Preparation Of Magnetic Nitrogen Doped Biochar And Its Coupling With Peroxymonosulfate For Degradation Of Metolachlor

Metolachlor(MET)is an amide herbicide that is widely used in agricultural production.During the production and application,MET will inevitably enter into the environment through various ways,causing harmness to the ecological environment and human health.Therefore,it is important to synthesize and prepare catalytic materials with strong catalytic ability and environmental benign.At present,the degradation technology of MET is mainly divided into biological and chemical methods.Although the cost of the biological treatment process is low,MET is not easily biodegradable,and it is difficult to separate efficient degradation strain.In contrast,chemical treatment processes have the advantages of high degradation rate and easy operation,such as photodegradation,Fenton oxidation and ozone oxidation.Peroxymonosulfate(PMS)is widely used to degrade organic pollutants because of its strong oxidizing capability.However,studies on the treatment of MET with PMS have rarely been reported,and the corresponding degradation mechanism is still unclear.Cobalt ion(Co2+)can efficiently activate PMS to produce SO4·-for the degradation of organic pollutants.However,but it is difficult to separate and recover Co2+ from the system after reaction.Co2+ by combining with iron ion(Fe2+),a Co/Fe bimetallic material can be prepared.Therefore,in this thesis,a magnetic Co/Fe bimetallic catalyst was firstlly prepared and its catalytic activity and stability were studied.Due to the fact that the material was easily to be agglomerated,a magnetic nitrogen-doped biochar catalyst is further prepared by using biocharas a carrier.Moreover,the catalytic mechanism and the degradation mechanism were systematically studied.The degradation pathway was elucidated by identifying degradation products.The main research contents and results were as follows:1.Co/Fe bimetallic catalyst activated PMS to degrade METCo/Fe bimetallic catalyst was prepared by hydrothermal method with a specific surface area of 22.4 m2 g-1.Characterized by XRD,Co and Fe in the catalyst mainly existed in the form of crystals of CoFe2O4 and Fe2O3.Under[MET]=10 mg L-1,[PMS]=3 mM,[Co/Fe]=0.2 g L-1,the degradation process could be completed within 40 min.The corresponding pseudo-first-order kinetic constant(kapp)is 0.11 min-1.The stability of the Co/Fe bimetallic catalyst was evaluated by the cycle experiment and the dissolved ion concentration.It was found that the reused catalytic activity of the catalyst was greatly improved after simple heat treatment(500?,4h).The effect of pH on the degradation process was studied and it was found that neutral and weak alkaline conditions were beneficial for the degradation process.Cl-and humic acid(HA)showed inhibitay effects on the degradation process.While HCO3has a dual effect on the degradation process,that was low concentration showed inhibitay effects on the degradation process and high concentration showed beneficial for the degradation process.Through radical quenching experiments and electron paramagnetic resonance(EPR)analysis,both SO4·-(major)and ·OH(minor)participated in the reaction.The second-order rate constant for the reaction between SO4·-and MET reaction was determined by competitive kinetics method,and it was 2.17×109 M-1 s-1.Eleven degradation intermediates were detected by LC-TOF-MS.MET was probably degraded by dechlorination,hydroxylation,dealkylation and so on.2.Nitrogen-doped biochar activated PMS to degrade METFour kinds of biochar(BC-400,BC-600,BC-800B,C-1000)with different pyrolysis temperatures were prepared by using a tube furnace in an inert atmosphere(N2).By comparing surface area,adsorption capacity and activation capacity,BC-1000 showed best performance.Therefore,Nitrogen-doped biochar(NBC),Sulfur-doped biochar(SBC)and Nitrogen-sulfur co-doped biochar(NSBC)were prepared at 1000?.The specific surface area and adsorption capacity were also analyzed and determined.It was found that NBC showed superior performance.High pyrolysis temperature was beneficial for the specific surface area of biochar.The specific surface area played an important role in the adsorption capacity and activation capacity of biochar.The catalytic degradation of MET was largely dependent on the adsorption of MET on the surface of biochar.N-doping was advantageous for adsorption and activation properties,while S-doping had a negative impact.The pseudofirst-order kinetic rate constants(kapp)of modified biochar followed NBC(0.0238 min-1)>BC(0.0223 min-1)>NSBC(0.0190 min-1)>SBC(0.0184 min-1).Under the condition of[MET]=5 mg L-1,[PMS]=4 mM,[NBC]=0.2 g L-1,MET could be completely degraded within 120 min reaction.Neutral and weak alkaline conditions were favorable for degradation and the degradation efficiency reached the highest at pH=9.Radical pathways(SO4·-and·OH)and non-radical pathways(1O2)were involved in the reaction as proved by radical quenching experiments and EPR analysis.Ten degradation intermediates were detected by LC-TOF-MS,which indicated the six degradation pathways of MET,i.e.hydroxylation,hydrolysis dechlorination,N-dealkylation,dehydroxylation,demethylation and amide bond cleavage.3.Magnetic nitrogen doped biochar activated PMS to degrade METTo further improve the catalytic activity of the Co/Fe bimetallic catalyst,it was decoratsd on nitrogen doped biochar.Magnetic nitrogen-doped biochar(MNBCX,x stands for preparation temperature)was prepared by impregnation method and characterized by XRD,XPS,BET and Raman spectra.From the characterization results of the catalyst,it was indicated that Co and Fe in MNBCX mainly existed in the form of CoFe2O4.MNBC800 possessed the largest specific surface area,the highest degree of graphitization,the highest content of graphite N and the best stability.The pseudo first-order reaction rate constants(kapp)of MNBCx followed MNBC800(0.1042 min-1)>MNBC700(0.0711 min-1)>MNBC600(0.0493 min-1)?MNBC500(0.0465 min-1)?MNBC400(0.0454 min-1).The catalytic activity of MNBCx was highly dependent on the pyrolysis temperature.Under[MET]=10 mg L-1,[PMS]=0.5 mM,[MNBC800]=0.2 g L-1,MET can be completely degraded within 40 min.Neutral and weak alkaline conditions were favorable for degradation,whilst the strong acid and alkali conditions were not beneficial for degradation.Through radical quenching experiments and EPR analysis,both radical pathways(SO4·-and ·OH)and non-radical pathway(1O2)were both involved in the reaction process,and the radical pathway played a crucial role.Eleven degradation intermediates were successfully detected by LC-TOF-MS and it was speculated that MET was mainly degraded via hydroxylation,dechlorination and dealkylation.The biological toxicity of MET before and after degradation was was found that the biological toxicity decreased after MET degradation.Finally,low ion dissolution rate,magnetic recyclability and good catalytic activity of the MNBC800 catalyst were expected.This study provided a theoretical basis for the treatment of metolachlor wastewater,and it also had important reference value for remediation of other organic pollutants.