Investigation On The Preparation,Adsorption Performance For Pb(?) And Mechanism Of Novel Type Iron-based Oxide Magnetic Fibers As High-efficiency Adsorbents

Iron-based oxide magnetic fibers not only possess excellent adsorption performance,but also have the advantages of large aspect ratio,self-supporting and non-agglomeration of the specifical fiber morphology and easy separation of the magnetic materials in an external magnetic field,so they have been widely used in water pollution treatment.Electrospinning is a commonly used technology for preparing fiber materials.With the sol-gel method to prepare precursor spinning solution,magnetic fibers with high saturation magnetization can be prepared.Fe3O4 and MgFe2O4 are the most commonly used magnetic materials,which have the advantages of high saturation magnetization,wide source of raw materials and environmental friendliness,etc.They are ideal adsorbent materials for the treatment of pollutants in wastewater.Firstly,starting from the formation principle and spinnability of the precursor spinning solutions,Fe3O4 precursor spinning solutions with excellent spinnability were prepared by adjusting their surface tension,conductivity and viscosity,and Fe3O4 fibers with high saturation magnetization were prepared by electrospinning technology and appropriate heat treatment process.This method is one of the few methods to fabricate Fe3O4 electrospinning fibers by sol-gel method.It overcame the drawbacks of the commonly used blend spinning method,such as the uneven distribution of magnetic nanoparticles in the Fe3O4 fibers,even the blocking of the spinning device in the spinning process and the low saturation magnetization of the fibers due to the limited amount of magnetic nanoparticles.The reason why the fibers were easily reduced to Fe fibers during heat treatment was explored,and the heat treatment process of fibers pretreatment in air was proposed.In order to further improve the adsorption performance of magnetic fibers,through loading MnO2 nanosheets on the surface of Fe3O4 fibers with polydopamine(PDA)as the intermediate platform,MnO2/PDA/Fe3O4 fibers were prepared.The optimal reaction time of loading PDA and MnO2 for the preparation of MnO2/PDA/Fe3O4 fiber was explored to obtain high adsorption efficiency.MgFe2O4 fibers were prepared by citric acid assisted sol-gel method and electrospinning technique.It overcame the disadvantages of the commonly used sol-gel method,such as low quality of MgFe2O4 fibers and easy pulverization during heat treatment.The influence of different raw materials on the fiber quality and adsorption performance was explored;the effects of the addition amount of surfactant cetyl trimethyl ammonium bromide(CTAB)and heat treatment temperature on the adsorption performance were studied.?.Preparation of Fe3O4 fibers by sol-gel method combined with electrospinning technology.1.Formation and spinnability of spinning solutions.The precursor spinning solution was prepared by using the newly prepared ferric hydroxide and citric acid as raw materials and the mixture of water and ethanol as solvent.The Fourier transform infrared(FT-IR)spectral results indicated that most of the citric acid chelated with iron ions in the formation of the spinning solutions,while the spinning solutions still contained a small amount of unreacted citric acid.The frequency separation(?v)between the asymmetric and symmetric stretching vibrations of carboxyl group in precursor fibers was 225 cm-1,which was larger than the ?v value of free RCOO(the?v value of RCOO in sodium citrate is 196 cm-1).This indicated that the iron ions in the spinning solutions were coordinated to carboxyl group in mono-dentate fashion.The coordination complex of iron ions and citric acid prepared by the sol-gel method was easily soluble in water,soluble in the mixture of water and ethanol,but insoluble in pure ethanol.The solutions used water as a solvent had a fine spinnability.However,it needs a higher environmental temperature to obtain dry precursor fibers.The effects of ethanol ratio and solvent amount on the surface tension,conductivity and viscosity of the spinning solution were studied.It was found that the spinning solutions had good spinnability and were easy to obtain dry precursor fibers when the amount of the solvent was 40?48 g of per 0.04 mol Fe3+,and the proportion of ethanol