Nanoporous-walled silica and alumina nanotubes derived from halloysite: controllable preparation and their dye adsorption applications

Applied Clay Science 112–113 (2015) 17–24

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Research paper

Nanoporous-walled silica and alumina nanotubes derived from halloysite: controllable preparation and their dye adsorption applications Zhu Shu a, Yun Chen a, Jun Zhou a,⁎, Tiantian Li a, Dongxue Yu a, Yanxin Wang b,⁎ a b

Faculty of Materials Science and Chemistry, China University of Geosciences, 430074 Wuhan, PR China School of Environmental Studies, China University of Geosciences, 430074 Wuhan, PR China

a r t i c l e

i n f o

Article history: Received 17 November 2014 Received in revised form 15 April 2015 Accepted 17 April 2015 Available online 25 May 2015 Keywords: Halloysite Selective etching Nanoporous Nanotube Adsorption

a b s t r a c t Acid/alkali selective-etching chemistry cooperated with a pre-calcination treatment are proposed and demonstrated, to process natural halloysite nanotubes (Hal) into nanoporous-walled silica or alumina nanotubes. The phase transformation behavior of Hal under the different condition of calcination was investigated. Acid or alkaline aqueous solution was used to selectively remove alumina or silica in the tube wall of pre-calcined Hal with resultant developed nanopores. Microporous/mesoporous-walled silica nanotubes of specific surface area (SBET) up to 414 m2/g were obtained by acid-etching Hal pre-calcined at 850 °C. Mesoporous-walled alumina-rich nanotubes of SBET up to 159 m2/g were developed by alkali-etching Hal pre-calcined at 1000 °C. In the experiment the materials exhibit enhanced adsorption for methylene blue (MB) in aqueous solution of elevated pH. Theoretical modeling based on the concepts of Langmuir, Freundlich and Redlich–Peterson was applied for their isotherms. The optimized monolayer MB adsorption capacities of 427 mg/g for silica and 249 mg/g for alumina products were achieved, showing potential in low-cost and high-efficient adsorbents. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Nanoporous materials are of great interests for scientific research and technical applications, with the benefits of well-defined nanoscale pores able to efficiently discriminate and interact with molecules and clusters. The International Union of Pure and Applied Chemistry (IUPAC) defines nanopores into three categories: micropores (diameter b2 nm), mesopores (2–50 nm) and macropores (N50 nm). Increasing efforts in the area of nanoporous materials have been driven by the rapid-growing wide applications, such as biotechnology, molecule adsorption/separation, energy storage/conversion, catalysis, and photonics (Schuth and Schmidt, 2002; Wan and Zhao, 2007; Liang et al., 2008; Ren et al., 2012; Li and Zhao, 2013; Ma et al., 2013). The production of nanoporous materials can be through either artificial synthesis or natural development. The synthesized mesoporous materials, with various kinds of frameworks such as carbon (Liang et al., 2008; Ma et al., 2013), silica (Wan and Zhao, 2007), metal oxide (Ren et al., 2012) or their hybrids (Guo et al., 2014), have been studied for two decades. Nevertheless, it is also acknowledged that many artificial nanoporous materials are suffering from high cost, complexity and pollution during the fabrication process, which thus hinders their

⁎ Corresponding authors at: 388 Lumo Road, 430074 Wuhan, PR China. Tel.: + 86 13018015320; fax: +86 27 67883731. E-mail addresses: [email protected] (J. Zhou), [email protected] (Y. Wang).

http://dx.doi.org/10.1016/j.clay.2015.04.014 0169-1317/© 2015 Elsevier B.V. All rights reserved.

wide applications. In contrast, the natural nanoporous materials are generally with low cost and environmental-friendly, benefiting from their richness in reserves and easiness in acquisition. Nowadays, many kinds of nanoporous minerals have been recognized and utilized, such as the microporous zeolite (Davis and Lobo, 1992) and sepiolite (Galan, 1996), the mesoporous halloysite (Joo et al., 2013; Kogure et al., 2013; Tan et al., 2013), and the macroporous pumice (Thomas et al., 1994) and diatomite (Khraisheh et al., 2004). Among all the natural nanoporous minerals, halloysite is of particular importance due to its unique tubular morphology, similar to the carbon nanotube. Halloysite generally exhibits a nanotube shape of 500–1000 nm in length and 10–100 nm in inner-diameter. The tube wall is consisted of curved layers of crystalline aluminosilicate belonging to the monoclinic system. The layer unit contains a corner-shared tetrahedral [SiO4] sheet, stacked with an edge-shared octahedral [AlO6] sheet with internal aluminol group (Al–OH). A water monolayer also exists between the adjacent two layers. The general stoichiometry of halloysite is expressed as Al2O3·2SiO2·nH2O, where n equals 6 for 10 Å halloysite and 2 for 7 Å halloysite (Oya et al., 1987; Barrientos-Ramirez et al., 2009; Abdullayev et al., 2012; Kadi et al., 2012; Zhang et al., 2012; Kogure et al., 2013). The interlayer water in 10 Å halloysite evaporates easily in the dry air with the resultant generation of 7 Å halloysite (Yuan et al., 2012; Ouyang et al., 2014) (Reaction (1)). Al2 O3
2SiO2
6H 2 O→Al2 O3
2SiO2
2H 2 O þ 4H 2 O

