- Email: [email protected]
C Nanocomposite for Adsorption and Separation of Organic Contaminants from Water
CHINESE JOURNAL OF ANALYTICAL CHEMISTRY Volume 44, Issue 2, February 2016 Online English edition of the Chinese language journal
Cite this article as: Chin J Anal Chem, 2016, 44(2), 224–231.
RESEARCH PAPER
An Acid‐resistant Magnetic Co/C Nanocomposite for Adsorption and Separation of Organic Contaminants from Water RUAN Chang-Ping1,2, AI Ke-Long1,*, LU Le-Hui1 1
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China 2 University of Chinese Academy of Sciences, Beijing 100049, China
Abstract: Magnetic adsorbents have recently been extensively investigated and applied in the field of water purification, because of their magnetic characters which are advantageous for the separation and recycle of these materials. Unfortunately, common magnetic materials are unstable and prone to dissolution in acid environment, thus limiting their practical applications in wide pH range, particularly in acidic condition. Therefore，it is highly imperative to exploit a novel magnetic adsorbent that is acid-resistant, to simplify separation process during the water purification. In present work, an acid-resistant magnetic Co/C nanocomposite was synthesized by using ZIF-67 as both template and precursor. The ZIF-67 was carbonized in an argon atmosphere at 800 ºC for 1 h, and then treated with acid. Upon calcination at an appropriate temperature in inert atmosphere, the generated Co nanoparticles were uniformly wrapped by graphite layers, due to the graphitization of carbon upon the catalysis effect of Co. The formed graphite layers were able to protect the Co particles from oxidation and acid environment, thus resulting in the generation of an acid-resistant magnetic adsorbent that can be applied in a wide pH range (pH 1‒13). Remarkably, the as-synthesized magnetic Co/C nanocomposite demonstrated excellent adsorption performance towards two typical organic dyes (rhodamine B and malachite green) over a wide pH range. The adsorption isotherms of rhodamine B and malachite green on Co/C nanocomposite were well fitted with the Langmuir model. Impressively, the maximum adsorption capacities towards rhodamine B and malachite green were estimated to be 400.0 and 561.8 mg g–1, respectively, far exceeding many previously reported adsorbents. Moreover, the adsorbent could be easily regenerated by washing with ethylene glycol (EG), suggesting its excellent reusability. Even after 5 cycles of reuse, no obvious capacity degradation was observed. Furthermore, practical application of the magnetic adsorbent was demonstrated by the removal of organic dyes from domestic wastewater with a superior removal efficiency of higher than 97%. Key Words:
1
Magnetic adsorbent; Acid resistance; Organic contaminants; Water purification
Introduction
With increasing industrial development, various environmental problems have caused a great negative impact on human survival and development[1–8]. Water pollution is one of the most challenging environmental issues, which not only causes serious environmental and ecological damage, but
also threatens human health. Organic dyes are one kind of the most extensively existed in organic contaminants, which are widely used in various industries, such as textiles, paper, tannery, plastics, and paints[2–9]. It is estimated that more than 700000 tons of organic dyes are produced every year, around 10%–15% of which are released into environment[8]. Not only deterioration in water quality, but also adverse effects on
________________________ Received 6 July 2015; accepted 16 July 2015 *Corresponding author. Email: [email protected] This work is supported by the National Natural Science Foundation of China (No. 21125521), the National Basic Research Program of China (973 Program, No.2010CB933600), and the Jilin Province Youth Foundation of China (No. 20130522131JH). Copyright © 2016, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-2040(16)60905-2
RUAN Chang-Ping et al. / Chinese Journal of Analytical Chemistry, 2016, 44(2): 224–231
human health would be caused by these dyes. Consequently, considerable attentions have been paid to the detection, enrichment and removal of these dye pollutants in wastewater in recent years. Among various wastewater treatment techniques (including photocatalytic degradation, precipitation, oxidative degradation and adsorption), adsorption by various porous adsorbents is one of the most efficient, economical, and easy-handling strategies. Diverse adsorbents have been developed and used for the removal of organic dyes, including activated carbon, porous silica, zeolite and porous polymers[2–9]. Activated carbon is one of the most extensively used adsorbents, because of its high specific surface area, porosity, and nontoxicity. However, the separation and recovery of activated carbon from water after the adsorption process remains a concern because the activated carbon is usually in powdered or granular form[5,6]. Recently, it was proven that the incorporation of magnetic material into adsorbents was beneficial for fast separation and convenient recovery of these adsorbents[8–15]. The magnetic separation will not only simplify the separation process, but also reduce the energy consumption by avoiding complicated separation steps, such as filtration, precipitation and centrifugation. A magnetic graphene/iron oxide nanocomposite (G/Fe3O4) was prepared by Lu’s group and demonstrated excellent performance for adsorption and separation of rhodamine B and malachite green from aqueous solutions[8]. However, common magnetic nanoparticles, including metal particles and metal oxides of Fe, Co, Ni and Mn, are unstable and prone to dissolution in acid environment, thus limiting their practical applications in acidic condition. Therefore，it is highly imperative to develop a novel magnetic adsorbent that is stable in acid environment, not only to simplify the separation process, but also to expand its application range. In the present work, a typical metal-organic framework (MOF), zeolitic imidazolate framework material (ZIF-67) was used as both template and precursor for the preparation of an acid-resistant magnetic Co/C nanocomposite[14,16]. Upon calcination at high temperature in inert atmosphere, the generated Co nanoparticles were uniformly wrapped by graphite layers, due to the graphitization of carbon under the catalysis of Co. The formed graphite layers could protect the Co particles from oxidation and acid environment, thus resulting in the generation of an acid-resistant magnetic adsorbent that could be applied in a wide pH range (pH 1‒13). Impressively, the as-synthesized magnetic Co/C nanocomposites demonstrated excellent adsorption performance towards two typical organic dyes (rhodamine B and malachite green) over a wide pH range.
