Journal of Hazardous Materials 265 (2014) 104–114

Contents lists available at ScienceDirect

Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat

Development of carbon nanotubes/CoFe2 O4 magnetic hybrid material for removal of tetrabromobisphenol A and Pb(II) Lincheng Zhou a,∗ , Liqin Ji a , Peng-Cheng Ma b , Yanming Shao a , He Zhang a , Weijie Gao a , Yanfeng Li a a State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Institute of Biochemical Engineering & Environmental Technology, Lanzhou University, Lanzhou 730000, P.R. China b The Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Urumqi 830011, P.R. China

h i g h l i g h t s • • • • •

Amino-functionalized CoFe2 O4 nanoparticles were deposited on MWCNTs in one-pot. Novel chitosan modified MWCNTs/CoFe2 O4 hybrid material were successfully synthesized. The hybrid material had high specific surface area and abundant functional groups. The hybrid material exhibited high adsorption properties for TBBPA and Pb(II). The hybrid material was an efficient, eco-friendly and reusable adsorbent.

a r t i c l e

i n f o

Article history: Received 16 September 2013 Received in revised form 19 November 2013 Accepted 28 November 2013 Available online 4 December 2013 Keywords: Magnetic carbon nanotube Chitosan Pb(II) Tetrabromobisphenol A Adsorption

a b s t r a c t Multi-walled carbon nanotubes (MWCNTs) coated with magnetic amino-modified CoFe2 O4 (CoFe2 O4 –NH2 ) nanoparticles (denoted as MNP) were prepared via a simple one-pot polyol method. The MNP composite was further modified with chitosan (CTS) to obtain a chitosan-functionalized MWCNT/CoFe2 O4 –NH2 hybrid material (MNP–CTS). The obtained hybrid materials were characterized by Transmission Electron Microscopy (TEM), Fourier Transform Infrared Spectrogram (FT-IR) Analysis and X-ray Photoelectron Spectroscopy (XPS) Analysis, Vibrating Sample Magnetometer (VSM) Analysis and the Brunauer–Emmett–Teller (BET) surface area method, respectively. The composites were tested as adsorbents for tetrabromobisphenol A (TBBPA) and Pb(II), and were investigated using a pseudo-second-order model. The adsorption of TBBPA was well represented by the Freundlich isotherm; the Langmuir model better described Pb(II) absorption. MNP–CTS adsorbed both TBBPA and Pb(II) (maximum adsorption capacities of 42.48 and 140.1 mg g−1 , respectively) better than did MNP without CTS. Magnetic composite particles with adsorbed TBBPA and Pb(II) could be regenerated using 0.2 M NaOH solution and were separable from liquid media using a magnetic field. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Highly adsorbent multi-functional composites that can be magnetically separated are increasingly useful in the disposal of waste electronic equipment (e-waste). TBBPA and lead are two representative components of e-waste [1–3]. TBBPA constitutes over half of brominated flame retardants used in various applications [4]; its global market was over 170,000 tons in 2004 [5]. However, TBBPA is a potentially harmful compound that has been detected in serum

∗ Corresponding author. Tel.: +86 931 8912528. E-mail address: [email protected] (L. Zhou). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.11.058

from computer technicians, electronic assembly workers, laboratory personnel, and the general population [6]. It is an endocrine disruptor, immunotoxin, and neurotoxin, and has been reported to affect adversely the health of mammals and humans [7]. Lead is a major heavy metal contaminant present in e-waste [8]; its average concentration in the plastic fraction of e-waste is more than 1000 mg kg−1 [9]. The release of lead into the environment can cause severe human health problems [10,11]. Therefore, the control and removal of toxic components in e-waste, such as TBBPA and Pb(II) that can be found in aqueous solutions, constitute an important research area. Carbon nanotubes (CNTs) are promising adsorbents for the removal of various contaminants from wastewater. Their suitability

L. Zhou et al. / Journal of Hazardous Materials 265 (2014) 104–114

arises from their unique structure and properties, including large surface area, excellent stability, and rich surface functionalities [12–14]. However, a major drawback comes from the difficulties associated with the separation of the very small CNTs from aqueous solutions [15]. The uncontrolled release of CNTs into the environment is also of concern because their environmental and ecological risks remain unclear [16]. Magnetic separation has been developed to facilitate the collection of CNTs [17]. Magnetic hybrid materials also have many advantages such as high adsorption, quick separation, and ease of use, making them compatible with environmental purification and other techniques [18]. Various magnetic materials can be used as adsorbents [19–22]. Cobalt ferrite (CoFe2 O4 ) with a cubic spinel structure can form magnetic nanoparticles that exhibit excellent chemical stability and saturation magnetization [20,23]; consequently, it has been prepared and used for contaminant adsorption. For example, Zhang et al. studied the adsorption and desorption of arsenic compounds on CoFe2 O4 nanoparticles, finding maximum adsorption capacities for arsenite and arsenate of 100 and 74 mg g−1 , respectively [24]. Li et al. reported that magnetic CoFe2 O4 -functionalized graphene sheets adsorbed methyl orange well [23], and Ai et al. synthesized activated carbon/CoFe2 O4 composites for the removal of malachite green dye from wastewater [25]. CoFe2 O4 /CNT composites have also been prepared. For example, El Rouby’s group produced CNTs decorated with CoFe2 O4 nanoparticles for the removal of methyl green dye from aqueous solutions [26]; however, the material demonstrated relatively poor adsorption capacity, possibly due to the agglomeration of the CoFe2 O4 nanoparticles on the surfaces of the CNTs and poor interactions between the CNTs and the nanoparticles. To maximize the performance of magnetic materials as absorbents, further modification is necessary [27]. Modification using organic ligands (e.g., thiol) [28], polymer grafting (e.g., chitosan) [29], and inorganic species (e.g., silica) [27] can enhance adsorbents. The aim of such modifications is to develop lowcost bio-sorbents that are suitable for the removal of pollutants [30]. Chitosan, a nontoxic and low-cost polysaccharide obtained from the deacetylation of chitin, has many significant biological properties (e.g., it is biocompatible and biodegradable) and physical properties. Its high content of amino and hydroxyl functional groups allows it to be chemically modified to serve as an effective bio-sorbent, and it is potentially useful for the adsorption of various aquatic pollutants [31]. Chitosan also exhibits high reactivity, excellent chelation, and high selectivity to various pollutants, such as phenolic compounds and heavy metals [32]. Hu et al. studied functionalized chitosan for the removal of phenol, p-nitrophenol, and p-chlorophenol from aqueous solutions, revealing that 80–94.2% of the adsorbate could be removed from the functionalized chitosan [33]. Liu et al. developed ethylenediamine-modified cross-linked magnetic chitosan resin, and found using the Langmuir model that its maximum adsorption capacity for Cr(VI) ions was 51.81 mg g−1 [34]. There is current interest in the development of magnetic CNTs composites materials with high specific surface area and chitosanbased active functional groups designed to improve the adsorption capacity of CNTs for organic pollutants and heavy metals [17,18]. In this paper, we have developed a facile synthetic one-pot route to decorate carbon nanotubes with –NH2 functionalized superparamagnetic CoFe2 O4 nanoparticles (denoted as MNP) for improved surface functionalization. MNP–CTS magnetic hybrid materials can be obtained by the reaction of this hybrid with terephthalaldehyde, followed by grafting of chitosan via a Schiff base reaction (Fig. 1). The synthesized material exhibited a relatively high specific surface area and was also rich in active functional groups. The two magnetic hybrid materials (i.e., one with and one without chitosan) were tested as adsorbents for the removal of TBBPA and Pb(II). The composites’ chemical functionalities, surface properties, and adsorption behaviors were studied and compared; their

