[email protected] nanocomposite for removal of Pb(II) from the aqueous solution

CLAY-03118; No of Pages 8 Applied Clay Science xxx (2014) xxx–xxx Contents lists available at ScienceDirect Applied Clay Science journal homepage: w...

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CLAY-03118; No of Pages 8 Applied Clay Science xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Applied Clay Science journal homepage: www.elsevier.com/locate/clay

Research paper

Magnetically separable Ni/[email protected] nanocomposite for removal of Pb(II) from the aqueous solution Zhenghui Xiao a,⁎, Qinqin Cui a, Xiangying Chen a, Xueliang Li a, Fangfang Peng a, Rui Zhang a, Taofa Zhou b,⁎⁎ a b

School of Chemical Engineering, Anhui key Laboratory of Controllable Chemistry Reaction & Material Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, PR China School of Resources & Environmental Engineering, Hefei University of Technology, Hefei, Anhui 230009, PR China

a r t i c l e

i n f o

Article history: Received 1 June 2013 Received in revised form 7 July 2014 Accepted 1 August 2014 Available online xxxx Keywords: Sepiolite Ternary nanocomposite Adsorption Pb(II) Magnetically separable

a b s t r a c t Magnetically separable nanocomposite adsorbents have been synthesized by a simple hydrothermal/solid-state method, in which sepiolite (abbr. SPL) clay, glucose and nickel nitrate were used as the role of template, carbon and the magnetic medium, respectively. In the present work, carbon firstly was covered onto the surface of rodlike SPL clays by hydrothermal treatment, and further ultrafine nickel particles (0.5–1 nm) were loaded into the above carbons, giving rise to unique ternary structures with various kinds of surface areas, pore volumes and pore size distributions. The Ni/[email protected]:10 sample possesses the largest surface area (92.0 m2 g−1) compared with that of Ni/[email protected]:1 (49.6 m2 g−1) and Ni/[email protected]n-1:5 (51.8 m2 g−1). The impact of several key factors, mainly including contact time, initial metal ion concentration and initial pH, upon the adsorptive performance was investigated in depth. The Ni/[email protected]:10 sample, as an effective adsorbent, thus exhibits the highest uptake amount (28.04 mg g−1) for the removal of Pb(II) ions from an aqueous solution. More importantly, the present adsorbents can be readily separated with an external magnetic field, implying their potential superiority in practical applications. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Nowadays, developments in technology have led to the release of heavy metals through industrial and agricultural processes and waste disposal, which are hazardous to the environment pollution (Da̧browski et al., 2004; Wong et al., 2007). The presence of heavy metals in the environment has been of great concern because of their growing discharge, toxicity and other adverse effects on receiving waters. Even low concentrations of heavy metals in natural water supplies have been known to cause several health problems to animals and human beings (Cheng, 2003). For example, Pb(II) ions can affect mental growth, red blood cells, the nervous system, and the kidneys in humans (Naseem and Tahir, 2001). Therefore, the removals of heavy metals are very important in environmental remediation and clean up. Various techniques have been widely used to the removals of these toxic metals from effluent sewage such as complexation (Varma et al., 2004), ion exchange (Mi et al., 2012), membrane filtration (Juang, 2000), electrodeposition (Kabdaşlı et al., 2009) and solvent extraction (Van de Voorde et al., 2004) methods. However, comparing with the above methods, adsorption is one of the most effective, economical and easily regenerated ways in the matter of the removals of heavy metal ions (Babel, 2003; Mudhoo et al., 2011; Wang et al., 2008). ⁎ Corresponding author. Tel./fax: +86 551 62901450. ⁎⁎ Corresponding author. E-mail address: [email protected] (Z. Xiao).

