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

Abstract

Abstract--Adsorption of uranyl to SWy-1 montmorillonite was evaluated experimentally and results were modeled to identify likely surface complexation reactions responsible for removal ofuranyl from solution. Uranyl was contacted with SWy-1 montmorillonite in a NaC104 electrolyte solution at three ionic strengths (I = 0.001, 0.01, 0.1), at pH 4 to 8.5, in a N2~) 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 A1OH2 + 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 UOz 2+ to fixed-charge sites. At higher pH, the most significant adsorption reactions were the binding of UP22+ to A1OH and of(UO2)3(OH)5 + tO SiOH edge sites. Near-saturation of A1OH sites by UOz 1+ allowed significant contributions of SiOH sites to uranyl adsorption.

Sorption of Uranium(VI) and Thorium(IV) by Jordanian Bentonite

“…e higher the metal charge causes a stronger interaction. is agrees very well with previous studies [5,6,11]. e correlation coefficient is a mathematical expression, which reveals the favorability of the sorption process, since the values of 2 for Langmuir isotherm and Freundlich are very good.…”

“…Adsorption constants for uranyl ions binding to gibbsite and silica were determined by �tting to experimental data, and these adsorption constants were then used to simulate SWy-1 adsorption results. e best simulations were obtained with an ionization model in which AlOH 2 + was the dominant aluminol surface species throughout the experimental range in pH [6].…”

Uranium(VI) Adsorption and Surface Complexation Modeling onto Background Sediments from the F-Area Savannah River Site

“…5,7,14,16,17 The strong adsorption by goethite at pH > 4.0 is consistent with many observations in the literature, where iron (oxy)hydroxides such as amorphous iron hydroxide, ferrihydrite and goethite in aquifer sediments have been identified as major sinks for U(VI). 6,13,15,18−20 Compared to the fine fractions (Figure 2c,d), kaolinite exhibits stronger U(VI) adsorption at pH < 4.0 and a similar adsorption extent at pH > 4.0 (up to 7.0), and goethite shows a similar adsorption extent at pH < 4.0 and stronger adsorption at pH > 4.0 (up to 8.0).…”

Development of a Composite Non-Electrostatic Surface Complexation Model Describing Plutonium Sorption to Aluminosilicates

“…The major assumption in this model is that the clay edge sites on montmorillonite that contain tetrahedral silica sites and octahedral alumina sites (Figure 1.3) will behave as aluminol (AlOH) and silanol (SiOH) sites in gibbsite and silica, respectively. A similar modeling approach was used by Turner et al (1998) and McKinley et al (1995) to model Np(V) and U(VI) sorption to montmorillonite, respectively. In the case of Pu, this modeling approach is complicated by the redox cycling between Pu(IV) and Pu(V).…”