Hydroxyls, edge-surface

Surface protonation at the kaolinite surfaces. The excess proton density, Th.v. at the surface hydroxyl group is displayed as a function of pH. Surface protonation is interpreted as a successive protonation of two distinct types of OH groups localized at the gibbsite and edge surfaces. The pHpzc of the edge surface is about 7.5. 

In summary, the model proposed on the basis of acid-base characteristics of kaolinite platelets explains the pH-dependent charge primarily to the protonation of the hydroxyl groups at the basal gibbsite and the edge surface. We will later illustrate how this charge characteristics (surface protonation) influences the reactivity (dissolution characteristics) of kaolinite. 

The protonation of surface hydroxyl groups at the gibbsite and edge surfaces is displayed in Figure 14. The surface proton concentration FH) v denotes the excess proton density with respect to pHZPC = 7.5 of the hydroxyl groups at the edge face. The total excess proton density (solid line) may be assigned to two successive protonation equilibria at the kaolinite surface (broken lines). 

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).

Surface areas determined by N2-BET methods most likely overestimate the amount of sorption sites on layered silicates such as montmorillonite and zeolitic minerals such as clinoptilolite. For example, it is believed that surface complex formation of U(VI) on montmorillonite occurs on the hydroxylated edge sites of the mineral (Zachara McKinley, 1993 Turner et al., 1996). Wanner et al. (1994) estimated that only 10% of the N2-BET specific surface area is accounted for by the crystallite edges of montmorillonite. Assuming that the effective surface area ( e.,) for montmorillonite and clinoptilolite is equivalent to about 10% of the measured 5a, sorption data for montmorillonite and clinoptilolite can be recast in terms of A., , where K. > is normalized to the mineral s 5., (i.e., K = A, /. ). For nonlayered and nonporous minerals such as quartz and a-alumina, A = A, . Figure 10 7 plots... 

Layer silicate minerals have a high selectivity of trace transition and heavy metals and greater irreversibility of their adsorption. Some chemisorbing sites such as -SiOH or AlOH groups may be at clay edges and form hydroxyl polymers at the mineral surface. Another possible reason for the high selectivity may be hydrolysis of the metal and strong adsorption of the hydrolysis ion species. 

The first chemical agents used in composite fabrication were amino acids [179] for synthesizing Nylon 6. Other agents are also used in the synthesis of nanomaterials, however, the most popular are alkylammonium ions due to their ability to exchange ions located between the layers of the clay. In addition, silanes have been widely used due to their ability to react with the hydroxyl groups on the surface and edges of the clay layers. 

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