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Applied Surface Science 276 (2013) 317– 322

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

j ourna l ho me page: www.elsev ier .com/ locate /apsusc

high activity adsorbent of ZnO–Al2O3 nanocomposite particles:ynthesis, characterization and dye removal efficiency

amid Tajizadegana,∗, Majid Jafari a, Mehdi Rashidzadehb, Ali Saffar-Teluri a

Department of Materials Engineering, Najafabad Branch, Islamic Azad University, P.O. Box 517, Isfahan, IranCatalysis & Nanotechnology Research Division, Research Institute of Petroleum Industry (RIPI), P.O. Box 14665-137, Tehran, Iran

a r t i c l e i n f o

rticle history:eceived 31 December 2012eceived in revised form 14 March 2013ccepted 14 March 2013vailable online 21 March 2013

eywords:nOl2O3

a b s t r a c t

In order to stabilize the ZnO species on surfaces of Al2O3 particles, the ZnO–Al2O3 nanocomposite parti-cles with different ZnO contents were prepared by heterogeneous precipitation method using bayeriteseed particles. The as-prepared nanocomposites were characterized in terms of crystal structure, mor-phology and surface area. The results indicated the formation of wurtzite-type ZnO nanoflakes (thicknessof 40–80 nm) on surfaces of �-Al2O3 particles, which led to nanocomposite particles with high surfaceareas depending on Al2O3 content. The obtained nanocomposites were used as promising adsorbents inadsorption of methyl orange (MO) from aqueous solution as an anionic dye and were compared with pureZnO and Al2O3 adsorbents. The nanocomposite adsorbents showed a superior MO removal efficiency than

omposite materialsrecipitationcanning electron microscopy

pure adsorbents, which was attributed to unique morphology of ZnO active sites with activated surfacecharge and also high surface area obtained in nanocomposite adsorbents. Moreover, it was found thatthere is an optimum between the amount of ZnO active sites and Al2O3 for the maximum of percentageremoval of MO, which was obtained for 40%ZnO–Al2O3 adsorbent with 98% efficiency and low equilib-rium time of 20 min with a fixed adsorbent concentration of 500 ppm and a fixed dye concentration of50 ppm.

. Introduction

Nowadays, many industries such as tanning, textile, paper andyestuff plants generate significant quantities of dye waste, which

ead to severe water pollution due to industrial effluent dischargedo the environment [1]. Since these dye wastes are hazardous, toxic,on-biodegradable and extremely carcinogenic, more stringentnvironmental regulations call for more efficient ways, especiallyith higher yield, to remove dye pollutants from industrial efflu-

nts to minimize their environmental impacts. Thus, the removalf dye compounds from industrial wastewater is one of the vitalecessities for the survival of human beings, which has attracteduch attention.Various techniques like filtration, coagulation, chemical oxida-

ion, sedimentation, ion exchange, precipitation and adsorption aresed for wastewater treatment [2,3]. Due to prominent propertiesuch as high efficiency, simplicity and reusability, the method ofdsorption has become one of the most promising techniques for

ye removal from industrial wastewaters [4,5]. The most importantommercial adsorbents are activated carbon, silica and activatedlumina. These porous materials are used extensively as support

∗ Corresponding author. Tel.: +98 331 229 1008; fax: +98 331 229 1008.E-mail address: hamid tjz@yahoo.com (H. Tajizadegan).

169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2013.03.089

© 2013 Elsevier B.V. All rights reserved.

and adsorbents for dye adsorption [1,4]. However, in spite of itshigh adsorption capacity, large surface area and pore volume, acti-vated carbon presents a high operating cost due to the high price[4]. Silica suffers drawback of high price of synthesis via sol–gel pro-cess due to the high price of the initial alkoxides [4]. Among them,activated alumina has a relatively large surface area, adequate sta-bility, and also, is used as a commercial adsorbent and support dueto its lower price [4,6,7]. In this regard, nano and micro-pores ofalumina form a continuous network to provide mass transfer ofdye molecules without significant diffusion resistance.

