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The definitive version is available at http://dx.doi.org/10.1016/j.materresbull.2013.04.093

Huang, W., Li, D., Zhu, Y., Xu, K., Li, J., Han, B. and Zhang, Y. (2013) Phosphate adsorption on aluminum-coordinated functionalized macroporous–mesoporous silica: Surface

structure and adsorption behavior. Materials Research Bulletin, 48 (12). pp. 4974-4978.

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Accepted Manuscript

Title: Phosphate Adsorption on Aluminum–coordinatedFunctionalized Macroporous–mesoporous Silica: SurfaceStructure and Adsorption Behavior

Author: Weiya Huang Dan Li Yi Zhu Kai Xu Jianqiang LiBoping Han Yuanming Zhang

PII: S0025-5408(13)00379-6DOI: http://dx.doi.org/doi:10.1016/j.materresbull.2013.04.093Reference: MRB 6699

To appear in: MRB

Received date: 20-1-2013Revised date: 30-4-2013Accepted date: 30-4-2013

Please cite this article as: W. Huang, D. Li, Y. Zhu, K. Xu, J. Li, B.H.</sup>,Yuanming Zhang, Phosphate Adsorption on Aluminum–coordinatedFunctionalized Macroporous–mesoporous Silica: Surface Structureand Adsorption Behavior, Materials Research Bulletin (2013),http://dx.doi.org/10.1016/j.materresbull.2013.04.093

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Phosphate Adsorption on Aluminum–coordinated Functionalized

Macroporous–mesoporous Silica: Surface Structure and Adsorption

Behavior

Weiya Huang1,2, Dan Li3, Yi Zhu1, Kai Xu1, Jianqiang Li1, Boping Han4,Yuanming

Zhang 1,*

1Department of Chemistry, Jinan University, Guangzhou 510632, P.R. China

2Department of Materials Science and Engineering, Taizhou University, Linhai

317000, P.R. China

3Environmetal Engineering, School of Engineering and Information Technology,

Murdoch University, Murdoch, Western Australia 6150, Australia

4Institute of Hydrobiology, Jinan University, Guangzhou 510460, P.R. China

* To whom correspondence should be addressed. E-mail: tzhangym@jnu.edu.cn (Y.M.

Zhang);

Fax: +86-20-85221264; Tel: +86-20-85221264.

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Abstract:

In this study, Al(Ш)-coordinated diamino-functionalized macroporous-mesoporous

silica was synthesized and characterized by X-ray diffraction, N2

adsorption-desorption, Fourier transform infrared spectroscopy, scanning and

transmission electron microscopy. Because of well-defined and interconnecting

macroporous-mesoporous networks, the resulting adsorbent (MM-SBA) exhibited a

significantly better phosphate adsorption performance and faster removal rate, as

compared with the mesoporous adsorbent (M-SBA). Based on the Freundlich and

Langmuir models, the phosphate adsorption capacity and the maximum adsorption

capacity of MM-SBA were 7.99 mg P/g and 23.59 mg P/g, respectively. In the kinetic

study of MM-SBA, over 95% of its final adsorption capacity reached in the first 1 min;

whereas that of M-SBA was less than 79%.

KEYWORDS: A. amorphous materials; A. structural materials; B. chemical

synthesis; D. diffusion

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1. Introduction

Phosphorus is an important nutrient for living species, which is typically present

at a low concentration in environment serving as the limiting factor for aquatic plant

growth. To date, the increasing discharge of contaminating nutrients, e.g. phosphate,

into the aquatic system has resulted in serious environmental problems around the

world, such as eutrophication and consequent algal bloom [1]. Hence, it is necessary

to decrease the concentration of phosphate in wastewater before its discharge. The

typical discharge limit for phosphorus in wastewater is less than 1 mg P/L [2];

however, this still causes the problem of eutrophication. Therefore, methods which are

capable to remove phosphate efficiently and rapidly at a relatively low concentration

are urgently needed. A series of methods have been studied to remove phosphate from

water [3], in which the adsorption-based process has attracted great interest, due to the

high removal efficiency and fast removal rate, especially at low phosphate

concentrations [4]. Among various types of adsorbents, including inorganic, organic

and inorganic-organic hybrid materials [5-7], well-ordered mesoporous silica

materials are useful in adsorption for their unique features, such as highly ordered

structures, ultrahigh surface areas, narrow mesopore size distributions, and tunable

pore size and structure [8,9]. However, the pure mesoporous silicas exhibit extremely

low or even immeasurable phosphate adsorption capacities, which are attributed to the

poor capacity of surface groups Si-OH capturing phosphate anions from aqueous

solution [10]. So far, organic-functionalization has been considered as an effective

