Soil Washing Enhanced by Humic Substances and Biodegradable Chelating Agents

Industrial timber treatment sites have resulted in widespread soil contamination by Cu, Cr, and As, presenting potential long-term liability and associated risks to human health and the environment. This study evaluated the roles of natural humic substances (lignite-derived humic substances, standard and commercially available humic acids) and biodegradable chelating agents (ethylenediamine-N,N-disuccinic acid (EDDS) and glutamic-N,N-diacetic acid (GLDA)) for soil washing. Batch kinetic experiments revealed that humic substances promoted Cu extraction at pH 8, but they were significantly adsorbed on the soil at pH 4, possibly posing impediment to soil remediation. The metal extraction by EDDS and GLDA was comparable to that of EDTA (ethylenediamine-tetraacetic acid), and it was more effective at pH 4 than pH 8, probably due to acidic dissolution of metal precipitates and oxides. Metal distribution analysis indicated that the carbonate fraction of Cu and the oxide fraction of As and Cr were mainly extracted, while the exchangeable fraction of Cu increased. The residual leachability tests showed that humic substances reduced the Cu and As leachability but the reduction was insufficient. In contrast, EDDS was able to reduce the leachate concentrations of Cu and As to below 5 mg L-1, meeting the waste acceptance criteria for landfill disposal. Nevertheless, soil washing methods and remediation strategy may need further modifications to facilitate site restoration and promote soil recycling.