《水污染控制原理》课程教学资源（文献资料）Facile synthesis of alumina hollow microspheres via trisodium citrate-mediated hydrothermal process and their adsorption performances for p-nitrophenol from aqueous solutions

Journal of Colloid and Interface Science 394 (2013) 509-514Contents lists available at SciVerse ScienceDirectJournal of Colloid andInterfaceScience.ELSEVIERwww.elsevier.com/locate/jcisFacile synthesis of alumina hollow microspheres via trisodium citrate-mediatedhydrothermal processand theiradsorptionperformancesfor p-nitrophenolfromaqueoussolutionsJiabin Zhou*, Lei Wang, Zhong Zhang, Jiaguo YuSchool of Resources and Environmental Engineering. Wuhan University of Technology.122 Luoshi Road, Wuhan 430070, PR ChindState Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan Universiy of Technology.122 Luoshi Road. Wuhan 430070, PR ChinaINFOARTICLEABSTRACTArticle history:Alumina hollow microspheres with high adsorption affinity toward p-nitrophenol in water were pre-Received 28 September 2012pared by using urea and trisodium citrate as precipitating and mediating agents, respectively, via a sim-Accepted 25November 2012ple one-pot hydrothermal synthesis followed by calcination. The as-prepared samples were characterizedAvailable online 5 December 2012by X-ray diffraction (XRD),field-emission scanning electron microscopy (FE-SEM),transmission electronmicroscopy (TEM), and nitrogen adsorption-desorption isotherms measurement. This study shows thatKeywords:the morphology,specific surface area, and the pore structure of the resulting materials can be controlledAl20g microspheresby varying the concentration of trisodium citrate.The result of adsorption of p-nitrophenol onto the as-Trisodium citrateprepared samples revealed that the pseudo-second-order kinetic equation can better describe theHydrothermal synthesisadsorption kinetics. Furthermore, adsorption isotherm studies indicated that the resulting aluminaAdsorption capacitymicrospheres are powerful adsorbents for the removal of p-nitrophenol from water with maximump-Nitrophenoladsorption capacity of 217.4 mg/g.@ 2012 Elsevier Inc. All rights reserved.1. Introductionapplications in adsorbents, composite materials, ceramics, cata-lysts, and catalyst supports [18-20]. Aiming at this goal, manyHollow sphere structures have attracted much attention in thekinds of interesting and delicate boehmite nanostructures, suchgeneral synthesis of functional materials because of their low den-as nanofibers [21],nanobelts [22],nanotubes[23],and bundlessity, high specific surface area and attractive optical properties,of aligned boehmite nanowires [24],as well as flower-like 3D nan-which have been widely used in various fields spanning from arti-oarchitectures [25] have been prepared, However, it is still a chal-ficial cells, energy-storage media, drug-delivery carriers [1-3], andlenge to search for an effective approach to develop aluminumcatalysis [4] to nanoscale chemical reactors [5-7].oxides with hierarchically porous nanostructures, which can pro-To date, most of synthetic approaches were essentially focusedvide high flow rate during the uptake of pollutants [26-29].Herein, we present a simple trisodium citrate-assisted hydro-on various removabletemplates toobtain thesefascinatinghollowspherical materials.These templates include spherical silica, car-thermal route to prepare Al2O; hollow microspheres. The effectsbonsphere,blockcopolymerlatex,emulsiondroplets/micellesoftrisodiumcitrateonthemorphologyandmicrostructureofthe[8-11]. and gas bubbles [12,13]. However, the introduction of tem-resulting product as well as its formation mechanism were studied.platesintothesyntheticroutesusuallysuffersfromdisadvantagesThepreliminaryadsorptionperformances oftheseAl.Omicro-relatedtohighcostandtroublesomesyntheticprocedures,whichspheresforremovalofp-nitrophenolfrom waterwerepositive,may prevent them from being used in large-scale applications. Aswhich makes them promising adsorbent materials for wastewateran alternative,template-free approaches based on different mech-treatment.anisms were also developed to fabricate hollow spherical materialswith desired pore structure and surface properties [14-17].2. ExperimentalfunctionalAmongdiversematerials,hollowboehmite(-AIOOH) and its oxide derivatives such as -Al,O, have been2.1.Sample preparationstudied intensively over a long period of time because of its broadAll chemicals and reagents used were in analytical grade* Corresponding author at: School of Resources and Environmental Engineering.(Shanghai Chemical Industrial Company) and used without furtherWuhan University of Technology, 122 Luoshi Road, Wuhan 430070, PR China. Fax:purification. In a typical synthesis, 3.5 mmol AINH(SO4)2-12H20+86 27 87885647.E-mail address: jbzhou@whut.edu.cn (. Zhou).and different amounts of trisodium citrate (0, 0.125, 0.25, 0.5,0021-9797/S - see front matter 2012 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/jcis.2012.11.050

Facile synthesis of alumina hollow microspheres via trisodium citrate-mediated hydrothermal process and their adsorption performances for p-nitrophenol from aqueous solutions Jiabin Zhou ⇑ , Lei Wang, Zhong Zhang, Jiaguo Yu School of Resources and Environmental Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, PR China State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, PR China article info Article history: Received 28 September 2012 Accepted 25 November 2012 Available online 5 December 2012 Keywords: Al2O3 microspheres Trisodium citrate Hydrothermal synthesis Adsorption capacity p-Nitrophenol abstract Alumina hollow microspheres with high adsorption affinity toward p-nitrophenol in water were pre￾pared by using urea and trisodium citrate as precipitating and mediating agents, respectively, via a sim￾ple one-pot hydrothermal synthesis followed by calcination. The as-prepared samples were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and nitrogen adsorption–desorption isotherms measurement. This study shows that the morphology, specific surface area, and the pore structure of the resulting materials can be controlled by varying the concentration of trisodium citrate. The result of adsorption of p-nitrophenol onto the as￾prepared samples revealed that the pseudo-second-order kinetic equation can better describe the adsorption kinetics. Furthermore, adsorption isotherm studies indicated that the resulting alumina microspheres are powerful adsorbents for the removal of p-nitrophenol from water with maximum adsorption capacity of 217.4 mg/g. 2012 Elsevier Inc. All rights reserved. 