Efficiency And Mechanism Of Electroche-mically Activated Peroxymonosulfate For Degradation Of Sulfonamide Antibiotics

Sulfonamide antibiotics are one of the longest produced and used antibiotics in human history.Due to the strong hydrophilicity and stability,sulfonamide antibiotics are easily involved in the water cycle and accumulate in organisms,which pose huge risks to ecological security and human health.Conventional water treatment processes have limited degradation efficiency for sulfonamide antibiotics.Therefore,developing new technologies for efficient removal of sulfonamide antibiotics from water has become a research hotspot in the field of water treatment.Electrochemically activated peroxymonosulfate（EA-PMS）can couple the advantages of electrochemistry and PMS and generate various reactive species to degrade organic pollutants effectively.Three EA-PMS systems were constructed in this dissertation,i.e.EA-PMS with boron-doped diamond anode（EA-PMS-BDD）,EA-PMS with mixed metal oxide electrodes（EA-PMS-MMO/MMO）and EA-PMS with three-dimensional electrodes based on pyrite（EA-PMS-3D）,and the efficacy,mechanism,and influencing factors of these three systems in degrading sulfonamide antibiotics were investigated.Besides,the degradation rules of sulfonamide antibiotics were clarified with the combination of theoretical calculations and experimental analysis.Sulfamethoxazole（SMX）could be effectively degraded by EA-PMS-BDD,with a removal rate of 84.67%within 30 minutes.The reaction followed pseudo-first-order kinetics,with a kinetic constant of 0.0636 min^-1,which was approximately 2.5 times that of the electrolysis process and7 times that of the oxidation by PMS,indicating an excellent synergistic effect between the BDD anode and PMS.The results of quenching experiments and electron paramagnetic resonance（EPR）tests showed that HO^·was the main free radical in the system and its contribution to the degradation of SMX was over 50%.It was speculated that a special transition structure,i.e.BDD（PMS*）was formed based on the electrochemical characterization,which was conducive to the activation of PMS and the generation of free radicals.The removal rate of SMX in the system increased with the increasing current density and the concentration of PMS,while the initial p H（3-11）of the solution had little effect on the removal rate of SMX.It was revealed by response surface analysis that Cl^-in water could promote the degradation of SMX,while the effect of SO₄^2-was inhibition first and then promotion.Besides,NO₃^-and natural organic matter（NOM）inhibited the degradation of SMX.A predictive model for the degradation of SMX under the coexistence of three anions and NOM was established based on these findings.EA-PMS-BDD also demonstrated good efficacy on the degradation of SMX in actual water bodies.Furthermore,the removal rates of emerging contaminates such as carbamazepine,bisphenol S,propranolol,and ibuprofen were all above 80%,indicating the broad-spectrum efficacy of the system towards emerging contaminates.Experimental results show that EA-PMS-MMO/MMO could effectively remove sulfadimidine（SDZ）from water,with a removal rate of over 80%in 30 minutes.PMS could be effectively activated by both the mixed metal oxide（MMO）anode and cathode.PMS was activated on the anode by an electric field.Besides,dissolved oxygen on the cathode gained electrons to generate O₂^·-,which then reacted with PMS to activate it.The main reactive species in the system included HO^·,SO₄^·-,O₂^·-,and ¹O₂,and ¹O₂ was the most abundant among them.The generation pathways of ¹O₂ included anodic activation of PMS,oxygen excitation,and reaction between PMS and OH^-.Increasing the current density and the concentration of PMS could accelerate the degradation of SDZ,but increasing the current density would increase energy consumption and reduce current efficiency.PMS could react with OH^-to generate ¹O₂,so alkaline conditions were more conducive to the degradation of SDZ.Ecological toxicity of the degradation products of SDZ was calculated based on the ecological structure-activity relationship（ECOSAR）,and it was found that the acute and chronic ecological toxicity of most degradation products of SDZ were lower than those of SDZ,indicating that EA-PMS-MMO/MMO could reduce the ecological toxicity of water contaminated by SDZ.Pyrite was introduced into EA-PMS-MMO/MMO as the third electrode to construct the EA-PMS-3D system.The system was used to degrade sulfamethazine（SMZ）and the removal rate of SMZ reached 98.74%within 30 minutes,which was 27.08%higher than that of EA-PMS-MMO/MMO,indicating that the introduction of pyrite significantly enhanced the degradation of SMZ.Pyrite could react with PMS and dissolved oxygen to release iron elements into the solution,and the Fe³⁺/Fe²⁺cycle was then constructed to enhance the activation of PMS in the solution.In addition,pyrite could be polarized to form micro-batteries,which increased the effective area of the electrodes,shortened the diffusion path,and enhanced the activation of PMS on the electrodes.The results of quenching experiments,EPR tests,and probe experiments showed that there were multiple reactive species such as HO^·,SO₄^·-,¹O₂,O₂^·-,and Fe（IV）coexisting in EA-PMS-3D.Increasing the dosage of pyrite and the concentration of PMS,and decreasing the p H of the solution could enhance the degradation of SMZ,while the change in current density(1-15 m A cm^-2)had little effect on the degradation efficiency of SMZ.In addition,pyrite had good stability and reusability.A comparison of the effectiveness and economic viability of three systems for the degradation of sulfonamide antibiotics was conducted.All these three systems exhibited good effectiveness and broad-spectrum activity against sulfonamide antibiotics,and EA-PMS-3D system performs the best among them.The degradation efficiency of sulfonamide antibiotics was positively correlated with their global hardness（η）and energy gap（ΔE）.The larger the values ofηandΔE,the lower the degradation efficiency.According to theoretical calculations based on density functional theory（DFT）and experimental analyses,the degradation pathway of sulfonamide antibiotics included nitration of amino on the benzene ring,hydroxylation of phenyl,and cleavage of sulfonamide,as well as“Smile”rearrangement for sulfadiazines.The EA-PMS system is an efficient and clean new technology for water treatment,which provides a new idea for controlling and removing sulfonamide antibiotics in water.