Inner-Sphere and Outer-Sphere Complexation of a Polycarboxylic Acid at the Water−Boehmite (γ-AlOOH) Interface:  A Combined Potentiometric and IR Spectroscopic Study

Abstract

Speciation and equilibria of 1,2,4,5-benzenetetracarboxylate (pyromellitate) at the water−boehmite (γ-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) were 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 ≤ pH ≤ 8.1). The IR spectroscopic characteristics of this complex were in agreement with an outer-sphere nonprotonated pyromellitate ion. No evidence was found for 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 model 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.

The Sorption of Anthracene onto Goethite and Kaolinite in the Presence of Some Benzene Carboxylic Acids

“…Figure 1 shows that the benzene carboxylic acids have a higher affinity for the goethite surface than kaolinite. The adsorption edges for phthalic acid onto goethite found in this study are similar to those found in previous studies (17,18,(21)(22)(23)(24).…”

“…Extended constant capacitance models have been used by Persson and coworkers (17)(18)(19) to describe the adsorption of benzene carboxylic acids onto a number of different surfaces. An ECCM allows adsorption to occur in two planes: "strongly" bound ions adsorb at the surface plane (0-plane) as inner-sphere complexes, and "weakly" bound ions adsorb in an outer plane (β-plane), separated from the surface by at least one layer of water molecules.…”

“…Adsorption of polar organic materials to mineral surfaces has been generally attributed to interactions with surface hydroxyl species (17)(18)(19)(20)(21)(22)(23)(24). The lesser adsorption onto kaolinite compared with goethite may be due, at least in part, to the fact that the siloxane face of the kaolinite crystals is not hydroxylated, and therefore will not interact strongly with carboxyl groups, while the aluminol surface has doubly coordinated hydroxyl groups that have been found to be less reactive than their singly coordinated counterparts.…”

Adsorption of fulvic acid on goethite

“…Nevertheless, at low pH inner sphere complexes are found to be important for the binding of organic material (Gu et al, 1995;Kaiser et al, 1997;Nordin et al, 1998). This can be explained by the changes in electrical energy.…”

Outer-sphere adsorption of Pb(II)EDTA on goethite

“…Zachara et al (1995) found that, at pH • 5, Co(II)EDTA adsorption on goethite and aquifer sands is followed by significant Co(II)EDTA dissociation within 30 minutes. Sigg (1996, 1997) (Chisholm-Brause et al, 1990;Roe et al, 1991;Manceau et al, 1992;Bargar et al, 1996;1997a,b,d; (Nordin et al, 1997;Nordin et al, 1998;Persson et al, 1998a;Persson et al, 1998b).…”