Adsorption of methylene blue on graphene oxide ...

Journal of Molecular Liquids 221 (2016) 82–87

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Adsorption of methylene blue on graphene oxide prepared from amorphous graphite: Effects of pH and foreign ions Weijun Peng, Hongqiang Li, Yanyan Liu, Shaoxian Song ⁎ School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei 430070, China Hubei Provincial Collaborative Innovation Center for High Efficient Utilization of Vanadium Resources, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei 430070, China Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei 430070, China

a r t i c l e

i n f o

Article history: Received 18 February 2016 Received in revised form 10 May 2016 Accepted 10 May 2016 Available online 25 May 2016 Keywords: Amorphous graphite Inorganic ions Graphene oxide Methylene blue Removal

a b s t r a c t The adsorption of methylene blue (MB) on graphene oxide (GO) prepared from an economical and resourceful amorphous graphite (AG) was studied. The effects of pH, foreign ions and concentrations of KClO4 on MB removal were also evaluated. XRD, FT-IR, XPS, AFM and an Analyzer were employed to characterize the prepared GO. The results indicated that the C/O mass ratio of GO reached 1.84 and that thin layers (thickness of which less than 2 nm) accounted for 83.76%. The results of batch experiments showed that the adsorption of MB in the presence − − of cations decreased in the sequence Li+ ≈ Na+ N K+, while it was reduced in the order ClO− 4 N NO3 N Cl in the presence of anions. MB removal in ClO− 4 was independent of solution pH, which may be ascribed to the synergistic effect between GO and ClO− 4 . The adsorption process was well described by a pseudo-second-order kinetics model, and the adsorption isotherm agreed well with the Langmuir model. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Graphene oxide is the oxidized form of graphene [1], functionalized by a range of active oxygenous groups, including epoxide and hydroxyl in the planes and carbonyl and carboxyl groups at the edges [2,3]. Conventionally, GO is prepared from flaky and synthetic graphites. AG is a type of abundantly reserved natural graphite with poor crystallinity and a small crystal particle size of less than 1 μm [4] and is not frequently used to synthesize GO. Notably, AG was oxidized much more easily than flaky graphite due to its poor crystallinity, large specific surface area and small lateral size [1,5]. The oxygenous functional groups (such as –OH, –COOH) on the GO surface are extremely beneficial to the hydrophilicity and high negative charge density, which were directly related to the removal of contaminants [6,7]. Thus, AG should be a potential raw material for the synthesis of a GO adsorbent. MB dye is a heteroaromatic compound that is harmful to the quality of water, resulting in various adverse outcomes [8]. Compared with other adsorbents, including neem leaf [9], red mud [10], bagasse fly ash, sawdust [11], conducting polymers [12], activated carbon [13] and carbon nanotubes [14], GO is regarded as the most promising absorbent to adsorb MB [6,15,16] due to its large theoretical surface area, surface hydrophobic π-π interactions, hydrophilicity, high negative charge density and ease of obtaining large quantities from

⁎ Corresponding author. E-mail address: [email protected] (S. Song).

http://dx.doi.org/10.1016/j.molliq.2016.05.074 0167-7322/© 2016 Elsevier B.V. All rights reserved.

abundant natural graphite [17,18]. Moreover, the high affinity of GO to MB dye is principally ascribed to the electrostatic interactions between negatively charged oxygenous functional groups on GO and positively charged amino groups on MB molecules, while the π-π interactions between the localized π electrons in the conjugated aromatic rings of GO and MB dye molecules might also contribute to the interaction [15,17,19]. Foreign ions have a vital effect on the adsorption of contaminants from wastewater. Yang et al. confirmed that foreign cations could modify the surface properties of oxidized multi-walled carbon nanotubes (MWCNTs) and influence the adsorption of Ni(II) [20]. Furthermore, Wang et al. reported that the competition between adsorbate and foreign ions (Na+ and K+) for the active surface sites on GO might affect its adsorption capacity [21]. Gao et al. found that the adsorption capacities of tetracycline on GO decreased with the addition of NaCl [22], whereas the existence of 100 mM Na+ could make the adsorption capacity of MB on GO increase by 26% at an MB concentration of 500 mg/L [15]. The impact of foreign ions on the adsorption capacity of other adsorbents has been reported in many studies [15,20–23]. However, systematic investigation of the effect of foreign ions on the removal of MB by GO is still lacking. Hence, the removal of MB by GO in the presence of various foreign − ions (Li+, Na+, K+, Cl−, NO− 3 and ClO4 ) was investigated. The objectives of this work were: (1) to characterize GO prepared from amorphous graphite; (2) investigate the effect of the initial solution pH, presence of foreign ions and concentrations of KClO4 on MB removal by the GO; and (3) study the adsorption kinetics and isotherms of MB on GO.

