Study On The Removal Of Emerging Contaminants During Transformation Of Chlorine Dioxide And Its By-products Under Ultraviolet Irradiation

With the issuance of the Action Plan for the Control of Emerging Contaminants by the General Office of the State Council,the control of emerging contaminants is attracting widespread social attention.For the past few years,emerging contaminants have been found throughout the social cycle of water.In particular,emerging contaminants are not effectively removed from drinking water by conventional water treatment processes,posing a potential health risk to the public.Currently,pre-oxidation processes are widely used in water treatment plants to remove various types of emerging contaminants from water.Chlorine dioxide（ClO₂）is a commonly used pre-oxidation agent,but chlorite（ClO₂^?）and chlorate（ClO₃^?）produced during the pre-oxidation of ClO₂ also threaten the safety of drinking water.Therefore,there is an urgent need for a simple,green,and efficient way of removing ClO₂^?,ClO₃^?,and emerging contaminants simultaneously from drinking water.In recent years,UV irradiation has been extensively investigated in the removal of OPMs from water due to the light-absorbing properties of various contaminants in water.In this study,different UV wavelength irradiation methods are proposed for the removal of ClO₂^?and ClO₃^?from water while simultaneously removing emerging contaminants through the generation of reactive radicals.The mechanisms involved in the generation and conversion of reactive radicals during UV irradiation of ClO₂ and its inorganic by-products have rarely been reported.In addition,the reaction pathways between reactive radicals and emerging contaminants in the above system and the contribution of reactive radicals to the degradation of emerging contaminants remain unclear and require further investigation.This research seeks to investigate the effectiveness of the UV-LED/ClO₂ process for the removal of typical emerging contaminants from water.Due to the maximum absorption peak of ClO₂ is between 320 and 400 nm,UV-LED at 365 nm was chosen as the UV light source for this study and three typical emerging contaminants,atrazine（ATZ）,metronidazole（MET）and carbamazepine（CBZ）,were used as the subjects.The results showed that the degradation efficiency of the UV-LED₃₆₅/ClO₂ process for ATZ,MET,and CBZ increased with decreasing p H and decreased with increasing natural organic matter and bicarbonate ion concentrations.Among the reactive species produced during the UV-LED₃₆₅/ClO₂ process,ozone makes the largest contribution to the degradation of CBZ and hydroxyl radicals（HO^·）play a major role in the degradation of ATZ and MET.The degradation mechanism of ATZ in the UV-LED₃₆₅/ClO₂ process is mainly based on reactions such as dichlorination and hydroxylation.In addition,during the removal of emerging contaminants by the UV-LED₃₆₅/ClO₂ process,there is a risk of ClO₂ being transformed into ClO₂^?and ClO₃^?.In this study,simultaneous removal of ClO₂^?and CBZ was achieved by irradiation with an ultraviolet lamp at 254 nm to address the risk of ClO₂^?and CBZ in drinking water.It was found that ClO₂^?could be converted to a large number of reactive oxygen species and reactive chlorine species under UV₂₅₄ irradiation for efficient degradation of CBZ.During the simultaneous removal of ClO₂^?and CBZ by UV₂₅₄ irradiation,an increase in the concentration of ClO₂^?increased the removal of CBZ and an increase in the concentration of natural organic matter and bicarbonate ions inhibited the removal of CBZ.The contribution of reactive chlorine species to the degradation of CBZ was greatest in the p H range 6～8 and the removal of CBZ decreased with increasing p H.The CBZ degradation pathway is dominated by reactions such as chlorine substitution and hydroxylation,while ClO₂^?is mainly converted to Cl^?and ClO₃^?.For the production of ClO₃^?,this study selected 14 model pollutants to verify that the ClO₃^?yields were highly correlated with the second-order reaction rate constants of emerging contaminants and HO^·.The formation of ClO₃^?is the main issue faced in this study.To further explore the effect of UV wavelength,this research systematically investigates the efficiency and mechanism of simultaneous removal of ClO₂^?and CBZ by UV-LED₂₅₅,UV-LED₂₈₅,and UV-LED₃₆₅.At the same UV dosage,UV-LED₂₅₅ has the highest removal efficiency for ClO₂^?and CBZ,while UV-LED₃₆₅ has almost no effect.During the simultaneous removal of ClO₂^?and CBZ by UV-LED₂₅₅ and UV-LED₂₈₅ irradiation,the increase in ClO₂^?concentration promoted the degradation of CBZ,while the presence of natural organic matter and bicarbonate ions inhibited the degradation of CBZ.The removal of CBZ under UV-LED₂₅₅ irradiation conditions decreased with increasing initial p H of the solution,while the removal under UV-LED₂₈₅ irradiation conditions increased with increasing initial p H solution.In addition,similar to UV₂₅₄ irradiation,there is also a risk of ClO₃^?formation under UV-LED₂₅₅and UV-LED₂₈₅ irradiation.Therefore,to address the risk of ClO₃^?formation in the above system,a systematic study of ClO₃^?in water using vacuum ultraviolet/ultraviolet（VUV/UV）at 185 nm+254 nm was carried out in this study and ClO₃^?was found to be converted to non-toxic and non-hazardous Cl^?.It was demonstrated that the removal of ClO₃^?depends mainly on the direct photolysis by VUV/UV and not on the indirect reduction induced by hydrated electrons(e_aq^-)and hydrogen atom radicals（H^·）.During VUV/UV irradiation of ClO₃^?,the presence of natural organic matter significantly increasing the removal of ClO₃^?.The addition of bicarbonate and the increase in solution p H inhibited the removal of ClO₃^?.In addition,the study also confirmed that VUV/UV irradiation could simultaneously remove ClO₂^?,ClO₃^?,and emerging contaminants,with Cl-being the end product of ClO₂^?and ClO₃^?photolysis by VUV/UV.In summary,based on the light absorption characteristics of various pollutants,a progressive and systematic study of the UV irradiation method has been carried out to effectively solve the risk of the presence of ClO₂^?and ClO₃^?in water,and to achieve the simultaneous degradation of emerging contaminants,which has certain application prospects in the field of water treatment.