"Etude comparative de la dégradation photochimique et photocatalytique de quatre colorants: Impact de la structure chimique et corrélation entre l'adsorption et l'activité photocatalytique de TiO2"

Abstract

"This study allows us to confirm the influence of the chemical structure of organic dyes on the effective of their photochemical degradation using (UV-C) at 254 nm and photocatalytic degradation at 365 nm (UV-A/TiO2 with two different types crystal (TiO2 Degussa-P25 and Millennium-PC500)). The four dyes chosen in this study are widely used in industry and are grouped under two types: azo compounds (methyl orange (MeO), methyl red (RMe)), triphenylmethanes (bromophenol blue (BBP) and bromothymol blue (BBT)). These dyes have different chemical structures and different substituents, which gives a significant impact on their reactivity depending the photochemical or photocatalytic processes used, the reaction pathways of degradation and the type of by-products of degradation. Adsorption is the first step in the process of photocatalytic degradation of an organic molecule on the surface of TiO2, it was studied to explain the behavior of some couples photocatalyst/organic compound during the photocatalytic degradation. The photochemical degradation of the four dyes using UV at 254 nm resulted to a slow decolorization of the substrates for the two azo dyes (MeO and RMe) which is not the same for the two carbonyl dyes (BBP and BBT) where the speed rate is more important. However, the COD values showed a relatively very low rate of mineralization, which means that the dyes studied photochemical process, did not lead to significant mineralization. The results also showed that the degradation rate increases with the decrease of the dye concentration. An improvement on the rate degradation was observed also when the intensity of the UV light increased. Decolorization of the four dyes studied depends on the variation of pH. Process efficiency can be increased by adding an oxidant as (H2O2) in the solution, to accelerate the degradation we obtained higher yields after 15 minutes of irradiation at a concentration of H2O2 10-2 mol l-1, with the highest for the four dyes studied at this concentration. This results show that the photochemical process in the presence of oxidizing hydrogen peroxide at 254 nm (UV/H2O2) was effective for the complete degradation of dyes. The adsorption kinetic study of the studied dyes shows that the adsorption mechanism over the two selected catalysts is described by a second pseudo-order kinetic and the dyes adsorption isotherms were found favorable by the Langmuir approach. The comparative study of the photocatalytic degradation of aqueous solutions of MeO, RMe, BBP, BBT dyes shows that the molecular formula of the dyes may be a determining factor influencing the degradation rate. The P25 catalyst is more effective than the PC500 at different operating conditions studied. The COD values measured confirm that the photocatalytic process leads to the reduction of the organic matter until the mineralization of contaminants. The effect of the initial dye concentration, solution pH, the type and the photocatalyst concentration, electron acceptor such as H2O2 and inorganic anions (Cl−, HCO3 − and CO3 2−) in the presence of both catalysts are discussed. The effect of solar energy on the photocatalytic decomposition of dyes and the correlation with the adsorption was examined. The disappearance of substrates follow a pseudo-first order kinetics and Langmuir- Hinshelwood model is well suited to describe the kinetics of photocatalytic disappearance of these dyes. The evolution of apparent rate constant Kapp as a function of pH indicates that it plays an important role on the kinetics of the dyes degradation. The degradation efficiency of MeO, RMe, BBP was found to increase at acidic medium, however the reaction of degradation of BBT increase at alkaline medium. The presence of low H2O2 concentrations improves the kinetics of photocatalytic disappearance dyes until an optimum. Addition of salts such as NaCl, Na2CO3 and NaHCO3 significantly reduces decolorization efficiencies. The sunlight has been found more efficient for decolorization and mineralization of model compounds than the UV light."