Effect And Mechanism Of Phenol Degradation By Nitrogen-doped Carbon Nanotubes Activated Persulfate

Persulfate-based advanced oxidation process,as a new advanced oxidation method,has great potential for application in environmental remediation.Transition metal activated persulfate systems have received widespread attention due to its high efficiency,however,there is a problem of secondary pollution of transition metals,which limits its application.Non-metallic nano-carbon materials as a new catalyst can activate persulfate to degrade pollutants,compared with metal catalysts,nano-carbon materials have the advantages of high specific surface area,thermal stability,acid and alkali resistance,no metal contamination,and easy adjustment of surface chemical structure.As a class of nano-carbon materials,carbon nanotubes have been widely used as new-type catalytic materials due to their unique lumen and adsorption characteristics,and unique electrical characteristics.Doping nitrogen on carbon nanotubes can simply and effectively regulate the properties and structure of carbon nanotubes to improve their reactivity.When using nitrogen-doped carbon nanotubes to activate persulfate to degrade organic matter,the degradation mechanism of the system is still controversial,and the relationship between the nitrogen doping species and content and the reactivity of the material is unclear.In this study,the prepared nitrogen-doped carbon nanotubes?NoCNT-700?were used to activate persulfate?peroxymonosulfate?PMS?and peroxodisulfate?PDS??to degrade phenol.The degradation effect,influencing factors and reaction mechanism of the system were investigated.The effects of nitrogen doped species and content on the activation performance of the material were analyzed.The degradation pathway of phenol and the reuse performance of the material were explored.The main conclusions are as follows:1.NoCNT-700 was prepared by a high-temperature calcination method using commercial multi-walled carbon nanotubes as raw materials and melamine as a nitrogen precursor;nitrogen doping was realized in NoCNT-700 about 3.33 at%,where the contents of pyridine nitrogen,pyrrole nitrogen,and graphite nitrogen were1.20 at%,1.06 at%,and 0.74 at%,respectively;nitrogen doping did not change the tubular structure and crystal structure of carbon nanotubes,but increased their specific surface area.2.NoCNT-700 can effectively activate PDS to degrade phenol,and the phenol degradation efficiency can reach 100%and the mineralization rate can reach 55.7%within 120 minutes.NoCNT-700 can effectively activate PMS to degrade phenol,and the phenol degradation efficiency can reach 100%and the mineralization rate can reach 61.6%within 60 minutes.3.NoCNT-700/persulfate system is effective under a wide range of pH conditions,and the degradation effect is better under acidic conditions than under neutral and alkaline conditions;the degradation rate of phenol increases with increasing temperature;the system is effective under aerobic and anaerobic conditions;Mg²⁺,Ca²⁺and Fe³⁺have no obvious effect on the degradation effect of the system;SO₄^2-has no obvious effect on the degradation of the system,addition Cl^-and HCO₃^-reduced the degradation rate of the system.4.The main active oxidizing substances produced by the NoCNT-700/PDS system are hydroxyl radicals?·OH?,sulfate radical(SO₄^·-),and singlet oxygen?¹O₂?.In the 88%oxidative degradation,the free radical effect accounts for about 57.8%,and the ¹O₂ effect accounts for about 30.2%.The NoCNT-700/PMS system is a combination of free radicals and non-free radicals.In the 91%oxidative degradation,free radicals account for about 49%,¹O₂ for about 18%,and active intermediates for about 24%.The phenol degradation pathway in the system is:phenol is oxidized to produce o-diphenol,p-diphenol and p-benzoquinone,which is further ring-opened to form small molecular acids,and finally decomposed into H₂O and CO₂ to achieve partial mineralization.5.The nitrogen doping modification of the material can improve the degradation efficiency and mineralization degree of phenol in the activated persulfate system,and increase the proportion of the non-radical part of the system.6.Calcination temperature affects the content and type of nitrogen doping in the material.The reactivity of material is directly proportional to the content of nitrogen doping.Further research has found that the reactivity of material is closely related to the nitrogen content of graphite.With the increasing number of reuses of NoCNT-700,the efficiency of activating PMS to degrade phenol gradually decreases,and the high temperature treatment can restore its reactivity.