Adsorption properties and mechanism of mesoporous adsorbents prepared with fly ash for removal of Cu(II) in aqueous solution

Applied Surface Science 261 (2012) 902–907

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Adsorption properties and mechanism of mesoporous adsorbents prepared with fly ash for removal of Cu(II) in aqueous solution Xiu-Wen Wu a,b,∗ , Hong-Wen Ma b , Lin-Tao Zhang b , Feng-Jiao Wang b a b

School of Science, China University of Geosciences, Beijing 100083, PR China National Laboratory of Mineral Materials, China University of Geosciences, Beijing 100083, PR China

a r t i c l e

i n f o

Article history: Received 1 June 2012 Received in revised form 29 August 2012 Accepted 31 August 2012 Available online 7 September 2012 Keywords: Adsorption Cu(II) Mesoporous materials Fly ash Adsorption mechanism

a b s t r a c t Mesoporous materials were hydrothermally prepared from fly ash in an alkaline condition with cetyltrimethylammonium bromide as synthesis directing agent. The structural properties of the mesoporous materials were characterized by X-ray powder diffraction, high-resolution transmission electron microscope, and N2 adsorption. The chemical contents of SiO2 in the mesoporous materials were determined by spectrometry of the silicone molybdenum and sulfosalicylic acid complexes, and Al2 O3 determined by complexometry with Ethylene Diamine Tetraacetic Acid in the presence of KF-Zn(Ac)2 tests. The removal of Cu(II) was studied under equilibrium and dynamic conditions, and the influence of the Al:Si molar ratio were also considered. The equilibrium data were fitted to Freundlich and Langmuir models and the models parameters were evaluated. The adsorption mechanism was clarified with the Dubnin–Radushkevich isotherm. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Copper is a heavy metal with broad range applications such as catalysts, pigments, pesticides, fertilizers, and stabilizers [1–5]. However, their toxicity [6] and potential to bio-accumulate have led the Environmental Protection Agency (EPA) to specify a threshold concentration limit, the maximum contaminant level (MCL) of Cu is 1.3 mg/L in discharged water in 1999, and the maximum recommended concentration of Cu(II) in drinking water is 1.0 mg/L. Various treatment technologies such as precipitation, ion exchange, adsorption, and solvent extraction have been commonly employed to remove metal pollutants from aqueous solutions [7–10]. Among these methods, adsorption is the most simple, so it is easy to be operated by laypeople. Many kinds of adsorbents have been studied for removing heavy metal ions, and the general adsorbent was activated carbon [9]. However, the predominantly non-polar surface of activated carbon often limits its effectiveness for removal of metal ions. Many studies were used mesoporous materials as catalyst in organic reactions [11–16], but limited researches have been reported on their use as adsorbents [17] in wastewater purification. The goal of this study was to investigate Cu(II) removal efficiencies by the multi-elemental mesoporous materials prepared with

∗ Corresponding author at: School of Science, China University of Geosciences, Beijing 100083, PR China. Tel.: +86 010 82321062. E-mail addresses: [email protected], [email protected] (X.-W. Wu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.122

fly ash. The adsorption properties of the adsorbents were studied in batch equilibrium conditions. Freundlich and Langmuir isotherms were used to analyze the equilibrium data. The influences of various parameters, such as pH, adsorption time and the molar ratio Al:Si, were studied statistically. The adsorption mechanism was clarified with the Dubnin–Radushkevich isotherm.

2. Materials and methods 2.1. Materials The fly ash (the average molar mass is 72.82 g mol−1 , and its main phase is mullite and amorphous silicon oxide) used in this study was obtained from the hancheng power plant in China and was used in each experiment after a pre-treatment in the presence of Na2 CO3 (molar ratio of Na2 CO3 and fly ash 1.05:1.00) at 1123 K for 2.5 h in an oven. The chemical composition of the fly ash and the pretreated fly ash were listed in Table 1. The main component of the fly ash is silica (52.37 mass%) and alumina (33.91 mass%), and that of the pretreated fly ash is silica (33.47 mass%), alumina (17.70 mass%) and sodium monoxide (42.02 mass%), with a small amount of other metal oxide, such as Fe2 O3 , FeO, CaO, Na2 O, and K2 O. The particle size of the fly ash was about 50 ␮m. Other chemicals used in the synthesis process (Na2 CO3 , Cetyltrimethylammonium bromide (C16 TMABr), hydrochloric acid, sodium hydroxide, silica, Cu(NO3 )·3H2 O and deionized water) were supplied by the Beijing Chemical Reagents Company (China).

