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Elucidating Bioreductive Transformations within Physically Complex Media: Impact on the Fate and Transport of Uranium and Chromium
The greatest challenge to elucidating geochemical and biological chromium reduction in natural sediments is to create a sterile environment without destroying the chemical and physical properties of the system. In this study we determined the potential for geochemical and biological chromium reduction in a naturally reducing soil using carbon amendments and sterilization. To minimize alterations to the sediment samples, soils were sterilized via exposure to ?-irradiation which causes fewer changes in the physical and chemical properties of the soil compared to other methods of sterilization. The objective of our research was to determine if the absence of viable microorganisms significantly affected the extent of chromium reduction in a reducing soil. Our hypothesis was that if geochemical reduction pathways dominated the system then soil sterilization should have little to no effect on the amount and rate of chromium that was reduced. However, if the reduction of chromium in these soils was a synergistic process then significantly different amounts of chromium should be reduced in the sterilized versus non-sterilized samples
Effects of gamma-sterilization on the physico-chemical properties of natural sediments
Batch U(VI) sorption/reduction experiments were completed on sterilized and non-sterilized sediment samples to elucidate biological and geochemical reduction mechanisms. Results from X-ray absorption nearedge structure (XANES) spectroscopy revealed that γ-sterilized sediments were actually better sorbents of U(VI), despite the absence of any measurable biological activity. These results indicate that γ-irradiation induced significant physico-chemical changes in the sediment which is contrary to numerous other studies identifying γ-sterilization as an effective and minimally invasive technique. To identify the extent and method of alteration of the soil as a result of γ-sterilization, untreated soil samples, physically separated size fractions, and chemically extracted fractions of the soil were analyzed pre- and post-sterilization. The effects of sterilization on mineralogy, pH, natural organic matter (NOM), cation exchange capacity (CEC), and iron oxidation state were determined. Results indicated that major mineralogy of the clay and whole sediment samples was unchanged. Sediment pH decreased only slightly with γ-irradiation; however, irradiation produced a significant decrease in CEC of the untreated sediments and affected both the organic and inorganic fractions. Mössbauer spectra of non-sterile and γ-sterilized sedimentsmeasuredmore reduced iron present in γ-sterilized sediments compared to non-sterile samples. Our results suggest that sterilization by γ-irradiation induced iron reduction that may have increased the sorption and/or reduction of U(VI) onto these sediments. However, Mössbauer and batch sorption data are somewhat contradictory, the former indicates that the iron oxide or iron hydroxide minerals are more significantly reduced while the later indicates that reduced clay minerals account for greater sorption of U(VI)