Spectrosmicroscopic and spectroscopic investigation of U(IV) speciation in model mineral-organic matter assemblages

Abstract:

Both nanocrystalline uraninite (UO₂) and non-crystalline U(IV) occur in anoxic sediments, controlling the fate and transport of U in contaminated aquifers. It is important to distinguish between these forms of U because non-crystalline species are more reactive towards oxidants and aqueous complexing ligands, increasing the likelihood of U re-release into groundwater in the presence of such solutes. Much work has been done to elucidate microbiological and geochemical conditions favoring non-crystalline U(IV) or UO₂ formation, primarily in model systems containing a single type of U(VI)-reducing bacterium. Research suggests that microbial biomass, including cell walls and exopolymeric substances (EPS), can adsorb U(IV), likely via phosphoryl groups. Furthermore, conditions that favor EPS formation appear to promote non-crystalline U(IV) formation. Non-crystalline U(IV) formation is also favored in the presence of phosphate. However, U(IV) behavior in complicated systems containing competing U(IV) sorbents has not been studied. Investigations of U(IV) behavior in such systems are needed to understand uranium mobility in natural sediments, which contain multiple sinks for U(IV).

We have developed a model system in which the native microbial consortia associated with partially decayed plant roots utilize homogenized root material to facilitate U(VI) reduction during anaerobic incubations. The model is intended to simulate an environment similar to that found in anoxic sediments where buried organic matter drives anaerobic respiration. We use this model to address the following questions: (1) Does U(IV) become associated with organic materials or minerals (or both)? (2) Does U(IV) form complexes with particular ligands, such as P? (3) Is UO₂ produced when aqueous U(VI) concentrations are relatively low (~1 μM), which is typical of even contaminated sites? We have found that U(IV) does not form UO₂ at low, environmentally relevant U:sorbent ratios. Furthermore, nanocrystalline and non-crystalline U(IV) associate with both inorganic and organic sorbents, suggesting that, although microbially-derived sedimentary organic matter can play an important role in U(IV) behavior, so can the inorganic sedimentary matrix.