Deprotonation aluminol sites

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. 

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

Application of Surface Complexation Models for External Surfaces The formation of surface charges in the surface complexation model is demonstrated on the example of aluminosilicates. Aluminosilicates have two types of surface sites, aluminol and silanol (van Olphen, 1977). These sites, depending on pH, may form both protonated and deprotonated surface complexes. From the thermodynamic equilibrium point of view, the protonated and deprotonated surface complexes can be characterized by the so-called intrinsic stability constants, considering the surface electric work. For aluminol sites,... 

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 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... 

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 alkaline medium, the aluminol sites are deprotonated (Alt) ). Also, the characteristic species of valine is negatively charged and cannot be adsorbed by negative A10 sites. 

When, however, the system contains a metal ion that can form stable positive complexes with valine (e.g., copper ion), then these complexes may be sorbed on the deprotonated edge sites. Calculations made on the basis of the stability constants show that positively charged CuVal+ complexes form at acidic pH where the silanol sites can be deprotonated and aluminol sites are protonated (Figure 2.3). As a result, the surface complex can be formed as follows ... 

Similar classification can be made on the basis of the surface acid-base properties of bentonite samples (Chapter 1, Section 1.3.2.1.1 and Chapter 2, Section 2.4). The number and ratio of the edge silanol and aluminol sites, as well as the intrinsic stability constants of the protonation and deprotonation constants (Chapter 1, Equations 1.54-1.56 Chapter 2, Equations 2.3-2.5) are very different for sedimentary bentonites (layers B-I.b. and B-II.a.) and for the bentonitic tuff (B-II.b. layer Table 3.3). 

As seen in Table 3.3, the intrinsic stability constants of the protolytic processes of aluminol sites are approximately the same for all bentonite types. Only the intrinsic stability constants of deprotonation of aluminol sites show some differences. The error in the deprotonation constants of aluminol sites, however, is quite large because the sites practically do not deprotonate at pH < 7 (in the pH range of the determination). The intrinsic stability constants of the deprotonation of the silanol site are different for sedimentary bentonites (B-I.b., B-II.a.) and the bentonitic tuff (B-II.b.). 

The surface acid-base parameters of bentonites from Sajobabony can be compared to similar parameters of other bentonites samples (Chapter 2, Section 2.4.3, Table 2.4). As mentioned in Chapter 2, Section 2.4.3, the parameters of SWy-2, BSAB, and MX-80 from the literature, cannot be compared to the data of bentonite samples from Sajobabony because the deprotonation of silanol and aluminol sites are treated together. Calcium bentonite (Istenmezeje, HU) shows similar characteristics as sedimentary bentonites, while the ratio of the amount of the edge site and the deprotonation intrinsic stability constant of SWy-1 bentonite is similar to the data of the bentonitic tuff. 

The concentration of aluminol and silanol sites and intrinsic stability constants of protonation and deprotonation are listed in Table 3.14. The data in Table 3.14 show that the number of surface silanol and aluminol sites is different for each soil, confirming that it is important to take into consideration the actual surface sites. 

Hydrogen ions participate in the cation-exchange processes of the interlayer space. As will be seen later (Section 2.7.1), they have a very large affinity for the layer charge. Hydrogen and hydroxide ions are potential-determining ions of the external surfaces via the protonation and deprotonation processes of aluminol and silanol sites. In acidic media, the degradation of aluminosilicates can be observed. 

As seen in Table 3.12, the humus content of soils varies within a rather wide concentration range (0.6%-6.6%). However, parameter adjustment is only successful when the protolytic processes of humus are neglected. Consideration of the protonation and deprotonation of aluminol and silanol sites (Chapter 1, Equations 1.54-1.56 Chapter 2, Sections 23-2.5) is sufficient. It is likely caused by the cations of the support electrolyte and the divalent and trivalent (aluminum and ferric) cations dissolved from the soil that react with the acidic functional groups of soil organic matter, limiting the protonation of functional groups (Hargrove and Thomas 1982 Sparks 2003). 

The intrinsic stability constants of protonation and deprotonation (Table 3.14) for most soils are the same within the experimental errors. Therefore, it can be concluded that these are thermodynamic parameters characterizing the surface aluminol and silanol sites. The values, however, are different some of them are used by Goldberg et al. (2005). It can be explained by the modifying effect of the silanol site, neglected by Goldberg et al. (2005). 

Concentration of Silanol- (SiOH) and Aluminol (AIOH) Sites, Intrinsic Stability Constants of the Deprotonation (Ig K (SiO ), Ig K (AIO )) and Protonation (IgK (AIOH2 ), the Specific Surface Area Used for Parameter Adjustment... 

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