Experimental evidence of the contrasting reactivity of external vs. interlayer adsorption sites on swelling clay minerals: The case of Sr2+-for-Ca2+ exchange in vermiculite

The potential contrasting reactivity of external (edge and basal) and interlayer adsorption sites is studied by investigating the Sr2 +-for-Ca2 + exchange in three different size fractions (0.1–0.2, 1–2, and 10–20 μm) of vermiculite. For a given equivalent fraction of Sr2 + in solution, the obtained experimental adsorption isotherms showed that the equivalent fraction of adsorbed Sr2 + on the solid decreased with decreasing particle size. Using a combination of bulk chemistry, X-ray diffraction profile modeling of 00l reflections and morphological data, the interlayer site compositions were found to be independent of the vermiculite size fraction for a given chemical composition of the solution and without any preference between Ca2 + and Sr2 + cations. The lack of selectivity of both of these cations for interlayer adsorption sites [log Kint.(Ca2 +/Sr2 +) = 0] is supported by molecular simulation results indicating that Ca2 + and Sr2 + have similar interactions with the clay surface and water molecules in the interlayer space of vermiculite. In contrast, external adsorption sites exhibited a clear preference for Ca2 + cations [− 2.5 < log Kext.(Ca2 +/Sr2 +) < − 1.0]. The two-site model (i.e., interlayer and external adsorption sites), which is constrained by the relative amounts of both types of sites derived from morphological analysis, is found to well interpret experimental ion-exchange experiments. The two-site ion-exchange model derived from this study represents a morphology-constrained model with clear evidence of contrasting reactivity between external and interlayer adsorption sites. This model can be used to improve our understanding and prediction of the behavior of Sr2 + cations in contaminated soils with a wide range of particle sizes.