Montmorillonite aluminol sites

Surface protonation/deprotonation reactions at the edge of the silanol and aluminol sites (>SOH) of montmorillonite, which can be exemplified by the following reactions ... 

Besides the cation exchange in the interlayer space, cations and anions can also undergo sorption on the edge charges of montmorillonite. The edge charges are formed by the protonation and deprotonation of silanol and aluminol sites, and thus they depend on the pH. 

The formation of edge charges of minerals have been discussed in Chapter 1, Section 1.3.21. It has been shown that aluminosilicates (including montmorillonite) have two types of surface (aluminol and silanol) sites, and their protolytic processes have been expressed by Chapter 1, Equations 1.54-1.56. For simplicity, the reaction equations are repeated here. For aluminol sites,... 

FIGURE 2.3 Potentiometric titration curve of copper-montmorillonite in 0.1 mol dm-3 NaC104 solution, m = 50 mg, V = 20 cm3 (upper left). Vs are the experimental points, line is the plotted curve by the surface complexation model. The concentration of surface sites—lower left interlayer cations upper right silanol sites lower right aluminol sites (Nagy and Konya 2004). 

The Concentration of Edge Sites and Intrinsic Stability Constants of Protonation and Deprotonation of Silanol and Aluminol Sites of Montmorillonite Samples Calculated by the Surface Complexation Model... 

For KSF montmorillonite, the number of silanol and aluminol sites was found to be less by an order of magnitude. It is in accordance with the ratios of specific surface areas (10 m2/g for KSF montmorillonite, and 93.5 m2/g for montmorillonite [Istenmezeje]). This is an interesting observation because KSF montmorillonite is an acid-treated substance. Thus, it seems that acidic treatment causes the decrease of the layer charges (the CEC decreases montmorillonite content of Ca-, Cu-, and Zn-montmorillonites is 91%, and that of KSF montmorillonite is 53%). The acidic treatment, however, does not change the nature of silanol and aluminol sites, the stability constants of the edge charge reactions remains the same, and the number of edge sites is proportional to the specific surface area. 

The parameters obtained by others for SWy-2, BSAB, and MX-80 cannot be compared to the previously discussed data because the silanol and aluminol sites as well as the deprotonation processes (Equations 2.4 and 2.5) were treated together. Calcium bentonite (Istenmezeje) shows similar intrinsic stability constant for SWy-1 bentonite, but the number of edge sites is different. Note, however, that the specific external surface areas are also very different 21.4 m2/g for SWy-1, and 93.5 m2/g for Istenmezeje montmorillonite (Table 2.1). The ratio of the specific surface area (Istenmezeje/SWy-1) is 4.4, and the ratio of the total number of edge sites (silanol + aluminol) is 5.3, which are in fairly good agreement if the surface charge density is the same. 

In the case of zinc-montmorillonite (Figure 2.18), valine is sorbed in the interlayer space and on the aluminol sites. As discussed in Sections 2.5.1.1 and 2.6, zinc ions are adsorbed on the deprotonated aluminol sites during the preparation of zinc-montmorillonite. These adsorbed zinc ions stimulate the sorption of valine on the edge sites, increasing the quantity of the total sorbed valine. This is shown by the value of KA10H2Valin Table 2.12, which is in fact KA10ZnVal. 

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