CNCs composite with adsorption performance towards [BMIM][Cl] from aqueous solution

Journal of Hazardous Materials 337 (2017) 27–33

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

Research Paper

Facile preparation of 3D GO/CNCs composite with adsorption performance towards [BMIM][Cl] from aqueous solution Hua Zhou a , Bin Gao b , Yanmei Zhou a,∗ , Han Qiao a , Wenli Gao a , Haonan Qu a , Shanhu Liu a , Qingyou Zhang a , Xiaoqiang Liu a a b

Institute of Environmental and Analytical Sciences, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, PR China Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL 32611, United States

h i g h l i g h t s • A three-dimensional crumpled graphene oxide adsorbent (GO/CNCs) was synthesized. • The loading of CNCs onto GO provides more chance for the sorption of [BMIM][Cl]. • GO/CNCs shows a maximum sorption capacity of 0.455 mmol/g for [BMIM][Cl].

a r t i c l e

i n f o

Article history: Received 4 December 2016 Received in revised form 4 April 2017 Accepted 3 May 2017 Available online 3 May 2017 Keywords: Graphene oxide Corncob Cellulose nanocrystals Three-dimensional structure [BMIM][Cl] removal

a b s t r a c t A novel three-dimensional crumpled graphene oxide/cellulose nanocrystals (GO/CNCs) composite was successfully synthesized and firstly used as adsorbent for the removal of ionic liquid [BMIM][Cl] from aqueous solution. The 3D crumpled structure and abundant oxygen of the functional groups on GO/CNCs composite can provide more chance for the sorption of [BMIM][Cl] compared with CNCs and GO, respectively. Therefore, a series of batch experiments were carried out to evaluate the adsorptive property of 3D GO/CNCs composite towards [BMIM][Cl], such as the GO mass content, the pH value and contact time. The results showed that pseudo-second-order kinetic model and Eovlich model were well fitted with the sorption kinetic. The isotherm adsorption data indicated that it was better described by Langmuir model, with the maximum sorption capacity of 0.455 mmol/g. This work provides a facile method for the preparation of 3D structure adsorbent from graphene oxide and cellulose nanocrystals which has high adsorption capacity of [BMIM][Cl] in aqueous solution. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Graphene oxide (GO) as a novel two-dimensional (2D) carbon nanostructure has attracted a large amount of attention due to its unique physical and chemical properties. Until now, various graphene oxide and graphene oxide composites have been widely used in different areas, such as supercapacitors [1,2], gas storage [3,4], catalysis [5,6], lithium storage [7,8] and sensors [9,10]. Recently graphene oxide and its composites have been developed to be a promising adsorbent for adsorption of heavy metal ions and organic pollutants [11–14] from aqueous solution, because the 2D structure, large surface area, and abundant oxygen-containing groups (epoxy, hydroxyl and carboxyl groups) of graphene oxide

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Zhou). http://dx.doi.org/10.1016/j.jhazmat.2017.05.002 0304-3894/© 2017 Elsevier B.V. All rights reserved.

can strongly enhance the combination between the pollutant and graphene oxide composite. In addition, graphene oxide can be stacked into three-dimensional (3D) graphene oxide composites through various methods and the composites have been applied in supercapacitors, sensors and adsorbents [15–18]. The threedimensional structure can provide more area for the diffusion of pollutant molecules into the 3D structure [19]. Therefore, it is interesting and challenging to develop 3D graphene composite as high efficient adsorbent for the removal of pollutant from wastewater. Ionic liquids (ILs) mostly made of organic cations and organic/inorganic anions at room temperature have been widely applied as catalysts and green solvents in chemical reactions and processing [20]. With the release into the environment, ionic liquids exhibit toxicities on organisms, bacteria and algae [21]. It has gained much attention because of its toxicities towards water as well as organisms and it was considered as a new kind of pollutant to environment. Thus how to remove or recover the ionic liquids

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Scheme 1. The synthesis of GO/CNCs.

