Adsorption of Methyl Orange on Magnetically Separable Mesoporous Titania Nanocomposite☆

Libo Gao,Qiang Zhang,Junyang Li,Ruiting Feng,Hongyan Xu,Chenyang Xue,*

Energy,Resources and Environmental Technology

Adsorption of Methyl Orange on Magnetically Separable Mesoporous Titania Nanocomposite☆

Libo Gao1,2,Qiang Zhang1,2,Junyang Li1,2,Ruiting Feng1,2,Hongyan Xu3,Chenyang Xue1,2,*

1Key Laboratory of Instrumentation Science and Dynamic Measurement of Ministry of Education,North University of China,Taiyuan 030051,China

2Science and Technology on Electronic Test&Measurement Laboratory,North University of China,Taiyuan 030051,China3School of Materials Science and Engineering,North University of China,Taiyuan 030051,China

A R T I C L EI N F O

Article history:

Mesoporous Fe3O4-SiO2-TiO2

Adsorption

Methyl orange

Pseudo-second order model

The adsorption of mesoporous Fe3O4-SiO2-TiO2(MFST),which can be separated easily from solution by a magnet,for theremovalofmethylorange(MO)wasinvestigated.Thenitrogenadsorption-desorption measurement shows successful synthesis of MFST with an average pore size of 3.8 nm and a large specif i c surface area of 55 m2·g?1.About 95%adsorption percentage of MO is achieved with an initial concentration of 10 mg·L?1in the dark and the MFST exhibits superior adsorption ability under acid conditions.The adsorption data f i t well with the pseudo-second order model for adsorption.After 4 cycles,the adsorption rate for MO remains 74%in the dark and the MFST can be recovered in a magnetic f i eld with a recovery of about 80%(by mass).It demonstrates that the samples have signif i cant value on applications of wastewater treatment.

?2014TheChemicalIndustry andEngineeringSocietyofChina,andChemicalIndustryPress.Allrightsreserved.

1.Introduction

Titaniumdioxide(TiO2)exhibitsastablechemicalstructure,anditis bio-compatible,nontoxic and cost effective[1,2].It is useful for removal of many organic contaminants from wastewater such as dyes and volatile organic compounds with photocatalytic oxidation[3-5].Considerable hydroxyl groups(OH)are present on TiO2surface and the pollutants in water can be adsorbed on the surface via interaction with OH.With large specif i c surface area,more active sites and highly porous structure,mesoporous TiO2should present high photocatalytic activity[6].It is expected to possess higher removal capability than non-porous TiO2.Therefore,mesoporous TiO2has been an area of intense interest for the past years,particularly for photocatalysis,water purif i cation,and environmental remediation[7-9].

However,much less research has been devoted to the adsorption of TiO2compared to photocatalysis for organic compounds[10-12].TiO2nanoparticles have large adsorption capacity due to their large surface area,sotheymaybeeffectivemetalsorbentscomparedtobulkparticles [13-15].Recently,TiO2nanomaterial is developed as adsorbent for high-chroma crystal violet,showing higher removal capacity than the raw material P25[16].Also,mesoporous TiO2is found to be an effective adsorbentfororganic compounds withcarboxy groups[17].Asuha etal. synthesized mesoporous TiO2by a hydrothermal method and investigated its adsorption for Cr(VI)and methyl orange(MO)[18].The adsorption capacity for MO was approximately 11 times higher than that of commercialTiO2and the specif i c surfacearea wasabout4 timeslarger,which may be the reason for high adsorption capacity.However, their mesoporous TiO2had a poor reusability for adsorption of MO.In addition,removing nano-sized TiO2from large volumes of water is diff i cult and expensive,which restricts its applications[19].Magnetic separation provides a convenient method for removing and recycling magnetic particles by applying external magnetic f i elds[20].Hence TiO2with magnetic particles may solve such a problem in separation from treated water,simply by applying an external magnetic f i eld [21].The crystallization of sol-gel TiO2is usually performed at high temperature and the magnetic materials,such as Fe3O4,may transform toα-Fe2O3rapidlyiftreatedconcurrently.Fe3O4ismuchmoresensitive and unstable compared with TiO2,especially under acidic conditions[22],so it is helpful to insert a passivation layer,such as a silica layer,between the magnetic core particle and TiO2coating.

