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CLAY-03118; No of Pages 8 Applied Clay Science xxx (2014) xxx–xxx
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Research paper
Magnetically separable Ni/SPL@Carbon 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/SPL@Carbon-1:10 sample possesses the largest surface area (92.0 m2 g−1) compared with that of Ni/SPL@Carbon-1:1 (49.6 m2 g−1) and Ni/SPL@Carbon-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/SPL@Carbon-1:10 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/SPL@Carbon), 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/SPL@Carbon 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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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/SPL@ Carbon and the Pb(II) ions. Experimental results reveal that the present Ni/SPL@Carbon 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/SPL@ 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/SPL@Carbon nanocomposites Ni/SPL@Carbon 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 (SPL@Carbon-1:1/1:5/1:10) were obtained by filtering, rinsing and drying. Subsequently, the obtained solid nanocomposites and Ni(NO3)2·6H2O at the certain mass ratio (SPL@Carbon/ 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/SPL@Carbon-1:1, Ni/SPL@Carbon-1:5 and Ni/SPL@Carbon-1:10) were obtained. 2.3. Characterization methods Power X-ray diffraction (XRD) patterns were performed for the Ni/ SPL@Carbon 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/SPL@Carbon 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/SPL@Carbon 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 SPL@Carbon 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 SPL@Carbon-1:1 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) SPL@Carbon-1:1, (c) Ni/SPL@Carbon-1:1, (d) Ni/SPL@ Carbon-1:5 and (e) Ni/SPL@Carbon-1:10.
Please cite this article as: Xiao, Z., et al., Magnetically separable Ni/SPL@Carbon 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) SPL@Carbon-1:1, (c) Ni/SPL@Carbon-1:1, (d) Ni/SPL@Carbon-1:5, (e) Ni/SPL@Carbon-1:10 as well as the corresponding EDS spectra.
Please cite this article as: Xiao, Z., et al., Magnetically separable Ni/SPL@Carbon 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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Fig. 3. TEM images of (a) SPL, (b) SPL@Carbon-1:1, (c) Ni/SPL@Carbon-1:1 and (d) is a magnified TEM of Ni/SPL@Carbon-1:1 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/ SPL@Carbon-1:1, Ni/SPL@Carbon-1:5 and Ni/SPL@Carbon-1:10 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/SPL@Carbon-1:5. Moreover, further information about the Ni/SPL@Carbon-1:10 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 SPL@Carbon-1:1 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/SPL@Carbon-1:1/1:5/ 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 SPL@Carbon-1:1 Ni/SPL@Carbon-1:1 Ni/SPL@Carbon-1:5 Ni/SPL@Carbon-1:10
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/SPL@ 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/SPL@Carbon-1:1, Ni/ SPL@Carbon-1:5 and Ni/SPL@Carbon-1:10 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/SPL@Carbon-1:1 to Ni/SPL@Carbon-1:10, 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/SPL@Carbon-1:10 sample has respectively been elevated as 185.46% and 177.58% compared with that of the Ni/SPL@Carbon-1:1 and Ni/SPL@Carbon-1:5 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/SPL@Carbon-1:10 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/SPL@Carbon 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/SPL@Carbon-1:1 Ni/SPL@Carbon-1:5 Ni/SPL@Carbon-1:10
175 150
25
adsorption capacity (mg/g)
200
Quantity Adsorbed (cm3/g STP)
5
125 100 75 50 25
20
15
Ni/SPL@Carbon-1:1 Ni/SPL@Carbon-1:5 Ni/SPL@Carbon-1:10
10
5
0 0.0
0.2
0.4
0.6
0.8
0
1.0
0
Relative Pressure (P/P0)
b
Ni/SPL@Carbon-1:1 Ni/SPL@Carbon-1:5 Ni/SPL@Carbon-1:10
0.0010
20
30
40
50
60
t (min)
3.5
b
Ni/SPL@Carbon-1:1 Ni/SPL@Carbon-1:5 Ni/SPL@Carbon-1:10
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/SPL@Carbon-1:1/1:5/1:10 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/SPL@Carbon 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/SPL@Carbon-1:10 is higher than Ni/SPL@Carbon-1:5 and Ni/ SPL@Carbon-1:1 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/SPL@ 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/SPL@Carbon 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/SPL@Carbon-1:1 Ni/SPL@Carbon-1:5 Ni/SPL@Carbon-1:10
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/SPL@C-1:1 Ni/SPL@C-1:5 Ni/SPL@C-1:10
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/SPL@Carbon 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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Z. Xiao et al. / Applied Clay Science xxx (2014) xxx–xxx
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/SPL@Carbon-1:1 Ni/SPL@Carbon-1:5 Ni/SPL@Carbon-1:10
adsorption capacity (mg/g)
150 125 100 75 50 25 0 25
Samples
Langmuir
Ni/SPL@Carbon-1:1 Ni/SPL@Carbon-1:5 Ni/SPL@Carbon-1:10
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/SPL@Carbon 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/SPL@Carbon-1:1 Ni/SPL@Carbon-1:5 Ni/SPL@Carbon-1:10
2.1
100 80
R (%)
log qt (mg/g)
1.8 1.5 1.2
60 40 20
0.9
Ni/SPL@Carbon-1:1 Ni/SPL@Carbon-1:5 Ni/SPL@Carbon-1:10
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/SPL@Carbon samples.
