphalt, fly ash, gypsum, iron based agents, organic agents and sulfur based agents (Wang et al., 2015, Cerbo et al., 2017, Li et al., 2017). Efficient Cr(VI) removal performance was obtained by Velasco et al. (2012) using sodium sulfide for the treatment of chromite ore processing residue. A comparative study on the Cr(VI) reduction efficiency using different sulfur based agents was conducted by Mahdieh et al. (2016), indicating that sodium sulfur delivers the best performance.

Previous studies have demonstrated that heavy metal solidification can be obtained by simple mechanical ball milling (Concas et al., 2007, Montinaro et al., 2007, Montinaro et al., 2008, Montinaro et al., 2009, Chen et al., 2016). In particular, Montinaro and Concas et al. investigated dry ball milling for the treatment of synthetic kaolinite, sandy and bentonite type soils (Concas et al., 2007, Montinaro et al., 2007, Montinaro et al., 2008, Montinaro et al., 2009) and indicated that a leaching concentration lower than the regulatory threshold was achieved after mechanical treatment. Concas et al. (2007) made three hypotheses to explain the phenomenon induced by mechanical ball milling: (1) entrapment of heavy metals into aggregates; (2) solid diffusion into the crystalline reticulum; (3) irreversibly absorbed onto fresh surfaces. Similarly, Chen et al. (2016) found that the heavy metal concentration in leachate significantly decreased after ball milling. In addition, Zhang (2008) works indicated that metal oxides and hydroxides could be sequestered with quartz to form structures that bind heavy metals even in the presence of boiling aqueous acids.

In the present study, Cr(VI) contaminated soil was treated with sodium sulfide by ball milling, and the degree of treatment was evaluated by analyzing the leachable fraction of Cr(VI) obtained through the “toxicity characteristic leaching procedure” (EPA TCLP). To elucidate the mechanism induced by the mechanical treatment, particle size, SEM analysis and XPS analyses were carried out. This study indicates the great future potential of mechanochemical reduction applied to Cr(VI) contaminated soil remediation.

Section snippets

Soil samples and reagents

In this study, Cr(VI) contaminated soil samples were collected from an abandoned chromic salt factory in Ji'nan, Shandong, China. Before use, the soil samples were air-dried and sieved with a 0.15 mm standard mesh. Next, the total Cr(VI) contents were determined following hexavalent chromium extraction by alkaline digestion (EPA METHOD 3060A) (US EPA, 1992a, US EPA, 1992b). After filtration through a 0.45 μm nylon syringe filter, the Cr(VI) contents of the filtrates were analyzed. The Cr(VI)

Effects of the mechanochemical parameters on the Cr(VI) leaching concentration

The leachability of heavy metals (such as Pb, Cd, and Zn) significantly decreased by mechanical ball milling, which was also demonstrated on synthetic contaminated soils by Montinaro et al., 2007, Montinaro et al., 2008, Montinaro et al., 2009. Notably, the leachable fraction of the heavy metals was decreased to the regulatory limit after a relatively short milling time. In the present research, the EPA TCLP was carried out to evaluate the effectiveness of the mechanochemical reduction. The

Concluding remarks

This work provides an effective technique for the remediation of Cr(VI) contaminated soil. This technique can sharply reduce the Cr(VI) content and immobilize Cr in soil upon the addition of Na₂S to the treatment. Compared to the conventional wet reduction method, mechanical treatment with Na₂S not only shortens the reaction time (only several hours or shorter) but also improves the additive utilization efficiency. The results of the single factor experiments indicated that this method greatly

Acknowledgements

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).

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Mechanochemical treatment of Cr(VI) contaminated soil using a sodium sulfide coupled solidification/stabilization process

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Soil samples and reagents

Effects of the mechanochemical parameters on the Cr(VI) leaching concentration

Concluding remarks

Acknowledgements

Int. J. Miner. Process.

Chemosphere

Chem. Eng. Sci.

Sci. Total Environ.

Colloids Surf., A

Int. J. Miner. Process.

Chemosphere

Chemosphere

Chemosphere

Chem. Eng. J.

Chem. Eng. J.

J. Colloid Interface Sci.

Powder Technol.

Chem. Eng. J.

J. Hazard Mater.

Electrochem. Commun.

J. Hazard Mater.

Environ. Pollut. Cont.

J. Taiwan Inst. Chem. Eng.

Sci. Total Environ.

J. Hazard Mater.

Bioremediation of chromium contaminated soil by a brown-rot fungus, gloeophyllum sepiarium

Res. J. Microbiol.

XPS study of the reaction of chromium (VI) with mackinawite (FeS)

Surf. Interface Anal.