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Table 1 Chemical composition of fly ash and pretreated fly ash (mass %). Sample

SiO2

Al2 O3

T(Fe2 O3 + FeO)

CaO

Na2 O

MgO

Total

Fly ash Pretreated fly ash

52.37 33.47

33.91 17.70

5.59 1.75

3.93 1.82

– 42.02

0.77 1.57

96.57 98.33

2.2. Synthesis of mesoporous adsorbents A main reaction occurred during the calcination process of fly ash and Na2 CO3 is shown in Eqs. (2-1) and (2-2). Na2 SiO3 and NaAlO2 formed are used as Si and Al sources for synthesizing the multi-elemental mesoporous adsorbents [18]. The synthesis was carried out as follows. Extraction of Si and Al sources: a mixture of 8.0 g pretreated fly ash and 40 ml deionized water was stirred for 2 h, and statically kept for 24 h. Then Si and Al solution was separated from the mixture by a filtration process. The amounts of Si, Al and Na in the extracted solution (denoted as Si solution) were 0.98, 0.30 and 1.35 mol L−1 , respectively. 3Al2 O3 · 2SiO2 (˛ mullite) + 5Na2 CO3 = 2Na2 SiO3 + 6NaAlO2 + 5CO2 ↑

Intensity (Cps)

100

110

MPF1 MPF2 MPF3 MPF4 0

2

4

6

8

10

2 (Degree) Fig. 1. XRD pattern of MPF1, MPF2, MPF3 and MPF4.

2.4. Adsorption studies (2-1)

SiO2 (amorphous silicon oxide) + Na2 CO3 = Na2 SiO3 + CO2 ↑ (2-2)

The preparation of mesoporous adsorbents was following the procedure described in the literature Wu et al. (2008) [18]. At 363 K and under stirring at 300 rpm, 40 ml of the Si solution was mixed with 20 ml of C16 TMABr solution (C16 TMABr 4.0 g, deionized water 20 ml) and silica (3.0–12.0 g, for adjusting the Si and Al molar ratio). The samples MPF1, MPF2, MPF3 and MPF4 are corresponding to Si: Al 7.5, 12.5, 17.5 and 19.9, respectively. The pH value of the mixture (measured by an acidometer PHS-3 C) was adjusted to 10.5 with hydrochloric acid (5 mol L−1 ). A gel was formed after 2 h during stirring. The gel was transferred into a 70-mL Teflon-lined stainless autoclave and crystallized at 378 K for 72 h. After crystallization, the autoclave was cooled to 293 K automatically, and the solid products were filtered and dried at 378 K for 10 h. Finally, the powders were calcined in air at 823 K for 5 h to remove the surfactant, using a ramping rate of 2 K min−1 .

The synthetic aqueous solutions of Cu(II) (5.0–200.0 mg/L) were prepared by dissolution of Cu(NO3 )·3H2 O in deionized water. For pH adjustments, hydrochloric acid and sodium hydroxide were used. The adsorption of Cu(II) onto the mesoporous materials was studied by batch technique. The adsorbents (0.01–0.90 g) were equilibrated with 15 ml of the copper ions solution at a room temperature 293 K for about 0–55 min. The pH of the solutions was adjusted between 3.0 and 5.5 with 0.2 M NaOH or 0.2 M hydrochloric acid at the beginning of experiments and not controlled afterwards. After equilibration, the suspension of the adsorbent was separated from solution by centrifugal separation. The concentration of Cu(II) remaining in solution was measured by Atomic Absorption Spectroscopy (AAS) using flame method. All experiments were conducted in triplicate and mean values were used. The amount of Cu(II) adsorbed was calculated using the following equation: Qe =

(Co − Ce )V m

(2-3)

where Qe is the equilibrium adsorption capacity (mg g−1 ), Co is the initial concentration (mg/L) of Cu(II) in solution, Ce is the equilibrium concentration (mg/L) of Cu(II) in solution, V is the volume of aqueous solution (L) and m is the dry mass of the adsorbent (g).

