Lead-montmorillonite, concentration

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. 

The lead concentration of the calcium-lead-montmorillonites increases with pH and with the initial lead concentrations of the suspensions. 

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.)... 

Figure 7 shows the representative bright field HRTEM images of nanocomposites of NR and unmodified montmorillonite (NR/NA) prepared by different processing and curing techniques. It is apparent that the methodology followed to prepare the nanocomposites by latex blending facilitates the formation of exfoliated clay structure, even with unmodified nanoclays. It has been reported in the literature that hydration of montmorillonite clay leads to extensive delamination and breakdown of silicate layers [94, 95]. It has also been shown that NA disperses fully into the individual layers in its dilute aqueous dispersion (clay concentration <10%)... 

The C.E.C. of montmorillonite does not appear to vary with grais size (Osthaus,1955) however, there is a relation between cation type and flake size. In most montmorillonite clays and shales the Na is concentrated in the finer fraction and the Ca in the coarser fraction (McAtee, 1958). This is probably because Na allows much greater interlayer expansion which leads to the shearing-off of thin flakes. 

Obviously, the dissolution of the elements leads to change in the crystal lattice and the mineral composition. This can well be seen during the acidic treatment of montmorillonite or bentonite for catalytic purposes (Section 2.1). The treatment is done using concentrated hydrochloric, sulfuric, or phosphoric acid. X-ray diffraction studies show that a commercially available montmorillonite has low montmorillonite content (53%). The other constituents are illite 10%, kaolinite 6%, quartz 10%, plagioclase 5%, gypsum 1%, anhydrite 4%, and amorphous 7%. 

The SEM picture shows two types of particles (1) lead enrichments without montmorillonite, and (2) montmorillonite particles with lead enrichment on it. The montmorillonite particles are much larger, their size can reach 10 pm, and their elemental SEM maps show even distributions of Al, Si, Mg, Fe, Ca, and O only the distribution of Pb shows spots with higher concentration (white spots in Figure 2.29). 

The precipitation and colloid formation of different metal oxide hydroxides is known in soils when the concentration of the ions reaches the value of stability products. In this case, the precipitation can be explained by the thermodynamic properties of the bulk solution. In the lead ion/calcium-montmorillonite system, however, the production of lead enrichments cannot be explained by the... 

Layers or platelets can also be associated into aggregates or floes, and in the extreme case of very concentrated solutions, a gel can also be formed.8-11 This is especially the case of montmorillonite, where edge-to-face interactions can lead to the formation of a gel at concentrations higher than about 4%. 

In contrast to kaolinite, increasing electrolyte concentration sharply reduces the stability of illite and montmorillonite particles, which leads to their active coagulation even in a 1% suspension of these minerals. As a result, a continuous structural network is formed with medium-sized cells in illite (Fig. 7b) and large-sized ones in montmorillonite (Fig. 7c). 

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