Surface Complexation of 2,5-Dihydroxybenzoic Acid (Gentisic Acid) at the Nanosized Hematite−Water Interface: An ATR-FTIR Study and Modeling Approach

Abstract

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.

Sorption of nalidixic acid onto micrometric and nanometric magnetites: Experimental study and modeling

“…Interparticle diffusion in NA sorption was determined experimentally by ultrasonic mixing which was employed to disperse loosely formed aggregates in the aqueous phase. In this case, the sorption rate should be faster if the interparticle diffusion is the rate-controlling step [22,25,31]. Sorption data with ultrasonication is in agreement with that of magnetic stirring which suggests that interparticle diffusion is not the rate-limiting step.…”

Iron Oxide Surface Chemistry: Effect of Chemical Structure on Binding in Benzoic Acid and Catechol Derivatives

“…There is evidence that mixed binding of 2,5-DHBA to the IONP surface occurs through the participation of ortho -hydroxyl and monodentate carboxylate groups and through a bidentate chelate coordination. The separations between the asymmetric and symmetric stretches observed in the bidentate chelate coordination of 2,5-DHBA with FeCl 3 at pH 5.2 (1549 – 1454 = 95 cm –1 ) correspond well with the calculated Δ v for both preparations of IONP-2,5-DHBA [89/87 cm –1 (HCl/DI water)] and provide further support for the chelate mode of coordination for 2,5-DHBA and Fe 3 O 4 nanoparticles. Likewise, mixed binding is also noted for 2,3-DHBA where both carboxylate (bridging) and a minor contribution from catechol binding are indicated, with more catechol binding occurring under HCl-free reaction conditions.…”

“…This is also consistent with prior work where a shift of this band to lower frequencies was observed for the binding of SA to goethite in a salicylate binding mode. 61 A red shift in this vibration is also seen in the binding of 2,5-DHBA to Fe(III), 28 compared with free 2,5-DHBA, suggesting that the ortho-phenol is involved in binding, further strengthening our claim here. Our conclusions are also consistent with the more pronounced shoulders at around 1630 cm −1 for v as (COO − ) in the DI water preparation (Figure 4), which have been associated with salicylate-type binding of 2,5-DHBA.…”

“…By studying a set of chemically related ligands, we are able to investigate the effect of identity, relative position of substituents, and substituent chain length on binding to IONPs. The binding of dihydroxybenzoic acids was previously studied for other metal oxides; − however, prior work on salicylic acid (SA, a similar benzoic acid derivative) has suggested that the chemical composition of the metal oxide determines which ligand surface complex forms . Therefore, an evaluation of ligand binding to magnetite is warranted because of its popular use in inorganic and biological nanomaterials, even if studies of the target ligands with other metal oxides had been done previously.…”

Chemical Force Spectroscopy Evidence Supporting the Layer-by-Layer Model of Organic Matter Binding to Iron (oxy)Hydroxide Mineral Surfaces

“…First, studies have used low molecular weight synthetic aliphatic and aromatic acids as models of natural organic matter. 3,4 Second, studies have used operationally extracted (on a pH solubility basis) humic substances to represent natural organic matter in chemical studies. 5,6 These humic and fulvic acids are thought to be primarily high molecular weight aromatic polymers formed through enzyme-mediated microbial processes.…”