Vermiculite 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]. 

Magnesium-vermiculite also forms monolayer hydrates with basal-plane spacings of 1.163 and 1.153 nm [23]. These hydrates are distinguished by the configuration of the Mg2+ solvation complexes (outer-sphere surface complexes) in them. The hydrate with the larger basal-plane spacing contains Mg2+ in the centers of flattened tetrahedra formed by water molecules the other clay hydrate contains Mg2+ at the apex of a pyramid whose base comprises three water molecules. 

Metals constitute an important source of pollution in soils. As discussed in Chapters 5 and 6, they can bind to soils through humic substances, surface complex-ation, or ion exchange. In some cases more than one type of interaction can occur, as in clay minerals (e.g., montmorillonite and vermiculite) that bind metals through ion exchange as well as surface com-... 

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. 

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+). 

FIGURE 9.2 Neutron-scattering-density profiles p(z) for a lithium-substituted vermiculite. Oxygen plus clay layer is the solid line, hydrogen is the dashed line, and lithium is the stars. The molecular model above shows two sections of clay surface and an undistorted octahedral Li+(H20)6 complex. In this model all six water molecules are hydrogen bonded directly to the clay plate in practice we find that, on average, two of the six water molecules are less strongly oriented toward the plate. 

But it was not to be. Try as we might, the difference in scattering lengths between the 6Li and 7Li isotopes was too small to permit us to measure the lithium ion distribution in the swollen state. We had to content ourselves with the results for the crystalline phase, where the behavior of the lithium ions is different from that of the larger alkali metal cations [27], Potassium and cesium ions bind directly to vermiculite clay surfaces rather than hydrating fully. Because only lithium-substituted vermiculites of the alkali metal series will swell macroscopically when soaked in water, it seems that interlayer cations must form fully hydrated ion-water complexes if the particles are to expand colloidally. This conclusion has since been supported... 

Clays with mica-type lattices, especially vermiculite and illite, can also react with many organic compounds to form complexes in which the organic substance is held in the clay lattice. The rate of reaction with vermiculite is fairly rapid but very slow with illite and mica. Differences in behavior of these minerals has been attributed to surface area and to the density of charge on the surface. 

The configuration of water molecules between the basal planes in vermiculite depends sensitively on the nature of the cation complexed on these surfaces to balance the negative charge produced by isomorphic substitution of for in the tetrahedral sheet. Because of this substitution, the siloxane ditrigonal cavities exhibit a relatively ItKulized... 

---