Yong Liu*,Jianfei BaiHongtao DuanXiaohong Yin

1College of Chemistry and Chemical Engineering,Tianjin University of Technology,Tianjin 300384,China

2Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion,Tianjin University of Technology,Tianjin 300384,China

1.Introduction

Manganese,as the second abundant metal in nature,is commonly used in ceramics, dry battery cells and alloys.Mn(II) is a primary speciesin aqueous solution of manganese[1].Mn(II)pollution in our environment especially in the underground water has attracted more attention in the recent years because of its harmful effects to ecosystems and human beings[2].Due to the uncontrolled disposal and untreated discharge into the water bodies,there even exists a “manganese triangle”in the southeast of China where the manganese concentration is excessive to the standard regulation from tens to hundreds times in many water systems.Conventional methods such as chemical precipitation,filtration,electrodialysis and ion exchange are not suitable for the trace levels of Mn(II)removal in water due to their high maintenance and operation costs[3,4].

Adsorption is expected to overcome such impediments,having a wide variety of applications[5].Of many adsorbents,magnetic nanoparticles(Fe3O4)have attracted more attention as effective adsorbents for the removal of heavy metals due to their large surface area,easy separation and low costs[6,7]The chemical routes for the synthesis of magnetic nanoparticles mainly include precipitation,micro-emulsions,sol–gel synthesis and hydrothermal reactions[8].Some surface modification of Fe3O4methods was used to reduce the problems of aggregation but not so well biocompatibility[9].It is an effective method in Fe3O4synthesis with magnetic field[10,11].The applied magnetic field during magnetic particles preparation could have effects on the origination which could reduce the aggregation and improve the separation efficiency[12].

In this study,Fe3O4nanoparticles were prepared in a 0.15 T static magnetic field without any templateviaco-precipitation method.The size,structure and magnetic property of Fe3O4nanoparticles in the presence of magnetic field(PMF)and in the absence of magnetic field(AMF)were characterized by X-ray diffraction(XRD),scanning electron microscope(SEM),transmission electron microscopy(TEM),specific surface area measured by nitrogen adsorption technique with Brunauer–Emmett–Teller(BET)and vibrating sample magnetometer(VSM).The kinetics and thermodynamics of Mn(II)adsorption with Fe3O4nanoparticles were also examined.

2.Materials and Methods

2.1.Materials

Ferric chloride(FeCl3·6H2O),ferrous chloride(FeCl2·4H2O),sodium hydroxide(NaOH),and manganese chloride(MnCl2·4H2O)were of analytical grade and purchased from Tianjin Chemical Reagent Co.,Ltd.Other chemicals were the analytic grade reagents commercially available and used without further purification.All solutions were prepared with distilled,deionized water.Nd–Fe–B permanent magnetic field with magnetic intensity of 0.15T was purchased from Beijing Three Circles World Trade Corporation(Beijing,China).The magnetic field was a hollow cylinder with 100 mm external diameter and 65 mm internal diameter.

2.2.Preparation of Fe3O4nanoparticles

Magnetic nanoparticles were prepared by chemical co-precipitation of ferrous chloride and ferric chloride with NaOH in PMF and AMF.Typically,10 ml of solution containing 1.22 g of ferric chloride and 0.5 g of ferrous chloride was dropped to 25 ml of degassed sodium hydroxide solution(1.3 mol·L-1)with vigorous stirring.During the experiment,the nitrogen gas was kept passing through the solution to prevent the oxidation of Fe2+in the system.After kept at 25°Cfor8h,the produced black solid powders were separated by a magnet,and washed with distilled water and ethanol three times,respectively.The final products were dried in vacuum atmosphere at 60°C for 10 h.

2.3.Batch adsorption experiments

Batch adsorption experiments were carried out by adding 0.5 g Fe3O4nanoparticles with the desired concentration in 100 ml of 0.1 mol·L-1NaCl solution at pH 4,6 and 8 for 6 h to reach equilibrium.Suspension of Fe3O4nanoparticles was mixed thoroughly with 150 ml of 25 mg Mn(II)in 0.1 mol·L-1NaCl solution,which had the same pH values as the Fe3O4nanoparticles suspension.The mixtures were then placed on a shaker of 130 r·min-1at 25 °C,and liquid solution samples(1.5 ml from each flask)were collected at regular intervals by centrifugation at10000r·min-1for10min and analyzed for residual concentrations of Mn(II).In the kinetic studies,the mixtures were stirred for different periods of time(0.5,1,2,3,5,7,10,15 and 23 h)at an initial pH of 4,6 and 8.The adsorption thermodynamics studies of Mn(II)onto Fe3O4nano particles were conducted using the same procedure during 10 h and varying the initial solution concentrations(50,100,200,300,400 and 500 mg·L-1).

