Uranium(6+) sorption on montmorillonite: Experimental and surface complexation modeling study

Abstract

Sorption interactions with montmorillonite and other clay minerals in soils, sediments, and rocks are potentially important mechanisms for attenuating the mobility of U(6+) and other radionuclides through the subsurface environment. Batch experiments were conducted (in equilibrium with atmospheric Pco2) to determine the effects of varying pH (2 to 9), solid-mass to solution-volume ratio (M/V = 0.028 to 3.2 g/L), and solution concentration (2 x 10 -7 and 2 x 10 -6 M 233U) on U(6+) sorption on SAz-1 montmorillonite. The study focused on U(6+) surface complexation on hydroxylated edge sites as the sorption mechanism of interest because it is expected to be the predominant sorption mechanism at pHs typical of natural waters (pH ~6 to ~9). Thus, the experiments were conducted with a 0.1 M NaNO3 matrix to suppress ion-exchange between U(6+) in solution and interlayer cations. The results show that U(6+) sorption on montmorillonite is a strong function ofpH, reaching a maximum at near-neutral pH (,~6 to ~6.5) and decreasing sharply towards more acidic or more alkaline conditions. A comparison of the pH-dependence of U(6+) sorption with that of U(6+) aqueous speciation indicates a close correspondence between U(6+) sorption and the predominance field of U(6+)-hydroxy complexes. At high pH, sorption is inhibited due to formation of aqueous U(6+)-carbonate complexes. At low pH, the low sorption values indicate that the 0.1 M NaNO3 matrix was effective in suppressing ion-exchange between the uranyl (UO] +) species and interlayer cations in montmorillonite. At pH and carbonate concentrations typical of natural waters, sorption of U(6+) on montmorillonite can vary by four orders of magnitude and can become negligible at high pH.The experimental results were used to develop a thermodynamic model based on a surface complexation approach to permit predictions of U(6+) sorption at differing physicochemical conditions. A Diffuse-Layer model (DLM) assuming aluminol (>A1OH °) and silanol (>SiOH °) edge sites and two U(6+) surface complexation reactions per site effectively simulates the complex sorption behavior observed in the U(6+)--H20----4202-montmorillonite system at an ionic strength of 0.1 M and pH > 3.5. A comparison of model predictions with data from this study and from published literature shows good agreement and suggests that surface complexation models based on parameters derived from a limited set of data could be useful in extrapolating radionuclide sorption over a range of geochemical conditions. Such an approach could be used to support transport modeling by providing a better alternative to the use of constant K~ s in transport calculations.

A thermal, sorptive and spectral study of HDTMA-bentonite loaded with uranyl phosphate

“…For all kinds of bentonite, with the exception of H-bentonite, the desorption isotherms differed from the sorption isotherms, which attested to the irreversibility of the entire sorption process. Surface precipitation and surface complexation play important roles in U(VI) immobilization by bentonite [26][27][28].…”

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

“…However, these previous studies did not consider the potential formation of uranyl-carbonate ternary surface complexes on clay surfaces because their experiments were performed either under CO 2 (g) free or low CO 2 (g) conditions. 5,29,30,32 Recently, more detailed studies by Catalano and Brown Jr. 33 and Arai et al 34 using EXAFS spectroscopy revealed that U(VI) adsorption onto montmorillonite and imogolite surfaces occurs as uranyl-carbonate ternary surface complexes in systems equilibrated with atmospheric CO 2 . Formation of polymeric surface complexes via adsorption of aqueous UO 2 ) 3 (OH) 5 + was not observed, which is consistent with our calculation that at low U(VI) concentration (≤1 μM) and in the presence of atmospheric CO 2 , the formation of aqueous UO 2 ) 3 (OH) 5 + is not favored (Figure S1 in SI).…”

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

“…At pH and carbonate concentrations typical of natural waters, sorption of U(6+) on montmorillonite can vary by four orders of magnitude and can become negligible at high pH. A Diffuse-Layer model (DLM) assuming aluminol (>AlOH ∘ ) and silanol (>SiOH ∘ ) edge sites and two U(6+) surface complexation reactions per site effectively simulates the complex sorption behavior observed in the U(6+)-H 2 O-CO 2 -montmorillonite system at an ionic strength of 0.1 M and pH > 3 5 [7].…”