Albite surface titration

The prediction by our speciation model that in multiple oxides (as in quartz) Si sites are not susceptible to adsorption by H+ is confirmed by the results of surface titrations of albite, labradorite, and anorthite (Fig. 6). One can see that the net adsorption of H + at the feldspar solution interface increases markedly with the relative number of Al sites to Si sites, which is reflected by the stoichiometry of Al and Si in the different feldspars. These results help clarify the typical features of the dependence on pH of silicate dissolution (Table 2) ... 

Figure 8. Comparison of experimental surface titrations of ground and restored albite (ground albite annealed for 80 h at 1000°C) with theoretical titrations of albite and anorthite N% = 12 adsorption sites nm 2). Note the good superposition of theoretical and experimental curves for restored albite. Possible explanations for the important surface charge exhibited by ground albite are that (I) surface defects produced by prolonged grinding can introduce extra adsorption sites and (2) weakening and breaking of surface Si O bonds due to grinding can increase pKSi, ()11 1, and thus allows proton adsorption on Si sites even in midly acid solutions.

As shown above, surface titration curves of reacted albite exhibit notable differences, particularly in the acid pH range, relative to those obtained with fresh albite (but they are very close to those obtained with silica gels). T hus, one understands that surface titrations performed on fresh silicates can only help elucidate initial dissolution mechanisms. 

The proton-promoted surface complexation model was applied to silicates by several research groups in the late 1980s (e.g., Blum and Lasaga, 1988, 1991 Brady and Walther, 1989, 1992 Schott, 1990). For example, Blum and Lasaga (1988, 1991) performed dissolution and titration experiments for albite and found that the surface charge under acid conditions. 

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