Accepted Manuscript Graphene oxide caged in cellulose microbeads for removal of malachite green dye from aqueous solution Xiaomei Zhang, Hongwen Yu, Hongjun Yang, Yuchun Wan, Hong Hu, Zhuang Zhai, Jieming Qin PII: DOI: Reference:
S0021-9797(14)00697-3 http://dx.doi.org/10.1016/j.jcis.2014.09.048 YJCIS 19856
To appear in:
Journal of Colloid and Interface Science
Received Date: Accepted Date:
10 June 2014 14 September 2014
Please cite this article as: X. Zhang, H. Yu, H. Yang, Y. Wan, H. Hu, Z. Zhai, J. Qin, Graphene oxide caged in cellulose microbeads for removal of malachite green dye from aqueous solution, Journal of Colloid and Interface Science (2014), doi: http://dx.doi.org/10.1016/j.jcis.2014.09.048
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Graphene oxide caged in cellulose microbeads for removal of malachite green dye from aqueous solution
Xiaomei Zhanga,b, Hongwen Yub, *, Hongjun Yangb, Yuchun Wana, Hong Hua, Zhuang Zhaia, Jieming Qina, *
a
Department of Material Scienceand Engineering, Changchun University of Sci-
ence and Technology, 7989 Weixing Rd, Changchun130022, China Tel.: +86 431 85583407; fax: +86 431 85583407 E-mail address: [email protected]
b
Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences,
4888 Shengbei Rd, Changchun, 130102, China Tel.: +86 431 85542290; fax: +86 431 85542290 E-mail address: [email protected]
Graphene oxide caged in cellulose microbeads for removal of malachite green dye from aqueous solution
Xiaomei Zhanga,b, Hongwen Yub,*, Hongjun Yangb, Yuchun Wana, Hong Hua, Zhuang Zhaia, Jieming Qina,* a
Department of Material Science and Engineering, Changchun University of Science and Technology, 7989 Weixing Rd, Changchun, 130022, China
b
Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, 4888 Shengbei Rd, Changchun, 130102, China
Abstract A simple sol-gel method using non-toxic and cost-effective precursors has been developed to prepare graphene oxide (GO)/cellulose bead (GOCB) composites for removal of dye pollutants. Taking advantage of the combined benefits of GO and cellulose, the prepared GOCB composites exhibit excellent removal efficiency towards malachite green (>96%) and can be reused for over 5 times through simple filtration method. The high-decontamination performance of the GOCB system is strongly dependent on encapsulation amount of GO, temperature and pH value. In addition, the adsorption behavior of this new adsorbent fits well with the Langmuir isotherm and pseudo-secondorder kinetic model. Keywords: Graphene oxide; Cellulose; Beads; Adsorption; Malachite green.
*
Corresponding author. Tel.: +86 431 85542290; fax: +86 431 85542290. E-mail address: [email protected] Tel.: +86 431 85583407; fax: +86 431 85583407. E-mail address: [email protected]
1. Introduction
Synthetic dyes have been widely used in many industries such as the textile, leather tanning, paper production, food technology, hair colorings, etc. Wastewater discharged from these industries are seriously polluted. As a common synthetic dye, malachite green (MG) is highly effective for cure of protozoal and fungal infections. It is widely used as antibacterial, antiparasitical agent in fish forming due to its low cost, ready availability, efficacy and lack of proper alternative [1, 2]. In response to the health risks, many efforts have been focused on removal of MG, including aerobic degradation [3], photochemical [4], flocculation [5]. However, these degradation processes are costly and difficult to be carried out in large scale to treat the wide range of dye wastewater. The adsorption provides simple and economical method for the removal of MG pollutant [2, 6]. Graphene oxide (GO) has been currently used as an attractive adsorbent due to its unique two-dimentional pattern structure [7]. It has been performed as a forceful adsorbent for the removal of heavy metals and dyes, such as, As3+ [8], Cs+ [9], Cu2+ [10], Cr6+ [11], methyl violet [12], orange-G, Rhodamine B [13], methylene blue [14], tetracycline antibiotics [15] and some toxic pollutants [16]. However, the difficulty in the recycling of GO limite its practical application in environmental remediation. Assembly of GO with macromolecule is an important strategy for its further application. Macromolecule helps not only turn GO into 3D architectures, but also control the morphology of the resultant macrostructure, and enhance decontamination performance. Cellulose, a renewable, biodegradable, and an inexhaustible source of biologic materials, can be shaped into well defined spherical particles that are known as cellulose bead (CB) [17].
The cellulose bead is the common form but displays outstanding adsorption ability. However, as far as we know, there are few studies on preparing GO in CB adsorbent for removal of dyes. The synthesis of GOCB composite has been reported in literatures [18-20], but the rigorous conditions and toxic, expensive raw materials were involved in these methods. In this paper, we report a simple sol-gel strategy for the production GOCB composite as an adsorbent with high adsorbtion efficiency to MG pollutant. Several advantages of this adsorbent make it especially attractive. First, it is fabricated using a simple and nontoxic synthetic route. Second, the adsorbent showed excellent performance at ambient contditons. Third, the adsorbent can be reused from the reactive solution by simple filtering method. Thus, GOCB is expected to play an important role in the removal of organic pollutants.
