Bargar and three co-authors reported late 2008 that biogenic uraninite is of interest to geoscientists for its importance to bioremediation strategies, remarkably small particle size, and biological origin. Recent studies have begun to illuminate the chemical/structural complexities of this important natural nanomaterial. Intriguingly, in spite of its incredibly diminutive size, the molecular-scale structure, energetics, and surface- area-normalized dissolution rates of hydrated biogenic uraninite appear to be similar to those of coarser-particle, abiotic, stoichiometric UO2.
These findings have important implications for the role of size as a moderator of nanoparticle aqueous reactivity and for the bioremediation of subsurface U(VI) contamination.
The authors concluded that successful uranium bioremediation is likely to require continued maintenance of strictly anaerobic conditions. As long as transport-limited removal of reaction products allows approach to equilibrium, particle size is anticipated to minimally impact the stability of biogenic uraninite. Moreover, it is likely that acquisition of impurity cations from groundwater and excess structural oxygens may significantly enhance the stability of uraninite with respect to oxidation by O2.
Although diminutive, the nanoparticles are closely associated with biomass and tend to aggregate, making it unlikely that they will be transported by advection in groundwater. Distortion of nanoparticle surface structure is a means by which surface energy can be mitigated (Banfield and Zhang
2001).
It also has been noted that hydration (Navrotsky et al. 2008) and aggregation (Huang et al. 2004) play important roles in reducing nanoparticle surface energy and extending the stability range of the most stable large crystal phase. It is plausible that each of these effects acts to mitigate the impact of particle size on the observed stability of biogenic uraninite.
The intrinsically faster carbonate-enhanced dissolution of biogenic uraninite relative to coarse-grained abiotic UO2.0 suggests that the interfacial chemistry of biogenic uraninite is different from that of larger-particle forms. These differences are likely to be problematic for predicting rates of uranium release in the field.
Research that improves our understanding of the fundamental factors governing the surface chemistry of nanoparticulate uraninite is therefore important for constructing accurate geochemical models of uranium behavior following bioremediation. These challenges will present opportunities for research in nano(bio) geoscience for some time to come.
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