Adsorption of Para‐substituted Benzoates on Iron Oxides

Abstract

In order to understand the binding mechanism and adsorptivity of benzoic acids on Fe oxides, adsorption of parasubstituted amino, nitro, methyl, methoxy, and chlorobenzoic acids onto noncrystalline Fe hydroxide and goethite from aqueous solution and vapor state was studied. Adsorption on noncrystalline Fe hydroxide from solution phase was quantified by the determination of adsorption isotherms using UV spectrometry to measure the concentration of unadsorbed organic. Adsorption of the benzoic acids from the vapor phase was further studied, using Fourier‐transform infrared spectroscopy (FTIR) of self‐supported goethite films to identify the binding mechanism and relative bond strength of the organic anion‐oxide complex. Results showed that the higher the pKa of the benzoic acid (i.e., the smaller the Hammett Constant of the substituent) the more readily the organic was adsorbed from solution. From FTIR study, an inner‐sphere coordination of the carboxylate anion to surface Fe involving a ligand exchange process was identified. Quantity of adsorption from solution was correlated to surface Fe‐benzoate bond strength as deduced from FTIR of dry oxide films, with bond strengths of para‐substituted benzoate following the order: amino > methoxy > methyl > chloro > nitro. Bidentate coordination was deduced to be the binding mechanism for chemisorption on air‐dry surfaces; however, physical adsorption was also identified in the absence of competing anions. It is concluded that electron‐with‐drawing groups on the aromatic ring weaken the carboxylate bond to surface Fe atoms by withdrawing electron density from the carboxylate group. Conversely, electron‐donating groups increase adsorption by increasing the electron density of the carboxylate, strengthening its Lewis basicity.

Adsorption of phenol and substituted phenols by iron oxides

“…The opposite result was recently found for p-substituted benzoic acids [14], probably due to the much lower pKa values of those aromatic acids. 3) is favored by electron-donating substituents (X) because they enhance the Lewis basicity of the phenolic oxygen ligand, the same electron-donating substituents weaken the acidity of the phenol and make the dissociation step (Eqn.…”

Interactions of aromatic acids with montmorillonite: Ca2+- and Fe3+-saturated clays versus Fe3+–Ca2+-clay system

“…Kubicki et al (1999) related strong complexation of benzoic acid with illite to the availability of ironreactive sites on the illite surface. Previously, we showed that increasing KCl concentration up to 100 mM does not affect the adsorption of benzoic acid on Fe 3+ -montmorillonite (Polubesova et al, 2010), which indicated strong inner-sphere complexation of benzoic acid with the Fe 3+ -montmorillonite surface (Kung and McBride, 1989). The pH of the Fe 3+ -montmorillonite increased from 4.02 ±0.01 (clay-water suspension) to 4.5± 0.04 (Fe 3+ -montmorillonite-benzoic acid suspensions) followed by adsorption of benzoic acid, indicating exchange of carboxylate anions for the surface hydroxyls.…”

Adsorption of Isoxaflutole Degradates to Aluminum and Iron Hydrous Oxides

“…6). Formation of weak carboxylate–cation complexes was expected due to the sulfonyl group on the aromatic ring, which withdraws electron density from the carboxylate group and diminishes the strength of carboxylate–cation complexes (Kung and McBride, 1989).…”