105

recyclability and reusability during cyclic operation were also tested. 2. Experimental 2.1. Materials MWCNT (diameter, 10–20 nm) were purchased from Chengdu Organic Chemical Co. Ltd., Chinese Academy of Sciences (China). TBBPA was obtained from Sichuan Chemical Co. Ltd. (China). Iron acetylacetonate (Fe(acac)3 , 99.9%) and cobalt chloride (99.9%) were purchased from Sigma-Aldrich. Ethylene glycol, ethanol amine, sodium acetate, terephthalaldehyde were purchased from Tianjin Chemicals Co. Ltd. (China). All reagents were analytical grade and used as received. Chitosan (CTS, degree of deacetylation: 95%) was purchased from Shipin Chemical Co., Ltd. (Shanghai). Deionized water was used in the experiment. 2.2. Synthesis of CoFe2 O4 –NH2 magnetic nanoparticles Amino-functionalized CoFe2 O4 nanoparticles were synthesized via modification of a previously reported one-pot polyol method [35]. In a typical procedure, Fe(acac)3 (0.3708 g), CoCl2 ·6H2 O (0.1440 g) and sodium acetate (2.0 g) were dissolved in 30 mL ethylene glycol. The mixture was stirred at 80 ◦ C for 30 min, followed by the addition of 15 mL ethanol amine. The entire solution was transferred into a Teflon-lined autoclave and the reaction temperature was increased to 200 ◦ C and maintained for 8 h. After cooling to room temperature, 30 mL ethanol was added into the mixture, following which a black material was precipitated and separated via a commercial magnet, washed with ethanol for 12 h in a Soxhlet extractor and dried in a vacuum oven at 30 ◦ C. 2.3. Synthesis of chitosan-functionalized MWCNT/CoFe2 O4 –NH2 (MNP–CTS) hybrid material 2.3.1. Synthesis of MWCNT/CoFe2 O4 –NH2 The synthesis of MWCNT/CoFe2 O4 –NH2 (MNP) hybrid materials was similar to that observed in the synthesis of CoFe2 O4 –NH2 magnetic nanoparticles. In a typical experiment, Fe(acac)3 (0.7416 g), CoCl2 ·6H2 O (0.2880 g), sodium acetate (4.0 g) and MWCNT (0.7416 g) were put into 30 mL ethylene glycol. The mixture was stirred by sonication at 80 ◦ C for 30 min followed by the addition of 15 mL ethanol amine. The entire solution was transferred into a Teflon-lined autoclave and the reaction temperature was maintained for 8 h at 200 ◦ C. After cooling to room temperature, 30 mL ethanol was added into the mixture, and a black material was precipitated and separated via commercial magnet and washed with ethanol for 12 h in a Soxhlet extractor and dried in a vacuum oven at 30 ◦ C. 2.3.2. Reaction with terephthalaldehyde A 0.5 g sample of the MNP hybrid material was added to 50 mL 2.0 wt% terephthalaldehyde solutions and allowed to react at room temperature for 6 h. The Schiff base MNP hybrid material was removed via magnetic separation and washed with deionized water (5 × 5 mL) and dried in a vacuum oven at 30 ◦ C overnight. 2.3.3. Grafting of chitosan An aliquot of 50 mL of chitosan solution (0.2 g) was mixed with the Schiff base MNP hybrid material and allowed to react at 30 ◦ C for 6 h. The chitosan-functionalized MWCNT/CoFe2 O4 –NH2 magnetic material (MNP–CTS) was subsequently collected by separating with an external magnet, and the product was washed with deionized water (5 × 5 mL) and dried in a vacuum oven at 30 ◦ C overnight.

106

L. Zhou et al. / Journal of Hazardous Materials 265 (2014) 104–114

Fig. 1. The scheme of the preparation of the MNP–CTS magnetic hybrid material.

2.4. Characterization

where Ci and Ce are the initial and equilibrium concentrations of TBBPA or Pb(II) (mg L−1 ), respectively. V is the volume of TBBPA or Pb(II) solution (L), and m is the mass of adsorbent (g).

The morphology of MNP and MNP–CTS magnetic hybrid material were examined on a transmission electron microscope (TEM, TecnaiG2 F30). All samples were prepared by evaporating dilute suspensions on a carbon-coated film. Powder X-ray diffraction (XRD, Rigaku D/MAX-2400 X-ray diffractometer with Ni-filtered Cu K␣ radiation) was used to investigate the crystal structure of magnetic nanoparticles. FT-IR (170-SX, American Nicolet Corp., KBr pellet) and X-ray photoelectron spectroscopy (XPS) verified the composition of MNP and MNP–CTS magnetic materials. The XPS spectra were obtained with an ESCALab220i-XL electron spectrometer (VG Scientific) using 300 W Al K␣ radiation. Thermogravimetric analysis (TGA) was performed on a STA 449 C system in nitrogen atmosphere with a heating rate of 20 ◦ C per minute in the temperature range of 30–1000 ◦ C. The N2 adsorption–desorption isotherm was measured at liquid nitrogen temperature (76 K) using a Micromeritics ASAP 2010 M instrument. The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method. Magnetic properties of magnetic hybrid materials were characterized via a vibrating sample magnetometry (VSM, LAKESHORE-7304) at room temperature.

2.5.3. Desorption experiments The desorption of TBBPA or Pb(II) in composites was carried out by washing the adsorbent with distilled water for several times, and then adding 0.2 M 50 mL NaOH solution to the adsorbent. Afterwards, the suspension was shaken at 130 rpm for 12 h in order to achieve complete desorption. The solid and liquid phases were separated by a magnet. The adsorption performances of composites for removal of TBBPA or Pb(II) were run through five cycles to evaluate their reusability.

2.5. Batch adsorption experiment

3. Results and discussion

2.5.1. Adsorption kinetic To evaluate the adsorption behavior of the composites, 125 mg adsorbent (MNP or MNP–CTS) were added into 250 mL TBBPA or Pb(II) solution with a fixed concentration of 20 mg L−1 . The mixtures were shaken at 30 ◦ C in an air-bath shaker at 130 rpm for 0.5, 1, 2, 4, 6, 8 and 12 h. The adsorbent was then separated from aqueous media using a magnet. The concentrations of TBBPA and Pb(II) in solution were determined using UV/Vis spectrophotometry at wavelengths of 227.5 and 580 nm, respectively. All experiments were carried out in duplicate. The absorption capacity (Q, mg g−1 ) was calculated by Eq. (1):

3.1. Characterization

Q =

(Ci − Ce ) × V m

(1)

2.5.2. Adsorption isotherm Adsorption isotherm tests were conducted by adding 0.5 g L−1 dosages of MNP and MNP–CTS magnetic composites into TBBPA or Pb(II) solutions with specific concentrations ranging from 10 to 60 mg L−1 . The flasks were then shaken in an air-bath shaker at 130 rpm at the desired temperature. The effect of pH value on the adsorption behavior of adsorbents was studied by adding a specific amount of MNP or MNP–CTS into the TBBPA or Pb(II) solutions. The pH value of solutions were adjusted with 0.1 M HCl or 0.1 M NaOH.