2.3. Characterization 3. Results and discussion The X-ray diffraction (XRD) patterns of the samples were recorded using a Siemens D5005 X-ray powder diffractometer, which employed CuK␣ radiation and was operated at 40 kV and 30 mA. The nitrogen sorption isotherms at 77.4 K were measured using an AUTOSORB-1 system. The meso-structures were analyzed from desorption branches of the isotherms by the Barrett–Joyner–Halenda (BJH) model with Halsey equation for multiplayer thickness. The samples were degassed for 10 h at 573 K before the measurement. The pore structure was visualized by a high-resolution transmission electron microscope (HRTEM) (JEOL, JEM-2011, acceleration voltage: 200 kV). The compositions of SiO2 and T(Fe2 O3 + FeO) were determined by spectrometry of the silicone molybdenum and sulfosalicylic acid complexes. The contents of Al2 O3 and CaO were determined by complexometry with EDTA in the presence of KF-Zn(Ac)2 tests. The contents of Na2 O and K2 O were measured by the flame atomic absorption spectrometry.

3.1. Textural properties of mesoporous adsorbents The XRD patterns of the samples MPF1, MPF2, MPF3 and MPF4 are shown in Fig. 1. There are two obvious diffraction peaks at 2 about 2.0◦ and 4.0◦ , denoted as (1 0 0) and (1 1 0), suggesting the regular mesoporous structure existed. The nitrogen adsorption–desorption isotherms of the mesoporous aluminosilicates of MPF1–MPF4 are given in Fig. 2. The nitrogen adsorption-desorption isotherms show a typical IV-type adsorption profile [19] consisting of a step condensation behavior in the medium relative pressure area between 0.3 and 04, due to the formation of mesopores. The X-ray diffraction parameters, BET surface areas and pore properties of mesoporous adsorbents were listed in Table 2. The basal spacing of MPF1–MPF4 was between 4.20 and 4.44, and the unit cell parameter of the 4 samples was between 4.85 and 5.13.

904

X.-W. Wu et al. / Applied Surface Science 261 (2012) 902–907

12

1200 10

8

800 -1

Qe (mg g )

3

-1

Volumeadsorbed (cm g )

1000

600 400 200

MPF1 MPF2 (Volume adsorbed + 250) MPF3 (Volume adsorbed + 500) MPF4 (Volume adsorbed + 750)

0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

6

MPF1 MPF2 (Qe+1) MPF3 (Qe+2) MPF4 (Qe+3)

4

2

0 0

5

10

Relative pressure (P/P) 0 Fig. 2. N2 adsorption–desorption isotherms of MPF1, MPF2, MPF3 and MPF4. Table 2 The X-ray diffraction parameters, BET surface areas and pore properties of mesoporous adsorbents. Sample

MPF1 MPF2 MPF3 MPF4

XRD parameters

N2 adsorption and desorption

d1 0 0 (nm)

a0 (nm)

SBET (m2 g−1 )

4.44 4.43 4.41 4.20

5.13 5.12 5.09 4.85

dBJH (nm)

T (nm)

808 3.96 1.17 704 3.58 1.54 725 3.70 1.39 721 3.61 1.24 √ d100 : basal spacing; a0 : unit cell; a0 = 2d100 / 3; SBET : BET specific surface area; dBJH : BJH mean pore diameter; T: pore wall thickness T = a0 − dBJH .