Among other metal oxides, nano-zinc oxide (ZnO) as a semicon-ductor is one of the appreciable materials with incredible features,which has been used in adsorption of dye less than other con-ventional material. ZnO is well-known by its unique structure atnanoscale. ZnO has a hexagonal structure that is viewed as a num-ber of alternating planes composed of tetrahedrally coordinatedO2− and Zn2+ ions [8]. This arrangement causes a non-symmetricstructure with polar surfaces along the c-axis (positively chargedZn-(0 0 0 1) and negatively charged O-(0 0 0 1) surfaces). As a result,polar and non-polar surfaces are coupled with anisotropic growthof ZnO, which can be controlled by various parameters [9,10]. How-

ever, it is well documented that ZnO nanoparticles can present awide range of morphologies such as nanorode, nanotube, nanonee-dle and nanowire due to preferential growth along c-axis [8] andalso, nanodisc and nanosheet with higher polarity than others due

318 H. Tajizadegan et al. / Applied Surfac

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Fig. 1. The structure of methyl orange.

o growth restriction along c-axis [11]. Thus, there is a great inter-st to use the nano-ZnO with its unique properties in wastewaterreatment for dyes adsorption. But it should be mentioned that

aterials in nanoscale tend easily to aggregate in order to reduceheir surface area and surface energy. Besides, with regard to theact that adsorption reaction takes place on surfaces of solid phasef adsorbents, synthesis of nanosized ZnO supported on porousaterial like activated alumina can be an effective way to increase

he surface area and aggregation resistance of nano-ZnO activepecies. Nowadays, various methods such as hydrothermal [12],ol–gel [13], flame synthesis [14], combustion [15] and precipi-ation [16,17] methods have applied in the field of synthesis ofnO–Al2O3 composite powder. Among them, the method of pre-ipitation from chemical solution has attracted more attention dueo its simplicity, low cost and homogeneous dispersion of Zn and Alpecies [18]. In this work, with the aim of stabilizing the ZnO speciesn surfaces of Al2O3 particles, heterogeneous precipitation usingeed particles was designed as synthesis method. In this regard,he promising increase in dye adsorption is the greatest benefit ofnO–Al2O3 nanocomposite, which is the main object of the presentork.

In this work, methyl orange (MO) serves as a model compoundor common water-soluble azo dyes (Fig. 1), which are widely usedn textile, paper and chemical industries. In this regard, the aimf this work is: (i) to synthesize the different weight ratios ofnO–Al2O3 nanocomposite powder via heterogeneous precipita-ion together with single phase of ZnO and Al2O3, (ii) to characterizehe as-prepared adsorbents in terms of phase detection, crys-al structure, surface area and morphology, (iii) to investigatehe possibility and ability of Al2O3, ZnO and ZnO–Al2O3 adsor-ents for adsorption of MO (an anionic dye), (iv) and to comparehem together in order to find a comprehensive understanding ondsorption behavior. However, literature review showed that noesearch has been reported on using ZnO and ZnO–Al2O3 for MOdsorption except one different literature on MO adsorption withow efficiency using calcined layered double hydroxides contain-ng mixed Zn/Al layers [18]. Thus, to the best of our knowledge,his work is a novel research for adsorption of dye over ZnO–Al2O3anocomposite adsorbents.

. Experimental

.1. Preparation of adsorbents

The precursor materials were zinc acetate dehydrateZn(CH3COO)2·2H2O, Merck, 99.5%), urea (NH2CONH2, Merck,8%) and bayerite powder (Al(OH)3, Ardakan Industrial Ceramicso., 98% purity, mean particle size 3 �m). All precursor materialas used as received. Also, methyl orange (C14H14N3NaO3S,

bbreviation as MO) was supplied by Merck Co. and was used aseceived.

Pure ZnO (100Z sample) was followed by mixing 0.3 M zinccetate dehydrate solution and an excess amount of solid ureay mole ratio of 1:6, respectively. Then, the solution was placed

n an oil bath and refluxed under magnetic stirring at 90 ◦C for

h. By heating the solution, urea is homogeneously hydrolyzedEq. (1)), and works as a precipitate agent. It is well known thatrea hydrolysis leads to the formation of zinc carbonate hydroxideZn5(CO3)2(OH)6) as an intermediate product (Eq. (2)) [19]. After

e Science 276 (2013) 317– 322

refluxing, the obtained precipitation was filtered and washed withdistilled water several times.