1. Introduction Hollow sphere structures have attracted much attention in the general synthesis of functional materials because of their low den￾sity, high specific surface area and attractive optical properties, which have been widely used in various fields spanning from arti- ficial cells, energy-storage media, drug-delivery carriers [1–3], and catalysis [4] to nanoscale chemical reactors [5–7]. To date, most of synthetic approaches were essentially focused on various removable templates to obtain these fascinating hollow spherical materials. These templates include spherical silica, car￾bon sphere, block copolymer latex, emulsion droplets/micelles [8–11], and gas bubbles [12,13]. However, the introduction of tem￾plates into the synthetic routes usually suffers from disadvantages related to high cost and troublesome synthetic procedures, which may prevent them from being used in large-scale applications. As an alternative, template-free approaches based on different mech￾anisms were also developed to fabricate hollow spherical materials with desired pore structure and surface properties [14–17]. Among diverse functional materials, hollow boehmite (c-AlOOH) and its oxide derivatives such as c-Al2O3 have been studied intensively over a long period of time because of its broad applications in adsorbents, composite materials, ceramics, cata￾lysts, and catalyst supports [18–20]. Aiming at this goal, many kinds of interesting and delicate boehmite nanostructures, such as nanofibers [21], nanobelts [22], nanotubes [23], and bundles of aligned boehmite nanowires [24], as well as flower-like 3D nan￾oarchitectures [25] have been prepared. However, it is still a chal￾lenge to search for an effective approach to develop aluminum oxides with hierarchically porous nanostructures, which can pro￾vide high flow rate during the uptake of pollutants [26–29]. Herein, we present a simple trisodium citrate-assisted hydro￾thermal route to prepare Al2O3 hollow microspheres. The effects of trisodium citrate on the morphology and microstructure of the resulting product as well as its formation mechanism were studied. The preliminary adsorption performances of these Al2O3 micro￾spheres for removal of p-nitrophenol from water were positive, which makes them promising adsorbent materials for wastewater treatment. 2. Experimental 2.1. Sample preparation All chemicals and reagents used were in analytical grade (Shanghai Chemical Industrial Company) and used without further purification. In a typical synthesis, 3.5 mmol AlNH4(SO4)212H2O and different amounts of trisodium citrate (0, 0.125, 0.25, 0.5, 0021-9797/$ - see front matter 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.11.050 ⇑ Corresponding author at: School of Resources and Environmental Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, PR China. Fax: +86 27 87885647. E-mail address: jbzhou@whut.edu.cn (J. Zhou). Journal of Colloid and Interface Science 394 (2013) 509–514 Contents lists available at SciVerse ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

510J.Zhou et al./Jourmal of Colloid and Interface Science 394 (2013) 509-514Table 12.3.Kinetic adsorption experimentsThe specific surface areas and pore characteristics of the Al2O, samples.Kineticstudieswere carriedouttoestablishthe effect ofcontactTrisodium citrateSBETPore volumeAverage poreSamples(mmol) (m°/g)(cm=/g)time on the adsorption process and to quantify the adsorption rate.size (nm)64.489888Adsorption kinetic experimentswerecarried outbyaddinga de-ABCDE81250.120.30160.3fined amount of adsorbent (25 mg) to a series of flask filled with162.00.4050mLp-nitrophenolsolution(25mg/L).Theflasksweresealed8.2172.3andkeptatair-bathoscillatorunder30C.Theflaskswerethenre-255.0movedfromtheoscillator,andthefinal concentration ofp-nitro-phenol in thesolution was determined at 317nm on UV-visspectrophotometer(ShimadzuUV/vis2550 Spectrophotometer,Ja-pan).The amount of p-nitrophenol adsorbed on samples at time t,q (mg/g), was calculated from the following equation:(Co - C.)V(1)qWwhere Co and C, (mg/L) are the concentrations of p-nitrophenol atinitial and any time t, respectively, V is the volume of the solution(L) and W is the mass of adsorbent used (g)2.4.Equilibrium adsorption experimentsTTES(s)Experiments for p-nitrophenol adsorption isotherms studyNwere carried out by adding a defined amount of adsorbent20305060104070(25mg)toaseriesofflasksfilledwith50mLp-nitrophenolsolutions (10-150 mg/L), The pH value of the solution was maintained2Theta (degree)at7byadding 0.1MNaOHor 0.1 MHCI solutions.Theflasks wereFig.1. XRD patterns of the Al2O: samples.placed in air-bath oscillator at 30 C for 24 h to reach equilibrium.The flasks were then removed from the oscillator, and the finalconcentration of p-nitrophenol in thesolution was determined1 mmol) were dissolved in 50 mL distilled water. And then,on UV-vis spectrophotometer.The amount of p-nitrophenol ad-0.028 mmol CO(NH2)2 and 10 mL ethanol were added forming asorbed at equilibrium qe (mg/g) was calculated on the followinghomogeneous solution.After vigorously stirred for 30 min, theequation:mixedsolutionwastransferredintoa1o0mLTeflon-linedstain-V(Co - C.)less autoclave.The autoclave was kept at 180°C for 3 h and then(2)qeair-cooledto room temperature.The whiteprecipitatewascol-mlected and washed with distilled water and anhydrous ethanolwhere V is the volume (L) of the p-nitrophenol solution, Co and C。