Simulated sunlight photodegradation of an organic pollutant by heterogeneous photo-Fenton-like processes

Abstract

The problem of water pollution has been an environmental concern for many years, which lead researchers around the world to looking for effective methods to solve this issue. The incapability of conventional methods to remove effectively biorecalcitrant and toxic pollutants, have promoted the research of more efficient and ecologically friendly water treatment technologies. In the same aspect, this thesis is a small contribution to the development of a new advanced oxidation process considered as promising and clean method for the destruction of water pollutants. In this process we investigated the efficiency of untreated local natural clay rich in iron oxides as a cost-free photocatalyst for the degradation of a persistent organic pollutant under simulated sunlight irradiation. The targeted pollutant was: 2-mercaptobenzothiazole (MBT). The study showed that: Firstly, the adsorption capacity, kinetics and isotherms models were examined in the dark. Secondly, experiments of degradation by heterogeneous Fenton process under various operational parameters were investigated. And then, we focused our research on the photodegradation of MBT by heterogeneous photo-Fenton process using the same natural clay powder (NCP) as photocatalyst and under simulated sunlight irradiation. Experiments were conducted at natural pH with a bath reactor equipped with a medium-pressure Hg lamp emitting mainly at 366 nm. The natural clay was crushed into small homogeneous particles (powder) and then characterized by SEM-EDS, UV-Vis diffuse reflectance spectroscopy, XRF and XRD analysis. The specific BET surface area measured for the clay was 30.22 m²·g-1. Our main results indicated that, the photodegradation of MBT follows first order (for direct photolysis) or pseudo-first order kinetics (for photocatalysis) Direct photolysis of MBT showed a negligible effect both upon 254 and 365 nm irradiation, while 42.5% and 62% of MBT was eliminated in 3 h under 310 nm irradiation in the presence of H2O2, and under sunlight irradiation (using NCP), respectively. Kinetic runs carried out with 5,0.10-5M MBT and 0.5g·L-1clay showed higher MBT conversion and photodegradation rate at basic pH and in oxygenated media. The photodegradation of MBT is mainly attributed to reaction with HO, leading to different intermediates that have been identified by HPLC-MS. A reaction mechanism is proposed at the end of our thesis. The highest TOC removals were obtained using UVA at low pHs, in the absence of O2 with 68% and 65% TOC removal respectively. The presence of oxalic acid and H2O2 enhanced significantly MBT photodegradation. Finally, the obtained results support the use of natural clay rich in iron oxides as a free, clean and efficient photocatalysts for water pollutants abatement.