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Mechanistic Study On The Degradation Of Chloramphenicol In Water By UV-Activated Sulfite

Posted on:2023-04-05Degree:MasterType:Thesis
Country:ChinaCandidate:X K YanFull Text:PDF
GTID:2531307118499134Subject:Environmental Science and Engineering
Abstract/Summary:PDF Full Text Request
UV/sulfite processes are commonly used as advanced reduction processes(ARPs)due to their ability to produce·H and eaq-with strong reducing power.The system is accompanied by the production of·OH and SO4·-in the presence of O2,making the UV/sulfite combination attract much attention as advanced oxidation processes(AOPs)or coupled oxidation-reduction processes.In this paper,dissolved oxygen(DO)in solution was controlled by nitrogen purging and air aeration to construct anoxic-oxygen-rich two-stage reaction to convert UV/S(IV)system from ARPs to AOPs,thus achieving segmental dechlorination and mineralization of chloramphenicol(CAP).The study focused on four aspects,degradation efficiency,reaction mechanism,degradation pathway and toxicity evaluation.The different effects of S(IV)concentration,air aeration and different water quality parameters on the two stages were investigated.The water quality adaptability of the UV/S(IV)system was evaluated by introducing two real water bodies.The main active substances in the different stages were identified by radical trapping and EPR,and HPLC-MS was used to identify degradation intermediates(TPs)in the different reaction stages and to speculate the possible degradation pathways.Finally,the biological toxicity of CAP and its degradation products were analyzed by computational toxicology and seed germination experiments.The main findings were as follows.(1)In the anoxic UV/S(IV)-ARPs system,the degradation and dechlorination of CAP can be promoted by increasing the quantum yield by increasing the sulfite concentration.CAP can achieve more than 90%removal in a wide p H range(3-11).The dechlorination efficiency was positively correlated with the solution p H and complete dechlorination was achieved at p H 11.Under alkaline conditions,the presence of SO42-significantly inhibited the dechlorination of CAP.HCO3-and HPO42-had similar reaction rate constants to eaq-and therefore had similar weak dechlorination inhibition,while Cl-had no significant effect on the degradation and dechlorination of CAP.Low concentrations of HA(≤5 mg/L)contributed to the dechlorination of CAP,while high concentrations of HA(>5 mg/L)inhibited the degradation and dechlorination of CAP.UV/S(IV)-ARPs system still achieved efficient degradation and dechlorination of CAP in tap water and South Lake water.eaq-was the main reducing radical for dechlorination of CAP under alkaline conditions.The combined action of·H and eaq-on CAP resulted in the denitrification,dechlorination,and dehydration of CAP benzene ring and the production of five TPs.(2)In the UV/S(IV)oxygen enrichment system:the optimal p H and air aeration for the UV/S(IV)oxygen enrichment process were determined to be 3 and 0.3 L/min,when the COD removal rate was 29.3%.Adequate DO is the key factor for SO5·-and SO4·-formation.Excessive aeration under acidic conditions would lead to a decrease in COD removal,while increasing aeration under neutral and alkaline conditions had a positive effect on the mineralization of CAP.Cl-had a small effect on the dechlorination efficiency of CAP.SO42-(≥5 m M),HCO3-(≥50 m M),HPO4-(≥1 m M)and NO3-(≥5 m M)significantly inhibited the dechlorination of CAP by consuming large amounts of SO42-(≥5 m M),HCO3-(≥50 m M),HPO4-(≥1 m M)and NO3-(≥5m M)significantly inhibited the dechlorination of CAP.The presence of SO42-has no significant effect on the mineralization of CAP.HPO4-activates the peroxymonosulfate(PMS)produced in the oxygen-rich system to generate·OH and SO4·-,which promotes the mineralization of CAP.HCO3-exhibits a dual effect on the mineralization of CAP.The UV/S(IV)oxygen-enriched system is susceptible to the interference of inorganic anions and organic components in the real water under acidic conditions,leading to the reduction of CAP dechlorination efficiency and mineralization.·OH and SO4·-are the main causes of CAP mineralization.They act on CAP to break CAP C-N bonds,denitrify and hydroxylate the benzene ring,and produce four TPs.(3)The UV/S(IV)reduction-oxidation coupled system avoided the formation of toxic by-products and reduced the acute toxicity to plants caused by CAP exposure to the environment.ECOSAR prediction results showed that the toxicity of TPs in the oxidation section was overall lower than that in the reduction section and the parent compound.Compared with the anoxic section,the increase of mineralization in the enriched phase further reduced the residual toxicity of the solution.The toxicity test results verified that the UV/S(IV)reduction-oxidation coupled system has good safety,and the system is promising for chloramphenicol wastewater purification applications.
Keywords/Search Tags:UV/sulfite, dechlorination, mineralization, oxygen enrichment, biotoxicity
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