CoFe2 O4 –NH2 nanoparticles and the MNP hybrid material were synthesized via a polyol method in ethylene glycol, which acted as both solvent and reducing agent, using ethanol amine as a modifier. This method has many advantages over other routes for nanoparticle synthesis. For example, the high concentration of amino groups on the surfaces of the CoFe2 O4 nanoparticles can be directly functionalized without multistep surface functionalization; the polyol route can also be used to prepare more monodispersed CoFe2 O4 nanoparticles with better size control. The morphologies of the CoFe2 O4 –NH2 nanoparticles and MNP composites were characterized by TEM (Fig. 2). Images of the CoFe2 O4 –NH2 nanoparticles under different magnifications show uniform particles of

L. Zhou et al. / Journal of Hazardous Materials 265 (2014) 104–114

107

Fig. 2. TEM images of CoFe2 O4 -NH2 nanoparticles (a–c) and MNP magnetic hybrid material (d–f).

ca. 37.2 nm (Fig. 2(a–c)). The MWCNTs were uniformly coated by CoFe2 O4 –NH2 nanoparticles of average diameter ca. 76.3 nm (Fig. 2(d–f)). The interaction between the CoFe2 O4 –NH2 nanoparticles and MWCNTs was sufficiently strong, as demonstrated by the absence of CoFe2 O4 –NH2 nanoparticles on the copper grids after prolonged sonication. The CoFe2 O4 –NH2 nanoparticles coated on the MWCNTs were larger than the original CoFe2 O4 –NH2 nanoparticles. This was because MNP was synthesized using twice the amount of metal ions as used during the preparation of the CoFe2 O4 –NH2 nanoparticles, suggesting that the concentration of the metal ions affected the size and distribution density of the CoFe2 O4 –NH2 nanoparticles [36]. XRD patterns of CoFe2 O4 –NH2 nanoparticles and the MNP hybrid are shown in Fig. 3. Both show diffraction peaks at 2 = 30.1◦ , 35.5◦ , 43.3◦ , 56.8◦ , and 62.5◦ , which can be respectively assigned to the (2 2 0), (3 3 1), (4 0 0), (5 1 1), and (4 4 0) planes of the cubic CoFe2 O4 [25]. These peaks are consistent with the database in the

JCPDS file (No. 22-1086) for the reflection of pure cobalt ferrites with the spinel structure (space group: Fd3m(2 2 7)). The XRD pattern in Fig. 2b also shows a peak at around 26◦ , corresponding to the graphite (0 0 2) lattice plane of the MWCNTs [37]. The average sizes of the pure CoFe2 O4 –NH2 particles and when coated on MWCNTs were calculated using the Debye–Scherrer formula to be 37.9 and 72.8 nm, respectively, in agreement with the TEM observations. FTIR spectra of the CoFe2 O4 –NH2 nanoparticles and the MNP and MNP–CTS composites are shown in Fig. 4. The CoFe2 O4 –NH2 nanoparticles showed a peak at 580 cm−1 , corresponding to metal–oxygen stretching vibrations in the ferrite lattice [23]. This peak broadened after the particles were coated onto the MWCNTs. The two bands at 3436 and 1630 cm−1 were assigned to the NH2 stretching vibration and the N–H bending mode of free NH2 groups, respectively [38]. The peaks at about 1071 and 1373 cm−1 shown by MNP–CTS were assigned to chitosan, specifically to the C–O stretching vibration in –COH and the –CH symmetric vibration in –CHOH

Fig. 3. XRD patterns of CoFe2 O4 -NH2 nanoparticles (a) and MNP magnetic hybrid material (b).

Fig. 4. FT-IR spectra of CoFe2 O4 -NH2 nanoparticles (a), MNP (b) and MNP–CTS magnetic hybrid material (c).

108

L. Zhou et al. / Journal of Hazardous Materials 265 (2014) 104–114

Fig. 5. XPS spectra of MNP and MNP–CTS: (a) wide scan, (b) C1s spectra of MNPs–CTS, (c) N1s spectra of MNPs–CTS, (d) O1s spectra of MNP–CTS, (e and f) Co2p spectra and (g) Fe2p spectra.

respectively. MNP–CTS showed broader C–N vibration bands at 1620–1558 cm−1 than did MNP, suggesting that the C–NH2 groups became C N groups. The peak at 2890 cm−1 is assigned to C–H stretching in chitosan. This change confirmed that MNP was successfully modified by chitosan. These FTIR spectra of CoFe2 O4 –NH2 , MNP, and MNP–CTS are consistent with those reported in other works [39,40]. XPS further confirmed the surface functionalities and elemental compositions of the MNP and MNP–CTS composites (Fig. 5). Wide scan spectra of the two magnetic hybrids (Fig. 5a) show photoelectron lines at binding energies of 286, 400, 533, 781, and 711 eV, which were attributed to C1s, N1s, O1s, Co2p, and Fe2p, respectively. The C1s peak of MNP–CTS could be deconvoluted into two individual peaks at 284.8 and 286.3 eV (Fig. 5b), which were assigned to C–C (or C C in aromatic rings) and C–N (or C–O, or C–O–C) carbons, respectively [41]. The N1s XPS spectrum of MNP–CTS (Fig. 5(b)) shows a peak at 398.5 eV, attributed to N–;

the peak at 399.4 eV was assigned to free amine groups, indicating the deposition of chitosan on the surface the MNP composite [42]. This is consistent with the FTIR analysis. Deconvolution of the broad and asymmetric O1s band reveals three distinct oxygen bands at 530.7, 532.4, and 533.6 eV. These bands are ascribed to M(metal)–O, C–O, and C–O–C, respectively, in agreement with previous work [35]. To assess the oxidation states of cobalt and iron on the surfaces of the composites, Co2p and Fe2p spectra were studied (Fig. 5(e–g)). The peaks centered at 781.3 eV (with a satellite peak at 786.6 eV) and at 796.4 eV (with a satellite peak at 803 eV) respectively correspond to Co2p3/2 and Co2p1/2 . The peaks centered at 713 and 727 eV respectively correspond to Fe2p3/2 and Fe2p1/2 . These sets of peaks further confirmed that CoFe2 O4 phases were present in the two samples [23]. These XPS results agree with the XRD and FTIR analyses. Results from the TGA of the two composites under nitrogen are shown in Fig. 6. The curve of MNP showed no significant peaks; its

L. Zhou et al. / Journal of Hazardous Materials 265 (2014) 104–114

Fig. 6. TGA curves of the MNP (red) and MNP–CTS magnetic hybrid material (black) under nitrogen atmosphere.

