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Biomass-derived multifunctional magnetite carbon aerogel nanocomposites for recyclable sequestration of ionizable aromatic organic pollutants
Chemical Engineering Journal 245 (2014) 210–216
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Biomass-derived multifunctional magnetite carbon aerogel nanocomposites for recyclable sequestration of ionizable aromatic organic pollutants Xilin Wu a,b, Wenbao Jia a,⇑ a b
Nanjing University of Aeronautics & Astronautics, 29 Yudao Street, Nanjing 210016, PR China School of Nuclear Science and Technology, University of Science and Technology of China, Hefei, Anhui 230031, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A novel magnetite carbon aerogel
(MCA) was synthesized by using crude biomass and watermelon. The porous MCA has large surface area of 323.8 m2 g1 and can be easily lift by a magnet. The MCA exhibited excellent adsorption capacity toward both 1-naphthol and 1-naphthylamine. The MCA can be easily regenerated by a simple Fenton reaction.
a r t i c l e
i n f o
Article history: Received 2 January 2014 Received in revised form 5 February 2014 Accepted 11 February 2014 Available online 18 February 2014 Keywords: Carbon aerogel Nanocomposites Organic pollutants Adsorption
a b s t r a c t In this article, a novel composite material based on biomass-derived carbon aerogel and iron oxide were prepared by a facile hydrothermal treatment of watermelon and stepped by incorporating iron oxide nanoparticles into the networks of the carbon aerogel. The as prepared magnetite carbon aerogel (MCA) was characterized by Fourier transformed infrared (FTIR), field emission scanning electron microscopic (FE-SEM) and X-ray diffraction (XRD). The MCA has high surface area of 323.8 m2 g1 and can be easily lift by a magnet. The multifunctional MCA was further applied as adsorbents for the removal of ionizable aromatic organic pollutants from aqueous solution. 1-Naphthol and 1-naphthylamine were used as model of the ionizable organic pollutants. The MCA exhibited excellent adsorption capacity toward both 1-naphthol and 1-naphthylamine and can be easily separated from aqueous solution by an external magnet. More importantly the adsorbed MCA can be easily regenerated by a simple Fenton reaction and still retained high adsorption capacity after cycles of reuse. The results imply that the MCA can be potential candidate as cost-effective adsorbents for the preconcentration and degradation of organic pollutants from aqueous solutions in environmental pollution cleanup. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Water contamination by aromatic compounds is recognized as a global environmental problem and has drawn growing public ⇑ Corresponding author. Tel.: +86 25 52112903; fax: +86 25 52112626. E-mail addresses: [email protected], [email protected] (W. Jia). http://dx.doi.org/10.1016/j.cej.2014.02.032 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.
concerns. Phenols and anilines are large class of ionizable aromatic compounds which are widely found in the effluents from the pharmaceuticals, petrochemicals, dyestuffs, pesticides and other industries [1]. Both phenols and anilines are classified as priority contaminants due to their toxicity to organisms at low concentrations. Besides, they are relatively high solubility in water and can easily transport in natural environmental which may boost human
X. Wu, W. Jia / Chemical Engineering Journal 245 (2014) 210–216
risk for cancer and acute toxicity by contact and uptake. Therefore, the removal of aromatic compounds from the discharged effluent has become a major focus of environmental protection and research. Many techniques including adsorption [1,2], biological treatment [3,4], chemical oxidation [3] and photochemical degradation [5] have been applied for the remove of aromatic pollutants from aqueous solution. Among these techniques, adsorption is one of the most effective technologies due to its operational simplicity, effectiveness, low energy requirements and low cost. In the past decade, the development of nanoscience and nanotechnology open up new avenues for the remediation of environmental problems. Nanomaterials based adsorbents have been studied extensively and showed much faster rates and higher efficiency toward pollutants as compared with the conventional adsorbents [6–9]. Various carbonaceous nanomaterials have been adopted as adsorbents for the removal of organic pollutants such as mesoporous carbon [10], carbon nanotubes [1,11], graphene [8] and carbon aerogels [12]. Among them, carbon aerogels were considered as one of the cost-effective carbon materials for water decontamination due to their low cost and excellent capability. Aerogels are unique three-dimensional (3D) solid networks with open pores and majority of the volume is air [13,14]. Carbon aerogels are large class of aerogels which have recently triggered many research activities due to their multifunctional properties such as high surface area, porous structure, low mass densities and high electrical conductivity [15,16]. Because of these special chemical and textural characteristics, carbon aerogels possess promising applications in adsorbents, catalysis and supercapacitors [17]. In recent years, growing attentions have been paid to the design and fabrication of carbon aerogels based composite materials with multifunctional properties for environmental applications. For example, TiO2/carbon aerogels composites have been synthesized and used as electrosorption–photocatalysis synergistic electrode for the degradation of organic pollutants in aqueous solutions [18]. Wang and co-workers prepared ferrite–carbon aerogel composites and applied as Fenton catalysis for the oxidation degradation of metalaxyl [14]. Conventional carbon aerogels are prepared by the pyrolysis of organic aerogels such as resorcinol and formaldehyde polymer aerogels. However, these methods are usually associated with problems of multiple step process and using expensive and poisonous chemical reagents. Thus, to develop a simple and environmental friendliness approach to carbon areogels for environmental applications is still challenging. In this work a green and template free rote was developed for the synthesis of low-cost carbon aerogels and further modified with iron oxide nanoparticles. The obtained magnetite carbon aerogel (MCA) composite material was applied as adsorbent for the efficient removal of ionizable aromatic compounds from aqueous solution. 1-Naphthol and 1-naphthylamine are used as model compounds of the harmful and water soluble aromatic organic pollutants, which are widely used as industrial intermediates [19]. The adsorption properties of 1-naphthol and 1-naphthylamine on MCA are investigated and the mechanisms were also discussed. To the best of our knowledge, it is the first example of using carbon aerogels based nanocomposites for the efficient removal of toxic ionizable aromatic compounds from aqueous solution. And the results showed that MCA can be suitable candidate as cost-effective adsorbents for practical application.
2. Experimental 2.1. Chemicals Chemicals including ferric chloride (anhydrous, FeCl3), ferrous sulfate heptahydrate (FeSO47H2O), ammonium hydroxide (25%,
211
NH3H2O), trisodium citrate (Na3Cit), sodium hydroxide (NaOH) and hydrochloric acid (38%, HCl) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). 1-Naphthol and 1-naphthylamine were purchased from Sigma–Aldrich (St Louis, MO). All above chemicals were used as received without further purification. Milli-Q ultrapure water (18.2 MX cm1) was used for all the experiments. 2.2. Sample preparation The original carbonaceous monolith was prepared by a wellestablished hydrothermal carbonization method by using crude biomass, watermelon, as the carbon source. Watermelon was first cut and put in into a Teflon-lined stainless-steel autoclave and hydrothermal treated at 180 °C for 12 h. After that, the 3D black monolith was taken out and washed with Milli-Q water several times. For preparing magnetite carbon aerogel (MCA), Fe3O4 nanoparticles were first loaded in the above carbonaceous monolith by using a simple solution process [20]. Briefly, a piece of carbonaceous monolith with dimension of about 2 3 3 cm3 was fully immersed in 20 mL aqueous solution of FeSO47H2O (1.25 g, 4.5 mmol) and FeCl3 (1.30 g, 8 mmol) for 15 min under N2 atmosphere. The mixture was heated to 80 °C under N2 protection and 10 mL of 30% ammonia solution was quickly added into the mixture. After that, the mixture was kept at 80 °C for 30 min and 3.0 g of trisodium citrate was added to the solution while the temperature rising to 95 °C. The obtained Fe3O4 loaded carbon monolith was rinsed with Milli-Q water and dried in a vacuum oven at 70 °C over night. The magnetite carbon aerogel (MCA) was obtained by calcination of the above Fe3O4 loaded carbon monolith in nitrogen atmosphere at 550 °C for 4 h. 2.3. Characterization and analysis Transmission electron microscopy (TEM) (JEOL-2010) and field emission scanning electron microscopic (FE-SEM) (JEOL JSM6330F) were applied to observe the microstructure of the original carbon monolith and MCA. X-ray diffraction (XRD) pattern was performed on0 a Philips X’Pert X-ray diffractometer (Cu Ka source (k = 1.54178 Å A)) in the range of 2h = 5–65°. The N2 adsorption– desorption curve of MCA was measured by a Micromeritics ASAP 2010 system at 77 K utilizing Barrett–Emmett–Teller (BET) methods to calculate the specific surface area. The pore size distribution of the MCA was calculated by applying the Barrett–Joyer–Halenda (BJH) model by using the desorption branch of the N2 adsorption– desorption isotherm. Magnetic measurement of the MCA was carried out on a (Quantum Design MPMS XL) magnetometer at 300 K with an applied magnetic field of 20 KOe. The UV–vis measurements were performed on a UV-3000 spectrophotometer (Japan). 2.4. Adsorption experiments The batch adsorption experiments of 1-naphthol and 1-naphthylamine on MCA were carried out at pH = 6.0 in glass vials. Before adsorption, calibration curves of 1-naphthol and 1-naphthylamine was obtained from the UV–vis spectra of the standard 1-naphthol and 1-naphthylamine solutions (5–60 mg L1) at pH 6, respectively. The stock suspensions of MCA, 1-naphthol or 1-naphthylamine solutions were added in the glass vials to achieve the desired concentrations of different components. The desired pH of the suspension in each vial was adjusted by adding negligible volumes of 0.01 or 0.1 M NaOH or HCl with a pH meter. After the suspensions were oscillated for 48 h, the solid and liquid phases were separated by centrifugation at 10,000 rpm for 5 min. The concentration of the aromatic organic compounds in the supernatant was determined by UV–vis spectrophotometer. The adsorbed
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amounts of aromatic organic compounds on MCA were calculated by using the following equation:
qe ¼
ðC 0 C e Þ V m
ð1Þ
where C0 is the initial concentration and Ce is the equilibrium concentration of the aromatic organic compounds (mg L1), respectively, V is the volume of the suspension in each vial (L) and m is the mass of MCA used in each vial (g).