2. Experimental
2.1 Materials All of chemicals were A. R. The particle size of α-cellulose (Aladdin) was about 25 μm. Sodium hydroxide (NaOH, Purity>96.0%) and Urea (CO(NH2)2, Purity>99.0%) were used to for solution. Nitric acid (HNO3, Purity>98.0%) and (NaCl, Purity>99.0%) were used to for coagulation. Hydrochloric acid (HCl, Purity>36.0%, Beijing chemical works) and NaOH aimed to adjust the final pH of samples. The cationic dye, malachite green (C23H25ClN2, Purity>99.0%, Sinopharm Chemical Reagent Co, Ltd.) was used without further purification, unless otherwise specified.
2.2 Preparation of GOCB The GO was prepared by chemical exfoliation of graphite powder based on a modified Hummer’s method [21]. Cellulose (3.7 mmol) was added to the GO aqueous (6.3 g L-1) by ultrasound and stirred vigorously for 10 min, respectively. A brown colored precipitate was gradually formed. The suspension was mixed with 5 ml solution of NaOH (18.75 mmol) and urea (22.64 mmol) with stirring vigorously for another 10 min. Subsequently, the colloidal precursors were cooled to -12 ℃ under stirring for 5 min until the sample became uniform. The colloidal solution was directly taken out and left to heat naturally ambient temperature. Finally the sol was injected through 5 ml syringe into HNO3 (2 mol L-1) containing NaCl (10 wt %). GOCB was washed several times with deionized water and soaked with deionized water for the following experiment. The whole synthesis of GOCB was shown in Fig. 1.
2.3 Removal of MG A weighed amount of different mass (0 g, 0.05 g, 0.1 g, 0.2 g, 0.3 g) of wet GOCB (90 wt % water) was added in a centrifuge tube containing 30 ml of dye solution (10 mg L-1) and the solution was shaken in a thermostate shaker at 180 rpm, for 24 h. The pH of the solution was 7.0. The samples were shaken for adequate adsorption, absorbance of each dye solution was measured at various wavelengths using UV/Vis spectrometer (PerkinElmer Lambda 25) to obtain λmax and their values are 617 nm corresponding to the maximum absorbance. The absorbance for each dye solution was recorded at their respective λmax values and the amount of dye adsorbed (qe) was calculated. The elimination efficiency (E %)
and adsorption capacity at equilibrium of MG can be calculated by using Eqs. (1) and (2), respectively [22,23]. E(%)= (C 0 − C e ) / C 0 × 100
(1)
qe = v(C 0 − C e ) / m
(2)
Where C0 is the initial dye concentration (mg L-1), Ce is the equilibrium concentration (mg L-1), V is the solution volume (L), m is the mass (g) of dry-weight GOCB.
2.4 Characterization methods of GOCB Scanning electron microscopy observation (SEM) was operated with samples sputtered with gold for observation at an accelerating voltage of 10.0 kV. Transmission electron microscopy (TEM) images were taken by using a TECNAI G2 high-resolution transmission electron microscope with an accelerating voltage of 200 kV. TEM samples were prepared by depositing a drop of diluted suspensions in water on a carbon-filmcoated copper grid. Fourier transform infrared (FT-IR) spectroscopy wavelength ranged from 500 to 4500 nm. Thermal gravity analysis (TGA) of samples was performed on a thermogravimetric analyzer (PerkinElmer Thermal Analysis). Samples were heated under O2 atmosphere from 50 to 800 ℃ at the rate of 8 ℃ min-1.
3. Results and discussion
3.1 Characterization analysis of GOCB SEM images (Fig. 2a-e) of freeze dried GOCB showed that all the full sized GOCB were remarkably similar in size (ranging from 1.9 to 2.3 mm in diameter). The cross section of the GOCB revealed that all the beads contained several porous channels in
interior structure. From embedded images of Fig. 2(a)-(e), we could see that they were gradully cross-linked and with immense porosity. As shown in Fig. 2(f) numerous GO were encapsulated in the GOCB compared with pure CB. Fig. S1(a) showed the TEM image of GO, it could be seen that GO was transparent film due to its single-atom layered structure. The surface of GO film was wrinkled due to the crumpling and scrolling of graphene sheets. Fig. S1(b) showed that GOC suspention had belt-like structrue. Most of the GO sheets were embedded into GOCB and had a rough surface. To confirm the coalescence of GO and cellulose, the FT-IR spectra of GO, cellulose and GOCB were taken respectively and shown in Fig. 3. The involvement of GO broadened the peaks at around 3441 cm-1 , which was assigned to stretching vibration of -OH groups. The bands at 1625 cm-1 and 1400 cm-1 indicated the existence of a symmetric and asymmetric stretching vibration of C=O groups. The band at 1110 cm-1could be attributed to the appearance of C-O groups. The band at 621 cm-1 reflected the C-H bending vibrations [24]. As compared, there is no obvious changes except the bands at 1731 cm-1 of GO and 1634 cm-1 of cellulose shifted to 1665 cm-1 of GOCB further indicateing the GO was encapsulated in the GOCB. To clarify the thermal performance of GOCB, the temperature-mass relation was studied by thermal gravity analysis (TGA) (Fig. 4). In fact, the temperature responsive mass evolution was divided into three-stage process: Ⅰ) GOCB hydrated water went lost at temperatures up to about 250 ℃. Ⅱ) decomposition of carbohydrate/the rupture of chains, fragments and monolayers of cellulose. Ⅲ) decomposition of GO. However, as shown in Fig. 4, GO on the thermal stability of GOCB enhancement effect was relatively limited, the research of cellulose decomposition heat could be known, cellulose in
heating first dehydration, decomposition of cellulose, GO smelting had no value in this stage the thermal decomposition of basic. With the increase of temperature, the thermal decomposition of GO, the cellulose chain was protected by carbon layer of GO on condition of thermal decomposition, so as to improve the thermal stability of cellulose. Due to the low content of GO in the composite, the effect was not very obvious. Direct evidence for the existence of GO in GOCB was shown in Fig. 4.