2
Experimental
2.1
Instruments and reagents
Cobalt nitrate (Co(NO3)2·6H2O) and 2-methylimidazole
were purchased from Aladdin Regeants Co. Ltd (Shanghai, China). Methanol was bought from Beijing Chemical Works (China). All these reagents were used as received without further purification. The deionized water (18.2 MΩ cm) was purified by Milli-Q system (Millipore Co., USA). The X-ray diffraction (XRD) patterns were recorded on a D8 ADVANCE (Germany) using Cu-Kα (0.15406 nm) radiation. The X-ray photoelectron spectroscopy (XPS) measurements were performed on a VG ESCALAB MKIIX-ray photoelectron spectrometer. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were collected on a Hitachi H-8100 TEM system. UV-Vis absorption spectra were collected on CARY 500 UV-Vis-NIR spectrophotometer. 2.2
Synthesis of materials
In a typical synthesis of ZIF-67, 24.38 g of 2-methylimidazole and 24.56 g of Co(NO3)2·6H2O were dissolved into 1 L of methanol, respectively. Then, the two solutions were mixed together and stirred for a few minutes. After being stored at room temperature for 24 hours without any disturbance, the formed purple powders at the bottom were collected by centrifugation and washed with methanol for several times. The obtained purple powders were dried at 110 ºC. During the carbonization process, the dried ZIF-67 was heated to 800 ºC at a heating rate of 5 ºC min-1 and maintained at this temperature for 1 h under Ar atmosphere. After cooling down to room temperature, the magnetic black powder was treated by 6 M HCl for 12 hours to remove the unstable metal particles and metal oxides. The resultant black powder was washed with deionized water for several times to remove any impurities and then dried at 110 ºC to obtain the magnetic Co/C nanocomposite. 2.3
Adsorption experiments
Aqueous solutions with different concentrations of dyes were treated by magnetic Co/C nanocomposites to remove the dyes. The mixtures were shaken vigorously for a period of time, and then separated by an external magnetic field. The concentrations of dyes before and after the treatment were monitored by UV-Vis absorption spectra. The removal efficiency was estimated according to changes of dye concentrations before and after the treatment. The adsorption capacity at equilibrium (qe, mg g‒1) was calculated by the following equation: qe = (Ci ‒ Ce)V/m (1) where, V is the volume of the treated solution (mL), m is the weight of used adsorbent (g), and Ci and Ce are the initial concentration and final equilibrium concentration of dyes, respectively. All the adsorption experiments were carried out at room temperature.
RUAN Chang-Ping et al. / Chinese Journal of Analytical Chemistry, 2016, 44(2): 224–231
3
Results and discussion
3.1
Optimization of synthetic condition
ZIF-67 is a typical MOF material with high porosity. Upon calcination at high temperature in inert atmosphere, the organic component in ZIF-67 was degraded and carbonized; whereas Co(II) was reduced to Co(0) nanoparticles[14,16,17]. The generated Co nanoparticles in the present work were uniformly wrapped by graphite layers, due to the graphitization of carbon under the catalysis of Co. The formed graphite layers were able to protect the Co particles from oxidation and acid environment. An appropriate temperature during the carbonization process is crucial, not only for the generation of magnetic Co particles with good stability and crystallinity, but also for the formation of protective graphite layers. To optimize the carbonization temperatures, five samples synthesized at different temperatures (500, 600, 700, 800 and 900 ºC) were prepared and investigated. The XRD patterns (Fig.1a) indicated that the protective graphite layers couldn’t form at relatively low temperatures (500, 600 and 700 ºC). The crystallinity of the resultant composite would be gradually improved with the increasing carbonization temperature. The sample prepared at 900 ºC demonstrated the highest graphitization degree and crystallinity, as evidenced by the corresponding diffraction peaks of graphite and Co nanoparticles (XRD, Fig.1a)[17]. However, the higher temperature (900 ºC) would lead to the degradation of adsorption performance (Fig.1b) because of the reduced specific surface area. Upon increasing the temperature from 800 ºC to 900 ºC, the specific surface area of nanocomposite was reduced from 654 m2 g–1 to 592 m2 g–1. The sample synthesized at 800 ºC exhibited the best removal efficiency (Fig.1b). Therefore, the sample prepared at a carbonization temperature of 800 ºC was used in the present work. 3.2
Characterization of materials
The structure and composition of the optimized sample was
Fig.1
investigated by TEM, XRD and XPS characterizations, as shown in Fig.2. The XRD patterns revealed that the sample had three characteristic peaks for metallic Co and one sharp characteristic peak (002) of graphitic carbon, agreeing well with previous reports[14,16,17]. XPS survey spectrum indicated that the Co/C nanocomposites were mainly composed of C, N and Co elements. High-resolution XPS further revealed that sp2 C was the dominant component of C and Co was composed of a majority of zero-valance Co and a small amount of Co(II), which was in well agreement with the XRD data. The size of ZIF-67 was in the range of 300–400 nm (see TEM image, inset in Fig.2a). After carbonization, the resultant Co/C nanocomposites were approximately 300 nm, as revealed by the corresponding TEM image (Fig.2a). High-resolution TEM (HRTEM) image showed that Co nanoparticles (about 10 nm) were uniformly wrapped by graphite layers. Moreover, the saturation magnetization of Co/C nanocomposites was determined to be 34 emu g‒1. Therefore, Co/C nanocomposites could be easily separated from the aqueous dispersion by external magnetic field. Magnetic separation will simplify the separation process by avoiding complicated separation steps[8]. 3.3
