THE INFLUENCE OF URANYL HYDROLYSIS AND MULTIPLE SITE-BINDING REACTIONS ON ADSORPTION OF U(VI) TO MONTMORILLONITE

Adsorption of uranyl to SWy-1 montmorillonite was evaluated experiment ally and results were modeled to identify likely surface complexation reactions responsible for removal of uranyl from solution. Uranyl was contacted with SWy-1 montmorillonite in a NaClO4 electrolyte solution at three ionic strengths (I = 0.001, 0.01, 0.1), at pH 4 to 8.5, in a N-2(g) atmosphere. At low ionic strength, adsorption decreased from 95 % at pH 4 to 75% at pH 6.8. At higher ionic strength, adsorption incre ased with pH from initial values less than 75%; adsorption edges for a ll ionic strengths coalesced above a pH of 7. A site-binding model was applied that treated SWy-1 as an aggregate of fixed-charge sites and edge sites analogous to gibbsite and silica. The concentration of fixe d-charge sites was estimated as the cation exchange capacity, and non- preference exchange was assumed in calculating the contribution of fix ed-charge sites to total uranyl adsorption. The concentration of edge sites was estimated by image analysis of transmission electron photomi crographs. Adsorption constants for uranyl binding to gibbsite and sil ica were determined by fitting to experimental data, and these adsorpt ion constants were then used to simulate SWy-1 adsorption results. The best simulations were obtained with an ionization model in which AlOH 2+ was the dominant aluminol surface species throughout the experiment al range in pH. The pH-dependent aqueous speciation of uranyl was an i mportant factor determining the magnitude of uranyl adsorption. At low ionic strength and low pH, adsorption by fixed-charge sites was predo minant. The decrease in adsorption with increasing pH was caused by th e formation of monovalent aqueous uranyl species, which were weakly bo und to fixed-charge sites. At higher ionic strengths, competition with Na+ decreased the adsorption of UO22+ to fixed-charge sites. At highe r pH, the most significant adsorption reactions were the binding of UO 22+ to AlOH and of (UO2)(3)(OH)(5)(+) to SiOH edge sites. Near-saturat ion of AlOH sites by UO22+ allowed significant contributions of SiOH s ites to uranyl adsorption.