Study On The Preparation Of Carbon Nanotubes Composite Electrodes And Electrochemical Degradation Of Cephalosporin Antibiotics In Wastewater

The electrocatalytic oxidation(EO)technology has attracted wide attention for its advantages of simple device,strong oxidation ability and environmental friendliness.Carbon nanocomposites,which combines the characteristics of strong catalysis of metal nanoparticles and large specific surface area of carbon nanomaterials,have broad application prospects in the field of treating organic pollutants in water.Carbon nanotubes(CNTs)have unique one-dimensional structure and physicochemical properties.The supported metal particles exhibit good dispersion and have higher electrocatalytic activity than traditional planar electrodes.However,the metal-nanoparticles/CNTs composite electrode has poor stability under hydraulic erosion,and the carbonaceous anode are easy to be oxidized to cause collapse and exfoliation.Therefore,this study is devoted to the preparation of CNT composite electrodes with longer serve lifespan and higher catalytic activity,as well as the promoted degradation efficiency of cephalosporins catalyzed by coupled cathodic electro-Fenton(EF)and sulfate radical catalytic process(PS).Firstly,the chemically stable TiO2 and Al2O3 were codoped with catalytic active CeO2 and Sn02 metals,preparing CeO2-ZrO2/TiO2/CNT and SnO2-Al2O3/CNT electrodes by solvothermal method.The surface morphologies and elemental composition of the novel electrodes were characterized by scanning electron microscopy(SEM)and X-ray photoelectron spectroscopy(XPS).The electrocatalytic activities were significantly improved evidenced by electrochemical analysis.The application of modified electrodes in the electrocatalytic degradation experiments shows that the removal rates of ceftazidime and TOC are enhanced by doping active metals.Repetitive degradation experiments at high current intensity proved that TiO2 and Al2O3 improved the stability of carbon material electrodes,and the optimum doping amount and proportion were obtained.In the process of electrocatalysis,p-hydroxybenzoic acid was used as the scavenger of hydroxyl radicals and the generation rate of ·OH was semi-quantitatively analyzed.CeO2 and Sn02 were found to be the main catalysis components of ·OH.The EO process was coupled with EF process and PS with CNT-PTFE used as cathode material.The degradation comparison of EO-Fenton and EO-PS processes demonstrated that although EO-PS had high removal efficiency towards ceftazidime,the mineralization efficiency of EO-PS process(45.7%)was much lower than that of EO-Fenton process(65.2%).In order to improve the catalytic performance of CNTs towards persulfate,a bamboo-configuration carbon nanotube with high nitrogen content and Ni encapsulated was synthesized by a simple pyrolysis method.Experiments show that Ni encapsulated in carbon nanotubes can effectively prevent the precipitation of toxic metal Ni and improve the stability of the material.At the same time,Ni and N modified carbon nanotubes have high catalytic effect towards persulfate.The electrocatalytic oxidation of cephalexin and sulfate radical catalytic degradation were carried out with Ni(Ni@NCNT)used as cathode material.It was found that the degradation effect of TOC was EO(48%)+PS(9.2%)<EO-PS(70.1%).The results show that the coupling process has obvious advantages and application potentials.CNTs can also be utilized as the intermediate layer of dimensionally stable anode(DSA),which can not only improve the electrochemical specific surface area,but also prevent the damage caused by oxidation of carbon materials.Graphite/CNT-Ce/PbO2-Ce electrodes and Ti/CNT/SnO2-Sb-Er electrodes were prepared by doping rare earth metal elements into the active layer during the preparation process.SEM and XRD analysis showed that the introduction of CNT interlayer resulted in smaller crystals sized and more stable morphology.Electrochemical analysis showed that doping CNT and rare earth metal could greatly increase the oxygen evolution potential and the electrochemical specific surface area,with OEP values of graphite/CNT-Ce/PbO2-Ce(2.3 V)and Ti/CNT/Sn02-Sb-Er(2.2 V)obtained respectively.The 20-hour recycling degradation proves that the introduction of CNT intermediate layer constructs a layered structure and a more compact morphology,which improves the service life of DSA electrodes.EO process with Graphite/CNT-Ce/PbO2-Ce electrodes were coupled with electro-Fenton,achieving an 88.4%TOC removal rate after 4 hours reaction.In comparison,the TOC removal rate of EF was 17.5%and the EO removal rate was 74.4%.The results show that hydrogen peroxide produced by cathode is catalyzed by Fe2+to produce ·OH,which enhances the degradation efficiency of ceftazidime.In summary,it was found that the coupling of electrocatalytic oxidation process with EF or PS catalysis could improve the catalytic efficiency.Therefore,Ti/CNT/SnO2-Sb-Er anode and Ni@NCNT-PTFE cathode were prepared,and a triple compound system of electrocatalytic oxidation+electro-Fenton+persulfate catalysis(EO-Fenton-PS)was established.The TOC removal rates of EO(68.4%),EO-Fenton(88.4%),EO-PS(79.3%)and EO-Fenton-PS(98.1%)were achieved towards cefotaxime,demonstrating that the coupling process system has increased degradation efficiency for antibiotics and higher industrial application value.The intermediate products of ceftazidime,cefalexin and cefotaxime in the degradation process were analyzed by means of HPLC-MS,GC-MS and IC.The changes of biotoxicity in the degradation process were studied by using Vibrio Fischer as indicator.The results showed that the inadequate electrocatalytic degradation was prone to produce toxic intermediate products.With the proceeding of electrolysis,the biotoxicity decreased rapidly,which proved that electrocatalytic oxidation can effectively destroy the toxic groups of antibiotics and reduce the impact on aquatic ecology.