three times, respectively. The washed samples were dried in a vac-are initial and equilibrium p-nitrophenol concentrations (mg/L).uum oven at 80 C for 24 h. Finally, the Al2O3 product can be ob-respectively,and m is the mass (g) of adsorbent added.tained by calcination at 600 °C for 2 h under air. Table 1 showsthe detailed experimental conditions for the preparation of the3.Results and discussionsamples.3.1.XRD studies2.2.CharacterizationThe phase structure and relative crystallinity of the samples ob-Powder X-ray diffraction (XRD) patterns of the as-preparedtained by varying Crc from O to 1 mmol were investigated by XRD.samples were obtained on an X-ray diffractometer (typeAs can be seen in Fig.1, all the diffraction peaks can be indexed asHZG41B-PC)using CuKa radiation at a scan rate of 0.0520 s-1.cubicphaseof-Al,O(JCPDSNo.10-0425).NootherdiffractionThe accelerating voltage and applied current were 40 kV andpeaks was detected in Fig. 1, which indicated the high purity oftheresultingy-AlzO3products.Furtherobservation indicatesthat80mA,respectively.TheBrunauer-Emmolett-Teller (BET)surfacearea of the powders was analyzed by nitrogen adsorption in athe width of the diffraction peaks becomeslightly wider withMicromeritics ASAP 2020 nitrogen adsorption apparatus (USA).increasing Crc, implying the formation of smaller Al2Og crystallites.All the samples were degassed at 180 C prior to nitrogen adsorp-tion measurements. The BET surface area was determined by a3.2. Morphologymultipoint BET method using the adsorption data in the relativepressure (P|Po) range of 0.05-0.3. A desorption isotherm was usedFig.2 shows the effect of concentration of trisodium citrate onto determine the pore-size distribution by the Barret-joyner-the morphology of the Al2O samples. As can be observed inHalender (BJH) method [30]. The nitrogen adsorption volume atFig. 2a and b, the irregular aggregated particles with rough surfacethe relative pressure (P/Po) of 0.994 was used to determine thewere obtained inthe absence of trisodium citrate.An increase inpore volumeand average pore size.Morphological analysis wasCrcto0.125mmolresultedinhollowmicrosphereswithdiameterperformedbyanS-4800field-emissionscanningelectronmicro-ofca.600nm,andits50nmthicknessshellscomposedoflooselyscope(FE-SEM,Hitachi,Japan)atanacceleratingvoltageof1okVconnected nanoflakes (Fig. 2c, d and their inset). A further increaseand linked with an Oxford Instruments X-ray analysis system.inCrcto0.25mmolresultedinmorehollowstructuresandsmallerTransmission electron microscopy (TEM) analyses were conductedpore sizes (Fig. 2e, f and their inset). When Crc approaches0.5 mmol, the morphology of the sample converted to well-definedonaJEM-2100Felectronmicroscope(JEOL,Japan)usinga200kVacceleratingvoltage.microsphereswithdiametersofca.2-3μm (Fig.2g).Finally.there

1 mmol) were dissolved in 50 mL distilled water. And then, 0.028 mmol CO(NH2)2 and 10 mL ethanol were added forming a homogeneous solution. After vigorously stirred for 30 min, the mixed solution was transferred into a 100 mL Teflon-lined stain￾less autoclave. The autoclave was kept at 180 C for 3 h and then air-cooled to room temperature. The white precipitate was col￾lected and washed with distilled water and anhydrous ethanol three times, respectively. The washed samples were dried in a vac￾uum oven at 80 C for 24 h. Finally, the Al2O3 product can be ob￾tained by calcination at 600 C for 2 h under air. Table 1 shows the detailed experimental conditions for the preparation of the samples. 2.2. Characterization Powder X-ray diffraction (XRD) patterns of the as-prepared samples were obtained on an X-ray diffractometer (type HZG41B-PC) using Cu Ka radiation at a scan rate of 0.05 2h s1 . The accelerating voltage and applied current were 40 kV and 80 mA, respectively. The Brunauer-Emmolett-Teller (BET) surface area of the powders was analyzed by nitrogen adsorption in a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA). All the samples were degassed at 180 C prior to nitrogen adsorp￾tion measurements. The BET surface area was determined by a multipoint BET method using the adsorption data in the relative pressure (P/P0) range of 0.05–0.3. A desorption isotherm was used to determine the pore-size distribution by the Barret-Joyner￾Halender (BJH) method [30]. The nitrogen adsorption volume at the relative pressure (P/P0) of 0.994 was used to determine the pore volume and average pore size. Morphological analysis was performed by an S-4800 field-emission scanning electron micro￾scope (FE-SEM, Hitachi, Japan) at an accelerating voltage of 10 kV and linked with an Oxford Instruments X-ray analysis system. Transmission electron microscopy (TEM) analyses were conducted on a JEM-2100F electron microscope (JEOL, Japan) using a 200 kV accelerating voltage. 2.3. Kinetic adsorption experiments Kinetic studies were carried out to establish the effect of contact time on the adsorption process and to quantify the adsorption rate. Adsorption kinetic experiments were carried out by adding a de- fined amount of adsorbent (25 mg) to a series of flask filled with 50 mL p-nitrophenol solution (25 mg/L). The flasks were sealed and kept at air-bath oscillator under 30 C. The flasks were then re￾moved from the oscillator, and the final concentration of p-nitro￾phenol in the solution was determined at 317 nm on UV–vis spectrophotometer (Shimadzu UV/vis 2550 Spectrophotometer, Ja￾pan). The amount of p-nitrophenol adsorbed on samples at time t, qt (mg/g), was calculated from the following equation: qt ¼ ðC0 CtÞV W ð1Þ where C0 and Ct (mg/L) are the concentrations of p-nitrophenol at initial and any time t, respectively, V is the volume of the solution (L) and W is the mass of adsorbent used (g). 2.4. Equilibrium adsorption experiments Experiments for p-nitrophenol adsorption isotherms study were carried out by adding a defined amount of adsorbent (25 mg) to a series of flasks filled with 50 mL p-nitrophenol solu￾tions (10–150 mg/L). The pH value of the solution was maintained at 7 by adding 0.1 M NaOH or 0.1 M HCl solutions. The flasks were placed in air-bath oscillator at 30 C for 24 h to reach equilibrium. The flasks were then removed from the oscillator, and the final concentration of p-nitrophenol in the solution was determined on UV–vis spectrophotometer. The amount of p-nitrophenol ad￾sorbed at equilibrium qe (mg/g) was calculated on the following equation: qe ¼ VðC0 CeÞ m ð2Þ where V is the volume (L) of the p-nitrophenol solution, C0 and Ce are initial and equilibrium p-nitrophenol concentrations (mg/L), respectively, and m is the mass (g) of adsorbent added. 