Phosphate amendments can enhance the immobilization of uranium in subsurface environments. The objectives of this study were to quantify the effect of phosphate on the transport of U(VI) and to identify the dominant mechanisms of U(VI)-phosphate reactions in subsurface environments. Column experiments were performed with sediments from the Hanford 300 Area in Washington and with artificial groundwater prepared to emulate the conditions at the site. For uranium-loaded sediments, a significant amount of uranium was retained in sediments when phosphate-containing groundwater was introduced. In contrast, most uranium was desorbed from the uranium-loaded sediments when phosphate-free influents were introduced. The benefits of phosphate addition for uranium immobilization could persist even after phosphate was no longer in the influent. Sequential extractions of sediments showed that the retained uranium was primarily in ion-exchangeable and weak acid-extractable forms, which indicates that uranium was retained mainly in adsorbed forms. A kinetic study of selected sequential extraction steps provided information on the chemical and physical processes controlling the rates of dissolution and desorption of solid-associated uranium. A one-dimensional reactive transport model was used to study the transport behavior of uranium in columns with different influent compositions and flow rates.

In situ immobilization of uranium in Hanford sediments with the amendment of phosphate

249th National Meeting (2015)

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