Degradation of Iron(III)-Salicylate complexes in Fenton systems. Efficiency of Iron(II) fotoproduction

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

La Serena

Encuentro; X ELAFOT, Encuentro Latinoamericano de Fotoquímica y Fotobiología; 2010

Photo-reduction by inner-sphere electron transfer (LMCT) of Fe(III)-aquo complexes provide an important route for Fe(III) reduction in photo-Fenton systems that substantially increase the rates of Fe(II) and HO• production: [Fe(III)(OH)-]+2 + hn → Fe(II) + HO• (R1) In the present work, photo-Fenton techniques have been used for the degradation of a set of hydroxyl-derivatives of benzoic acid. The studied model substrates (L) were the following ones: 2-hydroxybenzoic acid (2H-BA); 2,4-dihydroxybenzoic acid (24DH-BA); 2-hydroxy 5-nitrobenzoic (2H5N-BA) and 2-hydroxy 4-nitrobenzoic acid (2H4N-BA). Previous work has sown that in Fe(III)-containing solutions, the different substrates form highly stable complexes with stoichiometry 1:1 or 2:1 LH- + Fe(III) ® [Fe(III)L]+ + H+ (R2a) 2 LH- + Fe(III) ® [Fe(III)L2]- + 2 H+ (R2b) In this study we have focused on the efficiency of Fe(II) and HO• photo-production by the latter iron(III)-complexes. Concentration profiles of Fe(II) were studied in the absence of H2O2, since ferrous species react very quickly with hydrogen peroxide. Two sets of experimets were performed. In the first set we irradiated Fe(III) solutions of the different substrates and recorded the resulting Fe(II) profiles. The results showed the following trend. During the first minutes, the amounts of produced Fe(II) decrease in the following order: Fe(III).Aq > 24DH-BA > 2H-BA > 2H4N-BA > 2H5N-BA HPLC results showed an important degradation of the irradiated substrates. Since reaction intermediates may affect Fe(II) production, we performed a second set of experiments in the presence of 0.019 M of benzene as HO• scavenger. In this second set, we analyzed both Fe(II) and HO• productions. The following trend was observed: Fe(III).Aq > 2H-BA > 24DH-BA > 2H4N-BA > 2H5N-BA This confirms that reaction intermediates strongly affected Fe(II) production in the firs set of experiments. The analysis of Fe(II) photo-production results shows that Fe(III)-complexes formed with the model substrates is less efficient than the photo-reduction of Fe(III) aquo complexes.