Kinetics and reversibility of radionuclide sorption reactions with rocks. Progress report for fiscal year 1978

Sorption reactions of environmentally significant radionuclides with rocks from proposed waste repository sites were examined. The kinetics and reversibility of sorption were studied by measuring the distribution of radioactive tracers between synthetic groundwaters and crushed basalt, granite, and argillite in batch experiments. The initial rapid sorption reaction is followed by a slow reaction which results from changes in the composition of the rock surface and bulk solution. Equilibrium concentrations of radionuclides were not reached even after 91 days. Dissolution of some rock components is also incomplete after this time. A relatively large amount of silica is dissolved in groundwaters contacted with basalt due to dissolution of the glassy phase of the basalt sample. This silica appears to increase the solubility of plutonium and americium. Artificially weathering basalt greatly increases the sorption capacity for each of the radionuclides studied. Weathering of granite had no observable effect on sorption. However, weathered argillite sorbs cesium, plutonium, and americium more strongly than unweathered argillite. Several irreversible sorption reactions were identified. Cesium is irreversibly sorbed on granite and ruthenium is irreversibly sorbed on basalt, granite, and argillite. Strontium and neptunium sorption are reversible for each rock type