Montmorillonite cation-exchange processes

When studying the surface acid-base properties of montmorillonite, it is essential to understand that hydrogen ions and cations of the support electrolyte can also participate in cation-exchange processes. The processes on the internal and external surfaces have to be taken into consideration simultaneously, and they both have to be included into the equilibrium thermodynamical models. 

The other interfacial process involving hydrogen ion is the cation-exchange process in the interlayer space. When montmorillonite is suspended in water or in an electrolyte solution, a part of exchangeable cations can be dissolved. In Table 2.7, the relative quantity of calcium ions dissolved in water or in acidic solutions is shown. 

The effect of a complex-forming agent on the cation-exchange processes of montmorillonite is well demonstrated in calcium-montmorillonite, manganese(II) ion, and the sodium salt of the ethylene diamine tetraacetic acid (EDTA) system (K6nya and Nagy 1998 Konya et al. 1998). The reactions are illustrated in Figure 2.9. 

Kawasumi etal (1997) modified the montmorillonite clay by octadecyl-amine surfactant via cation exchange process [19]. Shi and Gan (2007) used cetryl trimethyl ammonium bromide for modifying the montmorillonite [20]. Clay can also be modified using hexadecyl trimethyl bromide [21]. 

Na-montmorillonite (MMT) was purified by dispersion of crude clay into deionized water and separation of non-colloidal impurities. To obtain cation exchange process, the purified MMT was swollen in deionized water for 24 h agitation at room temperature and a certain quantity of 1-octadecylamine was added. The system was maintained at around 68 °C for about 4 h and then filter and repeatedly washed with deionized water. The product was then dried, crushed, and sieved with 325-mesh to obtain organically modified montorillonite (OMMT). The main objective for the modification of nanoclay montmorillonite consists in or-ganophilization of clay in order to improve the compatibility with epoxy resin blends. Otherwise, the polymer and the clay will form a separate phase system with a low interface among the various components. 

The most important argument for the siUcate layers intercalation by protonated adducts was revealed by XRD analysis, which gives the value of the basal distance between silicate layers as shown in Fig. 3. From Fig. 3, one may observe that the basal distance of modified montmorillonite is always higher than of unmodified clay. The modification directed that aU the new protonated adducts were successively intercalated between the silicate layers during the cationic exchange process. 

A theoretical model for the adsorption of metals on to clay particles (<0.5 pm) of sodium montmorillonite, has been proposed, and experimental data on the adsorption of nickel and zinc have been discussed in terms of fitting the model and comparison with the Gouy-Chapman theory [10]. In clays, two processes occur. The first is a pH-independent process involving cation exchange in the interlayers and electrostatic interactions. The second is a pH-dependent process involving the formation of surface complexes. The data generally fitted the clay model and were seen as an extension to the Gouy-Chapman model from the surface reactivity to the interior of the hydrated clay particle. 

The negative layer charge is mostly neutralized by the hydrated cations in the interlayer space. These cations are bonded to the internal surfaces by electrostatic forces, and they are exchangeable with other cations. The interaction strength between the hydrated cation and the layers (the internal surface) increases when the charge of the cation increases, and the hydrated ionic radius decreases. Cations with hydrate shell can be considered as outer-sphere complexes. Cation exchange is the determining interfacial process of the internal surfaces of montmorillonite. 

The other type of transformation process of the interlayer cation is pillaring (Chapter 1, Section 1.3.5). In this process, metal oxide chains are formed in the interlayer space upon thermal treatment of different cation-exchanged mont-morillonites. As a result, the basal spacing, that is, the size of the interlayer space, increases. The pillared montmorillonites are widely applied in catalytic reactions, as evidenced by over 600 scientific publications in the last 10 years (e.g., Fetter et al. 2000 Johnson and Brody 1988 Perez Zurita et al. 1996). 

The most important industrial example of cation exchange is the preparation of sodium-montmorillonite/bentonite from calcium bentonite. As seen in Table 2.2, calcium ions have greater affinity to the layer charge than sodium ions, so the calcium-sodium cation exchange must be performed in the presence of carbonate ions. It means that calcium-montmorillonite/bentonite is suspended in sodium carbonate solution. Calcium ions precipitate with carbonate ions, so sodium ions can occupy the interlayer space. This process is known as soda activation of bentonite. The disadvantage of soda activation is that sodium-montmorillonite is contaminated with calcium carbonate. 

The acidic destruction of montmorillonite results in the release of silicon and aluminum. The initial fast exchange of surface cations by hydrogen ions is followed by the release of aluminum and silicon. The dissolution rate of Si is higher than that of A1 and is influenced by the relative ratios of basal siloxane and edge surfaces. The shift of pH to more basic values by the ion-exchange processes and the hydrolysis of dissolved species induce the formation of secondary amorphous solids, initiating the formation of amorphous aluminosilicates (Sondi et al. 2008). 

As seen earlier, cation exchange of montmorillonite has an important role in agricultural applications. In these cases, cations sorbed previously on bentonite are added to the soil. The opposite process, however, is also important in environmental applications, namely, when polluting cations have to be sorbed on bentonite. Such applications are the clay barriers of waste disposals. The high CEC is desirable. The sorption of cationic species decreases the migration rate of wastes, too (Section 3.2). 

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