Phosphate addition can be used to immobilize U(VI) contamination in situ in subsurface environments. Such addition is often motivated by a desire to precipitate low solubility U(VI) phosphate minerals, but phosphate can also affect the adsorption of U(VI) to minerals present in sediments. Batch and column experiments were performed with sediments from the Hanford 300 Area in Washington to quantify the effect of phosphate on U(VI) transport and to identify dominant immobilization reactions at U(VI) concentrations similar to those in contaminated Hanford vadose zone sediments (10-6 M – 10-5 M). In batch experiments, U(VI) sorption on the sediments was enhanced by phosphate addition. In column experiments, phosphate addition to the groundwater resulted in greater retention of U(VI) in the sediments. Although a slow release of U(VI) occurred after ending phosphate addition, concentrations stay below 20 μg/L with &gt; 85% of U(VI) remaining in the sediments. Sequential extractions of sediments suggested that the retained U(VI) was primarily in adsorbed forms both in the presence and absence of phosphate. Laser-induced fluorescence spectroscopy found no evidence of U(VI)-phosphate precipitation, further indicating that phosphate-enhanced U(VI) adsorption is the dominant mechanism of U(VI) immobilization. The rates of both adsorption and desorption were biphasic with a rapid adsorption/desorption period followed by a period of slower uptake or release that is likely controlled by mass transfer to and from intragranular adsorption sites. Simulations of selected data from the columns with a one-dimensional reactive transport model also suggested that the retention of U(VI) in columns was controlled by rates of mass transfer. X-ray adsorption spectroscopy measurements of samples from the batch experiments found U(VI) precipitates in Hanford sediments only at high initial U(VI) and phosphate loadings and that U(VI) was predominantly adsorbed under column-relevant conditions.

Impact of phosphate on immobilization of U(VI) in sediments

251st National Meeting (2016)

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