The BET surface area of MPF1–MPF4 was between 704 m2 g−1 and 808 m2 g−1 , the BJH mean pore diameter was between 3.58 nm and 3.96 nm, and the pore wall thickness was between 1.17 nm and 1.54 nm. TEM images along different orientations of the same crystal (tilting series) are usually helpful and required prior to any silicate framework structure assessment. TEM is used commonly to elucidate the nature of the pore structure of mesoporous materials. The TEM image of MPF1 was showed in Fig. 3. Here was not given the TEM images of other samples for their similarity. It can be noticed

15

20

25

30

35

45

55

Adsorption time (min) Fig. 4. Adsorption kinetics of Cu(II) in open atmosphere onto MPF1, MPF2, MPF3 and MPF4 (adsorbent mass m: 0.1 g; temperature T: 293 K; initial concentration of Cu(II) Co : 53.02 mg/L, pH 5.2).

that the mesopores possess a highly order hexagonal array (circle A), and parallel to each other (circle B). 3.2. Chemical composition The chemical compositions of MPF1, MPF2, MPF3 and MPF4 were listed in Table 3. The molar ratios of Si and Al in MPF1, MPF2, MPF3 and MPF4 were 7.05, 11.54, 15.02, and 17.66, respectively, which were a little decrease compared with those (7.5, 12.5, 17.5 and 19.9) in their synthesis aqueous solution. The reason of the relatively high mass loss about 11 mass% was the organic template not removed thoroughly in the sintering process. 3.3. Adsorption properties 3.3.1. Determination of equilibrium time Fig. 4 illustrates the kinetic profiles for Cu(II) adsorption onto the adsorbents. The four adsorbents differed in the adsorption capacity as characterized by initial rapid sorption (0–10 min) followed by a slower adsorption step (10–20 min) leading to equilibrium uptake levels (after 20 min). The equilibrium adsorption time was about 20 min, but 30 min was adopted at the later adsorption experiments for reliable. 3.3.2. Effect of pH The solution pH is a very important parameter in the adsorption process. The pH range 3.0–5.5 was chosen here for the weak acid resistance of the adsorbent under pH 3.0 and the formation of precipitate CuO above pH 5.2, but relatively less between pH 5.2 and 5.5 [20]. The effect of pH on copper adsorption by MPF2 was presented in Fig. 5. The adsorption capacity sharply increased with the pH increased at the value 3.0–4.0, reached the maximum 221 mg g−1 (by Gaussian fitting) at about pH 4.4, and decreased afterward.

Fig. 3. TEM images of MPF1.

3.3.3. Influence of Al:Si The molar ratio of Al:Si, the difference of which will result in a different surface environment of the adsorbents for the different valence state of Si4+ and Al3+ , is an important factor to the adsorption properties of mesoporous materials. In this study, the interferences of the different pore size and BET surface area to Al:Si molar ratio were ignored, for the four samples MPF1–MPF4 have a little difference in the pore size and BET surface area except MPF1.

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Table 3 Chemical compositions of mesoporous adsorbents (mass%). Sample

SiO2

Al2 O3

T(Fe2 O3 + FeO)

Na2 O

CaO

MgO

Mass loss

Total

MPF1 MPF2 MPF3 MPF4

72.60 77.35 78.56 79.30

8.74 5.69 4.44 3.81

0.40 0.37 0.39 0.37

2.59 2.22 2.55 2.24

0.97 0.80 0.82 0.90

0.88 0.79 0.77 0.85

11.99 11.32 11.43 11.33

98.17 98.54 98.96 98.80

3.3.4. Adsorption isotherms Freundlich and Langmuir models are the most common isotherms used to determine adsorption phenomena. Equilibrium data obtained for Cu(II) adsorption on MFP1 were applied to Freundlich and Langmuir equations. Freundlich model assumes that uptake or adsorption of metal ions occurs on a heterogeneous surface by monolayer adsorption. The model is described by the following equation:

300 280 260 240

-1

Qmax (mg g )

220 200 180

q = kf (Ce )1/n

160 140

log q =

120 100

60 3.5

4.0

4.5

5.0

5.5

pH Fig. 5. Influence of pH on maximum adsorption capacity (adsorbent: MPF2; adsorbent mass m: 0.01 g; temperature T: 293 K; initial concentration of Cu(II) Co : 200.0 mg/L; adsorption time 30 min).