As is well known, many sorbing materials were investigated to eliminate the toxic metals in the past decades all over the world, including the typical carbonaceous (Machida et al., 2006; Pyrzyńska and Bystrzejewski, 2010), bioadsorbents (Kuroda and Ueda, 2005), magnetic materials (Hua et al., 2012) and natural substances (Wang and Peng, 2010). In recent years, the natural mineral clays have been used for the purification of the wastewater due to their wide range of sources and other advantages. For instance, Erdemoğlu et al. (2004) and Demirbaş et al. (2007) made a series of adsorption tests by the pyrophyllite and sepiolite modified by organo-functional and 3-aminopropyltriethoxysilane. Chen et al. (2011) and Wu et al. (2011) have synthesized an attapulgite/ carbon nanocomposite adsorbent by a hydrothermal carbonization process and applied it in the removal of toxic metal ions and phenol from water, respectively. Cho et al. (2012) and Oliveira et al. (2003) prepared the magnetic clay adsorbents and studied the kinetics and thermodynamics behaviors between the heavy metal ions and the magnetic adsorbents. Obviously, researchers are dedicated in the efficient and potential adsorbents, which possess low cost, high adsorption capacity and recyclability. Herein, we have demonstrated a simple, economical and environmentally-benign synthetic method for preparing a magnetic ternary nanocomposite adsorbent (Ni/[email protected]), composed of magnetic substance (Ni(NO3)2·6H2O), clay (sepiolite, which is a powerful clay mineral sorbent with fibrous morphology that is abundant in nature; Giustetto et al., 2011) and carbon (glucose, which is a green and cheap chemical obtained from biomass). Subsequently, the further

http://dx.doi.org/10.1016/j.clay.2014.08.001 0169-1317/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: Xiao, Z., et al., Magnetically separable Ni/[email protected] nanocomposite for removal of Pb(II) from the aqueous solution, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.001

2

Z. Xiao et al. / Applied Clay Science xxx (2014) xxx–xxx

application of the nanocomposites as adsorption agents for the removal of Pb(II) ions in an aqueous solution was studied. Furthermore, the adsorption isotherms and kinetics were also studied in order to understand the adsorption mechanism between the synthesized Ni/[email protected] Carbon and the Pb(II) ions. Experimental results reveal that the present Ni/[email protected] exhibits a good adsorption efficiency of Pb(II) ions, and shows the advantage to be easily removed from the medium by a simple magnetic separation procedure after saturation is reached. 2. Experimental

research the equilibrium and kinetic parameters of Pb(II) ions on the magnetic nanocomposites, diverse initial concentrations were prepared ranging from 10 mg L−1 to 200 mg L−1. The adsorbents were filtered from the solutions after the end of adsorption behavior, and the concentrations of Pb(II) ions in the filtrate were measured by atomic absorption spectroscopy (AAS) analysis. The quantity of metal ion adsorbed by per unit mass of used Ni/[email protected] Carbon nanocomposites, qe (mg g−1), is calculated from the equation: qe ¼

ðC 0 −C e ÞV m

ð1Þ

2.1. Materials

2.2. Preparation of Ni/[email protected] nanocomposites Ni/[email protected] nanocomposites were synthesized via an economical and simple hydrothermal/solid-state method. In a synthetic procedure, the mixtures of SPL and glucose at various mass ratios (SPL/ glucose as 1:1/5/10) were put into a Teflon-lined stainless steel autoclave with a volume capacity of 50 mL. After that, the above mixed solutions were stirred sufficiently and closed maintaining at 160 °C for 24 h. Then, the autoclaves were cooled to room temperature naturally and the nanocomposites ([email protected]:1/1:5/1:10) were obtained by filtering, rinsing and drying. Subsequently, the obtained solid nanocomposites and Ni(NO3)2·6H2O at the certain mass ratio ([email protected]/ Ni(NO3)2·6H2O as 2:1) were carbonizated under Ar atmosphere for 2 h at 600 °C after ground enough in an agate mortar. Finally, the black products of samples (Ni/[email protected]:1, Ni/[email protected]:5 and Ni/[email protected]:10) were obtained. 2.3. Characterization methods Power X-ray diffraction (XRD) patterns were performed for the Ni/ [email protected] nanocomposites on a Rigaku Max-2200 with Cu Kα radiation. Field emission scanning electron microscopy (FESEM) images were obtained with a JSM-6490LV scanning electron microscope. High-resolution transmission electron microscope (HRTEM) images and selected area electron diffraction (SAED) patterns were performed with a JEM-2100F unit. The specific surface area and pore structures of the products were measured by N2 adsorption–desorption isotherms at 77 K (Micrometrics ASAP 2020 system). The specific surface area was calculated by the conventional BET (Brunauer–Emmett–Teller) method. To study the removal of Pb(II) ions by the prepared magnetic nanocomposites, the Pb(II) concentration in the solution was determined by an inductively coupled plasma atomic emission spectrophotometer (ICP-AES, AA800). 2.4. Adsorption measurements Batch adsorption experiments were conducted to study the effect of pH in the solutions between Ni/[email protected] adsorbents and Pb(II) ions. The pH value was adjusted from 2.5 to 6.5 with 0.1 M HNO3 and 0.1 M NaOH solution, and all of the pH measurements were carried out using a pH meter (LPH-802 Chinese desktop acidity meter). In order to investigate the adsorption capacity and behavior of the Ni/[email protected] nanocomposites, 20 mg of adsorbent samples were put into 25 mL of 20 mg L−1 Pb(II) ion solutions by stirring for definite time intervals (5, 10, 15, 20, 25, 30, 45, 60 and 120 min) at room temperature. To

