title: Identification of simultaneous U(VI) sorption complexes and U(IV) nanoprecipitates on the magnetite (111) surface.
abstract: Sequestration of uranium (U) by magnetite is a potentially important sink for U in natural and contaminated environments. However, molecular-scale controls that favor U(VI) uptake including both adsorption of U(VI) and reduction to U(IV) by magnetite remain poorly understood, in particular, the role of U(VI)-CO(3)-Ca complexes in inhibiting U(VI) reduction. To investigate U uptake pathways on magnetite as a function of U(VI) aqueous speciation, we performed batch sorption experiments on (111) surfaces of natural single crystals under a range of solution conditions (pH 5 and 10; 0.1 mM U(VI); 1 mM NaNO(3); and with or without 0.5 mM CO(3) and 0.1 mM Ca) and characterized surface-associated U using grazing incidence extended X-ray absorption fine structure spectroscopy (GI-EXAFS), grazing incidence X-ray diffraction (GI-XRD), and scanning electron microscopy (SEM). In the absence of both carbonate ([CO(3)](T), denoted here as CO(3)) and calcium (Ca), or in the presence of CO(3) only, coexisting adsorption of U(VI) surface species and reduction to U(IV) occurs at both pH 5 and 10. In the presence of both Ca and CO(3), only U(VI) adsorption (VI) occurs. When U reduction occurs, nanoparticulate UO(2) forms only within and adjacent to surface microtopographic features such as crystal boundaries and cracks. This result suggests that U reduction is limited to defect-rich surface regions. Further, at both pH 5 and 10 in the presence of both CO(3) and Ca, U(VI)-CO(3)-Ca ternary surface species develop and U reduction is inhibited. These findings extend the range of conditions under which U(VI)-CO(3)-Ca complexes inhibit U reduction.
authors.affiliation: Department Earth & Planetary Sciences, University of California, Berkeley, Berkeley, California 94720, USA. dmsinger@lbl.gov
authors.forename: David M
authors.initials: DM
authors.lastname: Singer
authors.forename: Shawn M
authors.initials: SM
authors.lastname: Chatman
authors.forename: Eugene S
authors.initials: ES
authors.lastname: Ilton
authors.forename: Kevin M
authors.initials: KM
authors.lastname: Rosso
authors.forename: Jillian F
authors.initials: JF
authors.lastname: Banfield
authors.forename: Glenn A
authors.initials: GA
authors.lastname: Waychunas
mesh_terms: D000327:Adsorption; D002118:Calcium / Q000737:chemistry; D011232:Chemical Precipitation; D052203:Ferrosoferric Oxide / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D016018:Least-Squares Analysis; D008956:Models, Chemical; D009038:Motion; D053758:Nanoparticles / Q000737:chemistry* / Q000648:ultrastructure; D010084:Oxidation-Reduction; D013052:Spectrometry, X-Ray Emission; D013499:Surface Properties; D014501:Uranium / Q000302:isolation & purification*; D056928:X-Ray Absorption Spectroscopy
chemical_list: D014501:Uranium; D002118:Calcium; D052203:Ferrosoferric Oxide