Studied On Fe-based Metal-organic Framework And Its Derivatives Activated Persulfate To Degrade Organic Micropollutants

In line with industrial and agricultural development and the increasing improvement of people's living standards,organic micropollutants?OMPs?are changing both quantitatively and qualitatively in the water environment,which has become an urgent social problem to be solved.The advanced oxidation technology based on persulfate?Persulfate,PS?has been proved to be an effective method to remove OMPs from water environment.It is widely concerned by researchers because of its advantages such as strong oxidation ability,multiple degradation ways,and a wide range of p H values.The existing methods to activate PS have the problems of introducing an external heating source,unstable activation performance,and easily causing secondary pollution.Therefore,it is necessary to develop a heterogeneous activator with stable activation performance,mild reaction conditions,and environmental friendliness.Fe-based metal-organic frameworks?MOFs?are relatively ideal heterogeneous catalyst for activating PS due to their characteristics of coordinated unsaturated active sites?CUS?,diverse structures,and easy functional modification.Based on those,this study focuses on the functional modification of Fe-based MOFs to solve the problems of unstable PS activation and poor cycle performance.The structural characteristics of Fe-based MOFs and their functional derivatives were analyzed by various characterization methods,and the catalytic performance was studied by degrading a variety of typical OMPs,simultaneously the activation mechanism of PS and the degradation mechanism of OMPs were further studied.The specific study content and main conclusions are listed as follows:?1?The modified Fe-based MOFs catalyst Fe???-MOFs was formed by regulating the valence state of metal atoms in traditional Fe-based MOFs.In all test experiments,Fe???-MOFs-1 synthesized under the conditions of 24 h&140?has the most complete crystal structure and high-density Fe???CUS.The optimal conditions for the degradation of DBP by activating PDS by Fe???-MOFs-1 are:0.018 mmol L^-1 DBP,0.40 g L^-1Fe???-MOFs,2.70 mmol L^-1 PDS,under a wide range of p H.Fe???-MOFs-1 has good stability and cycle performance,and the crystal structure and surface morphology have not changed much after 5 degradation cycle experiments.In the mechanism study,it is proposed that CUS does not participate in the activation process,but only provides the electron transfer channel for activation,and the unstable state of the metastable electron layer in the ferrous atom accelerates the speed of electron transfer.Therefore,the Fe???CUS has better activity.?2?Using Fe???-MOFs as a precursor,the MOFs derivative Fe???-MOFs@MIP with directional removal of organic micropollutants was formed by surface molecular imprinting.Orthogonal experiments were used to optimize the surface molecular imprinting process.Fe???-MOFs@MIP prepared under the conditions of 1 g Fe???-MOFs,1.25 mmol DBP,0.75mmol MAA,and 10 mmol EGDMA have the best surface imprinted layer thickness?300 nm?.Through the targeted imprinting,the adsorption capacity of Fe???-MOFs for DBP increases from 0.26 mg g^-1 to 48.52 mg g^-1,and the degradation rate constant of DBP increases from0.072 min^-1 to 0.131 min^-1.Moreover,in the advanced oxidation system of Fe???-MOFs@MIP/PDS/DBP,the mineralization rate of organic pollutants have also been improved to a certain extent.So Fe???-MOFs@MIP can indeed efficiently degrade a single target pollution and achieve the directional removal of organic micropollutants.In the mechanism study,it is found that the activation site Fe CUS in Fe???-MOFs@MIP can activate PDS to generate SO₄^-?and.OH,and the targeted imprinting layer quickly recognizes and adsorbs DBP molecules,reduces the mass transfer distance between active species and DBP molecules,thus achieves the efficient removal of target pollutants.?3?Four kinds of MOFs such as Fe???-MOFs,Fe-MIL-88A,Fe-MIL-53 and Fe-MIL-101 were synthesized and their application effects in PS advanced oxidation technology were analyzed.The results show that the order of the four MOFs'ability to activate PDS is:Fe???MOFs>Fe-MIL-88A>Fe-MIL-53>Fe-MIL-101.The experiment also proves that the process of Fe-based MOFs activating PDS to degrade CIP has a negative impact on the stability of the MOFs'structure.The results show that:·OH has a greater effect on the crystal structure of MOFs than SO₄^-?,PDS has a greater effect than PMS,and the destruction of the MOFs'crystal structure is more serious under acidic conditions.This is because in the reaction system,water molecules and free radicals attack the coordination bond between the metal atom and the organic ligand,resulting in the dissolution of the Fe element in MOFs.In addition,free radicals can also attack the organic structures in MOFs,thereby causing the MOFs crystals to collapse.?4?Fe???-MOFs was modified by pyrolysis method to form a new catalyst C-Fe-F.When the pyrolysis temperature is 641?,the spatial structure of C-Fe-F and the number of active sites is optimal.In the advanced oxidation system of PMS/C-Fe-F-4/OMPs,a variety of organic micropollutants are effectively removed and mineralized.The advanced oxidation system has good cycle performance,strong resistance to inorganic ions and p H value,and has potential value in practical wastewater treatment.In addition,the active sites present in C-Fe-F can activate PMS to generate SO₄^-?and ¹O₂ for degrading OMPs,or complex PMS to form complexes to directly degrade OMPs,and the complex plays a major role in oxidative degradation of OMPs,which can degrade about 2/3 of OMPs.?5?A novel MOFs derivative ZSM-5-?C@Fe?was generated by pyrolyzing the polymer of Fe???-MOF and carboxylated ZSM-5.Through generating Fe-C,C-O-Si or C-O-Al,the MOFs derivative ZSM-5-?C@Fe?with excellent water stability,thermal stability,and high activation performance is formed.The optimal conditions for ZSM-5-?C@Fe?to activate PMS to degrade CIP are:the dosage of ZSM-5-?C@Fe?is 4.0g l-1,and the molar concentration ratio of PMS to CIP is 30:1.ZSM-5-?C@Fe?has good activation performance,stability and recyclability.In the mechanism study,the Fe activation site in ZSM-5-?C@Fe?activates PMS to generate ¹O₂ which degrades CIP through electrophilic reaction.The C activation site modified by Fe doping forms electronegative oxidation groups with PMS to degrade CIP through nucleophilic reaction.The original C activation site can activate PMS to generate SO₄^-?,and assists the degradation of CIP through free radical reaction.