Surface charge of variable porosity Al₂O₃(s) and SiO₂(s) adsorbents

The surface charge properties of two SiO₂ and three Al₂O₃ mineral adsorbents with varying degrees of framework porosity were investigated using discontinuous titration and ion adsorption methodologies. Points of zero net change (p.z.n.c.) for porous SiO₂ were <2.82 and for Al₂O₃ minerals ranged from 6.47-6.87. Silica surfaces possessed very slight negative charge in the acid pH range (pH ≤ 7) and significant dissociation of silanol groups occurred at pH ≥7. Variation of surface charge density with aqueous proton concentration was nearly identical within a mineral type (i.e., SiO₂ or Al₂O₃) irrespective of the degree of framework porosity, indicating that the densities of dissociable surface sites are equivalent, when normalized to surface area. The results suggest that the use of titration methods alone may be insufficient for thorough surface charge characterization, particularly at low and high pH. Proton titrations should be coupled with concurrent ion adsorption measurements to confirm surfaced charge development. Discontinuous proton titration and ion adsorption data, which were in agreement in the slightly acidic through slightly basic pH range, both indicated that p.z.n.p.c. was equal to the point of zero net proton charge (p.z.n.p.c.) for the variable charge minerals investigated.