W. Peng et al. / Journal of Molecular Liquids 221 (2016) 82–87

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2. Experimental 2.1. Chemicals AG with a particle size of 45 to 38 μm and purity of more than 99% was used to synthesize GO. Sulfuric acid (98%) and hydrochloric acid (36%) were obtained from Xinyang Chemical (China). All of the other analytical grade reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Deionized water (18.25 MΩ cm) produced by a Milli-Q Direct 16 (Millipore Q, USA) was used. 2.2. Preparation of graphite oxide (GrO) and GO GrO was prepared from AG using Hummers' Method [24], as reported previously [25,26]. The GrO powder was re-dispersed in deionized water (0.67 mg/mL) to form a yellow-brown GrO suspension. Then, the GrO suspension was ultrasonically exfoliated by a Cole Parmer ultrasonic processor (750 W and 20 kHz) with a 60% amplitude for 9 min. Finally, the colloidal suspension was centrifuged at 2520 g for 20 min to remove unexfoliated GrO. The homogeneous supernatant contained GO.

Fig. 1. XRD patterns of AG before and after oxidation.

3. Results and discussion 2.3. Measurements 3.1. Characterization of GO X-ray diffraction (XRD) patterns were obtained using an advance diffractometer (D8, Bruker, Germany). The Fourier transform infrared (FT-IR) spectrum was detected by a Fourier transform infrared spectrometer (Vector-22, Bruker, Germany). X-ray photoelectron spectroscopy (XPS) was measured on a VG Multilab 2000 spectrometer, and the XPS spectra were corrected by the C1s line at 284.6 eV. Elemental analysis of carbon, oxygen and hydrogen in the prepared GO powder was performed with a Vario EL cube analyzer (Germany). Atomic force microscope (AFM) images of GO were obtained with a Bruker MultiMode 8 AFM with the peak force tapping-mode. Zeta potential analysis of GO in the presence of 5 × 10−3 mol/L NaCl was carried out on a Malvern Zetasizer Nano ZS90 (UK) equipped with a rectangular electrophoresis cell at 25 °C. 2.4. MB adsorption Analytical-grade MB was used to prepare a 0.5 g/L MB stock solution. The batch experiments of MB adsorption on GO were carried out in conical flasks into which the stock suspensions of GO (0.6 g/L), MB and foreign ion (LiCl, NaCl, KCl, KNO3 and KClO4) solutions were added to achieve the desired concentrations of different components. The desired pH of the suspension in each flask was adjusted by adding negligible volumes of 0.1 mol/L HCl, HNO3, HClO4, KOH or NaOH solutions. After the suspensions were shaken thoroughly by a mechanical shaker (HZQ-C, Hangzhou Chincan Trading Co., Ltd., Hangzhou, China) at an agitation speed of 160 rpm for a regular time interval at 30 °C, the solid phase was separated from the solution using 0.22-μm membrane filters. The concentration of MB in the filtrate was determined by a UV spectrophotometer (Orion Aquamate 8000, Thermo, US) at a wavelength of 665 nm. The amount of MB adsorbed was calculated using the following equation: qt ¼

C0 −Ct V m

Fig. 1 shows the XRD patterns of AG before and after oxidation. The pattern showed a strong and sharp peak at approximately 2θ = 26.51°, corresponding to the characteristic (002) reflection plane of graphite. In the XRD pattern of GrO, a dominant and broad peak at 2θ = 10° was observed, corresponding to an average d-spacing of 0.808 nm, which is significantly larger than that of AG (0.336 nm). This indicated that numerous oxygenous functional groups were successfully intercalated into the regular AG crystalline structure during the oxidation process [27]. The FT-IR spectrum of the GO powder is shown in Fig. 2. The peak at 3391 cm−1 is ascribed to the –OH stretching vibration from the hydroxyl group and water molecules [28]. The peak that appeared at 1722 cm−1 corresponded with C_O stretching vibrations from carbonyl and carboxylic groups. The peak that corresponded with the aromatic C_C stretching mode from unoxidized graphitic domains appeared at 1615 cm−1 [29]. The peak located at 1399 cm−1 arose from deformation vibration of the tertiary C\\OH groups [30]. In addition, the peaks at 1228 cm− 1 and 1053 cm− 1 belonged to the C\\O\\C and C\\O stretching vibrations, respectively [31]. This indicated that AG was

