Efficiency And Mechanism Of Sulfamethoxazole Degradation In Transition Metal-Activated Peracetic Acid System

China is the largest producer and consumer of antibiotics in the world.Various antibiotics were frequently detected in water due to their extensive use and improper disposal,which poses threats to the aquatic environment and human health.As antibiotics cannot be effectively removed via conventional water treatment,it is necessary to find new strategies to get rid of them.Peracetic acid-based advanced oxidation processes（PAA-based AOPs）possess potential to degrade antibiotics due to the generation of reactive species.In this study,activation of PAA by transition metals was developed to degrade sulfamethoxazole（SMX）and the related mechanism was evaluated.The main research contents and results are as follows:Activation of PAA by cobalt ion（Co（Ⅱ））was developed to degrade SMX.The influence factors of SMX degradation such as Co（Ⅱ）dose,PAA dose and initial pH were evaluated,and the mechanism of SMX degradation was investigated.Results showed that the Co（Ⅱ）/PAA process can effectively degrade SMX in water;89.4%of SMX was removed after 15 min reaction with the initial dosage of Co（Ⅱ）and PAA being 0.8μM and 0.1 m M,respectively.In the Co（Ⅱ）/PAA process,Co（Ⅱ）activating PAA produced CH₃C（O）O^●along with Co（Ⅱ）being converted to Co（Ⅲ）;the formed Co（Ⅲ）was reduced to Co（Ⅱ）by PAA with generating CH₃C（O）OO^●accordingly.Thus,Co（Ⅱ）/Co（Ⅲ）cycle was achieved while producing reactive radicals.Other reactive species including ¹O₂,HO^●,CH₃^●,and CH₃OO^●were also generated in this system.CH₃^●,CH₃OO^●,CH₃C（O）O^●and CH₃C（O）OO^●were viewed as organic radicals（RO^●）,and only CH₃C（O）O^●and CH₃C（O）OO^●were responsible for the SMX degradation.The optimum reaction p H was 7,and the increased dosages of PAA and Co（Ⅱ）promoted SMX degradation.During the degradation of SMX,the benzene ring,amino group and S-N bond of SMX are vulnerable to be attacked,and coupling reaction also occurred.The acute toxicity of SMX during degradation was not changed;the treated SMX solution could promote the growth of bacteria and algae.After determining the role of CH₃C（O）O^●and CH₃C（O）OO^●for SMX removal,a novel PAA-based AOP was developed using Mo S₂ as an activator.The influence factors of SMX degradation such as Mo S₂ dose,PAA dose and initial p H were evaluated,and reactive species in the Mo S₂/PAA process were identified to make clear the mechanism.Results illustrated that SMX can be effectively degraded in the Mo S₂/PAA process.76.1%of SMX was removed after reacting 15 min with the initial dosage of Mo S₂ and PAA being 0.3 g/L and 0.3 m M,respectively.Mo（IV）donated electrons and converted to Mo（VI）,and PAA was activated by Mo S₂ to produce CH₃C（O）O^●and HO^●simultaneously.¹O₂,CH₃^●,CH₃OO^●and CH₃C（O）OO^●were also generated in the Mo S₂/PAA process.HO^●,CH₃C（O）O^●and CH₃C（O）OO^●were critical radicals for SMX degradation.Elevating the initial dosages of PAA and Mo S₂ facilitated SMX degradation,and the optimum reaction p H was 3（p H 3～11）.The amino groups of sulfonamides can be oxidized to nitro groups in this system;the Mo S₂/PAA process also induced SO₂ extrusion reaction for sulfonamides containing six-membered heterocyclic moieties.To further improve the degradation efficiency of SMX,Fe（Ⅲ）was added to the Mo S₂/PAA system.Various factors such as the initial p H and the concentrations of Fe（Ⅲ）,Mo S₂ and PAA were studied to reveal the mechanism of SMX degradation.When the dosage of Fe（Ⅲ）,Mo S₂ and PAA were 0.1 m M,0.1 g/L and 0.3 m M,respectively,the removal rate of SMX reached 96.8%after 10 min reaction,which was higher than that（<40%）in the Mo S₂/PAA process.In the Fe（Ⅲ）/Mo S₂/PAA process,Fe（Ⅲ）was reduced to Fe（Ⅱ）by Mo S₂;the formed Fe（Ⅱ）further reacted with PAA and generated Fe（IV）,HO^●and CH₃C（O）O^●.¹O₂,CH₃^●,CH₃OO^●and CH₃C（O）OO^●also existed in this system.Among these reactive species,Fe（IV）,HO^●,CH₃C（O）O^●and CH₃C（O）OO^●played important roles for the SMX degradation.The optimum reaction p H was 3（p H 3～9）,and the removal efficiencies of SMX were positively correlated with the dosages of Fe（Ⅲ）,Mo S₂ and PAA.The main degradation pathways of SMX in this system include S-N bond breaking,amino group oxidation and hydroxylation.Finally,the developed PAA-based AOPs（Co（Ⅱ）/PAA,Mo S₂/PAA and Fe（Ⅲ）/Mo S₂/PAA）were employed to treat SMX under the background of real water（secondary effluent of WWTPs）;the removal efficiencies of the residual pharmaceuticals and organic matters with fluorescence characteristics in these processes were also investigated.When the dosage of PAA was 1 m M,more than 90%of SMX was removed after reaction in these systems.33 kinds of pharmaceuticals were detected in the effluent using HPLC-HRMS.There are 17 kinds of pharmaceuticals with removal rates over 50%in Co（Ⅱ）/PAA process;the number of these pharmaceuticals with removal rates over 50%reached 31 in Mo S₂/PAA and Fe（Ⅲ）/Mo S₂/PAA processes.Dissolved organic matters with fluorescence characteristics can also be degraded,but no obvious variations of the dissolved nitrogen and phosphorus were achieved during the reaction.Overall,PAA-based AOPs activated by transition metals are efficient in degrading sulfonamides antibiotics and have tremendous potential for organic pollutants treatment.