Fenton-like Degradation Of Organic Pollutants By NH₂-MIL-Fe Metal-organic Frameworks

Wastewater pollution has always been a worldwide environmental problem that needs to be solved urgently.Excessive discharge of dye and antibiotic wastewater harms aquatic organisms and human health.Therefore,it is imperative to solve these two types of organic pollutants.Fenton technology has become a popular organic wastewater treatment technology because of its high efficiency,low cost,easy operation and mild reaction conditions.However,the Fenton catalysts have the disadvantages of low catalytic activity,narrow pH working range,and difficulty in recovery,which limits their applications in industry.Thus,it is of great significance to research and develop efficient Fenton catalysts with recycling ability and pH tolerance.The metal-organic frameworks have the characteristics of adjustable active sites,easy functionalization,large specific surface area,and good stability,which is an excellent potential Fenton catalyst.Thus,in this paper,three new Fenton-like systems were constructed by using NH₂-MIL-Fe metal-organic frameworks as the Fenton catalysts,and used for the degradation and removal of organic pollutants such as dye and antibiotic.The full text consists of four parts.The main contents are as follows:Chapter 1:By searching the reported,the applications and advantages of Fenton technology in the removal of organic pollutants were briefly introduced from five aspects:organic pollutants,the methods of pollutant removal,Fenton technology and its development,Fenton catalysts and metal-organic frameworks.Chapter 2:NH₂-MIL-53?Fe?was synthesized by one-pot and characterized by X-ray diffraction?XRD?,scanning electron microscopy?SEM?,and infrared spectrosc-opy?FT-IR?.In this paper,NH₂-MIL-53?Fe?was used as Fenton catalyst to construct NH₂-MIL-53?Fe?/H₂O₂ Fenton-like system for degradation of high concentrations of tetracycline hydrochloride?TC?.The optimal experimental parameters for the degrad-ation of TC were obtained as follows:the catalyst concentration was 0.20 g L^-1,the H₂O₂ concentration was 50.0 mmol L^-1,and the temperature of water bath was 25 ^oC.In this condition,TC(100 mg L^-1)could be degraded by NH₂-MIL-53?Fe?/H₂O₂ system in a wide range of initial pH?3¹0?,and the corresponding total organic carbon?TOC?removal rate is 37.2%.The main active free intermediate species proved to be·OH through the capture experiment of active free radicals,and HO₂·/O₂^-·exists at the same time.This system not only has a strong catalytic activity and pH tolerance,but also has good stability.The catalyst activity only decreased by 5%after four times of use,and keeped the original structure through SEM.The construction of NH₂-MIL-53?Fe?/H₂O₂Fenton-like system effectively solved the problems of narrow pH and recovery.Chapter 3:A new ligand-enhanced high-efficiency Fenton-like system,PA/NH₂-MIL-53?Fe?/H₂O₂,was constructed by introducing pyrogallol?PA?as ligand.After optimization of experimental parameters,the most suitable conditions were obtained:concentration of PA:0.80 mmol L^-1,concentration of NH₂-MIL-53?Fe?:0.20 g L^-1,concentration of H₂O₂:0.04 mol L^-1,initial pH:5.7,reaction temperature:25^oC.The methylene blue?MB?of 30 mg L^-11 could be completely degraded by this system within20 min at pH 3¹1.In the mechanism study section,the contribution of NH₂-MIL-53?Fe?adsorption for MB in the removal of MB was studied and showed that the adsorption kinetics of NH₂-MIL-53?Fe?on MB is consistent to Psudo-second-order model,and isotherm in accordance with Langmuir model with adsorption capacity of 143.3 mg g^-1.Thus,MB is enriched to the surface of the catalyst.Then,·OH was found to be the main active free radical through the experiment of free radical trapping in the degradation of PA/NH₂-MIL-53?Fe?/H₂O₂ system.The·OH in this Fenton-like pcocess could attack MB and break it down.Finally,the role of ligands in this system was discussed.Firstly,PA could enhance the iron ion concentration in the solution through its chelation with iron ions,thereby enhancing the homogeneous Fenton degradation process.Then,PA could accelerate the Fenton's degradation due to its rapid reducibility by rapidly reducing the ferric on the surface of the catalyst and the solution to ferrous.Last,PA not only enhances the decolorization of MB,but also enhances the mineralization efficiency of MB.The PA/NH₂-MIL-53?Fe?/H₂O₂ Fenton-like system not only solved the problem of narrow working range of pH,but also enhanced the activity of the catalyst by introducing an external ligand.Chapter 4:NH₂-MIL-101?Fe?was prapared successfully by a simple method and was used to construct a ligand-enhanced Fenton-like system:HH/NH₂-MIL-101?Fe?/H₂O₂ by introducing hydroxylamine hydrochloride?HH?for the efficient degrada-tion of methyl orange?MO?.The removal rate of MO was improved 60%by addition of HH.In the optimization conditions,MO of 50 mg L^-1could be completely degraded in 10min,and the removal rate of organic carbon?TOC?could reach 72%in 30 min.In HH/NH₂-MIL-101?Fe?/H₂O₂ system,the adsorption of NH₂-MIL-101?Fe?contributed to the degradation of MO,thus the adsorption behavior of NH₂-MIL-101?Fe?on MO was studied.The results showed that the adsorption kinetic and isotherm of NH₂-MIL-101?Fe?was in accordance with the Psudo-second-order model and Langmuir model,respectively.And the adsorption capacity of NH₂-MIL-101?Fe?was 114.3 mg g^-1.The active intermediates for the degradation of MO in the HH/NH₂-MIL-101?Fe?/H₂O₂Fenton-like system have been shown to be mainly·OH by free radical trapping experiments.On the role of HH enhancement,it speculated that the addition of HH created an acidic environment for the degradation process,which facilitated the dissolution of iron ions,increasing the efficiency of Fenton degradation.On the other hand,HH,as a strong reducing agent,could accelerate the ferric and ferrous cycle and the increase·OH,accelerating the entire degradation process.The HH/NH₂-MIL-101?Fe?/H₂O₂ Fenton system constructed by ligand introduction showed a strong catalytic degradation ability to MO,which provided an strategy for building a highly efficient Fenton-like system.