CESIUM MIGRATION IN HANFORD SEDIMENT: A MULTI-SITE CATION EXCHANGE MODEL BASED ON LABORATORY TRANSPORT EXPERIMENTS AT 25 AND 80°C

Cs+ transport experiments carried out at 25 and 80°C in columns packed with uncontaminated Hanford Formation sediment from the SX tank farm provide strong support for the use of a multi-site, multicomponent cation exchange model to describe Cs+ migration in the Hanford vadose zone. The experimental results indicate a strong dependence of the effective Cs+ Kd on the concentrations of other cations, including Na+, which is present at high to extremely high concentrations in fluids leaking from the Hanford SX tanks. The data not only show a dependence of the Cs+ Kd with regard to the aqueous Cs+ concentration, but also with temperature. The focus of this presentation will be on the temperature effect of cesium exchange on Hanford sediments. At 80ºC, cesium mobility is about 2 times higher than at 25ºC for solutions of similar composition. At low cesium concentrations (5 x 10-7M) cesium retardation increases from Rd=117 at 80ºC to Rd=282 at 25ºC. Similar trends are observed at higher cesium concentrations and higher salt solutions. In 1 M NaNO3 and 10-4M Cs solutions, we report Cs Rd=17 at 80ºC and Rd=41 at 25ºC. In higher salt solutions that are more representative of tank liquours (7.5 M NaNO3) 10-4 M Cs is retarded by a factor of 4 at 80ºC and a factor of 8 at 25ºC. The decrease in retardation with increasing temperature may be attributed to solution chemistry temperature effects as well as temperature effects of the ion exchange reaction. We will employ Pitzer thermodynamic database together with our multi-site cation exchange model to determine contributions from solution chemistry and ion exchange with the Hanford sediments.

This work was performed under the auspices of the U.S. Department of Energy by the University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.