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College of Chemistry and Chemical Engin

225002, P. R. China. E-mail: quqishu@qq.c

† Electronic supplementary informa10.1039/c3ay41593g

Cite this: Anal. Methods, 2014, 6, 1397

Received 13th September 2013Accepted 4th December 2013

DOI: 10.1039/c3ay41593g

www.rsc.org/methods

This journal is © The Royal Society of C

Facile synthesis of hierarchical MCM-41 sphereswith an ultrahigh surface area and their applicationfor removal of methylene blue from aqueoussolutions†

Qishu Qu* and Zuli Gu

Hierarchical mesoporous silica spheres (HS) were prepared by a simple sol–gel method and used for the

removal of methylene blue from aqueous solutions. The macrostructure of HS can be tuned simply by

adding different amounts of ethanol as a reactant. The prepared HS were characterized by scanning

electron microscopy, transmission electron microscopy, X-ray diffraction, nitrogen adsorption–

desorption isotherms, and thermogravimetric analysis. The results showed that HS possessed both

macropores and mesopores, with an average diameter of 5.6 mm, a surface area of 2280 m2 g�1, and a

pore volume of 1.15 cm3 g�1. The influence of pH, temperature, dosage of adsorbent, and initial

methylene blue (MB) concentration on the adsorption behavior were investigated. The experimental

results showed that HS exhibited a high adsorption capacity (654.5 mg g�1) and extremely rapid

adsorption rate (<2 min) due to their unique hierarchical structure and very high surface area. The kinetic

studies showed that the experimental data fitted well to the pseudo-first-order kinetic model. The

Langmuir adsorption isotherm model was found to present an accurate description of these adsorption

data. The experimental results suggest that HS are potentially useful materials for effective adsorption

and removal of methylene blue in aqueous solution.

Introduction

Nowadays, dyes are widely used inmany industrial elds such astextiles, plastics, paper, coatings, and rubber. The discharge ofdyes into water has aroused concern because these compoundsand their degradation products can be toxic and carcinogenic.1

In addition, very small amounts of dyes in water are highlyvisible, leading to visual pollution. Therefore, the developmentof technologies to remove dyes from wastewater has been ofgreat importance in recent decades. A variety of methods such ascoagulation,2,3 chemical oxidation,4 membrane ltration,5,6

photocatalytic degradation,7–9 and adsorption10–12 have beendeveloped. Among all the approaches proposed, adsorption isgenerally preferred because of its high efficiency, ease ofhandling, and availability of different adsorbents. The mostcommonly used adsorbent for dye removal is activated carbondue to its high surface area and relatively high adsorptioncapacity.13–15 However, commercially available activated carbonsare very expensive. In addition, activated carbon suffers fromslow kinetics because of its microporous structure. Thus, the

eering, Yangzhou University, Yangzhou

om

tion (ESI) available. See DOI:

hemistry 2014

wide application of activated carbons has been restricted.16

Therefore, many researchers have focused their efforts ondeveloping novel adsorbents with high adsorption capacity, fastadsorption rate, and low cost.

Silica based mesoporous materials such as SBA-3,17 SBA-15,18–20 MCM-41,21–24 and zeolite25 have received much attentionowing to their high surface areas, large pore volumes, andordered pore structures. Both functionalized26–28 and unfunc-tionalized29 mesoporous silica can be used for dye removal. Anefficient adsorbent should possess a large adsorption capacity, afast adsorption rate, and should be easy to separate fromsolution aer the adsorption process. Compared to activatedcarbon, mesoporous silica provides a faster adsorption rate butrelatively low adsorption capacity. For example, the reportedmaximum adsorption capacity of activated carbon for methy-lene blue (MB) is 916 mg g�1,15 while the maximum adsorptioncapacity of mesoporous silica for MB is only 280 mg g�1.19 Thepossible reason is that activated carbon possesses a largersurface area and more active sites compared with mesoporoussilica. However, the adsorption kinetics of mesoporous silicaare faster than those of activated carbon due to the larger meanpore size of mesoporous silica. Both kinds of adsorbent aredifficult to separate from solution, since the sizes of theseadsorbents are normally less than 1 mm. Therefore, in order toobtain an adsorbent with high adsorption capacity and fast

