Preparation of mesoporous Al-MCM-41 from natural palygorskite and its adsorption performance for hazardous aniline dye-basic fuchsin

Preparation of mesoporous Al-MCM-41 from natural palygorskite and its adsorption performance for hazardous aniline dye-basic fuchsin

Accepted Manuscript Preparation of mesoporous Al-MCM-41 from natural palygorskite and its adsorption performance for hazardous aniline dye-basic fuchs...

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Accepted Manuscript Preparation of mesoporous Al-MCM-41 from natural palygorskite and its adsorption performance for hazardous aniline dye-basic fuchsin Yuan Guan, Shaomang Wang, Xin Wang, Cheng Sun, Yongbo Wang, Lingjie Hu PII:

S1387-1811(16)30119-6

DOI:

10.1016/j.micromeso.2016.04.025

Reference:

MICMAT 7687

To appear in:

Microporous and Mesoporous Materials

Received Date: 12 January 2016 Revised Date:

2 April 2016

Accepted Date: 18 April 2016

Please cite this article as: Y. Guan, S. Wang, X. Wang, C. Sun, Y. Wang, L. Hu, Preparation of mesoporous Al-MCM-41 from natural palygorskite and its adsorption performance for hazardous aniline dye-basic fuchsin, Microporous and Mesoporous Materials (2016), doi: 10.1016/ j.micromeso.2016.04.025. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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A novel mesoporous Si-Al material, Al-MCM-41 was synthesized through alkali

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calcination leaching coupled with hydrothermal synthesis and calcination using

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natural palygorskite.

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Preparation of mesoporous Al-MCM-41 from natural palygorskite and its adsorption performance for hazardous aniline dye-basic fuchsin

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Yuan Guana,b Shaomang Wangb,c* Xin Wanga* Cheng Sunc Yongbo Wangb Lingjie Hub

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a Key Laboratory for Soft Chemistry and Functional Materials of Ministry Education, Nanjing University of Science and Technology, Nanjing 210094, China.

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b School of Environment and Safety Engineering, School of Huai De, Changzhou University, Changzhou 213164, PR China.

c State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210023, PR China.

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Corresponding authors: Tel: +86 25 84305667

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E-mail addresses: [email protected], [email protected]

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Abstract A promising mesoporous Si-Al material, Al-MCM-41 was successfully prepared by

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alkali calcination leaching of natural palygorskite, and sequent hydrothermal synthesis coupled

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with calcination. The morphology, structure, surface area and pore-size distribution of the material

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were characterized by scanning electron microscopy, small-angle X-ray diffraction and N2

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adsorption–desorption isotherms. The Al-MCM-41 possessed better crystallinity and larger

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specific surface area with CTAB/SiO2 mass ratio of 0.1:1, pH of 5, crystallization temperature of

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110 °C, crystallization time of 12 h and calcination of 550 °C for 5 h. The Al-MCM-41 was used

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as an adsorbent for the removal of hazardous aniline dye-basic fuchsin from water. Adsorption

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kinetics and isotherms analysis indicated that adsorption of basic fuchsin onto Al-MCM-41

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followed pseudo-first-order model and Langmuir adsorption isotherm. In addition, intra-particle

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diffusion was rate determining step of the adsorption process. The results demonstrated that the

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as-synthesized Al-MCM-41 had potential applications in the treatment of waste water.

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Keywords: Mesoporous Al-MCM-41; Natural palygorskite; Basic fuchsin; Adsorption kinetics;

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Adsorption isotherm; Desorption

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1. Introduction

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Mesoporous materials have attracted considerable interest since the discovery of M41S family

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at Mobile Research & Development Corporation in 1992 [1]. MCM-41, the most extensively

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studied member of M41S family, gains special attention because of its large specific surface area, 1

ACCEPTED MANUSCRIPT thermal and hydrothermal stabilities and ordered mesoporous nature with uniform pore diameter.

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During the past two decades, MCM-41 has been used in a wide range of applications such as

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adsorption [2-4], catalysis [5-7], separation [8-10] and energy [11, 12]. However, the applications

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of MCM-41 are limited due to its low acidity, basicity and redox properties [13]. Research has

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shown that these problems can be solved by introducing heteroatoms into the MCM-41 framework.

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It is reported that the incorporation of Al into the MCM-41 framework can lead to the formation of

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Brönsted acid sites, and the resulting Al-MCM-41material exhibits good adsorption affinity [14].