ð1Þ

where C0 and Ct (mg/L) are the liquid-phase concentrations of MB at the initial and final concentrations, respectively. V is the volume of the solution (L) and m is the mass of adsorbent used (g). The removal was calculated using the following equation: Removal ð%Þ ¼

C0 −Ct 100% C0

ð2Þ

Fig. 2. FT-IR spectrum of the GO prepared from AG.

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thoroughly oxidized in the oxidation process, resulting in the introduction of abundant oxygenous functional groups on the surface of GO sheets. The XPS survey spectrum of the GO powder is presented in Fig. 3a. Only C1s and O1s peaks centred at 284.6 and 531.0 eV were observed in GO [32]. The C1s XPS spectrum of GO (Fig. 3b) could be deconvoluted into four peaks attributed to C\\C/C_C (284.6 eV) in the aromatic rings as well as the C\\O (286.4 eV) of epoxy and alkoxy groups and C_O (288.3 eV) and O\\C_O (289.3 eV) groups [7,27,33], which are in agreement with the results of the FT-IR spectrum. Moreover, the relative contents of carbon, oxygen and hydrogen in the prepared GO powder were measured and the values were 64.10%, 34.77% and 1.13%, respectively. The mass ratio of C and O (C/O) reached 1.84, indicating that numerous oxygenous functional groups existed in GO. Compared with the C/O ratios of 2.3 for GO prepared from natural graphite flakes [34] and 2.0 for GO synthesized from synthetic graphite [35], GO prepared from AG contained more oxygenous functional groups that have been confirmed to be beneficial for the removal of MB dye from aqueous solution [36]. Fig. 4a shows the representative 2D AFM image of GO. Notably, most of the sheets in the image had a similar colour contrast, indicating that their thickness was almost the same. The cross-section analysis along with the line in AFM image of individual sheet is shown in Fig. 4b. The typical thickness of GO was approximately 0.942 nm, identical to conventionally prepared single GO sheets (0.7–1.2 nm) [35,37,38]. Four-hundred sheets in the 2D AFM images of GO were analysed for determination of the distribution of the topographic height. The thickness distribution of GO sheets is shown in Fig. 4c; notably, GO sheets with thickness of less than 2 nm (approximately 1–2 layers) accounted for 83.76%.

Fig. 3. Wide-scan XPS spectrum (a) and C1s XPS spectrum (b) of GO prepared from AG.

Fig. 4. Topographical 2D AFM image (a), corresponding height profiles (b), and histograms of the thickness distribution of GO sheets (c).

3.2. Adsorption study 3.2.1. Effect of initial solution pH The effects of the initial solution pH on the MB removal by GO and zeta potential of GO are shown in Fig. 5. The removal of MB by GO clearly increased with the initial solution pH, and the zeta potential of GO was negatively charged in the pH range from 2 to 12. The negative charge value was sharply enhanced when the initial solution pH was increased from 2.0 to 5.0, and there was a small fluctuation over the pH range from 5.0 to 12.0. The GO surface was highly negatively charged in the pH range from 5.0 to 12.0, which might be ascribed to the deprotonation of anionic oxygenous functional groups on GO [39]. Highly negatively charged GO sheets in suspension could not only disperse homogeneously via electrostatic repulsion [40] but also enhance the electrostatic attraction between them and cationic MB molecules, obviously improving the adsorption efficiency [6,15,17]. In addition, more H+ was released from the carboxyl groups on the GO surface by deprotonation after the adsorption reached equilibrium. The released H+ would be depleted via a neutralization reaction at higher solution pH, which would additionally benefit the absorption [15].

Fig. 5. Effect of the initial solution pH on the zeta potential of GO and MB removal by GO (Initial condition: MB 100 mg/L, GO 60 mg/L).