Anal. Methods, 2014, 6, 1397–1403 | 1397

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adsorption rate, the surface area, pore size and particle sizeshould be as large as possible.10

In this work, micrometer sized hierarchical mesoporoussilica particles (HS) with surface areas as large as 2280 m2 g�1

were synthesized and characterized by X-ray diffractionpatterns, nitrogen adsorption isotherms, thermogravimetricanalysis, scanning electron microscopy, and transmissionelectron microscopy. A typical basic dye MB was used for testingthe adsorption behavior of HS. Kinetic and equilibriumisotherm models were used to establish the rate of adsorption,adsorption capacity, and the mechanism for MB adsorption.

ExperimentalSynthesis of HS

Hierarchical mesoporous silica was prepared in a mannersimilar to that of a previously reported method,30 except ethanolwas used to control the morphology of the particles. A sche-matic illustration of the preparation process of the HS wasshown in Fig. 1. In a typical synthesis batch, 1.0 g of cetyl-trimethylammonium chloride (CTAC) was dissolved in 20 mL ofdistilled water. Then 1 mL of ethanol, 1.48 g of sodium meta-silicate (Na2SiO3$9H2O) and 4.8 mL of formamide (HCONH2)were added at 35 �C with magnetic stirring, giving a clearsolution. The resulting solution was stirred for 5 min at 35 �Cand placed in a water bath for 3 h in a quiescent state. The solidproduct was recovered by ltration, washed with water anddried at 60 �C. The surfactant was removed by calcination in airat 550 �C for 5 h. For comparison, mesoporous silica (MS)particles were prepared using the same conditions, except noethanol was added into the mixture.

Characterization

The morphologies of the hierarchical mesoporous silica parti-cles were characterized by eld emission scanning electronmicroscopy (FESEM, Hitachi S-4800 II, 15 kV) and high-resolu-tion transmission electron microscopy (HRTEM, Tecnai G2 F30S-TWIN, 300 kV). Powder X-ray diffraction (XRD) patterns weretaken on a BRUKER D8 Advance X-ray diffractometer equippedwith a Cu Ka radiation source. Thermogravimetric analysis(TGA) was carried out on a Pyris 1 TGA thermoanalyzer (Perki-nElmer) with nitrogen as the carrier gas at a heating rate of10 �C min�1. The surface area, pore volume and pore size of theparticles were determined by nitrogen adsorption–desorptionmeasurements using a Micromeritics ASAP 2010 instrument.The Brunauer–Emmett–Teller (BET) surface area was obtained

Fig. 1 Schematic illustration of the preparation process of the HS.

1398 | Anal. Methods, 2014, 6, 1397–1403

by applying the BET equation to the adsorption data. The poresize distribution was calculated from the adsorption branch ofthe sorption isotherms using the Barret–Joyner–Halenda (BJH)method.

Adsorption studies

Adsorption studies to evaluate the removal of MB from solu-tions were carried out in triplicate using a batch adsorptionprocedure. In a typical study, 12.5 mg of HS microspheres wereadded to 50 mL of MB solution of a predetermined concentra-tion. Aer the adsorption processes, the samples were lteredthrough a 0.45 mm membrane, and the ltrates were immedi-ately analyzed by a UV spectrometer (Shimadzu UV-2550) at 663nm. The equilibrium adsorption capacity of MB, qe (mg g�1),was evaluated by using the mass balance equation:

qe ¼ ðC0 � CeÞ � V

m

where C0, Ce, V, and m are the initial and equilibrium concen-trations of MB (mg L�1), volume of solution (L), and mass ofadsorbent (g), respectively.

The removal efficiency of adsorbents was calculated usingthe following equation:

Removal efficiency ð%Þ ¼ C0 � Ce

C0

� 100

The inuence of pH on the removal of MB was determined bycarrying out the adsorption experiments in solutions withdifferent initial pH values at 25 �C. The pH of the solution wasadjusted with 0.1 M HCl or 0.1 M NaOH solution.