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strategy to precisely control the chemical properties of silica materials and generate

specific binding sites for phosphate ions, which can enhance the adsorption capacity

and removal rate of resulting adsorbents. For example, Chouyyok and co-workers

reported that Fe(III)-coordinated ethylenediamine-functionalized MCM-41 silica had

a phosphate adsorption capacity of 14.13 mg P/g with 99% of phosphate removal

within 1 min [9]. Zhang et al. utilized Fe(III) and La(III)-coordinated

diamino-functionalized MCM-41 adsorbents for phosphate removal, which showed

the maximum adsorption capacities of 16.90 mg P/g and 17.72 mg P/g, respectively

[11, 12]. Recently, we reported the synthesis of functionalized SBA-15 adsorbents

with various loadings of diamino group via a new NH4F-assisted co-condensation

method for the first time [13]. With 0.5:1 molar ratio of

N-(2-aminoethyl)-3-aminopropyltrimethoxysilane and tetraethoxysilane, the resulting

Fe(III)-coordinated sample showed the maximum phosphate capture capacity of 20.7

mg P/g and reached almost 90% of the final adsorption capacity in 10 min.

However, it is noted that the attachment of functional groups on the silica pore

surfaces decreased pore sizes of mesoporous silicas and subsequently limited the

diffusion [14]. Previous research has shown that the introduction of macropores into

mesoporous materials may act as rapid transport conduits to the active sites on the

surfaces of mesopores, thus favoring mass transfer and reducing transport limitations

[15]. The objective of our work is to investigate the utilization of Al(III)-coordinated

diamino-functionalized macroporous-mesoporous SBA-15 silica material for

phosphate removal. Herein, a dual-templating approach was applied to generate

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mesopores and macropores by using P123 and polystyrene beads in the preparation of

adsorbent. Afterwards, the diamino-functional groups were post-grafted onto the

macroporous–mesoporous SBA-15 supports, followed by the immobilization of Al(Ш)

active sites. The fabrication, characterization and phosphate adsorption performance

of Al(Ш)-coordinated diamino-functionalized macroporous-mesoporous SBA-15

silicas are detailed in this paper. The presence of interconnected macroporous and

mesoporous structures in our prepared adsorbent is expected to promote mass transfer

and enhance phosphate removal.

2. Experimental

2.1 Preparation and characterization of adsorbents

Monodisperse polystyrene (PS) beads with an average size of 380 ± 10 nm were

synthesized via the emulsion polymerization approach as reported [16].

Macroporous-mesoporous SBA-15 silica was prepared with the use of Pluronic P123

triblock co-polymer (PEO20-PPO70-PEO20, Aldrich) and our prepared PS beads as

templates which generated mesopores and macropores [17]. 1.0 g of P123 was

dissolved in a solution consisting of 7.5 mL of deionized (DI) water and 25.0 mL of

2.0 M HCl under stirring at 35 °C. After well dispersing 8.7 g of PS beads, 2.3 mL of

tetraethoxysilane (TEOS, Aladdin Reagent Inc., 95%) was added to the mixture,

which was continuously stirred at 35°C for 24 h and subsequently reacted at 100 °C

for another 24 h. Solid powder was filtered and washed with DI water 3 times,

followed by calcination in air at 550°C for 6 h to remove P123 and PS beads.