Fig. 6 shows the influence of molar ratio Al:Si on the adsorption capacity of MPF1–MPF4. The adsorption capacity increased with increasing Al:Si, and the increasing relationship was fitted by an exponential function as follows, y = 7.43 − 1.75e−x/0.032

1 log Ce + log kf n

(3-3)

where kf and n are Freundlich constants that can be related to the adsorption capacity and adsorption intensity, respectively. The Langmuir model assumes that uptake of metal ions occurs on a homogeneous surface by monolayer adsorption without any interaction between adsorbed ions. The model can be represented in linear form as:

80 3.0

(3-2)

(3-1)

This phenomenon was because the unsaturated negative charge surface environment was strengthened with increasing Al:Si molar ratio and the nonlinear relation was mostly resulted by the little difference in pore size and BET surface area of the four samples.

qe = qmax

KL Ce 1 + KL Ce

(3-4)

1 1 1 1 = · + qe qmax KL Ce qmax

(3-5)

where qe is the amount adsorbed at equilibrium and qmax is the Langmuir constant, which is equal to the adsorption capacity. The parameter KL represents the Langmuir adsorption equilibrium constant, and Ce is the equilibrium concentration. Plots of log Ce vs. log q and 1/Ce vs. 1/qe evaluated the Freundlich and Langmuir isotherms of MPF1 were shown in Figs. 7 and 8, respectively (The figures about MPF2–MPF4 were not shown here for too many figures in this manuscript and their similarity to those of MPF1). The Freundlich and Langmuir constants of MPF1–MPF4 which were concluded by least-squares from the plots of log Ce vs. log q and 1/Ce vs. 1/qe were summarized in Table 4 1.4

7.45

7.40

1.2

7.35

logq

-1

Qe (mg g )

1.0 7.30

7.25

0.8

y = 7.43 - 1.75 exp ( - x/0.032) 0.6

7.20

7.15

0.4 7.10 0.06

0.08

0.10

0.12

0.14

Molar ratio Ali: Si Fig. 6. Influence of molar ratio Si:Al on adsorption capacity of MPF1, MPF2, MPF3 and MPF4 (adsorbent mass m: 0.2 g; temperature T: 293 K; initial concentration of Cu(II) Co : 100.0 mg/L; adsorption time 30 min; pH: 4.5).

-1.0

-0.5

0.0

0.5

1.0

logCe Fig. 7. Freundlich isotherm for adsorption of Cu(II) on MPF1 (adsorbent mass m: 0.1–0.9 g; temperature T: 293 K; initial concentration of Cu(II) Co : 150.0 mg/L; adsorption time 30 min; pH: 4.5).

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X.-W. Wu et al. / Applied Surface Science 261 (2012) 902–907

0.45 -8.0

0.40

0.0106 2 7.8457

lnQ

0.35

2

R =0.946

-8.5

0.30 -9.0

lnQ

1/qe

0.25 0.20

-9.5

0.15 -10.0

0.10 0.05

-10.5 0.000

0.00 0

2

4

6

0.005

0.010

0.015

0.020

0.025

2

8

1/Ce Fig. 8. Langmuir isotherm for adsorption of Cu(II) on MPF1 (adsorbent mass m: 0.1–0.9 g; temperature T: 293 K; initial concentration of Cu(II) Co : 150.0 mg/L; adsorption time 30 min; pH: 4.5).

Fig. 9. Dubnin–Radushkevich isotherm for adsorption of Cu(II) on MPF1 (adsorbent mass m: 0.1–0.9 g; temperature T: 293 K; initial concentration of Cu(II) Co : 150.0 mg/L; adsorption time 30 min; pH: 4.5).

