| The huge demand for pesticides in modern agriculture has brought about a large amount of pesticide production wastewater and surface water pollution,which poses a great danger to human health and environmental safety.Advanced oxidation technologies(AOPs)are the core units for treating pesticide wastewater,which can effectively remove hard-to-biodegrade pesticide pollutants by strong oxidative species(free radical or non-radical pathways).Among them,heterogeneous AOPs are ideal for treating pesticide contaminants in aqueous phase because of the advantages of recyclable catalysts,low energy consumption and designable active sites.The technological core of heterogeneous AOPs lies in the design of environmentally friendly,low-cost and effective heterogeneous catalysts.In this paper,carbon materials are used to confine Fe-Mn metal for activation of persulfate and treatment of pesticide contaminants in water.The confined structure could protect the metal core from metal leaching,prevent metal nanoparticles or atom agglomeration,enhance catalytic activity,and regulate the activation mechanism of persulfate.Carbon,iron and manganese are abundant in the environment and available at low cost.Confined Fe-Mn catalysts exhibted excellent catalytic activity,stability and diverse catalytic mechanisms.In this paper,a series of carbon material confining ferromanganese catalysts(mainly including two types:carbon material confining ferromanganese nanoparticle catalysts(NP@C)and confining single-atom catalysts(SAC))were prepared for the activation of persulfate to degrade pesticide organic pollutants,and the catalysts were preferentially selected to develop water treatment devices.Among them,the carbon shell of NP@C can effectively protect the metal core from metal leaching and prevent agglomeration of nanoparticles,thus maintaining the stability and activity of the catalysts.In order to further improve the catalytic activity,enhance the metal atom utilization and obtain an ideal platform for the research of heterogeneous catalyst,two SACs were prepared to investigate the structure-activity relationship and catalytic mechanism.The selected pesticide contaminants were two types of products widely used in agroforestry and preservation of agricultural products,namely neonicotinoid insecticides and phenolic fungicides.The details of the study and the main findings are as follows.(1)N-doped carbon nanotubes confining iron-manganese carbides(FeMn@NCNT)were designed and synthesized for the activation of peroxymonosulfate(PMS)to degrade pesticide contaminants.It was found that altering the FeMn ratio and synthesis temperature during the synthesis process could modulate the specific surface area,crystal structure and defect degree of the catalyst to change the catalytic activity.The optimal synthesis temperature and mole ratio of FeMn were determined to be 950℃ and 5:1.FeMn@NCNT exhibited a free radical mechanism in the degradation of neonicotinoid insecticides and a nonradical mechanism(electron transfer mechanism)in the degradation of phenolic fungicides.This is because catalysts can activate peroxymonosulfate(free radicals and non-free radicals)in multiple pathways,and the mechanism exhibited in contaminant degradation depends on the structure of the contaminants.Neonicotinoid insecticides contain a variety of electron-absorbing groups,making themselves difficult to be removed by the weaker non-radical mechanism,so it can only be removed by the strongly oxidizing SO4·-and OH·.Whereas phenolic fungicides contain electron-rich phenolic hydroxyl groups,making the phenolic fungicides more electron-donating and can be removed by non-radical mechanisms.Further,for the first time,a distinction was made between the electron shuttle and the adjacent transfer in the electron transfer mechanism by using a twochamber electro-oxidation system.The mechanism of PMS activation by FeMn@NCNT was proved to be an electron shuttle mechanism by the data of in situ current and in situ cathodic potential.Benefiting from the confined structure,the catalyst exhibits good stability and low metal leaching,therefore,it is an environmentally friendly and efficient non-homogeneous catalyst.(2)To further achieve encapsulation of the whole Fe-Mn metal nanoparticles and investigate the commonality of NP@C,the N-doped graphite-like carbon encapsulating Fe-Mn metal oxides was prepared and synthesized by one-step pyrolysis of Prussian blue analogues for the activation of PMS degradation and removal of clothianidin(CTD).800℃ was the optimal synthesis temperature at which the encapsulated Fe-Mn metal was oxide and the carbon shell layer was Ndoped graphite-like carbon.Electrochemical analysis showed that the catalyst at this temperature had the best charge transfer capability.The degradation mechanism of the catalysts for clothianidin degradation was verified by ESR and quenching experiments as a free radical mechanism,and for other neonicotinoid insecticides as well.Similarly,the catalysts can also degrade phenolic fungicides by a non-free radical mechanism,which indicates that the N-doped graphite-carbon structure confined FeMn-Mn metal nanoparticle catalysts all have a multiple PMS activation mechanism.The catalysts were further used to degrade contaminants in complex background matrices.It was found that Cl-and HCO3-could inhibit the radical dominated degradation process,NO3-had insignificant effect,and phosphate could accelerate the degradation of pollutants.By further experiments,it was revealed for the first time that HPO32-and PO33-could activate PMS to generate SO4·-to degrade pollutants.And for the non-radical processes,anions have negligible influence.