ð1Þ

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Z. Shu et al. / Applied Clay Science 112–113 (2015) 17–24

In the past decade, there are many halloysite-related works, such as halloysite-based adsorbents (Viseras et al., 2008; Zhao and Liu, 2008; Kiani et al., 2011; Kadi et al., 2012; Kiani, 2014), catalysts (Barrientos-Ramirez et al., 2009; Li et al., 2013), advanced ceramics (Fu and Zhang, 2005), polymer nanocomposites (Liu et al., 2013; Pasbakhsh et al., 2013; Carli et al., 2014; Sarifuddin et al., 2014; Yang et al., 2014), hard template (Zhou et al., 2011), and drug loading and releasing (Tan et al., 2014). Nonetheless, the halloysite nanotubes (abbreviated as Hal hereafter) feature a solid tube wall of tens of nanometers in thickness, which restricts the Hal with a confined specific surface area SBET of around 50 m2/g (Zhang et al., 2012). The enlargement of the specific surface area of nanotubes by thinning the tube wall or constructing nanopores within the solid wall can improve their performance in interface-relative implementations. Recently, researchers have reported the progress of the nanoporosity improvement of Hal. For instance, Abdullayev et al. (2012) has applied the direct Hal treatment with sulphuric acid to selectively etch the aluminum oxide and enlarged the nanotube lumen diameter from 15 to 25 nm, with the specific surface area increased from 40 to 250 m2/g. Zhang et al. (2012) alternatively treated Hal with sulphuric acid and also improved the specific surface area from 48 to 267 m2/g by selectively etching the alumina. The silica-rich nanotube products were obtained in their works accordingly. Wormlike nanopores existed in the tube wall and lead to the enhanced specific surface area. Nevertheless, specific surface area (about 250 m2/g) is yet restricted in the reported nanoporous-walled silica nanotubes prepared from directly acidetching Hal. Considering the chemical difference between silica and alumina, Hal-based nanoporous-walled alumina nanotubes may also be obtained from Hal by the selective etching of the silica component in alkaline solution. This difference has been observed by White et al. (2012) during the research on the stability of the raw Hal in acidic and alkaline aqueous solution. In this paper, the authors proposed and demonstrated both the nanoporous-walled silica nanotubes, with effectively improved specific surface area, and a new type of nanoporous-walled alumina nanotubes originated from the raw Hal. The raw Hal was firstly heat-treated for activation and then treated by acid or alkali to selectively etch the alumina or silica component. Microporous/mesoporous-walled silica nanotubes with improved specific surface area up to 414 m2/g were obtained while mesoporous-walled alumina nanotubes with the specific surface area up to 159 m2/g. The effects of the thermal and chemical treatments on the composition, morphology and porosity characteristics of Hal were investigated. The adsorption performance of the nanoporouswalled silica or alumina nanotubes towards methylene blue, that is a cationic dye model, was also evaluated. 2. Experimental 2.1. Materials In the work, the Hal material was purchased from Danjiangkou Shunhe Clay Co. The concentrated hydrochloric acid (HCl, A. R.), sodium hydroxide (NaOH, A. R.) and methylene blue (MB, A. R.) were purchased from Sinopharm Chemical Reagent Co., Ltd. All the materials were used directly, without any further purification. 2.2. Preparation of nanoporous-walled silica or alumina nanotubes from Hal Hal went through a thermal treatment and then was etched by acid or alkaline aqueous solution. It changes into nanoporous-walled silica or alumina nanotubes. Hal was calcined for 4 h at a temperature of 750– 1150 °C with a heating rate of 10 °C/min. Then the calcined Hal (5 g) was magnetically stirring-mixed with 100 mL of 5 mol/L HCl (or 2 mol/L NaOH) aqueous solution in a sealed glass (or Teflon) vessel at 80 °C for 6 h. Finally, the resultant solid was filtered, washed with

deionized water, and dried at 110 °C overnight. The HCl (or NaOH) etched sample was labeled as Hal-x-HCl (or Hal-x-NaOH), where x represents the pre-calcination temperature. 2.3. Batch adsorption test on methylene blue Two representative samples, Hal-850-HCl (nanoporous-walled silica nanotubes) and Hal-1000-NaOH (nanoporous-walled alumina nanotubes), were chosen to investigate the adsorption behavior of methylene blue (MB). 20 mg of each adsorbent (dried at 110 °C overnight) was added into 20 mL MB solution in a 40 mL sealed vessel. The initial MB concentration was set at 100, 150, 200, 300, 400 or 550 mg/L, and the pH of solution was adjusted from 2 to 12 by HCl or NaOH solutions. The obtained mixture solution was shaken at a speed of 170 rpm for 24 h at 25 °C to attain equilibrium. Once the solution was centrifuged at 3800 rpm for 20 min, the supernatant liquid was characterized by the UV–Vis spectrophotometer (DR6000, HACH, US) at the wavelength of 664 nm to measure the MB concentration in solution. The amount of MB adsorption at equilibrium Qe (mg/g) was calculated by the following Eq. (2): Qe ¼

