Identification of Simultaneous U(VI) Sorption Complexes and U(IV) Nanoprecipitates on the Magnetite (111) Surface

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)-CO3-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 NaNO3; and with or without 0.5 mM CO3 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 ([CO3](T), denoted here as CO3) and calcium (Ca), or in the presence of CO3 only, coexisting adsorption of U(VI) surface species reduction to U(IV) occurs at both pH 5 and 10. In the presence of both Ca and CO3, only U(VI) adsorption (VI) occurs. When U reduction occurs, nanoparticulate UO2 forms only within and adjacent to surface microtopographic feature; 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 CO3 and Ca, U(VI)-CO3-Ca ternary surface species develop and U reduction is inhibited. These findings extend the range of conditions under which U(VI)-CO3-Ca complexes inhibit U reduction.