Acid-resistant ability
After acid-washing, unstable Co and CoO nanoparticles were washed off and the remaining Co nanoparticles were uniformly wrapped and protected by graphite layers. These graphite layers could protect the Co nanoparticles from oxidation and acid environment. Consequently, we presumed that the Co/C nanocomposites were acid-resistant and could be applied in a wide pH range. To verify this assumption, we investigated the acid-resistant property of Co/C nanocomposites by studying its stability in various pH conditions. As a comparison, the stability of G/Fe 3 O 4 was also investigated through exactly the same method. Remarkably, as shown in Fig.3, the as-synthesized Co/C nanocomposites demonstrated excellent stability in the pH range of 1–13, indicating their superior acid-resistant ability. ICP analysis
(a) XRD patterns of Co/C nanocomposites prepared at different temperatures and (b) Removal of rhodamine B by using Co/C nanocomposites prepared at different temperatures
RUAN Chang-Ping et al. / Chinese Journal of Analytical Chemistry, 2016, 44(2): 224–231
Fig.2 Characterization of Co/C nanocomposites. (a) TEM image, inset: TEM image of ZIF-67; (b) HR-TEM image of Co/C nanocomposites; (c) XRD patterns of Co/C nanocomposites before and after acid-washing; (d) C 1s XPS spectra; (e) N 1s XPS spectra; (f) Co 2p XPS spectra
Fig.3 Stability of Co/C nanocomposites (in comparison with G/Fe3O4). (a) Concentration of released metal ions at different pH values, (b) Stability of two absorbents in acid environment
revealed that no obvious Co leakage occurred during the test. In stark contrast, G/Fe3O4 had inferior stability in acid environment and was only stable in neutral and alkali environment. When the pH value was lower than 5, obvious leakage of Fe was observed. At pH lower than 3, almost all of
the metal oxide (Fe3O4) in G/Fe3O4 composites was dissolved. Therefore, compared with G/Fe3O4, the as-synthesized Co/C nanocomposites were expected to be applied in a much wider pH range. After adsorption of organic dyes (rhodamine B and malachite green, respectively), the stability of used Co/C
RUAN Chang-Ping et al. / Chinese Journal of Analytical Chemistry, 2016, 44(2): 224–231
nanocomposites was also investigated by treatment with an acid solution (pH 1). As expected, no obvious Co leakage occurred, further proving the excellent acid-resistance of Co/C nanocomposites after the adsorption of organic dyes. 3.4
Effect of time and pH
The effect of treatment time was firstly investigated to optimize the treatment process. The concentrations of dyes at different time intervals were monitored by UV-vis absorption spectra. As shown in Fig.4a, after the addition of Co/C nanocomposites, the characteristic UV-vis absorption of rhodamine B (maximum absorption wavelength at 554 nm) decreased quickly, suggesting that Co/C nanocomposites had fast adsorption kinetics towards rhodamine B. The UV-vis absorption of rhodamine B reached a stable value after 30 min of treatment, revealing that the adsorption of rhodamine B on Co/C nanocomposites reached equilibrium within 30 min (Fig.4b). The adsorption of malachite green on Co/C
Fig.4
nanocomposites was also saturated within 30 min. Therefore, the optimal treatment time in the present work was 30 min. Considering that the Co/C nanocomposites were stable in a wide pH range, adsorption performance of Co/C nanocomposites (towards rhodamine B and malachite green) at three different pH values (pH 2, pH 7 and pH 12) was investigated to probe the effect of pH (Because malachite green was unstable under high-alkaline environment, the adsorption behavior towards malachite green was only investigated under neutral and acidic conditions). The removal efficiency of rhodamine B and malachite green both increased with the increase of pH values (Fig.4, c‒f). Such observation might be attributed to the structural features of dyes. Both rhodamine B and malachite green are positively charged. Upon increasing the pH values, the functional groups on Co/C nanocomposites might be deprotonated, thus resulting in increased negative charge density on Co/C nanocomposites. As a consequence, electrostatic interaction between Co/C nanocomposites and positively charged dyes was enhanced with the increase of
(a, b) Removal efficiency of rhodamine B (1 mg mL‒1) at different intervals; (c, d) Effect of pH values on the removal of rhodamine B (1 mg mL‒1); (e, f) Effect of pH values on the removal of malachite green (1 mg mL‒1)
RUAN Chang-Ping et al. / Chinese Journal of Analytical Chemistry, 2016, 44(2): 224–231
parameters were listed in Table 1. The adsorption isotherms of rhodamine B and malachite green on Co/C nanocomposite were well fitted with the Langmuir model (R2 > 0.959). Impressively, the maximum adsorption capacities towards rhodamine B and malachite green were estimated to be 400 and 561.8 mg g‒1, respectively, far exceeding many previously reported adsorbents (Table 2), such as G/Fe3O4[8], polydopamine-functionalized graphene hydrogel[5], biomassderived activated carbon[18].
