Photocatalytic Degradation Of Polybrominated Diphenyl Ethers On Modified-TiO₂ Nanoparticle

As flame retardants, polybrominated diphenyl ethers (PBDEs) are usually added to polymeric materials such as polyvinyl chloride, polyurethane foam and resin, which are widely used in building materials, textiles, electronics and other products. Because of no chemically bound to these polymers, they are easy to separate or leach from the products into the environment. Moreover, PBDEs possess the typical characteristics of persistent organic pollutants (POPs), consisting of persistent, bioaccumulation, semi-volatility and biotoxicity, the elimination therefore attracts more attention of researchers at home and abroad.As we well know, PBDEs are high-toxic, stable and resistant to natural degradation, which could be attributed to the existence of bromine atom(s) on benzene ring. Thus, reductive debromination is a common strategy to treat PBDEs. Among these reductive methods, TiO2 photocatalysis is considered to be the most efficient degradation system. However, the debromination products with less bromine atoms are much more difficult to further reductive debromination than that with more bromine atoms, leading to a higher accumulation of less brominated products. Thus, the object of this paper is committed to study the reductive degradation of BDE47 on mediated-TiO2 photocatalysts. The main points of this thesis were summarized as follows:(1) RGO/TiO2 composites was synthesized through a UV-assisted photocatalytic reduction process by using graphene oxide (GO) and P2S as pecurors, and their activity of photocatalytical reduction of decabromodiphenyl ether (BDE209) was studied. The optimized composites yielded a BDE209 degradation of 72.0% and a debromination of 59.4% in deaerated water containing 0.25 mol L-1 CH3OH after 12 h of UV irradiation, being 2 and 4 times that in the UV-TiO2 system, respectively. The BDE209 reduction generated PBDEs congeners with 3-9 bromine atoms, which could be further debrominated. Moreover, by increasing UV time to 24 h, almost all the added BDE209 and the reduction intermediates disappeared in the UV-RGO/TiO2 system, resulting in the debromination efficiency reaching to 90%. Unlike the stepwise reduction manner commonly observed in UV-TiO2 system, the generation, accumulation and distribution of intermediates in the time course implied that the BDE209 reduction skipped some debromination steps. This was attributed to that the loaded RGO acted as both the electron trapper and transport medium could provide abundant electrons and a larger number of effective reduction sites for the reductive debromination of BDE209. The peculiar reduction pathway induced by multi-electron would provide a green and efficient method to remove halogenated pollutants.(2) Based on that Ag nanoparticles could both accelerate the generation of electrons and exert affinity interaction with bromine atoms, an efficient system for photocatalytic reductive degradation of 2,2’,4,4’-tetrabromodiphenyl ether (BDE47) over silver nanoparticle-loaded TiO2 (Ag/TiO2) was developed. Almost all the added BDE47 was removed on Ag/TiO2 photocatalysts after 13 min of UV irradiation in acetonitrile-water mixtures containing 0.25 mol L’1 of methanol as hole scavengers. However, no obvious degradation of BDE47 was observed during its photocatalytic reduction on TiO2. The analysis on the distribution of intermediates showed that ortho-substituted bromines were more susceptible to debromination than those at para-positions. DFT studies showed that the Ag had high affinity to the bromine atoms of BDE47. After trapping the photogenerated electrons of TiO2, the charged Ag nanoparticles could further elongate the C-Br bonds, thus accelerating the cleavage of C-Br bonds. Moreover, Ag/TiO2 needed an induction period to intiate the BDE47 debromination. This process not only accumulated electrons for injecting electron to BDE47, but also greatly stretched the C-Br bond. Therefore, three important factors including increasing photo-excitation power, properly Ag loading and the higher polar solvent could lead to a shorter induction period and a faster degradation rate of BDE47.(3) CuO-modified TiO2 (CuO/TiO2) photocatalysts were prepared by a simple impregnation method, and effective degradation of PBDEs were achieved on CuO/TiO2 photocatalysts. The photogenerated electrons on TiO2 could reduce BDE209, but was ineffective in reducing BDE47, while CuO/TiO2 could rapidly reduce both BDE209 and BDE47. Moreover, the photoreductive degradation of BDE47 on CuO/TiO2 needed a short induction time period (20 s) to initiate the reduction. During the induction time period, the CuO clusters trapped electrons from TiO2 to form Cu2O. Further surface photovoltage and phase spectroscopic analysis and flatband potential measurements were conducted for TiO2, CuO/TiO2 and Cu2O/TiO2, and it was found that CU2O possessed the higher reduction potential than CuO and TiO2. Thus, the in-situ composition change led to reduction potential shift to a more negative for injecting electrons to BDE47, resulting in the initiation of photocatalytic reduction of BDE47. Based on the mechanism of "switching reduction potential by the valence state of copper", photocatalysts for reductively degrading POPs with high reduction potential could be designed and regulated.(4) An efficient photocatalytic system for reduction and subsequent oxidation BDE47 over RGO/Ti02 was proposed. TiO2 alone could not photocatalytically reduce BDE47 in anoxic solvents, but the coating of RGO enabled a rapid photocatalytic reduction of BDE47, yielding to a complete reductive degradation of the added BDE47 after 3.5 h of UV irradiation in acetonitrile-water (1:1) mixture containing 0.25 mol L"1 methanol. However, only one bromine atom was eliminated from BDE47 molecule through the sole reduction route. In aerobic water without methanol, RGO/TiO2 was capable of oxidizing BDE47. It was noted that the oxidation of BE47 was much slower than its reduction, but it could remove all four bromine atoms by prolonging UV time to 24 h. By adding properly methanol to the above oxidation system, complete debromination of BDE47 was achieved within 14 h under optimized conditions. Intermediates analysis indicated that this process was much superior to the individual reductive/oxidative route. BDE47 is more difficult to oxidative, while its debrominated products trended to oxidation. The strategy of concerted reduction and subsequent oxidation will provide an alternative approach to treating toxic halogenated pollutants.