3. Results and discussion 3.1. Characterization Digital pictures of the original carbonaceous monolith are shown in Fig. 1. The base radius of the cylinder carbonaceous monolith is about 4.5 cm and the height is about 12 cm. The dimension of the carbonaceous monolith can be easily controlled by using different volume of the Teflon-lined stainless-steel autoclave for the hydrothermal process. The obtained carbonaceous monolith can also be cut into smaller sizes by a sharp knife. SEM and TEM images show the interconnected microstructure of the carbonaceous monolith consisting of catenulate nanospheres (Fig. 2). The formation of interconnected networks of the carbonaceous monolith could be due to the polymerization and carbonization of carbohydrates in the watermelon during the hydrothermal reaction. The interconnected networks of the carbonaceous monolith can serve as good substrate for the synthesis of functional composite materials by decorating other nanoparticles into the networks. The mass density of the dried carbonaceous monolith is about 0.06 g cm3, indicating the light weight of this carbonaceous material. The Fe3O4 nanoparticles loaded carbonaceous monolith was further transformed into magnetite carbon aerogel (MCA) by calcination. The mass density of MCA was measured to be about 0.031 g cm3. Fig. 3a shows the SEM image of MCA. It can be observed that MCA is also made of catenulate carbon networks and carbon nanosphere networks, indicating that the original carbonaceous monolith kept its microscopic structure during the calcination process. The TEM images (Fig. 3b and c) further show the interconnected microstructure of MCA with decorated Fe3O4 nanoparticles in the networks. As can be seen in Fig. 3c, the Fe3O4 nanoparticles were firmly imbedded into the phase of carbon. When crush the MCA monolith into powder, only a few of Fe3O4 nanoparticles fall off the surface of carbon due to the external mechanical force. The average size of the Fe3O4 nanoparticles is about 9 nm. The XRD pattern of the MCA is shown in Fig. 3d. The typical diffraction peaks at
2h = 30.1°, 35.6°, 43.2°, 53.4°, 57.1° and 62.5° can be assigned to the phase of Fe3O4 (JCPDS 75-0033) [20]. The pore size distribution of MCA obtained from the N2 adsorption–desorption curve is shown in Fig. 4a. The BET specific surface of MCA was measured to be 323.8 m2 g1. The BJH average pore diameter of the MCA is 8.3 nm which is located at the mesoporous region (2–50 nm). The large surface area and the presence of large numbers of micropores and mesopores could provide abundant active sites for the diffusion/adsorption of other phase such as gas and liquids. The XPS analysis was carried out to determine the surface elements states of the MCA. The wide survey scan of XPS spectrum shows the presence of high content of the C, O and Fe elements in the MCA (Fig. 4b). The high resolution XPS spectrum of Fe 2p (Fig. 4c) shows two characteristic peaks at binding energy of 711.2 and 724.7 eV which is attribute to Fe 2p3/2 and Fe 2p1/2, respectively, indicating the presence of Fe3O4. The atomic ratio of iron (Fe%) obtained from the XPS analysis is 1.83 %. The magnetism of the MCA is measured at room temperature within magnetic filed from 20,000 to 20,000 Oe (Fig. 4d). The magnetic remanence (Mr) and the specific saturation magnetization (Ms) of the MCA are obtained from the magnetic hysteresis loop. The value of Mr and Ms. are about 2.3 and 66.9 emu g1, respectively. The small value of the Ms indicates that the MCA possess a superparamagnetic behavior, that is, when the applied magnetic filed is removed that no magnetization remains. It can be clearly observed from the insert in Fig. 4d that the MCA monolith can be easily lifted by a magnet which also reveals the excellent magnetic property of the MCA. The multifunctional properties of MCA could endow its great application potential in the field of adsorbents, catalysis and energy storage. 3.2. Effect of solution pH on adsorption The pH dependent adsorption of 1-naphthol and 1-naphthylamine on MCA is shown in Fig. 5a. The results showed that the solution pH played an important role in the adsorption of 1-naphthol and 1-naphthylamine. As the pH increasing, both the adsorption of 1-naphthol and 1-naphthylamine increased at pH < 4.5 and reach the maximum adsorption percentage at pH = 4.5. While the adsorption gradually declined when solution pH increases from 4.5 to 10. The pH affects the degree of ionization of the adsorbate as well as the surface charge of the adsorbent [21]. The pKa of 1-naphthol and 1-naphthylamine is 9.34 and 3.92, respectively. To further investigate the influence of pH on 1-naphthol and 1-naphthylamine adsorption, the Zeta-potential of the MCA were measured at different pH (Fig. 5b). The electrostatic point of the MCA is about 4.5, indicating that the MCA carries positive charges at pH < 4.5 and negative charge at pH > 4.5. At pH < 4.5, the
Fig. 1. Digital pictures of the original carbonaceous monolith.
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Fig. 2. (a) SEM image and (b) TEM image of the original carbonaceous monolith.
(b)
2 m
(d)
(400)
(220)
Intensity (a.u.)
(222)
100 nm
10
20
30
40
50
(440)
(c)
(422) (511)
10 m
(311)
(a)
60
70
2 Theta (degree) Fig. 3. (a) SEM image of the MCA. TEM images of the MCA at low (b) and high (c) magnification. (d) X-ray diffraction pattern of the MCA.
increase of solution pH increases the dissociation of 1-naphthol and 1-naphthylamine which is beneficial for their interaction with the positive charged MCA. When pH > 4.5, the repulsion between the dissociated 1-naphthol and 1-naphthylamine and the negatively charged MCA increased which reduce the adsorption. The electrostatic interaction between the adsorbates and adsorbent may be the main mechanism for the pH dependent adsorption experiments. At pH < 4, Fe3O4 nanoparticles would partially dissolved (about 21% dissolution of the iron at pH = 2) [22]. At low pH, the dissolved Fe3+ ions may complex with the 1-naphthol and 1-naphthylamine in bulk solution. And it would inhibit the adsorption of the ionic organic compounds onto the surface of MCA. At pH > 4, the dissociation of the iron oxide is negligible and the adsorption processes are mainly dominated by the physical adsorption.
3.3. Adsorption kinetics The adsorption kinetics of 1-naphthol and 1-naphthylamine on MCA were carried out to understand the adsorption properties. The time dependent adsorption data and the corresponding fitting
curves are shown in Fig. 6. As can be seen in Fig. 6a, the adsorption amount of both 1-naphthol and 1-naphthylamine on MCA increased rapidly in 10 h, and then the adsorption rate slowed down until the sorption reached equilibrium at about 40 h. Similarly, Hu et al. [23] found that the adsorption of 1-naphthylamine on multiwalled carbon nanotubes reached adsorption equilibrium at about 50 h. The beginning rapid step of 1-naphthol and 1-naphthylamine adsorption might be due to the surface physical sorption (p–p interactions). While the subsequent slow step could be due to the limited active adsorption sites available on the surface of the adsorbents. The 1-naphthol and 1-naphthylamine molecules first migrate through the solution (i.e. film diffusion), followed by the transportation from the particle surface into the interior sites by pore diffusion [21]. The pore diffusion process is responsible for the relatively long equilibrium time for the adsorption. The pseudo-second-order model was applied to simulate the kinetics of 1-naphthol or 1-naphthylamine adsorption onto MCA (Fig. 6b). The pseudo-second-order kinetic model can be expressed as follows [20]:
t 1 t ¼ þ Q t k2 Q 2e Q e
ð2Þ
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0.20
(a)
(b)
0.15
C 1s
Intensity (a.u.)