3. Results and discussion 3.1. XRD studies The phase structure and relative crystallinity of the samples ob￾tained by varying CTC from 0 to 1 mmol were investigated by XRD. As can be seen in Fig. 1, all the diffraction peaks can be indexed as cubic phase of c-Al2O3 (JCPDS No. 10-0425). No other diffraction peaks was detected in Fig. 1, which indicated the high purity of the resulting c-Al2O3 products. Further observation indicates that the width of the diffraction peaks become slightly wider with increasing CTC, implying the formation of smaller Al2O3 crystallites. 3.2. Morphology Fig. 2 shows the effect of concentration of trisodium citrate on the morphology of the Al2O3 samples. As can be observed in Fig. 2a and b, the irregular aggregated particles with rough surface were obtained in the absence of trisodium citrate. An increase in CTC to 0.125 mmol resulted in hollow microspheres with diameter of ca. 600 nm, and its 50 nm thickness shells composed of loosely connected nanoflakes (Fig. 2c, d and their inset). A further increase in CTC to 0.25 mmol resulted in more hollow structures and smaller pore sizes (Fig. 2e, f and their inset). When CTC approaches 0.5 mmol, the morphology of the sample converted to well-defined microspheres with diameters of ca. 2–3 lm (Fig. 2g). Finally, there Table 1 The specific surface areas and pore characteristics of the Al2O3 samples. Samples Trisodium citrate (mmol) SBET (m2 /g) Pore volume (cm3 /g) Average pore size (nm) A 0 64.4 0.12 9.8 B 0.125 160.3 0.30 9.9 C 0.25 162.0 0.40 9.5 D 0.5 172.3 0.40 9.3 E 1 255.0 0.52 8.1 10 20 30 40 50 60 70 (311) (511) A B C D (220) (111) (222) (400) Relative intensity (a.u.) (440) 2Theta (degree) E Fig. 1. XRD patterns of the Al2O3 samples. 510 J. Zhou et al. / Journal of Colloid and Interface Science 394 (2013) 509–514

511LZhouetal./loumalofColloidandInterfaceScience394(2013)509-514Fig. 2. SEM and TEM images of samples A (a and b), B (c and d). C (e and f), D (g), E (h)appeared collapsed microspheres when Crc reaching1mmolloop,indicatingthepresenceofslit-likepores.Withtheconcen-(Fig. 2h).trationoftrisodiumcitrateincreasingfrom0.125to1mmol,thehysteresisloopsisconvertedtotypeH2atlowrelativepressuresbetween 0.4 and 0.9, suggesting the presence of ink-bottle pores3.3. Surface area and porosity of microsphereswith narrow necks and wider bodies. However, the hysteresisloop athigh relativepressuresbetween 0.9 and 1.0resemblesThe effect of Crc on the pore structures of alumina samplestype H3,which is associated with slit-like pores.Meanwhile,was studied on the basis of nitrogen adsorption-desorption anal-thecorrespondingpore-sizedistributionsofallthesamplesbe-ysis. As can be seen in Fig. 3, the microspheres obtained in thecome narrower and are shifted from single-peak to bimodalabsence of trisodium citrate display a type IV isotherm accordingdistribution.to the BDDT classification [30] and a small H3-type hysteresis

appeared collapsed microspheres when CTC reaching 1 mmol (Fig. 2h). 3.3. Surface area and porosity of microspheres The effect of CTC on the pore structures of alumina samples was studied on the basis of nitrogen adsorption–desorption anal￾ysis. As can be seen in Fig. 3, the microspheres obtained in the absence of trisodium citrate display a type IV isotherm according to the BDDT classification [30] and a small H3-type hysteresis loop, indicating the presence of slit-like pores. With the concen￾tration of trisodium citrate increasing from 0.125 to 1 mmol, the hysteresis loops is converted to type H2 at low relative pressures between 0.4 and 0.9, suggesting the presence of ink-bottle pores with narrow necks and wider bodies. However, the hysteresis loop at high relative pressures between 0.9 and 1.0 resembles type H3, which is associated with slit-like pores. Meanwhile, the corresponding pore-size distributions of all the samples be￾come narrower and are shifted from single-peak to bimodal distribution. Fig. 2. SEM and TEM images of samples A (a and b), B (c and d), C (e and f), D (g), E (h). J. Zhou et al. / Journal of Colloid and Interface Science 394 (2013) 509–514 511

512J.Zhou et al./Journal of Colloid and Interface Science 394 (2013)509-514(a) 20N400-A4.B1.53504c-DDW(/)paoeaoE7E3001.0250200150-0.5100 --1.0 507A-1.50:0501001502002500.00.20.41.00.60.8t (min)RelativePressure (P/P,)(b)-A1.24A-5VD1.0C-D一41yb(3/,)()8OIP/AP0.830.6 20.4 --0.2 0-5015020025001000.0t (min)10010Porediameter (nm)(C) 80 B(K.)(Ka).(K)4Fig. 3. Nitrogen adsorption-desorption isotherms and the corresponding pore-size:cA70distributionVD7E60A3.4.Possible mediation mechanism of trisodium citrate50(a/m)40 Onthebasis ofthe above experiment, we believe that the triso-dium citrate plays a key role in the formation of the as-prepared30 microsphere. In the absence of trisodium citrate, A13* and urea20 can alternatively hydrolyze and polycondense, which leads to the10 -precipitation of amorphous aluminum oxyhydroxide spheres pro-moted by sulfate anions [4,31-34]. An increase in reaction timeo010122N81416t" (min")80.BFig, 5. Pseudo first-order kinetics (a), second-order kinetics (b), and intra-particle、cdiffusion kinetics (c) for adsorption of p-nitrophenol onto samples (T=30C:D704Eadsorbent dose -500 mg/L: p-nitrophenol concentration-25 mg/L and pH=7).(8/u)'b60 facilitates transformation of the surface layer to thermodynami-cally more stable boehmite; consequently, an ultrathin and less-soluble shell is formed via secondary nucleation and subsequent50growth of boehmite crystal nuclei until the system reaches equilib-rium with the surrounding solution [29,35-38]. After addition oftrisodiumcitrate,asacomplexingagent,trisodium citrateconsistsof three carboxyl group,and a hydroxyl oxygen atom.The anion30 can coordinate Ai3+ to form a citrate-aluminum complex anion,050100150200250which may prevent A3* polymerization. Then, porous amorphoust (min)aluminum oxyhydroxide particles can selectively adsorb trisodiumanions, resulting in the formation of hydrogen bonds between car-Fig.4.The variation of adsorption capacity with adsorption time for p-nitrophenolboxyl and hydroxyl groups of trisodium citrate and the surface OHon samples (T-30-c: adsorbent dose =500 mg/L: p-nitrophenol concentra-groups of aluminum oxyhydroxide, and its nuclei growth may betion =25mg/Land pH=7)

