In Situ Synchronous Construction Of Iron@Nitrogen-doped Carbon Nanocomposite Catalyst And Its Activation Of Peroxymonosulfate For Antibiotic Degradation

Due to the massive use of antibiotic pollutants in recent years,antibiotics are present in wastewater（livestock,medical industry,domestic wastewater）,and as emerging pollutants,antibiotics themselves are highly persistent and cannot be effectively removed from antibiotic residues by conventional wastewater treatment plants（WWTP）,making them frequently detected in natural water environments（rivers,lakes,groundwater）worldwide.Antibiotic residues were commonly detected in natural water environments（rivers,lakes,groundwater）around the world.Although the detected concentrations are mostly at ng to μg levels,the practical contamination is complex and difficult to treat due to the wide variety of antibiotics.In addition,since antibiotic residues can stay in water for a long time,they can induce the production of resistant microorganisms and resistance genes in natural environments even at low concentrations,thus posing a serious threat to ecological safety.Persulfate-based advanced oxidation techniques are widely used in the study of antibiotics degradation due to their high oxidation capacity,wide applicability and relatively strong radical half-life.Persulfate can be activated by various methods,including UV,ultrasonic,thermal,metal-free and transition metals catalysts,among which Fe-based heterogeneous catalysts have received increasing attention due to their low cost,easy recycling,wide pH applicability and avoiding the generation of iron sludge.Iron-carbon nanocomposites are promising catalysts because of their good support of iron active sites and accelerated electron conduction.The nitrogen doping technique enhances the electrical conductivity and adsorption properties of the material by introducing the smallest possible defects in the carbon material and provides more active loading sites,which leads to a significant improvement in catalytic performance.However,how to prevent agglomeration during pyrolysis and improve particle dispersion in the in situ nitrogen doping method needs to be addressed.An environmentally friendly Fe@nitrogen-doped carbon nanocomposite catalyst（Fe@NCNs）was in situ prepared via a facile and economical process using carboxymethyl chitosan（CMCs）hydrogel as a template to achieve Fe-anchoring and N-doping simultaneously for peroxymonosulfate（PMS）activation to efficiently degrade antibiotics.The main research contents and results are as follows.1.Fe³⁺ was immobilized by cross-linking sites in CMCs and was fully dispersed and immobilized by calcination under an argon atmosphere.Meanwhile,in situ nitrogen doping was achieved by using the large number of amino groups contained in CMCs itself,and iron-based nitrogen-doped carbon nanocomposites were successfully prepared.The effects of different synthesis conditions(CMCs concentration,Fe³⁺ cross-linker concentration and calcination temperature)on the components,morphology and the performance of PMS activation were compared by characterization as XRD,Raman,FTIR,SEM,TEM,XPS and electrochemical tests.The results showed that the carboxymethyl chitosan hydrogel could fully disperse and immobilize Fe³⁺ while achieving a uniform distribution of nitrogen in the composites and accomplishing efficient nitrogen doping.The material has a graphene-like structure and exhibits a "core-shell" structure,which can prevent the leaching of Fe nanoparticles by wrapping them with the graphitic carbon layer.The formation of FexNy sites and the high content of graphitic N facilitated the catalytic reaction.Fe₃C and Fe₃N were identified by XRD,while the CMCs concentration can affect the proportion of Fe₃N,Fe³⁺concentration will regulate the degree of graphitization and surface defects,and the calcination temperature can greatly affect the morphology of Fe@N-CNs.SMX degradation experiment showed that the degradation rate decreased with the increasing chitosan concentrations,while increased first and then decreased with the increasing Fe³⁺concentration as well as the increasing calcination temperature.The best catalyst was obtained under CMCs concentration of 4 wt%,Fe³⁺concentration of 0.3 mol·L^-1,and calcination temperature of 800℃,by which the complete degradation of SMX could be achieved within 10 min.The material synthesized under these conditions will be used in the subsequent experiments.In addition,electron paramagnetic resonance（EPR）tests demonstrated the presence of sulfate radicals（SO₄^-）,hydroxyl radicals（·OH）superoxide radicals（O₂^-）and singlet oxygen（¹O₂）in the system,and DFT calculations of SMX molecules indicated that 1N atoms,aniline rings and S-N bond were most likely to be oxidized.2.Based on the experimental results in the previous chapter,Fe@N-CNs,the material with the best performance,was used as the catalyst,SMX and sulfadiazine（SDZ）were selected as the target contaminants to investigate the performance and mechanism of the Fe@N-CNs activated PMS system for the degradation of SAs.The effects of different environmental conditions（system temperature,pH,coexisting anions and organic matter）on the degradation performance were investigated.The results showed that Fe@N-CNs/PMS system exhibited stable catalytic degradation efficiency of SAs in the pH range of 3.0～9.0 and under the interference of coexisting anions.The non-radical pathway dominated by ¹O₂ and complemented by the radical pathway of SO₄^-,·OH and O₂^-in the reaction system were identified by quenching experiments,combined with the EPR results in the previous section.To evaluate the environmental impact of the system,the intermediates and products of SAs during and after the reaction were identified using liquid-phase mass spectrometry（LC-MS）analysis,and the toxicity of the intermediates was further evaluated based on quantitative structure-activity relationships（QSARs）,which showed that the system greatly reduced the potential environmental hazard of both SAs in four evaluation dimensions:acute toxicity,bioaccumulation,genotoxicity,and teratogenicity.3.The applicability of the Fe@N-CNs/PMS system for the degradation of quinolone antibiotics（QNs）was further investigated.Norfloxacin（NOR）and Ofloxacin（OFL）were selected as the target contaminants to investigate the degradation efficiency,mechanism and environmental impact of the system.The effects of system temperature,pH,coexisting anions and organic matter(Cl^-,SO₄^2-,H₂PO₄^-,HCO₃^-and HA)on the degradation performance were investigated.The results showed that QNs were efficiently decomposed in a wider pH range（3.0～11.0）and at high concentrations of coexisting anions and organic matter due to their dissociative properties.The facilitation of QNs degradation under strongly alkaline conditions gave them higher degradation efficiency compared to SAs in terms of HCO₃^-interference and practical water matrix.The active species in the system was dominated by ¹O₂,with SO₄^-,·OH and O₂^-together led to the degradation of QNs.Fe@N-CNs showed excellent recyclability,which achieved high removal efficiency after five cycles without regeneration.The degradation pathways of QNs were determined by LC-MS analysis,and the results showed that QNs were more difficult to be completely degraded by Fe@N-CNs/PMS system than SAs.The environmental risk of the catalytic system was evaluated,and the results showed that after degradation by the Fe@N-CNs/PMS system,the four toxicity indicators ofNORs,namely acute toxicity,bioaccumulation factor,and developmental toxicity and teratogenicity,were reduced.However,the degradation process of OFL may produce environmentally threatening products,which need to be degraded by specific pathways to achieve its ecological risk elimination.