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 ... 

HF is weakly ionized (pH <3.2), and soluble alumino-fluoro complexes are formed resulting in the presence of aluminum ions in the treated water and lowering of the active sites. At near neutral pHs, the uptake of fluoride is maximum. Assuming that the pHpzc of AA is about 8-9 as reported in several literatures, then at near neutral pHs the active sites consist of = AI-OH (protonated) and = AI-OH (non-protonated) aluminol sites. The interaction between fluoride and the protonated aluminol sites leads to the formation of inner-sphere complexes and elimination of water. The reaction can be represented by... 

The protonated aluminol sites are the most effective fluoride sorption sites and are usually responsible for the rapid kinetics due to coulombic attraction between the positively charged sites and the negatively charged fluoride species. The reaction with non-protonated sites involves ligand exchange, leads also to the formation of inner-sphere complexes, releases hydroxyl ions, is slow and characterized by a higher activation energy. 

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 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 Figure 2.13, the sum of the equivalent fractions (X total) is also plotted. It has a pH range (2.5-3.5) where X total > 1, with a maximum X total 1.2 at pH 2.9. It means that about 20% of the layer charge can sorb EDTA species. In Section 2.4.3 and Table 2.4 we could see that the number of aluminol sites is in a fairly good agreement with this value. 

In acidic medium, the aluminol sites are mainly present as A10H2+ sites (Figure 2.3). Valine molecules are also present in protonated ligands, so sorption can be neglected. The main part of the silanol sites is depro-tonated, so valine cannot sorb again. When the pH is close to neutral, aluminol sites and valine can form surface complexes as follows ... 

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 ... 

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. 

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.). 

Furthermore, the number and the ratio of silanol and aluminol sites are also very different. The ratio of silanol to aluminol is 1.3—1.5 for sedimentary bentonites that is, there are more silanol sites. In the case of the bentonitic tuff, the ratio of silanol/aluminol is reversed. For example, in the case of the B-II.b. upper sample, the ratio of silanol to aluminol is 0.06. It is interesting that the bentonitic tuff containing volcanic glass in high concentration has less silanol sites than those in sedimentary bentonites. It can probably be explained by the higher particle size (smaller specific surface area) of the bentonitic tuff (Table 3.2). 

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. 

The results of Table 3.14 show that there is a relation between the concentration of surface sites and composition (Table 3.12). The number of silanol sites is proportional to the sand content, except for freshly deposited alluvial soils with high primary silicate content (e.g., Tiszalok, Zahony). The sandy soils from wetland areas (soils near River Tisza) do not fit the usual tendencies that is, the concentrations of silanol and aluminol sites are significantly lower, as expected from similar data of other sandy soils. 

There is also a relation between the number of aluminol sites and the pH of natural soil solution (Table 3.12). The acidity of natural soil solution is the composite of all of the processes influencing soil acidity previously mentioned in this chapter. 

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