The Degradation Efficiency And Mechanism Of Sulfamethoxazole In Water By Peracetic Acid Advanced Oxidation Processes Based On Co-balt And Manganese

Sulfamethoxazole（SMX）is one of the most common sulfonamide antibiotics all over the world and often used as a broad-spectrum antibiotic or feed supplement.SMX could not be metabolized completely by animals or human bodies,besides,SMX and its metabolic byproducts could not be eliminated completely through traditional wastewater treatment techniques.As a result,SMX has been detected worldwide at concentrations of ng L^-1～μg L^-1.SMX in aquatic environment would induce biotoxicity to aquatic organisms and lead to gene mutation as well as the generation of resistant genes,posing a threat to the ecosystem.Peracetic acid（PAA）-based advanced oxidation processes（AOPs）is one of the most suitable options for SMX removal in water due to its high efficiency,environmental friendliness and wide range of operating p H.In this study,cobalt doped graphitic carbon nitride（Co-CN）was synthesized via a calcination method and Fe/Co spinel sulfide modified graphitic carbon nitride composite（FeCo₂S₄-CN）as well as Mn₃O₄ were synthesized via a hydrothermal pro-cedure.Nitrilotriacetate（NTA）was used to enhance the oxidation capacity of Mn（II）/PAA.SMX was selected as the model contaminant and the degradation efficiency and mechanism of SMX in water by these AOPs were systematically investigated,including Co-CN/PAA,FeCo₂S₄-CN/PAA,Mn（II）/PAA/NTA and Mn₃O₄/PAA.The main contents are as followed:（1）The degradation efficiency and mechanism of SMX by Co-CN/PAA were system-atically investigated.It was found that Co-CN exhibited a high efficiency towards PAA ac-tivation for SMX removal.CH₃COO^·and CH₃COOO^·were the dominant reactive species for SMX degradation in Co-CN/PAA system.Based on the X-ray diffraction（XRD）,X-ray photoelectron spectroscope（XPS）and scanning electron microscope coupled with an energy diffraction spectrum（SEM-EDS）analyses of fresh and used Co-CN,cobalt was found to be distributed uniformly in the structure of graphitic carbon nitride（g-C₃N₄）through chemical bonding（Co-N）in Co-CN.≡Co（II）and≡Co（III）were the main active sites to catalyze PAA.The near-neutral p H,higher cobalt doping content in Co-CN as well as higher dosages of PAA and Co-CN were all beneficial to SMX degradation in Co-CN/PAA system.The pres-ence of HCO₃^-and fulvic acid（FA）would inhibit SMX degradation severely,while the presence of SO₄^2-,NO₃^-,Cl^-,Ca²⁺and Mg²⁺had a negligible impact on SMX removal.Co-CN showed an excellent stability and reusability on PAA activation.Moreover,four possible transformation pathways of SMX were proposed based on eight detected degradation prod-ucts of SMX,including hydroxylation,amino oxidation,bond cleavage and coupling reac-tion.Co-CN/PAA system possessed a wide applicability in degrading various organic con-taminants.Compared with other cobalt-based catalysts for PAA activation,Co-CN exhibited a higher catalytic efficiency towards PAA and a lower concentration of leached cobalt ion.（2）The degradation efficiency and mechanism of SMX by FeCo₂S₄-CN/PAA were sys-tematically investigated.It was found that FeCo₂S₄-CN could activate PAA effectively to produce CH₃COO^·,CH₃COOO^·and HO^·,exhibiting a high efficiency towards PAA activa-tion for SMX removal.Organic radicals（i.e.,CH₃COO^·and CH₃COOO^·）were the primary reactive species for SMX degradation.FeCo₂S₄ was the main active ingredient of FeCo₂S₄-CN,and there was a synergistic effect between FeCo₂S₄ and g-C₃N₄ on PAA activation.Based on the SEM-EDS,the fourier transformation infrared spectroscopy（FT-IR）,XRD and XPS analyses of fresh and used FeCo₂S₄-CN,FeCo₂S₄ was found to be combined with g-C₃N₄ through Metal-Nitrogen bond（M-N bond）.