title: Reaction of U(VI) with titanium-substituted magnetite: influence of Ti on U(IV) speciation.
abstract: 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 a 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 Fe(3+) replacement by Ti(IV) yields a proportional increase in the relative Fe(2+) content in the metal sublattice to maintain bulk charge neutrality. In the absence of oxidation, the Ti content sets the initial bulk Fe(2+)/Fe(3+) ratio (R). Here, we demonstrate that Ti-doped magnetites (Fe3 - xTixO4) reduce U(VI) to U(IV). The U(VI)-Fe(2+) redox reactivity was found to be controlled directly by R but was otherwise independent of Ti content (xTi). However, in contrast to previous studies with pure magnetite where U(VI) was reduced to nanocrystalline uraninite (UO2), the presence of structural Ti (xTi = 0.25-0.53) results in the formation of U(IV) species that lack the bidentate U-O2-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 (U(IV)Ti2O6). The observed U(IV)-Ti coordination at a distance of 3.43 Å 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.
authors.affiliation: Biosciences Division, Argonne National Laboratory, Argonne, Illinois 60439, USA. dlatta@anl.gov
authors.forename: Drew E
authors.initials: DE
authors.lastname: Latta
authors.forename: Carolyn I
authors.initials: CI
authors.lastname: Pearce
authors.forename: Kevin M
authors.initials: KM
authors.lastname: Rosso
authors.forename: Kenneth M
authors.initials: KM
authors.lastname: Kemner
authors.forename: Maxim I
authors.initials: MI
authors.lastname: Boyanov
mesh_terms: D052203:Ferrosoferric Oxide / Q000737:chemistry*; D010084:Oxidation-Reduction; D014025:Titanium / Q000737:chemistry*; D014501:Uranium / Q000737:chemistry*; D056928:X-Ray Absorption Spectroscopy
chemical_list: D014501:Uranium; D014025:Titanium; D052203:Ferrosoferric Oxide