Do Iron(III)-Carboxylate Complexes Systematically Increase the Efficiency of the Photo-Fenton Process?

ESTHER OLIVEROS*; DANIELA NICHELA; MÉNANA HADDOU; FERNANDO S. GARCÍA EINSCHLAG

Sao Pãulo

Workshop; V EPOA, Encontro sobre Aplicações Ambientais de Procesos Oxidativos Avançados; 2009

Among Advanced Oxidation Technologies (AOTs), the Fenton and especially the photochemically enhanced Fenton (photo-Fenton) processes are considered to be among the most efficient for the remediation of wastewaters from chemical, pharmaceutical and dye industries [1]. The Fenton reaction efficiently produces a strong oxidizing species, the hydroxyl radical, by decomposition of H2O2 catalyzed by Fe2+-salts (reaction 1). Fe2+ + H2O2 → Fe3+ + HO- + HO• (1) The catalytic cycle derives from the reduction of the Fe3+-ions produced in reaction 1 by H2O2 (reactions 2 and 3), but the overall rate constant of the Fe2+ regeneration is small. Fe3+ + H2O2 → [Fe-OOH]2+ + H+ (2) [Fe-OOH]2+ → HO2• + Fe2+ (3) Moreover, only partial mineralization (40%-60%) of most organic pollutants may be achieved, mainly due to the formation of stable complexes between Fe(III) and some oxidation by-products, especially carboxylic acids of low molecular weight. In the photo-Fenton process, irradiation of the system with near-UV and visible light, including solar light, enhances the reduction of Fe(III) to Fe(II), and complete mineralization may be achieved [1c]. The case of Fe(III)-(poly)carboxylate complexes (e.g., ferrioxalate, Fe(III)-nitriloacetate, citrate, pyruvate) has attracted particular interest, due to their increased absorption in the UV-Vis. In this case, both reduction of Fe(III) and oxidation of the ligand may be achieved under irradiation (reaction 4, for a brief review see [2]). Fe(OOC-R)2+ + hv → [Fe(OOC-R)2+]* → Fe2+ + CO2 + R• (4) However, to the best of our knowledge, the case of Fe(III)-complexes with aromatic carboxylic acids has not been studied up to now. We have therefore investigated in detail the role of the complexation of Fe(III) by a series of benzoic acid derivatives in the photo-Fenton process, using the following substrates: salicylic (SA), 2,4-dihydroxybenzoic (24DHBA), 2-hydroxy-5-nitrobenzoic (2H5NBA), 4-hydroxy-3-nitrobenzoic (4H3NBA), 2-hydroxy-4-nitrobenzoic (2H4NBA) [3]. Their acid-base properties were characterized and the complexation constants determined by spectrophotometric methods. Complex formation (Fe(III):Acid = 1:1 and 1:2 in some cases) was only observed for derivatives with an OH group in ortho position of the COOH group. All complexes showed a typical ligand-to-metal absorption band (LMCT) in the visible spectral region. We investigated the kinetics of oxidation of the benzoic acid derivatives (absorption spectra, HPLC), the mineralization rate (TOC) and the H2O2 consumption during the photo-Fenton process. Apparent quantum efficiencies of Fe(II) production by photolysis of the 1:1 complexes were determined, using ferrioxalate as an actinometer. Experiments were also carried out in the presence of an excess of benzene (hydroxyl radical trap) and the formation of phenol monitored. The following conclusions may be drawn from the results obtained: i) The efficiencies of Fe(II) production by photolysis of the complexes are lower than for the aquocomplex ([Fe(H2O)5(HO)]2+), i.e. in decreasing order: Aquo > 4H3N-BA > 2H-BA > 2H4N-BA > 2H4N-BA > 2H5N-BA. ii) The rates of oxidation of the different substrates by the photo-Fenton process are not directly correlated to the efficiency of Fe(II)-production by photolysis of the complexes; the oxidation is dominated by dark reactions involving organic intermediates (e.g., hydroquinones) capable of reducing Fe(III). iii) The contribution of photo-induced reactions only becomes important during the last steps of the mineralization, when complexes of Fe(III) with low molecular weight carboxylates (ferrioxalate) are present (TOC degradation proceeds after total consumption of H2O2). iv) Photoreduction of Fe(III) in the excited states of the complexes mainly takes place by charge transfer from a H2O/HO- ligand to Fe(III), producing Fe(II) and HO•. References [1] a) Oliveros, E., Legrini, O., Hohl, M., Muller, T., Braun, A., Chem. Eng. Proc., 1997, 36, 397-405; b) Tarr, M.A., Environ. Sci. Poll. Control Ser., 2003, 26, 165-200; c) Pignatello, J.J., Oliveros, E., MacKay, A., Crit. Rev. Envi. Sci. Technol., 2006, 36, 1-84; Erratum 2007, 37, 273-275. [2] Wang, L., Zhang, C., Mestankova, H., Wu, F., Deng, N., Pan, G., Bolte, M., Mailhot, G., Photochem. Photobiol. Sci., 2009, 8, 1059-1065. [3] a) Nichela, D., M. Haddou, M., Oliveros, E.,García Einschlag, F. S., Congreso Argentino de Fisicoquímica, 18-21.05.2009, Salta, Argentina; b) Haddou, M., Nichela, D., Benoit-Marquié, F., Maurette, M-T., Oliveros, E., García Einschlag, F. S., ELAFOT IX, 2-7. 11. 2008, Cubatao/Santos, Brazil, Book of Abstracts: P30. The authors acknowledge support of their cooperation by the exchange program France-Argentina ECOS-MINCyT, N° A07E07 (2008-2010).