Identification of Simultaneous U(VI) Sorption Complexes and U(IV) Nanoprecipitates on the Magnetite (111) Surface
Journal Article
·
· Environmental Science and Technology
- Univ. of California, Berkeley, CA (United States); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States); The University of Chicago
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Univ. of California, Berkeley, CA (United States)
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Sequestration of uranium (U) by magnetite is a likely important sink for U in natural and contaminated environments. Nonetheless, 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 and 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 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 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.
- Research Organization:
- Univ. of Chicago, Chicago, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22). Chemical Sciences, Geosciences & Biosciences Division
- Grant/Contract Number:
- FG02-94ER14466
- OSTI ID:
- 1508074
- Alternate ID(s):
- OSTI ID: 1037911
OSTI ID: 1040672
- Report Number(s):
- DOE-UCHICAGO-14466--14
- Journal Information:
- Environmental Science and Technology, Journal Name: Environmental Science and Technology Journal Issue: 7 Vol. 46; ISSN 0013-936X
- Publisher:
- American Chemical Society (ACS)Copyright Statement
- Country of Publication:
- United States
- Language:
- English
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