where qe is the equilibrium adsorption capacity of adsorbent, C0 is the initial concentration of metal ion (mg L−1), Ce is the equilibrium concentration of the metal ion after adsorption (mg L−1), m is the mass of the samples (g), and V is the volume of the metal ion solutions (L). 3. Results and discussion 3.1. Characterization of magnetic nanocomposites The crystallinity and purity of the natural sepiolite clay and obtained samples were determined by powder X-ray diffraction (XRD), employing a scan from 10° to 70° (2θ). The characteristic XRD diffraction pattern of sepiolite clay is shown in Fig. 1a. By comparing with Fig. 1a, b has no obvious difference among the diffraction peaks from the two samples, which demonstrates that the generated carbon from the decomposition of glucose is noncrystalline and the natural SPL still exists in [email protected] playing the role as a template. However, the characteristic peaks for nickel (2θ = 44.5°), matching well with the (111) planes of the face-centered cubic (fcc) nickel by comparison with the JCPDS card (04-0850) are presented in Fig. 1c–e. It reveals that the nickel nitrate would be reduced to the elemental nickel particle and the prepared samples possess the magnetic property. In addition, the low peak intensities of the characteristic peaks for nickel may be due to the high intensities of the sepiolite clay, as well as the low crystalline. The morphological characteristics of the natural sepiolite clay and the obtained magnetic samples, as well as the corresponding Energy Dispersive X-ray Spectroscopy (EDS) spectra were determined by a field emission scanning electron microscopy (FESEM) and shown in Fig. 2. Fig. 2a shows that the natural sepiolite has the typical rod-like structure with a diameter of 40–80 nm and a length of 200–1000 nm, and correspondingly more vibrant and bright appearance for the clay is presented in Fig. 3a. The obtained [email protected]:1 sample by the hydrothermal method is shown in Fig. 2b, displaying the SPL clay template

(111) intensity (a.u)

Sepiolite clay was kindly supplied by Guoxing Colloidal Co. Ltd. Anhui Province, China. Sodium hydroxide (NaOH), nitric acid (HNO3), lead nitrate (Pb(NO3)2), nickel nitrate hexahydrate (Ni(NO3)2·6H2O), glucose used in the study are analytical reagent. All the analytical chemicals were purchased from Sinopharm Chemical Reagent (Shanghai) Co. Ltd. and used as received without further purification. The solutions were prepared by dissolving in deionized water.

e d c b a 10

20

30

40

50

60

70

2θ (degree) Fig. 1. XRD patterns of (a) SPL, (b) [email protected]:1, (c) Ni/[email protected]:1, (d) Ni/[email protected] Carbon-1:5 and (e) Ni/[email protected]:10.

Please cite this article as: Xiao, Z., et al., Magnetically separable Ni/[email protected] nanocomposite for removal of Pb(II) from the aqueous solution, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.001

Z. Xiao et al. / Applied Clay Science xxx (2014) xxx–xxx

Fig. 2. FESEM images of (a) SPL, (b) [email protected]:1, (c) Ni/[email protected]:1, (d) Ni/[email protected]:5, (e) Ni/[email protected]:10 as well as the corresponding EDS spectra.

