Abstract-The kinetics of hydroquinone oxidation by aqueous suspensions of pure hematite and goethiteferrihydrite mixtures at pH 6.0, 7.4, and 9 was studied using an on-line analysis system. The electron transfer between hydroquinone and the Fe oxides was monitored by UV-visible and electron spin resonance spectroscopy. The adsorption of organics on the Fe oxide surface was detected by Fourier-transform infrared spectroscopy. For different Fe oxides, a higher surface area was correlated with a greater oxidizing ability and greater adsorption of organics, suggesting that the oxidation reaction was a surface process. A reversal of the initially rapid redox reaction was found in this system, suggesting a delayed release of Fe E § into solution as the reduction of the Fe oxide proceeded. Redox potential calculations confirmed the thermodynamic favorability of the reaction reversal. A distribution of the reduced state over neighboring Fe atoms on the oxide surface probably was responsible for the initial suppression of Fe 2+ release into the aqueous phase. Based upon these observations and detection of the semiquinone radical as an intermediate of hydroquinone oxidation, an inner-sphere one-electron transfer mechanism for the oxidation of hydroquinone at the oxide surface is proposed.

The extent of chelation of aqueous Fe3+ by glyphosate [isopropylamine salt of N‐(phosphonomethyl)glycine] and the related ligands glycine, iminodiacetic acid and aminomethylphosphonic acid was estimated over a range of pH and ligand/Fe3+ ratios. Electron spin resonance (ESR) and UV‐visible spectroscopy were used to detect chelation, evidenced by changes in the inner‐sphere coordination environment of Fe3+. Generally, the spectroscopic methods confirmed the degree of chelation predicted from the known stability constants of these Fe3+complexes. The importance of the phosphonomoeity in conferring a high degree of stability to the complexes were confirmed and the Fe3+‐glyphosate soluble complex persisted in aqueous solution to pH 4 and higher. The Fe3+complexes without the phosphonate group failed to form or existed only in very acidic solution, since they were easily dissociated upon the hydrolysis of Fe3+ as the pH was raised. Infrared spectroscopy of glyphosate adsorbed on goethite confirmed direct coordination of the functional groups with surface Fe3+, and suggested that the strong adsorption of glyphosate on oxides is a consequence of the same chelation mechanism observed in solution.

Abstract--Adsorption ofp-hydroxybenzoate anion on synthetic Fe oxides, hydroxides, and oxyhydroxides (hereafter referred to as oxides) was measured at pH 5.5, and the organic-oxide interaction was characterized using diffuse-reflectance Fourier-transform infrared (DRIFT) spectroscopy. Surface complexes with ferrihydrite, hematite, goethite, and noncrystalline Fe oxide were investigated. Infrared (IR) spectra of the oxides after separation from p-hydroxybenzoate solutions showed the organic to be coordinated by an inner-sphere mechanism to the oxide surface through the carboxylate group. Bidentate binding of the carboxylate was identified to be the dominant type of complexation on the oxides. Although the IR study suggested that goethite formed the strongest surface iron-carboxylate bond, the total amount of organic adsorbed was the least on this oxide. This result suggested that bond energy was less important than adsorption site density in determining the amount of organic anion adsorption. Increasing ionic strength had little effect on adsorption by the noncrystalline Fe oxide, but dramatically decreased adsorption on hematite and goethite. This difference might have been due to anion competition for binding sites. At pH 5.5, the amount of organic adsorbed per unit weight of oxides (in 0.05 M NaC104) followed the order: ferrihydrite > noncrystalline Fe oxide > hematite >> goethite. Adsorption per unit of surface area however, followed the order: ferrihydrite > hematite > noncrystalline Fe oxide >> goethite. The hematite adsorption reaction appeared to be driven by entropy, inasmuch as the reaction was endothermic. The difference among the oxides in surface reactivity toward p-hydroxybenzoate is hypothesized to have been caused by differences in both quantity and structural arrangement of reactive sites on the oxide surface.

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.

Abstract--A kinetic study of the oxidation of hydroquinone by aqueous suspensions of hausmannite at pH 6 was conducted using an on-line analysis system. Electron transfer between hydroquinone and the oxide was monitored by ultraviolet and electron spin resonance spectroscopy to measure the loss of hydroquinone and the appearance of oxidation products. Although hydroquinone oxidized on the surface of the oxide and the oxide surface was altered after the reduction, hydroquinone and its oxidation products did not adsorb strongly on the surface. At a high concentration of hydroquinone, p-benzosemiquinone free radicals persisted in aqueous solution and were oxidized by dissolved 02. Calculations based on the thermodynamic stabilities of the oxide and the organic species involved show that the formation of p-benzosemiquinone radical by Mn reduction is feasible. The presence of the radicals indicates that the oxidation of hydroquinone by the oxide proceeded by a one-electron transfer process. At high organic/ oxide ratios, an increase in the amount of hausmannite dissolved with increasing hydroquinone concentration suggests that the reduction of the oxide by the organic was not limited to the surface layer of the oxide. At a high concentration of hydroquinone, polymers were detected in solution, suggesting that radical-mediated reactions played a role in the polymerization process. A reaction scheme is proposed to explain the effect of the Mn oxide to hydroquinone ratio on the consumption of O2 and the appearance of quinone, p-benzosemiquinone, and polymers in solution.