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The Influence of Uranyl Hydrolysis and Multiple Site-Binding Reactions on Adsorption of U(VI) to Montmorillonite

Published online by Cambridge University Press:  28 February 2024

James P. McKinley ,
John M. Zachara ,
Steven C. Smith  and
Gary D. Turner
James P. McKinley
Affiliation:
Pacific Northwest Laboratory, MSIN K3-61, P.O. Box 999, Richland, Washington 99352
John M. Zachara
Affiliation:
Pacific Northwest Laboratory, MSIN K3-61, P.O. Box 999, Richland, Washington 99352
Steven C. Smith
Affiliation:
Pacific Northwest Laboratory, MSIN K3-61, P.O. Box 999, Richland, Washington 99352
Gary D. Turner
Affiliation:
Pacific Northwest Laboratory, MSIN K3-61, P.O. Box 999, Richland, Washington 99352
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Abstract

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Adsorption of uranyl to SWy-1 montmorillonite was evaluated experimentally 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 NaCIO4 electrolyte solution at three ionic strengths (I = 0.001, 0.01, 0.1), at pH 4 to 8.5, in a N2(g) atmosphere. At low ionic strength, adsorption decreased from 95% at pH 4 to 75% at pH 6.8. At higher ionic strength, adsorption increased with pH from initial values less than 75%; adsorption edges for all 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 fixed-charge sites was estimated as the cation exchange capacity, and non-preference exchange was assumed in calculating the contribution of fixed-charge sites to total uranyl adsorption. The concentration of edge sites was estimated by image analysis of transmission electron photomicrographs. Adsorption constants for uranyl binding to gibbsite and silica were determined by fitting to experimental data, and these adsorption constants were then used to simulate SWy-1 adsorption results. The best simulations were obtained with an ionization model in which AlOH2+ was the dominant aluminol surface species throughout the experimental range in pH. The pH-dependent aqueous speciation of uranyl was an important factor determining the magnitude of uranyl adsorption. At low ionic strength and low pH, adsorption by fixed-charge sites was predominant. The decrease in adsorption with increasing pH was caused by the formation of monovalent aqueous uranyl species, which were weakly bound to fixed-charge sites. At higher ionic strengths, competition with Na+ decreased the adsorption of UO22+ to fixed-charge sites. At higher pH, the most significant adsorption reactions were the binding of UO22+ to AlOH and of (UO2)3(OH)5+ to SiOH edge sites. Near-saturation of AlOH sites by UO22+ allowed significant contributions of SiOH sites to uranyl adsorption.

Keywords

MontmorilloniteUranyl
Type
Research Article
Information
Clays and Clay Minerals , Volume 43 , Issue 5 , October 1995 , pp. 586 - 598
Copyright
Copyright © 1995, The Clay Minerals Society

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