3.2 Adsorption isotherms UV/Vis absorbance spectrum of MG aqueous after adsorption by CB and 5 wt %GOCB were indicated in Fig. 5(a) and (b). It was illustrated that the cellulose consisting of GOCB didn’t make a role in the removal of MG. Fig. 5(b) also demonstrated that the adsorption of GOCB for MG should be attributed to its GO inclusion, so cellulose had negligible adsorbance for MG. It is the fact that the adsorbent can ensure the experimental reproducibility, we had repeated the experiment under identical conditions five times. Recycling of GOCB in the removal of MG was shown in Fig. 6. The adsorption isotherms GOCB were studied at ambient conditions. GO of GOCB played the role of adsorbent, possessing high affinity and adsorbility of adsorbent. Adsorption isotherms were very effective ways to provide information about adsorption mechanisms, surface properties, the affinity and adsorbility of adsorbent. These were plotted to evaluate the adsorption of GOCB. In this work, the adsorptions of MG with the same initial concentration and different usage amount of adsorbant were studied. To determine the mechanistic parameters associated with MG adsorption, the adsorption data were analyzed according to the Langmuir isotherm models [25]. The linear form of the Langmuir isotherm is represented by:
Ce / qe = Ce / Qmax b + 1 / Qmax
(3)
where qe is the amount of adsorbate adsorbed per unit weight of adsorbent (mg g-1), Ce is the equilibrium concentration of the adsorbate (mg L-1), b and Qmax are Langmuir constants related to the energy of adsorption (L mg-1) and the maximum adsorption capacity (mg g-1), respectively. The experimental results showed that the adsorption data were well fitted with Langmuir isotherms (Fig. 6a), which suggested that the adsorbent surface was heterogeneous in nature. The Langmuir isotherm constants and their correlation coefficients for GOCB were listed in Table 1. The Qmax value for 5 wt %-GOCB was found to be 17.862 mg g-1. Comparison of various adsorbents, Table 2 indicated the adsorption capacity of different types of adsorbents used for removal of MG. The most important parameter is the Langmuir Qmax value, since it is a measure of adsorption capacity of the adsorbent. The value of Qmax in this study is larger than those in most of previous works [26-29]. This suggests that MG could be easily adsorbed on GOCB. The results indicated that the GOCB can be considered a promising adsorbent for the removal of MG from aqueous solutions.
3.3 Adsorption kinetics During the adsorption process, it is necessary to investigate the adsorption kinetics for the evaluating of adsorbability. The time dependent dye adsorption behavior was measured by varying the equilibrium time between adsorbate and adsorbent in the range of 1-24h. The adsorption capacity of the dye as a function of contact time was plotted in Fig. 6 (b) indicating that the equilibrium between the dye and the GOCB was attained within 5 h. Fig. 6 (c) showed the data for the adsorption of MG solutions by GOCB at different time intervals. Adsorption occurred at the first 30 min, suggesting that GOCB
had fast adsorption kinetics towards MG solutions. It was also regarded that initial rapid sorption occurs in the first stage (0-30 min) and then gradually increases to reach an equilibrium value in approximately 12 h. Though Sun [30] has reported that GO has been used to adsorb many dyes, in this paper, to describe changes in the adsorption of MG with time, kinetic data were analyzed using pseudo-second-order kinetics [31], based on the assumption that chemisorption is the rate-determining step [32]. It can be expressed as: t / qt =1/ k 2 qe2 + t/ qe
(4)
Where qe is the adsorption capacity at equilibrium (mg g-1), and qt is the amount of adsorbate adsorbed per unit weight of adsorbent (mg g-1) at time t. The parameter k2 (g mg-1 min-1) represents the pseudo-second-order rate constant for the kinetic model. The relation is linear, and the correlation coefficient all have high values (R2>0.99), which suggests a strong correlation between the parameters. The linearity of the plot of t/qt versus t (Fig. 6c) indicated the applicability of the second-order rate expression. The K value, calculated from the slope of the line in Fig. 6(c) is 0.0069 g mg-1 min-1 ( Table 3). Additionally, the initial adsorption rate V0 (mg g-1 min-1) can be calculated from: V0= k2q e2
(5)
3.4 Mechanism of adsorption This stronger adsorption ability on the characteristics of polar organic dye, is mainly due to the large specific surface area of GO. Three consecutive steps were involved in the removal of dye by GOCB. First, the adsorbate species migrated from the bulk liquid phase to the outer surface of adsorbent particles. Secondly, the dye moved within the mesopores and macropores of adsorbent particles. Thirdly, the adsorption of adsorbate
by adsorbent took place on the surface. In the physisorption adsorption process, ) the cationic dyes showed a high propensity to be adsorbed by GO; this fact could be attributed to the electrostatic interactions between the π-electrons of GO and the positively charged moieties of the cationic dyes. This phenomenon was referred to as π-π electron-donor (GO)/acceptor (cationic dyes) interaction.
