Simultaneous Multi-metal Removal from Soil with Washing Agents of Waste, Plant and Microbial Origin

Abstract

ABSTRACT In this study, batch washings of soil highly contaminated with Cd, Cu, Ni, Pb and Zn were performed with three washing agents (WAs) of different origin, i.e., humic-like substances (HLS) extracted from municipal sewage sludge, a plant polyphenol (tannic acid, TA) and microbial rhamnolipids (RAM). At optimum concentration and pH, i.e., 5 g C/L and pH 4 (HLS), 21 g C/L and pH 4 (TA), and 3 g C/L and pH 5 (RAM), the efficiencies of HM removal were 50% (Pb) to 89% (Cd) for HLS, 61% (Zn) to 89% (Cd) for TA, and 24% (Cu) to 62% (Cd) for RAM. HLS was competitive with commercially available TA, especially for the removal of Cd, which had the highest toxic-response factor. Although single washing with HLS and TA decreased total Cd, Cu, Ni and Zn concentrations below quality standards for agricultural soil and washing with RAM decreased the Zn concentration below the standard, most HMs did not have a low potential ecological risk index after washing (Er > 40). Thus, sequential soil washing with HLS followed by HLS or with TA followed by HLS should achieve low ecological risk from HMs in soil.