Preparation Of Nonmetal-doped TiO₂ Composites And Study On The Photocatalytic Mechanism By The Degradation Of Tipical PPCPs

As a new kind of pollutants, pharmaceuticals and personal care products ?PPCPs?have been continuously concerned and frequently detected in aqueous environment?suface water, drinking water and groundwater? in recent years. Most of the PPCPs are difficult to volatilize or characterize by the drug resistance and toxicity to aquatic organisms, which have aroused more and more concern in the scientific community.Photocatalysis is an important degradation pathway for PPCPs in aquatic environment,which is needed to be studied for the assessment of the ecological effects of PPCPs in aqueous environment. TiO₂, owing to its photostability, availability and nontoxicity, has been recognized as a most widely used photocatalyst for water treatment. However, the photocatalytic activity of TiO₂ under natural solar irradiation is largely limited,because it responds only ?5% ultraviolet light in entire solar spectrum. Because metal modification causes second pollution and threats the ecological environment, our study would choose non-metal modification. Thus, we prepared three metal free-TiO₂ composites and study the photocatalytic performance for the degradation of typical PPCPs, containing the kinetics and mechanism. It will provide the theoretical foundation for real-world situations. The main research work and research results are as follow:?1? In the present study,g-C₃N₄ was prepared by a calcination approach using double acrylic amine as precursor. The g-C₃N₄/P25 photocatalyst was prepared via a facile hydrothermal-calcination synthesis approach. The morphology of g-C₃N₄/P25 was revealed by TEM observation. It can be clearly seen that the g-C₃N₄ presented an obvious graphite-like layered structure with the agglomeration of P25 on the surface of g-C₃N₄. The g-C₃N₄ obviously enhanced the peak at approximately 27.5° of P25. It can be concluded that the g-C₃N₄/P25 composites have a two-phase composition of g-C₃N₄ and TiO₂. UV-Vis spectra showed that g-C₃N₄/P25 possessed the hybrid absorption features of g-C₃N₄ and P25, further confirming that the light absorption region of P25 can be effectively extended to ?460 nm after introducing the g-C₃N₄. FT-IR and XPS spectra showed that g-C₃N₄/P25 possesed C=N-C,C-O and O-H groups. The reactive species ?RSs?-induced degradation mechanism of clofibric acid ?CA? was investigated using a newly sunlight-driven g-C₃N₄/P25 photocatalyst. When the doping rate of g-C₃N₄ was 1%, 5%, 8%, 10% and 15%, the degradation rate of CA was 71.4%, 78.2%, 85.4%,81.3% and 69.5%, respectively. A very low g-C₃N₄ content of 8.0 weight percent resulted in a 3.36 and a 2.29 times faster reaction rate for CA photodegradation than for pristine g-C₃N₄ and P25, respectively. ESR and quenching experiments demonstrated the participation of ·OH, h+, e·, 1O2 and O2·- in the photocatalytic system, and the contribution rates were calculated to 73.3%, 15.3%, 5.1%, 6.7% and 33.1%, respectively.According to the pulse radiolysis measurements and the competitive kinetics approaches,the bimolecular reaction rate constants for ·OH, e-, and 1O2 with CA were?8.47±0.33?×109M-1s-1,?6.41±0.48?×109M-1s-1 and ?6.6±0.37?×106 M-1s-1, respectively.RSs were found to significantly influence the degradation of CA, and the degradation pathways occurred primarily via e- reduction, ·OH addition and 1O2 attack reactions on the basis of mass spectrometry and theoretical calculations.?2? An environmentally friendly and sunlight-driven photocatalyst is thought to be a promising alternative to conventional water treatment technology. In this study, carbon dots ?C-Dots?, a newly discovered material with the ability to upconvert light, were decorated to TiO₂ via a facile hydrothermal-calcination synthesis approach. Under simulated sunlight irradiation, a very low C-Dots content of 5.0 weight percent resulted in a 2.3 times faster reaction rate for gemfibrozil ?GEM? photodegradation than pristine TiO₂. Oxidative species, particularly ·OH, were the most important reactive species mediating the photocatalytic degradation of GEM. A notable observation was the higher formation rates of ·OH in the Ti02/C-Dots system than in pristine TiO₂, which was determined via ESR spectroscopy. Frontier electron density calculations and mass spectrometry were used to verify that the major degradation pathways of GEM contained ·OH addition, H abstraction and O2·- attack. The acute toxicity of the treated solution at two trophic levels first increased slowly and then decreased rapidly as the total organic carbon decreased during photocatalytic degradation. Compared to traditional advanced oxidation processes, TiO₂/C-Dots photocatalytic technology could reduce the generation of toxic by-products. These results highlight the potential application of sustainable sunlight-driven photocatalyst in GEM wastewater purification.?3? The visible-light driven RGO/P25 composites were prepared and peroxodisulfate ?PDS? was used as electron acceptor to accelerate this photocatalytic system due to the promotion of the electron-hole pairs separation. According to the results of XRD, the characteristic diffraction lines of the RGO/P25 composites were similar to P25 nanoparticles, indicating very little change in the phase of P25 in the RGO/P25 composites. When the doping rate was 0 wt%, 0.1 wt%, 0.5 wt% and 1.0 wt%,the BET was 48.94, 52.50, 57.27 and 56.88 m2·g-1, respectively, indicating that a lower RGO content increased the BET while a higher RGO content jammed up the inner pore of RGO/P25 composites and slightly decreased the BET. RGO enhanced the light absorption of the composites comparing with P25. After modification, the band gap decreased from 3.04 eV to ?2.66 eV and the effective incident wavelength redshifted from ?408 nm to ?466 nm. Ti-O-C bond existed in the RGO/P25 composites that meant the modification was via chemical bond. The asprepared RGO/P25 composites contained C-O, C-C and C-H groups, which was the basis to enhance the light absorption. When PDS was 2.0 mM, RGO/P25 composites was 0.3 g·L-1, RGO doping rate was 0 wt%, 0.1 wt%, 0.5 wt% and 1.0 wt%, the diclofenac ?DCF? degradation rate was 0.0635 min-1,0.0754 min*1, 0.106 min-1 and 0.109 min-1. The results demonstrated the outstanding property of PDS-RGO/P25 visible light system in the degradation of DCF. Acidic condition was in favour of the DCF degradation in PDS-RGO/P25 visible light system.When the pH value increased from 4.0 to 9.0, the DCF degradation rate decreased from 0.201 min-1 to 0.0453 min-1. HCO3- decreased the DCF degradation rate via radical quenching, while NO3- slightly affected the system. Cl-1 promoted the degradation of DCF. Low concentration of fulvic acid ?FA? promoted the degradation of DCF via generating excited states. However, high concentration of FA inhibited the degradation probably due to the light screening effect. Tap water slightly inhibited the degradation of DCF while lake water and river water seriously decreased the degradation of DCF.PDS-RGO/P25 visible light system not only degraded DCF quickly, but also had a better ability to degrade the TOC comparing to pure PDS or RGO/P25 visible light system. In our study, PDS was used as electron acceptor for the promotion of the electron-hole pairs separation, RGO acted as electronic conductor. h+, -OH, SO4·-,e- and O2·- were generated in this process and e-, O2·- and h+ played leading role during the degradation of DCF.