Removal of Cr(VI) from model wastewaters by electrocoagulation with Fe electrodes

摘要

The perf ormance of an electrocoagulation system with iron electrodes for Cr(VI) removal from model wastewaters in laboratory scale was studied systematically. Several parameters - such as initial metal concentration (10-50 mg/l Cr), charge loading, and applied current - and their influence on the electrocoagulation process were investigated. Cr concentration decreased only slightly by coagulation time at high currents (1.0-3.0 A), whereas at low currents (0.05-0.1 A), 10 mg/l Cr was removed completely from the solution after 45 min. Initial concentrations from 10 to 50 mg/l Cr did not influence the removal rates at low currents, whereas higher initial concentrations caused higher removal rates at high currents. At all investigated currents, the Cr(VI) concentration was always only a little lower than the Cr_(totoal) concentration and the Fe~(2+) concentration in solution was always below 0.1 mg/l. The study gave indications on two different removal mechanisms of Cr(VI) with iron electrodes at high and low currents. We propose that at high currents Cr(VI) was reduced directly at the cathode and precipitated afterwards as Cr(OH)3. The Cr removal depended on initial concentration and the removal rate ((μmol/As) was independent from the inserted current. At low currents the Cr(VI) removal was proposed to work by reduction by Fe~(2+). Under these conditions, iron was quantitatively dissolved as Fe~(2+) from the electrodes according to Faraday's law and the dissolved amounts were two orders of magnitude higher compared to the iron dissolution at high currents. The removal process at low currents was much more efficient and seemed to be applicable for the removal of Cr(VI) from industrial wastewaters.