ca. 10.2% weight loss at 800 ◦ C was attributed to the loss of residual solvents. The 1.2% weight loss below 220 ◦ C was attributed to the loss of physically absorbed moisture [43]. MNP–CTS showed a 23.3% weight loss from 250 to 650 ◦ C, suggesting that chitosan was present at around 22.1 wt% [44]. This result further confirmed that the chitosan was successfully modified on the surface of MNP, supporting the FTIR and XPS analyses. The specific surface areas and pore volumes of the MNP and MNP–CTS composites are shown in Fig. 7. Their specific surface areas calculated using the BET equation were 136.7 and 157.5 m2 g−1 , respectively; these values are much higher than many previously reported for magnetic CNT materials (Table 1). Pore size distribution curves were estimated from the adsorption isotherms using Bareett–Joyner–Halenda (BJH) analysis (inset, Fig. 7). MNP showed a pore volume of 0.63 cm3 g−1 and a pore size of 18.50 nm; the corresponding values for MNP–CTS increased to 0.87 cm3 g−1 and 22.30 nm owing to the increase in surface roughness due to the chitosan. Note that both composites exhibited high BET surface areas, pore volumes, and pore sizes, and these properties are expected to enhance the adsorption of wastewater pollutants.

Fig. 7. N2 adsorption–desorption isotherms measured 76 K of MNP and MNP–CTS magnetic hybrid material. Inset: pore size distributions from the adsorption branches through the BJH method.

109

Fig. 8. Magnetic hysteresis curves for CoFe2 O4 -NH2 nanoparticles, MNP and MNP–CTS magnetic hybrid material.

The magnetic hysteresis loops of the CoFe2 O4 –NH2 nanoparticles and the MNP and MNP–CTS composites are shown in Fig. 8. The samples’ saturation magnetizations (Ms ) were 67.8, 11.0, and 4.68 emu g−1 , respectively, suggesting that these materials could be easily separated from solution using an external magnetic field [16]. The decreased magnetic saturations of MNP and MNP–CTS were most likely due to the presence of MWCNTs and chitosan. The CoFe2 O4 –NH2 nanoparticles exhibited ferromagnetic behavior at room temperature with a coercivity (Hc ) close to 285 Oe, which is higher than that of bulk CoFe2 O4 nanoparticles due to the lower synthesis temperature used here. The MNP and MNP–CTS composites exhibited superparamagnetic behavior at room temperature with near-zero coercivity and remanence. 3.2. Adsorption properties of MNP and MNP–CTS magnetic hybrid materials The adsorption of TBBPA and Pb(II) by the composites was studied at different pH levels (Fig. 9); pollutant removal was highly dependent on pH. TBPPA was in its molecular form at pH < 7.5; its first deprotonation was at around pH 7.5, its second was at around pH 8.5 [52]. The maximum adsorption capacities of the composites were observed in near neutral solutions, at pH 6.3 (Fig. 9a). The increased adsorption of TBBPA with increasing pH from 2.4 to 6.3 was attributed to the decreasing presence of H+ ions in solution, as these ions competed with TBBPA to be adsorbed on the composites [52]. Adsorption capacities declined above pH 6.3, eventually dropping to 0.30 mg g−1 for MNP and 6.5 mg g−1 for MNP–CTS. This reduction of adsorption capacities may be attribute to enhanced electrostatic repulsion between the anionic TBBPA and the negatively charged surfaces of the composites [53]. The ␲–␲ bonding between TBBPA and the hexagonal skeleton of the MWCNTs and the H-bonding between hydroxyl and amino groups have been identified as the dominant interactions in similar systems [13]. In the present study, the greatest adsorptions of Pb(II) on MNP (27.5 mg g−1 ) and MNP–CTS (57.8 mg g−1 ) were observed at pH 6.0 (Fig. 9b). Note that MNP–CTS exhibited a higher adsorption capacity than MNP, suggesting a positive influence of the added CTS, which provided more functional groups for the attachment of metal ions [31]. The Pb(II) adsorption capacities on both composites increased as pH increased from 2.0 to 6.0. This was due to competition between Pb(II) and H+ for adsorption sites on the composites at lower pH. A typical ion-exchange mechanism for the adsorption

110

L. Zhou et al. / Journal of Hazardous Materials 265 (2014) 104–114

Table 1 Summary of BET surface areas of magnetic CNTs reported previously and those investigated in the present work. Sample

Preparation condition

Multi-walled carbon nanotubes filled with Fe2 O3 particles MWCNTs/Fe3 O4 -NH2 MWCNT/iron oxide magnetic composite MWCNTs–CoFe2 O4 composite MMWCNT nanocomposite NiFe2 O4 -MWCNTs MWCNTs/ferrite nanocomposite CNTs/Fe3 O4 nanocomposites MPTS-CNTs/Fe3 O4 nanocomposites MWCNTs/Fe3 O4 nanocomposites MWCNTs/Fe3 O4 -NH2 nanocomposites MNP MNP–CTS

◦

Sonication and thermal treatment at 450 C A chemical method Hydrothermal method In situ chemical co-precipitation method Chemical precipitation In situ chemical precipitation Physical mixing Thermal decomposition Thermal decomposition Thermal decomposition Thermal decomposition Polyol method Polyol method

SBET (m2 g−1 )

References

114–129 70.09 88.53 109.54 61.74 73.9 50.84 88.367 97.163 108.37 90.68 136.7 157.5

[45] [46] [47] [26] [48] [49] [50] [51] Our previous work Present work

Fig. 9. Effect of initial pH on the adsorption of (a) TBBPA and (b) Pb(II) on MNP and MNP–CTS magnetic hybrid material.

of Pb(II) with increasing pH involves a decrease of H+ remaining on the surface of the composites, which leaves sites available for Pb(II) [54]. At pH > 5.5, the composites had negative surface potentials, which increased electrostatic attraction between their surfaces and Pb(II), thereby increasing the adsorption capacity for Pb(II) [52]. In summary, the optimum pH values for adsorption by either composite were determined to be 6.3 for TBBPA and 6.0 for Pb(II). All subsequent adsorption tests were conducted at the relevant optimal pH value. The study of adsorption kinetics can aid, for example, the optimization of residence time for adsorption and the investigation of adsorption rates. The adsorption of TBBPA or Pb(II) was initially rapid, likely due to the abundant availability of active sites on the adsorbents [55]. As active sites subsequently decreased, adsorption slowed. The pseudo-second-order kinetics model (Eq. (2)) can be used to analyze experimental adsorption data: dQt = k2 (Qe − Qt )2 dt

(2)

where Qe and Qt are the adsorption capacity (mg g−1 ) at equilibrium and at any time t, respectively, and k2 is the pseudo-second-order rate constant (g mg−1 min). Integration leads to 1 t t = . + Qt Qe (k2 Qe2 )

(3)

Plots of t/Qt against t were linear for both TBBPA and Pb(II) (Fig. 10). Calculated values of h, k2 , Qe , and R2 are listed in Table 2; they show that the pseudo-second-order model performs well in representing the kinetic data (R2 > 0.99), indicating typical chemical adsorption [56]. The data also indicate that the concentrations of both adsorbates (the pollutants) and adsorbents (the composites) are involved in the rate determination of adsorption [29]. The validity of the description can be supported by the agreement of the calculated (Qe ) and experimental (Qexp ) values.