from wastewater has become an important topic. Some methods have been reported about disposing the ILs-containing wastewater, including photocatalysis [22], biodegradation[23], electrodialysis [24], and adsorption [25,26]. Among the available methods, adsorption has been considered as a cost-effective, non-destructive and eco-friendly way for removing ILs from aqueous solution. Therefore, it is an urgent need to develop new adsorbents with high adsorption capacity of ILs in aqueous solution. Herein, a 3D crumpled structure GO/CNCs composite was designed and used as adsorbent for the removal of ionic liquid [BMIM][Cl] from aqueous solution. Firstly, the one-dimensional rod-like CNCs was prepared from waste corncob via Soxhlet extraction and freeze drying. Then, it was cross-linked with GO nanosheets, receiving a 3D structure adsorbent with abundance of functionality groups for adsorption. Finally, a range of experiments were carried out to evaluate the adsorption abilities of the 3D structure GO/CNCs adsorbent onto [BMIM][Cl] in aqueous solution. More importantly, the loading of CNCs onto GO sheets can improve the adsorption capacity towards [BMIM][Cl] compared with CNCs and GO, respectively. Through this study, we attempt to develop a new 3D structure GO composite and investigate its excellent performance as adsorbent. 2. Experiment 2.1. Materials Corncob was received from Kaifeng of Henan Province. 1-butyl3-Methylimidazolium Chloride ([BMIM][Cl]) was purchased from Henan Lihua Pharmaceutical Co., Ltd. Graphite was obtained from Sinopharm Chemical Reagent Co., Ltd., China. All reagents used in this study were of analytical grade. Deionized water was used throughout the experiments. 2.2. Instrumentation Fourier transform infrared spectroscopy (FT-IR) was performed on an AVATAR360 (American Nicolet Instrument Corporation) FTIR spectroscopy in the form of KBr pellets. X-ray diffraction (XRD) analysis was performed on a Philips X-PertPro automatic powder diffractometer using Cu K␣ radiation. The scanning electron microscope (FE-SEM) was carried out by a NOVA NanoSEM450 (American FEI corporation) scanning electron microscope. Atomic Force Microscopy (AFM) was performed on NT-MDT Solver P47HPRO (Russia NT-MDT corporation) in a tapping mode. 2.3. Preparation of CNCs from corncob CNCs were synthesized according to our previous work [27]: 20.0 g of corncob power was extracted by soxhlet’s extracter, receiving the pure cellulose. Then 10.0 g of the pure cellulose was

mixed with 85 ml of sulfuric acid under vigorous mechanical stirring. The CNCs were obtained after washed by deionized water and freeze-dried. 2.4. Preparation of three dimensional GO/CNCs composite GO was prepared by the Hummers’ method [28] and was ultrasonically dispersed in deionized water to acquire GO dispersion with a concentration of 5.5 mg/ml. Then 97 ml of 16.0 mg/ml CNCs solution was added into 120 ml of GO dispersion, receiving a brown mixture solution. The brown solution was ultrasound for 1 h and stirred for 2 h at ambient temperature. The solid adsorbent GO/CNCs composite (with 30% GO in mass) was obtained by freeze-drying the final solution. Then a series of GO/CNCs composites were prepared by the same procedure with different GO content of 10%GO, 20%GO, 40%GO, 50%GO (Scheme 1). 2.5. Adsorption experiments of [BMIM][Cl] Adsorption studies were performed by adding 20 mg of each adsorbent to 50 ml polyethylene flask with 20 ml of [BMIM][Cl] at a constant speed of 190 rpm in a thermostatted shaker bath. After centrifuged, the supernatants were determined at 211 nm for [BMIM][Cl] on UV–vis spectrophotometer. The pH was adjusted by 0.1 mol/l HCl and NaOH. The adsorption kinetic experiments were investigated by mixing the adsorbent and [BMIM][Cl] at different reaction time in the range of 0.5–25 h. All the experiments were carried out in triplicate and the average values were reported. The amount of adsorbed [BMIM][Cl] and removal percentage were calculated as follows [29]: Removal percentage = q = (Co − Ce )

Co − Ce × 100% Co

V m

(1) (2)

Where C0 and Ce (mmol/l) are the initial and equilibrium concentration of [BMIM][Cl] respectively, q (mmol/g) is the adsorption capacity of adsorbent towards [BMIM][Cl], V (l) is the volume of solution, and m (g) is the mass of the adsorbent. 3. Results and discussion 3.1. Characterization of the 3D GO/CNCs structures The FTIR spectrum of CNCs, GO and GO/CNCs were shown in Fig. 1a. The peaks emerging at 3300–3500 and 2918 cm−1 were attributed to the stretching of OH and CH bonds of CNCs according with our previous work [30]. The peaks appeared at 1726, 1400 and 3422 cm−1 were related to the COOH, C O and the C O stretching vibrations of GO, indicating the carboxylic acid groups on the surface of GO consisted with literatures [31,32]. The bands

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Fig. 1. (a) FTIR spectra of CNCs, GO and GO/CNCs; (b)X-ray diffraction patterns of CNCs, GO and GO/CNCs.

