Electrochemical LED-UV/Chlorine Process For The Degradation Of Contaminants Of Emerging Concern In Wastewater:Kinetics And Mechanisms

Contaminants of emerging concern(CECs),biologically toxic and difficult to degrade in wastewater,pose a great threat to ecological and human health,which are difficult to be removed by the traditional water treatment technologies.Novel and effective technologies need to be developed to effectively degrade CECs in wastewater.UV/Chlorine can generate hydroxyl radicals(·OH)and active chlorine radicals(RCS),which can degrade CECs effectively.However,the need of transporting and managing liquid chlorine limits the wide application of the UV/Chlorine process,and the continuous consumption of chlorine leads to a decrease in the concentration of active substances,which cannot continue to effectively degrade CECs.The in-situ generated chlorine on the electrochemical anode(EC/Cl2)technology can effectively use Cl-in wastewater to produce free chlorine and RCS.The concentration of free chlorine can be controlled by electrochemical processes,which selectively remove CECs.Therefore,this study developed an innovative UV275nm-EC/Cl2 process,combining LED-UV275nm irradiation and EC/Cl2,which efficiently generated and utilized ’OH and RCS with a controlled concentration of free chlorine.This study investigated the generation mechanisms of free radicals and the degradation kinetics and pathways of CECs in UV275nm-EC/Cl2.This thesis firstly studied the degradation kinetics of CECs in the EC/Cl2 and UV27snm-EC/Cl2 processes.In the UV275nm-EC/Cl2 process,Cl-can be oxidized at the RuO2-IrO2/Ti anode to in-situ generate Cl’ and subsequently produce other RCS and free chlorine,which can be activated by UV275nm to produce a large amount of ·OH and RCS.The degradation rates and observed first-order rate constants of atrazine(ATZ),ibuprofen(IBU),carbamazepine(CBZ),estradiol(E2),diclofenac(DCF),and levofloxacin(LVX)were compared in the UV275nm,EC/Cl2,and UV275nm-EC/Cl2 progresses.The observed first-order rate constants of degradation of the six CECs in the UV275nm-EC/Cl2 process are generally high,which are 2.6～171 times higher than that in the UV275nm process,and 1.6～1.7 times higher than that in the EC/Cl2 process.The contributions of UV275nm,EC,free chlorine,·OH,and RCS to the CEC degradation by the UV275nm-EC/Cl2 system were analyzed using modeling:1)ATZ and IBU were mainly degraded by non-selective ·OH,contributing 50%and 70%to the CEC degradation,respectively;CBZ,E2,DCF,and LVX were mainly degraded by selective Cl·,contributing 56%,73%,51%and 94%to the CEC degradation,respectively;3)The contribution of UV275nm to the degradation of DCF was 33%.Nitrobenzene and HCO3-were used as quenching agents to analyze the effects of ·OH and RCS in the UV275nm-EC/Cl2 system on the degradation of ATZ with high Co.Besides,the steady-state concentration of ’OH in the UV275nm-EC/Cl2 system with 20μM ATZ(1.09 × 10-14 M)was much lower than that with 1 μM ATZ(4.33 × 10-14 M).The transformation products(TPs)of ATZ,IBU,CBZ,DCF,and LVX in the UV275nm-EC/Cl2 and EC/Cl2 systems were analyzed by Q-TOF-MS/LC,which were used to propose the possible degradation pathways of CECs.The degradation of CECs in the UV275nm-EC/Cl2 process had the following characteristics:1)·OH and Cl· could both react with organic pollutants through electron transfer,dehydrogenation and addition pathways;2)n-hydroxylation and carbonylation initiated by ’OH and Cl’ were the main pathways for the degradation of all CECs;3)decarboxylation was an important degradation pathway for CECs(IBU,DCF,and LVX)containing carboxyl structure;4)heteroatoms(N,Cl)affected the dechlorination of ATZ,the ring-opening of LVX,the ring-closing of DCF and the ring reorganization of CBZ during the oxidation process;and 5)UV275nm contributed to the breaking of C-Cl bond in the ATZ structure and the ring-closing of DCF.The difference in the generation mechanisms,concentrations of free radicals,and the presence of UV275nm led to different TPs of CECs in the EC/Cl2 and UV275nm-EC/Cl2 processes.UV275nm-EC/Cl2 can effectively use Cl-widely presenting in wastewater to generate high concentration ·OH and RCS in situ,which can effectively remove CECs.The technology,which can also be controlled by the electrochemical process with excellent electrical efficiency,has a wide application prospect in the degradation of CECs in the future.