ðC 0 −C e Þ V M

ð2Þ

where C0 and Ce (mg/L) are the initial and equilibrium concentrations of MB in solution, respectively. V (L) is the volume of solution, and M (g) is the dosage of dried adsorbent. 2.4. Characterization and analysis The samples are characterized by the X-ray powder diffraction (XRD) technique and the XRD patterns were recorded by the Rigaku D/Max-3B diffractometer using CuKα radiation at 35 kV and 40 mA. Thermo-gravimetric/differential scanning calorimetry (TG-DSC) measurements were conducted using a Netzsch STA409 PG/PC apparatus at a heating rate of 10 K/min in air atmosphere. Nitrogen adsorption/desorption isotherms at 77 K were measured by the Micromeritics TriStar 3020 porosimeter. All samples were pre-outgassed at 150 °C for 6 h under flowing nitrogen. The total specific surface area (SBET) was calculated using the Brunauer–Emmet–Teller (BET) method. The micropore surface area (Smicro) was determined by the t-plot method and the mesopore surface area (Sexternal) was obtained by subtracting Smicro from SBET. The total pore volume (Vt) was calculated at the relative pressure p/p0 of 0.97. Similarly, the micropore volume (Vmicro) was determined by the t-plot method and the mesopore volume (Vmeso) was obtained by subtracting Vmicro from Vt. The pore size distributions were analyzed by the Barrett–Joyner–Halenda (BJH) method. Field emission scanning electron microscopy (SEM) analysis were performed using the Magellon 400 electron microscope. Field emission transmission electron microscopy (TEM) analysis was conducted on the JEOL 200CX electron microscope operated at 200 KV. 3. Results and discussion 3.1. Design of constructing nanopores in the Hal tube wall The tube wall of Hal is a crystalline layered aluminosilicate with interlayer water. Its layer unit is consisted of a tetrahedral [SiO4] sheet stacked with an octahedral [AlO6] sheet, precisely similar to that of kaolinite. It is known that calcination activates the aluminosilicate network of kaolinite and improves its activity in the reaction with acid or alkali (Okada et al., 1995; Belver et al., 2002; Lenarda et al., 2007). Likewise, a temperature-dependent change of the Hal structure also happens in calcination (Yuan et al., 2012; Ouyang et al., 2014). Therefore, a thermal treatment will allow the activation of the aluminosilicate network of Hal nanotube wall. A subsequent treatment of acid or alkali

Z. Shu et al. / Applied Clay Science 112–113 (2015) 17–24

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XRD results of samples calcined at 700–900 °C in Fig. 2c. The further calcination at increased temperatures of 1000–1100 °C leads to the network reorganization, forming poorly crystallized γ-Al2O3 (PDF 100425), mullite (PDF 15-0776) and a silica-rich amorphous phase (shown in Fig. 2c). This transformation can be explained by Reaction (4) referring to the reported phase transformation of halloysite and kaolinite under the similar thermal treatment (Okada et al., 1995; Belver et al., 2002; Lenarda et al., 2007; Yuan et al., 2012). Additionally, the calcination at 1200 °C leads to the crystallization of mullite, illustrated by the growing and sharping peaks of the XRD data. Al2 O3
2SiO2
2H 2 O→Al2 O3
2SiO2 þ 2H 2 O

ð3Þ

Al2 O3
2SiO2 →xAl2 O3 þ yð3Al2 O3
2SiO2 Þ þ ð1−x−3yÞAl2 O3
ð2−2yÞSiO2

ð4Þ

Fig. 1. Schematic diagram of constructing nanopores in the tube wall of Hal.

etching may selectively remove the alumina or silica component and develop nanopores in the tube wall (shown in Fig. 1). 3.2. Characterization of Hal and the thermal-treated counterparts The Hal is of typical nanotube shapes as observed in Fig. 2a by SEM and Fig. 2b by TEM. It contains both 7 Å halloysite (PDF 29-487) and 10 Å halloysite (PDF 09-0451), coupled with quartz (PDF 46-1045) impurities as indicated in Fig. 2c by XRD. The XRF data (Table 1) presents the significantly restricted amount of impurities, since the silicon and aluminum oxides are absolutely dominant in volume and the Si/Al molar ratio (1.03:1) agrees with the theoretical value (1:1) for halloysite. The DSC curve in Fig. 2d exhibits an endothermic peak at 526 °C and an exothermic peak at 1010 °C. The endothermic peak can be assigned to the dehydroxylation process (Reaction (3)) (Yuan et al., 2012). Afterwards the crystalline halloysite becomes amorphous, verified by the

