Sulfamethoxazole Removal From Water By Peracetic Acid Through Diverse Iron Species As Activators

The antibiotics are essential emerging contaminants due to their massive existence in aquatic environments,which have posed a potential threat to human health and aquatic ecology.Sulfamethoxazole（SMX）,a widely used synthetic drug with broad spectrum bacteriostatic activity,has been considered as a persistent pollutant due to its inefficient removal during conventional wastewater treatment and self-purification of water.SMX causes the generation of resistant bacteria and genes,and accumulates in the biological bodies.Therefore,an economical technology that can efficiently remove SMX from wastewater would be highly desirable.In recent years,the emerging advanced oxidation technology based on peracetic acid（PAA）has attracted extensive attention of researches because of its excellent disinfection activity,non-carcinogenic and non-mutagenic by-products during application and adaptation to wide p H conditions.However,the relevant systematic research and its application in SMX removal from water are limited.The degradation efficiency and transformation pathway of SMX in PAA based advanced oxidation system need further exploration.In this paper,SMX is selected as the target contaminant,and four systems(i.e.,Fe²⁺/PAA,ascorbic acid（AA）/Fe³⁺/PAA,Fe²⁺-zeolite/PAA and Fe Cu modified coal gangue（Fe Cu-CG）/PAA)are used to remove SMX from contaminated wastewater.The mian contents are as followed:（1）The activation of an emerging radical precursor（PAA）by Fe²⁺and its efficiency,influencing factors and mechanism for SMX removal were systematically studied.The results showed that Fe²⁺ions could efficiently activate PAA,which led to the complete conversion of PAA to several reactive radicals（including HO^·,CH₃COO^·and CH₃COOO^·）within 5 s.HO^·was the main radical in the system,and organic radicals also contributed to the degradation of SMX.However,the SMX was hardly removed through the non-radical pathway.Degradation of SMX in Fe²⁺/PAA system could be divided into two stages,included rapid reaction stage dominated by activated PAA and subsequent reaction stage dominated by Fenton reaction.Fe²⁺/PAA system could effectively remove SMX in a wide initial p H range from 3.0 to 9.0.The increase of PAA and Fe²⁺dosage improved the SMX degradation,but excess PAA(>75μmol L^-1)and Fe²⁺(>600μmol L^-1)were ineffective to achieve a higher SMX removal.The cycle of Fe²⁺and Fe³⁺was observed in the Fe²⁺/PAA system,but PAA was mainly activated by Fe²⁺to form CH₃COO^·.The co-existing HCO₃^-and Cl^-inhibited the SMX removal by Fe²⁺/PAA,while NO₃^-,SO₄^2-,Ca²⁺,Mg²⁺and natural organic matter（NOM）had little effect on the degradation of SMX.Based on the density functional theory and mass spectrometry,the degradation pathway of SMX in Fe²⁺/PAA system was proposed including hydroxylation,bond cleavage,amino oxidation and ring opening.（2）Ascorbic acid（AA）,a natural reductant widely exists in natural water,was used to strengthen the activity of Fe species toward PAA.The removal efficiency,influencing factors and degradation mechanism of SMX by AA/Fe³⁺/PAA system were systematically investigated.The results showed that PAA could be completely activated within 5 s and formed several reactive radicals（including HO^·,CH₃COO^·and CH₃COOO^·）.The SMX removal was mainly attributed to the oxidization of HO^·and organic radicals.Compared with homogeneous Fe²⁺/PAA system,AA could enhance the transformation of Fe³⁺to Fe²⁺and exhibited solubilization effect on Fe species.Therefore,the AA/Fe³⁺/PAA system showed higher efficiency in SMX removal at a lower Fe dosage.AA/Fe³⁺/PAA system possessed a wide p H range adapting property（3.0～9.0）.The increasing PAA and Fe³⁺dosage improved the SMX degradation,but excess PAA(>75μmol L^-1)and