Protonation aluminol

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. 

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,... 

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. 

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... 

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 ... 

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 3.12, 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 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 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... 

Surface functional groups in the suspended particles determine the type of process that will take place. In the case of inorganic colloidal particles (e.g., clays) the main functional groups are silanol (=Si — OH) and aluminol (=AI — OH), whereas in metal oxides or hydroxides the functional groups are (=M — OH). These groups may become protonated or deprotonated, depending on the pH of the aqueous medium, by sorption of H+ or OH- ions as follows ... 

Figure 1.10. Surface hydroxyl groups (shaded) on kaolinite. Besides the OH groups on the basal plane, there are aluminol groups, associated with Lewis acid sites, and silanol groups protruding from the edge surface. The right side of the figure shows an outer-sphere surface complex between an ionized H2O and Na" ", as well as complexes between the silanol groups and OH (i.e., proton dissociation).

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