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Biogenic UO₂ (uraninite) nanocrystals may be formed as a product of a microbial reduction process in uranium-enriched environments near the Earth's surface. We investigated the size, nanometer-scale structure, and aggregation state of UO₂ formed by iron-reducing bacterium, Shewanella putrefaciens CN32, from a uranium-rich solution. Characterization of biogenic UO₂ precipitates by high-resolution transmission electron microscopy (HRTEM) revealed that the UO₂ nanoparticles formed were highly aggregated by organic polymers. Nearly all of the nanocrystals were networked in more or less 100 nm diameter spherical aggregates that displayed some concentric UO₂ accumulation with heterogeneity. Interestingly, pure UO₂ nanocrystals were piled on one another at several positions via UO₂-UO₂ interactions, which seem to be intimately related to a specific step in the process of growing large single crystals. In the process, calcium that was easily complexed with aqueous uranium(VI) appeared not to be combined with bioreduced uranium(IV), probably due to its lower binding energy. However, when phosphate was added to the system, calcium was found to be easily associated with uranium(IV), forming a new uranium phase, ningyoite. These results will extend the limited knowledge of microbial uraniferous mineralization and may provide new insights into the fate of aqueous uranium complexes.

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Batch sorption experiments of U(VI) onto the surfaces of granite rock were carried out. The distribution of the sorbed U(VI) on crushed granite particles was investigated via a sequential chemical extraction method to quantify the amount of U(VI) sorbed on the mineral phases and by X-ray diffraction characterization of dried samples of the remaining minerals after each extraction step to determine the significant mineral phases that sorb U(VI). Chlorite among the constituent minerals, even though being present in a small amount, is assumed to contribute to the strong U(VI) sorption and most of the sorbed U(VI) was weakly bound to the granite surfaces. However, a discriminative amount of the sorbed U(VI) on granite particles was difficult to desorb, in particular, when sorption took place in the alkaline region. The distribution of the sorbed U(VI) both on the fresh intact surface and on the natural fracture surface was also investigated by a X-ray image mapping technique. The result shows that dominant minerals hosting the majority of the sorbed uranium are mica for the intact surface of granite and chlorite for the fracture surface of the granite.