3.2.2. Effect of foreign ions Fig. 6 shows the removal of MB from solution by GO in the presence of 0.01 mol/L LiCl, NaCl, KCl, KNO3 or KClO4 as a function of the solution pH. As shown in Fig. 4a, the removal of MB was lower in the KCl solution than that in the LiCl and NaCl solutions in the pH range from 2 to 12, indicating that cations could change the surface properties of GO and influence the MB removal. The MB adsorption on GO could be considered to be a competition of MB with foreign cations (Li+, Na+, K+) on the GO surface. The hydration radius of K+, 2.32 Å, is smaller than those of Na+ (2.76 Å) and Li+ (3.40 Å) [41], so K+ had the highest affinity to the GO surface and highest tendency for counter-ion exchange with the oxygenous functional groups on the GO surface, which decreased the number of active interaction sites on the GO surface. The influence of K+ on MB removal was more obvious than that of Li+ and Na+. Furthermore, the foreign cations could reduce the interaction of MB with H2O, making it more amiable to GO [15]. The influence sequence of foreign alkali metal ions on MB removal at pH 2–12 was Li+ ≈ Na+ b K+, in accordance with the effects of foreign ions on the adsorption of Pb(II) to adsorbents at low pH [20,42,43]. Additionally, it can be seen from Fig. 6b that foreign anions drastically affected MB removal. MB removal was higher in the KNO3 and KClO4 solution than in the KCl solution, and the MB removal in the KClO4 solution was independent of the initial solution pH. This phenomenon may be ascribed to: (1) the order of inorganic acid radical radius of

85

Fig. 7. Effect of ionic strength on the removal of MB by GO (initial condition: MB 100 mg/L, solution pH 4.70). − Cl− bNO− 3 b ClO4 [44] as smaller radii inorganic acid radicals occupy more ionic exchange sites and result in a decrease of MB removal by GO, in agreement with the Ni2+ adsorption on an ammonium citrate tri− basic-attapulgite surface in the presence of Cl−, NO− 3 and ClO4 [45]. (2) − Cl was easily captured by the adjacent hydroxyl or carboxyl groups on the GO surface, which may change the GO surface state and decrease the availability of binding sites [20,46]. (3) ClO− 4 can react with MB by complexation in aqueous solution as in Eq. (3), forming a light green, slightly soluble and less charged complex, resulting in the enhancement of MB removal in solution by GO [47].

KClO4 þ C16 H18 N3 SCl→C16 H18 N3 SClO4 þ KCl

ð3Þ

3.2.3. Effect of the KClO4 concentration Fig. 7 shows that the removal of MB is affected by the concentration of KClO4. When the concentration of KClO4 increased from 0 to 0.01 mol/L and from 0.01 to 0.02 mol/L, the removal of MB in aqueous solution increased abruptly and slowly, respectively. When no adsorbent GO was added, the removal of MB was also increased in a similar manner due to ClO− 4 reacting with MB by complexation in aqueous solution; the maximum removal of MB was approximately 91%. When 30 or 60 mg/L GO was added, the variation in MB removal was almost the

Fig. 6. Effect of foreign ions on the removal of MB by the GO (initial condition: MB 100 mg/L, GO 60 mg/L, foreign ions 0.01 mol/L).

Fig. 8. Models of the Langmuir, Freundlich and Temkin isotherms for MB adsorption on GO (initial condition: GO 60 mg/L, solution pH 11.50).

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Table 1 Langmuir, Freundlich and Temkin isotherm model constants and correlation coefficients for MB adsorption onto GO. Langmuir

Freundlich

Temkin

qmax (mg·g−1)

KL (L·mol−1)

R2

n

KF (mol1−n·Ln/g)

R2

A (mg·g−1)

B (J·mol−1)

R2

2273.59

1.39

0.967

6.48

1220.54

0.734

1195.64

282.23

0.839

same with the increase of the KClO4 concentration and the maximum removal of MB was 97.61% and 98.61%, respectively. Compared with the condition of no adsorbent addition, the sharp increase of the MB removal may be ascribed to the synergistic effect between GO and ClO− 4 . 3.2.4. Equilibrium adsorption isotherms The Langmuir (Eq. (4)) [48,49], Freundlich (Eq. (5)) [49,50], and Temkin isotherm (Eq. (6)) [51,52] equations were applied to evaluate the experimental data equilibrium: qe ¼