3.4. Possible mediation mechanism of trisodium citrate On the basis of the above experiment, we believe that the triso￾dium citrate plays a key role in the formation of the as-prepared microsphere. In the absence of trisodium citrate, Al3+ and urea can alternatively hydrolyze and polycondense, which leads to the precipitation of amorphous aluminum oxyhydroxide spheres pro￾moted by sulfate anions [4,31–34]. An increase in reaction time facilitates transformation of the surface layer to thermodynami￾cally more stable boehmite; consequently, an ultrathin and less￾soluble shell is formed via secondary nucleation and subsequent growth of boehmite crystal nuclei until the system reaches equilib￾rium with the surrounding solution [29,35–38]. After addition of trisodium citrate, as a complexing agent, trisodium citrate consists of three carboxyl group, and a hydroxyl oxygen atom. The anion can coordinate Al3+ to form a citrate-aluminum complex anion, which may prevent Al3+ polymerization. Then, porous amorphous aluminum oxyhydroxide particles can selectively adsorb trisodium anions, resulting in the formation of hydrogen bonds between car￾boxyl and hydroxyl groups of trisodium citrate and the surface OH groups of aluminum oxyhydroxide, and its nuclei growth may be 0.0 0.2 0.4 0.6 0.8 1.0 0 50 100 150 200 250 300 350 400 A B C D E Volume adsorbed (cm3/g) Relative Pressure (P/P0) 1 10 100 0.0 0.2 0.4 0.6 0.8 1.0 1.2 A B C D E dV/dlog(w) (cm3/g) Pore diameter (nm) Fig. 3. Nitrogen adsorption–desorption isotherms and the corresponding pore-size distribution. 0 50 100 150 200 250 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 A B C D E (a) t (min) log (qe-qt ) 0 50 100 150 200 250 0 1 2 3 4 5 6 t/qt (min g mg-1 ) A B C D E (b) t (min) 0 2 4 6 8 10 12 14 16 0 10 20 30 40 50 60 70 80 A B C D E t 1/2 (min1/2) (Kd3) (Kd2 (K ) d1) q (mg/g) t (C) Fig. 5. Pseudo first-order kinetics (a), second-order kinetics (b), and intra-particle diffusion kinetics (c) for adsorption of p-nitrophenol onto samples (T = 30 C; adsorbent dose = 500 mg/L; p-nitrophenol concentration = 25 mg/L and pH = 7). 0 50 100 150 200 250 30 40 50 60 70 80 A B C D E qt (mg/g) t (min) Fig. 4. The variation of adsorption capacity with adsorption time for p-nitrophenol on samples (T = 30 C; adsorbent dose = 500 mg/L; p-nitrophenol concentra￾tion = 25 mg/L and pH = 7). 512 J. Zhou et al. / Journal of Colloid and Interface Science 394 (2013) 509–514

513J.Zhou et al./Joumal of Colloid and Interface Science 394 (2013) 509-514Table 2国1.2 Pseudo-second-order adsorption kinetic constants of samples.“CoSamplesPseudo-second-order modelgeexp1.0 (mg/L)(mg/g)K2R2qecalS.D. (%)(mg/g)(gmgmin-")0.8 ABCDE35543.645.450.00450.9993.1162.50.00160.9974.3466.670.00150.9986.81258770.920.00180.9982.0383.330.00170.9996.020.4 0.2afforded via hydrogen bonds along a preferential direction with thelowest growth energy [39-41].As a result, not only the formation0.0 of well-defined amorphous aluminum oxyhydroxidespheres but020406080120140100also their subsequent transformation into crystallized boehmiteCe (mg/L)and further self-assembly into boehmite hollowmicrospheres (byFig 6. Langmuir isotherms for p-nitrophenol adsorption onto samples (T= 30 °C:spatially coupled processes involving core dissolution and shelladsorbent dose = 500 mg/L; p-nitrophenolconcentration =10-150 mg/Landcrystallization) with randomly aggregated nanoflake-like surfacepH=7).can be reduced to some extent. This reduction trend can be gradu-ally strengthened with increasing Crc, resulting in smaller crystal-linityofboehmitemicrosphereswhichwerereadilyconvertedintogives a satisfactory fit to all of the experimental data. The linearintact -Al20, hollow spheres by calcinations at 600 °C.plotsof p-nitrophenol adsorptionkinetics tothekineticmodelsand the calculated kinetic parameters are given in Fig.5 and Table 2.respectively.3.5. p-Nitrophenol adsorption kineticsFurthermore,the intra-particlediffusion kinetic modelbased onthe equation proposed by Weber and Morris was tested [45]. ThisThe adsorption kinetics is important for adsorption studies be-empirical equation assumes the adsorbate uptake varies almostcause it can predict the rate at which a pollutant is removed fromproportionally with t/2 rather than with the contact time t.aqueous solutions and provides valuable data for understandingAccording to the following Weber-Morris's equation:the mechanism of sorption reactions [42-44]. The kinetics ofp-nitrophenol adsorption on Al2Os samples obtained by batch con-q, = KaiVt+ C(5)tact time studies for an initial p-nitrophenol concentration of25mg/LareshowninFig.4.Theresultsrevealthatadsorptionwhere ka is the rate parameter of stage i (mg/min/2 g). calculatedprocess is clearly time dependent such that the majority offrom the slope ofthe straightline ofq: versus/2.G,is the interceptp-nitrophenol adsorption from aqueous solutions was completedof stagei,giving an idea about the thickness ofboundary layer,thatin approximately 180 min. As can be seen from the figure, theis, the larger the intercept, the greater the boundary layer effectadsorptioncapacitiesofthesamplesweresignificantlyimproved[46]. Fig. 5c shows three separate regions. kdi. kd2, and kd3 are theby increasing Crc. It might be related to their higher specific surfacerateparametersof correspondingstage.Thefirststepindiffusionareasandporevolumeaswellasuniqueporousstructuresmodelisthemasstransferofp-nitrophenolmoleculefromthebulkThe kinetics of p-nitrophenol