Senko and three co-authors reported in mid-2005 that microbiological reduction of soluble U(VI) to insoluble U(IV) is a well-known means of preventing the migration of that element in groundwater, but the presence of nitrate in U(IV)- containing sediments leads to U(IV) oxidation and remobilizaton. Nitrite or iron(III) oxyhydroxides might oxidize U(IV) under nitrate-reducing conditions, and the authors determined the rate and extent of U(IV) oxidation by these
compounds.
Fe(III) oxidized U(IV) at a greater rate than nitrite (130 and 10 μM U(IV)/day, respectively). In aquifer sediments, Fe(III) may be produced during microbial nitrate reduction by oxidation of Fe(II) with nitrite, or by enzymatic Fe(II) oxidation coupled to nitrate reduction. To determine which of these mechanisms was dominant, the authors isolated a nitrate-dependent acetate- and Fe(II)-oxidizing bacterium from a U(VI)- and nitrate-contaminated aquifer. This organism oxidized U(IV) at a greater rate and to a greater extent under acetate-oxidizing (where nitrite accumulated to 50 mM) than under Fe(II)-oxidizing conditions.
The authors showed that the observed differences in rate and extent of U(IV) oxidation are due to mineralogical differences between Fe(III) produced by reaction of Fe(II) with nitrite (amorphous) and Fe(III) produced enzymatically (goethite or lepidocrocite). The authors' results suggested that the mineralogy and surface area of Fe(III) minerals produced under nitrate-reducing conditions affect the rate and extent of U(IV) oxidation. These results may be useful for predicting the stability of U(IV) in aquifers.
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