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

ESTHER OLIVEROS; DANIELA NICHELA; MÉNANA HADDOU; FERNANDO S. GARCIA EINSCHLAG

Sao Pãulo

Congreso; V EPOA, Encontro sobre Aplicações Ambientais de Procesos Oxidativos Avançados, 26-29/10/2009, Sao Pãulo; 2009

DO IRON(III)-CARBOXYLATE COMPLEXES SYSTEMATICALLY INCREASETHE EFFICIENCY OF THE PHOTO-FENTON PROCESS?Esther Oliveros* (PQ)1, Daniela Nichela (PG)2, Ménana Haddou (PG)1,Fernando S. García Einschlag (PQ)21Laboratoire des IMRCP, UMR CNRS 5623, Université Toulouse III (Paul Sabatier), 118route de Narbonne, 31062 Toulouse cédex, France2INIFTA, Departamento de Química, Facultad de Ciencias Exactas, UNLP, CONICET, CICCC 16 Suc 4, (1900), La Plata, Argentina*e-mail: oliveros@chimie.ups-tlse.frAmong Advanced Oxidation Technologies (AOTs), the Fenton and especially thephotochemically enhanced Fenton (photo-Fenton) processes are considered to be among themost efficient for the remediation of wastewaters from chemical, pharmaceutical and dyeindustries [1]. The Fenton reaction efficiently produces a strong oxidizing species, thehydroxyl 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 byH2O2 (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 beachieved, mainly due to the formation of stable complexes between Fe(III) and someoxidation 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 mineralizationmay 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 increasedabsorption in the UV-Vis. In this case, both reduction of Fe(III) and oxidation of the ligandmay 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 aromaticcarboxylic acids has not been studied up to now. We have therefore investigated in detail therole of the complexation of Fe(III) by a series of benzoic acid derivatives in the photoFentonprocess, 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 thecomplexation 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 OHgroup in ortho position of the COOH group. All complexes showed a typical ligand-to-metalabsorption band (LMCT) in the visible spectral region.We investigated the kinetics of oxidation of the benzoic acid derivatives (absorptionspectra, 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 1:1complexes were determined, using ferrioxalate as an actinometer. Experiments were alsocarried out in the presence of an excess of benzene (hydroxyl radical trap) and the formationof 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 forthe 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 notdirectly correlated to the efficiency of Fe(II)-production by photolysis of the complexes; theoxidation 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 stepsof 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 bycharge transfer from a H2O/HO- ligand to Fe(III), producing Fe(II) and HO•.[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 deFisicoquímica, 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-ArgentinaECOS-MINCyT, N° A07E07 (2008-2010).