| Electrochemical advanced oxidation processes are based on in situ production of strong oxidants such as hydroxyl radicals(·OH)and other reactive oxygen species by electrochemical reactions that react with organics,which eventually mineralize and degrade the organics to CO2,H2O and inorganic ions.Among them,electro-Fenton technology can produce hydrogen peroxide(H2O2)in situ by oxygen reduction reaction(ORR)and construct a cycle between Fe2+ and Fe3+,overcoming the shortco1ings of traditional Fenton which requires the addition of H2O2 and is prone to secondary contamination due to the production of iron sludge,and has broad application prospects in the treatment of refractory organic wastewater.However,electro-Fenton technology still faces many challenges.For instance,the yield and catalytic utilization efficiency of H2O2 generated by ORR are not high,which makes it difficult to increase the production of·OH and the current efficiency is low,which is one of the important reasons that limit the application of electro-Fenton technology.In this study,we investigated the catalytic activity of electrocatalytic oxygen reduction using a TiO2/graphite(TiO2/C)composite as a cathodic electrocatalytic material and applied the electro-Fenton like system based on this cathode material to the degradation of microplastics in water.The main contents of this study are as follows:(1)The prepared TiO2/C composite was analyzed by structural characterization combined with quantum calculations.The results of scanning electron microscope showed that TiO2 was successfully loaded on the graphite surface,and the diameter of TiO2 nanoparticles was approximately 50 nm.X-ray diffraction test showed that the crystal form of TiO2 and graphite before compounding was a mixed crystal form of rutile and anatase,while after compounding,it became a single anatase crystal form.This result was verified by density functional theory.The calculation showed that the binding energy between anatase and graphite was much stronger than that between rutile and graphite,so TiO2 was more easily transformed into the anatase phase after bonding.Cyclic voltammetry showed that the peak of oxygen reduction appeared at-0.7 V vs.Ag/AgCl,and only one reduction peak of Ti4+to Ti3+was observed under nitrogen saturation,which proved that an oxygen reduction reaction occurred on TiO2/C.(2)TiO2/C was used as the cathode in the EF-like system,and the process of oxygen reduction reaction and its mechanism were investigated.The existence of Ti3+during the reduction process was confirmed by X-ray photoelectron spectra.The electrochemical oxygen reduction behavior between oxygen and TiO2 was observed by in-situ Raman spectroscopy and the formation of Ti-OH bonds during the oxygen reduction process occurred.According to the electron paramagnetic resonance spectrum,the free radical product of oxygen reduction was determined to be·OH.After 2 hours of cathodic polarization,the·OH yield reached 35.8 mg/L,and the current efficiency was 92.8%.Then,the intermediate product of oxygen reduction was proved to be adsorbed hydrogen peroxide(H2O2ad).Therefore,it was speculated that the oxygen reduction mechanism was that oxygen was reduced to H2O2ad at the cathode,and then H2O2ad was reduced to·OH without desorption.This result was subsequently verified by DFT calculation.(3)TiO2/C was applied to the degradation of microplastics in the cathodic electro-Fenton like system.The common plastic polyvinyl chloride(PVC)was selected as the degradation target,and its degradation and reaction mechanism were studied.It was found through experiments that the optimum conditions for degradation were:applied voltage of-0.7 V vs.Ag/AgCl,pH=3,PVC concentration of 100 mg/L,reaction temperature of 100?,PVC particle size range of 100-200 ?m,magnetic stirring speed was 800 rpm.Under this condition,the dechlorination rate could reach 75%,and the mass loss was 56%approximately after 6 h of electrocatalysis.Scanning electron microscopy images show that cracks and holes appeared on the surface of the PVC particles after electrocatalytic degradation.Both the infrared spectrum and XPS results showed the formation of new C=O,O-H bonds.Besides,the total organic carbon(TOC)content in the solution gradually increased with time,and finally reached 11.4 mg/L after 6 h of electrocatalytic treatment The intermediate products of PVC electrocatalytic degradation were carboxylic acids,esters,alcohols,and other substances,and partially mineralized into CO2 and H2O. |
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