(3)To further improve the atom utilization efficiency of Fe-Mn metal and to obtain an ideal catalytic research platform,a confined single-atom catalyst was developed.Graphite carbon nitride(g-C3N4)anchored single-atom iron(SAFe-CN)was synthesized for the activation of PMS for the degradation of phenolic fungicides.It was demonstrated by XAS and HAADF-STEM that the iron site of the catalyst is atomically dispersed and the coordination environment is Fe-N2O2.The catalytic mechanism of the catalytic system was determined as electron transfer regime by probe experiments,ESR,quenching experiments,and photogenerated 1O2 system.The introduction of single-atom iron was confirmed to be more favorable for charge transfer by means of bandgap and Bader charge analysis.Laplacian charge density and charge difference density demonstrated that single-atom iron sites are the adsorption and activation sites for PMS,which was subsequently verified by the linear correlation between iron content and PMS adsorption capacity or catalytic activity.Further experiments showed that SAFe-CN could only activate PMS through the non-radical pathway and could not generate free radicals to degrade nicotinic insecticides.(4)Then,nitrogen-doped graph itized carbon nanotubes(NCNT)with better conductivity were selected as the substrate,and single-atom manganese(Mnsa-NCNT)was loaded on its surface for activating PMS to degrade pesticide contaminants.The interfacial process of PMS activation by Mnsa-NCNT and NCNT was investigated,and it showed that the amount of PMS adsorbed by NCNT was higher than that by Mnsa-NCNT,while previous literature has clearly indicated that there is a significant positive correlation between the adsorption amount of PMS and the catalytic degradation rate.To further investigate the mechanism of interface interaction,the adsorption capacity of both on PMS in the presence of 4-CP was measured,and it was found that 4-CP significantly inhibited the adsorption of PMS by NCNT,while MnsaNCNT was not significantly affected.Based on the experimental phenomena,the theoretical model of inner sphere complexation was proposed for the first time.It was concluded that the adsorption of PMS and 4-CP onto NCNT is by outer sphere complexation,and there is obvious competition between the two.However,the PMS adsorption onto Mnsa-NCNT is by inner sphere complexation,and the 4-CP adsorption is by outer sphere complexation.The inner sphere complexation of PMS adsorption is not affected by the outer sphere complexation,thus resulting in the higher catalytic activity of Mnsa-NCNT.Anion interference tests,XPS and XAS further confirmed the inner-sphere complexation mechanism of Mnsa-NCNT,and the R-space EXAFS indicated that the adsorption of PMS on the Mn-N4 site resulted in a Mn-O scattering path of approximately 2.08 A in length.Subsequent theoretical calculations further corroborate the proposed inner-sphere complexation mechanism.Mnsa-NCNT can also degrade neonicotinoid insecticides and phenolic fungicides by multipathway activation of PMS.Further analysis with the catalytic mechanism of SAFe-CN and previous studies,the conductivity of the substrate is the key to determine the multipathway activation of PMS.(5)Catalytic filters and catalytic ceramic membranes were further prepared by selected catalysts for continuous treatment of wastewater.The SAFe-CN loaded catalytic filters were used for continuous operation for 100 h to degrade organic pollutants with excellent performance.The catalytic ceramic membranes prepared based on the developed spray cross-linking method showed excellent stability.The surface coating catalyst was shaked in a water bath shaker for 72 h without peeling off.The catalytic ceramic membranes were effective in treating wastewater containing pesticide contaminants,and the metal leaching was lower than the limit value in the drinking water standard set by the Ministry of Health of China.Because the spraycrosslinking method can be used for large-scale catalytic ceramic membrane preparation,this method can be applied on the development of commercial catalytic reactors.The above-mentioned results obtained in this study are useful as a guide for the subsequent development of catalysts and the efficient treatment of pesticide wastewater. |