) among the cationic dyes, MG
is an ideally planar molecule and therefore could be easily adsorbed by π-π stacking interactions between the aromatic backbone of the dyes and hexagonal skeleton of GO. Thus, GOCB was expected to play a promising role in the field of organic pollutant removal. Furthermore, the oxygen functional group in GO, further improved the adsorption performance of GOCB for molecular organic pollutants. 3.5 Effect of temperature on malachite green removal The adsorption temperature of MG on GOCB ranged from 5
to 35
. The data of
Fig. 7(a) showed that the amount of adsorption increased with an increasing in temperature before 25
and decreased after 25
, indicating both the endothermic nature of the
process and the thermostability of materials. The variation in the extent of adsorption with temperature might be explained on the basis of the change in chemical potentials which was related to the solubility of the adsorbate, which increased with an increase in temperature, the chemical potential decreases also reacted on it, i.e. [25]. The decreasing trend of adsorption with temperature was mainly due to the weakening of adsorptive forces between the active sites of GOCB and adsorbate [26].
3.6 Effect of pH on malachite green removal To study the effect of pH on the adsorption performance of GOCB, experiments were conducted at different pH values, ranging from 4 to 10. Fig. 7(b) showed the amount of
MG adsorbed per unit weight of adsorbent (qt, mg g-1) at various pH values. These results showed that qt gradually increased as the pH increased from 6.0, and it reached its maximum at a certain pH=7.0, and decreased as the pH further increased. For GOCB, the maximum value appeared at pH=7.0, which was consistent with Mall’s previous research [33]. As the pH changed, the number of negatively charged surface sites on the GOCB changed. MG haved an overall positive charge in aqueous solution [34]. When the pH ranged from 6.0 to 8.0, this would increase the electrostatic attraction between the adsorbent and the MG. When the pH was under 6 or above 8, this occurred because part of the GOCB might decomposes in acidic conditions and it dissolve in alkaline conditions.
4. Conclusion
A simple sol-gel method has been developed for producing GOCB composites by using non-toxic and cost-effective precursors. Benefiting from the combined advantages of GO and cellulose, these GOCB composites were shown to be an efficient adsorbent to treat dye pollutants and could be reused from aqueous solution by a simple filtration in a short time. In addition, the adsorption behavior of this new adsorbant fitted well with the Langmuir isotherm and pseudo-second-order kinetic model. In particular, GOCB exhibited remarkably effect of MG removal. It is expected that the unique structure and advanced chemical constitution of GOCB have extensive applicability in the removal of dye from wastewater.
Acknowledgements
Xiaomei Zhang and Hongwen Yu, these authors attribuate equally to this work. This work was supported by “Hundred Talents” Program of Chinese Academy of Sciences, the National Natural Science Foundation of China (No.2177132), the Important Deployment Project of Chinese Academy of Sciences (KZZD-EW-TZ-16), the Natural Science Foundation of jilin Province,China (GrantNo20140101052JC).
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Fig. 1. Synthesis flow-chart of GOCB.
Fig. 2. SEM images of GOCB with different concentration of GO, 0 wt % (a), 1 wt % (b), 3 wt % (c), 5 wt % (d), 10 wt % (e), and photos of CB and 5 wt %-GOCB (f).
Fig. 3. FTIR spectra of CB, GO and 5 wt %-GOCB.
Fig. 4. TG curves of CB, GO and 5 wt %-GOCB.
Fig. 5. UV/Vis absorption spectra of MG (MG: 10mg L-1, pH: 7, temperature: 25 ℃, time: 24 h) aqueous after adsorption of (0 g-0.3 g) CB (a), and (0 g-0.3 g) 5 wt %GOCB (b).
Fig. 6. Recycling of GOCB in the removal of MG.
Fig. 7. Langmuir isotherms adsorption model (10 mg L-1, pH: 7, temperature: 25 ℃, beads mass: 0.1 g) (a), and adsorption kinetics adsorption model (b), pseudo-secondorder adsorption model (c).
Fig. 8. Effect of temperature (a) and pH (b) on the adsorption of MG on GOCB (10 mg L-1, time: 24 h, beads mass: 0.1g).
Table 1 Langmuir isotherm constants and their correlation coefficients. Qmax (mg g-1)
wt%
b (L mg-1)
R2
0%
-
-
-
1%
3.869
1.534
0.957
3%
8.467
0.612
0.987
5%
17.862
1.321
0.996
10%
30.090
1.107
0.991
Table 2 Comparison of adsorption capacities of various adsorbents for malachite green. Adsorbent
Qmax (mg g-1)
Activated Charcoal
Reference
0.179
[26]
Neem Sawdust
4.35
[27]
Bentonite Clay
7.724.
[28]
Polymeric Gel
4.900
[29]
Table 3 Rate constants and correlation coefficients of the pseudo-second-order kinetic model. qe (mg g-1) 14.0621
k2 (g mg-1 min-1) 0.0069
V0 (mg g-1 min-1)
R2
1.3551
0.99908
Graphical Abstract Graphene oxide/cellulose bead (GOCB) composites were prepared using a simple sol-gel method. GOCB adsorbent exhibited excellent removal efficiency (over 96% for malachite green) and can be rapid separated and reused from aqueous solution by a simple filtration. GOCB is expected to play an important role in the field of dye pollutants removal.
Highlights • Graphene oxide/cellulose beads (GOCB) are prepared by a simple sol-gel method. • GOCB is a facile, effective and economic adsorbent for removal of malachite green. • GOCB shows good performance at ambient temperature and neutral aqueous. • The new adsorbent of GOCB can be separated and reused from aqueous solution by filtration. • GOCB has extensive applicability in the removal of dye from wastewater.
