Comparative Study for Reduction of Hexavalent Chromium by High Carbon Iron Filings and Electrolytic Iron: Mass Transfer Limitations

摘要

Reduction of chromium(VI) by electrolytic iron and high carbon iron filings (HCIF) was carried out in batch reactors. The pseudo-first order reaction rates for reduction of Cr(VI) by HCIF were nearly one order of magnitude higher than electrolytic iron. It was revealed that the adsorption of Cr(VI) to graphite inclusions, present on HCIF, played an important role in quick decline of aqueous concentration in comparison to electrolytic iron. Further the reduction of Cr(VI) with HCIF was studied in completely mixed and non-mixed conditions. The pseudo-first order reduction rates of Cr(VI) in completely mixed conditions were nearly 20 times that of rates obtained in stationary batch reactors. The rates for electrolytic iron and HCIF stationary batch reactors were comparable which is due to reduction in adsorption of Cr(VI) to non reactive sites in non mixed conditions. This suggests that the reduction of Cr(VI) in permeable reactive barriers, built in sub-surface, for in situ remediation of groundwater, may be subjected to mass transfer limitations as the conditions in subsurface are poorly mixed. Further reduction of Cr(VI) lead to the formation of Cr(III). Elution of Cr(III) was also found to be lower in case of HCIF, probably due to adsorption of Cr(III) to non reactive sites on HCIF.