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In general, Al-MCM-41 molecular sieve can be prepared by hydrothermal transformation of

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basic silica aluminum gels in the presence of quaternary ammonium surfactants with different

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alkyl chain lengths. However, the source of silica for the traditional synthesis is mainly from

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costly organic silica precursors such as silicon alkoxides or costly efficient substances, for

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example, sodium silicate [15-17]. These silicate precursors not only increase the cost of

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production but also result in environmental pollution. Thus, it is urgent to seek cheaper raw

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materials for the fabrication of mesoporous Si-Al materials. Some materials such as coal fly ash

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[18], iron ore tailing [19], sepiolite [20] and bentonite [21], etc. have been reported to synthesize

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mesoporous Si-Al materials through mechanical chemistry and hydro-leaching processes.

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Palygorskite (Pal), a species of hydrated magnesium aluminum silicate nonmetallic mineral

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((H2O)4(Mg, Al, Fe)5(OH)2Si8O20•4H2O) with a lath or fibrous morphology, is a porous crystalline

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structure containing tetrahedral layers alloyed together along longitudinal sideline chains [22].

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Owning to high content of useful silica and aluminum [23], natural Pal clay can be used to

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fabricate ordered mesoporous Si-Al materials.

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Herein, we synthesize a mesoporous Si-Al material, Al-MCM-41 through alkali calcination

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leaching coupled with hydrothermal synthesis and calcination using Pal. The effects of preparation

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parameters on the formation of the material with ordered structure are systematically investigated.

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A hazardous aniline dye, basic fuchsin (BF) is chosen as the model adsorbate to evaluate

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adsorption performance of Al-MCM-41. The influences of various operational parameters

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including contact time, adsorbent quality, initial adsorbate concentration and temperature on the

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adsorption process are discussed by batch adsorption experiments. The adsorption kinetics and

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equilibrium investigations are also carried out.

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2. Experimental section 2

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2.1 Materials Pal (effective substance > 99 wt %) with an average diameter of 74 µm was provided by Jiangsu

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Pal Co. Ltd (china). Hydrochloric acid (HCl, 37%), ammonium acetate (CH3COONH4), sodium

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hydroxide (NaOH), cetyltrimethylammonium bromide (CTAB), polyethylene glycol 4000 (PEG)

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and basic fuchsin (BF) were purchased from sinopharm chemical reagent Co. Ltd (china). All the

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reagents were analytical grade chemicals.

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2.2 Synthesis

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20 g of Pal was treated by 80 mL of HCl (6 mol L-1) for 3 h at 60 with deionized water to pH value of 7, and then dried at 80

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achieved and stored in a desiccator for further use.

. It was filtered, washed

for 12 h. HCl-acidified Pal was

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10 g of acidified Pal and 15 g of NaOH were ball-milled together and calcined in air at 600

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for 5 h. The calcined mixture was added into 100 mL of deionized water and stirred vigorously for

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12 h to obtain the suspension. Then the extracted solution containing silica aluminum source was

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separated from the suspension by a filtration process.

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An equal amount of CTAB and PEG was dissolved into 30 mL of deionized water. The

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extracted solution was slowly added to the homogeneous solution with magnetic stirring. After

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stirring for 1 h, 6 mol L-1 of HCl was added to adjust pH value of the dispersion and continuously

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stirred for 2 h. The resultant slurry was transferred to a Teflon-lined stainless steel autoclave (100

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mL ) and aged at 110

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structure and properties of materials, various conditions including mass ratio of CTAB to SiO2 (R

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= 0.05–0.5), pH value (pH = 3–9), crystallization temperature (T = 60–210

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time (t = 6–48 h) were chosen.

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for 12 h. To study the influences of different synthesis conditions on the

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) and crystallization

After crystallization, the resulting gel was filtered, washed repeatedly with deionized water and

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dried at 80

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the template.

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2.3 Characterization

. Finally, the as-synthesized sample was calcined in air at 550

for 5 h to remove

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The crystalline phase structure of as-prepared mesoporous materials was determined by a ARL

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X'TRA X-ray diffractometer using Cu Kα radiation (λ = 0.154 nm) in the 2theta range of 1-10°.

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The XRD tests were performed using a 1.0° divergence slit, a 1.0° scatter slit and a 0.15 mm

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receiving slit. The data were recorded with a step size of 0.02° and scan speed of 1°/min in a 3

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proportional detector. The BET surface area was evaluated by N2 adsorption in a constant volume

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adsorption apparatus (Beekman Coulter SA 3100). The samples had been degassed at 200

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2 h prior to measurement. Scanning electron micrographs (SEM) were recorded on a Zeiss

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Supra55VP SEM microscope at 5 keV. The transmission electron microscopy (TEM) analysis was

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performed on a JEM-2100 microscope, operating at 200 kV.