S0021-9797(14)00697-3 http://dx.doi.org/10.1016/j.jcis.2014.09.048 YJCIS 19856
To appear in:
Journal of Colloid and Interface Science
Received Date: Accepted Date:
10 June 2014 14 September 2014
Please cite this article as: X. Zhang, H. Yu, H. Yang, Y. Wan, H. Hu, Z. Zhai, J. Qin, Graphene oxide caged in cellulose microbeads for removal of malachite green dye from aqueous solution, Journal of Colloid and Interface Science (2014), doi: http://dx.doi.org/10.1016/j.jcis.2014.09.048
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Graphene oxide caged in cellulose microbeads for removal of malachite green dye from aqueous solution
Xiaomei Zhanga,b, Hongwen Yub, *, Hongjun Yangb, Yuchun Wana, Hong Hua, Zhuang Zhaia, Jieming Qina, *
a
Department of Material Scienceand Engineering, Changchun University of Sci-
ence and Technology, 7989 Weixing Rd, Changchun130022, China Tel.: +86 431 85583407; fax: +86 431 85583407 E-mail address: [email protected]
b
Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences,
4888 Shengbei Rd, Changchun, 130102, China Tel.: +86 431 85542290; fax: +86 431 85542290 E-mail address: [email protected]
Graphene oxide caged in cellulose microbeads for removal of malachite green dye from aqueous solution
Xiaomei Zhanga,b, Hongwen Yub,*, Hongjun Yangb, Yuchun Wana, Hong Hua, Zhuang Zhaia, Jieming Qina,* a
Department of Material Science and Engineering, Changchun University of Science and Technology, 7989 Weixing Rd, Changchun, 130022, China
b
Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, 4888 Shengbei Rd, Changchun, 130102, China
Abstract A simple sol-gel method using non-toxic and cost-effective precursors has been developed to prepare graphene oxide (GO)/cellulose bead (GOCB) composites for removal of dye pollutants. Taking advantage of the combined benefits of GO and cellulose, the prepared GOCB composites exhibit excellent removal efficiency towards malachite green (>96%) and can be reused for over 5 times through simple filtration method. The high-decontamination performance of the GOCB system is strongly dependent on encapsulation amount of GO, temperature and pH value. In addition, the adsorption behavior of this new adsorbent fits well with the Langmuir isotherm and pseudo-secondorder kinetic model. Keywords: Graphene oxide; Cellulose; Beads; Adsorption; Malachite green.
*
Corresponding author. Tel.: +86 431 85542290; fax: +86 431 85542290. E-mail address: [email protected] Tel.: +86 431 85583407; fax: +86 431 85583407. E-mail address: [email protected]
1. Introduction
Synthetic dyes have been widely used in many industries such as the textile, leather tanning, paper production, food technology, hair colorings, etc. Wastewater discharged from these industries are seriously polluted. As a common synthetic dye, malachite green (MG) is highly effective for cure of protozoal and fungal infections. It is widely used as antibacterial, antiparasitical agent in fish forming due to its low cost, ready availability, efficacy and lack of proper alternative [1, 2]. In response to the health risks, many efforts have been focused on removal of MG, including aerobic degradation [3], photochemical [4], flocculation [5]. However, these degradation processes are costly and difficult to be carried out in large scale to treat the wide range of dye wastewater. The adsorption provides simple and economical method for the removal of MG pollutant [2, 6]. Graphene oxide (GO) has been currently used as an attractive adsorbent due to its unique two-dimentional pattern structure [7]. It has been performed as a forceful adsorbent for the removal of heavy metals and dyes, such as, As3+ [8], Cs+ [9], Cu2+ [10], Cr6+ [11], methyl violet [12], orange-G, Rhodamine B [13], methylene blue [14], tetracycline antibiotics [15] and some toxic pollutants [16]. However, the difficulty in the recycling of GO limite its practical application in environmental remediation. Assembly of GO with macromolecule is an important strategy for its further application. Macromolecule helps not only turn GO into 3D architectures, but also control the morphology of the resultant macrostructure, and enhance decontamination performance. Cellulose, a renewable, biodegradable, and an inexhaustible source of biologic materials, can be shaped into well defined spherical particles that are known as cellulose bead (CB) [17].
The cellulose bead is the common form but displays outstanding adsorption ability. However, as far as we know, there are few studies on preparing GO in CB adsorbent for removal of dyes. The synthesis of GOCB composite has been reported in literatures [18-20], but the rigorous conditions and toxic, expensive raw materials were involved in these methods. In this paper, we report a simple sol-gel strategy for the production GOCB composite as an adsorbent with high adsorbtion efficiency to MG pollutant. Several advantages of this adsorbent make it especially attractive. First, it is fabricated using a simple and nontoxic synthetic route. Second, the adsorbent showed excellent performance at ambient contditons. Third, the adsorbent can be reused from the reactive solution by simple filtering method. Thus, GOCB is expected to play an important role in the removal of organic pollutants.
2. Experimental
2.1 Materials All of chemicals were A. R. The particle size of α-cellulose (Aladdin) was about 25 μm. Sodium hydroxide (NaOH, Purity>96.0%) and Urea (CO(NH2)2, Purity>99.0%) were used to for solution. Nitric acid (HNO3, Purity>98.0%) and (NaCl, Purity>99.0%) were used to for coagulation. Hydrochloric acid (HCl, Purity>36.0%, Beijing chemical works) and NaOH aimed to adjust the final pH of samples. The cationic dye, malachite green (C23H25ClN2, Purity>99.0%, Sinopharm Chemical Reagent Co, Ltd.) was used without further purification, unless otherwise specified.
