Assessing Frayed-Edge Site Density and Contaminant Desorption In Hyperalkaline-Weathered Hanford Sediment.

The Department of Energy’s Hanford Site poses the largest environmental cleanup project in the U.S. due to the complexity and extent of radioactive contamination. Existing reactive-transport models that have been applied to the Hanford Site contamination do not adequately address the impact of de novo mineral formation generated by contact of the hyperalkaline (pH >13) waste with sediments beneath the storage tanks. These minerals, such as NO₃-feldspathoids, are known to sequester Cs and Sr and could serve as a potential long-term dissolution source should they ever be exposed to less alkaline pore waters. However, the long-term fate (e.g., transport mechanism) of these main contaminants in the new-formed minerals is largely unknown. The overall goal of this research is to use experimental methods to understand the mechanism of the main contaminant desorption from hyperalkaline-weathered Hanford sediments. We simulated the sediment contamination with high treatment (10^-3m Cs, 10^-3m Sr and 10^-5m I) and low treatment (10^-5m Cs, 10^-5m Sr and 10^-7m I), then conducted a three-month long leaching (~8000 pore volumes) of reacted Hanford sediments to determine if Cs and Sr desorption from the NO₃-feldspathoids [Na_(8-2x)Sr_x((NO₃)₂Al₆Si₆O₂₄), where x is the Sr stoichiometry] can occur without feldspathoids dissolution. We also directly measured the number (or density) of frayed-edge sites (FES) available for Cs sorption using multiple methods. Our results reveal sediment nitrate concentrations decreased from 0.016 mol kg^-1 soil to 0.005 mol kg^-1 soil (>50% reduction) whereas, NO₃-feldspathoid abundance was only reduced from 21 g kg^-1 to 13 g kg^-1. This suggests the sequestered contaminants Cs and Sr may be released from NO₃-feldspathoids via a desorption of an ion pair in addition to mineral dissolution. After 8000 PV’s Cs and Sr are released more than 85% from the sediments which have been contaminated with high Cs and Sr concentration (high treatments) while they are only released less than 50% from the low treatments. In addition, we constrained the FES density of pristine Hanford sediments within 0.10-1.04 mmol_c kg^-1, which yields to 0.11-1.18% of total CEC. Future work includes simulating the contaminant desorption by reactive-transport modeling ,and further constraining the FES density on all reacted sediments using column-based experiments and transmission electron microscopy.