Autocatalytic Behaviour in the Heterogeneous Fenton Reaction using Goethite as Catalyst

GUADALUPE B. ORTIZ DE LA PLATA; ORLANDO M. ALFANO; ALBERTO E. CASSANO

Palermo, Italia

Congreso; SPEA5, 5° European Meeting on Solar Chemistry and Photocatalysis: Environmental Applications.; 2008

Autocatalytic Behaviour in the Heterogeneous Fenton Reaction using Goethite as Catalyst.G.B. Ortiz de la Plata,a* O.M. Alfano,a A.E. Cassanoa a INTEC , CONICET and Universidad Nacional del Litoral, Güemes 3450, Santa Fe, Argentina.* guadaortiz@santafe-conicet.gov.arA new process among the Advanced Oxidation Technologies has received increasing attention in the last years. This application, a variation of one previously known, is called Heterogeneous Fenton and it employs hydrogen peroxide and a solid container / carrier of iron. As expected, it shares some advantages with the classical Fenton process: (i) it can be significantly improved with temperature and (ii) it can be combined with UV radiation (the Heteroge-neous Photo-Fenton reaction) which implies the possibility of using solar illumination. In the present work, the proposed reaction scheme for the degradation process as well as the corresponding kinetics of the model compound 2-Chlorophenol (2-CP), at constant temperature, is studied. The apparatus and the operating conditions have been described in a companion work1 and will not be repeated here. The latter are summarized in Table 1. Table 1 Reaction conditions and conversions for TOC, 2-CP and H2O2 after 6 hs. of reaction time. Typical examples. Experimental Results During the progress of the degradation of  2-CP, the reaction shows an unusual acceleration. This autocatalytic behaviour, with stronger tendencies at higher temperatures, implies a completely different behaviour than the one typically expected. The autocatalytic performance is successfully explained by the joint action off two factors. The evolution of the available iron in the homogeneous phase during the course of the reaction, 2 and the autocatalytic contribution of some of the reaction intermediates in the iron cycle.3 The expressions of the proposed reaction scheme can be seen in Table 2. Experimentally it is clearly observed the appearance of an intermediary, Cl-benzoquinone (ClBQ), which has been identified together with Cl-hydroquinone (ClHQ) by GCMS from reaction samples (See Figure 1). By adding small quantities of these intermediates at the beginning of the reaction (about 5% of initial 2-CP), an otherwise existing induction period is significantly decreased.  Table 2   Reaction mechanism. Constants 1 to 7, 9, 11, 13, 15 and 16 from literature [3-4]. Constants 8, 10, 12 and 14, from this work. Units in , except  and .  Reaction Modeling Table 2 sows the proposed reaction scheme. From this mechanistic assumption, the following set of differential equations has been obtained (Table 3). Table 3 Mass Balances. The values of the constants corresponding to reactions 8, 10, 12 and 14 were obtained using a Levenberg Marquardt non linear optimization procedure. Model predictions were compared with experimantal data for 2-CP, hydrogen peroxide and ClBQ concentrations in order to produce the values of the unknown paramters. For all runs the adjustment is satisfactory, as can be seen in Figure 2. Figure 1.   Experimental (♦: , s: , ¨: ) and model (solid lines). Values for a typical rFigure 2.   Experimental vs. Model concentrations . ♦: Hidrogen peroxide (molar). s: 2-CP (molar x 50).  ¨: ClBQ (molar x 500).  Under isothermal conditions and at 25ºC, when comparing the values of conversion of 2-CP under different operating conditions, it is possible to conclude that: greater loads of catalyst and higher ratios of hydrogen peroxide  to 2-CP concentrations, always implies higher conversions (See Table 1). The next step of this work is to generalize the kinetic model for a wider range of all the operating conditions, in particular the temperature. AcknowledgementsThe authors would like to thank to Universidad Nacional del Litoral, Consejo Nacional de Investigaciones Científicas y Técnicas and Agencia Nacional de Promoción Científica y Tecnológica for their financial support.Bibliography[1]    G.B. Ortiz de la Plata, O.M. Alfano, A.E. Cassano, Temperature Effects in the Fenton Reaction, using goethite as catalyst. Proc. of Solar Chemistry and Photocatalysis: Environmental Applications, Sicilia, Italy, 2008.[2]    Y.-T. Lin, M.-C. Lu, Water Sci. Technol. 55 (2007) 101.[3]    R. Chen, J.J. Pignatello, Environ. Sci. Technol, 31 (1997) 2309.[4]    N. Kang, D.S. Lee, J: Yoon, Chemosphere, 47 (2002) 915.