Pore volume (cm3/g, dV/dlog(D))
214
0.10
0.05
O 1s Fe 2p
0.00 1
10
200
100
300
Pore diameter (nm)
400
500
600
700
800
Binding Energy (eV) 80
(c)
(d)
Fe 2p3/2
Fe 2p1/2
Intensity (a.u.)
Magnetization (emu/g)
60 40 20 0 -20 -40 -60 -80
740
735
730
725
720
715
710
705
-20000
0
-10000
10000
20000
Magnetic field (Oe)
Binding Energy (eV)
Fig. 4. (a) Pore size distribution of the MCA. (b) XPS survey and (c) Fe 2p XPS spectrum of the MCA. (d) Room temperature magnetic hysteresis loop of the MCA and the insert shows the MCA monolith lifted by a magnet.
(a)
100
(b)
20
Zeta potential (mV)
80
Adsorption (%)
40
60
40
1-Naphthol 1-Naphthylamine
20
0
-20
-40
-60
0 1
2
3
4
5
6
7
8
9
10
11
2
3
4
5
6
7
8
9
10
11
pH
pH
Fig. 5. (a) Effect of solution pH on 1-naphthol and 1-naphthylamine adsorption on MCA. (b) Zeta-potential of MCA as a function of pH. In panel A, m/V = 0.5 g/L, C[1-naphthol]initial = 60 mg/L, C[1-naphthylamine]initial = 60 mg/L.
where Qt and Qe are the amounts of 1-naphthol or 1-naphthylamine adsorbed on per gram of the MCA (mg g1) at time t and equilibrium time (h), respectively, and k2 is the rate constant (g mg1 h). As can be seen in Fig. 6b, the plots of t/Qt versus t for both 1-naphthol and 1-naphthylamine on MCA achieve high correlation coefficient values (R2 = 0.989 and 0.987 for 1-naphthol and 1-naphthylamine, respectively), indicating that the pseudo-second-order kinetic model fits the experimental data very well. The values of Qe calculated from the intercept and slope of the plots of t/Qt versus t are 98.02 mg g1 for 1-naphthol and 50.47 mg g1 for 1-naphthylamine, respectively.
3.4. Adsorption isotherm Adsorption isotherms of 1-naphthol and 1-naphthylamine on MCA are shown in Fig. 7. The experimental data are fitted by Lang 1=n muir QC ee ¼ qm1kL þ qCme and Freundlich (Q e ¼ kC e ) isotherm equations, respectively (where Ce is the equilibrium concentration of 1-naphthol or 1-naphthylamine in the supernatant (mg L1); Qe is the amount of 1-naphthol or 1-naphthylamine adsorbed on per weight of adsorbent after equilibrium (mg g1); qm represents the maximum adsorption capacity of 1-naphthol or 1-naphthylamine
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1.2 80
(a)
1.0
t/q t ( h· g/mg )
Qt (mg/g)
60
40
1-Naphthol 1-Naphthylamine
20
(b) 1-Naphthol 1-Naphthylamine
0.8 0.6 0.4 0.2 0.0
0 0
10
20
30
40
50
60
0
10
Time (h)
20
30
40
50
60
Time (h)
Fig. 6. (a) Time dependent adsorption of 1-naphthol and 1-naphthylamine on MCA. (b) Pseudo-second-order kinetic plots for the adsorption of 1-naphthol and 1-naphthylamine on MCA. (m/V = 0.5 g/L, pH = 6.0, C[1-naphthol]initial = 60 mg/L, C[1-naphthylamine]initial = 60 mg/L.)
100
Qe (mg/g)
80
60
40
1-Naphthol 1-Naphthylamine
20 0
10
20
30
40
50
60
70
80
Ce (mg/L) Fig. 7. Adsorption isotherms of 1-naphthol and 1-naphthylamine on MCA (m/V = 0.5 g/L, pH = 6.0). Symbols denote experimental data, solid line represents the Langmuir model simulation and dashed line represents the Freundlich model simulation.
Table 1 Parameters for the Langmuir and Freundlich isotherm models of 1-naphthol and 1naphthylamine adsorption on MCA. Aromatic organic matter
Langmuir
Freundlich
qm (mg/g)
b (L/mg)
R2
k
n
R2
1-Naphthol 1-Naphthylamine
80.67 63.65
0.92 0.32
0.98 0.94
40.39 24.82
5.12 4.21
0.89 0.96
on per weight of adsorbent (mg g1); kL is the Langmuir constant which represents enthalpy of sorption (L mg1); and the Freundlich constant k and 1/n is correlated to the relative adsorption capacity
of the adsorbent (mg/g) and the adsorption intensity, respectively). The Langmuir and Freundlich isotherm models fitted curves are also shown in Fig. 7. The above parameters obtained from the two models are listed in Table 1. For 1-naphthol adsorption, the relative coefficient (R2) of the Langmuir model was higher than that of the Freundlich model, indicating that the adsorption of 1-naphthol on MCA follows the Langmuir isotherm model but not the Freundlich model. The adsorption data fitted with the Langmuir isotherm model indicates that the adsorption of 1-naphthol on MCA is a monolayer adsorption and the binding energy on the whole surface of MCA is uniform [24]. For 1-naphthylamine adsorption, the experimental data are well fitted by both the Langmuir and Freundlich isotherm models. The values of qm obtained by the Langmuir isotherm model are 80.67 and 63.65 mg g1 for the 1-naphthol and 1-naphthylamine on MCA, respectively. These values are quite in agreement with the experimental data shown in Fig. 7. The high adsorption capacities of the MCA toward both 1-naphthol and 1-naphthylamine could be due to its high surface area and the multifunctional properties. The obtained maximum adsorption capacity of 1-naphthol is higher than that on as prepared multi-walled CNTs (54.35 mg g1) [25], oxide multi-walled CNTs (30.58 mg g1) [25] and comparable to that on XC-72 carbon (83.54 mg g1) [26], and that of 1-naphthylamine is higher than on as prepared multiwalled CNTs (58.82 mg g1) [25], oxide multi-walled CNTs (58.14 mg g1) [25] and modified polymer (15.0 mg g1) [27]. For comparison, the maximum adsorption capacity of 1-naphthol or 1-naphthylamine on MCA or other adsorbents as list in Table 2. It can be seen that the MCA exhibit excellent adsorption capacity toward both 1-naphthol and 1-naphthylamine. The reusability of the MCA was also investigated by a simple and well-developed Fenton reaction [29]. Generally, 0.1 g of 1-naphthol or 1-naphthylamine adsorbed MCA was separated by a magnet and the wet black slurry
Table 2 Comparison of the maximum absorption capacities of 1-naphthol and 1-nNaphthylamine adsorption on MCA with other adsorbents. Adsorbents
Adsorbates
Adsorption capacity (mg g1)
pH
Ref.