2.2 Preparation of GOCB The GO was prepared by chemical exfoliation of graphite powder based on a modified Hummer’s method [21]. Cellulose (3.7 mmol) was added to the GO aqueous (6.3 g L-1) by ultrasound and stirred vigorously for 10 min, respectively. A brown colored precipitate was gradually formed. The suspension was mixed with 5 ml solution of NaOH (18.75 mmol) and urea (22.64 mmol) with stirring vigorously for another 10 min. Subsequently, the colloidal precursors were cooled to -12 ℃ under stirring for 5 min until the sample became uniform. The colloidal solution was directly taken out and left to heat naturally ambient temperature. Finally the sol was injected through 5 ml syringe into HNO3 (2 mol L-1) containing NaCl (10 wt %). GOCB was washed several times with deionized water and soaked with deionized water for the following experiment. The whole synthesis of GOCB was shown in Fig. 1.
2.3 Removal of MG A weighed amount of different mass (0 g, 0.05 g, 0.1 g, 0.2 g, 0.3 g) of wet GOCB (90 wt % water) was added in a centrifuge tube containing 30 ml of dye solution (10 mg L-1) and the solution was shaken in a thermostate shaker at 180 rpm, for 24 h. The pH of the solution was 7.0. The samples were shaken for adequate adsorption, absorbance of each dye solution was measured at various wavelengths using UV/Vis spectrometer (PerkinElmer Lambda 25) to obtain λmax and their values are 617 nm corresponding to the maximum absorbance. The absorbance for each dye solution was recorded at their respective λmax values and the amount of dye adsorbed (qe) was calculated. The elimination efficiency (E %)
and adsorption capacity at equilibrium of MG can be calculated by using Eqs. (1) and (2), respectively [22,23]. E(%)= (C 0 − C e ) / C 0 × 100
(1)
qe = v(C 0 − C e ) / m
(2)
Where C0 is the initial dye concentration (mg L-1), Ce is the equilibrium concentration (mg L-1), V is the solution volume (L), m is the mass (g) of dry-weight GOCB.
2.4 Characterization methods of GOCB Scanning electron microscopy observation (SEM) was operated with samples sputtered with gold for observation at an accelerating voltage of 10.0 kV. Transmission electron microscopy (TEM) images were taken by using a TECNAI G2 high-resolution transmission electron microscope with an accelerating voltage of 200 kV. TEM samples were prepared by depositing a drop of diluted suspensions in water on a carbon-filmcoated copper grid. Fourier transform infrared (FT-IR) spectroscopy wavelength ranged from 500 to 4500 nm. Thermal gravity analysis (TGA) of samples was performed on a thermogravimetric analyzer (PerkinElmer Thermal Analysis). Samples were heated under O2 atmosphere from 50 to 800 ℃ at the rate of 8 ℃ min-1.
3. Results and discussion
3.1 Characterization analysis of GOCB SEM images (Fig. 2a-e) of freeze dried GOCB showed that all the full sized GOCB were remarkably similar in size (ranging from 1.9 to 2.3 mm in diameter). The cross section of the GOCB revealed that all the beads contained several porous channels in
interior structure. From embedded images of Fig. 2(a)-(e), we could see that they were gradully cross-linked and with immense porosity. As shown in Fig. 2(f) numerous GO were encapsulated in the GOCB compared with pure CB. Fig. S1(a) showed the TEM image of GO, it could be seen that GO was transparent film due to its single-atom layered structure. The surface of GO film was wrinkled due to the crumpling and scrolling of graphene sheets. Fig. S1(b) showed that GOC suspention had belt-like structrue. Most of the GO sheets were embedded into GOCB and had a rough surface. To confirm the coalescence of GO and cellulose, the FT-IR spectra of GO, cellulose and GOCB were taken respectively and shown in Fig. 3. The involvement of GO broadened the peaks at around 3441 cm-1 , which was assigned to stretching vibration of -OH groups. The bands at 1625 cm-1 and 1400 cm-1 indicated the existence of a symmetric and asymmetric stretching vibration of C=O groups. The band at 1110 cm-1could be attributed to the appearance of C-O groups. The band at 621 cm-1 reflected the C-H bending vibrations [24]. As compared, there is no obvious changes except the bands at 1731 cm-1 of GO and 1634 cm-1 of cellulose shifted to 1665 cm-1 of GOCB further indicateing the GO was encapsulated in the GOCB. To clarify the thermal performance of GOCB, the temperature-mass relation was studied by thermal gravity analysis (TGA) (Fig. 4). In fact, the temperature responsive mass evolution was divided into three-stage process: Ⅰ) GOCB hydrated water went lost at temperatures up to about 250 ℃. Ⅱ) decomposition of carbohydrate/the rupture of chains, fragments and monolayers of cellulose. Ⅲ) decomposition of GO. However, as shown in Fig. 4, GO on the thermal stability of GOCB enhancement effect was relatively limited, the research of cellulose decomposition heat could be known, cellulose in
heating first dehydration, decomposition of cellulose, GO smelting had no value in this stage the thermal decomposition of basic. With the increase of temperature, the thermal decomposition of GO, the cellulose chain was protected by carbon layer of GO on condition of thermal decomposition, so as to improve the thermal stability of cellulose. Due to the low content of GO in the composite, the effect was not very obvious. Direct evidence for the existence of GO in GOCB was shown in Fig. 4.
