Inhibition effect of secondary phosphate mineral precipitation on uranium release from contaminated sediments.

The inhibitory effect of phosphate mineral precipitation on diffusion-limited uranium release was evaluated using a U(VI)-contaminated sediment collected from the U.S. Department of Energy Hanford site. The sediment contained U(VI) that was associated with diffusion-limited intragrain regions within its millimeter-sized granitic lithic fragments. The sediment was first treated to promote phosphate mineral precipitation in batch suspensions spiked with 1 and 50 mM aqueous phosphate and calcium in the stoichiometric ratio of the mineral hydroxyapatite. The phosphate-treated sediment was then leached to solubilize contaminant U(VI) in a column system using a synthetic groundwater solution with chemical components representative of Hanford groundwater. Phosphate treatment significantly decreased the extent of U(VI) release from the sediment. Within the experimental duration of about 200 pore volumes, the effluent U(VI) concentrations were consistently lower by over 1 and 2 orders of magnitude after the sediment was treated with 1 and 50 mM of phosphate, respectively. Measurements of solid-phase U(VI) using laser-induced fluorescence spectroscopy, scanning electron microscopy, and chemical extraction of the sediment collectively indicated that the inhibition of U(VI) release from the sediment was caused by (1) U(VI) adsorption to the secondary phosphate precipitates and (2) the transformation of original U(VI) mineral phases to less soluble forms.