Photocatalytic And Photosensitized Degradation Of Organic Pollutants In Water With Heteropoly Acid Under Visible Light Irradiation

The environmental problems caused by persistent and toxic organic pollutants have become the major problems affecting human survival and health in the21st century. Using an efficient, economic and green technology to eliminate them from wastewater is one of the mostly active topics in environmental science. Currently, the most international attention, the greatest potential technology for the degradation of pollutants is TiO2photocatalysis technology. However, because of low photocatalytic efficiency and excitation only under UV light, TiO2hasn’t been large-scale applied. Therefore, to finding an appropriate catalyst that is capable of utilizing the solar energy is a crucial task in photocatalysis field.In order to efficiently utilize visible light, iron substituted polyoxotungstate and iodine were chosen as photocatalysts and used them for the degradation of dyes and chlorophenols in the presence of H2O2. This dissertation are divided into two parts, four sections are contained as follows.(1) A novel system using iron(III) substituted polyoxotungstate (PW11Fe) to replace Fe3+in traditional photo-Fenton as photocatalyst for dye X3B and methyl orange degradation was developed. Compared with the traditional Fenton reaction, PW11Fe effectively active H2O2to degrade X3B and methyl orange at initial pH2～8under visible light at wavelengths longer than400nm, which sustained good activity after several times of recycle. Through a spin-trapping electron paramagnetic resonance spectroscopy and ethanol quenching experiments, hydroxyl radicals were found in the irradiated aqueous solution of PW11Fe and H2O2, a photo-Fenton like mechanism is proposed. Although X3B and methyl orange are azo dye, the degradation pathway was markedly different, and the latter existed autocatalytic process.(2) To eliminate the interference of dye sensitization, chlorophenols (CPs) were chosen as target compounds to evaluate the photocatalytic activity of PW11Fe. In an aerated aqueous solution, PW11Fe was active for the photodegradation of phenol, 4-chlorophenol (4-CP),2,4-dichlorophenol and2,4,6-trichlorophenol in the presence of H2O2at an initial pH from2.2to7.2under visible irradiation (λ≥>420nm) However, all the observed reactions of organic degradation were slow in the beginning, followed by a fast, first-order process. As the initial concentration of PW11Fe or H2O2increased, this induction period for4-CP degradation declined, together with increase in the overall reaction rate. With the aid of HPLC, IC and LC-MS, the main intermediaries or products of4-CP were identified,1,4-benzoquinone,2-hydroxy-BQ, catechol, and4-chlorocatechol and Cl". Moreover, through a spin-trapping electron paramagnetic resonance spectroscopy, hydroxyl radicals were found only in the irradiated aqueous solution of PW11Fe and H2O2. Accordingly, we propose a possible mechanism explaining the observed slow and fast stages of chlorophenols. The early slow degradation of chlorophenol was ascribed to the weak absorption of PW11Fe/HfeOz system toward visible light, the fast stage was caused by the role of quinone-like intermediates, which were identified to reduce PW11Fe(Ⅲ) to PW11Fe(Ⅱ) quickly, promote the OH radical generated, thus accelerating the degradation of chlorophenol.(3) Molecular iodine has been studied as a sensitizer for degradation of2,4,6-trichlorophenol (TCP) in aqueous solution under visible light (A.>450nm). In the presence of a commercial I2, TCP could degrade, but the reaction became to be very slow after2h. When the combined solution of Nal and H2O2was used as an iodine source, and phosphotungstic acid as a catalyst, TCP degradation was not only fast, but also followed the zero-order kinetics. Importantly, I2concentration remained unchanged with time, indicative of I2recycling as a kind of photocatalyst. During TCP degradation,2,6-dichloro-1,4-benzoquinone was produced as the main intermediate (76%), which slowly degraded in the irradiated solution. Further study on the effect of variables including the type of polyoxomatalates (POM) and the initial concentration of each component revealed that the rate of TCP degradation under visible light was determined by the rate of I2production in the dark. The optimum pH and apparent activation energy for TCP disappearance were4.5, and42.8kJ/mol, respectively. It is proposed that TCP degradation is initiated by iodine radicals produced from I2photolysis, followed by I2regeneration through a POM-catalyzed oxidation of iodide by H2O2.(4) Instead of PW12, PW11was used as assistant catalyst activing H2O2to improve the photodegradation rate of TCP by NaI/H2O2/PW system, which showed that the initial rate could rise up to～2.3μM/min in the range of pH1-6. Through the study of chlorine content of chlorinated phenols (phenol,4-chlorophenol,2,4-dichlorophenol and2,4,6-trichlorophenol) on their degradation rate, we found that the iodination and oxidation of phenols by molecular iodine made competition under visible light irradiation. The increasing number of chlorine atoms caused the decrease of iodination, while the oxidation of chlorophenols by iodine radicals became evident.