Kinetics of Cu, Pb and Zn removal during soil flushing with washing agents derived from sewage sludge

Flushing of Soils Highly Contaminated with Cd Using Various Washing Agents Derived from Sewage Sludge

The suitability of sewage-sludge derived washing agents (SS_WAs) (dissolved organic matter DOM; humic-like substances HLS; soluble humic substances SHS), was assessed for removing Cd from highly contaminated (300 mg/kg) sandy clay loam and clay. The soils were remediated via column flushing at two flow rates, 0.5 and 1.0 mL/min. The stability of the flow velocity (FV) depended on the type of SS_WA and decreased in the following order: DOM > HLS > SHS. Cd was most effectively removed during the first hours of flushing, and the process proceeded with a first-order kinetics. The overall process efficiency was higher at flow rate of 1.0 mL/min than at 0.5 mL/min and ranged from 65.7 (SHS) to 75.5% (DOM) for the sandy clay loam and from 64.7% (SHS) to 67.8% (DOM) for the clay. However, all SS_WAs at both flow rates removed the most mobile Cd fraction (F1) with an efficiency above 90%. Flushing improved soil characteristics in terms of the content of organic matter, humic substances and nutrients. Among all SS_WAs, DOM was the most suitable for remediation of highly Cd-contaminated soils due to high efficiency of Cd removal, the high stability of its FV during flushing and the simple manner of DOM recovery from sewage sludge.

Quality of heavy metal-contaminated soil before and after column flushing with washing agents derived from municipal sewage sludge

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.