Mechanistic Study And Process Optimization Of The Degradation Of Organic Micropollutants In UV/NH₂Cl Advanced Oxidation Process

UV-based advanced oxidation processes（AOPs）via photolysis of radical precursor chemicals（RPCs）have been of interest to numerous researchers over the past several decades due to their capacity to generate highly active radical species and interesting radical chemistry.Unsatisfying cost-effective energy consumption has inhibited the engineering applications of the UV-based AOPs.Upgrading the existing disinfection process,making it into an advanced oxidation processes has been a new trend for the water treatment facilities.Monochloramine（NH₂Cl）can be irradiated by UV to create an advanced oxidation condition（i.e.UV/NH₂Cl）for the elimination of organic micropollutants（OMPs）from source water.Therefore,this study is proposed with the aim of providing a comprehensive valuable knowledge for the UV/NH₂Cl AOP,and aid in the energy optimization of UV/NH₂Cl as well as other UV-based AOPs for engineering applications.Firstly,the generation and transformation mechanisms of reactive species in UV/NH₂Cl AOP were investigated.The investigation on the effect of water matrix and OMP molecular structures on UV/NH₂Cl performance was conducted.The degradation of five representative OMPs,including triclosan,carbamazepine,sulfamethoxazole,estradiol（E2）and ethinylestradiol（EE2）,was examined in different water matrices.All OMPs were rapidly removed by UV/NH₂Cl but exhibited different degradation mechanisms.Although·OH,·Cl,and direct photolysis mainly contributed to the overall degradation of OMPs in buffered nano-pure water,the contribution of reactive nitrogen species（RNS）generated from the photolysis of NH₂Cl was not negligible in the degradation of E2 and EE2.A phenolic group was identified as the moiety reactive towards RNS.Based on quantitative analysis of the impact on OMP degradation from co-solutes（including Cl^-,HCO₃^-,DOM）as well as p H and NH₂Cl doses,we developed a kinetic model for the prediction of OMP degradation in complex water matrices.In environmental water matrices,the performance and radical contributions in UV/NH₂Cl and UV/H₂O₂ systems were taken into comparison,which showed faster degradation of OMPs and a more significant contribution of CO₃·^-in the UV/NH₂Cl process.Afterwards,based on the study of radical chemistry of UV/NH₂Cl system,this paper further evaluated the generation and transformation of by-products in UV/NH₂Cl system.The degradation of 14 phenolic compounds was examined to reveal the effect of the substituted positions and the substituted number of the-OH and–COOH on the reactivity with RNS.Among the 14 selected phenolic compounds,except for the 4phenolic compounds can react with NH₂Cl in the dark,the other 10 selected phenolic compounds can be effectively removed under UV/NH₂Cl process.As the intermediates of TBA and acetate react with·OH could contribute to the degradation of target compounds,applying TBA/acetate only cannot obtain the contribution of·OH/·Cl in this study.By applying the TBA/acetate with UV/NH₂Cl,the phenolic compounds that may possibly react with RNS were selected and further determined with the UV-vis full scan results of the transformation products.Among the 10 compounds,phenol,2-HBA,4-HBA and 4-HPA were proved to react with RNS as well as·OH/·Cl,and can generate N-containing transformation products.Finally,the above results were further validated applying DFT calculation by employing the lowest activation free energy forms of each target compound.The generation potential of N-containing transformation products of phenolic compounds under the concern and need more investigation in the following research.The efficiency of UV/NH₂Cl and other UV-based AOPs should be compared in real application scenarios and under optimal operating conditions.Therefore,the optimal dose of radical precursor chemicals（RPCs）was obtained based on the primary reactive species（PRS）and energy consumption（EE/O）in engineering photoreactors,respectively.The theoretical investigations on process optimization,especially on RPC doses in the applications of UV-based AOPs were studied.A simple equation for UV/H₂O₂（·OH as the sole primary reactive species（PRS））to obtain the theoretical optimal concentration（Copt-radical）for H2O2 was derived (C_opt-radical=（（Ab·Sc）/（ε·k））^1/2).The equation was then validated for its accuracy in the calculation of C_opt-radical for H₂O₂ in the UV/H₂O₂AOP using a well-established comprehensive kinetic model.A competition kinetics method for the measurement of scavenging capacity（S_c,the unknown parameter for the simple equation）was designed,for which nitrobenzene was employed as the probe compound and tert-butyl alcohol was introduced as the standard compound.Based on this simple equation,we calculated the C_opt-radical of 77 environmental water samples and introduced the concept of a practical optimal RPC dose for the UV/H₂O₂ AOP,while minimizing the operation costs in engineering applications.Moreover,this study mathematically proved that the simple equation obtained from UV/H₂O₂ could be successfully extended to other UV-based AOPs,including UV/HOCl,UV/NH₂Cl,UV/PDS,and UV/PAA.The simple equation of C_opt-radical derived in this study may not only help to provide instructions for engineering applications,but also point out the ultimate treatment capability of each UV-based AOPs.Finally,the mathematical discussions on the kinetics and energy consumption for engineering applications of UV-based AOPs,especially the newly proposed UV-based AOPs（e.g.,UV/PDS,UV/HOCl,and UV/NH₂Cl）were conducted.The photoreactor with reflective surface was mathematically characterized and validated using clean and complex water matrices,in which the approaches for measuring effective light absorbance by RPC(I_RPC)and the reflectance of the photoreactor wall（η）were proposed.Applying the I_RPC,ηand employing geosmin as a model compound,the energy optimization of the UV-based AOPs were conducted based on an updated kinetic model.A new equation for UV/H₂O₂ to obtain the most cost-effective RPC dose with regards to EE/O(C_opt-EE/O)was derived,which was then proved to be successfully extended to other UV-based AOPs（including UV/PDS,UV/HOCl,and UV/NH₂Cl）.Moreover,the balance between the treatment goals and the operational cost was discussed in systems operated at the C_opt-radical and C_opt-EE/O.The UV fluence was then optimized together with RPC dose for designing new constructed UV-based AOPs.Overall,the new concepts and approaches proposed in this study may provide instructions for the energy optimization of UV-based AOPs for the engineering applications.In summary,OMPs can be effectively degraded in the UV/NH₂Cl system by a cocktail of radical species,including·OH,·Cl,RNS,and CO₃·^-.This study highlighted the importance of the OMP molecular structures and water matrix effect on the degradation of OMPs.The generated of RNS in UV/NH₂Cl may involve in the degradation of OMPs with phenolic group and react with DOM in the water matrix,which may bring the concern about the formation of harmful nitrogenous-disinfection byproduct.Moreover,the performance and energy consumption of UV/NH₂Cl were comparable with other UV-based AOPs.Of note,the obtained optimal dose of NH₂Cl in UV/NH₂Cl AOP was comparable with that in the drinking water treatment facilities(Cl₂,～4 mg·L^-1),suggesting an advantage of the UV/NH₂Cl AOP in the engineering applications.