Reaction of UVI with Titanium-Substituted Magnetite: Influence of Ti on UIV Speciation

Reduction of hexavalent uranium (U-VI) to less soluble; tetravalent uranium,(U-IV) through enzymatic. or abiotic redox reactions has the potential to alter U mobility in subsurface environments. As ubiquitous natural mineral, magnetite (Fe3O4) is of interest because of its ability to act as a rechargeable reductant for U-VI. Natural magnetites are often impure with titanium, and structural Fe3+ replacement by Ti-IV yields a proportional increase in the relative Fe2+ content in the metal sublattice to maintain bulk charge neutrality. In the absence of oxidation, the Ti content sets the initial bulk Fe2+/Fe3+ ratio (R). Here, we demonstrate that Ti-doped magnetites (Fe3-xTixO4) reduce U-VI to U-IV. The U-VI-Fe2+ redox reactivity was found to be controlled directly by R but was otherwise independent of Ti content (x(Ti)). However, in contrast to previous studies with pure magnetite where U-VI was reduced to nanocrystalline uraninite (UO2), the presence of structural Ti (x(Ti) = 0.25-0.53) results in the formation of U-IV species that lack the bidentate U-O-2-U bridges of uraninite. Extended X-ray absorption fine structure spectroscopic analysis indicated that the titanomagnetite-bound U-IV phase has a novel U-IV-Ti binding geometry different from the coordination of U-IV in the mineral brannerite ((UTi2O6)-Ti-IV). The observed U-IV-Ti coordination at a distance of 3.43 A suggests a binuclear corner sharing adsorption/incorporation U-IV complex with the solid phase. Furthermore, we explored the effect of oxidation (decreasing R) and solids-to-solution ratio on the reduced U-IV phase. The formation of the non-uraninite U-IV-Ti phase appears to be controlled by availability of surface Ti sites rather than R. Our work highlights a previously unrecognized role of Ti in the environmental chemistry of U-IV and suggests that further work to characterize the long-term stability of U-IV phases formed in the presence of Ti is warranted.