pH values, which led to higher removal efficiency of dyes at higher pH values. 3.5
Adsorption performance
The as-synthesized magnetic Co/C nanocomposite demonstrated excellent acid-resistance and good removal efficiency towards rhodamine B and malachite green over a wide pH range. Subsequently, adsorption performance of Co/C nanocomposite towards rhodamine B and malachite green at various pH values were investigated in detail. The corresponding adsorption isotherms were presented in Fig.5. An adsorption isotherm expresses the relationship between adsorption capacities (qe, mg g‒1) and the concentrations of adsorbate (Ce, mg L‒1) at a given temperature under equilibrium conditions[8,15]. The corresponding adsorption data in present work were analyzed using two well-known isotherm models, Langmuir isotherm model and Freundlich isotherm model, respectively. The corresponding fitting
3.6
Regeneration and reusability
The regeneration and reusability of the adsorbent are quite crucial for practical applications, especially in terms of cost control. We found that the adsorbent could be easily regenerated by washing with ethylene glycol (EG). Even after 5 cycles of reuse, no obvious capacity degradation was observed (Fig.6), suggesting the excellent reusability of Co/C nanocomposites.
Table 1 Langmuir and Freundlich model parameters for the adsorption of rhodamine B and malachite green at different pH values Isotherm model
Malachite green
Isotherm contants
pH 7
pH 2
pH 7
pH 12
qmax (mg g )
561.8
561.8
400.0
395.3
389.1
KL (mL mg-1)
6.684
42.329
6.419
8.714
15.541
R2
0.9662
0.9920
0.9591
0.9620
0.9761
k
484.5
878.0
309.4
337.1
351.2
n
2.602
2.818
3.943
3.597
4.204
0.9115
0.6901
0.9824
0.9821
0.8156
-1
Langmuir
Freundlich
Rhodamine B
pH 2
2
R
Fig.5 Adsorption isotherms of rhodamine B (a) and malachite green (b) on Co/C nanocomposites at different pH values Table 2 Comparision of adsorption performances of Co/C nanocomposites with some previous reported adsorbents Dyes
Rhodamine B
Malachite green
qmax (mg g‒1)
Adsorbents
Removal efficiency
Adsorption time (h)
Reusability
Co/C nanocomposite
> 99%
400.0
0.5
Yes (EG)
G/Fe3O4[8]
91%
13.15
2.0
Yes (EG)
Graphene hydrogel[5]
——
207.06
12.0
Yes (Ethanol)
Co/C nanocomposite G/Fe3O4[8]
> 99% 94%
561.8 22.0
0.5 2.0
Yes (EG) Yes (EG)
Biomass-derived activated carbon[18]
99%
48.48
24.0
No
RUAN Chang-Ping et al. / Chinese Journal of Analytical Chemistry, 2016, 44(2): 224–231
3.7
Fig.6
Reusability of Co/C nanocomposites after washed by EG (concentration of dyes: 1 mg mL‒1)
Application in real water samples
The as-synthesized magnetic Co/C nanocomposites have demonstrated excellent acid-resistance and adsorption performance towards rhodamine B and malachite green over a wide pH range. Magnetic separation of organic dyes (rhodamine B and malachite green) from water was achieved, with removal efficiency higher than 99% (Fig.7). Furthermore, the feasibility of magnetic separation by using Co/C nanocomposite in practical application was also demonstrated by the removal of organic dyes (rhodamine B and malachite green) from domestic wastewater with a superior removal efficiency of higher than 97%.
