INNER-SPHERE AND OUTER-SPHERE COMPLEXATION OF A POLYCARBOXYLIC ACID AT THE WATER-BOEHMITE (GAMMA-ALOOH) INTERFACE - A COMBINED POTENTIOMETRIC AND IR SPECTROSCOPIC STUDY

Speciation and equilibria of 1,2,4,5-benzenetetracarboxylate (pyromell itate) at the water-boehmite (gamma-AlOOH) interface were studied in 0 .1 M Na(Cl) at 298 K. Surface equilibrium analyses were based upon pot entiometric and adsorption measurements in the range 4.5 < pH < 10.0. To characterize the surface complexes at the molecular level, attenuat ed total reflectance infrared spectra were recorded. The IR data provi ded information on the structure and composition of the complexes. Thi s information could then be used to constrain the thermodynamic surfac e complexation model. The acid/base properties of pyromellitate in sol ution 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 th ese measurements were primarily used to investigate the effect of pyro mellitate 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 conc entration ranges studied. IR spectra of pyromellitate sorbed at the wa ter-boehmite interface suggested the existence of one dominating surfa ce complex over a wide pH range (4.4 less than or equal to pH less tha n or equal to 8.1). The IR spectroscopic characteristics of this compl ex were in agreement with an enter-sphere nonprotonated pyromellitate ion. No evidence was found fur protonation of the sorbed ion. Thus, th e nonprotonated form is greatly stabilized at the interface as compare d to the speciation in the aqueous solution. The IR data also showed s ome indications of a second minor surface complex. This was tentativel y assigned to an inner-sphere species. According to the constraints pr ovided by the IR data, the potentiometric and adsorption data were mod eled with two surface complexes, =AlOH2+L4- (outer-sphere) and =AlL- ( inner-sphere). The extended constant capacitance mod.el was used to ac count for the electrostatic effects at the interface. The agreement be tween the experimental data and the model was satisfactory. Furthermor e, the model was also in agreement with the minor changes observed in the IR spectra as a function of pH.