Molecular-Scale Structure of Uranium(VI) Immobilized with Goethite and Phosphate

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 ∼100 μM 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, Fe­(OH)₂UO₂, and bidentate corner-sharing, (FeOH)₂UO₂, surface complexes with respective U–Fe coordination distances of ∼3.45 and ∼4.3 Å. 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 μM 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 Å (U–P) and a single iron shell at ∼4.3 Å (U–Fe). While the data can be explained by a U-bridging ternary surface complex, (FeO)₂UO₂PO₄, it is not possible to statistically distinguish this scenario from one with P-bridging complexes also present