Inner-sphere and outer-sphere complexation of a polycarboxylic acid at the water-boehmite (γ-AlOOH) interface:: A combined potentiometric and IR spectroscopic study

Speciation and equilibria of 1,2,4,5-benzenetetracarboxylate (pyromellitate) at the water-boehmite (gamma-AlOOH) interface were studied in 0.1 M Na(Cl) at 298 K. Surface equilibrium analyses were based upon potentiometric and adsorption measurements in the range 4.5 < pH < 10.0. To characterize the surface complexes at the molecular level, attenuated total reflectance infrared spectra were recorded. The IR data provided information on the structure and composition of the complexes. This information could then be used to constrain the thermodynamic surface complexation model. The acid/base properties of pyromellitate in solution and its complexation with. Al(III) mere also studied by means of potentiometric titrations in 0.1 M Na(Cl) at 298 K. The results of these measurements were primarily used to investigate the effect of pyromellitate on the dissolution of boehmite. In aqueous solution Al(III) forms a series of mononuclear complexes, namely, AlHL, AlL-, and Al(OH)L2-. No indication of polynuclear complexes was found within the concentration ranges studied. IR spectra of pyromellitate sorbed at the water-boehmite interface suggested the existence of one dominating surface complex over a wide pH range (4.4 less than or equal to pH less than or equal to 8.1). The IR spectroscopic characteristics of this complex were in agreement with an enter-sphere nonprotonated pyromellitate ion. No evidence was found fur protonation of the sorbed ion. Thus, the nonprotonated form is greatly stabilized at the interface as compared to the speciation in the aqueous solution. The IR data also showed some indications of a second minor surface complex. This was tentatively assigned to an inner-sphere species. According to the constraints provided by the IR data, the potentiometric and adsorption data were modeled with two surface complexes, =AlOH2+L4- (outer-sphere) and =AlL- (inner-sphere). The extended constant capacitance mod.el was used to account for the electrostatic effects at the interface. The agreement between the experimental data and the model was satisfactory. Furthermore, the model was also in agreement with the minor changes observed in the IR spectra as a function of pH.