Photolysis of carboxylate anions adsorbed onto highly crystalline goethite (??-FeOOH) with 300-400-nm light produces Fe2+(aq) and ??OH from surface and solution redox reactions. The production of Fe2+(aq) and ??OH was studied in N2-purged and aerated aqueous suspensions, respectively, of goethite containing equimolar (10-3 M) concentrations of oxalate and benzoate (ob + g), succinate and benzoate (sb + g), benzoate alone (b + g), and goethite alone (g). The hydroxyl radical was measured by fluorescence analysis of salicylate formed in a reaction between benzoate and ??OH. Fe2+(aq) was determined colorimetrically. A proposed reaction mechanism includes the photoexcitation of Fe(III) surface complexes with OH-, HCO3-, and RCOO- ligands to form Fe2+(aq) and corresponding ligand radicals. Additional ??OH is made by the O2 oxidation of Fe2+(aq) (Haber-Weiss mechanism). Comparison of Fe2+(aq) and ??OH yields permitted an estimate of the relative importance of pathways leading to ??OH.
Additional publication details
Mechanisms for aqueous photolysis of adsorbed benzoate, oxalate, and succinate on iron oxyhydroxide (goethite) surfaces