adsorption onto Al2Os samplessolution to the external surface of the adsorbent or instantaneouswere analyzed using the pseudo-first-order kinetic models (Eq.stage, which was driven by high initial p-nitrophenol concentration(3)) and pseudo-second-order kinetic models (Eq. (4):difference.Furthermore,the linear lines of the second and thirdkistages did not pass through the origin and this deviation from the(3)log(qe q,) = log qeoriginornearsaturationmightbeduetothedifferenceinthemass2.303transfer rate in the initial and final stages of adsorption [47]. Kinetic1data obtained by the batch method has been described by theL(4)expressions given by Boyd et al. [48]. As can be seen in Fig. 5c, stageqrk2qe"qe1israpidlycompletedwithinabout10minandthestageof intra-where qe and qr are the amounts of p-nitrophenol adsorbed (mg/g)particle diffusion control (stage 2) is then attained. As discussedat equilibrium and at time t (min), respectively. k is the pseudo-above,p-nitrophenolisslowlytransported via intra-particle diffu-first-orderrateconstant(min-),andk2isthepseudo-second-ordersion into theparticles. The rate parameters for p-nitrophenolrate constant (g mg min-').The conformity between experimental(Table 3) show that the value of kd2 for the samples becomesdata and the model fitting values was expressed by the correlationlarger with Crc increasing from 0 to 0.5 mmol. This is due to theircoefficients R-.Itwasfound that thepseudo-second-order modellarger surface areas and pore volumes. Nevertheless, a deeperTable 3Intra-particle diffusion model constants and correlation coeffcients for adsorption of p-nitrophenol on Al2Os samplesSamplesC (mg/L)Intra-particle diffusion modelkai (mg/min'2 g)kaz (mg/min'/2 g)kds (mg/min'2 g)GG2G(Rt)(Ra)(Rz)A5532000010.4661.2240.48530.3836.901.0000.9470.820BCDE11.2722.5931.67527.9736.290.9691.0001.00012.1823.2901.22525.0344.280.9300.8561.0002514.6503.4880.82032.7358.101.0000.9860.74425017.2252.0841.4930.9930.99351.2456.811.000

afforded via hydrogen bonds along a preferential direction with the lowest growth energy [39–41]. As a result, not only the formation of well-defined amorphous aluminum oxyhydroxide spheres but also their subsequent transformation into crystallized boehmite and further self-assembly into boehmite hollow microspheres (by spatially coupled processes involving core dissolution and shell crystallization) with randomly aggregated nanoflake-like surface can be reduced to some extent. This reduction trend can be gradu￾ally strengthened with increasing CTC, resulting in smaller crystal￾linity of boehmite microspheres which were readily converted into intact c-Al2O3 hollow spheres by calcinations at 600 C. 3.5. p-Nitrophenol adsorption kinetics The adsorption kinetics is important for adsorption studies be￾cause it can predict the rate at which a pollutant is removed from aqueous solutions and provides valuable data for understanding the mechanism of sorption reactions [42–44]. The kinetics of p-nitrophenol adsorption on Al2O3 samples obtained by batch con￾tact time studies for an initial p-nitrophenol concentration of 25 mg/L are shown in Fig. 4. The results reveal that adsorption process is clearly time dependent such that the majority of p-nitrophenol adsorption from aqueous solutions was completed in approximately 180 min. As can be seen from the figure, the adsorption capacities of the samples were significantly improved by increasing CTC. It might be related to their higher specific surface areas and pore volume as well as unique porous structures. The kinetics of p-nitrophenol adsorption onto Al2O3 samples were analyzed using the pseudo-first-order kinetic models (Eq. (3)) and pseudo-second-order kinetic models (Eq. (4)): logðqe qtÞ ¼ log qe k1 2:303 t ð3Þ t qt ¼ 1 k2q2 e þ t qe ð4Þ where qe and qt are the amounts of p-nitrophenol adsorbed (mg/g) at equilibrium and at time t (min), respectively. k1 is the pseudo- first-order rate constant (min1 ), and k2 is the pseudo-second-order rate constant (g mg min1 ). The conformity between experimental data and the model fitting values was expressed by the correlation coefficients R2 . It was found that the pseudo-second-order model gives a satisfactory fit to all of the experimental data. The linear plots of p-nitrophenol adsorption kinetics to the kinetic models and the calculated kinetic parameters are given in Fig. 5 and Table 2, respectively. Furthermore, the intra-particle diffusion kinetic model based on the equation proposed by Weber and Morris was tested [45]. This empirical equation assumes the adsorbate uptake varies almost proportionally with t 1/2 rather than with the contact time t. According to the following Weber-Morris’s equation: qt ¼ Kdi ffiffi t p þ C ð5Þ where kdi is the rate parameter of stage i (mg/min1/2 g), calculated from the slope of the straight line of qt versus t 1/2. Ci is the intercept of stage i, giving an idea about the thickness of boundary layer, that is, the larger the intercept, the greater the boundary layer effect [46]. Fig. 5c shows three separate regions. kd1, kd2, and kd3 are the rate parameters of corresponding stage. The first step in diffusion model is the mass transfer of p-nitrophenol molecule from the bulk solution to the external surface of the adsorbent or instantaneous stage, which was driven by high initial p-nitrophenol concentration difference. Furthermore, the linear lines of the second and third stages did not pass through the origin and this deviation from the origin or near saturation might be due to the difference in the mass transfer rate in the initial and final stages of adsorption [47]. Kinetic data obtained by the batch method has been described by the expressions given by Boyd et al. [48]. As can be seen in Fig. 5c, stage 1 is rapidly completed within about 10 min and the stage of intra￾particle diffusion control (stage 2) is then attained. As discussed above, p-nitrophenol is slowly transported via intra-particle diffu￾sion into the particles. The rate parameters for p-nitrophenol (Table 3) show that the value of kd2 for the samples becomes