MWCNTs Oxide MWCNTs XC-72 carbon MWCNTs/Fe3O4/CD MCA MWCNTs Oxide MWCNTs Modified polymer MCA
1-Naphthol 1-Naphthol 1-Naphthol 1-Naphthol 1-Naphthol 1-Naphthylamine 1-Naphthylamine 1-Naphthylamine 1-Naphthylamine
54.35 30.58 83.54 57.47 80.67 58.82 58.14 15.0 63.65
6 6 7 5.5 6 6 6 – 6
[25] [25] [26] [28] Our work [25] [25] [27] Our work
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was redispersed in 200 mL H2O2 (8.3 102 M) and the pH was adjusted to 3.0 by 0.1 M HNO3. The suspension was stirred constantly for 8 h at 80 °C and after that the solid was separated by a magnet and dried in an oven at 80 °C. The above adsorption– regeneration process was repeated for 5 times. It was found that about 85% of the adsorption capacity was retained for both 1-naphthol and 1-naphthylamine after 5 cycles of reuse. The totally green synthetic method used, low-cost of the crude material and easily recyclable and reusable properties indicate that MCA could be a promising material for the removal of ionizable aromatic organic pollutants from aqueous solution. For further applications, MCA might be extended to treat other kinds of organic pollutants in future practical applications. 4. Conclusion In this study, a biomass-derived carbon monolith was prepared by a facile hydrothermal process. After that, magnetite nanoprticles were incorporated into the networks of the carbon monolith and further transformed into magnetite carbon aerogel (MCA). The obtained MCA is of very light weight and show high surface area and magnetic property. The multifunctional MCA showed excellent capacity for efficient removal of 1-naphthol or 1-naphthylamine from aqueous solution. The adsorbed MCA can be easily regenerated and reused which provide great potential as adsorbent for the removal of ionizable organic pollutants from aqueous solutions. And can be extended to treat other kinds of organic pollutants in future practical applications. Acknowledgements We acknowledge the financial support from National Natural Science Foundation of China (21272236, 21225730, 212207136), Hefei Center for physical science and technology (2012FXZY005) and the USTC Special Grant for Postgraduate Research, Innovation and Practice. References [1] K. Yang, W. Wu, Q. Jing, L. Zhu, Aqueous adsorption of phenol, and their substitutes by multi-walled carbon nanotubes, Environ. Sci. Technol. 42 (2008) 7931–7936. [2] W. Zhang, J. Chen, B. Pan, Q. Zhang, Cooperative adsorption behaviours of 1naphthol and 1-naphthylamine onto nonpolar macroreticular adsorbents, React. Funct. Polym. 66 (2006) 485–493. [3] R. Rosal, A. Rodríguez, J.A. Perdigón-Melón, A. Petre, E. García-Calvo, M.J. Gómez, A. Agüera, A.R. Fernández-Alba, Occurence of emerging pollutants in urban wastewater and their removal through biological treatment followed by ozonation, Water Res. 44 (2010) 578–588. [4] I. Mozo, G. Lesage, J. Yin, Y. Bessiere, L. Barna, M. Sperandio, Dynamic modeling of biodegradation and volatilization of hazardous aromatic substances in aerobic bioreactor, Water Res. 46 (2012) 5327–5342. [5] K. Nagaveni, G. Sivalingam, M.S. Hegde, G. Madras, Photocatalytic degradation of organic compounds over combustion-synthesized nano-TiO2, Environ. Sci. Technol. 38 (2004) 1600–1604. [6] J. Gong, T. Liu, X. Wang, X. Hu, L. Zhang, Efficient removal of heavy metal ions from aqueous systems with the assembly of anisotropic layered double hydroxide nanocrystals@carbon nanosphere, Environ. Sci. Technol. 45 (2011) 6181–6187.
[7] L. Wang, X.L. Wu, W.H. Xu, X.J. Huang, J.H. Liu, A.W. Xu, Stable organicinorganic hybrid of polyaniline/a-zirconium phosphate for efficient removal of organic pollutants in water environment, ACS Appl. Mater. Interf. 4 (2012) 2686–2692. [8] G.X. Zhao, L. Jiang, Y. He, J. Li, H. Dong, X. Wang, W. Hu, Sulfonated graphene for persistent aromatic pollutant management, Adv. Mater. 23 (2011) 3959–3963. [9] V. Chandra, J. Park, Y. Chun, J.W. Lee, I.C. Hwang, K.S. Kim, Water-dispersible magnetite-reduced graphene oxide composites for arsenic removal, ACS Nano. 4 (2011) 3979–3986. [10] L. Ji, F. Liu, Z. Xu, S. Zheng, D. Zhu, Adsorption of pharmaceutical antibiotics on template-synthesized ordered micro-and mesoporous carbons, Environ. Sci. Technol. 44 (2010) 3116–3122. [11] J. Chen, W. Chen, D. Zhu, Adsorption of nonionic aromatic compounds to single-walled carbon nanotubes: effects of aqueous solution chemistry, Environ. Sci. Technol. 42 (2008) 7225–7230. [12] D. Fairén-Jiménez, F. Carrasco-Marín, C. Moreno-Castilla, Adsorption of benzene, toluene, and xylenes on monolithic carbon aerogels from dry air flows, Langmuir 23 (2007) 10095–10101. [13] H.C. Chien, W.Y. Cheng, Y.H. Wang, S.Y. Lu, Ultrahigh specific capacitances for supercapacitors achieved by nickel cobaltite/carbon aerogel composites, Adv. Funct. Mater. 22 (2012) 5038–5043. [14] Y. Wang, G. Zhao, S. Chai, H. Zhao, Y. Wang, Three-dimensional homogeneous ferrite–carbon aerogel: one pot fabrication and enhanced electro-fenton reactivity, ACS Appl. Mater. Interf. 5 (2013) 842–852. [15] M.A. Worsley, J.H. Satcher, T.F. Baumann, Synthesis and characterization of monolithic carbon aerogel nanocomposites containing double-walled carbon nanotubes, Langmuir 24 (2008) 9763–9766. [16] S. Wei, D. Wu, X. Shang, R. Fu, Studies on the structure and electrochemical performance of Pt/carbon aerogel catalyst for direct methanol fuel cells, Energy Fuels 23 (2009) 908–911. [17] T. Bordjiba, M. Mohamedi, L.H. Dao, New class of carbon-nanotube aerogel electrodes for electrochemical power sources, Adv Mater. 20 (2008) 815–819. [18] M. Wu, Y. Jin, G. Zhao, M. Li, D. Li, Electrosorption-promoted photodegradation of opaque wastewater on a novel TiO2/carbon aerogel electrode, Environ. Sci. Technol. 44 (2010) 1780–1785. [19] W. Zhang, C. Hong, B. Pan, Z. Xu, Q. Zhang, L. Lv, Removal enhancement of 1naphthol and 1-naphthylamine in single and binary aqueous phase by acid– basic interactions with polymer adsorbents, J. Hazard. Mater. 158 (2008) 293– 299. [20] X.L. Wu, L. Wang, C.L. Chen, A.W. Xu, X.K. Wang, Water-dispersible magnetitegraphene–LDH composites for efficient arsenate removal, J. Mater. Chem. 21 (2011) 17353–17359. [21] B.H. Hameed, A.A. Rahman, Removal of phenol from aqueous solutions by adsorption onto activated carbon prepared from biomass material, J. Hazard. Mater. 160 (2008) 576–581. [22] Q. Chang, W. Lin, W.C. Ying, Preparation of iron-impregnated granular activated carbon for arsenic removal from drinking water, J. Hazard. Mater. 184 (2010) 515–522. [23] J. Hu, D. Shao, C. Chen, G. Sheng, X. Ren, X. Wang, Removal of 1-naphthylamine from aqueous solution by multiwall carbon nanotubes/iron oxides/ cyclodextrin composite, J. Hazard. Mater. 185 (2011) 463–471. [24] X.L. Wu, D.L. Zhao, S.T. Yang, Impact of solution chemistry conditions on the sorption behavior of Cu(II) on Lin’an montmorillonite, Desalination 269 (2011) 84–91. [25] G.D. Sheng, D.D. Shao, X.M. Ren, X.Q. Wang, J.X. Li, Y.X. Chen, X.K. Wang, Kinetics and thermodynamics of adsorption of ionizable aromatic compounds from aqueous solutions by as-prepared and oxidized multiwalled carbon nanotubes, J. Hazard. Mater. 178 (2010) 505–516. [26] L.M. Zuo, S.M. Yu, L. Cheng, E. Du, Adsorption of phenol and 1-naphthol onto XC-72 carbon, Korean J. Chem. Eng. 30 (2013) 714–723. [27] D. Zhao, L. Zhao, C.S. Zhu, X. Shen, X. Zhang, B. Sha, Comparative study of polymer containing b-cyclodextrin and –COOH for adsorption toward aniline, 1-naphthylamine and methylene blue, J. Hazard. Mater. 171 (2009) 241–246. [28] J. Hu, D. Shao, C. Chen, G. Sheng, J. Li, X. Wang, M. Nagatsu, Plasma-induced grafting of cyclodextrin onto multiwall carbon nanotube/iron oxides for adsorbent application, J. Phys. Chem. B 114 (2010) 6779–6785. [29] M.H. Do, N.H. Phan, T.D. Nguyen, T.T.S. Pham, V.K. Nguyen, T.T.T. Vu, T.K.P. Nguyen, Activated carbon/Fe3O4 nanoparticle composite: fabrication, methyl orange removal and regeneration by hydrogen peroxide, Chemosphere 85 (2011) 1269–1276.