Please cite this article as: Xiao, Z., et al., Magnetically separable Ni/[email protected] nanocomposite for removal of Pb(II) from the aqueous solution, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.001

3

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Z. Xiao et al. / Applied Clay Science xxx (2014) xxx–xxx

Fig. 3. TEM images of (a) SPL, (b) [email protected]:1, (c) Ni/[email protected]:1 and (d) is a magnified TEM of Ni/[email protected]:1 and the corresponding SAED pattern insets in (d).

coated with a layer of noncrystalline carbon with reference to the XRD diffraction pattern. Besides, the representative FESEM images of Ni/ [email protected]:1, Ni/[email protected]:5 and Ni/[email protected]:10 samples are shown in Fig. 2c–e. From the images, we can see that a degree of agglomerate phenomena are observed from the magnetic nanocomposites, especially the Ni/[email protected]:5. Moreover, further information about the Ni/[email protected]:10 structure is derived from the transmission electron microscopy (TEM) in Fig. 3. Fig. 3c and d exhibits obviously the role of SPL as a template in the nanocomposites, which take on large numbers of irregular carbons and further ultrafine granular nickel nanospheres (0.5–1 nm) were loaded into the above carbons, giving rise to unique ternary structures. To determine the contents of main elements (wt.%) in the samples, EDS measurements were conducted and the results were given in Table 1 and Fig. 2. Apparently, we can see that the content of carbon element in the [email protected]:1 has an increase compared to the SPL sample as the resultant of the decomposition of glucose after the hydrothermal reaction. However, there is an obvious trend of decreasing for the relative contents (wt.%) of the carbon element in the Ni/[email protected]:1/1:5/ 1:10 samples, which is in good accordance with the increasing of the contents of the Ni element depicted in Table 1. That is to say, increasing the contents of Ni element in the samples indeed favors for the decrease of the contents of carbon element due to its larger atomic weight.

Table 1 The contents of primary elements (wt.%) in the obtained samples. Samples

SPL [email protected]:1 Ni/[email protected]:1 Ni/[email protected]:5 Ni/[email protected]:10

Elemental analysis (wt.%) C

O

Si

Mg

Al

Ni

1.57 6.02 1.33 1.68 2.57

47.32 47.60 45.57 43.39 38.57

5.93 5.23 4.50 5.94 5.55

0.32 0.41 0.44 0.28 0.35

44.86 40.73 39.25 37.80 37.37

0.00 0.00 8.91 10.90 15.60

Besides, other native characteristic elements make an indistinctive change correspondingly. The specific surface areas and pore size distributions of the Ni/[email protected] Carbon-1:1/1:5/1:10 samples were measured by N2 adsorption–desorption isotherms at 77 K as displayed in Fig. 4. Fig. 4a shows the typical hysteresis loops of type-IV with sharp capillary condensation steps at relative pressure P/P0 N 0.45 toward the Ni/[email protected]:1, Ni/ [email protected]:5 and Ni/[email protected]:10 samples. All hysteresis loops are assigned to type-IV (H3 type) based on the IUPAC classification, which essentially corresponds to the mesoporous structures and demonstrates the plate–flake particles stacked to form the slit-shaped pores (Sangwichien, 2002). Furthermore, the capillary condensations become much wider in shape from Ni/[email protected]:1 to Ni/[email protected]:10, illustrating the occurrence of pore propagation and pore-widening on the channel-like mesopores (Xing et al., 2009). On the other hand, it is clearly seen that the total pore volumes (VT) of the samples have no significant change as displayed in Table 2. Nevertheless, the BET surface area (SBET) toward Ni/[email protected]:10 sample has respectively been elevated as 185.46% and 177.58% compared with that of the Ni/[email protected]:1 and Ni/[email protected]:5 samples. It can be the results of increasing ratio of micropore volumes in the total pore volumes and the smaller particles in an average pore width from Table 2. Moreover, the pore size distribution curves of the magnetic samples in Fig. 4b ranged from 2 nm to 150 nm, which obviously proves the occurrence of multi-modal structures containing major macropores, mesopores and micropores. From Fig. 4b, we can see that the pore size distribution is concentrating at the small diameter (b12 nm) and the Ni/[email protected]:10 particles have a smaller average pore width, which is well consistent with the data in Table 2. 3.2. Application on Pb(II) adsorption In this section, the obtained low-cost magnetic samples were applied as new adsorbents for the removal of Pb(II) ions from the aqueous