The Mn(II)uptake(q),expressed as Mn(II)adsorption amount per unit mass of Fe3O4nanoparticles,was calculated according to Eq.(1).

whereciis the initial Mn(II)concentration(mg·L-1),cfis the final concentration(mg·L-1),Vis the batch volume(L)andmistheFe3O4nanoparticles mass(g).

2.4.Characterization and analysis

Fig.1.XRD patterns of Fe3O4nanoparticles.

Fig.2.SEM images of Fe3O4nanoparticles(a,PMF and b,AMF).

The images of XRD and TEM were performed on a type of D8 FOCUS diffractometer(BRUKER Co.,Switzerland)with Cu Kα(λ =0.1541 nm)radiation,and scanning range from 10°–90°and transmission electron microscope(FEI,Tecnai G2 F20)with an accelerating voltage of 200 kV.The magnetization curve of the Fe3O4nanoparticles was measured on a VSM 6900-1(LDJ Electronics Co.,USA)at room temperature.SEM images of the products were taken using a field emission(FEI,Nanosem 430).Surface area measurement was taken with a BET analyzer(Gemini V,Micrometrics Co.,USA)at liquid nitrogen temperature using conventional gas sorption apparatus.Mn(II)concentration in aqueous solution was determined by atomic absorption spectroscopy(AAS)(Shimadzu,AA-6501).

3.Results and Discussion

3.1.Characterization of Fe3O4nanoparticles

Fig.3.TEM images of Fe3O4nanoparticles(a,PMF and b,AMF).

Fig.1 showed the XRD patterns for Fe3O4nanoparticles prepared with PMF and AMF.As seen from Fig.1,both Fe3O4nanoparticles had six characteristic peaks for Fe3O4(2θ =30°,36°,43°,53°,57°,and 62°),marked by their indices((220),(311),(400),(422),(511),and(440)).These peaks were consistent with the database in the JCPDS file(PDF standard cards,JCPDS88-0315)andrevealed that themagnetic field had no obvious different effect on the crystal structure of Fe3O4.The reflection peaks of XRD patterns became sharper and narrower with PMF,which indicated that Fe3O4nanoparticles prepared with PMF had better crystallinity.The followed SEM and TEM images also could con firm that.Fig.2 showed the SEM images for Fe3O4nanoparticles prepared with PMF(Fig.2a)and AMF(Fig.2b).From Fig.2,it could be found that the Fe3O4nano particles prepared with PMF appeared rodlike structures(Fig.2a),varied from those sphere-like nano particles obtained with AMF(Fig.2b).The TEM images in Fig.3 clearly revealed the effect of magnetic field on the nucleation and growth process of Fe3O4nanoparticles.Without the magnetic field,the shape of all Fe3O4nanoparticles was square and hexagonal with an average diameter of 10 nm(Fig.3b).However,the morphology changed drastically when an external magnetic field was applied.Beside the square particles,nanorods with the length of about 100 nm were observed when a magnetic field was added(Fig.3a).The introduction of the magnetic fieldaffected the chemical reaction rate and the process of crystal nucleation and growth[13].According to the principle of quantum mechanics,the chemical reactionrate was not entirely decided by the energy factor but also depended on the electron spin[14].Magnetic field affected on the rate of radical pair recombination and the phenomenon of magnetic fluid dynamic son the aqueous solution containing chargedions was the best understood mechanism by which magnetic fields interacted with chemical reaction systems.However,the principal mechanism about the effect of magnetic field on the rod-like nanostructure was complex and uncertain[15].Fig.4 showed magnetization curves of Fe3O4nanoparticles prepared with AMF and PMF at room temperature.As shown in Fig.4,the magnetization curves had no loops for both the magnetic nanoparticles,showing that they were superparamagnetic.TheMswas 42.67 emu·g-1for the Fe3O4nanoparticles prepared with PMF and 17.65 emu·g-1for the nanoparticles prepared with AMF.The higherMsvalue indicated that the time of magnetic separation for the Fe3O4nanoparticles in the aqueous solution was less.That could potentially benefit the application in the adsorption process.The specific surface area of Fe3O4nanoparticles calculated based on BET method was 60.4 and 74.6 m2·g-1for prepared with AMF and PMF,respectively.

3.2.Adsorption kinetics

Fig.4.Hysteresis loop of Fe3O4nanoparticles at room temperature.(1 Oe=79.5775 A·m-1)

Fig.5.Kinetic study of Mn(II)absorption at different pH values.Initial concentrations of Mn(II)and Fe3O4nanoparticles(PMF(a)and AMF(b))were 100 mg·L-1and 2 g·L-1,respectively.

