Degradation of Iron (III)-salicylate complexes in Fenton Systems. Efficiency of Iron (II) photoproduction

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

La Serena

Congreso; X Encuentro Latinoamericano de Fotoquímica y Fotobiología; 2010

DEGRADATION OF IRON(III)- SALICYLATE COMPLEXES IN FENTON SYSTEMS. EFFICIENCY OF IRON(II) PHOTOPRODUCTION.Daniela Nichela*, Menana Haddou#, Andrés Donadelli*, Esther Oliveros#, Fernando García Einschlag** Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA), Dep. de Química, Fac. de Cs Exactas, UNLP, CONICET, La Plata, Argentina. e-mail: dnichela@inifta.unlp.edu.ar# Laboratoire des IMRCP, Université Paul Sabatier (UPS, Toulouse III), 118 rte Narbonne, FranceKeywords: Fenton, benzoic acid derivatives, polychromatic quantum efficiencyPhoto-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  +  hv    →    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 shown that, in Fe(III)-containing solutions, the studied substrates form highly stable complexes with stoichiometries 1:1 or 2:1LH-  +  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 experiments were performed. In the first set we irradiated Fe(III) solutions of each substrate and recorded the Fe(II) profiles. 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-BAHPLC analyses 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. Both Fe(II) and HO• productions were analyzed and the following trend was observed:Fe(III).Aq >  2H-BA > 24DH-BA > 2H4N-BA > 2H5N-BAThis confirms that reaction intermediates strongly affected Fe(II) production in the first set of experiments. Thus, Fe(II) profiles obtained in the presence of benzene were used to evaluate the polychromatic quantum efficiencies. The results show that irradiation of Fe(III)-complexes is less efficient than the photo-reduction of Fe(III) aquo complexes.