Please cite this article as: Xiao, Z., et al., Magnetically separable Ni/[email protected] nanocomposite for removal of Pb(II) from the aqueous solution, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.001

Z. Xiao et al. / Applied Clay Science xxx (2014) xxx–xxx

30

a

a

Ni/[email protected]:1 Ni/[email protected]:5 Ni/[email protected]:10

175 150

25

adsorption capacity (mg/g)

200

Quantity Adsorbed (cm3/g STP)

5

125 100 75 50 25

20

15

Ni/[email protected]:1 Ni/[email protected]:5 Ni/[email protected]:10

10

5

0 0.0

0.2

0.4

0.6

0.8

0

1.0

0

Relative Pressure (P/P0)

b

Ni/[email protected]:1 Ni/[email protected]:5 Ni/[email protected]:10

0.0010

20

30

40

50

60

t (min)

3.5

b

Ni/[email protected]:1 Ni/[email protected]:5 Ni/[email protected]:10

3.0

0.0008 2.5 0.0006

2.0

t/qt

Pore Volume (cm3/g.Å)

0.0012

10

0.0004 0.0002

1.5 1.0

0.0000

0.5 0

20

40

60

80

100

120

140

160

0.0

Pore Diameter (nm)

0

10

20

30

40

50

60

t (min)

Fig. 4. (a) N2 adsorption–desorption isotherms and (b) corresponding pore size distribution curves of the Ni/[email protected]:1/1:5/1:10 samples.

Fig. 5. (a) Adsorption curves of Pb(II) effected by the contact time and (b) the corresponding pseudo-second order sorption kinetics on the Ni/[email protected] samples.

solution. It is known that the adsorption process is affected by several key factors, including contact time, initial metal ion concentration and initial pH. Thus, the corresponding experiments need to be investigated. 3.2.1. Effect of contact time In order to find the capacity of Pb(II) ions adsorbed on the nanocomposites at optimum contact time, batch adsorption tests on the effect of contact time were carried out first. The results are shown in Fig. 5(a). It can be seen that Pb(II) adsorption on the new adsorbents is a rapid process increasing with the contact time, where over 90% of the adsorption occurred within the first 30 min and equilibrium at time of about 45 min. Furthermore, the maximal equilibrium capacity of the Ni/[email protected]:10 is higher than Ni/[email protected]:5 and Ni/ [email protected]:1 samples due to its larger surface area (92.0 m2 g−1), more micropores and smaller pore width as acquired from the BET measures in Fig. 4 and Table 2. Meanwhile, the uptake value of the Ni/[email protected] Carbon-1:10 (28.04 mg g−1) sample is superior to that of the previously reported sample as displayed in Table 5. To study the adsorption behavior, the kinetic parameters for Pb(II) were analyzed using the pseudo-first order ( shannessy and Winzor,

1996), pseudo-second order kinetic (Ho, 1999) equations, and their linear forms were shown as follows. The pseudo-first order equation: ln ðqe −qt Þ ¼ −k1 t þ ln qe :

ð2Þ

The pseudo-second order equation: t 1 t ¼ þ qt k2 qe 2 qe

ð3Þ

where, qe and qt are the adsorption capacities (mg g− 1) at equilibrium and time t, respectively, k1 is the constant of first-order adsorption in min− 1 and k2 is the constant of second-order adsorption in g mg− 1 min− 1. The kinetic parameters for adsorption of Pb(II) ions and the regression correlations are given in Table 3. Considering these data, the values of qe2 are better suited to the experimental results. The values of the Table 3 The kinetic adsorption parameters obtained using pseudo-first order and pseudo-second order models for the adsorption of Pb(II) onto the Ni/[email protected] samples.

Table 2 Characteristic surface areas and pore structures of the magnetic samples. Samples

SBET/m2 g−1

VT/cm3·g−1

Vmicro/cm3·g−1

d/Å

Ni/[email protected]:1 Ni/[email protected]:5 Ni/[email protected]:10

49.6 51.8 92.0

0.141 0.145 0.142

0.003 0.006 0.009

113.6 112.3 61.6

Notes: 1. SBET represents the BET surface area; 2. VT represents the total pore volume measured at P/P0 = 0.99; 3. d represents the average pore width.

