Sorption of different cations onto clay minerals: Modelling approach with ion exchange and surface complexation

Abstract

The present article investigates the sorption of heavy metals, U(VI) and Eu(III), as an analog of An(III), on montmorillonite, illite and kaolinite suspensions in aqueous Na-electrolyte solutions by applying the Gibbs energy minimization computer code. Due to the complex nature of the sorption process, several simplifying assumptions should be made to solve the problem. The cation exchange capacity of clays, the density of the aluminol (>AlOH) and silanol (>SiOH) edge-type sites, responsible for the pH-dependent surface complexation, their acidity constants were derived from the reference data and fixed as non-adjustable parameters. The site-binding constants for all selected cations were calculated according to the direct correlations with their aqueous hydrolysis constants, and an exchange parameter K ex was accepted depending on the clay type, the valence of the exchanging cations and the solid to liquid ratio. The last parameters could be adjusted for goodness of fit in some cases. Relatively good correspondence between model sorption edges, distribution coefficients and those from experimental measurements was found; moreover, these observations are also consistent with the conceptual framework of the abundant reference data. The comparative model presented here is aimed to predict sorption uptake of cations over the whole spectrum of conditions in real systems having complex solid phase composition and water chemistries.