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Biomass-derived multifunctional magnetite carbon aerogel nanocomposites for recyclable sequestration of ionizable aromatic organic pollutants Xilin Wu a,b, Wenbao Jia a,⇑ a b
Nanjing University of Aeronautics & Astronautics, 29 Yudao Street, Nanjing 210016, PR China School of Nuclear Science and Technology, University of Science and Technology of China, Hefei, Anhui 230031, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A novel magnetite carbon aerogel
(MCA) was synthesized by using crude biomass and watermelon. The porous MCA has large surface area of 323.8 m2 g1 and can be easily lift by a magnet. The MCA exhibited excellent adsorption capacity toward both 1-naphthol and 1-naphthylamine. The MCA can be easily regenerated by a simple Fenton reaction.
a r t i c l e
i n f o
Article history: Received 2 January 2014 Received in revised form 5 February 2014 Accepted 11 February 2014 Available online 18 February 2014 Keywords: Carbon aerogel Nanocomposites Organic pollutants Adsorption
a b s t r a c t In this article, a novel composite material based on biomass-derived carbon aerogel and iron oxide were prepared by a facile hydrothermal treatment of watermelon and stepped by incorporating iron oxide nanoparticles into the networks of the carbon aerogel. The as prepared magnetite carbon aerogel (MCA) was characterized by Fourier transformed infrared (FTIR), field emission scanning electron microscopic (FE-SEM) and X-ray diffraction (XRD). The MCA has high surface area of 323.8 m2 g1 and can be easily lift by a magnet. The multifunctional MCA was further applied as adsorbents for the removal of ionizable aromatic organic pollutants from aqueous solution. 1-Naphthol and 1-naphthylamine were used as model of the ionizable organic pollutants. The MCA exhibited excellent adsorption capacity toward both 1-naphthol and 1-naphthylamine and can be easily separated from aqueous solution by an external magnet. More importantly the adsorbed MCA can be easily regenerated by a simple Fenton reaction and still retained high adsorption capacity after cycles of reuse. The results imply that the MCA can be potential candidate as cost-effective adsorbents for the preconcentration and degradation of organic pollutants from aqueous solutions in environmental pollution cleanup. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Water contamination by aromatic compounds is recognized as a global environmental problem and has drawn growing public ⇑ Corresponding author. Tel.: +86 25 52112903; fax: +86 25 52112626. E-mail addresses: [email protected], [email protected] (W. Jia). http://dx.doi.org/10.1016/j.cej.2014.02.032 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.
concerns. Phenols and anilines are large class of ionizable aromatic compounds which are widely found in the effluents from the pharmaceuticals, petrochemicals, dyestuffs, pesticides and other industries [1]. Both phenols and anilines are classified as priority contaminants due to their toxicity to organisms at low concentrations. Besides, they are relatively high solubility in water and can easily transport in natural environmental which may boost human
X. Wu, W. Jia / Chemical Engineering Journal 245 (2014) 210–216
risk for cancer and acute toxicity by contact and uptake. Therefore, the removal of aromatic compounds from the discharged effluent has become a major focus of environmental protection and research. Many techniques including adsorption [1,2], biological treatment [3,4], chemical oxidation [3] and photochemical degradation [5] have been applied for the remove of aromatic pollutants from aqueous solution. Among these techniques, adsorption is one of the most effective technologies due to its operational simplicity, effectiveness, low energy requirements and low cost. In the past decade, the development of nanoscience and nanotechnology open up new avenues for the remediation of environmental problems. Nanomaterials based adsorbents have been studied extensively and showed much faster rates and higher efficiency toward pollutants as compared with the conventional adsorbents [6–9]. Various carbonaceous nanomaterials have been adopted as adsorbents for the removal of organic pollutants such as mesoporous carbon [10], carbon nanotubes [1,11], graphene [8] and carbon aerogels [12]. Among them, carbon aerogels were considered as one of the cost-effective carbon materials for water decontamination due to their low cost and excellent capability. Aerogels are unique three-dimensional (3D) solid networks with open pores and majority of the volume is air [13,14]. Carbon aerogels are large class of aerogels which have recently triggered many research activities due to their multifunctional properties such as high surface area, porous structure, low mass densities and high electrical conductivity [15,16]. Because of these special chemical and textural characteristics, carbon aerogels possess promising applications in adsorbents, catalysis and supercapacitors [17]. In recent years, growing attentions have been paid to the design and fabrication of carbon aerogels based composite materials with multifunctional properties for environmental applications. For example, TiO2/carbon aerogels composites have been synthesized and used as electrosorption–photocatalysis synergistic electrode for the degradation of organic pollutants in aqueous solutions [18]. Wang and co-workers prepared ferrite–carbon aerogel composites and applied as Fenton catalysis for the oxidation degradation of metalaxyl [14]. Conventional carbon aerogels are prepared by the pyrolysis of organic aerogels such as resorcinol and formaldehyde polymer aerogels. However, these methods are usually associated with problems of multiple step process and using expensive and poisonous chemical reagents. Thus, to develop a simple and environmental friendliness approach to carbon areogels for environmental applications is still challenging. In this work a green and template free rote was developed for the synthesis of low-cost carbon aerogels and further modified with iron oxide nanoparticles. The obtained magnetite carbon aerogel (MCA) composite material was applied as adsorbent for the efficient removal of ionizable aromatic compounds from aqueous solution. 1-Naphthol and 1-naphthylamine are used as model compounds of the harmful and water soluble aromatic organic pollutants, which are widely used as industrial intermediates [19]. The adsorption properties of 1-naphthol and 1-naphthylamine on MCA are investigated and the mechanisms were also discussed. To the best of our knowledge, it is the first example of using carbon aerogels based nanocomposites for the efficient removal of toxic ionizable aromatic compounds from aqueous solution. And the results showed that MCA can be suitable candidate as cost-effective adsorbents for practical application.
2. Experimental 2.1. Chemicals Chemicals including ferric chloride (anhydrous, FeCl3), ferrous sulfate heptahydrate (FeSO47H2O), ammonium hydroxide (25%,
211
NH3H2O), trisodium citrate (Na3Cit), sodium hydroxide (NaOH) and hydrochloric acid (38%, HCl) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). 1-Naphthol and 1-naphthylamine were purchased from Sigma–Aldrich (St Louis, MO). All above chemicals were used as received without further purification. Milli-Q ultrapure water (18.2 MX cm1) was used for all the experiments. 2.2. Sample preparation The original carbonaceous monolith was prepared by a wellestablished hydrothermal carbonization method by using crude biomass, watermelon, as the carbon source. Watermelon was first cut and put in into a Teflon-lined stainless-steel autoclave and hydrothermal treated at 180 °C for 12 h. After that, the 3D black monolith was taken out and washed with Milli-Q water several times. For preparing magnetite carbon aerogel (MCA), Fe3O4 nanoparticles were first loaded in the above carbonaceous monolith by using a simple solution process [20]. Briefly, a piece of carbonaceous monolith with dimension of about 2 3 3 cm3 was fully immersed in 20 mL aqueous solution of FeSO47H2O (1.25 g, 4.5 mmol) and FeCl3 (1.30 g, 8 mmol) for 15 min under N2 atmosphere. The mixture was heated to 80 °C under N2 protection and 10 mL of 30% ammonia solution was quickly added into the mixture. After that, the mixture was kept at 80 °C for 30 min and 3.0 g of trisodium citrate was added to the solution while the temperature rising to 95 °C. The obtained Fe3O4 loaded carbon monolith was rinsed with Milli-Q water and dried in a vacuum oven at 70 °C over night. The magnetite carbon aerogel (MCA) was obtained by calcination of the above Fe3O4 loaded carbon monolith in nitrogen atmosphere at 550 °C for 4 h. 2.3. Characterization and analysis Transmission electron microscopy (TEM) (JEOL-2010) and field emission scanning electron microscopic (FE-SEM) (JEOL JSM6330F) were applied to observe the microstructure of the original carbon monolith and MCA. X-ray diffraction (XRD) pattern was performed on0 a Philips X’Pert X-ray diffractometer (Cu Ka source (k = 1.54178 Å A)) in the range of 2h = 5–65°. The N2 adsorption– desorption curve of MCA was measured by a Micromeritics ASAP 2010 system at 77 K utilizing Barrett–Emmett–Teller (BET) methods to calculate the specific surface area. The pore size distribution of the MCA was calculated by applying the Barrett–Joyer–Halenda (BJH) model by using the desorption branch of the N2 adsorption– desorption isotherm. Magnetic measurement of the MCA was carried out on a (Quantum Design MPMS XL) magnetometer at 300 K with an applied magnetic field of 20 KOe. The UV–vis measurements were performed on a UV-3000 spectrophotometer (Japan). 2.4. Adsorption experiments The batch adsorption experiments of 1-naphthol and 1-naphthylamine on MCA were carried out at pH = 6.0 in glass vials. Before adsorption, calibration curves of 1-naphthol and 1-naphthylamine was obtained from the UV–vis spectra of the standard 1-naphthol and 1-naphthylamine solutions (5–60 mg L1) at pH 6, respectively. The stock suspensions of MCA, 1-naphthol or 1-naphthylamine solutions were added in the glass vials to achieve the desired concentrations of different components. The desired pH of the suspension in each vial was adjusted by adding negligible volumes of 0.01 or 0.1 M NaOH or HCl with a pH meter. After the suspensions were oscillated for 48 h, the solid and liquid phases were separated by centrifugation at 10,000 rpm for 5 min. The concentration of the aromatic organic compounds in the supernatant was determined by UV–vis spectrophotometer. The adsorbed
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amounts of aromatic organic compounds on MCA were calculated by using the following equation:
qe ¼
ðC 0 C e Þ V m
ð1Þ
where C0 is the initial concentration and Ce is the equilibrium concentration of the aromatic organic compounds (mg L1), respectively, V is the volume of the suspension in each vial (L) and m is the mass of MCA used in each vial (g).
