Vermiculite inner-sphere surface complex

Variations on this overview conceptualization occur because of differing isomorphic substitution patterns in 2 1 clay minerals. For example, both Li- and Na-vermiculite can form mono- and bilayer hydrates, whereas K-, Rb-, and Cs-vermiculite cannot. For the latter, inner-sphere surface complexes are stable against solvation of the cations because of the softer Lewis acid character of the latter and a favorable siloxane-surface stereochemistry [23]. 

A. A surface that has the potential to form inner-sphere complexes with certain monovalent cations (e.g., K+ or NH4) shows stronger preference for such cations than any other cation (e.g., vermiculite-K+ or vermicu ite-NH4 versus vermiculite-Na+ or vermiculite-Ca2+). 

Bostick et al. (2002) studied Cs+ adsorption onto vermiculite and montmorillonite with EXAFS and found that Cs+ formed both inner-and outer-sphere complexes on both aluminosihcates. The inner-sphere complexes bound to the siloxane groups in the clay structure. Combes et al. (1992) found that NpOj adsorbed onto goethite as a mononuclear surface complex. Waite et al. (1994) were successful in describing uranyl adsorption to ferrihydrite with the diffuse layer model using the inner-sphere, mononuclear, bidentate surface complex observed with EXAFS. 

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