- Title
- Co-transformation mechanism of carbon and chlorine in the Co(II)/HSO5- advanced oxidation system
- Creator
- Xue, Ying
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2021
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Advanced oxidation processes (AOPs) are extensively explored as environmentally friendly technologies, which utilize highly reactive radicals (such as •OH or SO4•-) to rapidly degrade contaminants. It is known that the SO4•- has a higher standard redox potential (E0 = 2.5 - 3.1 V vs. NHE) compared with •OH. SO4•- selectively reacts with organic contaminants through an electron transfer mechanism and potentially mineralizes various diverse, highly toxic, and persistent environmental contaminants. Peroxymonosulfate (PMS) is a moderate oxidant with high reduction potential (E0(HSO5-/HSO4-) = +1.85 V vs. NHE) and can be activated to generate highly reactive SO4•-. Halides (e.g., chloride, Cl-; bromide, Br-; iodide, I-) are commonly detected in natural water and wastewater, and they are believed to play essential and complex roles in the SO4•--based AOPs. It is known that the degradation efficiency of AOPs was affected by the exogenic halides (i.e., from saline wastewater), and the significance of endogenic halides (i.e., intramolecular halogen) of halogenated organics gaining importance in the performance of AOPs. Nowadays, many studies mainly focus on the influence of the exogenic halides on the performance and degradation mechanism of AOPs, while the conversion mechanism of alkyl endogenic chlorine, catechols endogenous chlorine, and the interaction between exogenic chlorine and heterocyclic structure chlorine carrier were poorly understood. In this study, the co-transformation mechanism of carbon and chlorine were comprehensively explained in the Co(II)/HSO5- advanced oxidation system. 2 chloramphenicol (i.e., thiamphenicol (TAP) and florfenicol (FFC)) and seven chlorocatechols were selected as target pollutants to investigate the transformation mechanism of alkyl endogenic chlorine and catechols endogenic chlorine, respectively. In addition, the interaction between exogenic chlorine and the heterocyclic structure was also evaluated by selecting sulfadiazine (SDZ) and sulfisoxazole (SIZ) as substrate models. The main results are as follows: (1) Release and rechlorination of chlorine atoms on the side chain of the benzene ring was fully assessed by degradation of TAP and FFC in the Co2+/PMS process. High-resolution mass spectrometry (HRMS) analysis detected a few chlorinated and dechlorinated products during the oxidation process. The low efficiency of total organic carbon (TOC) removal in conjunction with slightly decreased absorbable organic halogen (AOX) values with high degradation rates indicated that TAP and FFC could be degraded via the destruction of a small portion of the carbon moiety. Small amount of chloride ions was detected after reaction for 120 min, and it implied that a very small amount of chloride on the side chain of the benzene ring was transformed into inorganic Cl- ions. The possible degradation intermediates and pathways of TAP and FFC were proposed, and some of the by-products were proved to be more toxic than parent compounds. The bond dissociation energy of the C-Cl bond on the benzene ring and the side chain of the benzene ring was calculated according to the density functional theory. All experimental results and further quantum chemistry calculations indicate dichlorination on the side chain of the benzene ring is more difficult than that directly on the benzene ring. This suggests organic pollutants containing chlorine atoms on the side chain of the benzene ring will be more resistant to oxidative attack and deserve more attention. (2) The transformation mechanism of endogenic chloride on the benzene ring in the Co2+/PMS process was assessed by employing seven chlorocatechols, including catechol (CA), 3-chlorocatechol (3CA), 4-chloroatechol (4CA), 3,5-dichlorocatechol (35CA), 4,5-dichlorocatechol (45CA), 3,4,5-trichlorocatechol (345CA), tetrachlorocatechol (TCC). The degradation kinetics were proved to be affected by the number and position of chlorine substituents in chlorocatechols, with the degradation rates following the order of CA > 3CA > 4CA > 45CA > 35CA > 345CA > TCC. The chlorine atoms substituted at the ortho position of -OH group of chlorocatechols bear more steric strain than that at the meta or para position. More -OH group substituents on the aromatic ring of chlorocatechols may offset the steric hindrance effect to a certain extent, as it is an electron-donating group. The degradation pathways were also presumed. Since catechol and maleic anhydride are the parent and common intermediates of chlorocatechols, respectively, further experiments on the chlorination of catechol and the oxidation of maleic anhydride were performed to verify the possibility of degradation pathways. The bio-toxicity assessment showed that the degradation intermediates containing ketone groups showed higher acute and chronic toxicity than the parent compounds, while the open-loop products could reduce toxicity and even mineralization. A quantitative structure activity relationship (QSAR) model was established to prove that the reaction rate constants of sulfate radical with chlorocatechols were related to the molecular physicochemical properties. The results showed that two descriptors (i.e., EHOMO and number of halogen atoms (#X)) played essential roles in affecting the kSO4•-. Experimental results indicated that Co2+ activated PMS oxidation could be an efficient method for chlorocatechols remediation, but some toxic by-products should be paid more attention to during the decomposition process. (3) The depletion kinetics of two selected sulfonamides containing different heterocyclic groups, SDZ and SIZ, were investigated in the Co2+/PMS system, and the results indicated the degradation efficiencies were increased with the increasing content of Co2+, PMS and exogenic chlorine. The removal rates of SDZ and SIZ were generally on the rise with the initial pH values increase from 2.0 to 7.0, and the SIZ degradation was most readily at pH=6. The degradation kinetics results showed that SDZ was more susceptible to free radical attack than SIZ, which is in line with the results of further theoretical study. Since SDZ and SIZ contain different heterocycles, namely a six-membered heterocyclic group (pyrimidine) and a five-membered heterocyclic group (isoxazole), the main reason for the different reaction mechanism is that SO4•- selectively reacts with different groups. However, the SIZ cation may undergo reform into parent compounds, thereby leading to lower degradation efficiency. The oxidation and chlorination by-products of SDZ and SIZ were speculated, as well as their degradation pathways. Only one chlorinated by-product was detected in the degradation process of SDZ in the presence of Cl-, while no chlorinated product was detected in the chlorination process of SIZ. Combined with the formation of a small amount of AOX, it indicated that the chlorination of SDZ and SIZ mainly occurs on the benzene ring, rather than the entire structures. The toxicity evaluation results showed that most of the degradation by-products of SDZ in the Co2+/PMS system were less toxic than their parent compound. Conversely, degradation of SIZ led to increased acute and chronic toxicity because most oxidized intermediates were more toxic than SIZ. Experimental results and theoretical calculations further supported that the Co2+/PMS process was more suitable for destructing SDZ than that of SIZ. It is worth noting that the presence of different kinds of chloride ions affects the degradation efficiency of AOPs to varying degrees. Therefore, it is necessary to monitor the halide ion content and consider the position of the halogen in the pollutant molecule before selecting and applying SO4•--based AOPs.
- Subject
- endogenic chlorine; exogenic chlorine; peroxymonosulfate; AOX; degradation pathway; DFT; QASR
- Identifier
- http://hdl.handle.net/1959.13/1509875
- Identifier
- uon:56322
- Rights
- Copyright 2021 Ying Xue
- Language
- eng
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