Samples

Ni/[email protected]:1 Ni/[email protected]:5 Ni/[email protected]:10

Pseudo-first order

Pseudo-second order 2

k1

qe1

R

k2

qe2

R2

0.0775 0.0905 0.0888

10.49 13.37 9.74

0.9729 0.9665 0.9561

0.0133 0.0137 0.0216

19.22 22.79 28.77

0.9989 0.9992 0.9998

Please cite this article as: Xiao, Z., et al., Magnetically separable Ni/[email protected] nanocomposite for removal of Pb(II) from the aqueous solution, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.001

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correlation coefficients (R2) from the pseudo-second order kinetic equation are all high and greater than 0.998 extremely, which indicates that the adsorption is fitted to a pseudo-second order model. The relative linear plots fitted to the experimental data are displayed in Fig. 5(b). In general, the pseudo-second order kinetic model assumes that the adsorption process occurs on localized sites with no interaction between adsorbates and maximum adsorption corresponds to a saturated monolayer of adsorbates onto the adsorbent surface (Shin et al., 2011). 3.2.2. Effect of initial Pb(II) concentration The adsorption of Pb(II) ions onto the obtained magnetic nanocomposite samples is determined in the Pb(II) concentrations ranging from 10 to 200 mg L−1 and the corresponding adsorption curves are plotted in Fig. 6. The curves illustrate the adsorption capacity increased with the Pb(II) initial concentration. The adsorption data have been subjected to different adsorption isotherms, namely, Langmuir (1918) and Freundlich (1939), which are to describe the adsorption process and understand the heterogeneity of the adsorbent surfaces. The linear forms of the Langmuir and Freundlich models are presented in Eqs. (4) and (5). Ce Ce 1 ¼ þ qe qm bqm

lnqe ¼

ð4Þ

1 lnC e þ ln K F n 200

a

Ni/[email protected]:1 Ni/[email protected]:5 Ni/[email protected]:10

adsorption capacity (mg/g)

150 125 100 75 50 25 0 25

Samples

Langmuir

Ni/[email protected]:1 Ni/[email protected]:5 Ni/[email protected]:10

Freundlich

qm

b

R2

KF

n

R2

346.02 413.22 334.45

0.0014 0.0017 0.0060

0.1008 0.1002 0.7244

0.6771 1.1509 4.1626

1.1251 1.1778 1.3797

0.9798 0.9779 0.9906

where, qm (mg L− 1) represents the maximum amount of adsorbate required to form a monolayer adsorbed per unit mass of adsorbent. b is the equilibrium Langmuir adsorption constant in L mg−1. n and KF (mg g−1) are the Freundlich isotherm constants. The Langmuir adsorption isotherm describes a homogeneous adsorption surface sites and each site binds to only a single adsorbate. On the contrary, the Freundlich model describes that the adsorbent has heterogeneous surface sites with multilayer sorption of the surface. The experimental data are analyzed with the above two isotherms and the parameters (qm, b, n, KF and R2) are calculated and displayed in Table 4. Clearly, the results show that Freundlich isotherm model can give better fitness to the experimental data with a high correlation coefficient and the linearity of the plot of the Freundlich model is shown in Fig. 6(b). This suggests that the adsorption behavior takes place on heterogeneous surfaces including the carbon nanolayer as well as the surface of the sepiolite clay in the adsorption process.

ð5Þ

175

0

Table 4 The kinetic adsorption parameters calculated from the Langmuir and Freundlich models for the adsorption of Pb(II) on the Ni/[email protected] samples.

