Effects of Electron Donors and Acceptors on Cr(VI) Removal in Biotic Fe0 Columns Preloaded with Microorganisms

The removal of Cr(VI) from groundwater by zero-valent iron (Fe0) and microorganisms under different nutrients and electron-accepting conditions was investigated using continuously-flowed column reactors with an abiotic Fe0 column as the control. Both abiotic and biotic Fe0 columns were operated at low and relatively high concentrations of organic substrates (sodium acetate and humic acid) and electron acceptors (nitrate and sulfate). Sodium acetate and humic acid were found to be effective substrates for the biotic Fe0 system to remove Cr(VI) since microorganisms were able to gain extra reducing capacity from two nutrients and simultaneously avoid the passivation effect of humic acids on Fe0 by metabolizing them. For nitrate and sulfate, varied influences were observed in the biotic Fe0 system. The former showed apparent inhibition on Cr(VI) removal as it competed with Cr(VI) for electrons, and the inhibition became more severe while nitrate concentrations increased. Differently, highly reactive minerals such as green rusts and iron sulfide could be generated on the biotic Fe0 surface under sulfate-reducing conditions. They significantly enhanced Cr(VI) removal by providing more adsorptive and reductive sites. Overall, though the biotic Fe0 column exhibited a longer life-span and higher Cr(VI) removal capacity than the abiotic Fe0 column, the effects of organic substrates and electron acceptors should still be taken into account when applying biotic Fe0 systems for Cr(VI)-contaminated groundwater remediation.