Cation exchange reactions controlling desorption of 90Sr2+ from coarse-grained contaminated sediments at the Hanford site, Washington

Authors

James Mckinley, Pacific Northwest National LaboratoryFollow
John M. Zachara, Pacific Northwest National LaboratoryFollow
Steven Smith, Pacific Northwest National LaboratoryFollow
Chongxuan Liu, Pacific Northwest National LaboratoryFollow

Date of this Version

2007

Citation

Geochimica et Cosmochimica Acta 71 (2007) 305–325; doi:10.1016/j.gca.2006.09.027

Abstract

Nuclear waste that bore ⁹⁰Sr²⁺ was accidentally leaked into the vadose zone at the Hanford site, and was immobilized at relatively shallow depths in sediments containing little apparent clay or silt-sized components. Sr²⁺, ⁹⁰Sr²⁺, Mg2+, and Ca²⁺ was desorbed and total inorganic carbon concentration was monitored during the equilibration of this sediment with varying concentrations of Na⁺, Ca²⁺. A cation exchange model previously developed for similar sediments was applied to these results as a predictor of final solution compositions. The model included binary exchange reactions for the four operant cations and an equilibrium dissolution/precipitation reaction for calcite. The model successfully predicted the desorption data. The contaminated sediment was also examined using digital autoradiography, a sensitive tool for imaging the distribution of radioactivity. The exchanger phase containing ⁹⁰Sr was found to consist of smectite formed from weathering of mesostasis glass in basaltic lithic fragments. These clasts are a significant component of Hanford formation sands. The relatively small but significant cation exchange capacity of these sediments was thus a consequence of reaction with physically sequestered clays in sediment that contained essentially no fine-grained material. The nature of this exchange component explained the relatively slow (scale of days) evolution of desorption solutions. The experimental and model results indicated that there is little risk of migration of ⁹⁰Sr²⁺ to the water table.