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

摘要

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 (0.5 mg/l) in a single process step without addition of any precipitating agent. The highest reduction rate of Cr(VI) was found to be 3.8 mg l~(-1) min~(-1) at lowest initial pH 2 at the beginning (treatment time <1 min) 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~(-1) min~(-1) at higher initial pH (4.9-10). Among the anions Cl~-, NO_3~-, SO_4~(2-) and PO_4~(3-), PO_4~(3-) 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 Fe~0 on the electrode surface(s) in reducing Cr(VI) and accumulation of deposit on electrodes explain the observations including progressive variation.