Substituent Effects Of Phenolic Compounds On Their Electrochemical Degradation:Behaviors And Mechanisms

The benzene ring is the most widely distributed chemical structure in nature,and the monobenzene ring compound is the precursor for synthesis of many widely used chemicals and the degradation intermediate of complex structural pollutants.The number,type and location of benzene ring substituents determine their role and behavior in the environment.This research is based on the current research status of phenolic organic pollutants,and the characteristics of aromatic phenolic organic pollutants in electrochemical degradation.First,this paper studies the effect of different operating parameters on the degradation of phenol,using the network of titanium-based lead dioxide?Ti/Pb O2?as the anode,sodium sulfate as the electrolyte,and investigated the current density,electrolyte concentration,initial concentration,p H,electrode The influence of operating parameters such as spacing on the treatment of phenol.The optimal operating experiment conditions were selected:current density 20 m A/cm2,sodium sulfate concentration 0.07 mol/L,p H=5,initial concentration 500 mg/L,and electrode spacing L=2cm.Using the membrane cell reactor,the degradation process of phenol was investigated under the best test conditions.The results show that in addition to p-phenol,the electrode also has a good catalytic oxidation effect on its intermediate product benzoquinone.By analyzing the oxidation process of phenol at the anode and proposing a possible degradation path.Secondly,on the basis of experimental research on the conditions of electrochemical degradation of phenol,four phenol substitutes of p-aminophenol,p-nitrophenol,p-methylphenol and p-chlorophenol were selected as model pollutants.Using Uv-vis spectroscopy,high performance liquid chromatography and other analytical methods to analyze the solution composition of the electrolysis process,especially the determination of intermediate products in the reaction process combined with the attribute analysis of the pollutant itself,it was found that the structural properties of organic matter react to the electrochemical reaction process and The degradation effect has an important effect.The electron-donating group can increase the electron cloud density on the benzene ring,which is beneficial to the attack of hydroxyl radicals.The electron-withdrawing group reduces the electron cloud density of the benzene ring,which is not conducive to the attack of hydroxyl radicals.In addition,the rate of electrolysis will also vary depending on the nature of the substituent.The methyl group will slow down due to the steric hindrance of the group.Although the chlorine group can passivate the benzene ring,it will undergo a dechlorination reaction during the reaction to generate some active substances?HCl O and Cl₂?,thereby accelerating the degradation rate of intermediate products.At the same time,it was found that cathode reduction plays a key role in certain situations.Overall degradation rate:electron-donating phenol>phenol>electron-withdrawing phenol.In view of the characteristics of substituents in organic degradation,we selected nitrophenol isomers to further study the effect of organic structure on electrochemical degradation.Cyclic voltammetry scan results showed that there was no oxidation peak for nitrophenol,indicating that nitrophenol electrolysis is mainly indirect oxidation,that is,the free radical generated in the anode electrolysis,a small part of the active oxygen chemically adsorbed on the anode,that is,enter Oxygen atoms of the oxide lattice oxidize.The results of the non-membrane experiment show that the degradation order of the three nitrophenol isomers is in the order of o-NP>m-NP>p-NP due to the difference in electron-withdrawing effect of substituents and molecular hydrogen bonding.The results of the membrane experiment showed that the removal rates of the three nitrophenol isomer substrates were the same with the membrane cathode>no membrane>membrane anode under the same reaction time.The removal rate of pollutants in the membrane electrolysis cell is higher than that in the membraneless electrolysis cell,while the removal rate of COD is membrane anode>membraneless>membrane cathode.During diaphragm electrolysis,it was known by liquid chromatogram analysis that nitrophenol was oxidized to resorcinol in the anode chamber without aminophenol.In the cathode compartment,nitrophenol is reduced to aminophenol.Experiments show that nitrophenol electrolysis is first reduced at the cathode,and the electrolytic oxidation reaction mainly occurs near the anode,and at the same time,the cathode can also remove part of the COD.The electrochemical oxidation degradation process is the joint action of the cathode and the anode.Sometimes the control of the cathode is critical and cannot be ignored.