Experimental Assessment of Chromium Mobilization from Polluted Soil by Washing

Abstract

Chromium contamination in soils mainly results from the discharge of chromium-containing wastes from several industrial activities. Chromium remediation in soils may involve extraction from the soil matrix by washing with an aqueous solution containing a chelating agent. Since any extraction treatment strongly affect soil chemical and physical properties, and the metals distribution into the different soil fractions, one of the major risks of applying this remediation approach may be the increase in the oxidation potential of the soil and, as a consequence, the enhancing of trivalent chromium conversion into the soluble and more toxic hexavalent form. This oxidation potential, in fact, will strongly depends upon several soil characteristics, mainly organics amount and manganese oxide content. The objectives of this paper were therefore to investigate the effectiveness of selected unspecific chelating agents (Ethylenediaminetetraacetic acid, EDTA, Ethylenediamine-N,N\'-disuccinic acid, EDDS, and Nitrilotriacetic acid, NTA) towards total chromium removal and to investigate the effect of the extraction treatment on the potential chromium oxidation capacity of the soil. Experimental batch tests of chromium extraction from soil slurries were performed at different pH and at a 1:10 liquid vs. solid ratio: chromium concentration was determined on the extracted solutions, and mass balances were performed. Results show that using EDTA and EDDS, the calculated maximum chromium extraction efficiencies were 25 % and 27.5 % respectively, depending upon the pH of the extracting solution, while a lower removal rate was observed by using NTA. According to the results of oxidation tests performed after the extractions, as a consequence of soil characteristics modifications induced by the extraction process, the soil showed a different potential of Cr(III) oxidation, depending on the operating conditions of the washing treatment.