Inthispaper,wereportthepreparationofFe3O4nanoparticlesasthe magnetic core via the carbon reduction method and the fabrication of mesoporous Fe3O4-SiO2-TiO2(MFST)with excellent adsorption ability and recyclability through the sol-gel method.The adsorption ability of MFST in the dark for removal of MO is examined after each cycle.The MO is chosen as the representative model compound(Fig.1)because it is a typical water-soluble anionic dye.

Fig.1.The structure of methyl orange dye.

2.Experimental

2.1.Materials

Ferric chloride,tetraethoxysilane(TEOS),tetra-butyl ortho-titanate (TBOT),ethanol,hydrochloric acid,nitric acid,and MO(AR)were purchased from Sinopharm Chemical Reagent Co.,Ltd(Shanghai,China).

2.2.Sample preparation and characterization

Fe3O4nanoparticles were prepared by a novel method named carbon reduction method as reported in our previous paper[23].Fe3O4particles and 100 ml of alcohol were put into a beaker in an oven. When the temperature was raised to 50°C,15 ml TEOS was added. Then 0.97 ml of hydrochloric acid and a little deionized water were addedintothecollosol.Theresidualcollosolwasremovedfromthebeakerwhiletheparticlesatthebottomwereattractedbymagnetafter3h. Finally,wet particles were put in a quartz Petri dish and kept under 500°C and 2.6×10?5MPa vacuum conditions for 2 h.Fe3O4-SiO2particles were obtained after milling.The MFST coated with three TiO2layers was prepared as follows.Fe3O4-SiO2particles were put into the beaker with TBOT and alcohol.The beaker with collosol and particles was in an ultrasonic bath for 10 min and stirred for 20 min.Deionized water was added with slow stirring.The ratio of ester,alcohol to water was 30.3:12.5:1.At last,the particles were heated at 500°C for 2 h at 2.6×10?5MPa.Aftermilling,theMFST wasobtained.Theprocedure was repeated for three times to obtain the MFST coated with three TiO2layers.

MFST particles were characterized by TEM and XRD.The components were measured by advanced X-ray diffraction(XRD)system (Bruker D8)using Cu Kαradiation of wavelength 0.15406 nm.Shapes and sizes of particles wereexamined usingJEOLJEM-1200EX TEMmeasurements(operated at 120 kV).Magnetic properties were characterized on a Lake Shore 7307 vibrating sample magnetometer.Nitrogen adsorption-desorption isotherms were taken by a Micromeritics ASAP 2020 instrument.Brunauer-Emmett-Teller(BET)surface area was calculated from adsorption branches and pore size distribution was from desorption branches using the Barrett-Joyner-Halenda(BJH)method. The ultraviolet visible adsorption spectra were measured with a UVVis-NIR spectrophotometer 5000(Varian).

2.3.Measurement of adsorption ability

The adsorption ability of MFST was evaluated by measuring the adsorptionpercentageofMOinanaqueoussolutionat room temperature. 0.2 g MFST was added with stirring into 50 ml of MO aqueous solution with concentration of 10 mg·L?1and maintained in the dark. 6 mol·L?1of HCl and 6 mol·L?1of NaOH were used to adjust the pH value of the solution.2 ml suspension solution was removed at regular intervals and f i ltered through a high-speed desktop centrifuge(TGL-16KZhuhai Black Horse Medical Equipment Co.,Ltd)for adsorption analysis.The UV-Vis adsorption spectra were measured with a UVVis-NIR spectrophotometer 5000(Varian)atλ=462.5 nm.The experiment was performed until nearly complete adsorption.All the tests were repeated three times;the presented results are the average of three measurements.The adsorption capacity of MFST is calculated by

where qtis the adsorbed amount of MO(mg·g?1),C0and Ctrepresent the initial concentration(mg·L?1)and that at time t,respectively,V is thevolumeofMOsolution(L),andWisthemass(g)ofMFSTadsorbent.

