Experimental study of uranium(6+) sorption of the zeolite mineral clinoptilolite

Experiments on the sorption of uranium(6+) on clinoptilolite from solutions in equilibrium with atmospheric CO2(g) were conducted to understand the fundamental controls on uranium sorption on zeolite minerals, including the effects of pH, aqueous uranium speciation, and uranium concentration in solution. The results indicate that uranium(6+) species are strongly sorbed on the zeolite mineral clinoptilolite at near-neutral pH. The amount of uranium sorbed is strongly dependent on pH and, to some extent, on the total concentration of uranium. Uranium sorption on clinoptilolite is important in the pH range where UO2(OH)2 degrees(aq) is the predominant uranium aqueous species, whereas sorption is inhibited at pH's where carbonate- and hydroxy-carbonate-complexes are the primary uranyl species. Surface adsorption appears to be the main sorption mechanism, although at pH<4 the results suggest ion exchange may occur between the UO22+ ions in solution and the cations in the intracrystalline cation exchange sites of clinoptilolite. The effectiveness of zeolite-rich horizons underneath Yucca Mountain, Nevada, as barriers to actinide transport through sorption processes will depend strongly on groundwater chemistry. Reliable predictions of radionuclide transport through these horizons will need to properly account for changes in solution chemistry