A holistic approach to remediation of soil contaminated with Cu, Pb and Zn with sewage sludge-derived washing agents and synthetic chelator

Abstract

Green soil washing is a new trend in remediation of heavy metal (HM) contaminated soils that is based on application of washing agents (WAs) derived from organic waste sources. This study compared for the first time the efficiency of sewage sludge-derived WAs (SS_WAs) (dissolved organic matter, DOM; soluble humic-like substances, HLS; soluble humic substances, SHS) extracted from sewage sludge with water or a NaOH solution. As a benchmark for comparison, Na2EDTA was used. The FT-IR/PAS spectra had higher intensities for SS_WAs than Na2EDTA, with a predominance of carboxyl groups in all WAs. The WAs were used to remove Cu, Pb and Zn from soil during double batch washings. The effects of these WAs on HM removal and the quality of remediated soil, including fertility indicators, content of humic substances, HM distribution, HM stability, soil phytotoxicity and activity of soil microorganisms (as dehydrogenase activity, DHA) were evaluated. The SS_WAs removed HMs with high efficiency (HLS 80–90%, SHS 61–89%), although slightly less efficiently than Na2EDTA (93–95%). DOM very efficiently removed Cu (7274 mg Cu·kg−1), but not Pb. Soil washing affected changes in HM distribution by increasing stability of most HM in washed soil. All SS_WAs, in contrast to Na2EDTA, increased soil content of organics and nutrients. The type of WA had no effect on germination of wheat seeds. However, Na2EDTA inhibited shoot and root growth (by 0.6% and 33%, respectively) and only slightly increased DHA in washed soil. In contrast, all SS_WAs stimulated shoot growth (by 29.4–40.6%), and HLS and SHS also stimulated root growth (by 5.6–11.6%) and increased DHA by the largest amount (6.5–7.4-fold compared to unwashed soil). Sewage sludge constitutes an important waste source of easily obtainable SS_WAs, which possess great potential for remediating highly polluted soils, especially Cu-polluted soils, with low toxicity and recovery of soil function. In conclusion, this holistic assessment of soil remediation indicates that SS_WAs can be an attractive alternative to Na2EDTA.