Comparison of the Adsorption ofo-Phthalate on Boehmite (γ-AlOOH), Aged γ-Al2O3, and Goethite (α-FeOOH)

Abstract

This work is concerned with the adsorption ofo-phthalate (1,2-benzenedicarboxylate) at the water–metal (hydr)oxide interface. Previously published infrared spectroscopic, potentiometric, and adsorption data characterizing the boehmite (γ-AlOOH) system are compared with new data collected foro-phthalate adsorption on aged γ-Al2O3and goethite (α-FeOOH). The study focuses on identifying bonding mechanisms, stoichiometries, and stabilities of the formed complexes, and comparing these among the three systems. Furthermore, the effects of ionic strength and composition of the ionic medium are investigated. The infrared spectroscopic data provided direct, molecular-level evidence for the existence of two dominating surface complexes on all three solids. One was shown to be a deprotonated outer-sphere species and the other was an inner-sphere surface complex. The inner-sphere complexes on the three solids were structurally related, and they were tentatively assigned to a mononuclear, chelating structure involving both carboxylate groups. The outer-sphere complexes were shown to increase in relative importance at high pH and low ionic strengths, while low pH and high ionic strengths favored the inner-sphere complexes. The information gained from the infrared spectroscopic investigations was used as qualitative input in the formulation of the surface complexation models. New models, based on the extended constant capacitance approach, were presented for theo-phthalate/aged γ-Al2O3ando-phthalate/goethite systems.