XAFS Investigation of the Interactions of UVI with Secondary Mineralization Products from the Bioreduction of FeIII Oxides

Biogenic Fe-II phases (magnetite, green rust siderite, vivianite, etc.) provide a reservoir of reducing capacity in many subsurface environments that may contribute to the reduction of contaminants such as U-VI. We have examined the uptake and reduction of U-VI in the presence of biogenic green rust(BioGR), magnetite (BioMAG), and siderite (BioSID) formed during the reduction of Fe-III oxides by Shewanella putrefaciens CN32. Within 48 h, total solution-phase U-VI concentrations decreased from 500 mu M to 1.5 mu M, 392 mu M, and 472 mu M in the U-BioGR, U-BioMAG, and U-BioSID systems, respectively. Analysis of the samples by U L-III extended X-ray absorption fine structure spectroscopy (EXAFS) indicated that despite a stoichiometric excess of Fe-II, no more than 6% of U-VI was reduced to U-IV in the U-BioSID system, and no more than 22% of U-VI was reduced in the U-BioMAG system. For comparison, in the U-BioGR system, >99% of U-VI was reduced to U-IV. Uptake of U-VI by BioGR and BioMAG was accompanied by formation of nanoparticulate uraninite. The U EXAFS data for the U-BioSID system were consistent with partial U-VI/U-IV substitution for Fe-II in the surface layer of siderite particles and adsorption of U-IV.