3. Results and discussion 3.1. Characterization Digital pictures of the original carbonaceous monolith are shown in Fig. 1. The base radius of the cylinder carbonaceous monolith is about 4.5 cm and the height is about 12 cm. The dimension of the carbonaceous monolith can be easily controlled by using different volume of the Teflon-lined stainless-steel autoclave for the hydrothermal process. The obtained carbonaceous monolith can also be cut into smaller sizes by a sharp knife. SEM and TEM images show the interconnected microstructure of the carbonaceous monolith consisting of catenulate nanospheres (Fig. 2). The formation of interconnected networks of the carbonaceous monolith could be due to the polymerization and carbonization of carbohydrates in the watermelon during the hydrothermal reaction. The interconnected networks of the carbonaceous monolith can serve as good substrate for the synthesis of functional composite materials by decorating other nanoparticles into the networks. The mass density of the dried carbonaceous monolith is about 0.06 g cm3, indicating the light weight of this carbonaceous material. The Fe3O4 nanoparticles loaded carbonaceous monolith was further transformed into magnetite carbon aerogel (MCA) by calcination. The mass density of MCA was measured to be about 0.031 g cm3. Fig. 3a shows the SEM image of MCA. It can be observed that MCA is also made of catenulate carbon networks and carbon nanosphere networks, indicating that the original carbonaceous monolith kept its microscopic structure during the calcination process. The TEM images (Fig. 3b and c) further show the interconnected microstructure of MCA with decorated Fe3O4 nanoparticles in the networks. As can be seen in Fig. 3c, the Fe3O4 nanoparticles were firmly imbedded into the phase of carbon. When crush the MCA monolith into powder, only a few of Fe3O4 nanoparticles fall off the surface of carbon due to the external mechanical force. The average size of the Fe3O4 nanoparticles is about 9 nm. The XRD pattern of the MCA is shown in Fig. 3d. The typical diffraction peaks at
2h = 30.1°, 35.6°, 43.2°, 53.4°, 57.1° and 62.5° can be assigned to the phase of Fe3O4 (JCPDS 75-0033) [20]. The pore size distribution of MCA obtained from the N2 adsorption–desorption curve is shown in Fig. 4a. The BET specific surface of MCA was measured to be 323.8 m2 g1. The BJH average pore diameter of the MCA is 8.3 nm which is located at the mesoporous region (2–50 nm). The large surface area and the presence of large numbers of micropores and mesopores could provide abundant active sites for the diffusion/adsorption of other phase such as gas and liquids. The XPS analysis was carried out to determine the surface elements states of the MCA. The wide survey scan of XPS spectrum shows the presence of high content of the C, O and Fe elements in the MCA (Fig. 4b). The high resolution XPS spectrum of Fe 2p (Fig. 4c) shows two characteristic peaks at binding energy of 711.2 and 724.7 eV which is attribute to Fe 2p3/2 and Fe 2p1/2, respectively, indicating the presence of Fe3O4. The atomic ratio of iron (Fe%) obtained from the XPS analysis is 1.83 %. The magnetism of the MCA is measured at room temperature within magnetic filed from 20,000 to 20,000 Oe (Fig. 4d). The magnetic remanence (Mr) and the specific saturation magnetization (Ms) of the MCA are obtained from the magnetic hysteresis loop. The value of Mr and Ms. are about 2.3 and 66.9 emu g1, respectively. The small value of the Ms indicates that the MCA possess a superparamagnetic behavior, that is, when the applied magnetic filed is removed that no magnetization remains. It can be clearly observed from the insert in Fig. 4d that the MCA monolith can be easily lifted by a magnet which also reveals the excellent magnetic property of the MCA. The multifunctional properties of MCA could endow its great application potential in the field of adsorbents, catalysis and energy storage. 3.2. Effect of solution pH on adsorption The pH dependent adsorption of 1-naphthol and 1-naphthylamine on MCA is shown in Fig. 5a. The results showed that the solution pH played an important role in the adsorption of 1-naphthol and 1-naphthylamine. As the pH increasing, both the adsorption of 1-naphthol and 1-naphthylamine increased at pH < 4.5 and reach the maximum adsorption percentage at pH = 4.5. While the adsorption gradually declined when solution pH increases from 4.5 to 10. The pH affects the degree of ionization of the adsorbate as well as the surface charge of the adsorbent [21]. The pKa of 1-naphthol and 1-naphthylamine is 9.34 and 3.92, respectively. To further investigate the influence of pH on 1-naphthol and 1-naphthylamine adsorption, the Zeta-potential of the MCA were measured at different pH (Fig. 5b). The electrostatic point of the MCA is about 4.5, indicating that the MCA carries positive charges at pH < 4.5 and negative charge at pH > 4.5. At pH < 4.5, the
Fig. 1. Digital pictures of the original carbonaceous monolith.
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Fig. 2. (a) SEM image and (b) TEM image of the original carbonaceous monolith.
(b)
2 m
(d)
(400)
(220)
Intensity (a.u.)
(222)
100 nm
10
20
30
40
50
(440)
(c)
(422) (511)
10 m
(311)
(a)
60
70
2 Theta (degree) Fig. 3. (a) SEM image of the MCA. TEM images of the MCA at low (b) and high (c) magnification. (d) X-ray diffraction pattern of the MCA.
increase of solution pH increases the dissociation of 1-naphthol and 1-naphthylamine which is beneficial for their interaction with the positive charged MCA. When pH > 4.5, the repulsion between the dissociated 1-naphthol and 1-naphthylamine and the negatively charged MCA increased which reduce the adsorption. The electrostatic interaction between the adsorbates and adsorbent may be the main mechanism for the pH dependent adsorption experiments. At pH < 4, Fe3O4 nanoparticles would partially dissolved (about 21% dissolution of the iron at pH = 2) [22]. At low pH, the dissolved Fe3+ ions may complex with the 1-naphthol and 1-naphthylamine in bulk solution. And it would inhibit the adsorption of the ionic organic compounds onto the surface of MCA. At pH > 4, the dissociation of the iron oxide is negligible and the adsorption processes are mainly dominated by the physical adsorption.
3.3. Adsorption kinetics The adsorption kinetics of 1-naphthol and 1-naphthylamine on MCA were carried out to understand the adsorption properties. The time dependent adsorption data and the corresponding fitting
curves are shown in Fig. 6. As can be seen in Fig. 6a, the adsorption amount of both 1-naphthol and 1-naphthylamine on MCA increased rapidly in 10 h, and then the adsorption rate slowed down until the sorption reached equilibrium at about 40 h. Similarly, Hu et al. [23] found that the adsorption of 1-naphthylamine on multiwalled carbon nanotubes reached adsorption equilibrium at about 50 h. The beginning rapid step of 1-naphthol and 1-naphthylamine adsorption might be due to the surface physical sorption (p–p interactions). While the subsequent slow step could be due to the limited active adsorption sites available on the surface of the adsorbents. The 1-naphthol and 1-naphthylamine molecules first migrate through the solution (i.e. film diffusion), followed by the transportation from the particle surface into the interior sites by pore diffusion [21]. The pore diffusion process is responsible for the relatively long equilibrium time for the adsorption. The pseudo-second-order model was applied to simulate the kinetics of 1-naphthol or 1-naphthylamine adsorption onto MCA (Fig. 6b). The pseudo-second-order kinetic model can be expressed as follows [20]:
t 1 t ¼ þ Q t k2 Q 2e Q e
ð2Þ
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0.20
(a)
(b)
0.15
C 1s
Intensity (a.u.)
