Molecular-scale structure of uranium(VI) immobilized with goethite and phosphate
- WU
The molecular-scale immobilization mechanisms of uranium uptake in the presence of phosphate and goethite were examined by extended X-ray absorption fine structure (EXAFS) spectroscopy. Wet chemistry data from U(VI)-equilibrated goethite suspensions at pH 4-7 in the presence of {approx}100 {micro}M total phosphate indicated changes in U(VI) uptake mechanisms from adsorption to precipitation with increasing total uranium concentrations and with increasing pH. EXAFS analysis revealed that the precipitated U(VI) had a structure consistent with the meta-autunite group of solids. The adsorbed U(VI), in the absence of phosphate at pH 4-7, formed bidentate edge-sharing, {triple_bond}Fe(OH){sub 2}UO{sub 2}, and bidentate corner-sharing, ({triple_bond}FeOH){sub 2}UO{sub 2}, surface complexes with respective U-Fe coordination distances of {approx}3.45 and {approx}4.3 {angstrom}. In the presence of phosphate and goethite, the relative amounts of precipitated and adsorbed U(VI) were quantified using linear combinations of the EXAFS spectra of precipitated U(VI) and phosphate-free adsorbed U(VI). A U(VI)-phosphate-Fe(III) oxide ternary surface complex is suggested as the dominant species at pH 4 and total U(VI) of 10 {micro}M or less on the basis of the linear combination fitting, a P shell indicated by EXAFS, and the simultaneous enhancement of U(VI) and phosphate uptake on goethite. A structural model for the ternary surface complex was proposed that included a single phosphate shell at 3.6 {angstrom} (U-P) and a single iron shell at {approx}4.3 {angstrom} (U-Fe). While the data can be explained by a U-bridging ternary surface complex, ({triple_bond}FeO){sub 2}UO{sub 2}PO{sub 4}, it is not possible to statistically distinguish this scenario from one with P-bridging complexes also present.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
- Sponsoring Organization:
- National Science Foundation (NSF)
- OSTI ID:
- 1045687
- Journal Information:
- Environ. Sci. Technol., Vol. 46, Issue (12) ; 06, 2012; ISSN 0013-936X
- Country of Publication:
- United States
- Language:
- ENGLISH
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