This is the final year (in a one-year extension) of a project with the principal goal of investigating the impact of clay surface alteration, resulting from hydroxy-aluminum (HyA) and hydroxyaluminosilicate (HAS) species intercalation on reactivity of soils towards Cs and Sr. Special emphasis has been accorded to the unique geochemical conditions that are representative of the Hanford site vadose zone (high ionic strength, high pH, high Al concentrations). Specific objectives of the research include: (1) Quantify the rate and extent of contaminant sorption to prevalent soil minerals as a function of system composition and contaminant concentration. (2) Determine the effects of intercalation of clays by HyA and HAS on the sorption/ion exchange process. (3) Determine the role of NOM (dissolved and mineral bound) on the particle retention of Cs and Sr. (4) Investigate the nature of mineral transformations induced by high pH and ionic strength conditions characteristic of waste impacted environments as it affects concurrent/subsequent retention of Cs and Sr. (5) Determine the coordination chemistry of contaminants bound into clay surfaces (as measured in objectives (1)-(4)) using NMR and X-ray absorption spectroscopy (XAS). Significant progress has been made in achieving these objectives. We have conducted long-term kinetic studies--reaction times ranging from 1 d to 2 yr--to examine relationships between aluminosilicate weathering in the presence of synthetic tank waste leachate (STWL) and Cs/Sr uptake and release. Our experiments employ a sequence of specimen clay minerals including illite, vermiculite, smectite and kaolinite, which are also important reactive solids in the Hanford sediments (Serne et al., 2001).
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