Radical Identification And Contaminants Transformation Mechanisms In The UV/Monochloramine Process

With the rapid development of the economic level and industrialization in recent years,many contaminants of emerging concern（CECs）have been discharged into natural water bodies by various means,causing biological and environmental hazards.UV/monochloramine（NH₂Cl）,as an advanced oxidation process（AOP）that can generate a variety of complex radicals,can effectively remove CECs with different structures.However,the degradation mechanism of multiple reactive radicals with CECs is not clear.Eight common CECs were studied in this study,focusing on reactive nitrogen radicals（RNS）.The degradation kinetics and mechanism of the UV/NH₂Cl process on CECs were explored in detail by combining theoretical calculations and experiments.First,the degradation efficiency of the UV/NH₂Cl process on CECs was investigated.The UV/NH₂Cl process showed good degradation efficiency for all CECs,which conformed to pseudo-first-order kinetics.Besides the easy photolysis of CECs,hydroxyl radicals（HO^·）and RNS were always the most important contributors to the degradation of CECs.The properties and reaction mechanism of HO^·have been extensively studied.Therefore,this study focused on RNS and compared the reactivity of^·NO,^·NO₂,^·NH₂with CECs.Thermodynamic and kinetic results from density functional theory（DFT）calculations indicate that^·NH₂reacts most readily with CECs.Therefore,in the follow-up study,RNS only considered the effect of^·NH₂on the degradation of CECs.This study predicted the second-order reaction rate constants between^·NH₂and eight CECs based on the NH₂Cl concentration model and found a good linear relationship between^·NH₂reactivity and the specific molecular descriptors（negative values of the summation of Hammett constants）of the CECs.Second,considering the reaction between^·NH₂and CECs,a new kinetic model of the UV/NH₂Cl process was established in this study.The model can accurately predict the changes of HO^·,chlorine radical（Cl^·）,RNS and other active components under different water matrices（anions,natural organic matter（NOM）,pH,and NH₂Cl dosage）to explain the effect of water matrix on the degradation of typical CECs.Specifically,the concentrations of HO^·,Cl^·,and RNS increase with increasing NH₂Cl concentration,thereby promoting the degradation;NOM,NO₂^–and HCO₃^–decrease the concentrations of HO^·,Cl^·,and RNS,thereby inhibiting the degradation;pH and NO₃^–did not significantly affect the concentration of the prominent free radicals.Since the main products of NH₂Cl photolysis are inorganic nitrogen components such as NO₃^–,NO₂^–,and NH₄⁺,this study also used this model to explore the effect of CECs on the conversion of inorganic nitrogen,and found that the existence of different CECs did not significantly affect the photolysis of NH₂Cl and the distribution of inorganic nitrogen components.In addition,this study combined DFT calculations with high-resolution mass spectrometry to investigate the reaction mechanism of acetaminophen（AAP）,bisphenol A（BPA）,and carbamazepine（CBZ）with active components such as HO^·,Cl^·,and RNS.The results showed that the reactions between HO^·and the three CECs were mainly completed by hydrogen atom transfer（HAT）and radical adduct formation（RAF）,while Cl₂^·–and Cl^·degraded CECs by single electron transfer and hydrogen atom transfer,respectively.The DFT calculation results verified the specificity of^·NH₂for the reaction of phenolic hydroxyl groups.^·NH₂degrades phenolic compounds by abstracting hydrogen atoms from phenolic hydroxyl groups.DFT calculations also show that^·NH₂can react with CBZ to abstract hydrogen atoms from double bonds and amino groups.Meanwhile,high-resolution mass spectrometry was used to identify multiple transformation products of CECs,including products containing nitro,nitroso,hydroxyl,and chlorine groups,indicating the important role of RNS as well as HO^·and RCS.Detailed degradation pathways were proposed to combine the active site and potential energy surface.Overall,this study provides a deep and comprehensive insight into the radical-mediated degradation mechanism of CECs,while deepening a new reference for RNS.