Afterwards, the resulting silica sample was functionalized via the post-grafting

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method, by refluxing in the mixture of 30.0 mL of toluene and 3.0 mL of

N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AAPTS, 95%, Aladdin Reagent

Inc.) at 130°C for 24 h. Following that, the diamino-functionalized sample was

filtered, washed with ethanol 3 times and mixed in 0.1 M AlCl3 aqueous solution for 2

h. The solid was separated, washed with DI water and 2-propanol, and dried under

vacuum at 60 °C for 12 h. The final product, Al(III)-coordinated

diamino-functionalized macroporous–mesoporous silica, is denoted as MM-SBA. For

comparison, by using the similar synthetic method, the Al(III)-coordinated

diamino-functionalized mesoporous silica which was fabricated without the addition

of PS beads is denoted as M-SBA.

Surface morphologies of samples were examined by scanning electron

microscopy (SEM, JSM-7401F, JEOL Ltd., Japan) and transmission electron

microscopy (TEM, JEM2010-HR, JEOL Ltd., Japan). X-ray powder diffraction (XRD)

patters were recorded in the 2θ range of 0.6–6° at a scan speed of 1°/min by using a

diffractometer (Bruker D8 Advance diffractometer, Germany) with Cu Kα radiation

(40 mA, 45 kV). Fourier transform infrared (FT-IR) measurements were performed

between 400 cm−1 and 4000cm−1 by using Shimadzu IR Prestige-21 instrument, in

which KBr pellets containing 0.50 % of the samples were used. Nitrogen

adsorption-desorption isotherms were measured at 77 K using ASAP2010

(Micromeritics Inc., USA). Prior to analysis, the samples were degassed at 120 °C for

12 h under vacuum. The specific surface area (SBET) was determined from the linear

part of the BET plot (P/P0 = 0.05-0.20). The pore size (DBJH) was calculated from the

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desorption branch of isotherm by using Barrett-Joyner-Hallenda (BJH) method. The

total pore volume (VTotal) was evaluated from the adsorbed nitrogen amount at a

relative pressure 0.98.

2.2 Phosphate adsorption of adsorbents

A series of batch tests were conducted to investigate the phosphate adsorption

performances of adsorbents. In the equilibrium experiment, 0.025 g of adsorbent was

added into 25.0 mL of phosphate solution with various initial concentrations in a

polypropylene bottle, which was prepared by dissolving anhydrous K2HPO4

(analytical grade, Sinopharm Chemical Reagent Co., Ltd) in DI water. After shaking

for 2 h, the solution was removed by filtering through a syringe nylon-membrane

filter (pore size 0.45 μm; Shanghai Xinya Purification Devices Factory). The

concentraiton of phosphate in filtrate was analyzed by Autoanalyzer 3 (Bran and

Luebbe Inc., Germany). The amount of phosphate adsorbed on the sample at

equilibrium (qe) was calculated by Eq. (1),

m

VCCq e

e

!!!

)( 0 (1)

where C0 and Ce are the initial and equilibrium phosphate concentrations in solution

(mg P/L), respectively; V is the volume of solution (L) and m is the mass of adsorbent

(g).

The equilibrium data were fitted to the well-known Langmuir and Freundlich

isotherm models, as shown in Eqs. (2) and (3), respectively [18],

Langmuir model: 00

1qC

KqqC e

Le

e !! (2)

Freundlich model: eFe Cn

Kq log1

loglog !! (3)

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where q0 (mg/g) and KL (L/mg) are constants in the Langmuir isotherm model which

are related to maximum adsorption capacity and energy or net enthalpy of adsorption,

respectively; KF (mg/g) and n are the constants in the Freundlich isotherm model,

which measure the adsorption capacity and intensity, respectively.

The adsorption kinetic experiment was conducted as follows: 0.10 g of adsorbent

was added in 100.0 mL of phosphate solution with initial concentration of 50.0mg P/L.

The sealed polypropylene bottle was then placed in the shaker bath for 4 h. About 2.0

mL of suspension was taken out of bottle over a given period of time, and the

phosphate concentration was analyzed as in the equilibrium experiment.

3. Results and discussion

3.1 Characterization of adsorbents

The X-ray diffraction patterns of M-SBA and MM-SBA are presented in Fig. 1.

