248-6 Assessing Frayed-Edge Site Density and Contaminant Desorption In Hyperalkaline-Weathered Hanford Sediment.



Tuesday, October 18, 2011: 2:55 PM
Henry Gonzalez Convention Center, Room 206A, Concourse Level

Yi-Ting Deng1, Aaron Thompson1 and Nicolas Perdrial2, (1)Department of Crop and Soil Sciences, University of Georgia, Athens, GA
(2)Department of Soil, Water and Environmental Science, University of Arizona, Tucson, AZ
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)Srx((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 mmolc 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.
See more from this Division: S02 Soil Chemistry
See more from this Session: Symposium--Sorption to Bioavailability: II