Modeling proton binding at the goethite (alpha-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 usin g surface complexation theory with the basic Stern model (BSM), The affinit y of the goethite surface functional groups for protons was evaluated using the multisite complexation model (MUSIC) framework considering singly, dou bly, and triply-coordinated surface oxygens with respect to underlying Fe(I II) atoms. The affinity of these functional groups for protons was investig ated first by using a calibration curve devised in Hiemstra et al. [J. Coll oid Interface Science, 184 (1996) 680]. The calibration curve correlates th e proton affinity constants of aqueous metal monomers to the undersaturatio n of the coordination environment of oxygens by considering the actual bond valences of Fe-O bonds in goethite, short hydroxyl bonds and hydrogen bond s. The results show that the predictions are sensitive to the range of shor t hydroxyl bonds;hydrogen bonds found in the literature. The singly- and on e type of the triply coordinated sites are, however, most likely responsibl e For the basic charging behavior of goethite in the pH 2-11. The proton af finity constants of the singly- and triply coordinated sites were also opti mized using titration data at different ionic strengths by co-optimizing va lues 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 parameter s are interdependent. A narrow range of C-Stern and electrolyte ion pairs w as chosen to model the charging behavior of goethite by considering, (i), t he range of proton binding constants from the aforementioned calibration cu rve; and (ii), the success of the models to predict zeta potential measurem ents assuming that the shear plane coincides with the head of the diffuse l ayer. Modeling parameters were also produced with the '1pK approximation' w hereby the proton affinity constant of the singly- and of the triply-coordi nated sites were set to the pH of zero charge and C-Stern and the electroly te 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 indicat es a larger proton uptake capacity of the 23 and 37 m(2) g(-1) goethites, p utatively resulting from the larger surface roughness at the termination of the particles. (C) 2001 Elsevier Science B.V. All rights reserved.