Characteristics And Mechanisms Of Sulfadiazine Degradation By Emerging Oxidants Activation With Sulfide-modified Zero Valent Iron

Sulfadiazine（SDZ）is a kind of sulfonamide antibiotics,which has low cost and stable effect,and is often used in human health treatment,livestock,poultry and aquiculture.However,excessive use of antibiotics can lead to immune dysfunction and increase the viral infection.In addition,the abuse of antibiotics will induce resistance genes,which pose a big threat to the ecological environment.Zero valent iron（Fe⁰）is safe,cheap,and strong reducing,so it can be used as an iron source to participate in advanced oxidation technologies（AOTs）to treat antibiotic wastewater.Compared with Fe²⁺directly added in traditional Fenton reaction,Fe⁰ is convenient to recycle and reuse,has strong reductivity,and can reduce Fe³⁺to Fe²⁺in time.Sulfide-modified zero valent iron（S-Fe⁰）can overcome the defects of Fe⁰,poor electron selectivity and easy passivation.In recent years,the research on the participation of S-Fe⁰ in AOTs has attracted widespread attention.S-Fe⁰ is an emerging iron-based functional material with Fe⁰ core and Fe S_X shell,which has a controllable synthesis method,strong electron transfer capacity,high reactivity and selectivity.S-Fe⁰ can activate a variety of oxidants and generate strong oxidizing reactive oxygen species（ROS）to degrade organic pollutants.At present,there are few reports on the relationship between sulfidation parameters and active radical species and their roles in degradation,as well as the mechanism of enhanced activation with different sulfur species.Therefore,in this paper,liquid impregnation method was used to modify microscale zero valent iron,and two catalytic oxidation systems,S-Fe⁰/peroxymonosulfate（PMS）and S-Fe⁰/periodate（PI）were constructed.The effects of different sulfidation parameters,dosage,initial p H and coexisting anions were systematically investigated to reveal the mechanisms of enhanced activation by sulfide modification in two different systems.The feasibility of their application in antibiotic wastewater treatment was also evaluated.For the S-Fe⁰/PMS/SDZ system,sulfide-modified zero valent iron with different molar ratio of sulfur to iron was optimally prepared.The optimal molar ratio in S-Fe⁰/PMS system was 1:18.The specific surface area of the modified material increased from 3.210 m²/g of original Fe⁰ to10.069 m²/g,and the contact angle increased from 35.60°to 68.19°.The Tafel curve showed that S-Fe⁰ had a lower corrosion potential,and the XPS fitting analysis showed that sulfur species mainly existed in the form of S^2-（22.98%）and SO₃^2-（42.55%）on the surface.The optimal reaction conditions were 0.05 g/L S-Fe⁰,0.5 m M PMS,and initial p H=5.4.99.4%of SDZ could be removed after 60 min with a initial concentration of 0.08 m M,and the reaction rate constant k value was as high as 0.296 min^-1.The higher the S-Fe⁰ dosage,the higher the SDZ removal rate and k value,and the sulfide modification could save 87.5%of the catalyst dosage.The S-Fe⁰/PMS system had a wide p H application range,with the removal rate of SDZ reaching more than 97.0%in the range of 3 to 9.The coexistence of 1～10 m M Cl^-,SO₄^2-and NO₃^-has negligible effect on the removal of SDZ,while HCO₃^-inhibited obviously and reduced the removal rate to 41.2%.The optimal amount of sulfur doping varied for different particle sizes of Fe⁰,the smaller the particle size,the stronger the surface effect,and the activity of atoms on the particle surface was high but unstable,so more sulfur source was needed for activation enhancement.The optimal molar ratio of sulfur to iron were 1:15,1:18 and 1:30 for 1μm,8μm and 100μm,respectively.The k values were increased to49.5,8.9 and 1.5 times,respectively.Quenching experiments and electron paramagnetic resonance（EPR）results confirmed that the dominant mechanism of enhanced activation by sulfide modification was:the surface S^2-promoted the reduction of Fe³⁺to Fe²⁺and the continuous activation of PMS,SO₃^2-reacted with Fe³⁺to produce SO₄^·-.The different sulfidation parameters did not change the mechanism,ROS that played the main role in the S-Fe⁰/PMS system were·OH and SO₄^·-.The concentration of SO₄^·-in the system increased by 27.56μM after sulfide modification,and the contribution to the SDZ degradation increased by 8.8%.For the S-Fe⁰/PI/SDZ system,sulfide-modified zero valent iron with different molar ratio of sulfur to iron was optimally prepared.The optimal molar ratio in S-Fe⁰/PI was 1:30.Electrochemical impedance spectroscopy（EIS）showed that the resistance of S-Fe⁰ was smaller than that of Fe⁰,and the electron conductivity was enhanced.The optimal reaction conditions were0.1 g/L S-Fe⁰,1.0 m M PI,and initial p H=3.0.97.4%of SDZ could be removed after 5 min with a initial concentration of 0.06 m M,and the reaction rate constant k value was as high as 3.161 min^-1.TOC removal rate was 71.5%.Compared with the Fe⁰/PI system,the S-Fe⁰/PI system could save95%of Fe⁰ dosage and 90%of PI dosage.The S-Fe⁰/PI system showed better catalytic oxidation under acidic conditions,and had excellent resistance to inorganic anion interference.When Cl^-,SO₄^2-,NO₃^-and HCO₃^-coexisted for 10 m M,the SDZ removal rate was still above 94%.S-Fe⁰had good recycling performance,and the removal rate of SDZ was still 80.2%after 7 times of use.Quenching experiments,EPR and density functional theoretical（DFT）calculations results confirmed the mechanism of enhanced activation by sulfide modification as follows:the Fe（II）site of Fe S formed by sulfide modification had higher adsorption affinity for PI than that of bare Fe⁰,sulfidation accelerated the electron transfer from the inner core of Fe⁰ to the outside,and rapidly activated PI adsorbed on the material to generate the active substances such as·OH,O₂^·-,¹O₂,Fe（IV）,IO₃^·-and SO₄^·-,among which·OH made the largest contribution to the SDZ degradation.The dominant routine of·OH production was PI→O₂^·-→H₂O₂→·OH.Surface S^2-and SO₃^2-reacted with PI and Fe⁰ respectively to generate SO₄^·-,which accelerated the oxidative degradation of SDZ.Mass spectrometry（LC-MS）results and DFT calculations confirmed the SDZ degradation pathway and mechanism,the S-Fe⁰/PI system generated various radicals with strong oxidizing properties,which attacked C-N bond of aniline group,S-N bond of sulfonamide group and C-N bond on pyrimidine ring of SDZ.The degradation process included hydroxylation,bond cleavage and ring opening reactions.The SDZ was degraded to less toxic substances,and part of it was mineralized to carbon dioxide and water.In summary,this study systematically investigated the performance and mechanisms of PMS and PI activation by S-Fe⁰ to degrade SDZ.The research results could provide theoretical guidance for the enhanced activation of emerging oxidants by modified zero valent iron and technical support for the practical application of S-Fe⁰ in the treatment of sulfonamide antibiotic wastewater.