3.3. Development of nanoporous-walled silica nanotubes by selective acid etching An acid treatment of the Hal calcined at 750–950 °C in an HCl solution (5 mol/L at 80 °C) was performed to selectively etch the activated aluminum oxide and develop nanopores in the tube wall. Nitrogen adsorption and desorption analyses were conducted to investigate the porosity features of the Hal samples before and after treatment. N2 adsorption–desorption isotherms and BJH pore size distributions are plotted in Fig. 3, and Table 2 shows the porosity parameters. To be noted in Table 2, a preferable maximum SBET up to 414 m2/g is achieved by the acid-etched samples and significantly exceeds the value of the raw Hal, i.e. 32 m2/g. The BJH pore size distributions (Fig. 3) show that the raw Hal exclusively have the large-size mesopores of diameters above 10 nm corresponding to the lumen of Hal nanotube (Fig. 2b). Nevertheless, besides the nanotube lumen, the acid-treated pre-calcined counterparts not only have more small-scale mesopores of diameters below 5 nm but also possess microporosity in general (shown in Fig. 3 and Table 2).

Fig. 2. (a) SEM image, (b) TEM observation, (c) XRD pattern and (d) TG-DSC curves of Hal and (c) XRD patterns of its counterparts calcined at different temperatures.

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Table 1 XRF results of the Hal, Hal-850-HCl, Hal-900-HCl and Hal-1000-NaOH. Composition

SiO2

Al2O3

Fe2O3

MgO

CaO

Na2O

K2O

P2O5

Ignition loss

Hal Hal-850-HCl Hal-900-HCl Hal-1000-NaOH

45.2 92.6 83.0 20.4

37.2 1.68 13.4 71.8

0.305 0.127 0.201 0.182

0.008 0.038 0.018 0.011

0.738 0.060 0.160 0.333

0.022 0.030 0.024 0.865

0.057 0.019 0.037 0.118

0.922 0.134 0.173 0.125

15.8 5.25 2.87 6.72

These small mesopores/micropores are observed by TEM. Hal-850HCl and Hal-900-HCl generally maintain the nanotube morphology of Hal, as shown in Fig. 4. Moreover, there are wormlike micropores/ mesopores evenly distributed in the tube walls. These nanoporous tube walls are mainly composed of SiO2 coupled with a spot of Al2O3 (Fig. 4), with a Si/Al molar ratio greatly higher than that of the raw Hal (Table 1). It indicates that, the development of nanopores in the tube walls is related to the selectively removing of the alumina component of the pre-calcined Hal. White et al. (2012) has previously reported the acid etching of raw Hal. It was found that, the amorphous silica nanoparticles containing micropores established inside the lumen of the raw Hal after the alumina composition being selectively acidetched. Such nanoparticles aggregated together and small mesopores were constructed between the particles. It is supposed that, during the acid etching of the pre-calcined Hal (750–900 °C) in this work, the amorphous alumina component was selectively resolved due to its high solubility in acid. Consequently, amorphous silica nanoparticles with inter-particle micropores/mesopores were formed, which presented the tubular morphology with nanoporous tube walls as a whole. Moreover, the pre-calcination temperature is found to be significantly influential on the nanoporosity of the acid-etched counterpart. In detail, Hal-750-HCl has the SBET of 351 m2/g, consisted of a micropore surface area (Smicro) of 219 m2/g and a mesopore surface area (Sexternal) of 132 m2/g (Table 2). The increase of pre-calcination temperature from 750 to 800 °C barely varies the porosity. Nevertheless, further elevation at temperatures of 850 and 900 °C leads to the sharp decrease of Smicro to 133 and 9 m2/g, and the increase of Sexternal to 218 and 315 m2/g. Accordingly, an evident shift of pore size distribution towards the larger sizes (below 5 nm) is observed for the acid-etched samples pre-calcined at increasing temperature from 750 to 900 °C (Fig. 3). Thus it is believed that, more large-size silica nanoparticles coupled with enlarged interparticle mesopores were formed in the acid-etched Hal pre-calcined at the higher temperature. Further increased temperature of 950 °C results in the greatly diminishing of nanoporosity (SBET of 26 m2/g), attributed to the reorganization of the Al–O network into crystallized γ-Al2O3 with enhanced acid resistance (Fig. 2c and Reaction (4)).

