PNNL

Abstract

Phosphate can be added to subsurface environments to immobilize U(VI) contamination. The efficacy of immobilization depends on the site-specific groundwater chemistry and aquifer sediment properties. Batch and column experiments were performed with sediments from the Hanford 300 Area in Washington State and artificial groundwater prepared to emulate the conditions at the site. These experiments quantified the effect of phosphate on U(VI) transport and suggested dominant immobilization reactions. Batch experiments revealed enhanced U(VI) sorption with increasing phosphate addition. X-ray absorption spectroscopy measurements of samples from the batch experiments observed U(VI) phosphate precipitates only at high initial U(VI) and phosphate loadings and found that U(VI) was predominantly adsorbed at the most environmentally relevant conditions. In column experiments more U(VI) was retained in sediments when phosphate-containing groundwater was introduced to U(VI)-loaded sediments than when the groundwater did not contain phosphate. The benefits of phosphate addition for U(VI) immobilization persisted for one month even after phosphate addition was stopped. Sequential extractions and laser-induced fluorescence spectroscopy of sediments from the columns suggested that the retained U(VI) was primarily in adsorbed forms.