| Iron-based metal organic frameworks(Fe-MOFs)are a new type of functional materials formed by the combination of iron metal centers and organic ligands.They have higher porosity,chemical stability,water stability,and more active metal sites,which have been used as Fenton-like catalyst for the catalytic degradation of sulfonamides in water.Although several studies have been added reducing agents to Fenton-like systems to accelerate the conversion between Fe(Ⅲ)/Fe(Ⅱ)and improve the catalytic efficiency of the system,there are few literatures about reducing agents/MOFs heterogeneous Fenton-like systems to degrade sulfonamides in water.Therefore,in this study,sulfamethoxazole(SMX)was used as the target pollutant,and MIL-88B(Fe)with rich iron active sites was used as the catalyst.MIL-88B(Fe)/H2O2 and the reducing agent ascorbic acid(ASA)optimized the MIL-88B(Fe)/H2O2 Fenton-like system for the catalytic degradation of SMX experiments were investigated.The effects of reaction conditions such as oxidant concentration,catalyst dosage,initial pH of the solution,common anions in water and their concentration on the degradation of SMX in the system were studied.The structure and performance of fresh and use MIL-88B(Fe)were characterized,and the reusability and stability of MIL-88B(Fe)in the reaction system were also tested.Meanwhile,the intermediate products of SMX and ASA during the degradation process were identified and analyzed,and the degradation pathways of SMX and ASA were deduced.The reactive free radicals and possible catalytic degradation mechanisms in the systems were analyzed and aim to provide relevant basic data for MIL-88B(Fe)as a Fenton-like catalyst for the removal of sulfonamides in water.MIL-88B(Fe)was successfully prepared by the hydrothermal synthesis method,X-ray diffraction(XRD)and Fourier infrared spectroscopy(FTIR)diagrams showed that MIL-88B(Fe)had a well structure and morphology,MIL-88B(Fe)showed a needle-like octahedral crystal form,with a specific surface area of 232.6 m2/g,and the pores were mainly micropores of about 1-2 nm.99%SMX was removed using MIL-88B(Fe)as a Fenton-like catalyst within 180 min as the optimal initial condition of 6 mmol/L H2O2,0.2 g/L MIL-88B(Fe),and pH=4 in the batch experiment.In the range of 0.6-6 mmol/L H2O2 and 0.1-0.2 g/L MIL-88B(Fe),increasing the concentration of H2O2 and the dosage of MIL-88B(Fe)can enhance the degradation rate of SMX;when the initial pH of the solution was in the range of 4-6,MIL-88B(Fe)still showed good catalytic activity,and in the range of initial pollutant concentration of 5-20 mg/L,SMX removal rate was still above 90%;after 5 cycles,the removal rate of SMX in the system can reach above 95%as before,and there were no significant changes in its structural morphology,functional groups,and crystal morphology,etc.It was indicated that MIL-88B(Fe)has a relatively stable performance as a heterogeneous Fenton-like catalyst;Electronic paramagnetic resonance(EPR)and radical quenching experiments have proven that the main reactive radicals in the MIL-88B(Fe)/H2O2 system was·OH,the catalytic activity of MIL-88B(Fe)was mainly attributed to its abundant coordinated unsaturated iron sites in the structure,which combine with H2O2 to form H2O2-Fe complexes on the catalyst surface,and the Fe(Ⅲ)was reduced to Fe(Ⅱ)by electron transfer,which catalyzed H2O2 to produce·OH,thereby effectively degrading SMX in water.Due to the limited amount of Fe(Ⅱ)in MIL-88B(Fe)and the slower rate of Fe(Ⅲ)reduced to Fe(Ⅱ),the application of MIL-88B(Fe)in water treatment was limited.Therefore,the reducing agent ASA was introduced into the MIL-88B(Fe)/H2O2 system in order to strengthen the recycling of Fe(Ⅲ)/Fe(Ⅱ)and improve the catalytic capacity of the system.In the MIL-88B(Fe)/H2O2 system,the SMX removal rate was only 14%at 60 min,and the addition of ASA increased the SMX removal rate to 99%at 60 min.The degradation of SMX in ASA/MIL-88B(Fe)/H2O2 system conformed to the pseudo-secondary reaction kinetics,and its degradation rate was 0.177 L/(mg·min),which was 60 times that of the MIL-88B(Fe)/H2O2 system.In the range of 0.06-0.2 mmol/L ASA,the addition of ASA promoted the degradation of SMX,but excessive ASA consumed ·OH in the system,which will inhibit the degradation of SMX;under the pH condition of 5-9,the removal rate of SMX can still reach 99%;HCO3-had a significant inhibitory effect on the degradation of SMX,and high concentration(50 mmol/L)of Cl-had a certain inhibitory effect on SMX degradation,and low concentrations(0.5-5 mmol/L)of SO42-,NO3-,Cl-had little effect on the degradation of SMX.ASA can promote the cycle of Fe(Ⅲ)/Fe(Ⅱ)in the system,thus catalyzed H2O2 to produce more ·OH,thereby increasing the degradation rate of SMX in the reaction system;free radical scavenging experiments showed that ·OH was the main reactive free radical in the system,and few O2-· also participated in the degradation reaction,and the addition of ASA promoted the system to produce more ·OH and O2-·.MIL-88B(Fe)can remove more than 98%of SMX in 5 cycles,and the iron dissolution was relatively stable,showing excellent stability and catalytic performance.High resolution liquid chromatography tandem mass spectrometry was used to identify and analyze the intermediate products of SMX and ASA during the degradation process.SMX degradation intermediate products mainly included 11 kinds of compounds,such as benzene ring hydroxylated derivative compounds,isoxazole ring hydroxylated derivative compounds,isoxazole ring ring-opened compounds,etc.;ASA degradation intermediate products mainly included 8 kinds of compounds such as ascorbic acid oxidative dehydrogenation compounds,2,3,4-trihydroxybutyric acid and oxalic acid,etc.Based on these results,it was inferred that the degradation pathway of SMX mainly includes:hydroxyl radical substitution reaction of benzene ring and isoxazole ring in SMX,sulfonamide bond cleavage,isoxazole ring opening,-SO2 group loss,hydroxyl dehydration and intramolecular rearrangement reactions;ASA degradation pathways include:five-membered epoxidation ring opening in the ASA,oxidative cleavage of the carbon chain on the ring,etc.Combining the degradation path of SMX and ASA and the obtained experimental results,the possible degradation mechanism of ASA/MIL-88B(Fe)/H2O2 system was deduced as follows:on the one hand,the Fe(Ⅲ)active site on MIL-88B(Fe)was combined with H2O2,and through the electron transfer between MIL-88B(Fe)and H2O2,the Fe(Ⅲ)was reduced to Fe(Ⅱ)to catalyze H2O2 to produce ·OH;on the other hand,the reducing agent ASA coordinated with MIL-88B(Fe)to form complex,which promoted Fe(Ⅲ)/Fe(Ⅱ)and Fe3+/Fe2+cycle in the system,and then catalyzed H2O2 to produce more ·OH,thereby increasing the degradation rate of SMX in the reaction system. |
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