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The functional groups on the surface were investigated on a Nicolet Nexus-870 FTIR

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spectrometer. The FTIR spectra were obtained in the range of 500–4000 cm−1 at 2 cm−1 resolution

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with 64 scans. The surface electronic state was identified through X-ray photo-electron

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spectroscopy (XPS) performed on a PHI 5000 Versa Probe electron spectrometer using Al Kα

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radiation. The scans with 10 sweeps were obtained with a pass energy of 100 eV and a step size of

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1 eV. For the high resolution scans, 20 sweeps were used with a pass energy of 20 eV and a step

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size of 0.1 eV.

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2.4 Batch adsorption

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To evaluate the adsorption properties of as-synthesized mesoporous samples, batch adsorption

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experiments were conducted. Typically, the adsorbent (0.05 g) was added into 50 mL of BF

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solution (90 mg L-1), and then the mixture was shaken in an orbital shaker (150 rpm) at 25

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4 h. At given time intervals, 3.0 mL of solution was taken out and the mixture was centrifuged at

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9000 rpm for 10 min. The BF concentration in the supernatants was determined via 722N

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spectrophotometer at a maximum wavelength of 543 nm. The equilibrium adsorption was

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quantified according to the equation (1).

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C − C V 1 m

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In which, qe is the adsorption amount of BF at equilibrium (mg g-1), C0 and Ce are the initial and

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equilibrium concentrations of BF (mg L-1), m is the mass of adsorbent (g), and V is the volume of

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solution (L).

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In order to determine the adsorption kinetic and thermodynamic characteristics, a set of

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experiments were carried out at three different temperatures (25, 35 and 45

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initial BF concentration, and at 25

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L-1).

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2.5 Desorption

) with 60 mg L-1 of

with three initial BF concentrations (60, 90 and 120 mg

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The desorption experiments were conducted in 250 mL flasks containing 200 mL of

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CH3COONH4 solution (NH4Ac) and 0.05 g of Al-MCM-41 powders after adsorption. Each sample

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was carried out three times to ensure desorption thoroughly. The desorption rate Rd is calculated

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by the following equation (2):

 × 100% 2 

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 = 3. Results and discussion

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3.1 The influences of different parameters on formation of mesoporous adsorbent

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Fig. 1.

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Fig. 2.

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Table 1

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3.1.1 Influence of CTAB /SiO2 mass ratios

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XRD patterns of Al-MCM-41 obtained at different CTAB/SiO2 mass ratios are recorded in °

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Fig. 1a. All the samples showed characteristic reflections of (1 0 0) crystal plane at 2θ = 2.43 ,

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which can be indexed to a two-dimensional hexagonal lattice [24]. With increasing mass ratio of

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CTAB/SiO2, the crystallinity of Al-MCM-41 was enhanced. When the mass ratio was 0.1, three

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small signals at 4.03 , 4.52 and 5.81 corresponding to (1 1 0), (2 0 0) and (2 1 0) crystal planes

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were observed. It confirms that the Al-MCM-41 with highly ordered mesoporous was successfully

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synthesized.

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Nitrogen adsorption–desorption isotherms and the corresponding pore size distribution of

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Al-MCM-41 (CTAB/SiO2 = 0.1) are presented in Fig. 2. The sample displayed a typical IV-type

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isotherm (Fig. 2a), indicating the presence of mesopores (2–50 nm). Adsorption amount increased

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at a relative low pressure (P/P0 < 0.4) in the isotherm, which is due to monolayer adsorption of

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nitrogen towards the walls of the mesopores. The sharp inflection between the relative pressure of

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0.4–0.5 corresponds to the uniformity of pore size distribution, further confirmed by the pore size

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distribution curve (Fig. 2b). The gradual increase at the pressure range of 0.3–1.0 is attributed to

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multilayer adsorption on the outer surface of the particles. The SBET, pore volumes and pore sizes

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of all the samples are listed in Table 1. When the mass ratio of CTAB/SiO2 was above 0.1, the

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SBET of samples located in the range of 946–985 m2 g-1.

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3.1.2 Influence of gelation pH

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intensity of characteristic reflection increased gradually with the increase of pH value. However,

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when pH = 9, the reflection intensity of (1 0 0) crystal plane was weaker. The reason may be that

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the polycondensation of the silicate species is lower at higher pH value, which is in agreement

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with the previous literature [25]. In this condition, it is hard to obtain Al-MCM-41. Also, when the

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value of pH was 5 or 7, the reflection of (2 0 0) crystal plane was also observed. The effect of

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gelation pH on the textural properties is summarized in Table 1. Generally, at lower pH value (pH

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= 3 and 5), the samples with larger specific surface area and pore volumes were obtained.

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3.1.3 Influence of crystallization temperature

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The effect of crystallization temperature on the structure of Al-MCM-41 is shown in Fig. 1c.