Fig.7 Removal of (a, b) rhodamine B and (c, d) malachite green by using magnetic Co/C nanocomposites and magnets
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Zhao D Y. Angew. Chem. Int. Ed., 2014, 53(11): 2888–2892 [12] Chen S, Yao J L, Guo Q H, Gu R A. Spectroscopy and Spectral Analysis, 2011, 31(12): 3169‒3174 [13] Li Y Z, Li Z Q, Zhu H L, Wang W P. Chinese J. Anal. Chem., 2015, 43(12): 1882‒1887 [14] Torad N L, Hu M, Ishihara S, Sukegawa H, Belik A A, Imura M, Ariga K, Sakka Y, Yamauchi Y. Small, 2014, 10(10): 2096–2107 [15] Wang P, Shi Q H, Shi Y F, Clark K K, Stucky G D, Keller A A. J. Am. Chem. Soc., 2009, 131(1): 182–188 [16] Lin K Y, Hsu F K, Lee W D. J. Mater. Chem. A, 2015, 3(18): 9480–9490 [17] Xia W, Zhu J H, Guo W H, An L, Xia D G, Zou R Q. J. Mater. Chem. A, 2014, 2(30): 11606–11613 [18] Nethaji S, Sivasamy A, Thennarasu G, Saravanan S. J. Hazard. Mater., 2010, 181(1-3): 271–280
Cite this article as: Chin J Anal Chem, 2016, 44(2), 224–231.
RESEARCH PAPER
An Acid‐resistant Magnetic Co/C Nanocomposite for Adsorption and Separation of Organic Contaminants from Water RUAN Chang-Ping1,2, AI Ke-Long1,*, LU Le-Hui1 1
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China 2 University of Chinese Academy of Sciences, Beijing 100049, China
Abstract: Magnetic adsorbents have recently been extensively investigated and applied in the field of water purification, because of their magnetic characters which are advantageous for the separation and recycle of these materials. Unfortunately, common magnetic materials are unstable and prone to dissolution in acid environment, thus limiting their practical applications in wide pH range, particularly in acidic condition. Therefore，it is highly imperative to exploit a novel magnetic adsorbent that is acid-resistant, to simplify separation process during the water purification. In present work, an acid-resistant magnetic Co/C nanocomposite was synthesized by using ZIF-67 as both template and precursor. The ZIF-67 was carbonized in an argon atmosphere at 800 ºC for 1 h, and then treated with acid. Upon calcination at an appropriate temperature in inert atmosphere, the generated Co nanoparticles were uniformly wrapped by graphite layers, due to the graphitization of carbon upon the catalysis effect of Co. The formed graphite layers were able to protect the Co particles from oxidation and acid environment, thus resulting in the generation of an acid-resistant magnetic adsorbent that can be applied in a wide pH range (pH 1‒13). Remarkably, the as-synthesized magnetic Co/C nanocomposite demonstrated excellent adsorption performance towards two typical organic dyes (rhodamine B and malachite green) over a wide pH range. The adsorption isotherms of rhodamine B and malachite green on Co/C nanocomposite were well fitted with the Langmuir model. Impressively, the maximum adsorption capacities towards rhodamine B and malachite green were estimated to be 400.0 and 561.8 mg g–1, respectively, far exceeding many previously reported adsorbents. Moreover, the adsorbent could be easily regenerated by washing with ethylene glycol (EG), suggesting its excellent reusability. Even after 5 cycles of reuse, no obvious capacity degradation was observed. Furthermore, practical application of the magnetic adsorbent was demonstrated by the removal of organic dyes from domestic wastewater with a superior removal efficiency of higher than 97%. Key Words:
1
Magnetic adsorbent; Acid resistance; Organic contaminants; Water purification
Introduction
With increasing industrial development, various environmental problems have caused a great negative impact on human survival and development[1–8]. Water pollution is one of the most challenging environmental issues, which not only causes serious environmental and ecological damage, but
also threatens human health. Organic dyes are one kind of the most extensively existed in organic contaminants, which are widely used in various industries, such as textiles, paper, tannery, plastics, and paints[2–9]. It is estimated that more than 700000 tons of organic dyes are produced every year, around 10%–15% of which are released into environment[8]. Not only deterioration in water quality, but also adverse effects on
________________________ Received 6 July 2015; accepted 16 July 2015 *Corresponding author. Email: [email protected] This work is supported by the National Natural Science Foundation of China (No. 21125521), the National Basic Research Program of China (973 Program, No.2010CB933600), and the Jilin Province Youth Foundation of China (No. 20130522131JH). Copyright © 2016, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-2040(16)60905-2
RUAN Chang-Ping et al. / Chinese Journal of Analytical Chemistry, 2016, 44(2): 224–231
human health would be caused by these dyes. Consequently, considerable attentions have been paid to the detection, enrichment and removal of these dye pollutants in wastewater in recent years. Among various wastewater treatment techniques (including photocatalytic degradation, precipitation, oxidative degradation and adsorption), adsorption by various porous adsorbents is one of the most efficient, economical, and easy-handling strategies. Diverse adsorbents have been developed and used for the removal of organic dyes, including activated carbon, porous silica, zeolite and porous polymers[2–9]. Activated carbon is one of the most extensively used adsorbents, because of its high specific surface area, porosity, and nontoxicity. However, the separation and recovery of activated carbon from water after the adsorption process remains a concern because the activated carbon is usually in powdered or granular form[5,6]. Recently, it was proven that the incorporation of magnetic material into adsorbents was beneficial for fast separation and convenient recovery of these adsorbents[8–15]. The magnetic separation will not only simplify the separation process, but also reduce the energy consumption by avoiding complicated separation steps, such as filtration, precipitation and centrifugation. A magnetic graphene/iron oxide nanocomposite (G/Fe3O4) was prepared by Lu’s group and demonstrated excellent performance for adsorption and separation of rhodamine B and malachite green from aqueous solutions[8]. However, common magnetic nanoparticles, including metal particles and metal oxides of Fe, Co, Ni and Mn, are unstable and prone to dissolution in acid environment, thus limiting their practical applications in acidic condition. Therefore，it is highly imperative to develop a novel magnetic adsorbent that is stable in acid environment, not only to simplify the separation process, but also to expand its application range. In the present work, a typical metal-organic framework (MOF), zeolitic imidazolate framework material (ZIF-67) was used as both template and precursor for the preparation of an acid-resistant magnetic Co/C nanocomposite[14,16]. Upon calcination at high temperature in inert atmosphere, the generated Co nanoparticles were uniformly wrapped by graphite layers, due to the graphitization of carbon under the catalysis of Co. The formed graphite layers could protect the Co particles from oxidation and acid environment, thus resulting in the generation of an acid-resistant magnetic adsorbent that could be applied in a wide pH range (pH 1‒13). Impressively, the as-synthesized magnetic Co/C nanocomposites demonstrated excellent adsorption performance towards two typical organic dyes (rhodamine B and malachite green) over a wide pH range.