Fig. 2. SEM images of GO/CNCs, (a) scale = 10 ␮m, (b) scale = 100 nm.

at 1726 and 2918 cm−1 due to the stretching modes of C O and CH were both observed in the spectrum of GO/CNCs, suggesting the successful synthesis of graphene oxide composite. Fig. 1b presents the X-ray diffractograms for CNCs, GO and GO/CNCs composite. The CNCs exhibits three main peaks at 2␪ = 14.9◦ , 22.6◦ and 34.2◦ , corresponding to the (101), (002), and (040) planes of cellulose [33,34]. The diffraction peak of GO was found at approximately 2␪ = 10.2◦ (001), which corresponds to the layer-to-layer distance of 0.86 nm [35]. Moreover, the diffraction peak of GO/CNCs at 22.6◦ gradually became weak and the peak at 34.2◦ disappeared, which indicated that the crystalline structure of the CNCs may be affected by the introduce of GO. The SEM images of the surface morphologies of GO/CNCs (with 30% GO) composite were illustrated in Fig. 2a and b. The 3D crumpled structure and abundant macropore of GO/CNCs can be clearly observed in Fig. 2a, with larger interlamellar spacing [36]. Fig. 2b was the high amplification of the smooth section of GO/CNCs. It can be seen clearly that a large number of one-dimensional rod-like CNCs were distributed as shown in Fig. 2b [37]. The nanocrystalline cellulose was embedded within the graphene oxide nanosheets, indicating the successful synthesis of the composite. AFM is one powerful tool to investigate the exfoliation rate, thickness and surfaces of graphitic flakes. 2D AFM images of CNCs, GO and GO/CNCs were shown in Fig. 3a–f. It can be observed that the CNCs was uniform distributed in dilute solution, presenting one-dimensional rod structure around 100 nm in Fig. 3a, with the height concentrated in 3–6 nm. In addition, the color of CNCs gradually changes from deep to light with the increasing of the CNCs

in length and height as shown in Fig. 3b [38]. Fig. 3c depicted the surface morphology and graphene oxide layers stacked together with the wrinkle on the edge. Fig. 3d was the AFM image of exfoliated GO sheet in a dilute solution, with a thickness around 1 nm. The increasing thickness of GO nanosheets was attributed to the functional groups on the surface [39]. The surface morphology of GO/CNCs composite was illustrated in Fig. 3e. It can be seen that the rod-like CNCs was cross-linked with the GO nanosheets. The high magnification AFM of GO/CNCs (Fig. 3f) clearly demonstrates that rod-like CNCs was homogeneously coated with the whole GO nanosheet, forming the 3D structure of GO/CNCs composite.

3.2. Removal of [BMIM][Cl] by GO/CNCs adsorbent 3.2.1. Effect of the content of GO on adsorption Fig. 4 shows the effect of different content of GO in mass for the adsorption of [BMIM][Cl]. The CNCs exhibited a low removal rate as low as 2.7%, meanwhile, GO showed a higher rate of 35.5%, respectively. With the addition of GO, the removal rate of GO/CNCs composite on [BMIM][Cl] was promoted with the increase of the mass content of GO in a range. When the mass content of GO was 30%, the removal rate reached the highest of 55.0%. However, the GO mass content was increased up to 50%, the adsorbent tended to be a much lower [BMIM][Cl] removal efficiency of 33.5%. The series experiments were investigated by the GO/CNCs adsorbent with the optimum content of 30% GO.

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Fig. 3. AFM images of CNCs (a and b), GO (c and d), GO/CNCs (e and f), (left: 10.0 × 10.0 ␮m, right: 1.0 × 1.0 ␮m).

