Calcium-montmorillonite cation-exchanged montmorillonites

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 Composition of Montmorillonites in Atomic Percent and the Selectivity Coefficient of Calcium-Lead Cation Exchange... 

The peculiar layer structure of these clays gives them cation exchange and intercalation properties that can be very useful. Molecules, such as water, and polar organic molecules, such as glycol, can easily intercalate between the layers and cause the clay to swell. Water enters the interlayer region as integral numbers of complete layers. Calcium montmorillonite usually has two layers of water molecules but the sodium form can have one, two, or three water layers this causes the interlayer spacing to increase stepwise from about 960 pm in the dehydrated clay to 1250, 1550, and 1900 pm as each successive layer of water forms. 

The apparent discrepancy could reside in the fact that if potassium ions are available at all, they will form a mica at temperatures near 100°C. Montmorillonite structures below these conditions (pressure and temperature) need not contain potassium at all. However, at the correct physical conditions the 2 1 portion of the montmorillonite must change greatly (increase of total charge on the 2 1 unit) in order to form a mica unit in a mixed layered mineral phase. Since neither Na nor Ca ions will form mica at this temperature, potassium will be selectively taken from solution. Obviously this does not occur below 100°C since cation exchange on montmorillonites shows the reverse effect, i.e., concentration of calcium ions in the interlayer sites. If potassium is not available either In coexisting solids or in solutions, the sodi-calcic montmorillonite will undoubtedly persist well above 100°C. 

By this method, the ion-exchange isotherms and selectivity coefficients can precisely be determined in a wide surface concentration range, which allows the construction of the ion-exchange isotherm and selectivity function, and the integration of the selectivity function (Chapter 1, Section 1.3.4.2.1, Equation 1.81). An example of a cation-exchange isotherm and isotherm parameters is shown in Figure 2.2 for the cation exchange of cobalt ions and calcium-montmorillonite. 

The structural parameters of cation-exchanged montmorillonites prepared from calcium-montmorillonite (Istenmezeje) are listed in Table 2.3. As seen in Table 2.3, the basal pacing of monovalent montmorillonite is approximately 1.25 nm, and the water content is approximately 1%. It means that there is one layer of water in the interlayer space. For bivalent montmorillonite, both basal spacing (>1.5 nm) and water content (>10%) are higher, showing two layers of water molecules in the interlayer space. The basal spacing of Pb-montmorillonite is 1.254 nm, which is similar to the value characteristic of monovalent montmorillonite (1.241 nm). However, it does not mean that lead is sorbed on the surface of montmorillonite as monovalent cation since the other parameters that are determined by the distance between the layers (hydration entropy, charge/ion radius value, water content in the interlayer space) lie between the values for bivalent and monovalent cations (Foldvari et al. 1998). 

The Structural Parameters of Cation-Exchanged Montmorillonites Prepared from Calcium-Montmorillonite (Istenmezeje, Hungary)... 

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. 

As seen in Table 2.7, the lower the pH, the greater quantity of calcium ions dissolved. The dissolved calcium ions are substituted by hydrogen ions in the interlayer space. In other words, the cations in the interlayer space of montmorillonite react with water itself, and a hydrogen-calcium cation-exchange reaction takes place ... 

FIGURE 2.7 The selectivity coefficients of hydrogen ion/calcium-montmorillonite cation exchange calculated from the experimental data and on the basis of Equation 2.24. 

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. 

The species distribution of the solution determines the cation composition of the interlayer space of montmorillonite. In equilibrium, the cation exchange sites of montmorillonite are covered by calcium, hydrogen, manganese, and sodium ions (Figure 2.9). Figure 2.13 shows the equivalent fractions (X) of these cations as a function of pH at the ratio of MrnEDTA =1 1. 

Here, the adsorption of valine on different cation-exchanged montmorillonites is described (Nagy and Konya 2004). A discussion of the kinds of interactions that are possible in the ternary system of montmorillonite/valine/metal ions will be presented, and a description how the metal ions can affect these interactions. The interlayer cations (calcium, zinc, copper ions) were chosen on the basis of the stability constants of their complexes with valine. The adsorption of valine on montmorillonite is interpreted using a surface-complexation model. 

It can be shown that the quantity of lead ions in montmorillonite in atomic percent is equal to the quantity of calcium ions in calcium-montmorillonite (Table 2.14). Since the quantity of calcium ions is equal to the CEC, it means total cation exchange. 

In the first row of Table 2.14, the average composition of calcium-montmoril-lonite is given. In the second row, the mean composition of lead-montmorillonite, where lead concentration is even (no enrichments), is provided. The atomic percent of lead in lead-montmorillonite is about equal, within the experimental error of 5% to 10%, of the atomic percent of calcium in calcium-montmorillonite. Since the interlayer cation of the original montmorillonite is calcium ion, lead ions can completely exchange calcium ions. 

In the natural clays the main interlayer cations (present to maintain the neutrality of total charges) are sodium and calcium. These cations can be exchanged by treatment with solutions of other ions such as, for instance, H+ leading to KIO and KSF montmorillonites [Eq. (30)]. 

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