Redox Interactions Between Cr(VI) and Fe(II) in Bioreduced Biotite and Chlorite

Contamination of the environment with Cr as chromate (Cr­(VI)) from industrial activities is of significant concern as Cr­(VI) is a known carcinogen, and is mobile in the subsurface. The capacity of Fe­(II)-containing phyllosilicates including biotite and chlorite to alter the speciation, and thus the mobility, of redox-sensitive contaminants including Cr­(VI) is of great interest since these minerals are common in soils and sediments. Here, the capacity of bacteria, ubiquitous in the surface and near-surface environment, to reduce Fe­(III) in phyllosilicate minerals and, thus, alter their redox reactivity was investigated in two-step anaerobic batch experiments. The model Fe­(III)-reducing bacterium Geobacter sulfurreducens was used to reduce Fe­(III) in the minerals, leading to a significant transformation of structural Fe­(III) to Fe­(II) of 0.16 mmol/g (∼40%) in biotite and 0.15 mmol/g (∼20%) in chlorite. The unaltered minerals could not remove Cr­(VI) from solution despite containing a larger excess of Fe­(II) than would be required to reduce all the added Cr­(VI), unless they were supplied in a very high concentration (a 1:10 solid to solution ratio). By contrast, even at very low concentrations, the addition of bioreduced biotite and chlorite caused removal of Cr­(VI) from solution, and surface and near surface X-ray absorption spectroscopy confirmed that this immobilization was through reductive transformation to Cr­(III). We provide empirical evidence that the amount of Fe­(II) generated by microbial Fe­(III) reduction is sufficient to reduce the Cr­(VI) removed and, in the absence of reduction by the unaltered minerals, suggest that only the microbially reduced fraction of the iron in the minerals is redox-active against the Cr­(VI).