2
Experimental
2.1
Instruments and reagents
Cobalt nitrate (Co(NO3)2·6H2O) and 2-methylimidazole
were purchased from Aladdin Regeants Co. Ltd (Shanghai, China). Methanol was bought from Beijing Chemical Works (China). All these reagents were used as received without further purification. The deionized water (18.2 MΩ cm) was purified by Milli-Q system (Millipore Co., USA). The X-ray diffraction (XRD) patterns were recorded on a D8 ADVANCE (Germany) using Cu-Kα (0.15406 nm) radiation. The X-ray photoelectron spectroscopy (XPS) measurements were performed on a VG ESCALAB MKIIX-ray photoelectron spectrometer. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were collected on a Hitachi H-8100 TEM system. UV-Vis absorption spectra were collected on CARY 500 UV-Vis-NIR spectrophotometer. 2.2
Synthesis of materials
In a typical synthesis of ZIF-67, 24.38 g of 2-methylimidazole and 24.56 g of Co(NO3)2·6H2O were dissolved into 1 L of methanol, respectively. Then, the two solutions were mixed together and stirred for a few minutes. After being stored at room temperature for 24 hours without any disturbance, the formed purple powders at the bottom were collected by centrifugation and washed with methanol for several times. The obtained purple powders were dried at 110 ºC. During the carbonization process, the dried ZIF-67 was heated to 800 ºC at a heating rate of 5 ºC min-1 and maintained at this temperature for 1 h under Ar atmosphere. After cooling down to room temperature, the magnetic black powder was treated by 6 M HCl for 12 hours to remove the unstable metal particles and metal oxides. The resultant black powder was washed with deionized water for several times to remove any impurities and then dried at 110 ºC to obtain the magnetic Co/C nanocomposite. 2.3
Adsorption experiments
Aqueous solutions with different concentrations of dyes were treated by magnetic Co/C nanocomposites to remove the dyes. The mixtures were shaken vigorously for a period of time, and then separated by an external magnetic field. The concentrations of dyes before and after the treatment were monitored by UV-Vis absorption spectra. The removal efficiency was estimated according to changes of dye concentrations before and after the treatment. The adsorption capacity at equilibrium (qe, mg g‒1) was calculated by the following equation: qe = (Ci ‒ Ce)V/m (1) where, V is the volume of the treated solution (mL), m is the weight of used adsorbent (g), and Ci and Ce are the initial concentration and final equilibrium concentration of dyes, respectively. All the adsorption experiments were carried out at room temperature.
