title: Stable U(IV) complexes form at high-affinity mineral surface sites.
abstract: Uranium (U) poses a significant contamination hazard to soils, sediments, and groundwater due to its extensive use for energy production. Despite advances in modeling the risks of this toxic and radioactive element, lack of information about the mechanisms controlling U transport hinders further improvements, particularly in reducing environments where U(IV) predominates. Here we establish that mineral surfaces can stabilize the majority of U as adsorbed U(IV) species following reduction of U(VI). Using X-ray absorption spectroscopy and electron imaging analysis, we find that at low surface loading, U(IV) forms inner-sphere complexes with two metal oxides, TiO2 (rutile) and Fe3O4 (magnetite) (at <1.3 U nm(-2) and <0.037 U nm(-2), respectively). The uraninite (UO2) form of U(IV) predominates only at higher surface loading. U(IV)-TiO2 complexes remain stable for at least 12 months, and U(IV)-Fe3O4 complexes remain stable for at least 4 months, under anoxic conditions. Adsorbed U(IV) results from U(VI) reduction by Fe(II) or by the reduced electron shuttle AH2QDS, suggesting that both abiotic and biotic reduction pathways can produce stable U(IV)-mineral complexes in the subsurface. The observed control of high-affinity mineral surface sites on U(IV) speciation helps explain the presence of nonuraninite U(IV) in sediments and has important implications for U transport modeling.
authors.affiliation: Biosciences Division, Argonne National Laboratory , Argonne, Illinois 60439, United States.
authors.forename: Drew E
authors.initials: DE
authors.lastname: Latta
authors.forename: Bhoopesh
authors.initials: B
authors.lastname: Mishra
authors.forename: Russell E
authors.initials: RE
authors.lastname: Cook
authors.forename: Kenneth M
authors.initials: KM
authors.lastname: Kemner
authors.forename: Maxim I
authors.initials: MI
authors.lastname: Boyanov
mesh_terms: D000327:Adsorption; D052203:Ferrosoferric Oxide / Q000737:chemistry; D019015:Geologic Sediments / Q000737:chemistry*; D060587:Groundwater / Q000737:chemistry*; D046529:Microscopy, Electron, Transmission; D008958:Models, Molecular; D010084:Oxidation-Reduction; D013499:Surface Properties; D014025:Titanium / Q000737:chemistry; D017974:Uranium Compounds / Q000032:analysis* / Q000737:chemistry; D014875:Water Pollutants, Radioactive / Q000032:analysis* / Q000737:chemistry; D056928:X-Ray Absorption Spectroscopy
chemical_list: D017974:Uranium Compounds; D014875:Water Pollutants, Radioactive; C009495:titanium dioxide; D014025:Titanium; C012597:uranium dioxide; D052203:Ferrosoferric Oxide