Elucidating the Structure and Reactivity of Bacteriogenic Iron Oxides to Better Understand Contaminant Cycling.

The iron (Fe) biogeochemical cycle is composed of numerous important environmental processes that partially govern carbon cycling, energy flow, and the adsorption and release of nutrients and potentially toxic metals in natural waters, soils, and sediments. The ability of Fe to act as a sink for toxicants is highly dependent upon its redox state, which is heavily influenced by bacteria in the environment. Oxidation of Fe(II) in most aerobic waters at circumneutral pH was long thought to be an abiotic process due to the rapid rates of Fe(II) oxidation. However, the oxidation of Fe(II) to Fe(III) at neutral pH is an exergonic reaction that allows bacteria to generate energy and grow. Recent microscopic and microbiological studies have shown that when anoxic groundwater with high concentrations of ferrous iron meets a physically quiescent oxic zone at circumneutral pH, communities of Fe oxidizing bacteria may form at the oxic-anoxic boundary and produce  bacteriogenic Fe(III) (oxyhydr)oxides which are quite diverse, form in many different environmental niches, and have unique chemical properties. However, our current understanding of the properties of bacteriogenic Fe(III) (oxyhydr)oxides  that form at circumneutral pH, such as crystal structure, reactivity, and binding mechanisms, is limited. This study used a combination of cutting-edge microscopic, spectroscopic, and X-ray scattering techniques to elucidate the structure, reactivity, and binding mechanisms of bacteriogenic iron (III) (oxyhydr)oxides to better understand how they impact the movement of contaminants in natural systems.