CO(NH2)2 + H2O → 2NH4+ + HCO3

− + OH− (1)

5Zn2+ + 2CO32− + 6OH− → Zn5(CO3)2(OH)6 (2)

ZnO–Al2O3 samples with ZnO contents of 20, 40, 60 and 80 wt%(20ZA, 40ZA, 60ZA and 80ZA samples, respectively) were preparedvia heterogeneous precipitation using bayerite particles as seed. Ina typical experiment, aqueous solution of zinc acetate dehydrate(0.3 M) was mixed with an excess amount of solid urea by moleratio of 1:6, respectively. In the next step, an appropriate amount ofbayerite powder was added to the solution containing zinc acetateand urea. Then, the solution was stirred at room temperature for2 h before refluxing. Subsequently, the same procedure, as men-tioned for pure ZnO, was followed. All samples were dried at 40 ◦Cfor 24 h and then were calcined at 400 ◦C for 3 h at a heating rateof 10 ◦C/min. Also, high purity Al2O3 (100A sample) was synthe-sized by calcination of bayerite powder at 400 ◦C for 3 h withoutany previous treatment.

2.2. Characterizations

The crystal structure was characterized by X-ray diffrac-tion (XRD, Philips) using Cu-Ka radiation. The morphology andmicrostructure of samples were studied by field-emission scanningelectron microscope (FE-SEM, Hitachi S-4160) and high-resolutiontransmission electron microscopy (HRTEM, Philips CM30, at anaccelerating voltage of 250 kV). The image analyzer softwareinstalled on FE-SEM device (Hitachi S-4160) was used to esti-mate the dimensions in FE-SEM images. Also, the particles sizeof TEM images was estimated using image analyzer software ofMicrostructure Measurement. The specific surface area (SBET) wasmeasured by BET method using N2 adsorption isotherms at 77 K(micromeritics ASAP-2010).

2.3. Batch adsorption experiments

Batch adsorption experiments were carried out to evaluate thepossibility and ability of Al2O3, ZnO and ZnO–Al2O3 adsorbents forMO adsorption from an aqueous solution. Stock solution of MO(50 mg/L) was prepared by dissolving 50 mg of MO in 1 L of distilledwater. The dye solution had a pH value of 6.5. In a typical experi-ment, 5 mg adsorbent was added to a glass flask containing 10 mlaqueous solution of MO (50 mg/L). The mixture was stirred mag-netically at room temperature and 400 rpm. At predetermined timeintervals, the mixture was centrifuged in order to separate the dyesolution from the adsorbent, and then the residual concentrationof MO was measured with a UV–vis spectrophotometer (UV-visspectrophotometer, Optizen 3220) at the wavelength of 465 nm.All experiments were carried out in triplicate. Also, the percentageremoval of MO was calculated by the following equation:

� = (C0 − Ct)C0

× 100 (3)

where C0 is the initial concentration of MO (in mg/L), Ct (in mg/L)is the instant concentration of MO at a predetermined time t, andV is the volume of the solution (in L).

3. Results and discussion

3.1. Characterization of adsorbents

Fig. 2 shows the X-ray diffraction patterns of samples with var-ious ZnO contents. In the XRD pattern of 100Z sample, all the sharp

H. Tajizadegan et al. / Applied Surface Science 276 (2013) 317– 322 319

t samples with various ZnO content.

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Table 1BET surface area (SBET) of as-prepared adsorbents.