in the solvent was 67%.2.Analysis of evolution process of Fe3O4 fibers.Through analysizing the X-ray diffraction(XRD)patterns,we found that when the precursor fibers were heat treated in nitrogen directly,they were amorphous at 400? and then were directly reduced to iron fibers at 600?.It was hard to form Fe3O4 crystalline phase due to the strong reduction ability of the organics in the fibers.According to the thermogravimetric analysis(TGA)results of the precursor fibers,there was a rapid weight loss process occurred at around 200? during the heat treatment of the precursor fibers.In order to reduce the reduction ability of organics in the fibers,combined with the results of thermal analysis,pretreated at 200-300? in air was set to promote the removal of organics.Fe3O4 fibers could be obtained by pretreating in air firstly,and then heat-treating to 600? in nitrogen,and 200? was the optimum temperature for pretreatment in air.The average diameter of Fe3O4 fibers was 0.759 ?m,the specific surface area was 51.03 m2/g,the saturation magnetization(MS)was up to 60 emu/g.3.Comparison of adsorption properties between Fe3O4 fibers and Fe3O4 powder.The adsorption performance of Fe3O4 fibers for Pb(?)was tested to characterize the adsorption for heavy metal,and Fe3O4 powder(milled from Fe3O4 fibers)was used as the contrastive adsorbent.According to the experimental data of adsorption kinetics and the fitting data of pseudo-first-order(PFO)and the pseudo-second-order(PSO)models,it was obtained that the adsorption rates of the fiber and the powder adsorbents were not much different during the adsorption process.The maximum adsorption capacity(Qm)calculated from Langmuir adsorption isotherm of Pb(?)adsorbed by Fe3O4 fibers and Fe3O4 powder were 16.78 and 15.80 mg/g,respectively,which were close to the experimental data.The Qm value of Pb(?)adsorbed by Fe3O4 fibers was slightly higher than that of adsorbed by Fe3O4 powder,which may be due to the fibers did not agglomerate in water.After six adsorption-desorption cycles,the Fe3O4 fibers and powder remained 95.3%and 89.5%of the original adsorption capacity,respectively.During the adsorption-desorption cycle experiments,the adsorption capacity of Fe3O4 powder decreased significantly compared with that of Fe3O4 fibers,which may be due to the loss of the powder form during the cleaning process.?.Preparation and Pb(?)adsorption properties of MnO2/PDA/Fe3O4 fibers.1.Preparation and optimization of MnO2/PDA/Fe3O4 fibers.Fe3O4 fibers were added into dopamine hydrochloride(DA)solution and then buffered with Tris-HCl solution to keep the pH value of the mixed solution at 8.5 to prepare the polydopamine-coated Fe3O4 fibers(PDA/Fe3O4 fibers).The obtained PDA/Fe3O4 fibers were subsequently immersed into potassium permanganate(KMnO4)solution to prepare MnO2/PDA/Fe3O4 fibers.The high magnetically responsive fiber carrier and a metal oxide nanomaterial with excellent adsorption properties were simultaneously introduced into the composite adsorbents,which can achieve rapid magnetic separation while achieving excellent adsorption performance.The results of scanning electron microscopy(SEM)and specific surface area showed that with the prolongation of reaction time in KMnO4 solution,the loading amount of MnO2 nanosheets on the fiber surface will increase.Excessive MnO2 nanosheets accumulated on the fiber surface would reduce the specific surface area of the fiber.We obtained that the MnO2/PDA/Fe3O4 fibers prepared in DA solution for 18 h and then KMnO4 solution for 6 h had the maximum adsorption capacity for Pb(?)by studying the adsorption capacity of fibers reacting in DA solution and KMnO4 solution for different time.The MnO2/PDA/Fe3O4 fibers prepared in this way had an average diameter of 935 nm,a saturation magnetization of 34.0 emu/g,and a specific surface area of 83.824 m2/g.2.Study on the the pH effect,adsorption kinetics,adsorption isotherm,thermodynamics.The maximum adsorption capacity of MnO2/PDA/Fe3O4 fibers for Pb(?)at 300 K,308 K and 318 K calculated from Langmuir model was 196.67,200.45 and 205.07 mg/g,respectively,all of which were higher than those of most reported MnO2 based Pb(?)adsorbents.The changes in standard Gibbs free energy(?G0)of Pb(?)adsorption on MnO2/PDA/Fe3O4 fibers was in the range of-40.52 and-43.43 kJ·mol-1,which indicated that the Pb(?)adsorption by fibers was a spontaneous physisorption together with chemisorption process.The priority of heavy metal ions adsorption on MnO2/PDA/Fe3O4 fibers was:Pb(?)