In addition, the size of the nanotube lumen was somewhat enhanced after the acid treatment, as proved by the increased amount of the pores larger than 10 nm (shown in Fig. 3). The lumen enlargement can be attributed to the reported fact that the dissolution of Hal in acid predominantly occurs at the inner surface of the tubes which consists of AlO6 octahedra (White et al., 2012). In summary, micropores and mesopores smaller than 5 nm are developed in the Hal pre-calcined at 750–900 °C by acid etching. The average pore size can be enlarged by elevating the pre-calcination temperature. The acid-etched sample pre-calcined at 850 °C has the highest total SBET of 414 m2/g. The sample processed at 900 °C has the largest pore size of about 3 nm at the maximum probability. Most of the initial alumina component in the tube wall was selectively removed after the acid etching, and amorphous silica nanoparticles with interparticle micropores/mesopores were formed with a morphology of nanoporous-walled nanotube. 3.4. Development of nanoporous-walled alumina nanotubes by selective alkali etching The nanoporosity development in the 950 °C-calcined Hal by acid etching was limited (Table 2 and Fig. 3). It is believed due to the reorganization of the Al–O network into poorly crystallized γ-Al2O3 and mullite at high temperatures (Fig. 2c and Reaction (4)). They are acid resistant and hinder the acid etching and the nanopore construction. In view of the formation of the alumina-rich crystals and a silica-rich amorphous phase (Reaction (3) and Fig. 2c) in the Hal calcined at high temperatures, alkali may be utilized to selectively remove the silicarich amorphous phase and generate nanopores. Thus, an alkali treatment of the Hal pre-calcined at 950–1150 °C in a NaOH solution (2 mol/L at 80 °C) was proposed and applied to develop nanoporouswalled alumina-rich nanotubes from Hal. Fig. 5 shows the N2 adsorption–desorption isotherms and BJH pore size distribution plots of the obtained counterparts. The porosity parameters are included in Table 3. For the alkali-etched samples, the typical Langmuir IV adsorption–desorption curves with hysteresis loops of the alkali-etched samples indicate their well-defined mesoporous structures. In Fig. 5, the BJH pore size distribution curves shows that all the alkali-etched counterparts have mesopores of 3–7 nm in diameter, which do not exist in the raw Hal (Fig. 5). It indicates the successful development of nanoporosity in Hal by alkali etching. The pre-calcination temperature has limited impact on the porosity of the alkali-etched samples, not as significant as the previous acid etching case. In detail, the SBET increases slightly from 136 to 159 m2/g when the precalcination temperature elevates from 950 to 1000 °C. It holds nearly Table 2 Porosity parameters of the Hal and the HCl etched counterparts pre-calcined at different temperatures.

Fig. 3. BJH pore size distribution plots N2 and adsorption–desorption isotherms (insert) of the HCl etched samples pre-calcined at different temperatures.

T °C

SBET m2/g

Smicro m2/g

Sexternal m2/g

Vt cm3/g

Vmicro cm3/g

Vmeso cm3/g

Hal 750 800 850 900 950

32.4 351 368 414 324 26.2

0.402 219 227 133 9.12 3.01

32.0 132 141 281 315 23.2

0.100 0.309 0.310 0.310 0.330 0.091

0.001 0.101 0.100 0.055 0.005 0.001

0.099 0.208 0.210 0.255 0.325 0.090

Z. Shu et al. / Applied Clay Science 112–113 (2015) 17–24

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Table 3 Porosity parameters of the Hal and the NaOH etched counterparts pre-calcined at different temperatures.

Fig. 4. TEM images and EDS data of (a) Hal-850-HCl, (b) Hal-900-HCl and their magnified images (inserts).

constant at further increased temperatures of 1050 and 1100 °C. The further elevated temperature of 1150 °C decreases SBET to 83 m2/g. This behavior is believed due to the reducing of alkali-etchable silicarich amorphous phase, resulting from the increase of crystalline mullite in the calcined counterpart (Fig. 2c). Moreover, mesoporosity dominants each alkali-etched sample and microporosity rarely exists. Considering the porosity and energy benefit, the pre-calcination temperature of 1000 °C is considered optimized, which results in an alkali-etched sample of 159 m2/g in SBET and 4.5 nm in pore-size at the maximum probability. The TEM data of Hal-1000-NaOH in Fig. 6 verifies that wormlike mesopores are adequately distributed in the walls of nanotubes. The nanoporous framework of the tube wall is mainly composed of Al2O3,

Fig. 5. BJH pore size distribution plots and N2 adsorption–desorption isotherms (insert) of the NaOH etched samples pre-calcined at different temperatures.

