Carbon Materials Modified By Nitrogen Doping Activate Persulfate To Degrade Wastewater Under Electric Fiel

One of the biggest problems in the environmental field in recent decades is the risk associated with contamination produced by persistent organic compounds（POPs）.POPs are a class of compounds that can resist physical,chemical,and biochemical degradation to varying degrees,which leads to their longer average presence in the environment.P-nitrophenol is an important organic synthetic raw material commonly used in pharmaceutical industries such as finasidine,manufacturing of paracetamol,manufacturing of pesticide 1605,and dyes.It can be retained in environmental samples for a long time,and has carcinogenic,teratogenic and mutagenic hazards.Therefore,if the wastewater containing p-nitrophenol is improperly treated,the high content will cause serious harm to the human body,water,environment and so on,etc.Activated persulfate oxidation method is a new advanced oxidation technology developed in recent years.It is done with the aid of external energy or catalysis.The manner of the agent breaks the dioxygen bond O-O in the persulfate(S₂O₈^2-)to produce a sulfate radical(SO₄^·-)with a solitary pair of electrons.The free radical has higher oxidation reduction potential（E₀=2.5-3.1V）and stronger oxidation capacity than the persulfate itself,and theoretically can degrade most organic pollutants.However,persulfate alone reacted slowly with most contaminants.So it is necessary to activate the persulfate to produce free radicals.Persulfate can be activated to generate a sulfate radical(SO₄^·-)by thermal,ultraviolet,transition metal ions,ultrasound,electrical,microwave,carbon,etc.Electrically activated persulfate technology can effectively activate persulfate to mineralize organic pollutants in waste water,and introduce no metal ions throughout the whole process,reducing the risk of secondary contamination.The efficiency of electro-activated persulfate depends largely on the selection of electrodes,and new electrodes may be more suitable for practical applications of the system.In the electro-activated persulfate system,whether the cathode material can achieve the efficient catalysis of persulfate has a decisive role on the pollutant degradation ability of the system.Recently,carbon-based materials have attracted attention in environmental applications.Currently commonly used carbon electrodes mainly include activated carbon,carbon felt,graphene,carbon nanotubes,etc.It has the advantages of low cost,huge specific surface area and strong adsorption capacity.Therefore,it is very worthwhile to study and develop to strengthen the mass transfer effect between the composition of the cathode material and the electrode and appropriately increase the defect of the carbon material as the active site.In this study,we synthesized three nitrogen-mixed carbon materials,nitrogen-mixed activated carbon plus graphene（NC）,nitrogen-doped activated carbon（NAC）,nitrogen-doped graphene（NGE）,and three nitrogen-doped carbon-mixed materials-graphite felt（GF）cathode.Three nitrogen-doped carbon materials（NC,NGE,NAC）were characterized using X-ray diffraction（XRD）,Raman spectrum（Raman）,X-ray electron spectrum（XPS）,infrared spectrum（FI-IR）,and nitrogen desorption-adsorption technology（BET）.Electron spin resonance techniques were used to identify the presence of hydroxyl radical（OH）,sulfate radical(SO₄^·-),and singlet oxygen species（¹O₂）.The results show that nitrogen-doped carbon materials（NC）are more favorable to the production of various types of free radicals.In addition,we applied NC-GF to systems where electro-activated persulfate degradation p-nitrophenol and investigated its performance for contaminant degradation under different conditions.The results showed that the degradation rate of PNP could reach91.69%,and the removal rates of TOC and COD could reach 81.41%and 73.33%at the condition of p H=5,voltage 5V,concentration of persulfate 15mmol/L and concentration of p-nitrophenol 50 mg/L.In common inorganic anions in water sources such as chloride,bicarbonate ions and phosphate ions have certain effects on PNP removal in the E-NC-PMS system:chloride has the promoting effect on the removal of PNP,bicarbonate ions have less effect on PNP removal,and phosphate ions inhibit the removal of PNP.Overall,the electro-activated persulfate process by nitrogen-doped carbon electrodes is a promising method for treating organic pollutants in wastewater.