Soil chemistry effect on feasibility of Cr-decontamination by acid-washing

Soil washing with simple acid has been proven to be effective for removal of cationic heavy metals from contaminated soils. Since the adsorption of anionic heavy metals is enhanced in acidic medium, the efficiency of acid-washing may not be guaranteed for soils that are doubly contaminated with cationic and anionic heavy metals. To evaluate the efficiency of acid-washing, nine soils were artificially contaminated with chromate and chromium was extracted with hydrochloric acid of 0.5 mmol L^-1 to 1 mol L ^-1. A part of spiked chromate was reduced to trivalent chromium and the remaining chromate ions were almost quantitatively extracted with the hydrochloric acid. However, increasing hydrochloric acid concentration was not effective for the extraction. Possible mechanisms underlying the observed low efficiency were considered to be the enhanced reduction in acidic solution, complexation by humic substance and adsorption via cation exchange reactions. We concluded that acid-washing satisfactorily works for chromate removal in non-allophanic soils but inefficient for removal of trivalent chromium particularly from soils having high effective cation exchange capacity and organic matter content.