Efficiency And Mechanism For The Degradation Of Carbamazepine In Drinking Water By UV-LED/NH₂Cl Process

Pharmaceutical active compounds（Ph ACs）are frequently detected in the global water,and their existence has caused potential risks to aquatic organisms and water supply safety,it has become an environmental problem which needs to be solved urgently.Carbamazepine（CBZ）is a typical Ph ACs with the characteristics of frequent detection and high occurrence concentration.CBZ control technology based on ozone oxidation,activated carbon adsorption,and membrane retention has been widely studied and applied in municipal water supply treatment.Compared with traditional chlorine disinfectants,monochloramine（NH₂Cl）has the ability to reduce the formation potential of trihalomethanes and maintain the residual chlorine requied in distribution network,the photochemical degradation and free radical oxidation characteristics of NH₂Cl make it promising in drinking water.The emerging Ultraviolet light-emitting-diode/monochloramine（UV-LED/NH₂Cl）offers new possibilities for mercury-free water treatment and photoreactor design.However,there are few studies on the degradation of pollutants in water using UV-LED/NH₂Cl,and the contribution of different free radicals to the degradation of target pollutants is still unclear.Therefore,in view of the residual problem of Ph ACs in the water,this study constructed UV-LED/NH₂Cl to realize the control of CBZ in drinking water.In this study,CBZ was selected as the target pollutant,the degradation efficiency of UV-LED/NH₂Cl on CBZ was investigated,and the effects of water quality conditions and process parameters on the degradation were explored.Through probe compounds and other means,the contribution of different active free radicals to the degradation of CBZ was analyzed;transformative products（TPs）of CBZ were identified,the degradation pathway of CBZ was deduced,and the relationship between active free radicals and TPs was explored.The purpose of this study is to comprehensively evaluate the efficiency and mechanism of UV-LED/NH₂Cl to degrade CBZ,and provide theoretical support and scientific basis for future research and application.In this study,it was found that NH₂Cl oxidation alone and UV-LED photolysis alone had little effect on the degradation of CBZ,whereas the UV-LED/NH₂Cl could achieve high-efficiency degradation of CBZ.The degradation of CBZ followed pseudo-first-order kinetics(R~2>0.99,k _CBZ=0.0043 cm~2·m J^-1,p H=7).The degradation of CBZ depends on the UV-LED wavelength.Among the three selected UV-LED wavelengths（265 nm,275 nm and 285 nm）,the best degradation efficiency was obtained at 265 nm.The common p H value（6.0～8.0）and the presence of chloride ions in actual water have no significant effect on the degradation efficiency,while the presence of bicarbonate ions and natural organic matter（NOM）inhibited the degradation of CBZ,and the higher the concentration,the stronger the inhibitory effect.Therefore,when the system is applied in actual water bodies,it is more suitable for advanced treatment.In this study,the TPs of CBZ degradation were identified by Ultra-high performance liquid chromatography quadrupole time-of-flight mass spectrometry（UPLC-Q-TOF-MS）and degradation pathways were deduced.A total of 8 main TPs were identified.During the degradation of CBZ,the benzene ring is gradually opened to generate fatty acid compounds under the action of free radicals through hydrogen extraction,ring shrinkage,deamination,etc.,and finally realizes the mineralization of CBZ.The acute and chronic toxicity of intermediates were predicted by Ecological structure and relationship（ECOSAR）software,and the results showed that the acute and chronic toxicity of most TPs tended to decrease with the increase of UV dose.This study confirmed that in the UV-LED/NH₂Cl system（p H=6.0～8.0）,hydroxyl radicals（·OH）and chlorine radicals（Cl·）played a major role in the degradation of CBZ.Electron paramagnetic response（EPR）spectra showed that·OH were the main free radicals in the UV-LED/NH₂Cl system（p H=6.0～8.0）.In addition,the probe compound experiment was used to distinguish the contributions of·OH and reactive chlorine species（RCS）.The contribution of·OH to CBZ degradation was over 50%,and the steady-state concentration of·OH was greater than that of Cl·.