The Mechanism Of Sulfadiazine Degradation By A Two-stage Electrofenton System Based On An Air-breathing Cathode With Multiple Hydrophobic Layers

The massive use of antibiotics has caused pollution of the water environment and endangered human health,which urgently needs to be dealt with effectively.Advanced oxidation is one of the effective methods for treating antibiotics.Among many advanced oxidation methods,electro-Fenton technology has attracted much attention because of the advantages of low agent dosing,green and easy operation.However,at present,electro-Fenton still suffers from low H₂O₂ production,insufficient antibiotic degradation efficiency and poor system durability,and the degradation mechanism of antibiotics in electro-Fenton system needs to be investigated in depth.Based on this,a multiple hydrophobic layers air-breathing cathode（MABC）was fabricated to increase the H₂O₂ production by increasing the number of three-phase reaction interfaces and the oxygen transfer rate of the electrode.Meanwhile,a homogeneous electro-Fenton system with MABC as the cathode was constructed to elucidate the degradation mechanism of the target pollutant sulfadiazine（SDZ）by combining the results of active species exploration,mass spectrometry analysis and theoretical calculations.Finally,the Fe⁰-Fe₃O₄/CF heterogeneous electro-Fenton cathode was prepared to construct a two-stage electro-Fenton system based on MABC and Fe⁰-Fe₃O₄/CF,which substantially improved antibiotic degradation efficiency and system durability.The study provides a theoretical basis for the popularization and application of electro-Fenton technology.The MABC was prepared by vacuum filtration.The electrode can spontaneously breathe the oxygen in the air for oxygen reduction reaction by the continuous hydrophobic layer and interrupted hydrophobic layer,eliminating the aeration device in the conventional electro-Fenton,and the energy consumption is only 1/14 of the conventional electro-Fenton reaction.The oxygen transfer coefficient of MABC_0.6（0.6 is PTFE doping ratio）is about 5.79×10^-6 m² s^-1,which is about 29%of the oxygen transfer coefficient in air and 3 orders of magnitude higher than the oxygen transfer coefficient of dissolved oxygen in water.The electrochemical impedance test（EIS）results showed that the low dissolved oxygen concentration in the solution limited the oxygen reduction reaction at the virgin carbon felt（V-CF）,while MABC_0.6 could spontaneously obtain sufficient oxygen for the reaction and oxygen transfer was no longer a limiting factor for the reaction.Due to the significant increase in oxygen transfer rate,the H₂O₂ accumulation of MABC_0.6 can reach 44.30 mg L^-1 cm^-2 h^-1under optimal conditions,which is 19 times higher than the H₂O₂ yield of V-CF under the same conditions,and the energy consumption is only 9.69 k W h kg^-1,which is4.08%of the energy consumption of V-CF under the same conditions.The MABC_0.6have good stability,with less than 5%decrease in electrode performance in 10replicate tests,and a significant increase in H₂O₂ production compared to the same type cathodes in other studies.A homogeneous electro-Fenton system was constructed with MABC_0.6 as the cathode to investigate the degradation mechanism of SDZ under the electro-Fenton system.Due to the ability of MABC_0.6 to produce H₂O₂ efficiently,the degradation and removal rates of SDZ at 60 min reached 91.81%and 68.78%,respectively,under optimal conditions(pH=3,Fe²⁺=300μM).The kinds of active species in cathode reaction at different pH and their contribution to SDZ degradation were investigated.Electron spin resonance（ESR）and probe tests demonstrated that four reactive groups·OH,¹O₂,·O₂^-and Fe（IV）were generated in the cathodic reaction,among which·OH was the most important active group at different pH,and ¹O₂ was the secondary reactive group can effectively degrade SDZ only at pH=3.Little SDZ was degraded by·O₂^-and Fe（IV）under different pH,so they are not the main active groups.The results of mass spectrometry analysis showed that SDZ mainly had four degradation pathways,namely oxidation of benzene ring,oxidation of amino group,smile rearrangement of SDZ,oxidation and ring opening of pyrimidine ring.The degradation process of SDZ was further analyzed by theoretical calculation.In the primary reaction of SDZ,the addition reactions of C1 and C5 sites accounted for 99.5%and 72.67%,which were the strongest active sites.Moreover,the formation process of key intermediate products TP267,TP281,TP187,TP243 and TP215 was further analyzed by means of thermodynamics,kinetics,charge analysis and spin density analysis to clarify the degradation mechanism of SDZ in the electro-Fenton system.The results of toxicity analysis showed that most of the products dropped to a lower concentration after the reaction,or even disappeared completely,and the biological toxicity of SDZ and its degradation products was effectively reduced.Fe⁰-Fe₃O₄/CF cathode was prepared by K₂Fe O₄ etching and high-temperature calcination.A two-stage electro-Fenton system was constructed based on MABC_0.6and Fe⁰-Fe₃O₄/CF to solve the problems of insufficient SDZ degradation efficiency and poor system durability.Through the characterization of Fe⁰-Fe₃O₄/CF,the pores were successfully etched in the carbon felt fibers and Fe⁰ and Fe₃O₄ were uniformly loaded in the pores of the carbon felt fibers.Fe（II）and Fe⁰ on CF accounted for 60%of the total iron,which is conducive to the efficient catalysis of H₂O₂ to generate·OH.The two-stage electro-Fenton system is divided into high concentration H₂O₂accumulation stage and H₂O₂ efficient catalysis stage.The first stage uses MABC_0.6as the cathode to run for 30 min to meet the requirements for H₂O₂ concentration in the second stage.The second stage replaces MABC_0.6 with Fe⁰-Fe₃O₄/CF cathode for efficient catalytic generation of·OH from H₂O₂.At pH=3,when the current density was 4 m A cm^-2,the degradation rate and mineralization rate of SDZ were 100%and80.42%,respectively,which significantly improved the degradation efficiency of SDZ compared to the homogeneous electro-Fenton system based on MABC_0.6.Due to the protection and reduction effect of cathodic current,the total iron concentration in the solution was only 1.67 mg L^-1,and the proportion of Fe（II）on Fe⁰-Fe₃O₄/CF was still 55.12%after the reaction,compared with that without cathodic current,the proportion of Fe（II）increased by 19.79%.According to the tests and calculations,compared with other electro-Fenton systems,the two-stage electro-Fenton reactor has obvious advantages in terms of SDZ degradation efficiency,system durability,energy consumption and reduce biological toxicity,which can provides a reference for the popularization and use of electro-Fenton technology.In summary,the study constructed a two-stage electro-Fenton system based on MABC and Fe⁰-Fe₃O₄/CF cathodes,which solves the problems of low H₂O₂production,insufficient antibiotic degradation efficiency and poor system durability.At the same time,the degradation mechanism of SDZ in the electro-Fenton system was clarified by means of active group exploration,mass spectrometry analysis and theoretical calculation.The research results are expected to provide a reference for the practical use of electro-Fenton technology and provide a comprehensive and feasible method for in-depth exploration of the degradation mechanism of pollutants.