In the XRD pattern of M-SBA, three well-resolved SBA-15 reflections, one intense

diffraction peak (100) and two relatively weak diffraction peaks (110) and (200),

suggest the presence of well-ordered hexagonal arrangement of mesoporous silica

framework [19]. In the XRD pattern of MM-SBA, the characteristic reflections of

SBA-15 peaks are relatively lower; however, all the peaks are distinguishable. This

indicates that the synthesized MM-SBA adsorbent has a well-ordered hexagonal

mesoporous silica framework as well.

Fig. 2 shows FT-IR spectra of M-SBA and MM-SBA. The bands at 1082 cm−1,

803 cm−1, and 466cm−1 are attributed to the Si-O-Si asymmetric and symmetric

vibrations of condensed silica network. The peak at 961cm−1 is ascribed to the

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non-condensed silanol Si-OH groups [20, 21]. The bands observed at 1408 cm−1 and

1635 cm−1 are corresponding to the NH2 vibrations [21, 22], which confirms the

successful incorporation of organic functional groups into the synthesized adsorbents

via the post-grafting method.

Fig. 3 shows N2 adsorption-desorption isotherms of M-SBA and MM-SBA, and

their corresponding Barrett-Joyner-Halenda (BJH) pore size distribution plots (the

inset). Table 1 summarizes the textual property of samples, including BET surface

area (SBET), pore diameter (DBJH) and total pore volume (VTotal). The N2

adsorption-desorption isotherm of M-SBA exhibits a type IV isotherm model with a

H1 hysteresis loop according to IUPAC classification standard, suggesting the sample

with uniform and even mesopores [23]. The BET surface area, pore diameter and total

pore volume of M-SBA are 219 m2/g, 4.98 nm and 0.32 cm3/g, respectively. Its BET

surface area is smaller than that (514 m2/g) of the corresponding parent SBA-15

without amino-functionalization and metal-impregnation; that is also observed in the

comparison of SBET between MM-SBA (187 m2/g) and its parent

macroporous-mesoporous SBA-15 (432 m2/g), which can be explained by the

diamino-functionalization on the silica and the Fe-coordination [9,11]. As compared

with M-SBA, MM-SBA exhibits a similar N2 adsorption-desorption isotherm as well

as textual properties. However, in the N2 adsorption-desorption isotherm of MM-SBA,

there is an additional prominent hysteresis above P/P0 = 0.9, which is attributed to the

presence of high concentration of macropores in the sample [24].

Fig. 4a-d shows SEM and HR-TEM images of M-SBA and MM-SBA. In Fig. 4a,

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the sample M-SBA exhibits a uniform and ropelike shape, which is the typical

SBA-15 morphology as reported in literature [19]. In MM-SBA, the addition of PS

beads modifies the SBA-15-like organization and creates a new macroporous network

(the inset of Fig. 4b). The formed macroporous network is open and interconnected

with SBA-15 mesoporous framework, which is expected to induce a rapid adsorption

through particle diffusion. The HR-TEM images (Fig. 4c and d) further confirms the

change of morphology between these two samples. As seen in Fig. 4c, M-SBA shows

a typical SBA-15 morphology with 2D periodically hexagonal mesostructure, which

are indicated by white arrows and lines. In Fig. 4d, the introduction of macropores,

which are highlighted by the yellow line, breaks up the extended mesoporous

channels. The size of observed macropore is approximately 350 nm, which is close to

the size of PS beads (Fig. 4e; 380 ± 10 nm). Moreover, the highly-organized

concentric mesoporous channels solely locate in the walls of macroporous framework

of MM-SBA, which are shown with white arrows and lines in Fig. 4d.

3.2 Phosphate adsorption of adsorbents

Fig. 5 shows the Langmuir (Fig. 5a) and Freundlich adsorption isotherms (Fig.

5b). In Fig. 5 and Table 1, both of the Freundlich model and Langmuir model can

well describe the experimental isotherm data, despite the use of the Freundlich model

(R2 ≈ 0.98) is slightly better as compared to the Langmuir model (R2 ≈ 0.94). As

shown in Table 1, the adsorption capacity (KF) of MM-SBA is 7.99 mg P/g, which is

40.42% greater than that of M-SBA. The quantitative Langmuir parameter q0 for

MM-SBA is 23.59 mg P/g, which is significantly greater than that of M-SBA (16.21

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mg P/g). These results can be explained by the incorporation of macropores into the

mesoporous adsorbent, which promotes the diffusion of phosphate to the active sites

of adsorbent, thus improving the adsorption efficiency as expected.

