Remediation of metal-contaminated aqueous systems by electrochemical peroxidation: an experimental investigation

An innovative technology, electrochemical peroxidation (ECP), was investigated for remediation of mixed metal-contaminated aqueous systems by application of direct electric current to steel electrodes and of dilute H2O2 solution to promote Fenton's reactions, forming sparingly soluble solid hydrous ferric oxides (HFO). Bench scale experiments evaluated the sorption and distribution of As, Be, Cd, Cr, Cs, Cu, Li, Ni, Pb, Se, V, and Zn among the soluble and solid state HFO formed as part of the ECP process. The effects of pH, hydrogen peroxide concentrations and electric current process times on the efficiency of metal removal were studied. The potential of this technology was demonstrated by effective removal at pH 3.5-4.6 and within 3 min of 0.25 A dc + 100 mg H2O2 l(-1), of As, Cr, Pb, Se and V with complete removal of As and Cr, > 95.0%. On increasing the pH of the solution to 6.0, the retention of Be and Cu by HFO increased from 0.9-1.9% at pH 3.5 to 76.8-80.7% at pH 6.0 while concentrations of other metals, such as Pb, decreased due to precipitation of Pb hydroxy-complexes. Experiments in the absence of H2O2 revealed that metals were adsorbed by HFO with the same order of affinity, As > Cr > Se > V > Be, as in the presence of H2O2, but, with the exception of Cr, to a lesser extent. H2O2 used in the ECP process was fundamental to increase the adsorption capacity of HFO for As, from 79.2 to 99.2%, due to the oxidation of As(III) to As(V), which has a stronger affinity for HFO. The reduced adsorption may be related to the formation of poorly ordered crystalline akaganeite, which has a lower surface area than ferrihydrite formed when H2O2 was used. The optimal operating conditions were pH < 6.0, an H2O2 concentration of 100 mg l(-1) and a current process time not exceeding 3 min. (C) 2001 Elsevier Science B.V. All rights reserved.