Adsorption of Oxalic Acid onto Hematite: Application of Surface Potential Measurements

Abstract

The surface potential at the hematite/aqueous oxalic acid interface was measured by means of a hematite Single Crystal Electrode. This allowed the simultaneous interpretation of the surface potential, electrokinetic potential and adsorption data for the adsorption of oxalic acid onto a hematite surface. Based on the Surface Complexation Model, this interpretation suggested that the oxalate ion is bound to a metal ion at the solid surface to form a singly-charged oxalate-surface complex, with the charge being exposed to the potential at the outer Helmholtz layer. A number of equilibrium parameters describing the interfacial equilibrium were obtained. Thus, for the two protonation steps of the surface sites, the thermodynamic equilibrium constants were log K1 = 7.1 ± 0.4 and log K2 = 5.2 ± 0.4, respectively. Two assumptions were tested with regard to the adsorption equilibrium constant, i.e. the charge of the surface complex is exposed to the inner surface potential, ψ0, or to the outer surface potential, ψd. The constancy of the interfacial equilibrium constants led to the conclusion the latter assumption was the more appropriate. The observed value of the adsorption equilibrium constant was log Kads = 3.0 ± 0.8. It was shown that measurement of the surface potential by Single Crystal Electrodes is a helpful tool in elucidating the equilibrium behaviour of the complex at the interface.