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 charge (p.z.n.c) for porous and non-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 surface 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.c. was equal to the point of zero net proton charge (p.z.n.p.c.) for the variable charge minerals investigated.