Fig. 6 shows the adsorption kinetic study with the use of M-SBA and MM-SBA.

In Fig. 6, over 95% of final adsorption capacity of MM-SBA reaches in the first 1 min.

In the following 10 min, the phosphate adsorption of MM-SBA slightly increases and

reaches the equilibrium for the rest of time. However, the adsorption rate of M-SBA is

slower as compared with that of MM-SBA; only 79% of final adsorption capacity is

recorded at 1 min and approximately 95% of final adsorption capacity of M-SBA

reaches at 3 min. The results of kinetic study suggest that faster diffusion of

phosphate to the active sites of the macroporous-mesoporous adsorbent MM-SBA

occurs, as compared with the single-sized mesoporous adsorbent M-SBA.

4. Conclusions

Al(III)-coordinated diamino-functionalized macroporous-mesoporous silica

adsorbent was synthesized by using a PS/TEOS weight ratio of 4:1. As compared with

the single-sized mesoporous M-SBA, MM-SBA possessed a

macroporous-mesoporous network, which exhibited an enhanced phosphate

adsorption performance, with a high maximum adsorption capacity (q0 = 23.59 mg

P/g) and a fast adsorption rate (> 95% of final adsorption capacity in the first 1min).

Therefore, the functionalized macroporous-mesoporous silica adsorbent shows great

potential for use in efficient phosphate removal.

Acknowledgements

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This work was supported by Natural Science Foundation of Guangdong (No.

S2011040001667), the Fundamental ResearchFunds for the Central Universities (No.

21611310), the National High Technology Research and Development Program of

China (863 Program) (No.2009AA064401), and the Key Laboratory of Mineralogy

and Metallogeny Cooperation Foundation (No. KLMM20110204). Support for Ms

Weiya Huang was partially provided by the program of China Scholarship Council

(No.201206780010).

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Table1. Textual property; and Langmuir and Freundlich isotherms parameters in the

phosphate adsorption by using MM-SBA and M-SBA.

Figure Captions:

Fig. 1. XRD patterns of M-SBA and MM-SBA.

Fig. 2. FT-IR spectra of M-SBA and MM-SBA.

Fig. 3. N2 adsorption-desorption isotherms of M-SBA and MM-SBA; and their

corresponding BJH pore size distribution plots (the inset).

Fig. 4. SEM images of M-SBA (a) and MM-SBA (b) at a low magnification (the inset

in (b) shows SEM image of MM-SBA at a high magnification); HR-TEM images of

M-SBA (c) and MM-SBA (d); TEM image of PS beads (e). The white arrows and

lines show the presence of mesopores; and the yellow line shows the observed

macropore in MM-SBA.

Fig. 5. (a) Langmuir adsorption isotherm and (b) Freundlich adsorption isotherm of

M-SBA and MM-SBA.

Fig. 6. Effect of contact time on the phosphate adsorption of M-SBA and MM-SBA.

Textual Property Langmuir Freundlich

Sample SBET

(m2/g)

DBJH

(nm)

VTotal

(cm3/g)

q0

(mg/g)

KL

(L/mg)

R2 n KF

(mg/g)

R2

M-SBA 219 4.98 0.32 16.21 0.449 0.944 3.88 5.69 0.978

MM-SBA 187 4.67 0.31 23.59 0.498 0.941 3.73 7.99 0.977

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Fig. 1.

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Fig. 2.

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Fig. 3.

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Fig. 4.

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Fig. 5.

Fig. 6.

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Research highlights

! ! Al-coordinated functionalized macroporous-mesoporous silica for phosphate removal

! ! It had the maximum adsorption capacity of 23.59 mg P/g

! ! Over 95% of the final adsorption capacity reached in the first 1 min

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*Graphical Abstract (for review)