2.4.Regeneration

The MFST wasrecovered in a permanentmagnetic f i eld and washed with sodium hydroxide and deionized water completely for further experimentalruns.Specif i cally,0.2gMFSTcontainingMOwascollectedin a permanent magnetic f i eld.Then it was added under stirring into 100 ml of NaOH aqueous solution with concentration of 1 mol·L?1andmaintainedinthedarkwithstirringfor1h.TheMFSTwasremoved from NaOH solution in a permanent magnetic f i eld and washed with 0.05 mol·L?1acid solution and deionized water.Finally,the recovered MFST was dried at 50°C in a vacuum oven for further experiments.

3.Results and Discussion

Fig.2.XRD pattern(a)and magnetization curves(b)of MFST.

3.1.Characteristics of synthesized MFST

Fig.2(a)shows the XRD pattern of MFST.The relatively strong and sharp diffraction characteristic peaks of Fe3O4and anatase structuralTiO2are clear according to JCPDS f i le.The relative intensities of(101) and(105)diffraction peaks indicate pure anatase of TiO2.There is a broad envelop between 20°-30°,suggesting the presence of amorphous silica in the sample.Fe3O4ensures the separation of MFST from the solution.Fig.2(b)reveals that the intensity of applied magnetic f i eld decreases to zero when the remanence magnetism of MFST is 7.7 emu·g?1,which is suff i ciently small to keep MFST particles dispersing without agglomeration.Besides,the MFST particles can be recovered easily in a permanent magnetic f i eld since the saturation magnetization of MFST is 39.5 emu·g?1.Ferrimagnetic is mixed with a superparamagnet,indicating slight hysteresis in the samples.Based on the above analysis,Fe3O4-SiO2-TiO2is successfully synthesized.

The TEM images of the MFST are presented in Fig.3,showing the average size of about 600 nm and similar square shape.Fe3O4-SiO2is covered by TiO2closely and holes form by accumulated particles, demonstrating the formation of MFST structure.

Fig.3.TEM images of MFST.

3.2.BET and BJH analyses

The nitrogen adsorption-desorption isotherms of MFST are shown in Fig.4(a),which are typical type IV isotherms with a H2 hysteresis loop due to capillary condensation,as def i ned by IUPAC.Mesoporous materials usually exhibit type IV-like isotherms with a hysteresis loop [24,25],so that the preparation of MFST is successful.The BET surface area of the MFST is estimated to be 55 m2·g?1according to Fig.4(a). Fig.4(b)shows the BJH pore size distribution of MFST.With only one narrow and strong peak,the pore size of MFST has a good distribution. The pore sizes are in the range between 2.5-10 nm,with average pore size of 3.8 nm and a pore volume of 0.033 cm3·g?1.

Fig.4.Nitrogen adsorption-desorption isotherms(a)and BJH pore size distribution(b)of MFST.

3.3.Adsorption ability

Fig.5(a)depicts the adsorption rate for MO in the dark,with the blank experiment without MFST.At the beginning,the adsorption capacity of MFST increases rapidlywithin13 minand thenslowly before the adsorption equilibrium is achieved at 48 min.The f i nal adsorption rate of MO on MFST can reach 95%.The adsorption rate is only about 2%in the blank experiment after 85 min,indicating that it is the MFST that contributes to the adsorption of MO.The MFST has excellent adsorption ability,since the rapid adsorption at the beginning is a surface effect,caused by the van der Waals force and hydrogen bond formed by O,N,and S atoms in MO[26].The slower adsorption later is due to the internal migration and diffusion in the pores of MFST.Fig.5(b)shows that the MO solution treated by Fe3O4exhibits an abnormal color resulted from Fe3+.

The pH effect on the adsorption of MFST is shown in Fig.6.At low solution pH,the MFST presents a high adsorption percentage of about 92%.The adsorption ability decreases with the increase of solution pH.As pH increases from 1.8 to 6.5,the adsorption percentagedecreases from 92%to74%,and changes little whenthesolution pH is over 6.5,illustrating that considerable hydroxyl groups(OH)are present on the TiO2surface.

Mesoporous TiO2under acid conditions:

Fig.5.Adsorption capacity for MO(a)and MO solutions at 84 min(b)(1—without particles;2—Fe3O4particles,3—SiO2particles;4—TiO2particles;5—MFST particles).