The inuence of temperature on the amount of MB adsorbedonto the HS was investigated under isothermal conditions inthe temperature range of 25–65 �C in a thermostatic shakerbath.

The adsorption of MB on HS was studied by varying theamount of HS in solution (0.15–0.35 g L�1), while maintainingthe initial MB concentration (30 mg L�1), temperature (25 �C),and pH value (11.0).

Thermodynamic isotherm model

Two isothermal equations were used in this work to analyze theadsorption data. One is the Langmuir isotherm equation basedon the assumption of adsorption on a homogeneous surface.The linear form of the Langmuir isotherm equation was rep-resented as:

Ce

qe¼ 1

qmaxKL

þ Ce

qmax

where qmax is the monolayer adsorption capacity (mg g�1) andKL is the Langmuir constant (L mg�1).

The other isotherm equation is Freundich isotherm equa-tion, describing a heterogeneous system. The linear form of theFreundlich isotherm is as follows:

ln qe ¼ ln KF þ 1

nln Ce

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where KF is the Freundlich parameter (mg1�1/nL1/n/g) and n isthe heterogeneity factor, indicating the adsorption capacity andthe adsorption intensity, respectively. If the value of 1/n < 1, itsuggests a normal Langmuir isotherm; otherwise, it is indica-tive of cooperative adsorption.

Adsorption kinetics of MB

The effect of contact time on the adsorbed amount of MB wasinvestigated at various initial concentrations of MB. Adsorptionequilibrium curves were obtained at an optimized pH value.12.5 mg of HS microspheres was added into a ask containing50 mL of MB solution at 25 �C. The mixture was shaken as afunction of time. At various intervals, samples were taken andthe residual amount of MB in the solution was determinedaer separation. The amount of MB adsorbed on HS particles(qt, mg g�1) at time t was calculated by the following equation:

qt ¼ ðC0 � CtÞ � V

m

where Ct (mg L�1) is the concentration of MB in the solution attime t.

Kinetic models

The adsorption dynamics presented by HS were tested with theLagergren pseudo-rst-order and the chemisorption pseudo-second-order models.

The linear form of the pseudo-rst-order equation isexpressed as follows:

logðqe � qtÞ ¼ log qe � k1

2:303t

where k1 is the rate constant of pseudo-rst-order adsorption(L min�1).

The pseudo-second-order kinetic model is represented by:

Fig. 2 SEM (a and c) and HRTEM (c and d) images of mesoporous silicaparticles prepared by adding 1 (a and b) and 0 mL (c and d) ethanol inthe initial solution.

This journal is © The Royal Society of Chemistry 2014

t

qt¼ 1

k2qe2þ 1

qet

where k2 is the rate constant of pseudo-second-order adsorption(g (mg�1 min)).

Results and discussionPreparation of HS

Fig. 2 shows the representative SEM and HRTEM images of themesoporous silica particles prepared using this approach.Fig. 1a shows that the HS are formed with aggregated rod-likestructures with a mean particle size of 5.6 mm. The large particlesize is very helpful for the separation of these particles from thesolution aer the adsorption process. The HRTEM image(Fig. 2b) conrms that these spheres from the inside out wereaggregated with loosely connected rods and thus formed aspecial interwoven macroporous structure. However, whenethanol was not used for the synthesis, only mesoporous silicaparticles with smooth surfaces were obtained (Fig. 2c and d).

Fig. 3 XRD pattern of mesoporous silica particles obtained by adding0 and 1 mL ethanol in the initial solution.

Fig. 4 Nitrogen adsorption–desorption isotherms of mesoporoussilica particles obtained by adding 0 and 1 mL ethanol in the initialsolution.

