Removal of hexavalent chromium by electrochemical reduction–precipitation: Investigation of process performance and reaction stoichiometry

Abstract

This work experimentally investigates aqueous Cr(VI) reduction/removal by electrotreatment with mild steel electrode in a batch stirred reactor. Cr(VI) removal is due to simultaneous reduction to Cr(III) followed by precipitation and adsorption of Cr(VI) on Fe(OH)3(s)/Cr(OH)3(s) sludge formed. Cr(VI) adsorbed on sludge increases to maximum 10% and decreases to negligible level with diminishing concentration of Cr(VI) in solution. Concentration of Cr(VI) can be decreased to its discharge limit (0.5mg/l) in a single process step without addition of any precipitating agent. The highest reduction rate of Cr(VI) was found to be 3.8mgl−1min−1 at lowest initial pH 2 at the beginning (treatment time <1min) of the run, but Cr(III) in solution remains high due to its high solubility. The Cr(VI) reduction rate varies between 1.38 and 0.89mgl−1min−1 at higher initial pH (4.9–10). Among the anions Cl−, NO3−, SO42− and PO43−, PO43− is found to inhibit Cr(VI) reduction significantly by around 27% as it enhances oxidation of Fe(II) by dissolved oxygen. Stoichiometry of Cr(VI) reduction was experimentally determined at lower pH (2–3.1) range, where oxidation of Fe(II) by dissolved oxygen and its precipitation can be neglected. Against stoichiometric ratio of ∼3 in case of reduction of Cr(VI) by Fe(II), the observed overall molar ratio varies from 2.49 to 2.92. Participation of Fe0 on the electrode surface(s) in reducing Cr(VI) and accumulation of deposit on electrodes explain the observations including progressive variation.