Photo (Electro) Catalytic Degradation Of Organic Pollutants By Novel Electrode Materials And Comprehensive Utilization Of The Chemical Energy Of Pollutants

With the rapid population increase and industrial development in the recent decades, the large amounts of toxic or hazardous waste, domestic sewage and industrial wastewater discharged into water bodies have caused serious environmental pollution and ecological destruction. However, the optimal efficiency of conventional wastewater treatment technologies (e.g., physico-chemical method and biological process) has been difficult to achieve. Hence, advanced systems have to be developed. Advanced oxidation technologies (AOTs), which the organic pollutants were oxidized by the generation of hydroxyl radicals (OH ), have been received considerable attention recently due to their strong oxidizability, fast oxidation rate, and high process efficiency. The photocatalytic oxidation technology, one of the common AOTs, is promising in organic pollutant treatment. Compared with conventional treatment processes, it has advantages of environmental friendliness, energy-saving property, high efficiency, non-secondary pollution, and complete mineralization of organic pollutants. Thus, the fabrication of a high-performance TiO₂ photocatalyst is a key issue in the photocatalytic technology. To overcome the existing deficiencies of traditional TiO₂ nanoparticulate materials (e.g., high recombination, poor visible-light activation, and low efficiency), in this study, a novel TiO₂ nanotube (pore) array was fabricated, modified, and applied in organic pollutant degradation. Meanwhile, the utilization of the chemical energy of organic pollutants released during the photocatalytic process was studied by designing a TiO₂-nanotube-array (TNA)-based photocatalytic fuel cell (PFC), which can achieve rapid organic pollutant degradation and simultaneously efficient chemical energy recovery.Fabrication of TiO₂ nanotube (pore) array and its photoelectrocatalytic performance. Traditional TNAs were synthesized via the anodization of Ti in HF aqueous (HF-H2O) and fluorinated dimethyl sulfoxide (HF-DMSO) solutions, and their photoelectrocatalytic properties were studied. The structural parameters (e.g., tube length, diameter, and thickness), substrate microstructure, and mechanical stability were found to be important factors affecting the electrode material performance. To fabricate a nanotubular electrode with strong stability, long working life and desirable electron transfer performance, the ultrasonication technology was integrated into the anodization process, and a short TNA (STNA) film was obtained. The self-organized STNA of approximately 12 nm to 65 nm in diameter and 75 nm to 280 nm in length were synthesized at an anodic voltage of 5 V to 20 V. Compared with the traditional TNA electrode prepared by magnetic agitation, the STNA electrode achieved an evidently enhanced photoelectrochemical performance. In particular, the saturated photocurrent of the STNA electrode was ¹.80 times higher than that of the traditional TNA electrode under similar working conditions, and the tetracycline removal rate of the STNA electrode was 20% higher than that of the traditional TNA electrode. Moreover, a highly ordered TiO₂ nanopore array (TNP) with controllable pore size and good uniformity was prepared via low-temperature anodization in HF-DMSO solution followed by a post-sonication treatment. In the TNP, the porous structure is directly connected to the Ti substrate, resulting in a lower transport resistance for photogenerated electrons. Hence, compared with the traditional TNA material, the TNP electrode possesses better photoelectrocatalytic properties. The effects of bias potential, electrolyte concentration, pH and initial concentration of the organics on the photoelectrocatalytic performance of the TiO₂ nanotube (pore) array electrode were also studied.Modification of the STNA electrode. To overcome the deficiencies of the conventional electrochemical deposition method (e.g., poor stability and distribution), the sonoelectrochemical deposition technology was applied to synthesize high-performance composite materials (e.g., CdS/STNA and Cu₂O/STNA) with high visible-light absorption and desirable stability. The SEM and XRD characterizations revealed that the hexagonal CdS nanoparticles are evenly distributed on the nanotubular surface. The UV-Vis absorption and photoelectrochemical measurements proved that the CdS coating extends the visible spectrum absorption (to 525 nm) and solar spectrum-induced photocurrent response. The photoelectrochemical test shows the optimum deposition of CdS (e.g., 10 min). Moreover, the SEM, EDS, XPS, and XRD characterizations showed that the Cu₂O particles are evenly distributed on the nanotubular surface. These results show that the Cu₂O coating improves the visible-light absorption property and photoelectrochemical performance. Under AM1.5 irradiation, the photocurrent density of the composite electrode with Cu₂O deposition for 5 min was more than 4.75 times higher than that for the pure nanotubular electrode.Utilization of the chemical energy of pollutants released during the degradation process. To utilize the chemical energy of the organic pollutants released during the degradation process, a novel PFC system was designed, based on the primary cell principle, using the high-performance STNA electrode as anode and a Pt-black/Pt electrode as cathode. The proposed PFC can achieve organic pollutant degradation and simultaneously recover the released chemical energy of pollutants. The PFC performance was studied using various model and refractory compounds, as well as real wastewater samples as substrate. The results demonstrate that the photoanode material, electrolyte concentration, initial solution pH value, and cathode material were found to be important factors influencing the cell performance. Moreover, a novel composite system composed of a photoelectrocatalytic reactor and a PFC was designed. The PFC was used to provide an external bias potential to the reactor. The results indicate that the composite system can evidently enhance the treatment efficiency of organic compounds. The external resistance, electrolyte concentration, PFC anode illumination area, and substrate were found to be important parameters influencing system performance.