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When the temperature was 60

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the sample. High crystallization temperature was more conducive to the formation of well-ordered

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structure of Al-MCM-41. For example, the XRD pattern of Al-MCM-41 obtained at t = 110

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exhibited stronger reflections of (1 0 0), (1 1 0) and (2 0 0) planes. This is because that high

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temperature accelerates the condensation rate of silicate species on the silica wall, resulting in

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pores shrinkage and reducing the specific surface area of the sample [26]. The results can be

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further confirmed by the SBET values listed in Table 1. Lower or higher crystallization temperature

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(60 or 210

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may be described that the optimal crystallization temperature slows down the condensation rate of

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silicate species on the silica wall, increasing the SBET of the sample [27].

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3.1.4 Influence of crystallization time

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, there was no characteristic reflection of (1 0 0) crystal plane in

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) led to the smaller specific surface area of samples. The SBET of 995 m2 g-1 at 110

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Fig. 1d illustrates XRD patterns of Al-MCM-41 achieved at different crystallization time. The

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results show that the reflection intensity of (1 0 0) plane was strengthened gradually with

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increasing crystallization time. During the crystallization of 24 h, the intensity of (2 0 0) crystal

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plane increased more rapidly. As time increases, silicate units may interact with the additional

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surfactant molecules and come into being new surfactant/silica aggregates, forming better ordered

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crystallites [28]. However, the longer time (> 24 h) increases pore wall thickness, which can

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prevent greater shrinkage of the mesoporous structure during calcination, resulting in less ordered

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pore structure [29]. The effect of crystallization time on the pore structure of Al-MCM-41 is

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remarkable (Table 1). There are maximum values in the specific surface area and pore volume at 6

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crystallization time of 12 h. The results imply that suitable crystallization time is beneficial for

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increasing crystallinity of Al-MCM-41.

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3.2 Characterization of mesoporous adsorbent

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3.2.1 SEM and TEM analysis Fig. 3.

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SEM images of natural Pal, acidified Pal, Al-MCM-41, and TEM images of Al-MCM-41are

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shown in Fig. 3. It is seen that many rods or fibers of about 0.5–5 µm in length stacked up into

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bulk bundles and aggregations of natural Pal clay (Fig. 3a). After acidification, the

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above-mentioned bundles of fibrous structures disappeared and numerous rods or fibers with

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shorter length were uniformly dispersed resulting in pores or voids with different sizes (Fig. 3b)

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[22]. The Al-MCM-41 from Pal clay consisted of evenly distributed lamellar or spherical particles

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with cross-linked network, indicating that the rearrangement of Pal occurred after hydrothermal

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treatment and calcination (Fig. 3c and d). TEM images of the Al-MCM-41 are presented in Fig. 3e

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and f. The regular and ordered straight-channels characteristics of MCM-41 can be observed in

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TEM images. This is consistent with the low-angle XRD results presented before.

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3.2.2 XPS analysis

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Fig. 4.

The surface chemical composition and valence state of acidified Pal and Al-MCM-41 were

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analyzed by XPS. The C 1s peak at 284.8 eV originates from adventitious hydrocarbon in the XPS

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instrument [30]. The acidified Pal was composed of Al, Si, O and Mg elements, and the

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Al-MCM-41 consisted of Al, Si and O elements (Fig. 4a). The results manifest that the obtained

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Al-MCM-41 retains the original components of Pal. For acidified Pal, the characteristic peaks of

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Al 2p, Si 2p and O 1s located at 74.44 eV, 103.02 eV and 532.64 eV, respectively (Fig. 4b, c and

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d). Compared to acidified Pal, the above-mentioned characteristic peaks in Al-MCM-41 had a

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shift to high energy due to the stronger interaction among Al 2p, Si 2p and O 1s.

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3.2.3 FTIR analysis

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Fig. 5. Before BF adsorption, FTIR spectra of Al-MCM-41 is displayed in Fig. 5a.

The vibrational

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bands at 1092 and 778 cm−1 correspond to TO4 (T = Si) tetrahedral symmetric and asymmetric

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stretch vibrations, respectively. The absorption band at 468 cm−1 is attributed to the bending 7

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aluminosilicate skeleton. The bands between 1100 and 400 cm−1 are typical bands of mesoporous

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material. The absorption band around 1635 cm−1 is caused by deformational vibrations of

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adsorbed water molecules, and the broad band around 3445 cm−1 corresponds to the surface

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structural hydroxyl groups and adsorbed water, illustrating the hydrophilic characteristic of the

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material. For the sample after BF adsorption (Fig. 5b), FTIR spectra possessed the same shape as

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Al-MCM-41, except for the bands at 2918 and 2850 cm−1, which are caused by the stretching and

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bending vibration of C–H from BF. The results show that the BF was adsorbed onto Al-MCM-41.

