COMPARISON OF THE ADSORPTION OF O-PHTHALATE ON BOEHMITE (GAMMA-ALOOH), AGED GAMMA-AL2O3, AND GOETHITE (ALPHA-FEOOH)

This work is concerned with the adsorption of o-phthalate (1,2-benzene dicarboxylate) at the water-metal (hydr)oxide interface. Previously pu blished infrared spectroscopic, potentiometric, and adsorption data ch aracterizing the boehmite (gamma-AlOOH) system are compared with new d ata collected for o-phthalate adsorption on aged gamma-Al2O3 and goeth ite (alpha-FeOOH). The study focuses on identifying bonding mechanisms , stoichiometries, and stabilities of the formed complexes, and compar ing these among the three systems. Furthermore, the effects of ionic s trength and composition of the ionic medium are investigated. The infr ared 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 th e three solids were structurally related, and they were tentatively as signed to a mononuclear, chelating structure involving both carboxylat e groups. The outer-sphere complexes were shown to increase in relativ e importance at high pH and low ionic strengths, while low pH and high ionic strengths favored the inner-sphere complexes. The information g ained from the infrared spectroscopic investigations was used as quali tative input in the formulation of the surface complexation models. Ne w models, based on the extended constant capacitance approach, were pr esented for the o-phthalate/aged gamma-Al2O3 and o-phthalate/goethite systems. (C) 1998 Academic Press.