Surface complexation of 2,5-dihydroxybenzoic acid (gentisic acid) at the nanosized hematite-water interface: an ATR-FTIR study and modeling approach.

In this study, characteristic interactions of 2,5-dihydroxybenzoic acid (or gentisic acid, GA) with the surface of 15-nm-sized hematite (α-Fe2O3) were studied by combining batch macroscopic experiments, in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopic investigations, and surface complexation modeling. A correlation between the pH, the amount of adsorbed GA, and the amount of Fe(III) released from the hematite surface was observed, whereas the dissolution of hematite nanoparticles became significant only at low pH and high ligand loading. From the ATR-FTIR results, two aqueous complex structures have been identified depending on pH. At the hematite-water interface, the occurrence of one deprotonated inner-sphere "bidentate" complex and one outer-sphere complex was suggested through all of the investigated pH range. At high surface coverage, variations of vibrational band intensities were observed, suggesting the occurrence of nonspecific molecular interactions. The macroscopic results (i.e., GA batch sorption and the ligand-promoted dissolution of hematite) obtained under a wide range of experimental conditions corroborated the ATR-FTIR microscopic findings. GA adsorption was described by a surface complexation model fitted to pH-adsorption curves with 1 mM sorbate concentration in the pH range of 3-9. Two surface complexes (one outer-sphere species (≡FeOH2)2···H2L((1+,1-)) and one inner-sphere species (≡Fe)2H2L) were proposed using the three-plane model. The inner-sphere complexes were predominant at low pH values, and the relative concentrations of the outer-sphere species increased with the pH increase. The formation of inner-sphere complexes at acidic pH values can promote the dissolution of nanosized hematite. At high solute loading, GA oxidation into carboxybenzoquinone compounds by ferric species was suspected, suggesting the occurrence of a redox reaction analogous to that of hydroquinone compounds.