Performance And Mechanism Of Removal Of 2,4-Dichlorophenol From Water By Activated Peroxymonosulfate With Nitrogen-Modified Biochar

Chlorophenol compounds（CPs）are widely used in agricultural and industrial production,but they can cause strong toxic effects on human beings and other organisms,and bring great risks to human health and ecological environment.Therefore,it is urgent to remove CPs pollution in water environment.Compared with other CPs pollutant removal techniques,the modified biochar prepared by nitrogen atom doping method contains rich nitrogen and oxygen functional groups and defect sites,which can be used as a metal-free catalyst to activate persulfate effectively without causing secondary pollution problems.In this study,2,4-dichlorophenol（2,4-DCP）was used as the model pollutant of CPs,and melamine was used as nitrogen source to prepare nitrogen-modified biochar（NBC）under high temperature pyrolysis condition.The catalytic performance of its surface active site was utilized to enable it to activate peroxymonosulfate（PMS）efficiently.The adsorption and degradation effects of 2,4-DCP on biochar/PMS system were investigated.The effects of biochar and PMS dosage,initial pH,common anions and different water bodies on biochar/PMS/2,4-DCP system were investigated.In addition,the correlation between the surface structure of biochar,such as N bond configuration,oxygen functional group and defect site,and the reaction rate constant was analyzed.Combined with electron spin resonance（ESR）spectroscopy,quenching experiment and electrochemical experiment,the dominant active site in the interaction between NBC and PMS was explored.The main conclusions are as follows:（1）Compared with the original biochar（BC）,nitrogen-modified biochar（NBC）formed a more fluffy and porous structure,and its surface chemical composition,types and number of functional groups and structural defects changed greatly.（2）The maximum adsorption capacity(Q_max)of biochar for 2,4-DCP was linearly fitted with the characterization data,and the correlation between Q_max and S_BET and I_D/I_G was relatively fitting.Compared with other biochar（BC,NBC-3,NBC-4,and NBC-6）,NBC-5 showed the best catalytic degradation performance,and its catalytic activity was related to the specific surface area of the material,the ratio of C=O to C-O content(A_C=O/A_C-O),and the I_D/I_G value of Raman spectrum.NBC with larger specific surface area will absorb more 2,4-DCP to its surface and interact with PMS to participate in the catalytic reaction,thus improving the catalytic performance.The electron-rich C=O and defect structure are the active sites for catalytic reaction.（3）By analyzing the effects of NBC and PMS dosage,pH,anion and actual water environment on NBC/PMS/2,4-DCP system,it can be concluded that the system can also show a high 2,4-DCP degradation ability under a small amount of NBC and PMS,and can maintain excellent degradation performance in acidic and neutral pH environment.Common anions in water and different water environment have little effect on degradation.（4）By analyzing the characterization of the materials before and after the reaction,it was found that the surface morphology,number of functional groups and defect sites of NBC were greatly changed,which were the direct reasons affecting the degradation effect.The activation mechanism of PMS was studied by ESR,quenching and electrochemical experiments.The results showed that NBC surface groups can activate PMS to produce SO₄·^-,·OH,O₂·^-and ~1O₂,which are reactive oxygen species（ROS）that cause the degradation of 2,4-DCP.In addition,with the help of NBC as an electron transfer medium,electrons are transferred from 2,4-DCP to persulfate anions and degradation of contaminants is completed.Combined with experiments and analysis,the oxidation of O₂·^-and ~1O₂ in the reaction system is the main way of pollutant degradation.NBC is a good activator of PMS from the perspective of bio-waste resource recovery and recycling bio-economy,which provides a new idea for rational design of functional nitrogen-doped biochar for efficient activation of persulfate.