Pore volume (cm3/g, dV/dlog(D))
214
0.10
0.05
O 1s Fe 2p
0.00 1
10
200
100
300
Pore diameter (nm)
400
500
600
700
800
Binding Energy (eV) 80
(c)
(d)
Fe 2p3/2
Fe 2p1/2
Intensity (a.u.)
Magnetization (emu/g)
60 40 20 0 -20 -40 -60 -80
740
735
730
725
720
715
710
705
-20000
0
-10000
10000
20000
Magnetic field (Oe)
Binding Energy (eV)
Fig. 4. (a) Pore size distribution of the MCA. (b) XPS survey and (c) Fe 2p XPS spectrum of the MCA. (d) Room temperature magnetic hysteresis loop of the MCA and the insert shows the MCA monolith lifted by a magnet.
(a)
100
(b)
20
Zeta potential (mV)
80
Adsorption (%)
40
60
40
1-Naphthol 1-Naphthylamine
20
0
-20
-40
-60
0 1
2
3
4
5
6
7
8
9
10
11
2
3
4
5
6
7
8
9
10
11
pH
pH
Fig. 5. (a) Effect of solution pH on 1-naphthol and 1-naphthylamine adsorption on MCA. (b) Zeta-potential of MCA as a function of pH. In panel A, m/V = 0.5 g/L, C[1-naphthol]initial = 60 mg/L, C[1-naphthylamine]initial = 60 mg/L.
where Qt and Qe are the amounts of 1-naphthol or 1-naphthylamine adsorbed on per gram of the MCA (mg g1) at time t and equilibrium time (h), respectively, and k2 is the rate constant (g mg1 h). As can be seen in Fig. 6b, the plots of t/Qt versus t for both 1-naphthol and 1-naphthylamine on MCA achieve high correlation coefficient values (R2 = 0.989 and 0.987 for 1-naphthol and 1-naphthylamine, respectively), indicating that the pseudo-second-order kinetic model fits the experimental data very well. The values of Qe calculated from the intercept and slope of the plots of t/Qt versus t are 98.02 mg g1 for 1-naphthol and 50.47 mg g1 for 1-naphthylamine, respectively.
3.4. Adsorption isotherm Adsorption isotherms of 1-naphthol and 1-naphthylamine on MCA are shown in Fig. 7. The experimental data are fitted by Lang 1=n muir QC ee ¼ qm1kL þ qCme and Freundlich (Q e ¼ kC e ) isotherm equations, respectively (where Ce is the equilibrium concentration of 1-naphthol or 1-naphthylamine in the supernatant (mg L1); Qe is the amount of 1-naphthol or 1-naphthylamine adsorbed on per weight of adsorbent after equilibrium (mg g1); qm represents the maximum adsorption capacity of 1-naphthol or 1-naphthylamine
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1.2 80
(a)
1.0
t/q t ( h· g/mg )
Qt (mg/g)
60
40
1-Naphthol 1-Naphthylamine
20
(b) 1-Naphthol 1-Naphthylamine
0.8 0.6 0.4 0.2 0.0
0 0
10
20
30
40
50
60
0
10
Time (h)
20
30
40
50
60
Time (h)
Fig. 6. (a) Time dependent adsorption of 1-naphthol and 1-naphthylamine on MCA. (b) Pseudo-second-order kinetic plots for the adsorption of 1-naphthol and 1-naphthylamine on MCA. (m/V = 0.5 g/L, pH = 6.0, C[1-naphthol]initial = 60 mg/L, C[1-naphthylamine]initial = 60 mg/L.)
100
Qe (mg/g)
80
60
40
1-Naphthol 1-Naphthylamine
20 0
10
20
30
40
50
60
70
80
Ce (mg/L) Fig. 7. Adsorption isotherms of 1-naphthol and 1-naphthylamine on MCA (m/V = 0.5 g/L, pH = 6.0). Symbols denote experimental data, solid line represents the Langmuir model simulation and dashed line represents the Freundlich model simulation.
Table 1 Parameters for the Langmuir and Freundlich isotherm models of 1-naphthol and 1naphthylamine adsorption on MCA. Aromatic organic matter
Langmuir
Freundlich
qm (mg/g)
b (L/mg)
R2
k
n
R2
1-Naphthol 1-Naphthylamine
80.67 63.65
0.92 0.32
0.98 0.94
40.39 24.82
5.12 4.21
0.89 0.96
on per weight of adsorbent (mg g1); kL is the Langmuir constant which represents enthalpy of sorption (L mg1); and the Freundlich constant k and 1/n is correlated to the relative adsorption capacity
of the adsorbent (mg/g) and the adsorption intensity, respectively). The Langmuir and Freundlich isotherm models fitted curves are also shown in Fig. 7. The above parameters obtained from the two models are listed in Table 1. For 1-naphthol adsorption, the relative coefficient (R2) of the Langmuir model was higher than that of the Freundlich model, indicating that the adsorption of 1-naphthol on MCA follows the Langmuir isotherm model but not the Freundlich model. The adsorption data fitted with the Langmuir isotherm model indicates that the adsorption of 1-naphthol on MCA is a monolayer adsorption and the binding energy on the whole surface of MCA is uniform [24]. For 1-naphthylamine adsorption, the experimental data are well fitted by both the Langmuir and Freundlich isotherm models. The values of qm obtained by the Langmuir isotherm model are 80.67 and 63.65 mg g1 for the 1-naphthol and 1-naphthylamine on MCA, respectively. These values are quite in agreement with the experimental data shown in Fig. 7. The high adsorption capacities of the MCA toward both 1-naphthol and 1-naphthylamine could be due to its high surface area and the multifunctional properties. The obtained maximum adsorption capacity of 1-naphthol is higher than that on as prepared multi-walled CNTs (54.35 mg g1) [25], oxide multi-walled CNTs (30.58 mg g1) [25] and comparable to that on XC-72 carbon (83.54 mg g1) [26], and that of 1-naphthylamine is higher than on as prepared multiwalled CNTs (58.82 mg g1) [25], oxide multi-walled CNTs (58.14 mg g1) [25] and modified polymer (15.0 mg g1) [27]. For comparison, the maximum adsorption capacity of 1-naphthol or 1-naphthylamine on MCA or other adsorbents as list in Table 2. It can be seen that the MCA exhibit excellent adsorption capacity toward both 1-naphthol and 1-naphthylamine. The reusability of the MCA was also investigated by a simple and well-developed Fenton reaction [29]. Generally, 0.1 g of 1-naphthol or 1-naphthylamine adsorbed MCA was separated by a magnet and the wet black slurry
Table 2 Comparison of the maximum absorption capacities of 1-naphthol and 1-nNaphthylamine adsorption on MCA with other adsorbents. Adsorbents
Adsorbates
Adsorption capacity (mg g1)
pH
Ref.
