Degradation Efficiency And Reaction Mechanisms Of Pharmaceuticals By Different Chlorine-Based Advanced Oxidation Processes

Pharmaceutical micropollutants,due to their relative low concentration in water and persistent to conventional water treatment process,can easily accumulate and metabolize in human and animal body,which can result in serious side effects,including immunodeficiency,growth deformities,neurobehavioral deficiency,and even tumors and cancer.Therefore,it is urgent to find a water treatment process that can efficiently remove pharmaceuticals in different water.Active chlorine is commonly used in traditional water treatment process as disinfectant,but it is also a strong oxidant.The combination of active chlorine and advanced oxidation technology can achieve the efficient degradation of pharmaceuticals in water while ensuring the disinfection effect.However,there are few comprehensive and in-depth comparisons of the pharmaceuticals degradation efficiency and mechanism of different chlorine-based advanced oxidation technologies.Therefore,there is a lack of certain theoretical basis in practical application and selection.Basing on the utilization of active chlorine,three different pharmaceuticals,namely carbamazepine,sulfamethoxazole and trimethoprim,who has different reactivity with active chlorine,was chosen as model compounds.Firstly,the degradation efficiency,reaction mechanism,and reaction routes of pharmaceuticals in UV/Cl₂ and in-situ chlorination systems were compared.Besides,the degradation energy efficiency and degradation mechanisms of model compounds,the influence of water matrix,and the generation of disinfection by-products（DBPs）in in-situ chlorination systems with different anodes were investigated.The main results obtained through the study are as follows（1）Carbamazepine was used as model compound to compare the removal efficiency,energy consumption and reaction mechanism of collimated low pressure（LP）UV/Cl₂,collimated medium pressure（MP）UV/Cl₂,submerge LP UV/Cl₂and in-situ chlorination respectively.Macroscopically,all the tested process in the research could achieve a complete removal of CBZ.However,the steady state concentration of·OH and Cl·produced in UV/Cl₂ processes were one to two magnitudes higher than that produced in in-situ chlorination,thus resulting in the faster degradation rate of CBZ.In short operating times,LP UV/Cl₂ was the process with the highest overall efficiency.But considering long-term operation,due to the relatively short life of the UV lamp and the need of constant addition of oxidants,the operation and maintenance costs of UV/Cl₂processes would increase.In addition,the concentration of chlorate produced in MP UV/Cl₂ and LP UV/Cl₂ processes was about 15 times higher than that of in-situ chlorination process.From a microscopic perspective,no matter in which chlorine-based process,Reactive chlorine species（chlorine-based radicals,RCS）was the dominant reactive species contributing to more than 70%of CBZ removal.According to the DFT calculation,CBZ had more reaction routes compared to·OH,and in the two important CBZ degradation routes:the generation of IMS and CBZ-epoxide,RCS showed higher reactivity and lower reaction barriers,which could correspond with its contribution to CBZ removal.（2）According to the comparison results above,the research focusing on in-situ chlorination was carried out.In this part,two pharmaceuticals,sulfamethoxazole（SMX）and trimethoprim（TMP）,which showed different reactivity towards chlorine were used as model compounds individually to investigate the energy efficiency and reaction mechanisms of in-situ chlorination systems with DSA,BDD and graphite as anodes respectively.Both SMX and TMP showed the highest removal rate constant in BDD system and the lowest in graphite system,while the highest EE/O of both micropollutants were achieved in graphite system.According to the EPR results,no characteristic peaks of SO₄·^-and ¹O₂ were found in in-situ chlorination systems.The degradation of micropollutants was achieved by·OH,RCS,active chlorine and the direct electrolysis on the anodes.Due to the relative high reactivity between SMX and active chlorine,active chlorine and RCS together were the dominant reactive species contributing to mare than 90%of SMX removal.While for TMP,who showed relative low reactivity to chlorine,RCS was the dominant reactive species contributing to 50%-93%of TMP removal in all three systems with different anodes.In chlorine-based AOPs,the degradation rate of micropollutants were positively related to their reactivity towards chlorine.（3）Combing the scavenger experiment and micropollutants removal results,it could be inferred that,in in-situ chlorination systems with different anodes,the generation mechanisms of reactive species were different.In DSA and graphite systems,active chlorine and RCS could be generated through two ways,firstly,it could be generated by the direct electron transfer on the anodes;secondly,it could also be generated through the chain reaction between Cl^-/ClO^-and·OH.While in BDD system,active chlorine and RCS could only be generated by the chain reaction between Cl^-and·OH.During micropollutants removal,RCS was always the dominant radical and·OH mainly participated in the degradation of micropollutants indirectly.（4）Furthermore,basing on the density functional theory（DFT）calculation of the reaction routes and reaction barriers between CBZ/SMX/TMP and different reactive species,it is concluded that the initial reactions between micropollutants and·OH are mainly hydroxylation,and the energy barrier of these reactions is relative low.When attacked by OCl·and OCl^-,with the insertation of-OCl and the leave of Cl·/Cl^-,the molecular structure rearrangement will occur and further lead to ring-opening and ring-forming reaction.The reaction routes between RCS and micropollutants are more diverse and the original structure showed higher degree of demage after reaction.This series of reaction routes had certain universality,which could provide a theoretical basis for the prediction of the degradation routes of more micropollutants in the chlorine-based advanced oxidation systems.（5）The influence of pH,Br^-,CO₃^2-,NH₄⁺and natural organic matter（NOM）on micropollutants removal efficiency and energy consumption were investigated in this research.Besides,the DBPs produced in different anode systems were compared,in all three systems,the concentration of HAA9 and THMs produced were below the related regulation,but the chlorate generated in BDD and graphite system was 68 and 86 times higher than that generated in DSA system respectively.In summary,micropollutants with different reactivity towards chlorine could achieve great degradation efficiency in all the tested chlorine-baed AOPs.And the degradation rate of micropollutants were positively related to their reactivity towards chlorine.In different reaction systems,RCS was always the dominant reactive species contributing to micropollutants removal,and the direct contribution of·OH to micropollutants removal was relative low.Under long-term operation,the operating and maintenance costs of in-situ chlorination was lower than UV/Cl₂processes.Besides,no extra chlorine needed to be added and less DBPs were generated in in-situ chlorination process,thus the process was of higher safety and wider application prospects.Therefore,combining the results of scavenger experiments,probe chemical degradation and electron paramagnetic resonance（EPR）spectroscopy,different reactive species and their concentration generated in in-situ systems with different anodes were determined,and the generation mechanisms of these reactive species were also revealed.In the meantime,the influence of water matrix on in-situ chlorination system was determined,all the results above layed a theoretical foundation for the application of this system.According to the DFT calculations,and the degradation routes between micropollutants and reactive species generated in chlorine-based AOPs were summarized.This provided a new idea for predicting the degradation path of more micropollutants in chlorine-based AOPs.There are 62 Figureures,33 tables,and 191 references in this thesis.