3.2.2. Effect of pH on adsorption for [BMIM][Cl] The stability of [BMIM][Cl] in different pH values has been investigated in our previous work [40]. The investigation of initial solution pH was from 3 to 9 as shown in Fig. 5. The removal rate increased with the increasing of pH value and gradually achieved balance around pH of 6. In alkaline solution, the adsorption of [BMIM] cation on the adsorbent can be promoted because the oxygen of the functional groups ( OH, COOH) can get deprotonated easily [41]. The pH of [BMIM][Cl] solutions was around 6, therefore the experiments were carried out without adjusting the value of pH.

kinetic reaction models [42] in their linear form (Fig. 6b, c and d) were applied for the experiments. The pseudo-first-order (PFO), pseudo-second-order (PSO) and Elovich equations are generally expressed as follows: Pseudo-first-order model: 1n (qe − qt ) = 1nqe − k1 t

(3)

Pseudo-second-order model: t 1 1 = + t qt qe k2 qe 2

(4)

Elovich model: 3.2.3. Effect of contact time and adsorption kinetic The effect of contact time on adsorption of [BMIM][Cl] was shown in Fig. 6a from 0.5 h to 25 h. The adsorption amount increases rapidly with the time and then reaches equilibrium at 15 h. Therefore, the following experiments were carried out at 15 h. In order to evaluate the adsorption kinetics onto [BMIM][Cl], three different

qt = ␣+␤lnt

(5)

where k1 is the pseudo-first-order rate constant (h/), k2 (g/mmol H) is the pseudo-second-order adsorption rate constant; qe (mmol/g) and qt (mmol/g) are the amounts of [BMIM][Cl] adsorbed at equilibrium and at time t (h), ␣ (mmol/g) is the Elovich initial sorption rate

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Fig. 4. The effect on removal rate of CNCs, GO and GO/CNCs with different mass content of GO in composite (t = 15 h, 20 ml 0.2 mmol/g [BMIM][Cl] solution, 20 mg adsorbent).

Fig. 5. The effect of solution pH on adsorption for [BMIM][Cl] (t = 15 h, 20 ml 0.2 mmol/g [BMIM][Cl] solution, 20 mg adsorbent).

at zero coverage, and ␤ (g/mmol) is the Elovich desorption constant related to the extent of surface coverage and activation energy for chemisorption. The date and the parameters values obtained from the kinetic modeling were illustrated in Table 1. The values of the correlation coefficient showed good agreement for both the pseudo-second-

order model (R2 = 0.997) and the Elovich model (R2 = 0.990) for describing the experiment, indicating that chemisorption occurred due to the strong interactions between the [BMIM][Cl] cation and the oxygen-containing groups of adsorbent [43,44]. In addition, the adsorption process may be controlled by multiple adsorption mechanisms.

Fig. 6. (a) Effect of contact time on [BMIM][Cl] adsorption (pH around 6, 20 ml 0.2 mmol/g [BMIM][Cl] solution, m = 20 mg adsorbent); (b) pseudo-first-order kinetic linear fit for [BMIM][Cl] onto GO/CNCs (pH around 6, t = 15 h, 20 ml 0.2 mmol/g [BMIM][Cl] solution, 20 mg adsorbent, T = 25 ◦ C); (c) pseudo-second-order kinetic linear fit for [BMIM][Cl] onto GO/CNCs; (pH around 6, t = 15 h, 20 ml 0.2 mmol/g [BMIM][Cl] solution, 20 mg adsorbent, T = 25 ◦ C); (d) Elovich equation kinetic linear fit for [BMIM][Cl] onto GO/CNCs (pH around 6, t = 15 h, 20 ml 0.2 mmol/g [BMIM][Cl] solution, 20 mg adsorbent, T = 25 ◦ C).

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Table 1 The kinetic parameters for the pseudo-first-order, pseudo-second-order and Elovich models for [BMIM][Cl] adsorption onto GO/CNCs. qe (exp)(mmol/g)

0.110

Pseudo-first-order

Pseudo-second-order

model

Eovlich model

k1 (/ h)

qe (mmol/g)

model

k2 (g/mmol h)

qe (mmol/g)

a(mmol/g)

b(g/mmol)

0.009

0.073

3.91

0.120

0.441

0.022

Table 3 Comparison of maximum [BMIM][Cl] adsorption capacities onto various kinds of adsorbents.

