| Author: | Gong, Han |
| Title: | Photodegradation of pharmaceuticals with a recyclable catalyst CoFe₂O₄/TiO₂ in aqueous phase |
| Degree: | Ph.D. |
| Year: | 2018 |
| Subject: | Hong Kong Polytechnic University -- Dissertations Water -- Purification -- Photocatalysis Photodegradation |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xxiii, 224 pages : color illustrations |
| Language: | English |
| Abstract: | Pharmaceuticals as emerging contaminants receive great concern because of their easy entry to the waterbody, frequent detection, low biodegradability, and considerable toxicity to the environment. The treatment of pharmaceuticals by advanced oxidation processes (AOPs) was explored due to the low removal efficiency of traditional methods. In this study, a novel recyclable photocatalyst CoFe₂O₄/TiO₂ was developed. Three pharmaceuticals, including sulfamethoxazole (SMX), antipyrine (AP) and ibuprofen (IBP) were selected as the target compounds to evaluate the application of the photocatalyst in the degradation of pharmaceuticals. The effects of reaction parameters (light source, UV light intensity, initial target compound concentration, catalyst dosage and solution pH), anions, and commonly used oxidants on the destruction performance were studied. The reaction mechanism including the organic intermediates, TOC reduction and inorganic ions release was determined. The toxicity of the degradation products to aquatic organisms, including the green alga Chlorella vulgaris and the brine shrimp Artemia salina was investigated. Firstly, the characteristic and magnetic property of the obtained catalyst were revealed. The catalyst was demonstrated to be homogenous spherical aggregates. The phases of CoFe₂O₄, anatase TiO₂ and rutile TiO₂, and the atomic ratios of Co, Fe, Ti and O from the catalyst were well justified. Reduced band gap energy and increased specific surface area compared to TiO₂ was found for the obtained catalyst. The average particle size and the zero point of charge value was determined to be around 30nm and 8.0, respectively. The catalyst showed good magnetic property. Secondly, based on the degradation of SMX, UV wavelength at 350 nm shows the best performance. The pseudo first-order rate constant of SMX destruction becomes predictable by using a proposed model in terms of SMX initial concentration and catalyst dosage. A light attenuation was found and analyzed by a linear correlation between the reaction rate k and transmission via a common factor (i.e. the catalyst dosage). The photocatalyst was found stable within the pH range of 5.2 and 8.8. The photocatalytic activity of the recycled catalyst remained intact after extensive reuses. The reaction mechanism during the treatment was determined. About 50% TOC reduction was detected as SMX was completely removed. Sixteen intermediates were detected, from which four of them were reported for the first time in this study. Four main destruction pathways, i.e. hydroxylation, cleavage of S-N bond, nitration of amino group, and isomerization were proposed. About 45% of the total mass sulfur source transformed to sulfate ion, and around 25%, 1%, and 0.25% of the total nitrogen transformed to ammonium, nitrogen, and nitrite ions. The toxicity of the treated solution was significantly reduced compared to that of the parent compound SMX. A variation of the algae growth was observed, which was due to the combination of generation of toxic intermediates (i.e. sulfanilamide) and the release of inorganic substances and carbon source as additional nutrients. The adverse effect on the clearance rate of the brine shrimp was also observed, but it can be eliminated if longer degradation time is used. Thirdly, in terms of AP degradation, the catalyst shows the optimal removal efficiency under the conditions of wavelength at 350 nm, the catalyst dosageat 0.5g/L, and pH value at 5.5. Ten organic intermediates were identified, and five of them were newly reported in AP treatment process. Hydroxylation, demethylation and the cleavage of the pentacyclic ring were included in the decomposition pathways. The ring opening was certified by the 45% TOC reduction and 60% ammonia release during the process. The parent compound AP and its degradation products show positive effects on the growth of the algae. However, acute toxicity of AP was detected on brine shrimps A. salina. The toxicity was eliminated gradually with the decomposition of AP and the generation of the byproducts. Fourthly, for IBP photodegradation, the UV wavelength of 300 nm, the catalyst dosage of 0.5 g/L, and acidic condition were favorable for IBP destruction by photocatalysis. The removal efficiency was significantly elevated with the addition of oxone or persulfate, and the reaction was faster in the presence of oxone than that with the same concentration of persulfate. However, the reaction was significantly slowed down with the introduction of H₂O₂. The anion NO₃- accelerated the reaction while the anions Cl-, F- and CO₃2- slowed down the IBP photodegradation. Approximately 35% TOC reduction was detected when the parent compound IBP was totally removed. The adverse effect of the parent compound IBP to C. vulgaris was gradually eliminated with the decomposition of IBP. No adverse effect was detected for the survival and the feeding of A. salina exposed to IBP and the degradation products. Finally, SMX degradation under UV/CoFe₂O₄/TiO₂ catalyzed permanganate oxidation was investigated in detail for the first time. Sole permanganate showed no effect in SMX degradation, while its introduction to the photocatalytic process doubled the reaction rate at the optimal dosage. It is interesting to find that the reaction rate showed a fluctuation trend in terms of permanganate dosage due to the summation of positive effect of permanganate oxidation and the negative effect of the formation of MnO₂ (at catalyst surface) and light attenuation due to overdosed permanganate. The determined intermediates, the higher inorganic ions release and TOC reduction provided a clue on a higher mineralization compared to SMX degradation in the same process without permanganate. Permanganate above 1 µM may pose threat to the algae growth, therefore a good monitoring and control of residual permanganate dosage should be incorporated into the process design. A good toxicity reduction to A. salina was observed in the treated effluent; a longer detention is suggested ifthe complete removal of toxicity is required. The results indicate that the photocatalysis process is effective in pharmaceutical removal, TOC reduction and toxicity elimination. |
| Rights: | All rights reserved |
| Access: | open access |
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| File | Description | Size | Format | |
|---|---|---|---|---|
| 991022068658803411.pdf | For All Users | 6.27 MB | Adobe PDF | View/Open |
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