Electrodialytic versus acid extraction of heavy metals from soil washing residue

The feasibility of electrodialytic remediation (EDR) for treatment of suspended sludge after soil washing is in focus in the present paper. Five industrially contaminated soils were treated in laboratory scale remediation experiments, and the toxic elements of the investigation were: As, Cd, Cu, Cr, Ni, Zn and Pb. The results showed that all investigated elements could be removed from all soils to some extent by EDR. For all anthropogenic contaminants, higher extraction can be obtained under the influence of the direct current during EDR than by washing. During EDR most elements were transferred primarily to the cathode, where Cu and Pb precipitated at the cathode, while Cd, Cr, Ni and Zn primarily, or solely (for Ni), were dissolved in the catholyte, showing how cationic species dominated the chemistry of these elements. Despite the differences between the soils, the remediation results were explained well by the hydrolytic chemistry of the elements, with a difference between the soils. The only element transported primarily towards the anode, was arsenic suggesting that As(V) is the dominating species, and showing that As(III) is oxidized during the remediation process. In contrast the kinetic stability of Cr(III) hinders oxidation of this element, and leaves this as the least removable of the seven.