3.2 Adsorption isotherms UV/Vis absorbance spectrum of MG aqueous after adsorption by CB and 5 wt %GOCB were indicated in Fig. 5(a) and (b). It was illustrated that the cellulose consisting of GOCB didn’t make a role in the removal of MG. Fig. 5(b) also demonstrated that the adsorption of GOCB for MG should be attributed to its GO inclusion, so cellulose had negligible adsorbance for MG. It is the fact that the adsorbent can ensure the experimental reproducibility, we had repeated the experiment under identical conditions five times. Recycling of GOCB in the removal of MG was shown in Fig. 6. The adsorption isotherms GOCB were studied at ambient conditions. GO of GOCB played the role of adsorbent, possessing high affinity and adsorbility of adsorbent. Adsorption isotherms were very effective ways to provide information about adsorption mechanisms, surface properties, the affinity and adsorbility of adsorbent. These were plotted to evaluate the adsorption of GOCB. In this work, the adsorptions of MG with the same initial concentration and different usage amount of adsorbant were studied. To determine the mechanistic parameters associated with MG adsorption, the adsorption data were analyzed according to the Langmuir isotherm models [25]. The linear form of the Langmuir isotherm is represented by:
Ce / qe = Ce / Qmax b + 1 / Qmax
(3)
where qe is the amount of adsorbate adsorbed per unit weight of adsorbent (mg g-1), Ce is the equilibrium concentration of the adsorbate (mg L-1), b and Qmax are Langmuir constants related to the energy of adsorption (L mg-1) and the maximum adsorption capacity (mg g-1), respectively. The experimental results showed that the adsorption data were well fitted with Langmuir isotherms (Fig. 6a), which suggested that the adsorbent surface was heterogeneous in nature. The Langmuir isotherm constants and their correlation coefficients for GOCB were listed in Table 1. The Qmax value for 5 wt %-GOCB was found to be 17.862 mg g-1. Comparison of various adsorbents, Table 2 indicated the adsorption capacity of different types of adsorbents used for removal of MG. The most important parameter is the Langmuir Qmax value, since it is a measure of adsorption capacity of the adsorbent. The value of Qmax in this study is larger than those in most of previous works [26-29]. This suggests that MG could be easily adsorbed on GOCB. The results indicated that the GOCB can be considered a promising adsorbent for the removal of MG from aqueous solutions.
3.3 Adsorption kinetics During the adsorption process, it is necessary to investigate the adsorption kinetics for the evaluating of adsorbability. The time dependent dye adsorption behavior was measured by varying the equilibrium time between adsorbate and adsorbent in the range of 1-24h. The adsorption capacity of the dye as a function of contact time was plotted in Fig. 6 (b) indicating that the equilibrium between the dye and the GOCB was attained within 5 h. Fig. 6 (c) showed the data for the adsorption of MG solutions by GOCB at different time intervals. Adsorption occurred at the first 30 min, suggesting that GOCB
had fast adsorption kinetics towards MG solutions. It was also regarded that initial rapid sorption occurs in the first stage (0-30 min) and then gradually increases to reach an equilibrium value in approximately 12 h. Though Sun [30] has reported that GO has been used to adsorb many dyes, in this paper, to describe changes in the adsorption of MG with time, kinetic data were analyzed using pseudo-second-order kinetics [31], based on the assumption that chemisorption is the rate-determining step [32]. It can be expressed as: t / qt =1/ k 2 qe2 + t/ qe
(4)
Where qe is the adsorption capacity at equilibrium (mg g-1), and qt is the amount of adsorbate adsorbed per unit weight of adsorbent (mg g-1) at time t. The parameter k2 (g mg-1 min-1) represents the pseudo-second-order rate constant for the kinetic model. The relation is linear, and the correlation coefficient all have high values (R2>0.99), which suggests a strong correlation between the parameters. The linearity of the plot of t/qt versus t (Fig. 6c) indicated the applicability of the second-order rate expression. The K value, calculated from the slope of the line in Fig. 6(c) is 0.0069 g mg-1 min-1 ( Table 3). Additionally, the initial adsorption rate V0 (mg g-1 min-1) can be calculated from: V0= k2q e2
(5)
3.4 Mechanism of adsorption This stronger adsorption ability on the characteristics of polar organic dye, is mainly due to the large specific surface area of GO. Three consecutive steps were involved in the removal of dye by GOCB. First, the adsorbate species migrated from the bulk liquid phase to the outer surface of adsorbent particles. Secondly, the dye moved within the mesopores and macropores of adsorbent particles. Thirdly, the adsorption of adsorbate
by adsorbent took place on the surface. In the physisorption adsorption process, ) the cationic dyes showed a high propensity to be adsorbed by GO; this fact could be attributed to the electrostatic interactions between the π-electrons of GO and the positively charged moieties of the cationic dyes. This phenomenon was referred to as π-π electron-donor (GO)/acceptor (cationic dyes) interaction.