Adsorption isotherms can describe surface properties and the affinity of adsorbents, aiding the design of adsorption systems. Two conventional adsorption isotherms, the Langmuir (Eq. (4)) and Freundlich (Eq. (5)) models, were employed here to described TBBPA and Pb(II) adsorption equilibria [57]: 1 1 1 1 = + × , Qe Qmax KL Qmax Ce log(Qe ) =

(4)

1 log(Ce ) + log(KF ), n

(5)

where Qe is the adsorption capacity (mg g−1 ) at equilibrium, Ce the equilibrium concentration of the adsorbate (mg L−1 ), KL the adsorption equilibrium constant including the affinity of binding sites (L mg−1 ), and Qmax is the maximum adsorption capacity (mg g−1 ). Qmax and KL can be determined from the slope and intercept of a linearized plot of 1/Qe vs. 1/Ce (Fig. 11). KF and n are empirical constants that indicate the relative adsorption capacity and the adsorption intensity, respectively. The values of KF (mg1−1/n L1/n g−1 ) and n can be calculated from the intercept and slope of the linearized plot of log Qe vs. log Ce (Fig. 12). Table 2 Parameters of kinetics model for the adsorption of TBBPA and Pb(II) on MNP and MNP–CTS magnetic hybrid material. MNPs

−1

Qexp (mg g ) Pseudo-second-order h (mg g−1 min) k2 (mg mg−1 min) Qe (mg g−1 ) R2

MNPs–CTS

TBBPA

Pb(II)

TBBPA

Pb(II)

17.37

27.56

24.62

57.76

0.7041 2.107 18.28 0.9976

0.4458 0.4280 32.25 0.9955

1.084 1.657 25.57 0.9982

0.9267 0.2090 66.67 0.9930

L. Zhou et al. / Journal of Hazardous Materials 265 (2014) 104–114

111

Fig. 10. Plot of pseudo-second-order kinetic model for TBBPA (a) and Pb(II) (b) on MNP and MNP–CTS magnetic hybrid material.

Fig. 11. Adsorption isotherm of TBBPA (a) and Pb(II) (b) on MNP and MNP–CTS magnetic hybrid material following Langmuir model.

Fig. 12. Adsorption isotherm of TBBPA (a) and Pb(II) (b) on MNP and MNP–CTS magnetic hybrid material following Freundlich model.

Table 3 summarizes the Langmuir and Freundlich adsorption constants and the calculated regression coefficients (R2 ). The Freundlich isotherm provided much higher R2 values (R2 > 0.99) than did the Langmuir isotherm for the adsorption of TBBPA onto either composite, indicating its better fit with the experimental data. The Freundlich constants n were found to be greater than 1, which is favorable for adsorption [33]. The maximum monolayer adsorption capacity (Qmax ) of TBBPA onto MNP was calculated to be 30.65 mg g−1 and the corresponding value for MNP–CTS was 42.48 mg g−1 . This indicates the favorable effects of CTS on the removal of TBBPA, similar to the observations of Nobuyoshi et al. [58]. Hydroxyl groups in CTS are electron-donating functional groups that can increase the ␲-donating strength of the aromatic

Table 3 Parameters of adsorption isotherms for the adsorption of TBBPA and Pb(II) on MNP and MNP–CTS magnetic hybrid material. MNPs

Langmuir Qmax (mg g−1 ) KL (L mg−1 ) R2 Freundlich KF (mg1−1/n L1/n g−1 ) n R2

MNPs–CTS

TBBPA

Pb(II)

TBBPA

Pb(II)

30.65 0.1189 0.9800

66.31 0.0725 0.9955

42.48 0.1750 0.9831

140.4 0.6013 0.9978

3.796 1.5740 0.9967

4.955 1.484 0.9778

6.671 1.573 0.9934

47.51 1.609 0.9885

112

L. Zhou et al. / Journal of Hazardous Materials 265 (2014) 104–114

Fig. 13. The reusability of the MNP and MNP–CTS magnetic hybrid material for TBBPA (a) and Pb(II) (b).

ring of TBBPA. The adsorption of TBBPA onto MNP–CTS may be dominated by the ␲–␲ interaction between the electrons of the benzene-ring of TBBPA and the hexagonal skeleton of MWCNTs [32]. The Qmax value of TBBPA on MNP–CTS is higher than that previously reported for MWCNT/Fe3 O4 (22 mg g−1 ) by our research team [52]. The correlation coefficients for Pb(II) adsorption suggest that the Langmuir model better fits the adsorption data than does the Freundlich model. This indicates that the adsorption of Pb(II) onto either composite occurs via monolayer adsorption. Qmax values calculated from the Langmuir equation for Pb(II) adsorption were 66.31 mg g−1 for MNP and 140.1 mg g−1 for MNP–CTS. Compared to the crosslinking of chitosan adsorbent, the crosslinking method may reduce the presence of chitosan amino groups and reduce the adsorption capacity for Pb(II) [54]. Additionally, the aminofunctionalized Fe3 O4 exhibited relatively low surface area with limited distribution of amino groups, which reduced the adsorption capacity for Pb (II) [56]. Moreover, increases in the specific areas of carbon nanotubes with large numbers of –COOH groups resulted in increased adsorption capacity for Pb(II) [60]. Furthermore, the adsorption capacity of MNP–CTS was found to be higher than for other adsorbents (Table 4). These results indicated that MNP modified with CTS exhibits improved Pb(II) adsorption due to the high specific surface area and the increased availability of coordinating functional groups provided by Chitosan and amine residues on MNP–CTS. Overall, the MNP–CTS composite showed superior adsorption properties, and appears to be a promising adsorbent for the removal of pollutants. The reusability of the two composites for TBBPA and Pb(II) adsorption was also evaluated (Fig. 13). Five repeated adsorption and desorption cycles were run. Both composites adsorbed TBBPA consistently well, maintaining at least 91% of adsorption capacity after five cycles (Fig. 13a). After similar cycling, MNP showed a loss of adsorption capacity for Pb(II) of 12.2%, while MNP–CTS showed an 8% loss (Fig. 13b). The fact that both composites exhibited excellent reusability suggests that they remained stable during desorption. The magnetic hybrid materials with adsorbed TBBPA or

