Adsorption of Para‐substituted Benzoates on Iron Oxides

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.