MWCNTs Oxide MWCNTs XC-72 carbon MWCNTs/Fe3O4/CD MCA MWCNTs Oxide MWCNTs Modified polymer MCA
1-Naphthol 1-Naphthol 1-Naphthol 1-Naphthol 1-Naphthol 1-Naphthylamine 1-Naphthylamine 1-Naphthylamine 1-Naphthylamine
54.35 30.58 83.54 57.47 80.67 58.82 58.14 15.0 63.65
6 6 7 5.5 6 6 6 – 6
[25] [25] [26] [28] Our work [25] [25] [27] Our work
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was redispersed in 200 mL H2O2 (8.3 102 M) and the pH was adjusted to 3.0 by 0.1 M HNO3. The suspension was stirred constantly for 8 h at 80 °C and after that the solid was separated by a magnet and dried in an oven at 80 °C. The above adsorption– regeneration process was repeated for 5 times. It was found that about 85% of the adsorption capacity was retained for both 1-naphthol and 1-naphthylamine after 5 cycles of reuse. The totally green synthetic method used, low-cost of the crude material and easily recyclable and reusable properties indicate that MCA could be a promising material for the removal of ionizable aromatic organic pollutants from aqueous solution. For further applications, MCA might be extended to treat other kinds of organic pollutants in future practical applications. 4. Conclusion In this study, a biomass-derived carbon monolith was prepared by a facile hydrothermal process. After that, magnetite nanoprticles were incorporated into the networks of the carbon monolith and further transformed into magnetite carbon aerogel (MCA). The obtained MCA is of very light weight and show high surface area and magnetic property. The multifunctional MCA showed excellent capacity for efficient removal of 1-naphthol or 1-naphthylamine from aqueous solution. The adsorbed MCA can be easily regenerated and reused which provide great potential as adsorbent for the removal of ionizable organic pollutants from aqueous solutions. And can be extended to treat other kinds of organic pollutants in future practical applications. Acknowledgements We acknowledge the financial support from National Natural Science Foundation of China (21272236, 21225730, 212207136), Hefei Center for physical science and technology (2012FXZY005) and the USTC Special Grant for Postgraduate Research, Innovation and Practice. References [1] K. Yang, W. Wu, Q. Jing, L. Zhu, Aqueous adsorption of phenol, and their substitutes by multi-walled carbon nanotubes, Environ. Sci. Technol. 42 (2008) 7931–7936. [2] W. Zhang, J. Chen, B. Pan, Q. Zhang, Cooperative adsorption behaviours of 1naphthol and 1-naphthylamine onto nonpolar macroreticular adsorbents, React. Funct. Polym. 66 (2006) 485–493. [3] R. Rosal, A. Rodríguez, J.A. Perdigón-Melón, A. Petre, E. García-Calvo, M.J. Gómez, A. Agüera, A.R. Fernández-Alba, Occurence of emerging pollutants in urban wastewater and their removal through biological treatment followed by ozonation, Water Res. 44 (2010) 578–588. [4] I. Mozo, G. Lesage, J. Yin, Y. Bessiere, L. Barna, M. Sperandio, Dynamic modeling of biodegradation and volatilization of hazardous aromatic substances in aerobic bioreactor, Water Res. 46 (2012) 5327–5342. [5] K. Nagaveni, G. Sivalingam, M.S. Hegde, G. Madras, Photocatalytic degradation of organic compounds over combustion-synthesized nano-TiO2, Environ. Sci. Technol. 38 (2004) 1600–1604. [6] J. Gong, T. Liu, X. Wang, X. Hu, L. Zhang, Efficient removal of heavy metal ions from aqueous systems with the assembly of anisotropic layered double hydroxide nanocrystals@carbon nanosphere, Environ. Sci. Technol. 45 (2011) 6181–6187.
[7] L. Wang, X.L. Wu, W.H. Xu, X.J. Huang, J.H. Liu, A.W. Xu, Stable organicinorganic hybrid of polyaniline/a-zirconium phosphate for efficient removal of organic pollutants in water environment, ACS Appl. Mater. Interf. 4 (2012) 2686–2692. [8] G.X. Zhao, L. Jiang, Y. He, J. Li, H. Dong, X. Wang, W. Hu, Sulfonated graphene for persistent aromatic pollutant management, Adv. Mater. 23 (2011) 3959–3963. [9] V. Chandra, J. Park, Y. Chun, J.W. Lee, I.C. Hwang, K.S. Kim, Water-dispersible magnetite-reduced graphene oxide composites for arsenic removal, ACS Nano. 4 (2011) 3979–3986. [10] L. Ji, F. Liu, Z. Xu, S. Zheng, D. Zhu, Adsorption of pharmaceutical antibiotics on template-synthesized ordered micro-and mesoporous carbons, Environ. Sci. Technol. 44 (2010) 3116–3122. [11] J. Chen, W. Chen, D. Zhu, Adsorption of nonionic aromatic compounds to single-walled carbon nanotubes: effects of aqueous solution chemistry, Environ. Sci. Technol. 42 (2008) 7225–7230. [12] D. Fairén-Jiménez, F. Carrasco-Marín, C. Moreno-Castilla, Adsorption of benzene, toluene, and xylenes on monolithic carbon aerogels from dry air flows, Langmuir 23 (2007) 10095–10101. [13] H.C. Chien, W.Y. Cheng, Y.H. Wang, S.Y. Lu, Ultrahigh specific capacitances for supercapacitors achieved by nickel cobaltite/carbon aerogel composites, Adv. Funct. Mater. 22 (2012) 5038–5043. [14] Y. Wang, G. Zhao, S. Chai, H. Zhao, Y. Wang, Three-dimensional homogeneous ferrite–carbon aerogel: one pot fabrication and enhanced electro-fenton reactivity, ACS Appl. Mater. Interf. 5 (2013) 842–852. [15] M.A. Worsley, J.H. Satcher, T.F. Baumann, Synthesis and characterization of monolithic carbon aerogel nanocomposites containing double-walled carbon nanotubes, Langmuir 24 (2008) 9763–9766. [16] S. Wei, D. Wu, X. Shang, R. Fu, Studies on the structure and electrochemical performance of Pt/carbon aerogel catalyst for direct methanol fuel cells, Energy Fuels 23 (2009) 908–911. [17] T. Bordjiba, M. Mohamedi, L.H. Dao, New class of carbon-nanotube aerogel electrodes for electrochemical power sources, Adv Mater. 20 (2008) 815–819. [18] M. Wu, Y. Jin, G. Zhao, M. Li, D. Li, Electrosorption-promoted photodegradation of opaque wastewater on a novel TiO2/carbon aerogel electrode, Environ. Sci. Technol. 44 (2010) 1780–1785. [19] W. Zhang, C. Hong, B. Pan, Z. Xu, Q. Zhang, L. Lv, Removal enhancement of 1naphthol and 1-naphthylamine in single and binary aqueous phase by acid– basic interactions with polymer adsorbents, J. Hazard. Mater. 158 (2008) 293– 299. [20] X.L. Wu, L. Wang, C.L. Chen, A.W. Xu, X.K. Wang, Water-dispersible magnetitegraphene–LDH composites for efficient arsenate removal, J. Mater. Chem. 21 (2011) 17353–17359. [21] B.H. Hameed, A.A. Rahman, Removal of phenol from aqueous solutions by adsorption onto activated carbon prepared from biomass material, J. Hazard. Mater. 160 (2008) 576–581. [22] Q. Chang, W. Lin, W.C. Ying, Preparation of iron-impregnated granular activated carbon for arsenic removal from drinking water, J. Hazard. Mater. 184 (2010) 515–522. [23] J. Hu, D. Shao, C. Chen, G. Sheng, X. Ren, X. Wang, Removal of 1-naphthylamine from aqueous solution by multiwall carbon nanotubes/iron oxides/ cyclodextrin composite, J. Hazard. Mater. 185 (2011) 463–471. [24] X.L. Wu, D.L. Zhao, S.T. Yang, Impact of solution chemistry conditions on the sorption behavior of Cu(II) on Lin’an montmorillonite, Desalination 269 (2011) 84–91. [25] G.D. Sheng, D.D. Shao, X.M. Ren, X.Q. Wang, J.X. Li, Y.X. Chen, X.K. Wang, Kinetics and thermodynamics of adsorption of ionizable aromatic compounds from aqueous solutions by as-prepared and oxidized multiwalled carbon nanotubes, J. Hazard. Mater. 178 (2010) 505–516. [26] L.M. Zuo, S.M. Yu, L. Cheng, E. Du, Adsorption of phenol and 1-naphthol onto XC-72 carbon, Korean J. Chem. Eng. 30 (2013) 714–723. [27] D. Zhao, L. Zhao, C.S. Zhu, X. Shen, X. Zhang, B. Sha, Comparative study of polymer containing b-cyclodextrin and –COOH for adsorption toward aniline, 1-naphthylamine and methylene blue, J. Hazard. Mater. 171 (2009) 241–246. [28] J. Hu, D. Shao, C. Chen, G. Sheng, J. Li, X. Wang, M. Nagatsu, Plasma-induced grafting of cyclodextrin onto multiwall carbon nanotube/iron oxides for adsorbent application, J. Phys. Chem. B 114 (2010) 6779–6785. [29] M.H. Do, N.H. Phan, T.D. Nguyen, T.T.S. Pham, V.K. Nguyen, T.T.T. Vu, T.K.P. Nguyen, Activated carbon/Fe3O4 nanoparticle composite: fabrication, methyl orange removal and regeneration by hydrogen peroxide, Chemosphere 85 (2011) 1269–1276.