Adsorbent ZnO ratio (wt%) SBET (m2/g)

100A 0 34120ZA 20 31540ZA 40 276

Fig. 2. XRD pattern of differen

nd strong peaks corresponded to wurtzite-type ZnO (JCPDS 36-451). In agreement with literature [20], all the wide and weakeaks in the XRD pattern of 100A sample can be indexed to �-l2O3 (JCPDS 10-0425). In the XRD pattern obtained from 20ZAample, the wide peaks corresponded to �-Al2O3 (JCPDS 10-0425)re clearly observed. While, a sharp peak, corresponding to the for-ation of wurtzite-type ZnO (JCPDS 36-1451), appear to emerge. In

he XRD pattern of 40ZA sample, all the sharp peaks correspond tourtzite-type ZnO, and those corresponding to �-Al2O3 have low

ntensity due to intrinsic weak peaks of Al2O3.As seen in the 60ZA and 80ZA samples, by further increasing ZnO

ontents, the intensities of the XRD peaks corresponding to Al2O3ecome much lower, whereas those corresponding to ZnO becomeuch higher. In addition, by decreasing the �-Al2O3 content, wide

nd weak �-Al2O3 peaks cannot be detected easily in the compositeamples due to lower portion of �-Al2O3 phase in X-ray diffraction,nd also intense ZnO peaks.

Considering the different morphologies of ZnO, the morphologyf as-prepared adsorbents was investigated by FE-SEM analysis.s shown in Fig. 3a, 100Z sample is composed of agglomeratednO nanoparticles with spherical shapes and a size distributionf 40–60 nm. Fig. 3b represents the FE-SEM micrograph of the00A sample, which clearly shows alumina particles with irregularhapes. After the deposition of ZnO active components on Al2O3upport, spherical morphology of ZnO particles changed to a sheetorphology. As can clearly be seen in Fig. 4(a)–(c), all 20ZA, 40ZA

nd 60ZA samples consist of ZnO nanoflake grown on alumina par-icles. These ZnO nanoflakes have a thickness of about 40–80 nmnd a diameter of about 1–2 �m. The FE-SEM observations of 80ZAample showed that together with the formation of ZnO nanoflakesn alumina particles (like the other composite samples), in someases and due to high ratio of ZnO to Al2O3 in this sample, theseanoflakes grow and cover the alumina particles. The other mor-hology obtained from 80ZA sample is shown in Fig. 4(d), in whichhe porous structure of ZnO nanosheets that cover alumina parti-les is clearly observed. Therefore, in comparison to pure ZnO, thenO–Al2O3 composite adsorbents showed a superior morphologyn nanoscale with formation of ZnO nanoflakes and nanosheets,

hich can maximize the effect of polar surfaces by enhancing sur-ace charges [8]. In addition, the ZnO–Al2O3 composite adsorbentshowed a suitable morphology with great stability against agglom-

ration, which can enhance the contact area between dye moleculend active sites.

To further investigate the microstructure of ZnO–Al2O3 samples,0ZA sample was further characterized by TEM analysis. Fig. 5a

60ZA 60 21080ZA 80 96100Z 100 16

shows a low magnification TEM micrograph of 80ZA sample, indi-cating nano-sized particles with less than 40 nm. The selected-areaelectron (SAED) pattern also indicates fine crystalline sizes (Fig. 5b).Fig. 5c shows a high-resolution TEM (HRTEM) micrograph of 80ZAsample that can show lattice planes. The distance between theparallel lattice planes was calculated to be 0.52 nm, well corre-sponding to a d-spacing of the (0 0 0 1) plane of wurtzite ZnO [11],which is in good agreement with XRD results. Also, the TEM anal-ysis showed that a relatively good agreement exists between theparticle size estimated from TEM observations and the thickness ofZnO nanosheets and nanoflakes estimated from FE-SEM analysis.

The BET analysis was carried out on as-prepared adsorbent todetermine the agglomeration resistance of different adsorbents.The measurement BET surface area of as-prepared samples is listedin Table 1. Pure ZnO adsorbent has the lowest surface area; andin contrast, pure alumina sample has the highest surface area. It isobvious that by deposition of ZnO on alumina particles, the BET sur-face area of composite adsorbents sharply increase in comparisonto pure ZnO sorbent. Also, it is obvious that the increase in aluminacontent in ZnO–Al2O3 nanocomposite adsorbents enhances theamount of BET surface area. In this regard, the textural propertiesof ZnO–Al2O3 nanocomposite adsorbents are superior to pure ZnOadsorbent. This result is affected by the incorporation of high sur-face area of alumina particles. In other word, this high surface areaintroduces more surface area for dispersing of ZnO nanoparticles,which increases the stability against agglomeration in compositeadsorbent compared with pure ZnO adsorbent.