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3.3 Adsorption

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3.3.1 Adsorption isotherms

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Fig. 6.

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Table 2

The adsorption isotherm is effective approach to evaluate adsorption capacity of adsorbent, and

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understand the interaction between adsorbate and adsorbent. The Langmuir and Freundlich

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adsorption models are useful tools to express adsorption phenomenon of adsorbent. The Langmuir

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equation is given as follows:

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1 1 C = + C 3 q qb q  226

The Freundlich equation can be written as follows:

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lnq  = lnK $ +nlnC 4 Where Ce (mg L-1) and qm (mg g-1) are the concentration of equilibrium adsorption in the solution

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and the maximum adsorption of adsorbent, respectively. The adsorption constant, b (L mg-1)

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describes the intensity of adsorption process. KF (mg1-n Ln g-1) represents the adsorption capacity

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of adsorbent. In addition, n, which has a range between 0 and 1, reflects the adsorption intensity.

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The theoretical parameters (qm, b, KF and n) of adsorption isotherms along with correlation

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coefficients (R2) are summarized in Table 2. In terms of R2 values, the experimental data can be

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well fitted to the Langmuir model rather than the Freundlich model, suggesting the surface

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homogeneity of adsorbent. Besides, the maximum adsorption capacities calculated using the

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Langmuir model at 25, 35 and 45

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adsorption capacities of adsorbent decreased with increasing temperature. It is obvious that the

were 23.81, 17.24 and 12.21 mg g-1, respectively. Maximum

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calculated values of qm obtained from Langmuir plots (Fig. 6a) are consistent with the

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experimental results, indicating that the adsorption process was monolayer.

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3.3.2 Thermodynamic study

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Table 3 Thermodynamic considerations can give very valuable insight into nature of the adsorption

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process such as its spontaneity, randomness, endothermicity or exothermicity, etc. The free energy

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change (∆G0), enthalpy change (∆H0) and entropy change (∆S0) for the adsorption process are

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calculated by the following equations [31]:

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ΔG = −RTlnK 5 −ΔH  ΔS  lnK = + 6 RT R

where R is the universal gas constant (8.314 J mol-1 K-1) , T is an absolute temperature (K) and

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K is the equilibrium constant at temperature T. According to the reported adsorption

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thermodynamics studies, the values ofΔG0 is estimated in Table 3. The values ofΔH0 andΔS0

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can be calculated from the slope and the intercept of the plot of lnK versus 1/T, respectively. The

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values of these parameters are also summarized in Table 3[32].

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After BF adsorption onto Al-MCM-41 reached equilibrium, negative values of ∆G0 confirm the

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spontaneity of the adsorption process. The increase in values of ∆G0 with an increase in

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temperature shows the adsorption process is more favorable at lower temperatures. The negative

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value of ∆H0 suggests that the process is exothermic. The negative value of ∆S◦ reveals the

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decreased randomness at the solid-solution interface during the fixation of the dye on the active

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sites of the adsorbent [31].

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3.3.3 Adsorption kinetics

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Fig. 7. Table 4

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The kinetic curves (Fig. 7) show that the influences of initial concentration and temperature on

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adsorption kinetics for BF. It can be seen that the adsorption of BF was quite rapid initially (< 30

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min), but it became slower with prolonging time and reached plateaus at 90 min. The greater BF

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concentration increased adsorption rate as well as equilibrium concentration. Besides, the

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adsorption capacities of adsorbent decreased with gradually increasing temperature. This confirms 9

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the exothermic nature of the process and may be due to the tendency of dye molecules to escape

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from the solid phase to bulk phase with increasing temperature of the solution.

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In order to elucidate the adsorption kinetic process, the pseudo-first-order and pseudo-second-order kinetics models are employed. The pseudo-first-order is given [33]:

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lnq  − q .  = lnq  − K/ t 7 268

Where t is the contact time (min) and qt (mg g-1) is the adsorption amount of aniline at time t. The

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capacity of equilibrium adsorption (qe) and the pseudo-first-order rate constant (K1) were obtained

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from the intercept and slope of the plot of ln (qe − qt) against t (Fig. 6 b).

The pseudo-second-order kinetic model in its integrated and linearized form has been used [34]:

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t 1 t = + 8 2 q. K2q q

Where, K2 (g mg-1 min-1) is the rate constant of pseudo-second-order adsorption. The values of qe

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and K2 were calculated by the slope and intercept of the plot of t/qt versus t.

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Table 4 lists the results of rate constant studies at different temperatures by the

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pseudo-first-order and pseudo-second-order models. The values of correlation coefficient R2 using

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the pseudo-first-order adsorption model for the adsorbent were relatively high (> 0.99), and the

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adsorption capacities calculated by the model were also close to those determined by experiments.