RUAN Chang-Ping et al. / Chinese Journal of Analytical Chemistry, 2016, 44(2): 224–231
3
Results and discussion
3.1
Optimization of synthetic condition
ZIF-67 is a typical MOF material with high porosity. Upon calcination at high temperature in inert atmosphere, the organic component in ZIF-67 was degraded and carbonized; whereas Co(II) was reduced to Co(0) nanoparticles[14,16,17]. The generated Co nanoparticles in the present work were uniformly wrapped by graphite layers, due to the graphitization of carbon under the catalysis of Co. The formed graphite layers were able to protect the Co particles from oxidation and acid environment. An appropriate temperature during the carbonization process is crucial, not only for the generation of magnetic Co particles with good stability and crystallinity, but also for the formation of protective graphite layers. To optimize the carbonization temperatures, five samples synthesized at different temperatures (500, 600, 700, 800 and 900 ºC) were prepared and investigated. The XRD patterns (Fig.1a) indicated that the protective graphite layers couldn’t form at relatively low temperatures (500, 600 and 700 ºC). The crystallinity of the resultant composite would be gradually improved with the increasing carbonization temperature. The sample prepared at 900 ºC demonstrated the highest graphitization degree and crystallinity, as evidenced by the corresponding diffraction peaks of graphite and Co nanoparticles (XRD, Fig.1a)[17]. However, the higher temperature (900 ºC) would lead to the degradation of adsorption performance (Fig.1b) because of the reduced specific surface area. Upon increasing the temperature from 800 ºC to 900 ºC, the specific surface area of nanocomposite was reduced from 654 m2 g–1 to 592 m2 g–1. The sample synthesized at 800 ºC exhibited the best removal efficiency (Fig.1b). Therefore, the sample prepared at a carbonization temperature of 800 ºC was used in the present work. 3.2
Characterization of materials
The structure and composition of the optimized sample was
Fig.1
investigated by TEM, XRD and XPS characterizations, as shown in Fig.2. The XRD patterns revealed that the sample had three characteristic peaks for metallic Co and one sharp characteristic peak (002) of graphitic carbon, agreeing well with previous reports[14,16,17]. XPS survey spectrum indicated that the Co/C nanocomposites were mainly composed of C, N and Co elements. High-resolution XPS further revealed that sp2 C was the dominant component of C and Co was composed of a majority of zero-valance Co and a small amount of Co(II), which was in well agreement with the XRD data. The size of ZIF-67 was in the range of 300–400 nm (see TEM image, inset in Fig.2a). After carbonization, the resultant Co/C nanocomposites were approximately 300 nm, as revealed by the corresponding TEM image (Fig.2a). High-resolution TEM (HRTEM) image showed that Co nanoparticles (about 10 nm) were uniformly wrapped by graphite layers. Moreover, the saturation magnetization of Co/C nanocomposites was determined to be 34 emu g‒1. Therefore, Co/C nanocomposites could be easily separated from the aqueous dispersion by external magnetic field. Magnetic separation will simplify the separation process by avoiding complicated separation steps[8]. 3.3
Acid-resistant ability
After acid-washing, unstable Co and CoO nanoparticles were washed off and the remaining Co nanoparticles were uniformly wrapped and protected by graphite layers. These graphite layers could protect the Co nanoparticles from oxidation and acid environment. Consequently, we presumed that the Co/C nanocomposites were acid-resistant and could be applied in a wide pH range. To verify this assumption, we investigated the acid-resistant property of Co/C nanocomposites by studying its stability in various pH conditions. As a comparison, the stability of G/Fe 3 O 4 was also investigated through exactly the same method. Remarkably, as shown in Fig.3, the as-synthesized Co/C nanocomposites demonstrated excellent stability in the pH range of 1–13, indicating their superior acid-resistant ability. ICP analysis
(a) XRD patterns of Co/C nanocomposites prepared at different temperatures and (b) Removal of rhodamine B by using Co/C nanocomposites prepared at different temperatures
RUAN Chang-Ping et al. / Chinese Journal of Analytical Chemistry, 2016, 44(2): 224–231
Fig.2 Characterization of Co/C nanocomposites. (a) TEM image, inset: TEM image of ZIF-67; (b) HR-TEM image of Co/C nanocomposites; (c) XRD patterns of Co/C nanocomposites before and after acid-washing; (d) C 1s XPS spectra; (e) N 1s XPS spectra; (f) Co 2p XPS spectra
Fig.3 Stability of Co/C nanocomposites (in comparison with G/Fe3O4). (a) Concentration of released metal ions at different pH values, (b) Stability of two absorbents in acid environment
revealed that no obvious Co leakage occurred during the test. In stark contrast, G/Fe3O4 had inferior stability in acid environment and was only stable in neutral and alkali environment. When the pH value was lower than 5, obvious leakage of Fe was observed. At pH lower than 3, almost all of
the metal oxide (Fe3O4) in G/Fe3O4 composites was dissolved. Therefore, compared with G/Fe3O4, the as-synthesized Co/C nanocomposites were expected to be applied in a much wider pH range. After adsorption of organic dyes (rhodamine B and malachite green, respectively), the stability of used Co/C
RUAN Chang-Ping et al. / Chinese Journal of Analytical Chemistry, 2016, 44(2): 224–231
nanocomposites was also investigated by treatment with an acid solution (pH 1). As expected, no obvious Co leakage occurred, further proving the excellent acid-resistance of Co/C nanocomposites after the adsorption of organic dyes. 3.4
Effect of time and pH