Fig. 7. The adsorption isotherms of GO/CNCs for [BMIM][Cl] at 10 ◦ C, 30 ◦ C and 50 ◦ C (pH around 6, t = 15 h, m = 20 mg adsorbent). Table 2 Parameters of the adsorption isotherm models for Langmuir and Freundlich models for [BMIM][Cl] adsorption onto GO/CNCs. Temp.(◦ C)

10 30 50

Langmuir

Freundlich 2

Adsorbent

Raw materials

qmax (mmol/g)

Reference

FCM ␣-CM Forest Soils SPS-200 SPC-100 IER AC-MkU AC-MkS AC CSCM CNGD CNGS CNCs GO GO/CNCs

Cellulose Cellulose Soil Resin Resin Resin Activated carbon Activated carbon Activated carbon Corn stalk Corncob Corncob Corncob Graphite Cellulose, Graphene oxide

0.171 0.322 0.108 0.118 0.112 1.025 0.170 0.250 0.206 0.520 0.473 0.499 0.105 0.276 0.455

[26] [47] [48] [49] [49] [49] [50] [50] [26] [41] [27] [27] this work this work this work

functional groups and three-dimensional structure [46]. In order to further estimate the adsorption property of GO/CNCs, a comparison with other kinds of adsorbent was taken out in Table 3. It can be seen clearly that GO/CNCs showed a higher adsorption capacity of [BMIM][Cl] compared with other types of adsorbent. 4. Conclusion

qm (mmol/g)

KL L/mmol

R

kF (mmol/g)

1/n

R2

0.370 0.390 0.455

0.851 1.780 1.788

0.997 0.996 0.998

0.153 0.216 0.251

0.463 0.362 0.365

0.974 0.941 0.935

3.2.4. Adsorption isotherms The adsorption isotherm was an important method to investigate the interactive behavior between the solution and the adsorbent. In this study, Langmuir and Freundlich isotherm models [45] were applied in their nonlinear form under different temperature (10 ◦ C, 30 ◦ C and 50 ◦ C) in order to simulate and understand the adsorption mechanism. The Langmuir and Freundlich equation were given as follows: Ce 1 Ce = + qe KL qmax qmax

(6)

1 logqe = logkF + logCe n

(7)

Where qmax (mmol/g) is the theoretical maximum adsorption, qe (mmol/g) is the adsorption amount at equilibrium, Ce (mmol/l) is the concentration at equilibrium, KL (l/mmol) is the adsorption equilibrium constant, kF (mmol/g) and n are constants related to the adsorption capacity and adsorption intensity, respectively. Fig. 7 and Table 2 show the adjustment of Langmuir and Freundlich isotherms to the experiment data. As shown in Fig. 7, the adsorption capacity increased with the increase of the concentration, and then reached equilibrium. Meanwhile, the rise of temperature could promote the sorption process. It can be seen clearly that the Langmuir model fits the experiment data better than the Freundlich model according to the correlation coefficients R2 (Table 2). The Langmuir maximum adsorption capacity of [BMIM][Cl] was 0.455 mmol/g at 50 ◦ C, suggesting the monolayer adsorption processes between [BMIM][Cl] onto the rich oxygen

In this study, an effective method of 3D structure GO/CNCs composite for direct removal towards [BMIM][Cl] from aqueous solution has been demonstrated. The material is fully characterized by FTIR, XRD, SEM and AFM. The adsorption kinetics was better described by the pseudo-second-order model and the Elovich model. The equilibrium isotherm data fitted well with Langmuir isotherms, predicting a maximum sorption capacity of 0.455 mmol/g. The 3D structure and wrinkled surfaces provided an advantageous condition and improved the removal rate compared with CNCs and GO respectively. This composite was easily synthesized and firstly served as an adsorbent for the removal of [BMIM][Cl]. It has a higher adsorption capacity compared to other adsorbents. The unique structure of the composite indicated that it has potential application in the adsorption of heavy metal ions and organics. Meanwhile, it has potential application in supercapacitor, lithium-sulfur battery or other areas. Acknowledgments The authors are grateful to the National Natural Science Foundation of China(21576071, U1504215), and the International Science and Technology Cooperation Project of Henan Province(152102410023). References [1] S.H. Lee, H.W. Kim, J.O. Hwang, W.J. Lee, J. Kwon, C.W. Bielawski, R.S. Ruoff, S.O. Kim, Three-dimensional self-assembly of graphene oxide platelets into mechanically flexible macroporous carbon films, Angew. Chem. Int. Ed. 49 (2010) 10084–10088. [2] Z.S. Wu, A. Winter, L. Chen, Y. Sun, A. Turchanin, X.L. Feng, K. Mullen, Three-dimensional nitrogen and boron co-doped graphene for high-performance all-solid-state supercapacitors, Adv. Mater. 24 (2012) 5130–5135.

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