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The values of R2 through the pseudo-second-order model were not satisfactory. Thus, it is

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concluded that the pseudo-first-order adsorption model is more suitable to describe the adsorption

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kinetics of BF over Al-MCM-41.

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3.4 Mechanisms of the adsorption process

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Fig. 8.

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The intra-particle diffusion model is also employed to investigate the adsorption process. The intra-particle diffusion model is described as follows:

q . = K 4 t .6 + C 9

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Where qt is the adsorption amount at time t (mg g-1), ki is the diffusion rate constant (mg g-1 min-0.5)

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and C is the intercept for any experiment. The results in Fig. 8 indicate that intra-particle diffusion

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model can represent the data well.

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The analysis shows that, prior to adsorption equilibrium, there are three distinct diffusion

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pathways controlling the adsorption kinetics at different time, a fast diffusion for the initial 10

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adsorption followed by two relatively slow diffusion processes. Considering the large BET areas

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and mesoporous structure of Al-MCM-41, BF molecule diffusion on the outer surface of the

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particles possibly represents the fast process, while that inside the pores results in the slow

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diffusion. As far as well-known, for adsorption onto the solid surface, six mechanisms may be supposed

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to exist, such as electrostatic interaction, ion exchange, ion–dipole interactions, coordination of

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surface metal cations, hydrogen bonding and hydrophobic interaction. The electrostatic interaction

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existed between the BF cations and Si−O− from the dissociating of the Si−OH on the surface of

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Al-MCM-41. In addition, the coordination of surface metal cations also promotes adsorption,

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because the Brönsted acid sites have been drawn into the structure of Al-MCM-41 through the

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introduction of aluminum. Thence, the BF adsorption mechanisms can be summarized to the

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electrostatic interaction and heteroatoms bond mechanisms [18].

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3.5 Desorption studies

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Fig. 9.

With different concentrations of NH4Ac solution, the final adsorption and desorption amount of

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BF are depicted in Fig. 9. It shows that the amounts from the elution process reach a stable value

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(42.5 mg g-1) with NH4Ac concentration above 0.4 mol L-1. With the adsorption equilibrium

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amount 54.4 mg g-1, the desorption rate stayed around 78%, and there was still 11.9 mg g-1 BF

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adsorbed on mesoporous Al-MCM-41 which can not be desorbed by ion-exchange effect. The

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regeneration and reuse of adsorbent indicates that the mesoporous Al-MCM-41 is efficient and

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stable adsorbent for dye removal.

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3.6 Adsorption performance comparison

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The adsorption capacities of BF onto Al-MCM-41 and natural pal were compared and the

313

results are displayed in Fig. S1. The adsorption amount of BF onto Al-MCM-41 and natural pal

314

was gradually increased with the increase in time. The adsorption amount of BF onto Al-MCM-41

315

was significantly greater than that onto natural pal at the same adsorption time, which presents that

316

the Al-MCM-41 has excellent adsorption property.

317

4. Conclusions

318

In summary, a highly ordered mesoporous molecular sieve, Al-MCM-41 was successfully

319

synthesized via alkali calcination leaching of natural palygorskite coupled with hydrothermal 11

ACCEPTED MANUSCRIPT reconstruction and calciantion. Its BET surface area reached 997 m2 g-1 and the pore volume was

321

0.82 cm3 g-1. As expected, the mesoporous Al-MCM-41 exhibited a good adsorption capability for

322

cationic BF. The kinetic study revealed that the rate determining step of the adsorption process

323

was intra-particle diffusion and increased mobility of adsorbate was observed with increasing

324

temperature. The Al-MCM-41 adsorbent from natural Pal is efficient and fast for the removal of

325

dyes from contaminated wastewater.

326

Acknowledgements

327

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This work was financially supported by the Natural Science Foundation of China

(No.

51472120 and No. 51322212), the Natural Science Foundation of Jingjiang Industrial Technology

329

Research Institute (No. KYH15020017) and the Natural Science Foundation of ChangZhou

330

University (No. ZMF15020099).