T °C

SBET m2/g

Smicro m2/g

Sexternal m2/g

Vt cm3/g

Vmicro cm3/g

Vmeso cm3/g

Hal 950 1000 1050 1100 1150

32.4 136 159 163 160 83.4

0.402 12.1 8.20 15.9 12.4 0.316

32.0 124 151 147 148 83.1

0.100 0.345 0.346 0.338 0.355 0.255

0.001 0.004 0.002 0.006 0.004 0.001

0.099 0.341 0.344 0.332 0.351 0.254

coupled with a small amount of SiO2 (Fig. 6), and its Si/Al molar ratio is significantly lower than that of the raw Hal (Table 1). It is of great interest to compare the alkali etching result of the precalcined Hal obtained in this work with that happened on the raw Hal as reported by White et al. (2012). Being different from the formation of mesoporous-walled alumina-enriched nanotube after the alkali etching of pre-calcined Hal at above 950 °C (Figs. 5, 6 and Table 3), the Al(OH)3 nanosheets were formed after the raw Hal was etched in alkaline solution. This difference may be attributed to the crystalline dissimilarity of the alumina component in the raw Hal and the high-temperature precalcined Hal. The alumina in the raw Hal exists as single-layer sheets of AlO6 octahedra (b 1 nm in thickness), which become dissolved significantly in the alkaline solution. It recrystallized into Al(OH)3 nanosheets on the surface of partially dissolved and layered fragments of the Hal walls (White et al., 2012). Nevertheless, the alumina in the precalcined Hal (950–1150 °C) exists principally as crystallized γ-Al2O3 nanoparticles of 5–40 nm in diameter (Yuan et al., 2012), which are more alkali-resistant and better sustained in alkali etching. Hence, once the silica component in the pre-calcined nanotube wall was partially etched by OH−, mesopores were developed within the retained γ-Al2O3 nanoparticles, along with the mesoporous-walled aluminaenriched nanotubes obtained. The nanotube lumen size somewhat enlarged after the alkali treatment, indicated by the increasing of pores larger than 10 nm (Fig. 5). It is ascribed to the synchronously etching of the amorphous alumina phase formed after the high temperature calcination (Reaction (3)). 3.5. Adsorption of MB by nanoporous-walled silica or alumina nanotubes After the abovementioned stages, the nanoporous-walled silica or alumina nanotubes were prepared from the pre-calcined Hal by the acid-etching or alkali-etching routes. Two representatives, Hal-850HCl and Hal-1000-NaOH, were selected for the MB adsorption application. Hal-850-HCl is the nanoporous-walled silica nanotube with 414 m2/g SBET and ≤2 nm pore size at the maximum probability. Hal1000-NaOH is the nanoporous-walled alumina-rich nanotube with 159 m2/g SBET and 4.5 nm pore size at the maximum probability. The pH value of aqueous solution is a crucial factor to determine the adsorption property of adsorbent. In this work, the solution pH (2–12) of Hal-850-HCl and Hal-1000-NaOH for a 100 mg/L MB solution was studied considering the adsorption efficiency, shown in Fig. 7. The data present that, when the solution pH elevates from 2 to 8, the MB removal efficiency increases sharply from 35.7% to 96.7% for Hal-850-HCl and from 38.9% to 92.9% for Hal-1000-NaOH. The favorable adsorption efficiencies around 88–95% are maintained for both adsorbents at the higher pH of 8–12. Therefore, the basic environments are of advantage to the MB adsorption using the nanoporous-walled silica or alumina nanotubes. The increased negative charges on adsorbent at elevated pH improve the adsorption of positively charged cations through the electrostatic attraction forces (Harris et al., 2006). The solution pH of 8 was chosen to analyze the subsequent adsorption isotherm, which adopted a series of initial MB concentrations of 100, 150, 200, 300, 400 and 550 mg/L. Fig. 6 shows the equilibrium concentrations of MB in the liquid and solid phases after the adsorption by

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Z. Shu et al. / Applied Clay Science 112–113 (2015) 17–24

Fig. 6. TEM image and EDS data of Hal-1000-NaOH and the magnified image (insert).

Hal-850-HCl and Hal-1000-NaOH. The adsorption isotherms are studied using Langmuir (Langmuir, 1916), Freundlich (Freundlich, 1906) and Redlich–Peterson (Redlich and Peterson, 1959) models, which include the non-linear elements shown in the Eqs. (5), (6) and (7), respectively.

Langmuir :

Qe ¼

Q m bC e 1 þ bC e

enlarged KF value implies the more probable uptake of the adsorbate. The slope 1/n between 0 and 1 indicates the surface heterogeneity of the adsorbent, with a negative proportion relationship. The Freundlich equation is an empirical equation based on adsorption occurring on heterogeneous surfaces with different energy of adsorption and nonidentical sites.

ð5Þ

where Qe (mg/g) is the equilibrium MB concentration on adsorbent, Ce (mg/L) is the equilibrium MB concentration in solution, and Qm (mg/ g) is a constant standing for the maximum monolayer adsorption capacity of adsorbent. The parameter b is constant, with increased value corresponding to stronger affinity of adsorbent towards adsorbate. It is assumed in the Langmuir isotherm model that adsorption occurs on a homogenous surface and no interaction happens between adsorbates on the plane of surface.

Redlich−Peterson:

Qe ¼

AC e 1 þ BC e g

ð7Þ

where the constant KF represents the amount of adsorbate in per unit adsorbent for a unit equilibrium concentration of the adsorbate. The

where A, B, and g are three constants, and the Redlich–Peterson model equals to the Langmuir model when g = 1. Thus, g indicates the tendency towards Langmuir or Freundlich models. The Redlich–Peterson isotherm model involves the features of both Langmuir and Freundlich isotherm models. Fig. 8 and Table 4 present the non-linear fitting plots and the detailed parameters of these three models for the MB adsorption isotherms on Hal-850-HCl and Hal-1000-NaOH. Although Langmuir model also gives an acceptable fitness in consideration of the Langmuir R2 values above 0.9, it is concluded that the order of fitness in a declined manner is Redlich–Peterson, Freundlich and Langmuir, in view of the correlation coefficient R2. Consequently, the MB adsorption on the nanoporous-

Fig. 7. Effect of solution pH on MB adsorption efficiency of Hal-850-HCl and Hal-1000NaOH.

