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Abstract
At the Department of Energys Hanford Site, hyperalkaline (pH > 13) and radioactive waste leaking from storage tanks causes rapid sediment weathering and de novo mineral formationboth of which can alter the mobility of the principal radioactive contaminants, Cs and Sr. These new-formed minerals, such as NO3-feldspathoids, sequester contaminants in their structures and the frayed-edge sites (FES) of the sediment clays strongly bind Cs. However the long-term stability of the sequestered contaminants is unknown, so existing reactive-transport models cannot address weathering related impact of the sediments. Our goal is to determine the mechanisms of contaminant release from NO3-feldspathoids and to quantify how the weathering process alters the FES density. Our results suggest that Cs and Sr are released from NO3-feldspathoids through both ion exchange and mineral dissolution. Furthermore, the hyperalkaline-weathering process increases the density of high-affinity Cs sorption sites in Hanford sediments, which may include the FES and also the high-affinity sites on the neo-formed minerals.