Proton adsorption mechanism at the gibbsite and aluminium oxide solid/solution interface.

Abstract

Titration curves of a synthetic pure crystalline gibbsite suspension have been made at three NaCl levels. The synthesized gibbsite is characterized by TEM, TGA and X-ray diffraction. The overall BET surface area and the surface area of the edges of the hexagonal crystals are determined. The surface structure of gibbsite is discussed. The surface charge ( sigma 0)-pH data are analysed with a recently proposed one-step charging model for proton adsorption (one-pK model). Analysis of the data suggests that the singly coordinated surface hydroxyls are probably the dominant reactive surface group. Considering these groups as reactive only the titration data could be fitted well with the one-pK model if it is extended with pair formation. Only two adjustable parameters are needed, the Stern layer capacitance and the pair formation constant. Analysis of titration data of aluminium oxides, as presented in recent literature, showed that all sigma 0-pH curves could be described rather well with one and the same set of two parameters, the capacitance of the Stern layer and the pair formation constant. The difference in proton adsorption behaviour between different aluminium (hydr)oxides is mainly caused by differences in the site distribution and the site densities of the aluminium (hydr)oxides involved. The results suggest that pair formation should be taken into account. (Abstract retrieved from CAB Abstracts by CABI’s permission)