title: Selective dissolution followed by EDDS washing of an e-waste contaminated soil: Extraction efficiency, fate of residual metals, and impact on soil environment.
abstract: To enhance extraction of strongly bound metals from oxide minerals and organic matter, this study examined the sequential use of reductants, oxidants, alkaline solvents and organic acids followed by a biodegradable chelating agent (EDDS, [S,S]-ethylene-diamine-disuccinic-acid) in a two-stage soil washing. The soil was contaminated by Cu, Zn, and Pb at an e-waste recycling site in Qingyuan city, China. In addition to extraction efficiency, this study also examined the fate of residual metals (e.g., leachability, bioaccessibility, and distribution) and the soil quality parameters (i.e., cytotoxicity, enzyme activities, and available nutrients). The reductants (dithionite-citrate-bicarbonate and hydroxylamine hydrochloride) effectively extracted metals by mineral dissolution, but elevated the leachability and bioaccessibility of metals due to the transformation from Fe/Mn oxides to labile fractions. Subsequent EDDS washing was found necessary to mitigate the residual risks. In comparison, prior washing by oxidants (persulphate, hypochlorite, and hydrogen peroxide) was marginally useful because of limited amount of soil organic matter. Prior washing by alkaline solvents (sodium hydroxide and sodium bicarbonate) was also ineffective due to metal precipitation. In contrast, prior washing by low-molecular-weight organic acids (citrate and oxalate) improved the extraction efficiency. Compared to hydroxylamine hydrochloride, citrate and oxalate induced lower cytotoxicity (Microtox) and allowed higher enzyme activities (dehydrogenase, acid phosphatase, and urease) and soil nutrients (available nitrogen and phosphorus), which would facilitate reuse of the treated soil. Therefore, while sequential washing proved to enhance extraction efficacy, the selection of chemical agents besides EDDS should also include the consideration of effects on metal leachability/bioaccessibility and soil quality.
authors.affiliation: Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China.
authors.forename: Jingzi
authors.initials: J
authors.lastname: Beiyuan
authors.affiliation: Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China. Electronic address: dan.tsang@polyu.edu.hk.
authors.forename: Daniel C W
authors.initials: DCW
authors.lastname: Tsang
authors.affiliation: School of Chemical and Biomolecular Engineering, The University of Sydney, NSW, 2006, Australia.
authors.forename: Marjorie
authors.initials: M
authors.lastname: Valix
authors.affiliation: School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou, 510275, China.
authors.forename: Weihua
authors.initials: W
authors.lastname: Zhang
authors.affiliation: School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou, 510275, China.
authors.forename: Xin
authors.initials: X
authors.lastname: Yang
authors.affiliation: Department of Biological Environment & Korea Biochar Research Center, Kangwon National University, Chuncheon, 24341, Republic of Korea.
authors.forename: Yong Sik
authors.initials: YS
authors.lastname: Ok
authors.affiliation: Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China.
authors.forename: Xiang-Dong
authors.initials: XD
authors.lastname: Li
mesh_terms: D002614:Chelating Agents / Q000378:metabolism; D002681:China; D059029:Electronic Waste / Q000032:analysis*; D004787:Environmental Pollution; D052918:Environmental Restoration and Remediation; D019216:Metals, Heavy / Q000737:chemistry*; D059027:Recycling; D012987:Soil / Q000737:chemistry*; D012989:Soil Pollutants / Q000737:chemistry*; D012995:Solubility
chemical_list: D002614:Chelating Agents; D019216:Metals, Heavy; D012987:Soil; D012989:Soil Pollutants
keywords: Chelating agents; Enzyme activity; Leachability/bioaccessibility; Organic acids; Reductants; Soil remediation