Abstract
Removal of heavy metals (HMs) from soil is a priority in soil washing/soil flushing. However, for further management of remediated soil, it should be characterized in detail. This study presents, for the first time, an evaluation of soil quality after column flushing with new-generation washing agents (WAs) recovered from municipal sewage sludge (dissolved organic matter, DOM; soluble humic-like substances, HLS; soluble humic substances, SHS) and Na2EDTA as a standard benchmark. Sandy loam soil was spiked with industrial levels of Cu, Pb and Zn, then flushed in a column reactor at two WA flow rates (0.5 and 1.0 ml/min). Soil quality was assessed by determining both physico-chemical (pH, total HMs and their mobility, soil organic matter, OM, humic substances, HS and their fractions, macroelements) and biological indicators (dehydrogenase activity, DHA; germination rate, GR; and inhibition factors for roots and shoots of Triticum aestivum). Total residual HMs contents and HMs contents in the mobile fraction were significantly lower in soil flushed at 1.0 ml/min than in soil flushed at 0.5 ml/min. With all WAs, the decrease in Cu content was larger than that of the other HMs, however this HM most effectively was removed with DOM. In contrast, Pb most effectively was removed by HLS and Na2EDTA, and DOM should not be used to remediate Pb-contaminated soil, due to its very low effectiveness. Flow rate did not appear to affect the fertilizing properties of the soil, DHA activity or soil toxicity indicators. Soil flushing with all SS_WAs increased OM, HS, and exchangeable P, K and Na content in remediated soils, but decreased exchangeable Ca content, and in most cases, exchangeable Mg content. Soil flushing substantially improved DHA activity and GR, but only slightly improved the shoot and root inhibition factors.
Highlights
Removal of heavy metals (HMs) from soil is a priority in soil washing/soil flushing
The residual HM contents in soil flushed at a flow rate of 1.0 ml/min. were significantly lower than those in soil flushed at 0.5 ml/min (p < 0.05)
In soil flushed with dissolved organic matter (DOM) or humic-like substances (HLS), the residual Cu content was lower than that in soil flushed with soluble humic substances (SHS) or N a2EDTA
Summary
Removal of heavy metals (HMs) from soil is a priority in soil washing/soil flushing. for further management of remediated soil, it should be characterized in detail. Some WAs that efficiently remove HMs change soil fertility and affect microbial activity, which plays an important role in soil organic turnover, element circulation, and phytotoxicity. The extent of these changes depends on the kind of WAs used. In order to effectively remove both HMs, those authors tested sequential use of the optimal dose of HCl followed by the optimal dose of N a2EDTA This sequential washing removed Cd and Pb with 87.3% and 73.2% efficiency, respectively. With the use of soap solution the removal efficiency of Cd, Cu, Pb and Zn was 62.0–69.0%, while with the use of rhamnolipids, it was 59.0–63.0%12
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