Fe³⁺(>200μmol L^-1)were ineffective to achieve a higher SMX removal.The increasing AA dosage in the range of 0-150μmol L^-1enhanced the PAA activation and the SMX degradation,but excess AA inhibited the SMX degradation due to the its quenching effect on radicals and chelation for Fe（II）.The loss of total Fe from liquid phase in AA/Fe³⁺/PAA system was only 18.3μmol L^-1 within 10 min.The co-existing HCO₃^-and Cl^-inhibited the degradation of SMX,while NO₃^-,SO₄^2-,Ca²⁺,Mg²⁺and NOM had little effect on SMX removal.The degradation pathway of SMX in AA/Fe³⁺/PAA system included hydroxylation,bond cleavage and amino oxidation.（3）A novel Fe²⁺-zeolite material with abundant active sites was prepared through a simple impregnation method,and the Fe²⁺-zeolite/PAA system was developed for the degradation of SMX.The degradation efficiency,influencing factors and reaction pathways of SMX in this system were investigated.The results showed that the newly prepared Fe²⁺-zeolite could effectively activate PAA to produce various radicals.The HO^·was the predominant reactive oxygen species,and the organic radicals also contributed to the SMX removal.The non-radical pathway was not the main pathway for SMX degradation.The degradation of SMX in Fe²⁺-zeolite/PAA system followed the pseudo first-order kinetic model.SMX could be efficiently removed under the optimum initial p H condition（7.0）,while acidic or alkaline environment inhibited its degradation.When the PAA and Fe²⁺-zeolite doses increased,the degradation efficiency of SMX was gradually enhanced.Excessive Fe²⁺-zeolite(>0.8 g L^-1)led to the decrease of SMX removal due to the aggregation and sedimentation of Fe²⁺-zeolite at high doses.The presence of NO₃^-,SO₄^2-,Ca²⁺and Mg²⁺had almost no influence on SMX removal,while the NOM,Cl^-and low concentration of CO₃^2-showed obvious negative effect on SMX degradation.After three times of recycling,Fe²⁺-zeolite maintained satisfactory activity for PAA activation and achieved 71.9%SMX removal under the optimum conditions.A possible SMX degradation mechanism was proposed including hydroxylation,bond cleavage,amino oxidation and self-coupling.（4）A new Fe Cu modified CG composite（Fe Cu-CG）with highly exposed sites and easy mass transfer was successfully prepared and used to activate PAA for SMX degradation.The Fe Cu-CG material was characterized,and the degradation efficiency,influencing factors and degradation mechanism of SMX by Fe Cu-CG/PAA system were systematically discussed.The results showed that the newly prepared Fe Cu-CG had porous structure,which loaded with abundant Fe Cu microspheres.The PAA could be activated by accepting electrons and reducing by Fe（II）,which were generated during the Fe Cu bimetallic corrosion reaction.HO^·was the main radical in the system,but organic radicals also contributed significantly to the degradation of SMX.Non-radical pathway had little contribution to SMX removal.The degradation of SMX in Fe Cu-CG/PAA system could be described by pseudo first-order kinetic model.The efficient removal of SMX could be achieved in the wide p H range of 5.0-9.0.The increase of PAA and Fe Cu-CG doses could improve the degradation efficiency of SMX,but excessive Fe Cu-CG(>1.0 g L^-1)competed with SMX for radicals.The HCO₃^-,Cl^-and NOM inhibited the degradation of SMX due to their quenching effect on reactive radicals,while NO₃^-,SO₄^2-,Ca²⁺and Mg²⁺showed little influence on the SMX removal.Trace co-existing Fe³⁺in water may be involved in PAA activation and showed synergistic effect with Fe Cu-CG.After three times of recycling,Fe Cu-CG maintained satisfactory activity for PAA activation and achieved 71.0%SMX removal under optimum conditions.The degradation pathways of SMX in Fe Cu-CG/PAA system included hydroxylation,bond cleavage,amino oxidation and self-coupling.