TiO2-based Photoelectrocatalysis Technology for Degradation and Detection of Organics in Wastewater

Abstract

With industrialization rapidly progressing in recent decades, great amounts of refractory organic pollutants are found in water bodies, which severely jeopardizes ecosystem health. Monitoring the organic compounds in water bodies and removing organic pollutants from wastewater is essential for ameliorating threats to aquatic life and human health. Photocatalytic (PC) and photoelectrocatalytic (PEC) degradation and detection of organic pollutants in wastewater are promising strategies for fulfilling these goals sustainably, since PC and PEC technologies can take advantage of solar energy, which is one of the most abundant energy sources on earth. Titanium dioxide (TiO2) is a commonly used photocatalyst due to its appropriate band position, high chemical stability, low cost, and nontoxicity. However, pristine TiO2 photocatalysts can only be stimulated by UV irradiation because the band gap of pristine TiO2 is higher than 3.0 eV, which seriously impedes its development with low-cost and environmentally friendly solar energy. There are several efficient strategies to overcome these disadvantages of pristine TiO2, such as morphology modification, bandgap engineering, and applying co-catalysts with the host photocatalysts. Herein, this thesis aims to utilize different strategies to enhance the photocatalytic performance of TiO2-based photocatalysts under visible light irradiation and apply those modified photocatalysts to degradation and detection of organics in wastewater.\nIn the first study, a photoelectrochemical Chemical Oxygen Demand (COD) sensor based on a linear photocurrent-concentration analytical principle was designed for the on-site determination of COD. A high-performance anatase-branch@hydrogenated rutile-nanorod TiO2 (AB@H-RTNR) photoelectrode was fabricated. The as-prepared photoanodes successfully achieved sensitive determination of COD with a detection limit of 0.2 ppm (S/N = 3), an RSD% of 1.5 %, a wide linear detection range of 1.25−576 ppm, and an average recovery rate fluctuating between 100% ± 4% for artificial wastewater sample analyses. The satisfying results of this work suggest that AB@H-RTNR can serve as a promising photocatalyst for fast and accurate detection of organic compounds in water bodies.\nIn addition to detecting COD in water bodies, the degradation of refractory organic compounds in water is also crucial for healthy ecosystems. Highly efficient, low-cost, and portable wastewater treatment and purification solutions are urgently needed for aqueous pollution removal. Herein, in the second study, we coupled a TiO2-based PC system with a persulphate (PS) oxidation system into a portable advanced oxidation device for rapid and deep degradation of organic contaminants in wastewater. Using hydrogenation, we fabricated hydrogenated anatase branched-rutile TiO2 nanorod (H-AB@RTNR) photocatalysts that enable PC degradation to occur under visible light to improve the utilization of solar energy. A degradation rate of 100% and a reaction rate constant of 0.0221 min−1 for degrading 1 L Rhodamine B (20 mg L-1) was achieved in 120 min in a specially designed thin-layer cell under visible light irradiation. These encouraging results suggest that the H-AB@RTNR photocatalysts/PS synergistic degradation system could be an alternative approach for the efficient degradation of organic pollutants in wastewater.\nMotivated by the result of the PC/PS synergistic degradation system, we further employed a PC/chlorination system for synergistic degradation of antibiotics. In the third study, we demonstrated the use of visible light (>420 nm) to produce •HO and •ClO through the assistance of photocatalysts (TiO2/WO3 nanofibers) and free chlorine (HOCl/ClO−). The introduction of visible-light-driven photocatalysts can significantly boost the yield of active radicals, which favors the degradation of antibiotics in water bodies. The synergistic PC/chlorination degradation system obtained a pseudo-first-order degradation rate constant of a model antibiotic, tetracycline hydrochloride, of 21.438 min-1, which is 3.66 and 86.57 times higher than that in the pure TiO2/WO3 photocatalysis and traditional chlorination processes, respectively. The stability test exhibits that the performance decline is negligible after multiple use cycles. The results of this study suggest that PC/chlorine degradation is a feasible, lowcost, and environmental-friendly strategy for antibiotics removal under visible irradiation.\nAnother approach to improve the degradation efficiency of TiO2-based photocatalysts is using co-catalysts to assist the host photocatalysts. In the fourth study, we fabricated noble-metal free co-catalysts, i.e., N-doped carbon wrapped FeNi nanoparticles (FeNi@NGC), via a pyrolysis method. Hydrogenated TiO2 (H-TiO2) was synthesized as the host photocatalysts and coupled with as-prepared FeNi@NGC to obtain superior PC activity in the degradation of tetracycline hydrochloride (TC-HCl), a model antibiotic. The FeNi@NGC/H-TiO2 system achieved a degradation rate of 100% within 120 min on degrading 100 mL 20 mg L-1 TC-HCl under visible light irradiation (λ>420 nm). The degradation rate constant of the FeNi@NGC/H-TiO2 system reached 23.18 min-1, which was 33.99, 26.98, and 2.23 times compared to that of TiO2, FeNi@NGC/TiO2, and H-TiO2 system. The favorable performance of the FeNi@NGC/H-TiO2 system can be ascribed to two reasons: the secondary electron transfer in the FeNi intermetallic compounds that facilitates the photo-induced charge separation in photocatalysts; and the enhanced visible light absorption ability resulted from the N-doped graphitized carbon shell. Moreover, the oxygen vacancies brought by the hydrogenation process also improves the visible light absorbance of the photocatalysts, which favors the degradation performance. This study suggests that coupling FeNi@NGC cocatalyst with H-TiO2 is a promising strategy for improving the photocatalytic degradation performance on antibiotics.\nIn summary, the strategies presented in this thesis show that the morphology and electronic properties of TiO2 can be manipulated to resolve the problems of poor visible light absorption and the large recombination rate of photogenerated charge carriers. Moreover, the degradation performances of organic compounds show that applying a synergistic system with TiO2-based materials is promising in the removal of refractory organics in the wastewater. The strategies utilized in the thesis (i.e., morphology manipulation, bandgap engineering, and applying co-catalysts) can be utilized in other members of the semiconductors family (such as SnO2, BiVO4, SrTiO3) for developing more sustainable and low-cost approaches and techniques to improve human and aquatic ecosystem health.