3.2. Adsorption experiments

Adsorption experiments were carried out on all as-prepared

adsorbents. Fig. 6 shows the adsorption results for different samplesin term of percentage removal of MO from the aqueous solution asa function of time. As shown, in spite of having the highest surfacearea, pure alumina (100A sample) does not show any dye removal.

320 H. Tajizadegan et al. / Applied Surface Science 276 (2013) 317– 322

Fig. 3. FE-SEM micrographs: (a) 100Z and (b) 100A sample.

A, (b)

TaAemn[hct

Fig. 4. FE-SEM micrographs: (a) 20Z

his is because the alumina surface with negative charge cannotdsorb the anionic ion of MO, as reported by other researchers [21].s seen in Fig. 6, the 100Z sample (pure ZnO) exhibits a dye removalfficiency, which can reach 11% efficiency after 120 min. This resulteans that anionic ion of MO can be adsorbed on surfaces of ZnO

anoparticles with positive charge, agreeing with other studies

5,22,23]. However, compared to 100A sample, the 100Z sampleas the lowest surface area. Thus, it can be concluded that surfaceharge of adsorbent have relatively higher adsorption competitionhan surface area of adsorbent on dye removal. In this regard, it is

Fig. 5. (a) Low-magnification TEM micrograph of 80ZA sampl

40ZA, (c) 60ZA and (d) 80ZA sample.

necessary to improve dye removal efficiency of ZnO adsorbents byimproving its surface charge and the surface area.

Promising and interesting results were obtained by compositeadsorbents. As shown in Fig. 6, the percentage removal of MO dyeusing all of the composite adsorbent is higher than pure adsor-bents. This higher efficiency, especially in lower times, can be

first interpreted in the presence of the nanosized active sites ofmodified zinc oxide. In detail, this modification occurs with theformation of nanoflake morphology of zinc oxide with an acti-vated surface charge [11], which can act as the active sites for

e, (b) related SAED pattern, and (c) HRTEM micrograph.

H. Tajizadegan et al. / Applied Surface Science 276 (2013) 317– 322 321

Fig. 6. The percentage removal of MO from an aqueous solution over different adsorbents.

bents

Mfnapsepc

otasmteatsssiwfs

f

Fig. 7. The MO removal efficiency over as-prepared adsor

O adsorption. Furthermore, Alumina particles with high sur-ace area have a significant role in introducing these charged ZnOanoflakes. In other words, alumina particles enhance the contactrea between active sites of ZnO and MO molecules in adsorptionrocess, which was predicted before by BET surface area mea-urements and microstructural observations. Therefore, this greatfficiency in all composite adsorbent was attributed to unique mor-hology of ZnO active sites and also high surface area obtained inomposite adsorbents.

It is obvious that the ratio of ZnO to Al2O3 has significant effectn percentage removal of MO and related adsorption equilibriumime. The 20ZA sample adsorbs 87% of MO dye in time of 60 minnd is almost unchanged after that time, indicating an equilibriumtate. By increasing the ZnO content to 40 wt% (40ZA sample), theaximum of percentage removal was attained 98% at equilibrium

ime of 20 min. This appreciable result in improving dye removalfficiency was attributed to increasing the active sites of ZnO andlso the presence of an adequate surface area of adsorbent. By fur-her increasing the ZnO content to 60 and 80 wt% (60ZA and 80ZAample, respectively), the maximum dye removal (98% efficiency)till is reachable due to the presence of adequate amount of activeites of nanosized ZnO. However, the equilibrium time graduallyncreased to 40 min for 60ZA sample and 90 min for 80ZA sample,

hich can be explained by reducing the amount of adsorbent sur-

ace area. Herein, it can be concluded that more decrease in theurface area will lengthen the equilibrium time of dye adsorption.