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References

332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357

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[6] S. Shen, J. Chen, R.T. Koodali, Y. Hu, Q. Xiao, J. Zhou, X. Wang, L. Guo, Activation of MCM-41 mesoporous silica by transition-metal incorporation for photocatalytic hydrogen production, Appl.Catal. B-Environ. 150 (2014) 138-146. [7] L. L. Lou, H. Du, K. Yu, S. Jiang, W. Yu, S. Liu, Immobilized chiral Mn(III) salen complexes on co-condensed imidazole-functionalized mesoporous materials: Effective catalysts for asymmetric epoxidation of olefins, J. Mol. Catal. a-Chem. 377 (2013) 85-91. [8] A. L. Khan, C. Klaysom, A. Gahlaut, I. F. J. Vankelecom, Polysulfone acrylate membranes containing functionalized mesoporous MCM-41 for CO2 separation, J. Membrane Sci. 436 (2013) 145-153. [9] A. Galve, D. Sieffert, C. Staudt, M. Ferrando, C. Gueell, C. Tellez, J. Coronas, Combination of ordered mesoporous silica MCM-41 and layered titanosilicate JDF-L1 fillers for 6FDA-based copolyimide mixed matrix membranes, J. Membrane Sci. 431 (2013) 163-170. [10] E. Kim, H. E. Kim, S. J. Lee, S. S. Lee, M. L. Seo, J. H. Jung, Reversible solid optical sensor based on acyclic-type receptor immobilized SBA-15 for the highly selective detection and separation of Hg(II) 12

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[17] C. Bernal, M. Mesa, M. Jaber, J. L. Guth, L. Sierra, Contribution to the understanding of the formation mechanism of bimodal mesoporous MCM41-type silica with large defect cavities, Micropor. Mesopor. Mater. 153 (2012) 217-226.

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[21] H. Yang, Y. Deng, C. Du, S. Jin, Novel synthesis of ordered mesoporous materials Al-MCM-41 from bentonite, Appl. Clay Sci. 47 (2010) 351-355. [22] J. L. Cao, G. S. Shao, Y. Wang, Y. Liu, Z. Y. Yuan, CuO catalysts supported on attapulgite clay for low-temperature CO oxidation, Catal. Commun. 9 (2008) 2555-2559.

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[25] A. C. Voegtlin, A. Matijasic, J. Patarin, C. Sauerland, Y. Grillet, L. Huve, Room-temperature synthesis of silicate mesoporous MCM-41-type materials: Influence of the synthesis pH on the porosity of the materials obtained, Micropor. Mater.10 (1997) 137-147. [26] L. Z. Wang, J. L. Shi, J. Yu, W. H. Zhang, D. S. Yan, Temperature control in the synthesis of cubic mesoporous silica materials, Mater. Lett. 45 (2000) 273-278. [27] Z. Yu, Y. Wang, X. Liu, J. Sun, G. Sha, J. Yang, C. Meng, A novel pathway for the synthesis of ordered mesoporous silica from diatomite, Mater. Lett. 119 (2014) 150-153. [28] R. Mokaya, Improving the stability of mesoporous MCM-41 silica via thicker more highly 13

ACCEPTED MANUSCRIPT condensed pore walls, J. Phys. Chem. B, 103 (1999) 10204-10208. [29] T. W. Kim, P. W. Chung, V. S. Y. Lin, Facile Synthesis of Monodisperse Spherical MCM-48 Mesoporous Silica Nanoparticles with Controlled Particle Size, Chem. Mater. 22 (2010) 5093-5104. [30] Y. Guan, H. Qian, J. Guo, S. Yang, X. Wang, S. Wang, Y. Fu, Synthesis of acidified palygorskite/BiOI with exceptional performances of adsorption and visible-light photoactivity for efficient treatment of aniline wastewater, Appl. Clay Sci. 114 (2015) 124-132. [31] G. K. Ioannis Anastopoulos, Are the thermodynamic parameters correctly estimated in

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[33] V. K. Gupta, B. Gupta, A. Rastogi, S. Agarwal, A. Nayak, A comparative investigation on adsorption performances of mesoporous activated carbon prepared from waste rubber tire and activated carbon for a hazardous azo dye-Acid Blue 113, J. Hazard. Mater. 186 (2011) 891-901.

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[34] Y. S. Ho, G. McKay, Pseudo-second order model for sorption processes, Process Biochem. 34 (1999) 451-465.

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402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420

424 425 426 427 428

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Captions of figures and tables:

433

Fig. 1. XRD patterns of Al-MCM-41: (a) different CTAB/SiO2 mass ratios of (1) 0.05, (2) 0.1, (3)

434

0.3 and (4) 0.5; (b) different gelation pH values of (5) 3, (6) 5, (7) 7 and (8) 9; (c) different

435

crystallization temperature of (9) 60

436

crystallization time of (13) 6 h, (14) 12 h, (15) 24 h and (16) 48 h.

437

Fig. 2. (a) N2 adsorption-desorption isotherms and (b) BJH pore size distribution of Al-MCM-41

438

(CTAB/SiO2 = 0.1).

439

Fig. 3. SEM images of (a) natural Pal, (b) acidified Pal, (c, d) Al-MCM-41, and TEM images of (e,

440

f) Al-MCM-41.

