Study On The Mechanism Of Removing Typical Pyridine Pesticides By Advanced Oxidation Technology Based On Sulfate Free Radical

China has a large scale of production and use of pesticides.The extensive use of pesticides leaded to their frequent detection in surface water,groundwater and soil,causing environmental hazards and destroying ecological balance.Through enrichment in drinking water and food,pesticide contaminants posed a threat to human health.Pesticides whose contained pyridine ring structures were founded in almost all types of pesticides.For the disposal of pyridine compounds,it was difficult for traditional water treatment measures to degrade the pollutants to complete mineralization.It was widely known that trace organics in water could be efficiently and stably removed by SR-AOPs(Sulfate radical based advanced oxidation processes).By using SR-AOPs,this study attemptted to achieve the removal of pyridine pesticides in the environment,and to explore the degradation mechanism,which would be of great significance for the removal of other type pyridine compounds.In this contribution,clopyralid(CLP),picloram(PCLO)and aminopyralid(AMP)selected as the target pollutants were subjected to degradation studies by thermally activated PS process.In the meanwhile,the removal efficiency and oxidation mechanism of three pesticide pollutants in SR-AOPs were comprehensively studied in detail.The research contents included:research on the ring-opening cleavage mechanism of SO4·-with pyridine ring;research on the influence mechanism of typical amino functional groups on pyridine pesticides degradation;research on the influence mechanism of dissolved oxygen(DO)on SR-AOPs system;a comparative study on the degradation mechanism of typical pyridine pesticides.The main conclusions were as followed:(1)The target pollutant CLP could be effectively removed by thermally activated PS process.The removal rate accelerated with the increase of the activation temperature and the dosage of PS,and the acidic conditions were favorable for the degradation of CLP.The degradation of CLP was in accordance with the pseudo-first-order kinetics.Quenching experiments and kinetic modeling results sugggestted that SO4·-was the main reactive species responsible for CLP degradation.Based on SPE-HR-MS(Solid phase extraction-High resolution LC-MS),DNPH(2,4-Dinitrophenylhydrazine)derivatization method and DFT(Density functional theory)calculation results,the main pathway including hydroxylation,dechlorination and decarboxylation were proposed on the degradation of CLP.With the deepening of the hydroxylation of CLP,o-quinone and p-quinone structures were formed on the pyridine ring,followed by cleavage of the pyridine ring.DFT calculations indicated that the formation of such quinone structures was particularly important for the ring-opening reaction.When cleavage of the CLP pyridine ring happened,further oxidation would be achieved by breaking each bond.The ketones generated after cleavage will continue to be oxidized to carboxylic acids.These small molecular products of cracking of CLP ring were mostly volatile potential air pollutants,whose cross-media effect needed to be paid attention to.need to be paid attention to.(2)AMP and PCLO could be achieved efficiency removal by using thermally activated PS processed.Compared to the chemical structure of CLP,the amino group was occupied by AMP and PCLO pyridine rings.The oxidaiton rate accelerated with the increase of the activation temperature and the dosage of PS,and the acidic conditions were favorable for the degradation.The presence of NO2-,HCO3-,Cl-inhibited the removal of AMP/PCLO.NO3-had a little effect on the degradation of both pollutants.Quenching experiments showed that SO4·-rather than HO· was the main active radical.It was found that AMP and PCLO exhibited similar oxidation pathways to SO4·-,including hydroxylation,decarboxylation,deamination and the formation of pyridine amino radicals.This radical with high reactivity could couple PCLO and AMP to form self-coupling dimer.Under attack by SO4·-,the N atom on the dimer was oxidativly dehydrated to form an azo compound.Such azo products could further undergo deep oxidation,which lead to the identificaiton a series of decarboxylation and hydroxylated azo derivatives in this system.(3)In thermally activated PS,UV/PS and Co2-/PMS systems,the presence of oxygen had a positive effect on the removal of CLP,however,the conversion of AMP and PCLO was faster under anaerobic conditions.Three pollutants showed different result on DO effect,possibly due to the participation of DO in the reaction degradation.The presence of DO promotes the hydroxylation process in the SR-AOPs system.Specifically,CLP-+OH·-radicals were generated in the interaction with CLP-and SO4·-.Such radical was further combined with oxygen to generate the CLP-+OH+O2·-intermediate,followed by the dissociation of HOO to the hydroxylated product CLP-OH.An accelerated degradation of CLP was observed in the kinetic study.As for PCLO and AMP,the vacancies on the pyridine ring were occupied by chlorine and amino groups.Since there was no position on the ring to interact with oxygen,the degradation of PCLO and AMP appearred to inhibit.In addition,under anoxic conditions,the S2O8·-generated in the system will be more inclined to undergo dechlorination reduction reaction with AMP and PCLO,which played a role in promoting the degradation of pollutants.By using the model pollutant TMP,DO was verified to participate in the reaction pathway and induce the degradation trend in SR-AOPs.TOC(total organic carbon)experiments revealed that the presence of DO had a beneficial effect on the mineralization of pollutants.It was crucial for practical application of SR-AOPs.(4)The degradation efficacy of CLP,PCLO and AMP under the same reaction conditions was comprehensively evaluated.The overall trend was CLP>AMP>PCLO.The MEP(Molecular electrostatic potential)results showed that all three pollutants were prone to decarboxylation.The FEDs(Frontier electron densities)results indicated that the reactive sites for CLP were the vacant carbon atom of the pyridine ring.When the amino group existed in the system,the reactive site of AMP and PCLO interacting with SO4·-was the electron-rich amino group.Both the ECOSAR(Ecological Structure-Activity Relationship Model)model and the TEST(Toxicity Estimation Software Tool)model showed that the deep cracking of pollutants would contribute to the reduction of system toxicity.When self-coupling and azo products were generated,the system showed severe toxicity.In general,the oxidation processes of three structurally similar pesticide pollutants,CLP,AMP,and PCLO,were comprehensively evaluated.When there was no other group in the pyridine ring,SO4·-would interact with the vacancy position.When the pyridine was filled with functional groups,SO4·-would first react with electron-rich groups instead.Oxidation on ring included decarboxylation,hydroxylation,deamination and nitration.Moreover,with the deepening of hydroxylation,the pyridine ringopening reaction would take place.When the electron-donating amino groups were existed,the highly reactive amino group could be oxidized to produce self-coupling and azo derivatives.ECOSAR model pointed out significance hazard when the formation of these substances.This phenomenon needed to be reminded in water treatment.The results obtained in this paper would help us better understand the oxidation behavior of pyridine compounds in SR-AOPs,and provide a scientific basis for the removal of pyridine organic pollutants in practical engineering.