Benzenecarboxylate Surface Complexation at the Goethite (α-FeOOH)/Water Interface:  I. A Mechanistic Description of Pyromellitate Surface Complexes from the Combined Evidence of Infrared Spectroscopy, Potentiometry, Adsorption Data, and Surface Complexation Modeling

An investigation combining IR spectroscopy, potentiometric titrations, and adsorption experiments was carried out to study pyromellitate (1,2,4,5-benzenetetracarboxylate) sorption at the goethite (α-FeOOH)/water interface. The IR spectra show evidence of outer-sphere complexation throughout the pH range from 3 to 9. Below pH 6 additional IR spectroscopic features appear, which are tentatively assigned to inner-sphere complexes. A normalized IR peak area plot for each peak indicative of inner- and of outer-sphere complexes as a function of pH provided a semiquantitative surface speciation scheme. This scheme was successfully reproduced using surface complexation theory with a multisite complexation model calibrated on potentiometric titration and on adsorption data. The surface speciation was described with a binuclear outer-sphere complex on the {110} plane of goethite and a mononuclear inner-sphere complex on the {001} plane. Furthermore, as the IR spectra also indicated partial protonation of pyromellitate complexes at low pH, a partially protonated outer-sphere species on the {110} plane was included in the model.