Characteristics of montmorillonite

The following characteristics of clay are highly important for dispersion in the polymer matrix [11-12]. 

Ability of the silicate platelets to disperse into individual layers. 

Ability to fine-tune their surface chemistry through ion exchange reactions with organic and inorganic cations. 

The silicate platelets in the montmorillonite clay possess the characteristics mentioned above. These two characteristics are interrelated and provide the possibility of using these clay platelets at nano-level in the 

Microcomposites the clay filler remains as micro tactoids without any changes in interlayer spacing of the clay platelets. 

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FIGURE 2.27 The concentration profiles of different elements of lead-montmorillonite. Upper Only the concentration of lead increases the other elements show concentrations characteristic of montmorillonite. Lower The increase of lead concentration is parallel to the increase of iron concentration. (Reprinted from Nagy et al. 2003a, with permission from Elsevier.)... 

In order to convert the montmorillonite clay into a nanoday compatible with organic polymers, an ion exchange process is performed to treat the day surfaces. Generally, an organic cation, such as from a quaternary ammonium chloride, is used to change the hydrophilic/hydrophobic characteristics of the clay (Figure 9.3). Typical characteristics of montmorillonite clays are as follows ... 

XRD spectra recorded for Sn02/montmorillonite nanocomposites are presented in Fig. 7. The figure demonstrates that intercalation of nanoparticles among the silicate layers indeed occurred, as the peak intensity of the basal spacing characteristic of montmorillonite gradually decreases and, in the presence of Sn02, thevalues of the basal distances fall in the range fl L = 2.7-6.6 nm. Nanocomposites with hectorite are shown in Fig. 8 intercalation of nanoparticles is amply... 

Montmorillonite is a laminar and expandable clay with wet binding properties and widely available throughout the world. The layers have permanent negative charges due to isomorphic substitutions. The scientific interest of montmorillonite lies in its physical and chemical properties as well as its low price. Consequently, the industrial application of montmorillonite is an attractive process [1]. On the other hand, among numerous reports published so far, crystallization of zeolite Beta draws much attention because of its unique characteristics, in particular, acidity and acid catalysis. It is reasonable to conceive that a catalyst system based on Beta/montmorillonite composite with suitable composition should provide a good catalytic capacity. 

As has been confirmed by XRD, the framework of montmorillonite has been partly destroyed due to the calcination under high temperature. Most diffraction peaks of montmorillonite are faint. After hydrothermal crystallization the characteristic Bragg reflections for zeolite Beta structure at 7.7° and 22.42° 20 are detected in the composite, indicating the presence of the Beta phase. 

Fig. 7 shows the progressive transformation of montmorillonite to iliite/smectite interlayers by the gradual development of both the characteristic Cs and Rb high selectivity profiles observed for pure illite and the high Cs-Rb selectivity at+ race fadings. The data can be simulated (see table VI for the Ca - Cs case) using a consistent set of intrinsic selectivity coefficients and identical site group capacities for the Ca-Cs and... 

The two series of phase relations deduced above result in, at a first approximation, two "facies" for the expandable dioctahedral minerals— that of low temperature where fully expandable minerals exist and where the tie-line or association beidellite-montmorillonite persists. More elevated conditions produce a kaolinite-illite tie-line characteristic of sequences of buried rocks. 

Bacterial desiccation and survival of other species in dried soils and mineral powders has been reported (Bitton et al., 1976, Labeda et al., 1976 Dupler Baker, 1984 Moll Vestal, 1992). In many instances the authors did not report soil mineralogical characteristics however, Bitton et al. (1976) showed greater survival of Klebsiella aerogenes under desiccation stress when in soils dominated by montmorillonite as compared to kaolinite. Amendment of montmorillonite to a sandy soil also increased the survival of K. aerogenes and thus produced the greatest increase in survival in these studies as well as those conducted with rhizobia. 

Figure 3.7. Phenanthrene sorption isotherms on (A) the whole Amherst peat soil humic acid, (B) montmorillonite and a montmorillonite-humic acid complex (5 1 ratio), and (C) kaolin-ite and kaolinite-humic acid complex (5 1 ratio). Insets in parts B and C are the respective isotherms presented on a linear scale. Reprinted from Wang, K., and Xing, B. (2005). Structural and sorption characteristics of adsorbed humic acid on clay minerals. J. Environ. Qual. 34, 342-349, with permission from the Soil Science Society of America.

Table XXXVIII). Brindley (1955) has suggested that stevensite is a mixed-layer talc-saponite however, Faust et al. (1959) considered it to be a defect structure with a random distribution of vacant sites in the octahedral sheets. A small proportion of domains with few or no vacancies would then be present having characteristics of talc. The layer charge in stevensite is due to an incompletely filled octahedral sheet (Faust and Murata, 1953). This deficiency is minor (0.05—0.10) and the resulting cation exchange capacity is only about one-third that of the dioctahedral montmorillonites (100 mequiv./lOO g.).

Many of these weathered micas and illites are completely leached of K and have the swelling characteristics of a montmorillonite. This material is usually leached to such an extent that the layer charge of some of the layers is sufficiently lowered so that they will no longer contract to 10 A. When these clays are exposed to sea water or K from any source, only a portion of their layers will contract and a mixed-layer clay is formed. 

Some basic properties, such as basal spacing (d001), internal and total specific surface area, and cation-exchange capacity (CEC) of some natural montmorillonite or bentonite with high montmorillonite samples are listed in Table 2.1. Similar characteristics of different cation-exchanged montmorillonites are given in Section 2.3. 

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). 

In this chapter the sorption of two organic substances, EDTA and valine amino acid, are discussed. They were selected for two reasons as seen in Figure 2.8, EDTA can be sorbed on the edge sites of montmorillonite. The other reason is the presence of characteristic functional groups in both molecules. The carboxylic and amine groups are very important in organic syntheses and also in humic substances. 

FIGURE 2.26 SEM picture of montmorillonite treated with 5e-4 mol/dm3 lead perchlorate solution. Lower left side morphology of the sample made by backscattered electrons. Lower right side lead map made by characteristic x-ray photons. Upper morphology of the sample made by backscattered electrons, enlarged from site No. 2. 

Some characteristic properties of bentonites (CEC, sorption properties) are mainly governed by the montmorillonite content and the layer charge of montmorillonite. Other properties, however, depend on the circumstances under which the rock is formed. These are particle size distribution, external specific surface area, and surface acid-base properties. The quantity of the edge sites mainly depends on the specific surface area. The protonation and deprotonation reactions take place on the edge sites of other silicates and aluminosilicates present beside montmorillonite, so their effects manifest via surface reactions. Consequently, the origin of bentonite determines all properties that are related to external surfaces. 

The agricultural and environmental applications are mostly based on water permeability (swelling) and sorption properties. Therefore, the montmorillonite content (CEC) and the quality of interlayer cations are the most important characteristics that influence the usability of montmorillonite in these cases. 

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