Reduction of uranium(VI) by mixed iron(II)/iron(III) hydroxide (green rust): formation of UO2 nanoparticles

Green rusts, which are mixed ferrous/ferric hydroxides, are found in many suboxic environments and are believed to play a central role in the biogeochemistry of Fe. Analysis by U LIII-edge X-ray absorption near edge spectroscopy of aqueous green rust suspensions spiked with uranyl (U(VI)) showed that U(VI) was readily reduced to U(IV) by green rust The extended X-ray absorption fine structure (EXAFS) date for uranium reduced by green rust indicate the formation of a UO2 phase. A theoretical model based on the crystal structure of UO2 was generated by using FEFF7 and fitted to the data for the UO2 standard and the uranium in the green rust samples. The model fits indicate that the number of nearest-neighbor uranium atoms decreases from 12 for the UO2 structure to 5.4 forthe uranium-green rust sample. With an assumed four near-neighbor uranium atoms per uranium atom on the surface of UO2, the best-fit value for the average number of uranium atoms indicates UO2 particles with an average diameter of 1.7 +/- 0.6 nm. The formation of nanometer-scale particles of UO2, suggested by the modeling of the EXAFS data, was confirmed by high-resolution transmission electron microscopy, which showed discrete particles (approximately 2-9 nm in diameter) of crystalline UO2. Our results clearly indicate that U(VI) (as soluble uranyl ion) is readily reduced by green rust to U(IV) in the form of relatively insoluble UO2 nanoparticles, suggesting that the presence of green rusts in the subsurface may have significant effects on the mobility of uranium, particularly under iron-reducing conditions.