larger with CTC increasing from 0 to 0.5 mmol. This is due to their larger surface areas and pore volumes. Nevertheless, a deeper Table 2 Pseudo-second-order adsorption kinetic constants of samples. Samples C0 (mg/L) qe,exp (mg/g) Pseudo-second-order model qe,cal (mg/g) K2 (g mg min1 ) R2 S.D. (%) A 25 43.6 45.45 0.0045 0.999 3.11 B 25 59 62.5 0.0016 0.997 4.34 C 25 61 66.67 0.0015 0.998 6.81 D 25 69 70.92 0.0018 0.998 2.03 E 25 77 83.33 0.0017 0.999 6.02 Table 3 Intra-particle diffusion model constants and correlation coefficients for adsorption of p-nitrophenol on Al2O3 samples. Samples C0 (mg/L) Intra-particle diffusion model kd1 (mg/min1/2 g) kd2 (mg/min1/2 g) kd3 (mg/min1/2 g) C1 C2 C3 (R1) 2 (R2) 2 (R3) 2 A 25 10.466 1.224 0.485 0 30.38 36.90 1.000 0.947 0.820 B 25 11.272 2.593 1.675 0 27.97 36.29 1.000 0.969 1.000 C 25 12.182 3.290 1.225 0 25.03 44.28 1.000 0.930 0.856 D 25 14.650 3.488 0.820 0 32.73 58.10 1.000 0.986 0.744 E 25 17.225 2.084 1.493 0 51.24 56.81 1.000 0.993 0.993 0 20 40 60 80 100 120 140 0.0 0.2 0.4 0.6 0.8 1.0 1.2 A B C D E Ce /qe (g/L) Ce (mg/L) Fig. 6. Langmuir isotherms for p-nitrophenol adsorption onto samples (T = 30 C; adsorbent dose = 500 mg/L; p-nitrophenol concentration = 10–150 mg/L and pH = 7). J. Zhou et al. / Journal of Colloid and Interface Science 394 (2013) 509–514 513

514J.Zhou etal./Journal of Colloid and InterfaceScience394 (2013)509-514Table 4AcknowledgmentsAdsorption isotherm parameters of samples.This work was partially supported by the National Natural Sci-SamplesLangmuir isotherm modelFreundlich isotherm modelenceFoundation of China(41173092,21177100,21277108andKLR2R2qmaxKen(mg/gXL/mg)/laNatural Science Foundation of51272201)HubeiProvince(L/mg)(mg/g)(2010CDA078).National Basic ResearchProgramofChinaABCDE137.00.04220.99013.232.10350.967(2009CB939704 and 2011CB933401),and the Fundamental Re-185.20.03380.99911.601.77050.964200.00.02810.99411.461.74950.975search Funds for the Central Universities (2011-IV-098)181.80.04990.99515.851.93500.925217.40.99916.850.9300.04761.8372References[1] Y.F. Zhu, D.H. Fan, W.Z. Shen, J. Phys. Chem. C 111 (2007) 18629-18635.[2] ZW. Deng. M. Chen, C.X. Gu, LM. Wu.J. Phys. Chem.B 112 (2008) 16-22[3]J.Zhang.S.R.Wang.YWang.MJ.Xu.HJ.ia,S.H.Zhang.W.P.Huang.Zuoinvestigation is needed to better understand the lower value of ka2S.H. Wu, Actuators B 39 (2009) 411-417.for sample E and gradually reduction of the value of ks with increas-[4] H.C.Yu.J.G.Yu.S.W.Liu,S.Mann.Chem.Mater.19(2007)43274334.[5j Y.Liu, X. Tan, K. Li, Ind. Eng.Chem. Res. 45 (2006) 3782-3790.ing Crc. Further observation indicates the order of uptake rate was[6] s.W. Cao, YJ. Zhu, M.Y. Ma, L Li, L. Zhang, J Phys. Chem. C 112 (2008) 1851-as follows: kai>ka2>kd3, possibly because the concentration of1856.p-nitrophenol left in the solutions gradually decreases.[7] T. Nakashima, N. Kimizuka. J. Am. Chem. Soc. 125 (2003) 63866387.[8] Q.Zhang. W. Li, S.X Liu, Powder Technol.212 (2011) 145-150[9] N.L.Yue,M.Xue,S.L.Qiu, Inorg.Chem.Commun.14 (2011) 1233-1236.[10] S.Q, Liu, M.Y. Wei, JC. Rao, Mater. Lett. 65 (2011) 2083-2085.[HJianguWnllsurymgs3.6. p-nitrophenol adsorption isotherms20111228-232[12] S.T. Chen, X.L Zhang. Growth Des. 10 (2010) 1257-1262.[13] Maciej Mazur,J. Phys. Chem.C 112 (2008) 13528-13534.Adsorption isotherms of p-nitrophenol on Al2O; samples were[14] J.B. Zhou, S.L Yang. J.C. Yu, Z Shu,J. Hazard.Mater. 192 (2011) 1114-1121j15j j.B. Zhou, S.L Yang. j.G. Yu, Colloids Surf. A 379 (2011) 102-108.presentedinFig.6.Aclearimprovementinp-nitrophenoluptake[16] S.G. Li, C.H. Li,F.Z. Huang.J. Nanopart Res. 13 (2011) 2865-2874.at equilibrium was observed. The equilibrium adsorption data of[17] C.L. Zheng.X.Z.Zheng. Mater. Lett. 65 (2011) 1645-1647p-nitrophenol on Al2Og adsorbent were analyzed using Langmuir[18] J.B.Zhou, Y. Cheng. J.G. Yu, G. Liu,J. Mater. Chem. 21 (2011) 19353-19361.and Freundlich models. Nonlinear regression is used to determine[19] A,Khanna, D.Bhat, Surf.Coat.Technol. 201 (2006)168-173[20j D.H.M. Buchold, C. Feldmann, NanoLetter 7 (11) (2007) 3489-3492.the best-fitting isotherm, and the applicability of isotherm equa-[21] XY. Wu, D.B. Wang, Z.S. Hu, G.H. Gu, Mater.Chem. Phys. 109 (2008) 560-564.tion is compared by judging the correlation coefficients R2. The[22j P. Gao, Y. Xie, Y. Chen, LN. Ye, QX. Guo,J. Cryst. Growth. 285 (2005) 555-560.resulting plots were shown in Fig. 6. Table 4 summarizes the Lang-[23] (a) D.B. Kuang, Y.P. Fang, H.Q, Liu, C. Frommolen, D. Fenske,J. Mater. Chem. 13(2003) 660-662:muirand Freundlichisothermalparametersfortheadsorptionof(b) H.W. Hou, Y. Xie, Q. Yang. QX. Guo, CR. Tan, Nanotechnology 16 (2005)p-nitrophenol on Al2Os samples. It was found that the Langmuir741-745equation gives more satisfactory fitting to the adsorption iso-[24] J.Zhang.S.Y.Wei.J.Lin.JJ. Luo,SJ. Liu,H.S.Song.E.Elawad, XX. Ding.J.M. Gao,S.R. Qi. C.C. Tang. J. Phys. Chem. B 110 (2006) 21680-21683.therms of p-nitrophenol with correlation coefficient R? higher than[25] J.Zhang, SJ. Liu, J. Lin, H.S. Song. JJ. Luo, E.M. Elissfah, E Ammolar, Y. Huang.0.99. It indicates the homogeneous nature of Al,Og surface andXX Ding. J.M. Gao, S.R.Qi,C.C.Tang.J.Phys.Chem.B110 (2006) 14249demonstratestheformationofmonolayercoverageofp-nitrophe-14252.[26] YL.Feng. W.C. Lu, LM.Zhang. X.H. Bao, B.H. Yue, Cryst. Growth Des. 8 (2008)nol moleculeon the outer surface of adsorbent.Based on the values1426-1429.of qmax (Table 4), theoretical p-nitrophenol adsorption capacity for[27] Q.Yuan,AXin,CLuo, D.Sun,Y.W.Zhang, W.T.Duan,H.C.Lu,C.H.Yan.JAl203 samples were obviously improved from 137.0 to 217.4 mg/gAm.Chem.Soc.130(2008)3465-3472.[28] W.Q.Cal.JG.Yu,Micropor.Mesopor.Mater.122(2009)42-47.by increasing Cre from 0 to 1 mmol.[29] W.Q.Cai. J.G. Yu, B. Cheng.J. Phys. Chem.C113 (2009) 14739-14746.3oj K.S.W.Sing.D.H.Everett,R.A.W.Haul,Moscou,RAPierott,J.Rouquerol,.Siemieniewska, Pure Appl. Chem. 57 (1985) 603-619.[31] C. Brosset, G. Biedermann, Acta Chem. Scand. 8 (1954) 1917-1926.[32] M. Egon, Chem. Res. 14 (1981) 22-29.4.Conclusions[33j C. Maurizio, M. Egon, Chem. Mater.1 (1989) 78-82.