Calcium montmorillonite

Robertson RHS (1986) Fuller s Earth A history of calcium montmorillonite. Voltuma Press, Hythe, Kent... 

Calcium magnesium acetate, 1 127 Calcium magnesium carbonate, health and safety factors related to, 15 74 Calcium monosulfoaluminate, 5 477t Calcium montmorillonite, 6 686, 696 structure and composition, 6 668-669 Calcium nitrate, in nitrogen fertilizers,... 

Cruz M, Kaiser A, Rowxhat PG, et al. 1974. Absorption and transformation of HCN on the surface of copper and calcium montmorillonite. Clays Clay Mineral 22 417-425. 

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. 

Wahlberg, et al., (7) report very similar loading effects on a calcium montmorillonite. The value of Pgy- they report for their clay at trace Sr(II), from 0.01 M Ca(II), is about 40% higher than the value in Figure 7. 

Keren, R. and M. J. Singer. 1988. Effect of low electrolyte concentration on hydraulic conductivity of sodium/calcium montmorillonite-sand system. Soil Sci. Soc. Am. J. 52 368-373. 

K6nya, J., and N. M. Nagy. 1998. The effect of complex-forming agent (EDTA) on the exchange of manganese ions on calcium-montmorillonite. I. Reaction scheme and Ca-montmorillonite/Na2EDTA system. Coll. Surf. 136 297-308. 

Stadler, M., and P. W. Schindler. 1993a. Modeling of H+ and Cu2+ adsorption on calcium-montmorillonite. Clays Clay Miner. 41 288-296. 

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. 

Calcium-Montmorillonite (Istenmezeje, Hungary) Equilibrium Constant ... 

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 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 layer charges (X ) of montmorillonite are neutralized by cations, namely, sodium or calcium ions in natural minerals or any other cations in cation-ex-changed montmorillonites. As our model substance is calcium-montmorillonite, the neutralization of the layer charge can be described as follows ... 

The acid-base properties of calcium-, copper-, zinc-, and manganese-montmoril-lonites produced from calcium-montmorillonite were studied by potentiometric titration (Nagy and Konya 2004 Nagy et al. 2004). The neutralization reactions of Cu, Zn, and Mn ions can be described by Equations 2.6-2.8 ... 

Similar experiments with dual radioisotopic labeling show that there is a small manganese(II) ion excess in the manganese(II) ion/calcium-montmorillonite (labeled with 54Mn and 45Ca isotopes) interfacial reaction. It means that there is an adsorption reaction besides ion exchange, but it has a very low contribution. For this reason, the presence of adsorption can be observed, but its quantitative treatment is difficult. 

The Estimated Isotherm Parameters of Zinc lon/Calcium Montmorillonite Interfacial Reactions (pH 5.8)... 

FIGURE 2.5 Composite (upper), ion exchange, and adsorption isotherms (lower) of zinc ion/calcium-montmorillonite interfacial reaction. T = 20°C, pH 5.8. 

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. 

Logarithm of Stability Constants of the Species Formed in the Solution Phase of the Manganese(ll) lon/EDTA/ Calcium-Montmorillonite System... 

As seen in the figure, the sorbed quantity of valine strongly depends on the quality of the cation in the interlayer space. Calcium-montmorillonite shows a rather low valine adsorption (about 10 7 mol valine/g Ca-montmorillonite) compared to the other montmorillonites (10 3 mol valine/g Cu-, and Zn-montmorillonites). 

In the case of calcium-montmorillonite, the quantity of valine adsorbed is very low, but, similar to zinc-montmorillonite, it is adsorbed in the form of H2ValX and A10H2Val. 

As a conclusion, we can say that the interlayer cations have an important role in the valine sorption mechanism. The total quantity of the adsorbed valine increases in the following order calcium-montmorillonite < copper-montmorillonite < zinc-montmorillonite. This order can be explained by the cumulative effect of the affinity of cations to the layer charges (Table 2.2) and the stability constants of metal ion-valine complexes in the solution (Table 2.11). 

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. 

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