Elucidating the Mechanisms of Bacteriogenic Iron Oxide Reduction and Its Effects on Cr(VI) Sorption.

The iron (Fe) biogeochemical cycle is composed of myriad important environmental processes that partially govern carbon cycling, energy flow, and the adsorption and release of nutrients and potentially toxic metals into the environment. Recent studies have shown that when anoxic groundwater with high concentrations of ferrous iron meets a physically quiescent oxic zone at circumneutral pH, communities of iron oxidizing bacteria can out compete abiotic Fe oxidation to form bacteriogenic iron oxides (BIOS) with unique chemical properties that distinguish them from their abiotic counterparts. Although much is known about the biotic oxidation of Fe in sub-oxic, circumneutral pH environments, less is known about their redox reactivity with nutrients and contaminants in waterways. For this study, we investigated the mechanisms of BIOS reduction and its effects on Cr(VI) sorption. In order to tease out the mechanism of BIOS reduction, a series of kinetic sorption studies were conducted using synthetic ferrihydrite, BIOS, BIOS with 0.01 M sodium azide, and BIOS with 0.135 M ferrozine hydrate (all treatments had 50 mg Cr(VI) L^-1), adsorbent concentrations of 1 g L^-1, and sampling times at day 0, 1, 3, 7, and 14. X-ray absorption spectroscopy, specifically X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) were used to elucidate the valence state of Fe and Cr, and the local molecular structure of Fe pre and post sorption, respectively. By day 14, synthetic ferrihydrite sorbed ca. two-fold more Cr than all BIOS treatments (mass basis) and was sorbed as 100% Cr(VI), whereas the sorbed Cr on BIOS ranged from 50-100% Cr(VI). Understanding these mechanisms of BIOS reduction will improve our knowledge of environmental contaminant dynamics.