The effect of treatment time was firstly investigated to optimize the treatment process. The concentrations of dyes at different time intervals were monitored by UV-vis absorption spectra. As shown in Fig.4a, after the addition of Co/C nanocomposites, the characteristic UV-vis absorption of rhodamine B (maximum absorption wavelength at 554 nm) decreased quickly, suggesting that Co/C nanocomposites had fast adsorption kinetics towards rhodamine B. The UV-vis absorption of rhodamine B reached a stable value after 30 min of treatment, revealing that the adsorption of rhodamine B on Co/C nanocomposites reached equilibrium within 30 min (Fig.4b). The adsorption of malachite green on Co/C
Fig.4
nanocomposites was also saturated within 30 min. Therefore, the optimal treatment time in the present work was 30 min. Considering that the Co/C nanocomposites were stable in a wide pH range, adsorption performance of Co/C nanocomposites (towards rhodamine B and malachite green) at three different pH values (pH 2, pH 7 and pH 12) was investigated to probe the effect of pH (Because malachite green was unstable under high-alkaline environment, the adsorption behavior towards malachite green was only investigated under neutral and acidic conditions). The removal efficiency of rhodamine B and malachite green both increased with the increase of pH values (Fig.4, c‒f). Such observation might be attributed to the structural features of dyes. Both rhodamine B and malachite green are positively charged. Upon increasing the pH values, the functional groups on Co/C nanocomposites might be deprotonated, thus resulting in increased negative charge density on Co/C nanocomposites. As a consequence, electrostatic interaction between Co/C nanocomposites and positively charged dyes was enhanced with the increase of
(a, b) Removal efficiency of rhodamine B (1 mg mL‒1) at different intervals; (c, d) Effect of pH values on the removal of rhodamine B (1 mg mL‒1); (e, f) Effect of pH values on the removal of malachite green (1 mg mL‒1)
RUAN Chang-Ping et al. / Chinese Journal of Analytical Chemistry, 2016, 44(2): 224–231
parameters were listed in Table 1. The adsorption isotherms of rhodamine B and malachite green on Co/C nanocomposite were well fitted with the Langmuir model (R2 > 0.959). Impressively, the maximum adsorption capacities towards rhodamine B and malachite green were estimated to be 400 and 561.8 mg g‒1, respectively, far exceeding many previously reported adsorbents (Table 2), such as G/Fe3O4[8], polydopamine-functionalized graphene hydrogel[5], biomassderived activated carbon[18].
pH values, which led to higher removal efficiency of dyes at higher pH values. 3.5
Adsorption performance
The as-synthesized magnetic Co/C nanocomposite demonstrated excellent acid-resistance and good removal efficiency towards rhodamine B and malachite green over a wide pH range. Subsequently, adsorption performance of Co/C nanocomposite towards rhodamine B and malachite green at various pH values were investigated in detail. The corresponding adsorption isotherms were presented in Fig.5. An adsorption isotherm expresses the relationship between adsorption capacities (qe, mg g‒1) and the concentrations of adsorbate (Ce, mg L‒1) at a given temperature under equilibrium conditions[8,15]. The corresponding adsorption data in present work were analyzed using two well-known isotherm models, Langmuir isotherm model and Freundlich isotherm model, respectively. The corresponding fitting
3.6
Regeneration and reusability
The regeneration and reusability of the adsorbent are quite crucial for practical applications, especially in terms of cost control. We found that the adsorbent could be easily regenerated by washing with ethylene glycol (EG). Even after 5 cycles of reuse, no obvious capacity degradation was observed (Fig.6), suggesting the excellent reusability of Co/C nanocomposites.
Table 1 Langmuir and Freundlich model parameters for the adsorption of rhodamine B and malachite green at different pH values Isotherm model
Malachite green
Isotherm contants
pH 7
pH 2
pH 7
pH 12
qmax (mg g )
561.8
561.8
400.0
395.3
389.1
KL (mL mg-1)
6.684
42.329
6.419
8.714
15.541
R2
0.9662
0.9920
0.9591
0.9620
0.9761
k
484.5
878.0
309.4
337.1
351.2
n
2.602
2.818
3.943
3.597
4.204
0.9115
0.6901
0.9824
0.9821
0.8156
-1
Langmuir
Freundlich
Rhodamine B
pH 2
2
R
Fig.5 Adsorption isotherms of rhodamine B (a) and malachite green (b) on Co/C nanocomposites at different pH values Table 2 Comparision of adsorption performances of Co/C nanocomposites with some previous reported adsorbents Dyes
Rhodamine B
Malachite green
qmax (mg g‒1)
Adsorbents
Removal efficiency
Adsorption time (h)
Reusability
Co/C nanocomposite
> 99%
400.0
0.5
Yes (EG)
G/Fe3O4[8]
91%
13.15
2.0
Yes (EG)
Graphene hydrogel[5]
——
207.06
12.0
Yes (Ethanol)
Co/C nanocomposite G/Fe3O4[8]
> 99% 94%
561.8 22.0
0.5 2.0
Yes (EG) Yes (EG)
Biomass-derived activated carbon[18]
99%
48.48
24.0
No
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3.7
Fig.6
Reusability of Co/C nanocomposites after washed by EG (concentration of dyes: 1 mg mL‒1)
Application in real water samples
The as-synthesized magnetic Co/C nanocomposites have demonstrated excellent acid-resistance and adsorption performance towards rhodamine B and malachite green over a wide pH range. Magnetic separation of organic dyes (rhodamine B and malachite green) from water was achieved, with removal efficiency higher than 99% (Fig.7). Furthermore, the feasibility of magnetic separation by using Co/C nanocomposite in practical application was also demonstrated by the removal of organic dyes (rhodamine B and malachite green) from domestic wastewater with a superior removal efficiency of higher than 97%.
Fig.7 Removal of (a, b) rhodamine B and (c, d) malachite green by using magnetic Co/C nanocomposites and magnets
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