Modeling proton binding at the goethite (α-FeOOH)-water interface

The basic charging behaviours of goethite particles with different surfaces area (23, 37 and 85 m(2) g(-1)) in 0.003-2.0 M NaNO3 were interpreted using surface complexation theory with the basic Stern model (BSM), The affinity of the goethite surface functional groups for protons was evaluated using the multisite complexation model (MUSIC) framework considering singly, doubly, and triply-coordinated surface oxygens with respect to underlying Fe(III) atoms. The affinity of these functional groups for protons was investigated first by using a calibration curve devised in Hiemstra et al. [J. Colloid Interface Science, 184 (1996) 680]. The calibration curve correlates the proton affinity constants of aqueous metal monomers to the undersaturation of the coordination environment of oxygens by considering the actual bond valences of Fe-O bonds in goethite, short hydroxyl bonds and hydrogen bonds. The results show that the predictions are sensitive to the range of short hydroxyl bonds;hydrogen bonds found in the literature. The singly- and one type of the triply coordinated sites are, however, most likely responsible For the basic charging behavior of goethite in the pH 2-11. The proton affinity constants of the singly- and triply coordinated sites were also optimized using titration data at different ionic strengths by co-optimizing values for electrolyte ion pairs and the capacitance of the Stern Layer. The optimal proton binding constants were in the range of the predicted values using the aforementioned calibration curve, although the modeling parameters are interdependent. A narrow range of C-Stern and electrolyte ion pairs was chosen to model the charging behavior of goethite by considering, (i), the range of proton binding constants from the aforementioned calibration curve; and (ii), the success of the models to predict zeta potential measurements assuming that the shear plane coincides with the head of the diffuse layer. Modeling parameters were also produced with the '1pK approximation' whereby the proton affinity constant of the singly- and of the triply-coordinated sites were set to the pH of zero charge and C-Stern and the electrolyte ion binding constants were co-optimized. In both cases, the values of C-Stern and of the electrolyte ion binding constants are slightly larger for the 23 and 37 m(2) g(-1) goethites than for the 85 m(2) g(-1). This indicates a larger proton uptake capacity of the 23 and 37 m(2) g(-1) goethites, putatively resulting from the larger surface roughness at the termination of the particles. (C) 2001 Elsevier Science B.V. All rights reserved.