Relative Reactivity of Biogenic and Chemogenic Uraninite and Biogenic Noncrystalline U(IV)

Aqueous chemical extractions and X-ray absorption spectroscopy (XAS) analyses were conducted to investigate the reactivity of chemogenic uraninite, nanoparticulate biogenic uraninite, and biogenic monomeric U­(IV) species. The analyses were conducted in systems containing a total U concentration that ranged from 1.48 to 2.10 mM. Less than 0.02% of the total U was released to solution in extractions that targeted water-soluble and ion exchangeable fractions. Less than 5% of the total U was solubilized via complexation with a 0.1 M solution of NaF. Greater than 90% of the total U was extracted from biogenic uraninite and monomeric U­(IV) after 6 h of reaction in an oxidizing solution of 50 mM K₂S₂O₈. Additional oxidation experiments with lower concentrations (2 mM and 10 mM) of K₂S₂O₈ and 8.2 mg L^–1 dissolved oxygen suggested that monomeric U­(IV) species are more labile than biogenic uraninite; chemogenic uraninite was much less susceptible to oxidation than either form of biogenic U­(IV). These results suggest that noncrystalline forms of U­(IV) may be more labile than uraninite in subsurface environments. This work helps fill critical gaps in our understanding of the behavior of solid-associated U­(IV) species in bioremediated sites and natural uranium ore deposits.