Etude comparative de la photodégradation d’un colorant cationique et d’un colorant anionique en phase homogène et hétérogène

Abstract

Industrial effluents resulting from the textile, tannery or the printing activities present a major source of coloring pollutant which is difficult to biodegrade. In this context, the removal of two model dyes, an anionic dye of the xanthene family (Eosin Yellow), and a cationic dye of the triphenylmethane family (Malachite Green) by the photochemical process in homogeneous medium under irradiation (UV-C) at 254 nm and by the photocatalytic process in a heterogeneous medium at 365 nm (UV-A/TiO2), was studied. The main objective of this study is to evaluate the effectiveness of these processes in degrading target pollutants at different operating conditions and to compare and determine the effect of the ionic charge and the molecular structure of the compound to be degraded on the effectiveness of its degradation. The first part of this study focused on the removal of Malachite Green and Eosin Yellow in a homogeneous medium by direct photochemistry at 254 nm and by the UV/H2O2 processes. The results showed that the two dyes are photodegradable compounds in presence of UV irradiation at 254 nm, in particular in the case of Eosin Yellow. However, the COD values show a relatively low mineralization rate. The effect of certain parameters on the rat of discoloration of the two dyes by direct photochemistry was studied (pH, intensity of the photonic flux and the concentration of the dye). The photochemical reaction was significantly improved in the presence of hydrogen peroxide (UV/H2O2 process) with an optimum concentration of H2O2 at 10-2 mol L-1. Modeling of the effect of three parameters: dye concentration, H2O2 concentration and photon flux on degradation rate of Malachite Green by the process (UV/H2O2), showed that the concentration of H2O2 is the most determining factor and that the interactions between the three parameters are not significant. The second part of this study concerns the removal of Malachite Green and Eosin Yellow in a heterogeneous medium by the UV/TiO2 processes. A kinetic study of the adsorption of the two dyes studied shows that the mechanism of adsorption on TiO2-P25 is described by kinetics of the pseudo-second order and that the adsorption isotherms of the dyes on the catalyst are of type L (Langmuir). The photocatalytic process UV/TiO2 at 365 nm is found to be very effective for the discoloration and mineralization of Malachite Green and Eosin Yellow. Their degradation seems to follow the Langmuir-Hinshelwood mechanism and the rate of their degradation is well described by pseudo-first order kinetics. A more important discoloration and mineralization of the anionic dye (Eosin Yellow) compared to the cationic dye (Malachite Green) is observed at natural pH. The operating parameters (the catalyst type (P25, PC100, PC105, PC50, PC500), pH, dye concentration, temperature, photon flux, catalyst concentration, presence of the anions (Cl−, HCO3−, SO42−, HPO4−, NO3−) and the addition of H2O2) all influenced the degradation of Eosin Yellow and Malachite Green, but in a different way. The use of sunlight as a source of energy in the process (UV/TiO2) has been found to be very effective for the discoloration and mineralization of model compounds with respect to UV light. In all experiments, photocatalytic degradation of both dyes was generally found to correlate with adsorption on the surface of TiO2 in the dark. The application of the experimental design methodology to model the influence of three experimental parameters (catalyst particle size, catalyst concentration and pH of the solution) on the photocatalytic process, showed that pH is the most determining factor and that all the interactions between the three parameters studied are significant. The optimum values of the parameters giving maximum yield were also determined.