Please cite this article as: Xiao, Z., et al., Magnetically separable Ni/SPL@Carbon 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/SPL@Carbon-1:1 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/SPL@Carbon-1:1 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/SPL@Carbon 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/SPL@Carbon 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/SPL@Carbon-1:10 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/SPL@Carbon
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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Shin, K.Y.,Hong, J.Y.,Jang, J., 2011. Heavy metal ion adsorption behavior in nitrogen-doped magnetic carbon nanoparticles: isotherms and kinetic study. J. Hazard. Mater. 190, 36–44. Terdkiatburana, T., Wang, S., Tadé, M.O., 2008. Competition and complexation of heavy metal ions and humic acid on zeolitic MCM-22 and activated carbon. Chem. Eng. J. 139, 437–444. Unlu, N., Ersoz, M., 2006. Adsorption characteristics of heavy metal ions onto a low cost biopolymeric sorbent from aqueous solutions. J. Hazard. Mater. 136, 272–280. Unuabonah, E.I., Adebowale, K.O., Olu-Owolabi, B.I., 2007. Kinetic and thermodynamic studies of the adsorption of lead(II) ions onto phosphate-modified kaolinite clay. J. Hazard. Mater. 144, 386–395. Van de Voorde, I., Pinoy, L., De Ketelaere, R.F., 2004. Recovery of nickel ions by supported liquid membrane (SLM) extraction. J. Membr. Sci. 234, 11–21. Varma, A.J., Deshpande, S.V., Kennedy, J.F., 2004. Metal complexation by chitosan and its derivatives: a review. Carbohydr. Polym. 55, 77–93. Wang, S.,Peng, Y., 2010. Natural zeolites as effective adsorbents in water and wastewater treatment. Chem. Eng. J. 156, 11–24. Wang, S., Terdkiatburana, T.,Tadé, M.O., 2008. Adsorption of Cu(II), Pb(II) and humic acid on natural zeolite tuff in single and binary systems. Sep. Purif. Technol. 62, 64–70. Wong, M.H.,Wu, S.C.,Deng, W.J.,Yu, X.Z.,Luo, Q., Leung, A.O.W.,Wong, C.S.C., Luksemburg, W.J.,Wong, A.S., 2007. Export of toxic chemicals: a review of the case of uncontrolled electronic-waste recycling. Environ. Pollut. 149, 131–140. Wu, X., Zhu, W., Zhang, X., Chen, T., Frost, R.L., 2011. Catalytic deposition of nanocarbon onto palygorskite and its adsorption of phenol. Appl. Clay Sci. 52, 400–406. Xing, W.,Huang, C.C.,Zhuo, S.P.,Yuan, X.,Wang, G.Q.,Hulicova-Jurcakova, D.,Yan, Z.F.,Lu, G.Q., 2009. Hierarchical porous carbons with high performance for supercapacitor electrodes. Carbon 47, 1715–1722.