Fig. 8. Langmuir, Freundlich and Redlich–Peterson isotherms for MB adsorption on (a) Hal-850-HCl and (b) Hal-1000-NaOH.

Freundlich:

Q e ¼ K F C e 1=n

ð6Þ

Z. Shu et al. / Applied Clay Science 112–113 (2015) 17–24 Table 4 Parameters of three adsorption isotherm models for the MB adsorption on two representative samples. Isotherm models

Langmuir

Freundlich

Redlich–Peterson

Parameters

Hal-850-HCl

Hal-1000-NaOH

SBET (m2/g)

414

159

Qm (mg/g) b (L/mg) R2 Qm/SBET (mg/m2) b/SBET KF 1/n R2 g A (L/g) B (L/mg) R2

427 0.0710 0.947 1.03 0.000171 78.9 0.319 0.973 0.782 70.6 0.526 0.977

249 0.0971 0.907 1.57 0.000611 68.2 0.242 0.992 0.802 93.8 1.08 0.996

walled silica or alumina nanotubes primarily occurs in monolayer adsorption on heterogeneous surfaces with different intensity and energy. The active energetic heterogeneity of the samples may originate from the irregular pore shapes, widely distributed pore sizes, surface functional groups and impurities. Hal-850-HCl has a substantially higher maximum monolayer adsorption capacity (Langmuir Qm of 427 mg/g) than that of Hal-1000NaOH (249 mg/g), which is believed due to the SBET difference of the former (414 m2/g) over the latter (159 m2/g). It is also found that the nanoporous-walled alumina nanotube Hal-1000-NaOH exhibits a specific monolayer adsorption capacity (Qm/SBET of 1.57 mg/m2) higher than that of the nanoporous-walled silica nanotube Hal-850-HCl (Qm/ SBET of 1.03 mg/m2). It is principally due to the superior affinity of alumina over silica towards MB, which is manifested by the differences of the specific affinity of adsorbent towards adsorbate on unit surface area (b/ SBET, shown in Table 4). Additionally, the enlargement of pore size at the maximum probability of Hal-1000-NaOH (4.5 nm) over Hal-850-HCl (≤2 nm) also leads to an improved diffusion of MB within the former. The as-synthesized nanoporous-walled silica or alumina nanotubes have comparable adsorption capability to the literatures with even greater specific surface areas (shown in Table 5). It signifies not only the favoring adsorption capacity but also the strong affinity of the nanotubes towards MB. With benefits of advanced adsorption capacity and low production cost, the as-prepared nanoporous-walled silica or alumina nanotubes are perspective as potential adsorbents in the future. 4. Conclusion In summary, the nanoporous-walled silica nanotubes of SBET up to 414 m2/g are prepared from the raw Hal by a calcination at 750– 900 °C along with a subsequent HCl etching treatment. The acid selectively removed the alumina of pre-calcined nanotube, and amorphous silica nanoparticles with inter-particle micropores/mesopores (b5 nm) were formed with a morphology of nanoporous-walled nanotube. The calcination at increased temperatures of 950–1150 °C followed with a subsequent NaOH etching established the nanoporous-walled

Table 5 MB adsorption capacities of Hal-850-HCl, Hal-1000-NaOH and other excellent adsorbents. Adsorbents

Qm SBET Qm/SBET References (mg/g) (m2/g) (mg/m2)

Hal-850-HCl Hal-1000-NaOH Bamboo activated-carbon SBA-15 Graphene Modified ball clay Carbon nanotube

427 249 454 280 154 100 48.1

414 159 1896 668 295 92 /⁎

⁎ SBET data was not given in this reference.

1.03 1.57 0.240 0.418 0.522 1.09 /

This work This work Hameed et al. (2007) Dong et al. (2011) Liu et al. (2012) Auta and Hameed (2012) Ai et al. (2011)