Pb(II) could be separated from solution under an external magnetic field. Overall, both composites showed high adsorption capabilities, good desorption properties, and high magnetic responses, which should significantly reduce the overall cost of their use as adsorbents, making them potentially useful magnetic adsorbents for the removal of pollutants from wastewater. To investigate the sorption mechanisms of TBBPA and Pb on the MNP–CTS material, the FTIR spectra of MNP–CTS, Pb loaded MNP–CTS and TBBPA loaded MNP–CTS were presented in Fig. 14. After TBBPA sorption, the peaks at 1630 and 1445 cm−1 due to the skeletal vibration of aromatic C C bonds were found to shift to 1635 cm and 1464 cm−1 , respectively, indicating that the ␲–␲ interaction might be formed between the benzene rings of TBBPA and the hexagonal skeleton of MWCNTs on the MNP–CTS [61], and this was similar to the absorption of TBBPA on graphene oxide [62]. The peak of N–H bond at 1558 cm−1 shifted to 1567 cm−1 , indicating that the hydrogen bonding was formed between amine groups provided by chitosan and amine residues on the hybrid materials and TBBPA. Furthermore, the peak of C–O at 1071 cm−1 shifted to 1039 cm−1 , which can be assigned to the hydrogen bond between O-H and TBBPA [63,64]. These results from the FTIR analysis demonstrated that the ␲–␲ bonding between TBBPA and the benzene rings on the MWCNTs. Moreover, the H-bonding between hydroxyl and amino groups might be involved in the sorption process between TBBPA and MNP–CTS [52]. While for the sorption of lead on MNP–CTS, the peaks of O–H and N–H groups at 3436 cm−1

Table 4 The maximum adsorption capacities Q (mg g−1 ) of Pb(II) on MNP–CTS and other adsorbents. Adsorbents

Adsorption capacity (mg g−1 )

References

MWCNTs/PAAM Crosslinked chitosan MNPs-NH2 Polysaccharide CNTs-iron oxide magnetic composite MWCNTs/Fe3 O4 -NH2 MNP–CTS

29.71 34.13 40.10 73.76 105.67

[59] [54] [56] [57] [60]

75.02 140.4

Our previous work Present work

Fig. 14. FTIR spectra of MNP–CTS (a), Pb loaded MNP–CTS (b), TBBPA loaded MNP–CTS (c) and TBBPA (d).

L. Zhou et al. / Journal of Hazardous Materials 265 (2014) 104–114

were shift to 3419 cm−1 , which might be attributed to the interaction between lead and oxygen in hydroxyl groups and nitrogen in amine groups. Both nitrogen and oxygen atoms possessed lone pair of electrons, which could bind to the Pb2+ through sharing the electron pair and form stable complexes. And the peak of N–H bond at 1558 cm−1 was found to shift to 1575 cm−1 , indicating that the Pb2+ –N complexes might be formed between Pb2+ and nitrogen in amine groups [65]. In addition, the peaks of C–O groups at 1071 and 1191 cm−1 shifted to 1043 and 1079 cm−1 , respectively, which may be caused by the chelating and ion exchange between lead and O–H groups [66]. 4. Conclusions A simple one-pot polyol method was developed for the synthesis of CoFe2 O4 –NH2 nanoparticles coated onto MWCNTs. The resulting magnetic composites were then modified using a Schiff base reaction to graft CTS onto the MNP composites. The CoFe2 O4 –NH2 nanoparticles on MNP showed almost uniform morphology with a mean diameter of 72.8 nm. MNP–CTS exhibited better adsorption properties for TBBPA and Pb(II) than did MNP. Adsorption onto either composite was optimal at pH 6.3 for TBBPA and at pH 6.0 for Pb(II). Pseudo-second-order modeling showed that the adsorption process is controlled by the chemical adsorption (R2 > 0.99). The Freundlich isotherm better describes the adsorption of TBBPA (R2 > 0.99) than does the Langmuir isotherm; however, the Langmuir model fits the adsorption of Pb(II) better than does the Freundlich model. This indicates that the adsorption of Pb(II) onto either composite was monolayer adsorption. MNP–CTS exhibited higher adsorption performance for both pollutants because of the presence of CTS. Repeated testing showed that both composites were stable, reusable, and durable for the adsorption of TBBPA and Pb(II). Overall, MNP–CTS appears a promising adsorbent for the removal of TBBPA and Pb(II) from wastewater. Acknowledgments The authors thank the financial supports from the Fundamental Research Funds for the Central Universities (lzujbky-2013-65) and the National Science Foundation for Fostering Talents in Basic Research of the National Natural Science Foundation of China (Grant No. J1103307). PC Ma was supported by the Western Light Program of CAS (Project No. RCPY201103) and the 1000-Talent Program (Recruitment Program of Global Expert, In Chinese: QianRenJiHua). References [1] R. Widmer, H. Oswald-Krapf, D. Sinha-Khetriwal, M. Schnellmann, H. Böni, Global perspectives on e-waste, Environ. Impact Assess. Rev. 25 (2005) 436–458. [2] S.L. Morf, J. Tremp, R. Gloor, Y. Huber, M. Stengele, M. Zennegg, Brominated flame retardants in waste electrical and electronic equipment: substance flows in a recycling plant, Environ. Sci. Technol. 39 (2005) 8691–8699. [3] A.O.W. Leung, N.S. Duzgoren-Aydin, K.C. Cheung, M.H. Wong, Heavy metals concentrations of surface dust from e-waste recycling and its human health implications in Southeast China, Environ. Sci. Technol. 42 (2008) 2674–2680. [4] L.S. Birnbaum, D.F. Staskal, Brominated flame retardants: cause for concern? Environ. Health Perspect. 112 (2003) 9–17. [5] A. Covaci, S. Harrad, M.A. Abdallah, N. Ali, R.J. Law, D. Herzke, C.A. de Wit, Novel brominated flame retardants: a review of their analysis, environmental fate and behaviour, Environ. Int. 37 (2011) 532–556. [6] M. Gorga, E. Martinez, A. Ginebreda, E. Eljarrat, D. Barcelo, Determination of PBDEs, HBB, PBEB, DBDPE, HBCD, TBBPA and related compounds in sewage sludge from Catalonia (Spain), Sci. Total. Environ. 444 (2013) 51–59. [7] A. Nakajima, D. Saigusa, N. Tetsu, T. Yamakuni, Y. Tomioka, T. Hishinuma, Neurobehavioral effects of tetrabromobisphenol A, a brominated flame retardant, in mice, Toxicol. Lett. 189 (2009) 78–83. [8] L. Zheng, K. Wu, Y. Li, Z. Qi, D. Han, B. Zhang, C. Gu, G. Chen, J. Liu, S. Chen, X. Xu, X. Huo, Blood lead and cadmium levels and relevant factors among children from an e-waste recycling town in China, Environ. Res. 108 (2008) 15–20.

