Montmorillonite exchangeable cations

Extensive intercalation of polar molecules takes place in this substance in an irreversible manner, and marked hysteresis results (Fig. 4.28). The driving force is thought to be the interaction between the polar molecules and the exchange cations present in the montmorillonitic sheets, since non-polar molecules give rise to a simple Type B hysteresis loop with no low-pressure hysteresis. 

The clay used is a montmorillonite referenced KC2 provided from CECA (France). Its general formula is (Si8 O2o AI4.X Mx)(OH)24 CEy. n H20, where CE are exchangeable cations. 

Rearrangement reactions catalyzed by the clay surface were observed for par-athion (an organophosphate pesticide) when it was adsorbed on montmorillonite or kaolinite in the absence of a liquid phase. The rate of rearrangement reactions increased with the polarization of the hydration water of the exchangeable cation (Mingelgrin and Saltzman 1977). Table 14.1 summarizes a series of reactions catalyzed by clay surfaces, as reported in the literature. 

The M+ component was calculated as being equal to the residual charge on the lattice. This eliminates problems of exchangable cations which have not been analyzed, such as H Oi. This procedure proved necessary for montmorillonites also. 

Fig. 77. The arrangement of layers of water molecules (dashed lines) in hydrated montmor-illonites and the corresponding interlayer spacings in (a) a one-layer clay (b) a two-layer clay (c) a three-layer clay. The spacing adopted by a particular montmorillonite depends on the nature of the silicate layer and on the exchangeable cation as well as on certain other conditions of intercalation (e.g., relative humidity and temperature) (470).

Montmorillonites (smectite clays) have structures resembling that of pyrophyllite but the structure is not electrically neutral. Exchangeable cations are located in interlamellar regions of the clay and, furthermore, the clay can be flocculated such that the plate-like crystals compact with parallel c-axes to give coherent layers. The smectites are then attractive materials with which to modify electrodes. 

The relationship of surface acidity of montmorillonites to the state of hydration and nature of the exchangeable cation was investigated by Mortland and Raman (243). By a combination of infrared spectroscopy and gravimetric and elemental analyses the direction of the equilibrium in the following reaction ... 

Hance, R.J. (1969c). Influence of pH, exchangeable cation and the presence of organic matter on the adsorption of some herbicides by montmorillonite. Can. J. Soil Sci., 49 357-364. 

Keller (1964) reported that Ca is the dominant exchangeable cation on mont-morillonite in equilibrium with river water. In sea water Ca and also Na (Carroll and Starkey, 1960) tend to be replaced by Mg which becomes the dominant exchangeable cation. In many ancient clays, Na is the most abundant exchangeable cation therefore, this abundance of Na appears to be inconsistent with the above data. Recently Hanshaw (1964) conducted exchange experiments with compacted clays and found that the order of cation selectivity is dependent upon whether a clay is dispersed or compacted. He found that compacted montmorillonite preferred cations in the following order K+> Na+>H+>Ca2 +>Mg2 +. It may be that in dispersed marine mont-morillonites Mg is the predominant exchange cation, but as the mud is compacted by burial, Na replaces a portion of the Mg. This has been confirmed by Weaver and Beck (1971a). 

The most powerful methods for the study of adsorption mechanism of nitroaromatic compounds on clay minerals have become in situ spectroscopic investigations. Handerlein et al. [152, 153] and Weissmahr et al. [154-156] have investigated the adsorption of NACs particularly on illites, montmorillonites and homoionic kaolinites. The substituted nitrobenzenes on the surface of smectites were investigated by Boyd et al. [157, 158], The main focus in the experimental study of adsorption of NACs on the surface of clay minerals is the influence of the type of clay mineral, the effect of exchangeable cation of the mineral, the effect of the structure and the kind of substituents of NAC compound on the position and orientation of NACs to the surface of mineral, the character of interaction between NACs and the surface of mineral, the adsorption energy. 

Fig. 9 The optimized structure of adsorbed nitrobenzene molecule on the hydrated surface of montmorillonite with Na+ exchangeable cation [200].

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