Anal. Methods, 2014, 6, 1397–1403 | 1399

Table 1 Properties of MS and HS

SampleSBET(m2 g�1)

Pore volume(cm3 g�1)

Pore diameter(nm)

d100(nm)

Mass loss(wt %)

MS 1244 0.96 3.1 4.1 40.7HS 2280 1.15 2.0 3.5 45.3

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This suggests that the morphology of the particles is mainlyaffected by the amount of ethanol in the solution. The texturalproperties of HS were further analyzed using XRD and N2

adsorption–desorption isotherms. The XRD patterns of both HSand MS exhibited one distinct peak indexed to (100) along witha broad shoulder around 4–5 (2q/deg) which was attributed tohigher-order reection peaks (110) and (200) (Fig. 3). Thesepatterns are characteristic of a mesophase with a pore systemlacking long-range order. Nitrogen sorption isotherms of HSshown in Fig. 4 are of a transitional type, between type I andtype IV, indicating the presence of a super microporous struc-ture.31 The N2 desorption isotherms of HS showed no hysteresis,suggesting that the architecture of the individual pores isuniform with no bottlenecking. The nitrogen sorptionisotherms of MS, however, show a pronounced hysteresis loopindicating a partial disintegration of the pore structure. As lis-ted in Table 1, both the specic surface area and the specicpore volume of MS show a marked decrease. Fig. 5 shows theTGA traces of MS and HS. It can be found that the mass loss ofsurfactant from the particles amounts to ca. 40.7% of MS whilethat of HS is ca. 45.3%. This indicates that the amount ofsurfactant required to construct the MS is less than that neededto make the HS. Therefore, it is not surprising that the surfacearea and pore volumes of HS were larger than those of MS.

Based on the above results, it is proposed that the particleswere formed through the deposition of self-assembled silicatemicelles, while the deposition process was mainly controlled bythe delicate competition between the rate of polymerization ofsilica micelles and the rate of mesostructure formation.Different deposition processes lead to the formation of different

Fig. 5 Representative TG and DTG curves for mesoporous silicaparticles obtained by adding 0 (MS, green dot line) and 1 mL (HS, bluesolid line) ethanol in the initial solution.

1400 | Anal. Methods, 2014, 6, 1397–1403

morphologies. When the silica source and the surfactant weremixed, spherical silica/surfactant composites were soon formedthrough a cooperative templating mechanism32 via an S+I�

pathway.33 When both the concentration of silica source and thesurfactant are high, the interaction between them is strong, andconsequently the surfactant/silica composites grow quickly intonanospheres. Driven by the minimum energy principle, thesenanospheres have a tendency to aggregate into large spheres inorder to minimize the surface area. However, the trend ofmesostructure formation is always present even though thespherical structure is the nal shape. Therefore, the attempts ofthese nanospheres to elongate into a rodlike structure wouldbecome apparent when the polymerization rate decreasedslightly.34 When ethanol was added into the reactant, the solu-bility of Na2SiO3$9H2O decreased, which then decreased theinteraction between the silica source and surfactant . Thus,silica spheres composed of small rodlike submicrometerparticles were obtained. In addition, the existence of ethanolcould disrupt the self-assembly of surfactant micelles due to thedecrease in the surfactant aggregates for the weak solvophobiceffect.35 Therefore, the amount of surfactant incorporated in theformation of silica/surfactant composites decreased, which is inline with the results of thermogravimetry analysis (Fig. 5). Whenthe volume of ethanol was further increased to 2 mL, onlyhexagonal or hexagonal-like silica rods were obtained (ESI,Fig. S1–4†). These experimental results conrmed our proposal.

Due to the hierarchical structure of HS, a BET surface area ashigh as 2280 m2 g�1 was obtained. The existence of macroporesin HS makes the mass transfer between adsorbate and adsor-bent become facile. Furthermore, the existence of the macro-pores helped to expose a greater inner surface area to theadsorbate, which could provide more Si–OH groups as theactive adsorption sites. Thus, this kind of particle is expected tobe a promising adsorbent for removal of dyes ormetal ions fromwastewater.

