PNNL

Abstract

Phosphate can be added to subsurface environments to immobilize U(VI) contamination. The e?cacy of immobilization depends on the site-speci?c groundwater chemistry and aquifer sediment properties. Batch and column experiments were performed with sediments from the Hanford 300 Area in Washington State and arti?cial groundwater prepared to emulate the conditions at the site. Batch experiments revealed enhanced U(VI) sorption with increasing phosphate addition. X-ray absorption spectroscopy measurements of samples from the batch experiments found that U(VI) was predominantly adsorbed at conditions relevant to the column experiments and most ?eld sites (low U(VI) loadings, 25µM) and phosphate loadings. While batch experiments showed the transition of U(VI) uptake from adsorption to precipitation, the column study was more directly relevant to the subsurface environment because of the high solid:water ratio in the column and the advective ?ow of water. In column experiments, nearly six times 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. This enhanced retention persisted for at least one month after cessation of phosphate addition to the in?uent ?uid. Sequential extractions and laser-induced ?uorescence spectroscopy of sediments from the columns suggested that the retained U(VI) was primarily in adsorbed forms. These results indicate that in situ remediation of groundwater by phosphate addition provides lasting bene?t beyond the treatment period via enhanced U(VI) adsorption to sediments.