441

Fig. 4. XPS spectra of (a) Survey, (b) Al 2p, (c) Si 2p and (d) O 1s for acidified Pal and

442

Al-MCM-41.

443

Fig. 5. FTIR spectra of Al-MCM-41 (a) before and (b) after BF adsorption.

444

Fig. 6. (a) Langmuir adsorption isotherms and (b) pseudo-first-order plots at three different

445

temperatures for Al-MCM-41 adsorbent.

446

Fig. 7. (a) Effect of initial concentration on adsorption kinetics for BF (Temperature = 25

447

adsorbent dose = 1 g L-1, pH = 6.5); (b) Effect of temperature on adsorption kinetics for BF (Initial

448

concentration on of BF = 60 mg L-1, adsorbent dose = 1 g L-1, pH = 6.5).

449

Fig. 8. Intra-particle diffusion modeling results of BF adsorption onto Al-MCM-41 with

450

different initial concentrations (Temperature = 25

451

Fig. 9. Equilibrium adsorption and desorption concentration with NH4Ac solution. (Initial

452

concentration of BF = 60 mg L-1, adsorbent dose = 1 g L-1, Temperature = 25

453

Table 1 The effects of different synthesis parameters on textural properties of Al-MCM-41.

454

Table 2 The constants of Langmuir and Freundlich isotherms and respective correlation

455

coefficients for BF adsorption onto Al-MCM-41.

456

Table 3 Thermodynamic parameters for BF adsorption onto Al-MCM-41.

457

Table 4 Kinetic parameters for BF adsorption onto Al-MCM-41.

, (11) 160

and (12) 210

; (d) different

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458

15

,

, adsorbent dose = 1 g L-1, pH = 6.5).

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, (10) 110

).

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459 460 461

462 463 464

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Fig. 1.

Fig. 2.

16

Fig. 3.

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465 466 467

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468 469

Fig. 4. 17

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472 473

Fig. 6.

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474 475 476

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Fig. 5.

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470 471

477 478 479

Fig. 7.

18

480 481 482

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Fig. 8.

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484 485 486

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483

Synthesis conditions

Fig. 9. Table 1

Parameters

SBET (m2 g-1)

Pore volume (cm3 g-1)

Pore Diameter (nm)

0.05

434.85

0.31

6.91

0.1

984.82

0.75

5.82

0.3

965.86

0.68

6.04

0.5

945.65

0.61

6.52

3

931.64

0.58

6.73

5

997.72

0.79

5.75

7

350.35

0.41

7.21

9

117.35

0.10

9.73

60

182.96

0.16

9.15

110

995.43

0.82

5.58

CTAB/SiO2 mass ratio

pH value

Crystallization temperature (

)

19

ACCEPTED MANUSCRIPT 160

364.74

0.43

6.22

210

96.41

0.08

9.89

6

221.56

0.27

8.43

12

911.51

0.71

5.95

24

351.14

0.39

7.16

48

242.97

0.31

8.47

487 488

Table 2 Langmuir equation Temp ( )

-1

-1

Freundlich equation 2

1-n

b (L mg )

R

25

54.44

0.20

0.9936

8.18

35

46.35

0.11

0.9912

4.58

45

40.90

0.06

0.9979

2.39

Table 3 ◦

35

0.28

0.9617

0.31

0.9734

0.35

0.9467

-∆H◦ (KJ mol-1)

-1

-∆G (KJ mol ) 25

R2

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Adsorbent

n

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KF ( mg

Ln g-1)

qm (mg g )

489 490

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Crystallization time ( h )

45

Al-MCM-41

45.35

6.16

4.83

491 492

0.13

3.39

Table 4

Pseudo-first-order equation Temp ( )

(exp) qe (mg g-1)

K1 (min-1)

qe (mg g-1)

Pseudo-second-order equation

R2

K2 (g mg-1 min-1)

(theoretical)

4.01

35

3.89

R2

(theoretical)

3.92

0.9973

4.06×10-4

4.14

0.9052

0.019

3.89

0.9985

3.61×10-4

4.07

0.9131

0.9915

-4

3.86

0.8982

0.023

3.59

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3.69

qe (mg g-1)

0.017

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25

45

493 494

-∆S◦ (KJ K-1 mol-1)

20

3.50×10

ACCEPTED MANUSCRIPT

Highlights 1. A novel mesoporous material, Al-MCM-41 was synthesized using natural palygorskite.

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2. The Al-MCM-41 possessed good crystallinity and large specific surface area.

3. The Al-MCM-41 exhibited excellent adsorption property for

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hazardous dye removal.

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4. The adsorption behavior of dye onto Al-MCM-41 was systematically

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investigated.