) among the cationic dyes, MG
is an ideally planar molecule and therefore could be easily adsorbed by π-π stacking interactions between the aromatic backbone of the dyes and hexagonal skeleton of GO. Thus, GOCB was expected to play a promising role in the field of organic pollutant removal. Furthermore, the oxygen functional group in GO, further improved the adsorption performance of GOCB for molecular organic pollutants. 3.5 Effect of temperature on malachite green removal The adsorption temperature of MG on GOCB ranged from 5
to 35
. The data of
Fig. 7(a) showed that the amount of adsorption increased with an increasing in temperature before 25
and decreased after 25
, indicating both the endothermic nature of the
process and the thermostability of materials. The variation in the extent of adsorption with temperature might be explained on the basis of the change in chemical potentials which was related to the solubility of the adsorbate, which increased with an increase in temperature, the chemical potential decreases also reacted on it, i.e. [25]. The decreasing trend of adsorption with temperature was mainly due to the weakening of adsorptive forces between the active sites of GOCB and adsorbate [26].
3.6 Effect of pH on malachite green removal To study the effect of pH on the adsorption performance of GOCB, experiments were conducted at different pH values, ranging from 4 to 10. Fig. 7(b) showed the amount of
MG adsorbed per unit weight of adsorbent (qt, mg g-1) at various pH values. These results showed that qt gradually increased as the pH increased from 6.0, and it reached its maximum at a certain pH=7.0, and decreased as the pH further increased. For GOCB, the maximum value appeared at pH=7.0, which was consistent with Mall’s previous research [33]. As the pH changed, the number of negatively charged surface sites on the GOCB changed. MG haved an overall positive charge in aqueous solution [34]. When the pH ranged from 6.0 to 8.0, this would increase the electrostatic attraction between the adsorbent and the MG. When the pH was under 6 or above 8, this occurred because part of the GOCB might decomposes in acidic conditions and it dissolve in alkaline conditions.
4. Conclusion
A simple sol-gel method has been developed for producing GOCB composites by using non-toxic and cost-effective precursors. Benefiting from the combined advantages of GO and cellulose, these GOCB composites were shown to be an efficient adsorbent to treat dye pollutants and could be reused from aqueous solution by a simple filtration in a short time. In addition, the adsorption behavior of this new adsorbant fitted well with the Langmuir isotherm and pseudo-second-order kinetic model. In particular, GOCB exhibited remarkably effect of MG removal. It is expected that the unique structure and advanced chemical constitution of GOCB have extensive applicability in the removal of dye from wastewater.
Acknowledgements
Xiaomei Zhang and Hongwen Yu, these authors attribuate equally to this work. This work was supported by “Hundred Talents” Program of Chinese Academy of Sciences, the National Natural Science Foundation of China (No.2177132), the Important Deployment Project of Chinese Academy of Sciences (KZZD-EW-TZ-16), the Natural Science Foundation of jilin Province,China (GrantNo20140101052JC).
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Fig. 1. Synthesis flow-chart of GOCB.
Fig. 2. SEM images of GOCB with different concentration of GO, 0 wt % (a), 1 wt % (b), 3 wt % (c), 5 wt % (d), 10 wt % (e), and photos of CB and 5 wt %-GOCB (f).
Fig. 3. FTIR spectra of CB, GO and 5 wt %-GOCB.
Fig. 4. TG curves of CB, GO and 5 wt %-GOCB.
Fig. 5. UV/Vis absorption spectra of MG (MG: 10mg L-1, pH: 7, temperature: 25 ℃, time: 24 h) aqueous after adsorption of (0 g-0.3 g) CB (a), and (0 g-0.3 g) 5 wt %GOCB (b).
Fig. 6. Recycling of GOCB in the removal of MG.
Fig. 7. Langmuir isotherms adsorption model (10 mg L-1, pH: 7, temperature: 25 ℃, beads mass: 0.1 g) (a), and adsorption kinetics adsorption model (b), pseudo-secondorder adsorption model (c).
Fig. 8. Effect of temperature (a) and pH (b) on the adsorption of MG on GOCB (10 mg L-1, time: 24 h, beads mass: 0.1g).
Table 1 Langmuir isotherm constants and their correlation coefficients. Qmax (mg g-1)
wt%
b (L mg-1)
R2
0%
-
-
-
1%
3.869
1.534
0.957
3%
8.467
0.612
0.987
5%
17.862
1.321
0.996
10%
30.090
1.107
0.991
Table 2 Comparison of adsorption capacities of various adsorbents for malachite green. Adsorbent
Qmax (mg g-1)
Activated Charcoal
Reference
0.179
[26]
Neem Sawdust
4.35
[27]
Bentonite Clay
7.724.
[28]
Polymeric Gel
4.900
[29]
Table 3 Rate constants and correlation coefficients of the pseudo-second-order kinetic model. qe (mg g-1) 14.0621
k2 (g mg-1 min-1) 0.0069
V0 (mg g-1 min-1)
R2
1.3551
0.99908
Graphical Abstract Graphene oxide/cellulose bead (GOCB) composites were prepared using a simple sol-gel method. GOCB adsorbent exhibited excellent removal efficiency (over 96% for malachite green) and can be rapid separated and reused from aqueous solution by a simple filtration. GOCB is expected to play an important role in the field of dye pollutants removal.
Highlights • Graphene oxide/cellulose beads (GOCB) are prepared by a simple sol-gel method. • GOCB is a facile, effective and economic adsorbent for removal of malachite green. • GOCB shows good performance at ambient temperature and neutral aqueous. • The new adsorbent of GOCB can be separated and reused from aqueous solution by filtration. • GOCB has extensive applicability in the removal of dye from wastewater.