Please cite this article as: Xiao, Z., et al., Magnetically separable Ni/SPL@Carbon 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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Research paper
Magnetically separable Ni/SPL@Carbon 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/SPL@Carbon-1:10 sample possesses the largest surface area (92.0 m2 g−1) compared with that of Ni/SPL@Carbon-1:1 (49.6 m2 g−1) and Ni/SPL@Carbon-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/SPL@Carbon-1:10 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/SPL@Carbon), 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/SPL@Carbon 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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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/SPL@ Carbon and the Pb(II) ions. Experimental results reveal that the present Ni/SPL@Carbon 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/SPL@ 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/SPL@Carbon nanocomposites Ni/SPL@Carbon 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 (SPL@Carbon-1:1/1:5/1:10) were obtained by filtering, rinsing and drying. Subsequently, the obtained solid nanocomposites and Ni(NO3)2·6H2O at the certain mass ratio (SPL@Carbon/ 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/SPL@Carbon-1:1, Ni/SPL@Carbon-1:5 and Ni/SPL@Carbon-1:10) were obtained. 2.3. Characterization methods Power X-ray diffraction (XRD) patterns were performed for the Ni/ SPL@Carbon 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/SPL@Carbon 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/SPL@Carbon 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 SPL@Carbon 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 SPL@Carbon-1:1 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) SPL@Carbon-1:1, (c) Ni/SPL@Carbon-1:1, (d) Ni/SPL@ Carbon-1:5 and (e) Ni/SPL@Carbon-1:10.
Please cite this article as: Xiao, Z., et al., Magnetically separable Ni/SPL@Carbon 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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Fig. 2. FESEM images of (a) SPL, (b) SPL@Carbon-1:1, (c) Ni/SPL@Carbon-1:1, (d) Ni/SPL@Carbon-1:5, (e) Ni/SPL@Carbon-1:10 as well as the corresponding EDS spectra.
Please cite this article as: Xiao, Z., et al., Magnetically separable Ni/SPL@Carbon 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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Fig. 3. TEM images of (a) SPL, (b) SPL@Carbon-1:1, (c) Ni/SPL@Carbon-1:1 and (d) is a magnified TEM of Ni/SPL@Carbon-1:1 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/ SPL@Carbon-1:1, Ni/SPL@Carbon-1:5 and Ni/SPL@Carbon-1:10 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/SPL@Carbon-1:5. Moreover, further information about the Ni/SPL@Carbon-1:10 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 SPL@Carbon-1:1 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/SPL@Carbon-1:1/1:5/ 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 SPL@Carbon-1:1 Ni/SPL@Carbon-1:1 Ni/SPL@Carbon-1:5 Ni/SPL@Carbon-1:10
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/SPL@ 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/SPL@Carbon-1:1, Ni/ SPL@Carbon-1:5 and Ni/SPL@Carbon-1:10 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/SPL@Carbon-1:1 to Ni/SPL@Carbon-1:10, 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/SPL@Carbon-1:10 sample has respectively been elevated as 185.46% and 177.58% compared with that of the Ni/SPL@Carbon-1:1 and Ni/SPL@Carbon-1:5 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/SPL@Carbon-1:10 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/SPL@Carbon 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
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a
a
Ni/SPL@Carbon-1:1 Ni/SPL@Carbon-1:5 Ni/SPL@Carbon-1:10
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Fig. 4. (a) N2 adsorption–desorption isotherms and (b) corresponding pore size distribution curves of the Ni/SPL@Carbon-1:1/1:5/1:10 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/SPL@Carbon 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/SPL@Carbon-1:10 is higher than Ni/SPL@Carbon-1:5 and Ni/ SPL@Carbon-1:1 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/SPL@ 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/SPL@Carbon 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/SPL@Carbon-1:1 Ni/SPL@Carbon-1:5 Ni/SPL@Carbon-1:10
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/SPL@C-1:1 Ni/SPL@C-1:5 Ni/SPL@C-1:10
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/SPL@Carbon 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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Z. Xiao et al. / Applied Clay Science xxx (2014) xxx–xxx
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/SPL@Carbon-1:1 Ni/SPL@Carbon-1:5 Ni/SPL@Carbon-1:10
adsorption capacity (mg/g)
150 125 100 75 50 25 0 25
Samples
Langmuir
Ni/SPL@Carbon-1:1 Ni/SPL@Carbon-1:5 Ni/SPL@Carbon-1:10
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/SPL@Carbon 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/SPL@Carbon-1:1 Ni/SPL@Carbon-1:5 Ni/SPL@Carbon-1:10
2.1
100 80
R (%)
log qt (mg/g)
1.8 1.5 1.2
60 40 20
0.9
Ni/SPL@Carbon-1:1 Ni/SPL@Carbon-1:5 Ni/SPL@Carbon-1:10
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/SPL@Carbon samples.
Please cite this article as: Xiao, Z., et al., Magnetically separable Ni/SPL@Carbon 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/SPL@Carbon-1:1 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/SPL@Carbon-1:1 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/SPL@Carbon 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/SPL@Carbon 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/SPL@Carbon-1:10 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/SPL@Carbon
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/SPL@Carbon 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