113

[9] L.S. Morf, J. Tremp, R. Gloor, F. Schuppisser, M. Stengele, R. Taverna, Metals, nonmetals and PCB in electrical and electronic waste—actual levels in Switzerland, Waste Manage. 27 (2007) 1306–1316. [10] X. Huo, L. Peng, X. Xu, L. Zheng, B. Qiu, Z. Qi, B. Zhang, D. Han, Z. Piao, Elevated blood lead levels of children in Guiyu, an electronic waste recycling town in China, Environ. Health Perspect. 115 (2007) 1113–1117. [11] R. Nevin, How lead exposure relates to temporal changes in IQ, violent crime, and unwed pregnancy, Environ. Res. 83 (2000) 1–22. [12] X. Yang, J. Lee, L. Yuan, S.R. Chae, V.K. Peterson, A.I. Minett, Y. Yin, A.T. Harris, Removal of natural organic matter in water using functionalised carbon nanotube buckypaper, Carbon 59 (2013) 160–166. [13] C.J.M. Chin, L.C. Shih, H.J. Tsai, T.K. Liu, Adsorption of o-xylene and p-xylene from water by SWCNTs, Carbon 45 (2007) 1254–1260. [14] Y.H. Li, S. Wang, Z. Luan, J. Ding, C. Xu, D. Wu, Adsorption of cadmium(II) from aqueous solution by surface oxidized carbon nanotubes, Carbon 41 (2003) 1057–1062. [15] B. Jia, L. Gao, Fabrication of tadpole-like magnetite/multiwalled carbon nanotube heterojunctions and their self-assembly under external magnetic field, J. Phys. Chem. B 111 (2007) 5337–5343. [16] C. Chen, J. Hu, D. Shao, J. Li, X. Wang, Adsorption behavior of multiwall carbon nanotube/iron oxide magnetic composites for Ni(II) and Sr(II), J. Hazard. Mater. 164 (2009) 923–928. [17] J.H. Jung, J.H. Lee, S. Shinkai, Functionalized magnetic nanoparticles as chemosensors and adsorbents for toxic metal ions in environmental and biological fields, Chem. Soc. Rev. 40 (2011) 4464–4474. [18] M. Zhu, G. Diao, Review on the progress in synthesis and application of magnetic carbon nanocomposites, Nanoscale 3 (2011) 2748–2767. [19] Y. Hou, J. Yu, S. Gao, Solvothermal reduction synthesis and characterization of superparamagnetic magnetite nanoparticles, J. Mater. Chem. 13 (2003) 1983–1989. [20] Y. Meng, D. Chen, X. Jiao, Synthesis and characterization of CoFe2 O4 hollow spheres, Eur. J. Inorg. Chem. 25 (2008) 4019–4023. [21] Y. Lu, Y. Yin, B.T. Mayers, Y. Xia, Modifying the surface properties of superparamagnetic iron oxide nanoparticles through a sol–gel approach, Nano. Lett. 2 (2002) 183–186. [22] D. Yang, F. Yang, J. Hu, J. Long, C. Wang, D. Fu, Hydrophilic multi-walled carbon nanotubes decorated with magnetite nanoparticles as lymphatic targeted drug delivery vehicles, Chem. Commun. 444 (2009) 7–444, 9. [23] N. Li, M. Zheng, X. Chang, G. Ji, H. Lu, L. Xue, Preparation of magnetic CoFe2 O4 functionalized graphene sheets via a facile hydrothermal method and their adsorption properties, J. Solid. State Chem. 184 (2011) 953–958. [24] S. Zhang, H. Niu, Y. Cai, X. Zhao, Y. Shi, Arsenite and arsenate adsorption on coprecipitated bimetal oxide magnetic nanomaterials: MnFe2O4 and CoFe2O4, Chem. Eng. J. 158 (2010) 599–607. [25] L. Ai, H. Huang, Z. Chen, X. Wei, J. Jiang, Activated carbon/CoFe2 O4 composites: facile synthesis, magnetic performance and their potential application for the removal of malachite green from water, Chem. Eng. J. 156 (2010) 243–249. [26] A.A. Farghali, M. Bahgat, W.M.A. El Rouby, M.H. Khedr, Decoration of MWCNTs with CoFe2 O4 nanoparticles for methylene blue dye adsorption, J. Solution Chem. 41 (2012) 2209–2225. [27] Y. Deng, D. Qi, C. Deng, X. Zhang, D. Zhao, Superparamagnetic highmagnetization microspheres with an Fe3 O4 @SiO2 core and perpendicularly aligned mesoporous SiO2 shell for removal of microcystins, J. Am. Chem. Soc. 130 (2007) 28–29. [28] W. Yantasee, C.L. Warner, T. Sangvanich, R.S. Addleman, T.G. Carter, R.J. Wiacek, Removal of heavy metals from aqueous systems with thiol functionalized superparamagnetic nanoparticles, Environ. Sci. Technol. 41 (2007) 5114–5119. [29] L. Zhou, Y. Wang, Z. Liu, Q. Huang, Characteristics of equilibrium, kinetics studies for adsorption of Hg(II), Cu(II), and Ni(II) ions by thiourea-modified magnetic chitosan microspheres, J. Hazard. Mater. 161 (2009) 995–1002. [30] A. Bhatnagar, M. Sillanpaa, Applications of chitin- and chitosan-derivatives for the detoxification of water and wastewater–a short review, Adv. Colloid Interface Sci. 152 (2009) 26–38. [31] Z. Liu, H. Wang, C. Liu, Y. Jiang, G. Yu, X. Mu, Magnetic cellulose-chitosan hydrogels prepared from ionic liquids as reusable adsorbent for removal of heavy metal ions, Chem. Commun. 48 (2012) 7350–7352. [32] N. Aoki, M. Nishikawa, K. Hattori, Synthesis of chitosan derivatives bearing cyclodextrin and adsorption of p-nonylphenol and bisphenol A, Carbohydr. Polym. 52 (2003) 219–223. [33] J.M. Li, X.G. Meng, C.W. Hu, J. Du, Adsorption of phenol, p-chlorophenol and p-nitrophenol onto functional chitosan, Bioresour. Technol. 100 (2009) 1168–1173. [34] X.J. Hu, J.S. Wang, Y.G. Liu, X. Li, G.M. Zeng, Z.L. Bao, Adsorption of chromium (VI) by ethylenediamine-modified cross-linked magnetic chitosan resin: isotherms, kinetics and thermodynamics, J. Hazard. Mater. 185 (2011) 306–314. [35] S. Mohapatra, S.R. Rout, S. Maiti, T.K. Maiti, A.B. Panda, Monodisperse mesoporous cobalt ferrite nanoparticles: synthesis and application in targeted delivery of antitumor drugs, J. Mater. Chem. 21 (2011) 9185–9193. [36] S. Sun, H. Zeng, D.B. Robinson, S. Raoux, P.M. Rice, S.X. Wang, Monodisperse MFe2 O4 (M = Fe, Co, Mn) Nanoparticles, J. Am. Chem. Soc. 126 (2003) 273–279. [37] H. Wu, G. Liu, X. Wang, J. Zhang, Y. Chen, J. Shi, Solvothermal synthesis of cobalt ferrite nanoparticles loaded on multiwalled carbon nanotubes for magnetic resonance imaging and drug delivery, Acta Biomater. 7 (2011) 3496–3504. [38] L. Lai, G. Huang, X. Wang, J. Weng, Solvothermal syntheses of hollow carbon microspheres modified with –NH2 and –OH groups in one-step process, Carbon 48 (2010) 3145–3156.

