Performance And Stabilization Of MOFs For Removal Of Different Arsenic And Selenium Species In Water

The efficient removal of toxic elements such as arsenic(As)and selenium(Se)from water is an important research topic for drinking water safety and sewage treatment.Different species of arsenic and selenium have different environmental behaviors and toxicological characteristics,and their removal mechanisms and processes are different.Therefore,the development of efficient materials to remove different species of arsenic and selenium is the key of arsenic and selenium pollution control.Metal organic framework compounds(MOFs)have great potential for pollutants removal in water due to their high specific surface area,high porosity,and tunable pore channel.However,the functional performance of mono-metal MOFs materials is relatively homogeneous because of the limitation of their metal active components.Moreover,solid(powder)MOFs are difficult to recover and recycle from water and cause secondary pollution,which also result in the application limitation of MOFs materials.Compared with unitary metal MOFs,bimetallic MOFs have topological defects and produce synergistic effects through tight integration,which in turn enhance the intrinsic properties of MOFs and improve the removal capacity to arsenic and selenium.Therefore,exploring the adsorption properties of binary metal MOFs for different forms of arsenic and selenium,and constructing a stabilization and recoverable strategy based on composite materials will be of great significance for developing new methods for arsenic and selenium pollution control and safe application of MOFs.In this paper,we successfully prepared Fe/Zr binary metal MOFs to address the practical problems faced by MOFs materials in applications,deeply investigated their adsorption properties and mechanisms on different species of arsenic and selenium,established a new method of electrostatic spinning to prepare nanocomposite fiber membranes for MOFs stabilization,and innovatively developed a new technique for the intrinsic performance enhancement and dissolution of encapsulated MOFs.The main results of this study are as follows:(1)Based on the principle that unsaturated metal sites of MOFs can be replaced,a one-step solvothermal method was used to prepare a water-stable bimetal Fe/Zr MOF[UiO-66(Fe/Zr)],in which Fe element was successfully introduced into the topology of UiO-66 by replacing some of the Zr sites.UiO66(Fe/Zr)can fully exploit the synergistic effect of two functional centers of Fe/Zr and large specific surface area,and exhibits rapid adsorption kinetic behavior and large adsorption capacity for different forms of arsenic.The maximum adsorption capacities of UiO-66(Fe/Zr)to As(Ⅲ)and As(Ⅴ)reached 101.7 mg/g and 204.1 mg/g at pH=7,respectively.The leaching procedure(TCLP)experiment indicated the leaching amount of As(Ⅴ)(3.19 mg/L)from used UiO-66(Fe/Zr)was lower than the US EPA hazardous waste leaching standard(5 mg/L),which demonstrated higher adsorption capacity and effective immobilization capacity of UiO-66(Fe/Zr),and greatly reduced the risk of secondary contamination of the adsorbent after use.Combined XPS characterization and DFT theoretical calculation,this study revealed the possible reaction mechanism between UiO-66(Fe/Zr)and As at the molecular level.The results showed that the affinity of As to Zr was better than that of Fe,and the introduction of Fe active sites enhanced the adsorption of As on the adsorbent surface and improved the adsorption efficiency of Zr at the reaction sites.(2)The bimetallic UiO-66(Fe/Zr)adsorbent also showed excellent adsorption performance for different species of Se.The maximum adsorption capacities of UiO-66(Fe/Zr)for Se(Ⅳ)and Se(Ⅵ)reached 196.77 mg/g and 258.81 mg/g at pH=3 and pH=5,respectively.UiO-66(Fe/Zr)had an excellent pH applicability and good anti-interference ability to coexisting ions,as well as good regeneration performance and stability.The adsorption efficiency higher than 90%was maintained after 5 adsorption/regeneration cycles.The adsorption mechanism was explored by the instrumental characterization results and DFT theoretical calculations.The results indicated that the adsorption of Se(Ⅵ)mainly occurred on the unsaturated sites of Zr.The binding energy of Fe and Se(Ⅵ)was similar with Zr sites,and both of them played a synergistic role in the adsorption of SeO42-.The conclusions by parts(1)and(2)showed that the Fe/Zr bimetallic MOF materials were successfully prepared based on the bimetallic synergistic mechanism,and a new material for the efficient removal of different species of arsenic and selenium from water was provided.(3)A strategy for preparing UiO-66/PAN nanofiber membranes by electrostatic spinning blending was established,and the preparation process parameters of the fiber membranes were optimized to resolve the practical problems such as the difficulty of separating solid MOFs after use,difficulty of recovery and recycle caused secondary pollution.XRD,mapping and other characterization methods demonstrated that UiO-66 was successfully embedded in PAN nanofibers.UiO-66/PAN nanofibers retained the complete structure and intrinsic properties of UiO-66,as well as made the composite macroscopically present as thin films with good flexibility,easy handling and recyclability.The adsorption processes of UiO-66/PAN composite fiber membranes to As(Ⅲ)and As(V)were mainly based on the chemisorption of single molecular layers,and the maximum adsorption amounts were 32.90 mg/g and 42.17 mg/g under neutral conditions,respectively.The UiO-66/PAN nanofibers can be regenerated by methanol after adsorption,and the material can maintain the intact material structure and good adsorption performance after five cycles.UiO-66/PAN nanofibers improved the anti-interference ability of MOFs,and can be easily separated and recycled from the treatment system,effectively solved the difficulty of solid MOFs separation and avoided secondary pollution.However,the intrinsic performance of MOFs was somewhat limited due to the encapsulation of MOFs by polymers.(4)A new method of swelling to improve the overall adsorption performance of the composite membrane was explored,which solved the problem that MOFs encapsulated in nanofibers cannot full play their intrinsic properties.The experimental results showed that the adsorption efficiency of As(V)was significantly improved by the swelling of pyridine(NFs-P)and nitrobenzene(NFs-N)compared with the unswollen membranes.The results of XRD and SEM showed that the expanded fiber film not only maintained the complete morphological characteristics,but also maintained the original structure of UiO66.In addition,the swelling and adsorption processes do not cause Zr leaching.Pyridine had the best swelling effect,and the adsorption efficiency could be increased by 45%after 1 h swelling.The swelling method effectively solved the problem of performance degradation caused by encapsulation of active components in the electrospun MOFs composite,and provided a new way to improve the intrinsic performance of active components in electrospun nanofiber membranes.In summary,the study successfully prepared Fe/Zr based binary MOFs material,which realized the rapid adsorption and effective immobilization of different species of As and Se in water.Combined with the experimental characterization and DFT theoretical calculation results,the adsorption performances and mechanism were thoroughly investigated.In addition,this study established a strategy for preparing UiO-66/PAN nanofiber membrane by electrospinning,and explored a new method of swelling for composite fiber membrane,which not only minimized the secondary pollution risks,avoided the agglomeration and loss of MOFs during use,but also effectively solved the problem of MOFs cannot fully play their intrinsic properties.This study provided a new material for the efficient removal of arsenic and selenium in water and a new strategy for the safe utilization and sustainable development of nanomaterials.