Therefore,-TiOH and-TiOH2+concentrations will increase when the solution pH is low,and they adsorb SO3?in MO through electrostatic force.However,as pH increases,the TiO2surface presents higher-TiO?concentration,which has electrostatic repulsion with -SO3?in MO,lowering the adsorption rate of MO.

Fig.7showstheadsorptionabilityofMFST forMOinseveraladsorption regeneration cycles with a magnetic f i eld.The adsorption rate of MFST remains 74%after 4 cycles,indicating that the MFST maintains a good stability for applications.The decrease in adsorption ability can be explained by the adsorption mechanism of MO on the MFST.Asuha et al.have proved that desorption of MO is diff i cult in NaOH solution [18].Hence,the adsorption ability of MFST decreases slightly because some MO adsorbs on the surface of MFST[27].

Fig.6.Solution pH effect on adsorption ability.

Pseudo-second order model for adsorption processes has been widely used since it was put forward by Ho and McKay[28].We also use it to f i t the data of adsorption rate of MO on MFST in four different cycles to examine the adsorption ability.

where qeis the equilibrium adsorption capacity,qtis the adsorption capacity at time t,and k represents the pseudo-second order adsorption rate constant,which reveals the adsorption ability.

Fig.8 and Table 1 present good linear relations for adsorption of MO on MFST(R2＞0.99),so the pseudo-second order model f i ts our experimental data well.It shows that the adsorption of MO on MFST includes the adsorption of MO molecules on the MFST surface and the migration and diffusion in the pores of MFST.The correlation coeff i cient R2and k decline as the recycle increases,probably due to theMO carbonizationon thesurfaceof MFST and shorter equilibrium time for lower equilibrium adsorption capacity.The equilibrium adsorption capacities of MFST with 1,2,3,and 4 cycles are calculated as 2.49,2.35,2.04,and 1.95 mg·g?1,respectively,decreasing slightly according to Eq.(4).It further conf i rms that our sample has a good stability for water treatment.

Fig.7.Recycle of MFST in the dark.

Fig.8.Curves of pseudo-second-order kinetic equation.

Table 1Adsorption parameters of kinetics for adsorption of MO on MFST

Table 2Maximum adsorption capacities(qmax)of MO on different adsorbents

Table 2 shows the comparison of maximum adsorption capacities (qmax)of MO on various adsorbents.It reveals that the MFST prepared in this study has a relatively high adsorption capacity and excellent recyclability.

The reproducibility of MFST is shown in Fig.9.The turbid solution became transparent within 5 s,resulting from the quick capture of MFST particles by the cubic magnet.The MFST was recovered in a magnetic f i eld with a recovery of about 80%(by mass)after 4 cycles.The loss of MFST is unavoidable in the process of regeneration using NaOH solution and acid solution(or deionized water).It shows that the MFST presents excellent reproducibility.

4.Conclusions

For adsorption of MO with mesoporous Fe3O4-SiO2-TiO2(MFST), the following results and conclusions are obtained.

(1)The MFST prepared in this study shows good adsorption property.The adsorption percentage of MO is nearly 92%at an initial concentration of 10 mg·L?1in 48 min in the dark and the MFST has the best adsorption ability under acid conditions. The pseudo-second order model for adsorption f i ts the adsorption data of MO(R2＞0.99).The equilibrium adsorption capacity is 2.49 mg·g?1.

(2)After 4 cycles in the dark,the adsorption rate of MO remains

to be 74%.The MFST is recovered in a magnetic f i eld with a recovery of about 80%(by mass).It indicates that the sample prepared has a promising potential for wastewater treatment.

Fig.9.Recovery of MFST with different cycles(a)and separation process of MFST by using a cubic magnet(b).

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28 December 2013

☆Supported by the National Natural Science Foundation of China(2011011013-2)and the Youth Foundation of Science and Technology Agency of Shanxi Province,China (2011021020-2).

*Corresponding author.

E-mail address:xuechenyang@foxmail.com(C.Xue).

http://dx.doi.org/10.1016/j.cjche.2014.09.015

1004-9541/?2014 The Chemical Industry and Engineering Society of China,and Chemical Industry Press.All rights reserved.

Received in revised form 28 February 2014

Accepted 18 March 2014

Available online 2 October 2014