Adsorption behaviour of HS

Effect of pH. The pH of the dye solution plays an importantrole in the whole adsorption process. It affects the degree ofionization of the adsorbent and the adsorbate, thereby alteringthe reaction kinetics and equilibrium characteristics of theadsorption process. MB is a cationic dye. In aqueous solution itexists in the form of positively charged ions.36 The pKa of silica isabout 1.5.37 Therefore, the silica particles are negativelycharged, and the cationic groups increase with increasing pHvalue. Meanwhile, with an increase in pH, the competition of H+

with positively charged MB cations decreased. As a result, theadsorption capacity increased with increasing pH value from 6to 10, as shown in Fig. 6. When the pH value further increasedfrom 10 to 11, both silica and MB were fully ionized. Thus, theadsorption capacity remained steady.

Effect of temperature. The effect of temperature on theadsorption capacity reects the thermodynamics of MB removalon the HS. It can be seen from Fig. 7 that there is a slightdecrease in adsorption capacity as the temperature increasedfrom 25 to 45 �C, while a more signicant decrease was

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Fig. 6 Effect of initial pH on the removal of methylene blue (C0, 30 mgL�1; adsorbent dosage, 0.25 g L�1; temperature, 25 �C; contact time,15 min).

Fig. 8 Effect of adsorbent dosage on the adsorption capacity andremoval efficiency of MB (C0, 30 mg L�1; temperature, 25 �C; contacttime, 15 min; pH 11).

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obtained over the temperature range of 45 to 65 �C. Theseexperimental results suggest that the adsorption and desorp-tion rate between HS andMB was very quick, while the former islarger than the latter when the temperature is low. However, asthe temperature increases, the enhancement of the desorptionrate was larger than that of the adsorption rate. Therefore, theadsorption capacity decreased with decreasing temperature.The fast adsorption rate of HS at low temperatures is due to itsunique hierarchical structure which enables facile masstransfer.

Effect of adsorbent dosage. The adsorption of MB onto HSwas studied by varying the HS concentration (0.15–0.35 g L�1)while maintaining the initial MB concentration, temperature,pH value, and contact time constant. As shown in Fig. 8, theadsorption capacity decreased with increasing amount of

Fig. 7 Effect of temperature on the removal of methylene blue (C0, 30mg L�1; adsorbent dosage, 0.25 g L�1; temperature, 25 �C; pH 11.0).

This journal is © The Royal Society of Chemistry 2014

adsorbent, whereas the removal efficiency increased as theadsorbent concentration increased. The increase inMB removalwith increasing adsorbent mass can be attributed to theincrease in the total surface area. However, the decreasedadsorption capacity indicates that adding too much adsorbentin the solution is wasteful, since the removal efficiency onlyincreased a little (96.1–98.5%) when the adsorbent concentra-tion was increased from 0.15 to 0.35 g L�1. Considering that inindustrial processes a greater amount of adsorbent is used toensure that all the dyes in the wastewater could be removed, inour case, the adsorbent concentration was xed at 0.25 g L�1.

Kinetic studies

In order to investigate the mechanism of adsorption andpotential rate controlling steps, kinetic studies were carried out

Fig. 9 Effect of initial concentration of MB on the adsorption rate(adsorbent dosage, 0.25 g L�1; temperature, 25 �C; pH 11, MBconcentrations were in the range of 30–240 mg mL�1).

Anal. Methods, 2014, 6, 1397–1403 | 1401

Table 2 Pseudo-first-order and pseudo-second-order kinetic parameters for the adsorption of MB onto HS

C0

(mg L�1)qe,exp(mg g�1)

Pseudo-rst-order model Pseudo-second-order model

k1 (min�1)qe,cal(mg g�1) R2

k2(g mg�1 min�1)

qe,cal(mg g�1) R2

30 119.6 23.22 119.6 0.9999 0.0225 120.8 0.999960 237.9 2.233 237.9 0.9999 0.1167 239.2 0.9999120 476.7 36.42 474.5 0.9997 0.0446 480.8 0.9999160 619.0 425.8 605.5 0.9996 0.0088 632.9 0.9998200 665.9 306.6 652.3 0.9998 0.0083 675.7 0.9998