50

75

100

125

150

175

200

225

3.2.3. Effect of pH It is well known that pH of the initial suspension is an important factor that affects the adsorption process. In addition, it is reported that the dominant species of the Pb(II) speciation is Pb(OH)2 at pH N 6.5 and Pb2+ and Pb(OH)+ at pH b 6.5 in an aqueous solution (Nassar, 2010). In order to establish the effect of pH on the adsorption of Pb(II) ions onto the nanocomposite adsorbents, batch experiments at different pH values were carried out in the range of 2.0–7.0 and the corresponding adsorption curves are illustrated in Fig. 7. As seen from the figure, the adsorption of Pb(II) increases slowly with an increase in pH from 2.5 to 3.5, and then increases sharply up to 5.5. At a lower pH, excess H+ ions compete with Pb(II) ions for the adsorption sites on the adsorbent surface resulting in lower adsorption capacity in an acidic environment. As the pH increases further, the positive charge sites on the adsorbent surface decrease while the negatively charged sites increase, which enhances attractive forces between the adsorbent surface and

C0 (mg/L) 2.4

b

Ni/[email protected]:1 Ni/[email protected]:5 Ni/[email protected]:10

2.1

100 80

R (%)

log qt (mg/g)

1.8 1.5 1.2

60 40 20

0.9

Ni/[email protected]:1 Ni/[email protected]:5 Ni/[email protected]:10

0

0.6 0.6

0.9

1.2

1.5

1.8

2.1

2.4

log Ce (mg/L)

2

3

4

5

6

7

pH Fig. 6. (a) Effect of the initial concentrations at pH 3.5 and (b) the fitted Freundlich isotherms for the adsorption of Pb(II).

Fig. 7. Effect of pH for the Pb(II) adsorption on the Ni/[email protected] samples.

Please cite this article as: Xiao, Z., et al., Magnetically separable Ni/[email protected] nanocomposite for removal of Pb(II) from the aqueous solution, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.001

Z. Xiao et al. / Applied Clay Science xxx (2014) xxx–xxx

7

Fig. 8. Photos showing the magnetic response of the Ni/[email protected]:1 sample toward the external magnet at the beginning (a) and after 8 s (b), respectively.

the Pb(II) ions and results a sharp increasing adsorption (Roonasi and Holmgren, 2009).

nanolayer could further increase their adsorption capability and selectively remove other contaminants in effluents.

3.2.4. The magnetic property of new adsorbent and the adsorption capacity compared with others on Pb(II) We have made a test for Ni/[email protected]:1 to realize the magnetic property and the result is shown in Fig. 8. From the figure, we can see that the magnetic adsorbent material is readily separated from the aqueous solution with an external magnet due to its magnetization. Furthermore, several studies have been investigated using various types of adsorbents for Pb(II) ion adsorption. Table 5 displays a comparison of the uptake amount on Pb(II) ions among the adsorbents. It can be seen from the table that Ni/[email protected] sample exhibits a higher value than those of reported low-cost adsorbents, revealing that the magnetic nanocomposite prepared here is one potential adsorbent material for the removal of toxic metal ions.

Acknowledgments

4. Conclusions In summary, magnetically ternary Ni/[email protected] nanocomposite adsorbents were synthesized by a simple hydrothermal/solid-state method using cheap and easily available resources. The magnetic samples have several advantages as potential and conventional adsorbents for the rapid removal of Pb(II) ions from the wastewater due to the use of low-cost materials and their high adsorption capacity. The adsorption processes can be completed in a short time and the adsorbents can be separated by an external magnetic field after adsorption behaviors. The experimental adsorption behaviors are better fitted to the pseudosecond order kinetics and Freundlich model. Furthermore, altering the proportion of the magnetic medium or modifying the surface of carbon

Table 5 Maximum adsorption capacity of Pb(II) ions onto Ni/[email protected]:10 sample comparing of various adsorbents. Adsorbents

Pb(II) (mg g−1)

Reference

Modified kaolinite Na-montmorillonite Modified pyrophyllite Expanded perlite Magnetic chitosan Activated carbon Biopolymeric sorbent Magnetic carbonaceous nanoparticles Grafted silica Zeolitic MCM-22 Ni/[email protected]

16.37 9.58 0.5 13.39 25 8.9 8.5 99.38

Unuabonah et al. (2007) Abollino et al. (2003) Erdemoğlu et al. (2004) Sarı et al. (2007) Liu et al. (2009) Kikuchi et al. (2006) Unlu and Ersoz (2006) Nata et al. (2010)

38.1 94 28.04

Chiron et al. (2003) Terdkiatburana et al. (2008) Present study

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Please cite this article as: Xiao, Z., et al., Magnetically separable Ni/[email protected] nanocomposite for removal of Pb(II) from the aqueous solution, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.001