114

L. Zhou et al. / Journal of Hazardous Materials 265 (2014) 104–114

[39] X. Liu, Q. Hu, Z. Fang, X. Zhang, B. Zhang, Magnetic chitosan nanocomposites: a useful recyclable tool for heavy metal ion removal, Langmuir 25 (2008) 3–8. [40] L. Wang, R. Xing, S. Liu, H. Yu, Y. Qin, K. Li, Recovery of silver (I) using a thioureamodified chitosan resin, J. Hazard. Mater. 180 (2010) 577–582. [41] P.L. Lee, Y.K. Chui, Y.C. Sun, Y.C. Ling, Synthesis of a hybrid material consisting of magnetic iron-oxide nanoparticles and carbon nanotubes as a gas adsorbent, Carbon 48 (2010) 1397–1404. [42] N. Li, R. Bai, A novel amine-shielded surface cross-linking of chitosan hydrogel beads for enhanced metal adsorption performance, Ind. Eng. Chem. Res. 44 (2005) 6692–6700. [43] Y. Wang, J. Park, B. Sun, H. Ahn, G. Wang, Wintersweet-flower-like CoFe2 O4 /MWCNTs hybrid material for high-capacity reversible lithium storage, Chem. Asian J. 7 (2012) 1940–1946. [44] J. Zhi, Y. Wang, Y. Lu, J. Ma, G. Luo, In situ preparation of magnetic chitosan/Fe3 O4 composite nanoparticles in tiny pools of water-in-oil microemulsion, React. Funct. Polym. 66 (2006) 1552–1558. [45] S. Qu, F. Huang, S. Yu, G. Chen, J. Kong, Magnetic removal of dyes from aqueous solution using multi-walled carbon nanotubes filled with Fe2 O3 particles, J. Hazard. Mater. 160 (2008) 643–647. [46] A.K. Mishra, S. Ramaprabhu, Nano magnetite decorated multiwalled carbon nanotubes: a robust nanomaterial for enhanced carbon dioxide adsorption, Energ. Environ. Sci. 4 (2011) 889–895. [47] C.L. Chen, X.K. Wang, M. Nagatsu, Europium adsorption on multiwall carbon nanotubeiron oxide magnetic composite in the presence of polyacrylic acid, Environ. Sci. Technol. 43 (2009) 2362–2367. [48] J.L. Gong, B. Wang, G.M. Zeng, C.P. Yang, C.G. Niu, Q.Y. Niu, et al., Removal of cationic dyes from aqueous solution using magnetic multi-wall carbon nanotube nanocomposite as adsorbent, J. Hazard. Mater. 164 (2009) 1517–1522. [49] M. Bahgat, A.A. Farghali, W.El. Rouby, M. Khedr, M.Y. Mohassab-Ahmed, Adsorption of methyl green dye onto multi-walled carbon nanotubes decorated with Ni nanoferrite, Appl. Nanosci. 3 (2012) 251–261. [50] M.A. Salam, M.A. Gabal, A.Y. Obaid, Preparation and characterization of magnetic multi-walled carbon nanotubes/ferrite nanocomposite and its application for the removal of aniline from aqueous solution, Synth. Met. 161 (2012) 2651–2658. [51] C. Zhang, J. Sui, J. Li, Y. Tang, W. Cai, Efficient removal of heavy metal ions by thiol-functionalized superparamagnetic carbon nanotubes, Chem. Eng. J. 210 (2012) 45–52. [52] L. Ji, L. Zhou, X. Bai, Y. Shao, G. Zhao, Y. Qu, et al., Facile synthesis of multiwall carbon nanotube s/iron oxides for removal of tetrabromobisphenol A and Pb (ii), J. Mater. Chem. 22 (2012) 15853–15862. [53] D. Shao, J. Hu, C. Chen, G. Sheng, X. Ren, X. Wang, Polyaniline multiwalled carbon nanotube magnetic composite prepared by plasma-induced graft technique

[54]

[55]

[56]

[57]

[58]

[59]

[60]

[61]

[62] [63]

[64]

[65]

[66]

and its application for removal of aniline and phenol, J. Phys. Chem. C 114 (2010) 21524–21530. A.H. Chen, S.C. Liu, C.Y. Chen, C.Y. Chen, Comparative adsorption of Cu (II), Zn (II), and Pb (II) ions in aqueous solution on the crosslinked chitosan with epichlorohydrin, J. Hazard. Mater. 154 (2008) 184–191. H.Y. Zhu, Y.Q. Fu, R. Jiang, J. Yao, L. Xiao, G.M. Zeng, Novel magnetic chitosan/poly (vinyl alcohol) hydrogel beads: preparation, characterization and application for adsorption of dye from aqueous solution, Bioresour. Technol. 105 (2012) 24–30. Y. Tan, M. Chen, Y. Hao, High efficient removal of Pb (II) by aminofunctionalized Fe3 O4 magnetic nano-particles, Chem. Eng. J. 191 (2012) 104–111. Z. Reddad, C. Gerente, Y. Andres, P. Le Cloirec, Adsorption of several metal ions onto a low-cost biosorbent: kinetic and equilibrium studies, Environ. Sci Technol. 36 (2002) 2067–2073. N. Aokia, M. Nishikawab, K. Hattorib, Synthesis of chitosan derivatives bearing cyclodextrin and adsorption of p-nonylphenol and bisphenol A, Carbohydr. Polym. 52 (2003) 219–223. S. Yang, J. Hu, C. Chen, D. Shao, X. Wang, Mutual effects of Pb(II) and humic acid adsorption on multiwalled carbon nanotubes/polyacrylamide composites from aqueous solutions, Environ. Sci. Technol. 45 (2011) 3621–3627. X. Peng, Z. Luan, Z. Di, Z. Zhang, C. Zhu, Carbon nanotubes-iron oxides magnetic composites as adsorbent for removal of Pb (II) and Cu (II) from water, Carbon 43 (2005) 880–883. M. Abdel Salam, R. Burk, Thermodynamics of pentachlorophenol adsorption from aqueous solutions by oxidized multi-walled carbon nanotubes, Appl. Surf. Sci. 255 (2008) 1975–1981. Y. Zhang, Y. Tang, S. Li, S. Yu, Sorption and removal of tetrabromobisphenol A from solution by graphene oxide, Chem. Eng. J. 222 (2013) 94–100. L. Ai, C. Zhang, F. Liao, Y. Wang, M. Li, L. Meng, J. Jiang, Removal of methylene blue from aqueous solution with magnetite loaded multi-wall carbon nanotube: kinetic, isotherm and mechanism analysis, J. Hazard. Mater. 198 (2011) 282–290. Y.H. Kim, B. Lee, K.H. Choo, S.J. Choi, Selective adsorption of bisphenol A by organic–inorganic hybrid mesoporous silicas, Micropor. Mesopor. Mater. 138 (2011) 184–190. ˇ ´ A.D. Marinkovic, ´ S.D. Skapin, ´ R. Aleksic, ´ A.A. Peric´ G.D. Vukovic, M.Ð. Ristic, ´ Removal of lead from water by amino modified multi´ P.S. Uskokovic, Grujic, walled carbon nanotubes, Chem. Eng. J. 173 (2011) 855–865. J.R. Rangel-Mendez, R. Monroy-Zepeda, E. Leyva-Ramos, P.E. Diaz-Flores, K. Shirai, Chitosan selectivity for removing cadmium (II), copper (II), and lead (II) from aqueous phase: pH and organic matter effect, J. Hazard. Mater. 162 (2009) 503–511.