Iron-based Metal-organic Framework As Heterogeneous Fenton-like Catalyst For Degradation Of Organic Dyes

Water pollution due to various industrial effluents is a global environmental problem.Due to the rapid industrialization,the use of coloring chemicals like dyes also increases day by day.These dyes are chemically,photolytically and biologically highly stable and are highly persistent in nature.Therefore,it is of great significance to seek technology with high efficiency and low-cost to remove organic dyes from wastewater.Among the removal methods of organic dyes,the Fenton system has been studied extensively because of its high efficiency,low cost,fast reaction rate,non-selectivity,simple operation and environmental friendliness.However,the homogeneous Fenton reaction has some critical drawbacks,i.e.,rigorous operating pH range,the formation of iron sludge and the difficulty for reclaiming.In recent years,the heterogeneous Fenton-like catalysts have been widely explored in wastewater treatment because they have already overcome the above drawbacks of homogeneous Fenton reaction.For example,many iron-based heterogeneous Fenton-like catalysts have been explored,including iron supported catalyst,iron oxide,iron sulfide and other iron-containing materials.Fe-based metal-organic frameworks?Fe-MOFs?possess a three-dimensional well defined structure and are constructed by iron-containing nodes connected by organic bridging ligand.Fe-MOFs would be an outstanding heterogeneous Fenton-like catalysts due to its characteristics of high surface area,unsaturated Fe site and tunable structure,which is expected to be used in actual wastewater treatment and has potential application value.In the present work,we construct Fe-MOFs heterogenous Fenton-like system for the removal of dyes by choosing refractory organic pollutants such as Rhodamine B?RhB?and Methylene blue?MB?dyes as model pollutants.The main research activities are as follows:?1?The MOF-235?Fe?was used as a heterogenous Fenton-like catalyst for degrading RhB;?2?The MIL-101?Fe?@PAN nanofibers was used as Fenton-like catalyst for catalytic degrading organic pollutants MB;?3?L-cysteine promotes the catalytic degradation of MB by NH₂-MIL-101?Fe?.This paper is divided into four parts:Chapter 1:By searching the reported work on removal of dyes and Fenton oxidation,we briefly introduce the development of the degredation of dyes via Fenton oxidation Processe from three parts?the damage of dyes,the homogeneous Fenton reaction,the heterogeneous Fenton-like reaction?.Chapter 2:MOF-235?Fe?was synthesized by a simple hydrothermal method.The prepared MOF-235?Fe?was characterized by X-ray diffraction?XRD?,scanning electron microscopy?SEM?,fourier transform infrared?FTIR?spectroscopy and thermogravimetric analysis?TGA?.The characterization results showed that the MOF-235?Fe?with high crystalization and crystal integrity was synthesized.The catalytic performance of MOF-235?Fe?was evaluated in the degradation of RhB at pH5.0 at 25 ^oC.Different reaction operating parameters were explored,like initial pH,catalyst dosage,H₂O₂ dosage and temperature.The acquired results demonstrated that the best reaction conditions were RhB concentration of 10 mg/L,pH of 5.0,MOF-235?Fe?dosage of 0.20 g/L,H₂O₂ concentration of 0.20 mol/L and temperature of25 ^oC.Under the optimal conditions,RhB can be totally decolorized after degradation for 60 min,and a removal rate of 63%for TOC was attained.MOF-235?Fe?can degrade RhB in the pH range from 3.0 to 10.0.The degradation kinetic curves can be expressed as the pseudo-first order model.Chapter 3:In this study,we synthesized MIL-101?Fe?by hydrothermal method,and then immobilized MIL-101?Fe?nanoparticles on electrospun polyacrylonitrile?PAN?forming MIL-101?Fe?@PAN nanofibers by electrospinning technique.The MIL-101?Fe?@PAN nanofibers were characterized by XRD,SEM,FTIR and TGA measurements.Detailed characterization showed that MIL-101?Fe?was successfully immobilized on polyacrylonitrile nanofibers,and the obtained MIL-101?Fe?@PAN nanofibers possessed a flexible one-dimensional structure.To demonstrate the catalytic performance of the MIL-101?Fe?@PAN nanofibers,the activation of H₂O₂ for the catalytic degradation of Methylene blue was chosen as the model catalytic reaction.The results show that 98%of MB?10 mg/L?was removed by MIL-101?Fe?@PAN in 60 min,indicating that MIL-101?Fe?@PAN nanofibers retain the catalytic properties of MIL-101?Fe?.More interestingly,the flexible MIL-101?Fe?@PAN composite nanofibers catalysts were not only easy to separate from solution but they also retained high catalytic stability,and still retained good catalytic activity with 81%degradation of MB at the seventh cycle.The influencing factors for MB degradation were also investigated,including the catalyst dosage,solution pH,H₂O₂ concentration and reaction temperature.The degradation mechanism was elucidated by free radical scavenging experiments.The degradation kinetic curves can be expressed as the pseudo-first order model.Chapter 4:We synthesized NH₂-MIL-101?Fe?by hydrothermal method.The synthesized NH₂-MIL-101?Fe?was characterized by XRD,SEM,FTIR and TGA measurements.In order to improve the catalytic activity of NH₂-MIL-101?Fe?,L-cysteine?L-Cys?,a green natural organic ligand,was innovatively introduced into the NH₂-MIL-101?Fe?/H₂O₂ Fenton-like system to construct an excellent catalytic oxidation system.The introduction of L-Cys into the Fenton-like system expanded the effective pH range up to 10.0 and achieved a superior oxidation efficiency,representing about 50%higher removal ratio and 20%higher TOC removal ratio,and 8 times higher reaction rate constant for methylene blue.An investigation of the reaction mechanism indicated that the addition of L-Cys into the system accelerated the Fe?III?/Fe?II?cycle,which enhanced the generation of hydroxyl radicals?·OH?.In addition,we also studied the influencing factors for MB degradation,including L-Cys concentration,initial pH,catalyst dosage,H₂O₂ concentration and reaction temperature.The degradation kinetic curves can be expressed as the pseudo-first order model.