4. Conclusions along with the regression coefficients. The data of kf and qmax , related to the adsorption capacity, increased with increasing Al:Si molar ratio. These theoretical results were consistent with the experimental results in Section 3.3.3. Both models indicate a good representation of the experimental results by linear Langmuir or Freundlich isotherm equations. But from the regression coefficients (R2 ), Freundlich isotherm provided better fitting to the equilibrium data than Langmuir isotherm. 3.4. Adsorption mechanisms of Cu(II) can be explained by the Adsorption Dubnin–Radushkevich (D–R) isotherm, given by the following equation: ln Q = ln Qm − ke2

(3-6)





where the Polanyi potential ε is RT ln(1 + (1/C) (R: gas constant, J mol−1 K−1 ; T: absolute temperature, K; and C: equilibrium concentration, mmol L−1 ), Q is the amount adsorbed per unit mass of adsorbent (mol g−1 ). k is a constant related to the adsorption energy (mol2 k J−2 ) and Qm is the adsorption capacity (mol g−1 ). The D–R isotherm of ε2 vs. ln Q was given in Fig. 9. The adsorption energy (E) can be calculated using D–R equation and the following relationship has been used. E = (−2k)

−0.5

(3-7) 6.868 kJ mol−1 .

It was between The value of E was found to be 8 kJ mol−1 and 16 kJ mol−1 , which suggested that chemical ion exchange mechanism was significant [21].

Table 4 Freundlich and Langmuir constants and correlation coefficients for adsorption of copper on MPF1–MPF4. Freundlich constants

Langmuir constants

Sample

kf (mg1−1/n L1/n g−1 )

n

R2

qmax (mg g−1 )

KL R2 (L mg−1 × 103 )

MPF1 MPF2 MPF3 MPF4

7.104 7.052 6.979 6.485

2.018 2.001 1.975 1.963

0.96 0.97 0.98 0.97

9.677 9.607 9.526 9.451

2.666 2.679 2.695 2.706

0.87 0.90 0.90 0.90

The mesoporous adsorbents were hydrothermally prepared from fly ash in an alkaline condition with cetyltrimethylammonium bromide as synthesis directing agent. The BET surface area of all adsorbents was between 704 m2 g−1 and 808 m2 g−1 , the BJH mean pore diameter was between 3.58 nm and 3.96 nm, and the pore wall thickness was between 1.17 nm and 1.54 nm. The equilibrium time of all adsorbents was about 20 min, and the appropriate pH value of the adsorption solution was about 4.4. The adsorption properties were influenced by Al:Si ratio in the adsorbents, and the adsorption capacity increased with increasing Al:Si molar ratio. The maximum adsorption capacity for Cu(II) was about 221 mg g−1 (by Gaussian fitting the experimental data). Both Langmuir and Freundlich isotherm models exhibited a good representation of the experimental results. But the Freundlich isotherm provided better fitting to the equilibrium data than Langmuir isotherm. The adsorption energy calculated with the Dubnin–Radushkevich isotherm was about 6.868 kJ mol−1 , suggested a chemical ion exchange mechanism. Acknowledgment The work was funded by the National Laboratory of Mineral Materials, China University of Geosciences, Beijing, PR China (Grant 06006, H03088 and H03122). References [1] V. Aragão-Leoneti, V.L. Campo, A.S. Gomes, R.A. Field, I. Carvalho, Application of copper(I)-catalysed azide/alkyne cycloaddition (CuAAC) ‘click chemistry’ in carbohydrate drug and neoglycopolymer synthesis, Tetrahedron 66 (2010) 9475–9492. [2] E. Manzano, J. Romero-Pastor, N. Navas, L.R. Rodrígue-Simón, C. Cardell, A study of the interaction between rabbit glue binder and blue copper pigment under UV radiation: a spectroscopic and PCA approach, Vib. Spectrosc. 53 (2010) 260–268. [3] T. Liu, C. Sun, N. Ta, J. Hong, S. Yang, C. Chen, Effect of copper on the degradation of pesticides cypermethrin and cyhalothrin, J. Environ. Sci. 19 (2007) 1235–1238. [4] X. Wei, M. Hao, M. Shao, Copper fertilizer effects on copper distribution and vertical transport in soils, Geoderma 138 (2007) 213–220. [5] H.-I. Du, M.-J. Kim, Y.-J. Kim, D.-H. Lee, B.-S. Han, S.-S. Song, AC over-current characteristics of YBCO coater conductor with copper stabilizer layer considering insulation layer, Physica C: Supercond. 470 (2010) 1626–1630. [6] M.M. Rao, A. Ramesh, G.P.C. Rao, K. Seshaiah, Removal of copper and cadmium from the aqueous solutions by activated carbon derived from Ceiba pentendra hulls, J. Hazard. Mater. 129 (2006) 123–129.