In order to investigate the Mn(II)adsorption on the Fe3O4nanoparticles PMF and AMF,the batch experiments were carried out.Fig.5 showed the Mn(II)adsorptiondataover Fe3O4nanopartic lesat different time intervals.As seen from Fig.5,Mn(II)adsorption capacity for the Fe3O4nanopartic les PMF and AMF was reached at 36.81 and 28.36 mg·g-1Fe3O4,respectively.This could be explained that Fe3O4nanoparticles PMF had a larger surface area than the control due to the ordered assembly structure under the magnetic field.At the same time,solution pH was one of the most important variables affecting the adsorption characteristics.As shown in Fig.5,for both Fe3O4nanoparticles PMF and AMF exhibited higher Mn(II)adsorption capacity under basic condition(pH 8)than acidic condition(pH 4).The pH dependent adsorption mechanism could be explained that the surface of metal oxides was generally covered with hydroxyl groups which vary in form at different pH levels.With an increase of pH of the solution,the adsorption capacity of Mn(II)increased,which was due to the negative higher concentration of OH-present in the surface of Fe3O4nanoparticles.

To further investigate the adsorption kinetics of Mn(II)adsorption on the Fe3O4PMF,two kinetic models,pseudo- first-order and pseudosecond-order[2],were introduced and compared in this study.The pseudo- first-order kinetic model indicated that the rate of uptake with time was directly proportional to the difference in the saturation concentration and the amount of solute uptake with time.The general equation was expressed as:

whereqeqandqtare the amounts of metal ions adsorbed(mg·g-1)at equilibrium and at timet,respectively,andK1is the pseudo- first order adsorption rate constant(min-1).The pseudo-second-order kinetic model that was based on the rate of adsorption between adsorbent and adsorbate was a second order mechanism.The pseudo-secondorder kinetics rate equation was expressed as:

whereK2is the second-order adsorption rate constant(mg·g-1·min-1).The fitting results obtained from different models at pH6 and pH 8 were summarized in Table1.At pH 6 and pH 8,the correlation coefficients for the pseudo- first order and pseudo-second-order models were 0.886,0.999 and 0.904,0.999,respectively.The higherR2value indicated that the pseudo-second-order model was more applicable to the kinetics of Mn(II)adsorption on Fe3O4nanoparticles.Meanwhile,the results ofqeqcalculated and experimental values also indicated that pseudo first-order kinetic model was not quite satisfactory with the adsorption process.

3.3.Adsorption isotherms

To establish the mechanism for Mn(II)adsorption onto Fe3O4nano particles PMF,the Langmuir and Freundlich isotherm models were introduced.These models were given by Eqs.(6)and(7),respectively.

In this study, Langmuir and Freundlich data fitting were taken by linearizationand given by Eqs. (8) and (9), respectively.

Table 1Mn(II)adsorption kinetic parameters by Fe3O4nanoparticles PMF

Fig.6.Mn(II)adsorption isotherms on the Fe3O4nanoparticles PMF.Initial concentration of Fe3O4nanoparticles was 2 g·L-1,equilibrium time was 24 h,temperature was at 25 °C.

Table 2Parameters for Mn(II)adsorption by Fe3O4nanoparticles PMF

whereqwas the equilibrium amount of Mn(II)adsorbed,cewas the equilibrium concentration,qmaxandKLwere the Langmuir maximum adsorption capacity and the equilibrium constant,respectively.The parametersKFandnwere the Freundlich adsorption capacity and intensity parameter,respectively.

Fig.6 showed adsorption isotherms of Mn(II)on the Fe3O4nanoparticles under different pH conditions which fitted with the Langmuir and Freundlich adsorption models.The values of the constants in the models and correlation coefficients obtained were summarized in Table 2.As seen from Table 2,theR2values of the Freundlich adsorption isotherms were larger than the Langmuir adsorption isotherms.Hence,the equilibrium data of Mn(II) fit the Freundlich adsorption isotherm well.Therefore Mn(II)adsorption was the result as a combination of several interfacial reactions namely,ion exchange,chemi-sorption besides monolayer adsorption.The adsorption capacities(KF)and adsorption intensities(n)were presented in Table 2.Value ofnwas greater than 1 showed a favorable adsorption of Mn(II)on the Fe3O4nanoparticles.Higher value forKFat pH 8 indicated higher affinity between Mn(II)and magnetic nanoparticles under the basic condition.

4.Conclusions

The Fe3O4nanoparticles were prepared with PMF and AMFviacoprecipitation method.The results showed that the crystal structure of Fe3O4nanoparticles was not influenced whereas the morphology was significantly changed by the magnetic field.The well-order rod like Fe3O4nanoparticles prepared with PMF showed the higherMsthan AMF and would need a shorter time in the solution with magnetic separation method.The larger surface area of Fe3O4PMF showed higher Mn(II)adsorption capacity.The adsorption results showed that the amount of Mn(II)adsorbed increases with pH and that the adsorption kinetics study of the Mn(II)followed a pseudo-second-order model.The adsorption isotherm indicated that the Freundlich models fit well for the adsorption process.All there sults found that the Fe3O4nanoparticles PMF showed a good potential as adsorbent for Mn(II).

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