>Cu(?)>Zn(?)>Cd(?)>Ni(?).The Pb(?)adsorption property of MnO2/PDA/Fe3O4 fibers was slightly decreased due to the presence of the coexisting heavy metal ions or the cations(K+,Na+,Mg2+,Ca2+)and anions(Cl-)of the different ion concentration background electrolytes.MnO2/PDA/Fe3O4 fibers had excellent treatment ability for Pb(?)containing wastewater.Under the laboratory simulation test conditions,the prepared MnO2/PDA/Fe3O4 fibers exhibited an excellent treated capacity of?2500 kg wastewater/kg adsorbent.This could dramatically purify wastewater from 10mg/L to below 0.01mg/L to reach the drinking water standard recommended by WHO.Furthermore,treated capacity of?6500 kg wastewater/kg sorbent,could decontaminate wastewater from 10mg/L to below 0.1mg/L to reach the sewage discharge standard recommended by China.After six adsorption-desorption cycles,the obtained fiber adsorbents still remained more than 83%of the original adsorption capacity,and had good recycling performance.The adsorption mechanism of MnO2/PDA/Fe3O4 fibers adsorbed Pb(?)included ion exchange and surface complexation.?.Preparation of MgFe2O4 fibers by citric acid assisted sol-gel method.1.A simple citric acid-assisted sol-gel method was developed to prepare MgFe2O4 fibers.In previous report,researchers prepared electrospun MgFe2O4 fibers by dissolving magnesium nitrate and ferric nitrate in N,N-dimethylformamide(DMF)and increased viscosity with polyethylene pyrrolidone(PVP)ethanol solution,and then electrospined.However,to the best of our knowledge,MgFe2O4 fibers obtained by this method were brittle and would be pulverized in the heat treatment process.In this original study,we used different magnesium and iron sources,prepared the four kinds of different spinning solutions by citric acid assisted sol-gel method,and prepared MgFe2O4 fibers through electrospinning.The results of infrared spectrum showed that citric acid complexed with metal ions to form spinning solution,which was the reason that the fibers prepared did not pulverized during heat treatment.The effects of different spinning solutions on fiber morphology,saturation magnetization and specific surface area were explored.Among the four kinds of fibers,the MgFe2O4 fibers prepared by newly prepared iron hydroxide and magnesium acetate assisted by citric acid had the most fluffy appearance,the largest specific surface area and highest heavy metal adsorption performance.The order of adsorption capacities of different MgFe2O4 fibers for different heavy metal ions was all Pb(?)>Cu(?)>Zn(?)>Cd(?)>Ni(?).2.The application of CTAB in precursor fibers could promote the thermal decomposition of organic matter.The saturation magnetizations of MgFe2O4 fibers,prepared from the spinning solutions with CTAB contents of 0%,4%and 8%,were 6.7,10.6,and 14.0 emu/g,respectively.The specific surface area of the fibers was basically not affected by the content of CTAB,but decreased rapidly with the increase of heat treatment temperature.The maximum adsorption capacities of MgFe2O4 fibers prepared from 0%,4%and 8%CTAB content spinning solutions for Pb(?)were 263.27,276.87 and 283.13 mg/g,respectively.Among the MgFe2O4 based adsorbents,the flexible MgFe2O4 fibers prepared in this work had the highest maximum adsorption capacity for Pb(?).Since the fiber adsorbents were self-supporting in aqueous solutions,they did not agglomerate,and therefore,have no hinderance on adsorption performance.The competitive adsorption experiments of bivalent coexisting heavy metal ions showed that Cu(?)had the greatest influence on the adsorption of Pb(?)by MgFe2O4 fibers,indicating that the fibers also had a higher affinity to Cu(?).However,Cd(?),Ni(?)and Zn(?))had weak effects on the adsorption of Pb(?).The FT-IR,XRD and XPS spectral results of MgFe2O4 fibers before and after Pb(?)adsorption proved that Pb(?)ions approached to the surface of fibers due to electrostatic interaction,and then exchanged with H+in the hydroxyl or Mg2+ and Fe3+ to form PbO,or combined with OH-to form Pb(OH)2 precipitation on the surface of the adsorbent.Subsequently,due to the CO2 adsorbed on the fibers surface and dissolved in the water system,Pb(OH)2 was converted into more stable Pb3(CO3)2(OH)2 and PbCO3.