[34] s. Ramanathan, S.K. Roy, R. Bhat, D.D. Upadhyaya, AR. Biswas, Ceram. Int. 23In summary, we have demonstrated the fabrication of alumina(1997) 45-53.[35] j.G. Yu, H.T. Guo, S.A Davis, S. Mann, Adv. Funct. Mater. 16 (2006) 20352041.microspheres via a facile approach involving the trisodium cit-[36j W.S. Wang.LZhen, C.Y.Xu, B.Y.Zhang. W.Z.Shao.J.Phys. Chem,B 110 (2006)rate-mediated hydrothermal synthesis and calcination. Well-23154-23158[37] J.G. Yu, S.W. Liu, H.G. Yu, J. Catal. 249 (2007) 5966.crystallized alumina hollow microspheres with a high specific[38] JG.Yu, S.W. Liu,M.H.Zhou.J. Phys.Chem.C112 (2008) 2050-2057.surface area, pore volume, and pore size of 160.3-255.0 m/g.[39]R.L.Penn,1.F.Banfield,Science281(1998)969-9710.30-0.52 cm'/g. and 8.1-9.9 nm were obtained by varying con-[40] X. Krokidis, P. Aybaud, A.E. Gobichon, B. Rebours, P. Euzen, H. Toulhoat, X.centration of trisodium citrate.Furthermore,the maximum uptakeKrokidis, J. Phys. Chem. B 105 (2001) 5121-5130.Guzman-Castillo, B.MarMar, F.[41] XBokhimi, J.A. Toledo-Antonio,M.Lof p-nitrophenol on resulting microspheres from water could reachHernandez-Beitran, J. Navarrete,J. Solid State Chem. 161 (2001)319-326.to217.4mg/gbyeasilyadjustingCrc=1mmol.Experimentalre-[42] R.C. Wu. J.H. Qu, Y.S. Chen.J. Water Res. 39 (2005) 630-638.sults show that trisodium citrate acts as a structure-directing agent43j LA.W.Tan, B.H.Hameed,A.L Ahmad.J. Chem.Eng.127 (2007) 111-119.[44] YS. Ho, G. McKay.J. Chem. Eng, 70 (1998) 115-124.inhydrothermal processwhereinvolvesthecomplexationand[45] wWJ Weber,J.C, Morris, Advances in water pollution research: removal ofadsorption of tartrate anions, self-transformation of metastablebiologically resistant pollutants from waste waters by adsorption.Inaluminum oxyhydroxide particles, core dissolution and shell crys-Proceedings of International Conference on Water Pollution Symposium, vol.2 Pergamon,Oxford, 1962, pp. 231266.tallization followed by calcination. This novel synthesis strategy[46] F. Wu, R. Tseng. C. Hu,J. 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investigation is needed to better understand the lower value of kd2 for sample E and gradually reduction of the value of k3 with increas￾ing CTC. Further observation indicates the order of uptake rate was as follows: kd1> kd2> kd3, possibly because the concentration of p-nitrophenol left in the solutions gradually decreases. 3.6. p-nitrophenol adsorption isotherms Adsorption isotherms of p-nitrophenol on Al2O3 samples were presented in Fig. 6. A clear improvement in p-nitrophenol uptake at equilibrium was observed. The equilibrium adsorption data of p-nitrophenol on Al2O3 adsorbent were analyzed using Langmuir and Freundlich models. Nonlinear regression is used to determine the best-fitting isotherm, and the applicability of isotherm equa￾tion is compared by judging the correlation coefficients R2 . The resulting plots were shown in Fig. 6. Table 4 summarizes the Lang￾muir and Freundlich isothermal parameters for the adsorption of p-nitrophenol on Al2O3 samples. It was found that the Langmuir equation gives more satisfactory fitting to the adsorption iso￾therms of p-nitrophenol with correlation coefficient R2 higher than 0.99. It indicates the homogeneous nature of Al2O3 surface and demonstrates the formation of monolayer coverage of p-nitrophe￾nol molecule on the outer surface of adsorbent. Based on the values of qmax (Table 4), theoretical p-nitrophenol adsorption capacity for Al2O3 samples were obviously improved from 137.0 to 217.4 mg/g by increasing CTC from 0 to 1 mmol. 4. Conclusions In summary, we have demonstrated the fabrication of alumina microspheres via a facile approach involving the trisodium cit￾rate-mediated hydrothermal synthesis and calcination. Well￾crystallized alumina hollow microspheres with a high specific surface area, pore volume, and pore size of 160.3–255.0 m2 /g, 0.30–0.52 cm3 /g, and 8.1–9.9 nm were obtained by varying con￾centration of trisodium citrate. Furthermore, the maximum uptake of p-nitrophenol on resulting microspheres from water could reach to 217.4 mg/g by easily adjusting CTC = 1 mmol. Experimental re￾sults show that trisodium citrate acts as a structure-directing agent in hydrothermal process where involves the complexation and adsorption of tartrate anions, self-transformation of metastable aluminum oxyhydroxide particles, core dissolution and shell crys￾tallization followed by calcination. This novel synthesis strategy would pave the way for controllable synthesis of versatile adsor￾bents with high surface area and porous property for a variety of applications such as water purification. Acknowledgments This work was partially supported by the National Natural Sci￾ence Foundation of China (41173092, 21177100, 21277108 and 51272201), Natural Science Foundation of Hubei Province (2010CDA078), National Basic Research Program of China (2009CB939704 and 2011CB933401), and the Fundamental Re￾search Funds for the Central Universities (2011-IV-098). References [1] Y.F. Zhu, D.H. Fan, W.Z. Shen, J. Phys. Chem. C 111 (2007) 18629–18635. [2] Z.W. Deng, M. Chen, G.X. Gu, L.M. Wu, J. Phys. Chem. B 112 (2008) 16–22. [3] J. Zhang, S.R. Wang, Y. Wang, M.J. Xu, H.J. Xia, S.H. Zhang, W.P. Huang, X.Z. Guo, S.H. Wu, Actuators B 39 (2009) 411–417. [4] H.G. Yu, J.G. Yu, S.W. Liu, S. Mann, Chem. Mater. 19 (2007) 4327–4334. [5] Y. Liu, X. Tan, K. Li, Ind. Eng, Chem. Res. 45 (2006) 3782–3790. [6] S.W. Cao, Y.J. Zhu, M.Y. Ma, L. Li, L. Zhang, J. Phys. Chem. C 112 (2008) 1851– 1856. [7] T. Nakashima, N. Kimizuka, J. Am. Chem. Soc. 125 (2003) 6386–6387. [8] Q. Zhang, W. Li, S.X. 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Samples Langmuir isotherm model Freundlich isotherm model qmax (mg/g) KL (L/mg) R2 KF (mg/g)(L/mg)1/n n R2 A 137.0 0.0422 0.990 13.23 2.1035 0.967 B 185.2 0.0338 0.999 11.60 1.7705 0.964 C 200.0 0.0281 0.994 11.46 1.7495 0.975 D 181.8 0.0499 0.995 15.85 1.9350 0.925 E 217.4 0.0476 0.999 16.85 1.8372 0.930 514 J. Zhou et al. / Journal of Colloid and Interface Science 394 (2013) 509–514