Table 3 Comparison of the maximummonolayer adsorption capacityof MB on various silica based adsorbents

Adsorbent

Maximumadsorptioncapacity (mg g�1) Reference

Silsesquioxane functionalized silica gel 54 40Silica nano-sheets 11.8 41Carboxylate functionalized SBA-15 159 27SBA-15 49.3 18Multicarboxylic hyperbranchedpolyglycerol functionalized SBA-15

160 20

Amino functionalized MCM-41 54 21SBA-15 280 19SBA-3 ca. 284 17MCM-41 654.5 This

work

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over different contact times at different initial MB concentra-tions. It appears from Fig. 9 that, for all concentrations of MB,the adsorption equilibrium could be achieved within 2 minutes,and no signicant change was observed from 2 to ca. 40 min,which suggests the rate of uptake of MB is considerably fast.

The pseudo-rst-order model is based on the hypothesis thatthe rate of occupation of the adsorption sites is proportional tothe number of unoccupied sites, while the pseudo-second-ordermodel assumes that the adsorption rate is controlled bychemical adsorption through sharing or exchange of electronsbetween the adsorbent and adsorbate.18 Parameters of the twokinetic models are given in Table 2. As can be seen in Table 2,the correlation coefficients calculated from both of the kineticmodels were very high (>0.9996) for all MB concentrations, andthe theoretical (qe,cal) value calculated from the pseudo-rst-order model agreed well with the experimental (qe,exp) values.Therefore, the adsorption process of MB on the HS is mainly aphysical process. However, this result is different from theresults reported previously, which state that a pseudo-second-order model is more suitable for describing the similaradsorption process17,18,20 when mesoporous silica was used asthe adsorbent. This difference can be attributed to the fact thatthe hierarchical structure of HS leads to a faster adsorptionprocess rate compared with other silica based mesoporousmaterials. So the adsorption was nished aer the initial stage,

Fig. 10 Nonlinear fits of Langmuir and Freundlich isotherm models(adsorbent dosage, 0.25 g L�1; temperature, 25 �C; pH 11).

1402 | Anal. Methods, 2014, 6, 1397–1403

and t well to the pseudo-rst-order model. The samephenomenon was obtained using hierarchical activatedcarbon14 or macroporous silica gel as adsorbents38 for theadsorption of MB.

Adsorption isotherms

The adsorption isotherms describe the distribution of adsor-bate between the liquid phase and the solid phase when thesystem reaches equilibrium.39 As shown in Fig. 10, HS has alarge adsorption capacity for MB. The saturation adsorptioncapacity of HS to MB is 654.5 mg g�1, which is much larger thanthose of the same dye onto most conventional silica basedmesoporous materials, as shown in Table 3. The adsorptionequilibrium data were tted to the Langmuir and Freundlichisotherms. The parameters and correlation coefficients calcu-lated from the models are listed in Table 4. It can be seen thatthe least-squares correlation coefficient (R2) value of the

Table 4 Parameters of adsorption isotherms of MB onto HS

Isotherms Parameters Value

Langmuir model qmax (mg g�1) 654.5KL (L mg�1) 0.7583R2 0.9566

Freundlich model KF (mg�1/nL1/n/g) 334.71/n 0.1772R2 0.7501

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Langmuir model is signicantly higher than that of theFreundlich model for MB removal, indicating that the Lang-muir model should better describe the MB adsorption onto HS.

Conclusions

Mesoporous silica particles with hierarchical structures weresynthesized by using ethanol as a cosurfactant. The presentstudy shows that the thus prepared hierarchical mesoporoussilica particles are an efficient adsorbent for the removal ofmethylene blue from aqueous solution. The adsorption rate isvery fast and adsorption equilibrium could be reached in 2 min.The adsorption isotherms can be tted well to a Langmuirmodel, showing a maximum monolayer adsorption capacity of654.5 mg g�1. The adsorption process follows the pseudo-rst-order model, which suggests that the process is controlled byphysisorption.

Acknowledgements

This work was supported by NSFC (20975090).

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