Résumé:
In this study we have used dimethylphenols (xylenol) as models pollutants to show the effectiveness of the natural hematite (Fe2O3 natural) on the photocatalytic processes in heterogeneous environment.
The degradation of 3,5-dimethylphenol by Fenton and photo-Fenton processes was investigated, starting with the study of its photolysis at 254 nm and by H2O2/UV process.
Different physico-chemical parameters (concentrations of H2O2, Fe2+ and the pH) were tested in order to optimize the efficiency of Fenton process for degradation of 3,5-DMP. A Comparison of the performance of mineralization by different processes studied (Fenton and photo-Fenton) has shown that 70% and 80% of DOC removal was obtained after only 4 hours of photo-Fenton process at 365 nm and 254 nm respectively.
In the field of heterogeneous photocatalysis, a naturally occurring mineral was used. It was characterized by XRD, BET, IR and SEM. The XRD analysis showed the presence of 9 characteristic peaks attributed to hematite. Its photocatalytic efficiency was tested by the oxidation of 2,6-dimethylphenol (2,6-DMP). Photocatalysts based on iron (Fe2O3) have photocatalytic efficiency in the visible region. The results showed good efficiency under solar irradiation for the degradation of 2,6-DMP. Experiments carried at 365 nm required activation of the iron oxide with carboxylic acids. oxalic acid was the most effective carboxylic acid in terms of dissolution of natural iron oxide, and degradation of 2,6-DMP. Fe (III) - oxalate complex produce Fe2+ species by the photo-reduction of Fe3+ species and also lead to the
formation of hydrogen peroxide which reacts with Fe2+ to give •OH (Fenton reaction). We have optimized the experimental parameters affecting the degradation rate (oxalic acid concentration, pH, O2, temperature, inorganic ions ...) and propose a mechanism of degradation. The abatement rate of COD was 85% obtained for a treatment time of 14 hours in natural iron oxide - oxalate system under irradiation at 365 nm, demonstrating the ability of this technique to clean up effluents loaded with organic matter. The coupling of the system with the solar light has increased the rate of degradation of 2,6-DMP to 92% after 5 hours.
The use of natural iron oxide alone or in the presence of oxalic acid seems to be favored under solar irradiation which can reduce energy costs, especially in a country like Algeria.