Study of major ions sorption equilibria to characterize the ion exchange properties of kaolinite

The content of clays as well as their intrinsic properties will play a key role on retention processes during transport of contaminants through subsurface soil and sediments. Since contaminant release to the surrounding environment is at trace level compared to major ions in interstitial waters, the extent of their sorption is conditioned by the occupancy of surface sites by groundwater solutes. This work focused on characterizing the ion exchange properties of a common phyllosilicate mineral, kaolinite, using a theoretical approach based on ion exchange formalisms. The ion exchange equilibria occurring at the mineral-solution interface were studied for Na/H, K/H, Cs/H, Ca/H, Mg/H, Cl/OH, and ClO₄/OH ion couples over a wide pH range (from 2 to 12). Cation saturation curves revealed five major sorption sites, of which three were identified as hydroxylated sites as shown by an increase of Cl and ClO₄ sorption at pH ≤ 7. Sorption isotherms with Cs confirmed the presence of a minor sorption site of lower concentration but higher affinity for elements at trace level. The sorption data were processed according to a general multi site sorbent/multi species sorbate model that provided type and concentration of sorption sites, fundamental sorption equilibria stoichiometry, and the associated corrected selectivity coefficients. (authors)