|Title:||Integrated remediation of metal-contaminated soils : biodegradable chelant-enhanced extraction and in-situ stabilization|
|Advisors:||Tsang, Dan (CEE)|
Li, Xiang-dong (CEE)
|Subject:||Soils -- Heavy metal content.|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Civil and Environmental Engineering|
|Pages:||xxvi, 207 pages : color illustrations|
|Abstract:||Soil contamination by metal(loid)s has drawn increasing attention worldwide. A national soil contamination survey by the government of China in 2014 showed that around 1/5 of the surveyed lands were considered contaminated according to the national standards. As reported, 82.4% of the surveyed lands were contaminated by metal(loid)s (particularly by Cd, Ni, As, Hg, Cu, Pb, Cr, and Zn). Remediation techniques like soil washing by a readily biodegradable chelant, EDDS (S,S-ethylene-diaminedisuccinic acid), has a low extraction efficacy of Pb in comparison with EDTA (ethylenediamine tetraacetic acid) which is hard to biodegrade naturally. Besides, it is found that residual metals become more mobilized after chelant-assisted extraction. At the same time, the lability of metal(loid)s after soil stabilization under varying environmental conditions is still an uncertainty. Individual studies were conducted using an e-waste contaminated soil to investigate the feasibility of EDDS washing using various washing schemes with mixed use of chelants and an integration with selective dissolution. Stabilization effects of As and Pb in a field soil sample was also studied. Low-cost amendments were used in the stabilization under dynamic redox potential (EH) conditions which is one of the most crucial environmental factors. Based on the above results, an integrated study of EDDS extraction and stabilization was proposed and conducted to evaluate the mobility, leachability, bioaccessibility, and phytoavailaiblity of residual metals in the soil.|
In this research work, the combined use of chelants successfully reduced the use of EDTA by half while keeping high extraction efficiency of all target metals (Cu, Zn, and Pb), which further studied by the modeling results of Visual MINTEQ. Besides, the multi-step washing demonstrated the high extraction efficacy, reduced the undesired mineral washing and readsorbed metal-chelant complexes which might increase metal leachability, in comparison with the 24-h continuous washing. The pretreatment using reductants and organic acids to remove the metals strongly bound on Fe/Mn oxides and organic matter facilitated the subsequent EDDS extraction. Higher metal extraction efficiencies and low leachability and bioaccessibility of the residual metals were observed, yet the reductants and organic acids increased the cytotoxicity and decreased the microbial activities and the amount of available nutrients in the soil. Frequently changing environmental conditions like EH altered the stabilization effects of As and Pb by pine sawdust biomass and pine sawdust-induced biochar. An increasing mobility of As was observed with the decrease of EH, which is associated with the increased mobility of arsenite and dissolved Fe oxides. The increment of pH related to the alkalinity of biochar also contributed to the enhanced mobility of As, especially for the biochar produced at a higher temperature. Meanwhile, the increasing pH suppresses the mobility of Pb through forming Pb-containing precipitations. An integration of 2-h EDDS extraction and 2-month stabilization by low-cost amendments actually removed the labile metals, which in turn reduced the metal bioaccessibility and phytoavailability. Meanwhile, the immobilization by biochar and coal fly ash facilitated alleviation the potentially mobilized metals by EDDS extraction.
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