≡Co（II）,≡Co（III）,≡Fe（II）and≡Fe（III）were the main active sites to catalyze PAA in FeCo₂S₄-CN and the presence of S^2-in FeCo₂S₄-CN could promote the redox cycles of≡Co（II）/≡Co（III）and≡Fe（II）/≡Fe（III）.The near-neutral p H and higher dosages of PAA and FeCo₂S₄-CN were all beneficial to SMX degradation in FeCo₂S₄-CN/PAA system.The presence of HCO₃^-and FA would inhibit SMX degradation severely,while the presence of SO₄^2-,NO₃^-,Cl^-,Ca²⁺and Mg²⁺had a negligible impact on SMX removal.FeCo₂S₄-CN showed an excellent stability and good reusability on PAA activation.Moreover,four possible transformation pathways of SMX were proposed based on five detected degradation products of SMX,including amino oxidation,hydroxyla-tion,bond cleavage and coupling reaction.FeCo₂S₄-CN/PAA system possessed a wide ap-plicability in degrading various organic contaminants.FeCo₂S₄-CN still possessed a high efficiency towards PAA activation and the concentration of cobalt ion leached from FeCo₂S₄-CN further decreased than that of Co-CN.（3）The degradation efficiency and mechanism of SMX by NTA enhanced Mn（II）/PAA were systematically investigated.It was found that NTA could enhance the oxidizability of Mn（II）/PAA system significantly and Mn（II）/PAA/NTA system exhibited an extremely high efficiency towards PAA activation for SMX removal.Based on the results of reactive radi-cals quenching experiments,UV-vis spectra,ethanol quenching experiments and methyl phenyl sulfide（PMSO）oxidation experiments,Mn（III）-NTA complex,Mn（V）-NTA com-plex and CH₃COO^·were proved to be generated in Mn（II）/PAA/NTA system through elec-tron transfer.High-valent Mn species（Mn（V）-NTA complex）was the dominant reactive spe-cies for SMX removal in Mn（II）/PAA/NTA system.Acidic and near-neutral conditions were both favourable to SMX degradation,while the degradation of SMX was inhibited severely at alkaline condition.Higher dosage of reagents could enhance SMX degradation in Mn（II）/PAA/NTA system.HCO₃^-and FA had a negligible impact on SMX degradation,which further proved that the degradation of SMX in Mn（II）/PAA/NTA system was domi-nated by Mn（V）-NTA complex rather than reactive radicals.Moreover,four possible trans-formation pathways of SMX were proposed based on six detected degradation products of SMX,including hydroxylation,amino oxidation,bond cleavage and coupling reaction.Mn（II）/PAA/NTA system exhibited a strong selectivity towards various organic contami-nants.（4）The degradation efficiency and mechanism of SMX by Mn₃O₄/PAA were systemat-ically investigated.It was found that Mn₃O₄ exhibited a high efficiency towards PAA acti-vation for SMX removal.CH₃COO^·and CH₃COOO^·were the dominant reactive species for SMX degradation in Mn₃O₄/PAA system.Based on XRD,SEM-EDS and XPS analyses of fresh and used Mn₃O₄,≡Mn（II）,≡Mn（III）and≡Mn（IV）were proved to be the main active sites to catalyze PAA.The near-neutral p H and higher dosages of PAA and Mn₃O₄ were all beneficial to SMX degradation in Mn₃O₄/PAA system.The presence of HCO₃^-and FA would inhibit SMX degradation severely,while the presence of SO₄^2-,NO₃^-,Cl^-,Ca²⁺and Mg²⁺had a negligible impact on SMX removal.Mn₃O₄ showed an excellent stability and reusa-bility on PAA activation.Moreover,four possible transformation pathways of SMX were proposed based on six detected degradation products of SMX,including hydroxylation,amino oxidation,bond cleavage and coupling reaction.Mn₃O₄/PAA system possessed a wide applicability in degrading various organic contaminants.Compared with other manganese-based catalysts for PAA activation,Mn₃O₄ exhibited a higher catalytic efficiency.In conclusion,four AOPs,i.e.,Co-CN/PAA,FeCo₂S₄-CN/PAA,Mn（II）/PAA/NTA and Mn₃O₄/PAA all exhibited a high efficiency towards SMX removal.Besides,Co-CN/PAA,FeCo₂S₄-CN/PAA and Mn₃O₄/PAA possessed a wide applicability in degrading various or-ganic contaminants,indicating that they had a promising application prospect in practical wastewater treatment containing SMX or other organic contaminants.