Biotic and abiotic transformations of alkyl halides in iron reducing environments

Transformations of carbon tetrachloride (CT), and other common alkyl halides, were investigated in a model iron reducing system which employed Geobacter metallireducens as a representative dissimilative iron reducing bacteria (DIRB), hydrous Fe(OH)₃ (s) as an electron acceptor, and acetate as a substrate. The contributions of cell-mediated (biotic) and mineral-mediated (abiotic) reactions to CT transformation were investigated separately in resting cell suspensions of G. metallireducens, or in suspensions of mineral particles recovered from spent cultures. Rates of CT transformation by G. metallireducens were described by a two-site Michaelis-Menten model. The biotic reaction products included chloroform (CF) (11–30%) and unidentified cell-bound trichlorocarbon species (70–89%). Specific labeling of membrane proteins by 14CT, and mutual inhibition effects observed for FeIII-citrate and CT, suggested that CT was transformed within the cell membranes via a co-metabolic mechanism.; Microscopic and crystallographic studies revealed that the biogenic minerals consisted predominantly of magnetite particles (3–20 nm diameter) with a small fraction of siderite (FeCO₃) as a co-precipitate. The changes in geochemistry and mineralogy accompanying microbial reduction of Fe(OH) ₃ (s) suggested that magnetite formed via a topotactic (solid-state) transformation. CT transformation by biogenic magnetite obeyed pseudo-first-order kinetics and was proportional to mineral surface area concentration. Products of the mineral-mediated (abiotic) reaction included chloroform (CF)(45–50%), carbon monoxide (CO)(37–39%) and methane (CH₄)(8–10%). Trichloromethyl free-radical and dichlorocarbene intermediates were trapped during the reaction, consistent with CF formation via hydrogenolysis, and CO and CH₄ formation via a carbene intermediate.; Measurements of protein and mineral surface area during growth of G. metallireducens on Fe(OH)₃ indicate that the transformation of CT in the model system (cells + minerals) is due, almost entirely, to abiotic surface-mediated reactions. This conclusion was further supported by an Arrhenius study, which showed linearity for the reaction rate in whole culture up to 70°C, where most enzymes should denature. The findings suggest that reactive biogenic minerals could play a significant role in the natural attenuation of chlorinated solvents in iron reducing environments. A novel approach for remediating alkyl halides, and other groundwater contaminants, may be to enhance the formation of reactive biogenic minerals in-situ.