Mechanisms of the Photochemical Enhancement of the Fenton Process and Role of Iron(III)-Carboxylate Complexes

DANIELA NICHELA; MENANA HADDOU; ESTHER OLIVEROS; FERNANDO GARCÍA EINSCHLAG

Encuentro; 7th European Meeting on Solar Chemistry and Photocatalysis: Environmental Applications; 2012

The formation and the photochemical reactivity of Fe(III)-complexes of a family of structurally related hydroxylated derivatives of benzoic acid (HBAs) have been investigated, in the context of our work on the oxidative degradation of a series of HBAs by Fenton-like and photo-Fenton processes. As part of this study, the acid-base properties of five HBAs in iron free aqueous solutions and the effect of the substituents on the pKa values of both -COOH and ?OH groups are presented. Spectrophotometric studies in the presence of Fe(III) show that ?COOH and ?OH groups must be in ortho position relative to each other for the formation of ferric complexes at pH 3.0. The values of the conditional formation constants obtained for the salicylate complexes correlate well with the pKa values of the ?COOH, suggesting that the equilibrium is dominated by the fraction of mono-deprotonated HBA ligand. The photochemical studies have shown that the ferric complexes are inert upon irradiation of the LMCT bands involving the organic ligand (i.e., l > 400 nm). However, these complexes are capable of producing hydroxyl radicals (HO?) upon excitation in the 300-400 nm wavelength range. These results indicate that, unlike what is observed for ferric complexes of aliphatic carboxylates, the irradiation of ferric complexes of aromatic carboxylates does not lead to a photo-induced decarboxylation, but yields Fe(II) through the oxidation of water molecules in the coordination sphere of the metal center