The percentage removal of MO dye was represented in Fig. 7or different adsorbents at two times: after 20 min and equilibrium

at two times: (i) after 20 min and (ii) at equilibrium time.

time. By comparing the 20ZA sample and 80ZA sample, it can befound that the 20ZA sample can attain an equilibrium state at lowertime (60 min) due to higher surface area, whereas the 80ZA sam-ple has longer equilibrium time (90 min) due to lower surface area.However, the MO removal efficiency for the 80ZA sample is higherthan the 20ZA sample which is the result of the presence of moreactive sites of ZnO in 80ZA sample. Therefore, it can be confirmedthat the surface area has a significant role in the adsorption kinetic,which can control the ratio of adsorption process. Also, it was con-cluded that there is an optimum between density of active sites ofZnO and the amount of surface area. The optimum was obtainedfor 40ZA adsorbent with lowest equilibrium time, as clearly shownin Fig. 7.

Ni et al. investigated adsorption of MO from an aqueous solutionon Zn/Al layered structure synthesized by coprecipitation method[18]. Table 2 shows the comparison of MO removal efficiency using40%ZnO–Al2O3 nanocomposite adsorbent (40ZA sample) preparedin present work and that of adsorbent reported by Ni et al. [18].It can be clearly seen, at fixed adsorbent and dye concentration,that ZnO–Al2O3 composite synthesized by heterogeneous precipi-tation method in present work has higher removal efficiency thanthat of ZnO–Al2O3 composite (containing Zn/Al layered structure)synthesized by coprecipitation method in the literature [18]. Thisgreater efficiency can be related to the synthesis method. In con-

ventional coprecipitation method, the precursors of both Zn andAl species were chosen soluble in aqueous solvent, which led to ahomogeneous dispersion of metal cations (Zn2+ and Al3+), result-ing in severe mass transfer resistance of dye molecules into bulk

322 H. Tajizadegan et al. / Applied Surface Science 276 (2013) 317– 322

Table 2Comparison of MO removal efficiency using 40ZA nanocomposite adsorbent with other literature reported by Ni et al. [18].

Ref. Adsorbent concentration (ppm) Dye concentration (ppm) Time (min) Dye removal (%)

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f solid adsorbent (ZnO–Al2O3). Herein, using bayerite particlesalumina precursor) as seed and also related heterogeneous pre-ipitation method lead to the formation of composite particles withistinct phases (Al2O3 cores, and ZnO shells), which can facilitatehe access of dye molecules to active sites of ZnO.

. Conclusion

In this work, ZnO–Al2O3 nanocomposite adsorbents were syn-hesized by heterogeneous precipitation method using bayeriteeed particles and urea precipitation agent. With regard to con-entional coprecipitation method, the motivation for designinghis synthesis method containing heterogeneous precipitation waso facilitate the access of dye molecules to active sites of ZnOn surfaces of alumina particles, which is the first novelty of theresent work. In this regard, this expected goal was successfullyroved with the formation of ZnO nanoflakes on surfaces of alu-ina particles. The other novelty of the present work was related

o modification of ZnO nanoparticles by means of morphologyodification. This modification occurred with the formation of

nO nanoflake resulting in the activation of surface charges ofnO polar surfaces. Moreover, alumina particles play a signifi-ant role in increasing the surface area in order to enhance theontact area between dye molecules and active sites. The adsorp-ion experiments indicated that the ZnO–Al2O3 nanocompositedsorbents have great dye removal efficiency with respect to pureorms of ZnO and Al2O3, which was attributed to unique mor-hology of ZnO active sites with activated surface charge andlso high surface area obtained in composite adsorbents. In thisegard, it was believed that morphology in nanoscale is a mea-ure of potential surface charges of the ZnO, and that surfacerea is a measure of how much of this potential can be real-zed. However, the maximum dye removal (98% efficiency) withowest equilibrium time (20 min) was obtained for 40ZA adsor-ent, which was the best result of the present work. Therefore,

t can be confirmed that the low cost ZnO–Al2O3 nanocom-osite prepared with ZnO content of 40 wt% is a very efficientdsorbent for the removal of azo dyes in industrial wastewaterreatment.

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