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alumina-rich nanotubes of SBET up to 159 m2/g from the Hal. The alkali selectively removed the amorphous silica in the pre-calcined nanotube wall and developed mesopores of 3–7 nm in diameter. The increment of temperature in pre-calcination process results in the increased firstly and then decreased SBET of the nanoporous-walled silica or alumina nanotubes. The elevating of solution pH promotes the MB adsorption on the asprepared nanoporous-walled silica or alumina nanotubes. The numerical simulations of Redlich–Peterson, Freundlich and Langmuir isotherms are demonstrated and the data show the agreement with the experiment results in a downward manner. Hal-850-HCl and Hal1000-NaOH exhibited favorable monolayer adsorption capacity of 427 and 249 mg/g, respectively. This technique provides a route for lowcost and high-efficient adsorbents in the future. Acknowledgment The research work was supported by the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. CUGL140811), Hubei Environmental Protection Bureau (No.2013HB10) and the National Natural Science Foundation of China (No.41002124). References Abdullayev, E., Joshi, A., Wei, W., Zhao, Y., Lvov, Y., 2012. Enlargement of halloysite clay nanotube lumen by selective etching of aluminum oxide. ACS Nano 6, 7216–7226. Ai, L., Zhang, C., Liao, F., Wang, Y., Li, M., Meng, L., Jiang, J., 2011. Removal of methylene blue from aqueous solution with magnetite loaded multi-wall carbon nanotube: kinetic, isotherm and mechanism analysis. J. Hazard. Mater. 198, 282–290. Auta, M., Hameed, B.H., 2012. Modified mesoporous clay adsorbent for adsorption isotherm and kinetics of methylene blue. Chem. Eng. J. 198, 219–227. Barrientos-Ramirez, S., Ramos-Fernandez, E.V., Silvestre-Albero, J., Speulveda-Escribano, A., Pastor-Blas, M.M., Gonzalez-Montiel, A., 2009. Use of nanotubes of natural halloysite as catalyst support in the atom transfer radical polymerization of methyl methacrylate. J. Mater. Sci. 120, 132–140. Belver, C., Munoz, M.A.B., Vicente, M.A., 2002. Chemical activation of a kaolinite under acid and alkaline conditions. Chem. Mater. 14, 2033–2043. Carli, L.N., Daitx, T.S., Soares, G.V., Crespo, J.S., Mauler, R.S., 2014. The effects of silane coupling agents on the properties of PHBV/halloysite nanocomposites. Appl. Clay Sci. 87, 311–319. Davis, M.E., Lobo, R.F., 1992. Zeolite and molecular-sieve synthesis. Chem. Mater. 4, 756–768. Dong, Y.L., Lu, B., Zang, S.Y., Zhao, J.X., Wang, X.G., Cai, Q.H., 2011. Removal of methylene blue from coloured effluents by adsorption onto SBA-15. J. Chem. Technol. Biotechnol. 86, 616–619. Freundlich, H.M.F., 1906. Over the adsorption in solution. J. Phys. Chem. 57, 385–470. Fu, Y.B., Zhang, L.D., 2005. Simultaneous deposition of Ni nanoparticles and wires on a tubular halloysite template: a novel metallized ceramic microstructure. J. Solid State Chem. 178, 3595–3600. Galan, E., 1996. Properties and applications of palygorskite–sepiolite clays. Clay Miner. 31, 443–453. Guo, L., Ida, S., Hagiwara, H., Daio, T., Ishihara, T., 2014. Direct soft-templating route to crystalline mesoporous transition-metal oxides. Colloids Surf. A Physicochem. Eng. Asp. 451, 136–143. Hameed, B.H., Din, A.T.M., Ahmad, A.L., 2007. Adsorption of methylene blue onto bamboobased activated carbon: kinetics and equilibrium studies. J. Hazard. Mater. 141, 819–825. Harris, R.G., Johnson, B.B., Wells, J.D., 2006. Studies on the adsorption of dyes to kaolinite. Clay Clay Miner. 54, 435–448. Joo, Y., Sim, J.H., Jeon, Y., Lee, S.U., Sohn, D., 2013. Opening and blocking the inner-pores of halloysite. Chem. Commun. 49, 4519–4521. Kadi, S., Lellou, S., Marouf-Khelifa, K., Schott, J., Gener-Batonneau, I., Khelifa, A., 2012. Preparation, characterisation and application of thermally treated Algerian halloysite. Microporous Mesoporous Mater. 158, 47–54. Khraisheh, M.A.M., Al-Degs, Y.S., McMinn, W.A.M., 2004. Remediation of wastewater containing heavy metals using raw and modified diatomite. Chem. Eng. J. 99, 177–184. Kiani, G., 2014. High removal capacity of silver ions from aqueous solution onto halloysite nanotubes. Appl. Clay Sci. 90, 159–164. Kiani, G., Dostali, M., Rostami, A., Khataee, A.R., 2011. Adsorption studies on the removal of Malachite green from aqueous solutions onto halloysite nanotubes. Appl. Clay Sci. 54, 34–39. Kogure, T., Mori, K., Drits, V.A., Takai, Y., 2013. Structure of prismatic halloysite. Am. Mineral. 98, 1008–1016. Langmuir, I., 1916. The constitution and fundamental properties of solids and liquids. J. Am. Chem. Soc. 38, 2221–2295. Lenarda, M., Storaro, L., Talon, A., Moretti, E., Riello, P., 2007. Solid acid catalysts from clays: Preparation of mesoporous catalysts by chemical activation of metakaolin under acid conditions. J. Colloid Interface Sci. 311, 537–543.

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