X.-W. Wu et al. / Applied Surface Science 261 (2012) 902–907 [7] F. Fu, R. Chen, Y. Xiong, Application of a novel strategy-Coordination polymerization precipitation to the treatment of Cu2+ -containing wastewaters, Sep. Purif. Technol. 52 (2006) 388–393. [8] H. Shin, J. Song, E. Shin, C. Kang, Ion-exchange adsorption of copper(II) ions on functionalized single-wall carbon nanotubes immobilized on a glassy carbon electrode, Electrochim. Acta 56 (2011) 1082–1088. [9] G. Issabayeva, M.K. Aroua, N.M. Sulaiman, Study on palm shell activated carbon adsorption capacity to remove copper ions from aqueous solutions, Desalination 262 (2010) 94–98. [10] H. Hu, C. Liu, X. Han, Q. Liang, Q. Chen, Solvent extraction of copper and ammonia from ammoniacal solutions using sterically hindered ˇ-diketone, Trans. Nonferrous Met. Soc. China 20 (2010) 2026–2031. [11] S. Ito, H. Yamaguchi, Y. Kubota, M. Asami, Mesoporous aluminosilicate- catalyzed allylation of aldehydes with allylsilanes, Tetrahedron Lett. 50 (2009) 2967–2969. [12] W. Zhan, Y. Guo, Y. Wang, Y. Guo, G. Lu, Synthesis of lathanum or La-B doped KIT-mesoporous materials and their application in the catalytic oxidation of styrene, J. Rare Earths 28 (2010) 369–375. [13] T. Xiu, J. Wang, Q. Liu, Single-walled carbon nanotubes and polyaniline composites for capacitive deionization, Micropor. Mesopor. Mater. 143 (2011) 362–367. [14] S. Yao, C. Sui, Z. Shi, Preparation and characterization of visible-light- driven europium doped mesoporous titania photocatalyst, J. Rare Earths 29 (2011) 929–933.

907

[15] J.A. Bae, K.-C. Song, J.-K. Jeon, Y.S. Ko, Y.-K. Park, J.-H. Yim, Effect of pore structure of amine-functionalized mesoporous silica-supported rhodium catalysts on 1-octene hydroformylation, Micropor. Mesopor. Mater. 123 (2009) 289–297. [16] Zhonglin Zhao, Yueming Liu, Haihong Wu, Xiaohong Li, Mingyuan He, Peng Wu, Hydrothermal synthesis of mesoporous zirconosilicate with enhanced textural and catalytic properties with the aid of amphiphilic organosilane, Micropor. Mesopor. Mater. 123 (2009) 324–330. [17] H. Sepehrian, S.J. Ahmadi, S. Waqif-Husain, H. Faghihian, H. Alighanbari, Adsorption studies of heavy metal ions on mesoporous aluminosilicate, novel cation exchanger, J. Hazard. Mater. 176 (2010) 252–256. [18] X. Wu, L. Zhang, H. Ma, K. Yuan, Z. Li, Synthesis of mesoporous aluminosilicate with fly ash, J. Chin. Ceram. Soc. 36 (2) (2008) 123–127. [19] W.J.J. Stevens, V. Meynen, E. Bruijn, O.I. Lebedev, G. Van Tendeloo, P. Cool, E.F. Vansant, Mesoporous material formed by acidic hydrothermal assembly of silicalite-1 precursor nanoparticles in the absence of meso-templates, Micropor. Mesopor. Mater. 110 (2008) 77–85. [20] J. Guzman, I. Saucedo, J. Revilla, R. Navarro, E. Guibal, Copper sorption by chitosan in the presence of citrate ions: influence of metal speciation on sorption mechanism and uptake capacities, Int. J. Biol. Macromol. 33 (2003) 57–65. [21] X.-W. Wu, H.-W. Ma, Y.-R. Zhang, Adsorption of chromium(VI) from aqueous solution by a mesoporous aluminosilicate synthesized from microcline, Appl. Clay Sci. 48 (2010) 538–541.