Mechanism Study Of TiO₂ Photocatalytic Degradation Of Pesticide Pyridaben

Heterogeneous photocatalysis is that irradiation of TiO₂ particles under UV irradiation that carries energy greater than the band gap (3.2 eV) induces conduction band electrons and valance band holes,which can further generate various oxidizing species and degrade pollutants to a great extent. This dissertation studied the reaction mechanism of TiO₂ photocatalytic degradation of pesticide pyridaben in acetonitrile/water suspension. For applying to really polluted waters, photocatalytic degradation pyridaben in surfactant suspension was investigated. Ultimately, small-scale experiments in the model reactor were carried out. The main works and achievements are presented as follows:Underλ≥300nm UV light irradiation, intermediates identification and reaction route proposition were conducted in TiO₂ photodegradation of pyridaben in organic/water suspension. Eight kinds of degradation products (DPs) were identified by GC/MS analysis, which were further confirmed by matching with authentic or synthesized compounds. The proposition of reaction pathway suggested that the initial degradation of pyridaben proceed via C-S bond cleavage between phenyl ring and heterocyclic group, and then promote a serious of reactions such as substitution, oxidation and dealkylation. Besides, on the basis of Langmuir-Hinshelwood equation, kinetic constant k 4.3×10^-5 mol.L^-1.min^-1and equilibrium adsorption constant K 3.1×10³ L.mol^-1 were derived.By comparing photodegradation behavior of pyridaben byλ≥300nm andλ≥360nm UV irradiation in different systems, we concluded that the photodecay of pyridaben byλ≥300 nm irradiation was dominated by a direct photolysis whereas a photocatalytic nature was confirmed byλ≥360nm irradiation. Mechanistic investigation of photocatalytic degradation pyridaben was mainly conducted byλ≥360nm irradiation. A qualitative study of degradation products generated during the photocatalytic process was performed by GC-MS and up to fifteen compounds were detected. The evolution of intermediates was analyzed semiquantitatively by plotting areas of the corresponding GC peaks as functions of irradiation time. Moreover, photocatalytic degradation kinetics were studied such as solvent effect, pH, catalyst concentration as well as radiant flux, which indicated that the degradation rates were strongly influenced by these parameters.For better application to industry, photocatalytic degradation pyridaben in surfactant cetyltrimethyl ammonium bromide(CTAB)suspension was studied.1HNMR was used to determine the quantitative information about the microstructure of CTAB micelles. The result indicated that pyridaben molecule mainly located in long chain groups of CTAB, which presented reasonable explanations for adsorption model of hemi-micelles on TiO₂ surface and the effect of pH values. The photocatalytic degradation rate could be significantly improved in the presence of hydrogen peroxide,which was higher than that of summation photocatalysis with H2O2 effect and was called synergic effect. In addition, a degradation pathway of photocatalysis pyridaben in CTAB suspension was proposed upon identified intermediates by GC/MS.Electron Spin Resonance (ESR)was used to detect active oxygen radicals formed during photocatalytic reaction. Evidence for the DMPO spin-trapping adducts DMPO-O2?- signals were presented in the pure acetonitrile suspension and the production of DMPO-?OH adducts increased as the water content increased gradually in photocatalytic degradation pyridaben. However, continuously increasing H2O content to a certain extent could decrease the signal intensity. It confirmed that the reaction rates of various water content in acetonitrile suspension followed the sequence: acetonitrile /H2O (80/20)> (60/40) > (95/5) > (100/0). In addition, strong DMPO-?OH signals were detected in pyridaben/CTAB/TiO₂/H₂O₂ system by ESR, which indicated that more hydroxyl radicals were generated in synergic effect of H₂O₂.Quantum chemical calculations were performed for a better understanding of the reaction mechanism of photocatalytic degradation pyridaben. On the basis of the optimal geometry conformation of pyridaben molecule obtained at UHF/PM3 level, the highest FED HOMO2 value of the highest occupied molecular orbital (HOMO) was calculated at S atom where would be the most readily extracted an electron to form pyridaben cation radical, and then occur C-S bond cleavage in TiO₂ photocatalysis. The bond length results indicated that C11-S12 bond was the longest among all others, where possessed less stability and easier to cleave under the attacking of oxidative radicals. The atom charge values manifested that the most negative point charge of pyridaben molecule was located at oxygen atom O23 so that the adsorption involved the negative carbonyl group of pyridaben on TiO₂ surface by static interaction in nature medium (pH6.0).Experiments of degradation pyridaben in CTAB suspension were carried out in the model reactor, which was in good performance and could degrade pyridaben entirely. Furthermore, impact factors such as pH value, the additive amount of oxidant H₂O₂ and disposal capacity of wastewater were investigated, which provided basis for industrialization of photocatalytic degradation pyridaben.