Photocatalysis of aqueous perfluorooctanoic acid by TiO₂ and Ga₂O₃ assisted with peroxymonosulfate under UV and visible light
- Publication Type:
- Thesis
- Issue Date:
- 2020
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Perfluorooctanoic acid (PFOA) has attracted considerable attention worldwide due to its widespread occurrence and environmental impacts. However, a suitable technology for PFOA controlling is worthwhile to be investigated nowadays. This thesis studied the photocatalysis by different catalysts and found that Ga₂O₃ and TiO₂ had better performance for PFOA removal than CeO₂, In₂O₃ and CdS. In addition, Ga₂O₃ mixed with peroxymonosulfate (PMS) was investigated for the PFOA degradation under UV light. It showed excellent performance and that 100% of PFOA was degraded within 90 min and 60 min under 254 nm and 185 nm UV irradiation, respectively. PFOA in real wastewater exhibited similar degradation efficiency and 75-85% TOC was removed by Ga₂O₃/PMS under 254 nm UV irradiation. Thus, a good method with well degradation efficacy was established in this thesis for aqueous PFAS removal. Moreover, this thesis investigated the PFOA photodegradation by using powerful visible light (300 W, 829.6 mW cm⁻²) in the presence of catalyst TiO₂ with PMS activation, which achieved 100% PFOA removal within 8 h. The presence of organic compounds in real wastewater reduced the degradation efficacy of PFOA by 18-35% in Vis/TiO₂/PMS system. Therefore, PFOA could be controlled under no matter UV light or visible light by TiO₂/PMS system.
Gallium oxide (Ga₂O₃), titanium dioxide (TiO₂), cerium dioxide (CeO₂), indium oxide (In₂O₃), and cadmium sulfide (CdS) are commonly used under UV light as photocatalyst for the pollutants degradation. In this study, these five catalysts were applied for the photodegradation of PFOA and the performance decreases as: Ga₂O₃ > TiO₂ > CeO₂ > In₂O₃ > CdS. Notably, CdS had almost no capability for PFOA removal. The initial pH, quantum yield and band gap energy were used to explain the various catalytic ability among these catalysts. Significantly, the band gap energy decreases as: Ga₂O₃ > TiO₂ > CeO₂ > In₂O₃ > CdS, which exactly matched their degradation performance. Thus, band gap energy was significantly related to the photocatalytic ability for PFOA removal. Further, according to the scavenger experiments, photogenerated holes rather than electrons played the main roles in degrading PFOA by TiO₂, CeO₂ and In₂O₃. In comparison, photogenerated conduction band electrons were more important when photocatalysis was carried out with Ga₂O₃.
This research focused on the photocatalytic process for the treatment of PFOA in water by Ga₂O₃ and peroxymonosulfate (PMS) mixed directly in the PFOA solution under different light sources. The results showed excellent performance that 100% of PFOA was degraded within 90 min and 60 min under 254 nm and 185 nm UV irradiation, respectively. Moreover, the degradation efficacy was unaffected by initial PFOA concentration from 50 ng L⁻¹ to 50 mg L⁻¹. Acidic solution (pH 3) improved the degradation process as high amount of PFOA was adsorbed on the surface of Ga₂O₃ via Coulombic attraction, leading to the promoted photocatalytic efficacy. The quantum yield in the PMS/Ga₂O₃ system under UV light (254 nm) was estimated to be 0.009 mol Einstein⁻¹. Scavengers such as 𝘵𝘦𝘳𝘵-butanol (𝘵-BuOH), disodium ethylenediaminetetraacetate (EDTA-Na₂) and benzoquinone (BQ) were added into PFOA solution to assess the roles of sulfate radicals (SO₄̇⁻), superoxide radical (O₂̇⁻) and photogenerated electrons (e⁻) as the active species with strong redox potentials for PFOA degradation in PMS/Ga₂O₃/UV system. Through the analysis of the intermediates, PFOA was degraded stepwise from long chain compound to shorter chain intermediates. In addition, PFOA in the real wastewater exhibited similar degradation efficiency and 75-85% TOC was removed by Ga₂O₃/PMS under 254 nm UV irradiation. Therefore, Ga₂O₃/PMS system was highly effective for PFOA photodegradation under UV irradiation, which has potential to be applied for the perfluoroalkyl substances (PFAS) treatment in water and wastewater.
This research also studied the PFOA photodegradation by using powerful visible light (300 W, 829.6 mW cm⁻²) in the presence of catalyst TiO₂ with PMS activation. The addition of PMS induced a significant degradation of PFOA on TiO₂ under visible light compared with sole TiO₂ or PMS treatment. Under powerful visible light, 0.25 g L⁻¹ TiO₂ and 0.75 g L⁻¹ PMS in the solution at initial pH 3 was advantageous for the PFOA degradation, and achieved 100% PFOA removal within 8 h. Under UV light irradiation at 254 and 185 nm wavelength, TiO₂/PMS resulted in an excellent performance of almost 100% PFOA removal within 1.5 h, attributed to the high absorbance ability of UV light by the catalyst. The intermediates analysis showed that PFOA was degraded from long carbon chains to shorter chains in a stepwise manner. Furthermore, scavenger experiments indicate that SO₄̇⁻ radicals from PMS and photogenerated holes from TiO₂ played an essential role in degrading PFOA. The presence of organic compounds in real wastewater reduced the degradation efficacy of PFOA by 18-35% in visible-TiO₂-PMS system. In general, TiO₂-PMS could be an ideal and effective catalyst for the degradation of PFOA from wastewater using either visible or UV light source.
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