q max K L C e 1 þ K L Ce

qe ¼ K F C e 1=n

ð4Þ ð5Þ

qe ¼ A þ B lnC e

ð6Þ

where Ce is the equilibrium concentration of MB in solution (mg·L−1), qe is the amount of MB adsorbed on GO (mg·g−1), qmax is the maximum amount of MB adsorbed per unit weight of GO, KL represents the enthalpy of adsorption and should vary with temperature (L·mol− 1). KF (mol1−n·Ln/g) and n represent Freundlich constants related to the adsorption capacity and adsorption intensity, respectively. A (mg·g−1) and B (J·mol−1) are the Temkin constants. As shown in Fig. 8 and Table 1, the lower Freundlich and Temkin R2 determination coefficients indicated that the experimental data did not fit the Freundlich and Temkin models. In contrast, the adsorption experimental data of MB on GO fit the Langmuir model well, with the highest regression correlation of R2 = 0.967. This suggested that the Langmuir equation can be used to evaluate the maximum MB adsorption capacity on GO; the calculated value was approximately 2273.59 mg/g. 3.2.5. Adsorption kinetics Adsorption kinetics experiments in solution at pH 11.50 were conducted to investigate the effect of contact time on the adsorption of MB on GO and to obtain the relevant kinetics parameters. The kinetics parameters of MB adsorption on GO were evaluated using the pseudofirst-order (Eq. (7)) and pseudo-second-order (Eq. (8)) kinetics models [9,16,53,54]. The kinetics models used for describing the adsorption behaviour are expressed as ln ðqe −qt Þ ¼ ln qe −k1 t

ð7Þ

t 1 t ¼ þ qt k2 qe 2 qe

ð8Þ

where k1 (min−1) and k2 (g·mg−1·min−1) are the pseudo-first-order and pseudo-second-order rate constants, respectively. qe (mg·g− 1) and qt (mg·g−1) are the adsorption capacity of MB on GO at equilibrium and at time t (min), respectively. Linear plots of ln(qe-qt) vs. t and t/qt vs. t are shown in Fig. 9. All of the kinetics parameters that were obtained by linear fitting using the kinetics models are summarized in Table 2. The determination coefficient R2 of the pseudo-first-order model was 0.832, suggesting that the pseudo-first-order model was inapplicable to fit the experimental kinetics data. The R2 correlation coefficient of the pseudo-second-order model was 0.997 and the calculated value of equilibrium capacity qe was 2302.81 mg/g, in accordance with the experimentally measured capacity of 2255.35 mg/g. This suggested that MB adsorption on GO could be well-described by the pseudo-second-order kinetics model. The adsorption capacities of MB on GO prepared from other types of graphite are listed in Table 3 for comparison. The adsorption capacity of MB on GO prepared from AG was obviously better than those prepared from natural flaky and expandable graphite, indicating that the

Table 2 Kinetics models parameters for adsorption of MB on the GO.

Fig. 9. Pseudo-first-order (a) and pseudo-second-order (b) adsorption kinetics models for MB adsorption on GO (Initial condition: GO 60 mg/L, MB 150 mg/L, solution pH 11.50).

qe(exp) (mg/g)

Pseudo 1st kinetic model

Pseudo 2nd kinetic model

k1(min−1)

qe(mg/g)

R2

k2(g·mg−1·min−1)

qe(mg/g)

R2

2255.35

0.045

369.46

0.832

1.59 × 10−4

2302.81

0.997

W. Peng et al. / Journal of Molecular Liquids 221 (2016) 82–87 Table 3 Comparison of MB adsorption capacities on GO prepared from the other types of graphite. Type of graphite

Adsorbent

Adsorption capacity (mg/g)

Reference

Expandable graphite Expandable graphite Expandable graphite Flaky graphite Flaky graphite Flaky graphite Flaky graphite Synthetic graphite Amorphous graphite

Graphene GO GO GO GrO GO GO/poly GO GO

153.85 470 243.90 714 351 1939 1530 2415 2273

[16] [18] [54] [15] [55] [56] [33] [57] This work

economical and resourceful AG was preferable for synthesis of GO adsorbent. 4. Conclusions (1) An abundant reserve and infrequently used amorphous graphite was employed to prepare GO. Numerous oxygenous functional groups and 83.76% thin layers less than 2 nm were detected in GO, indicating that amorphous graphite is preferable for the synthesis of the GO adsorbent. (2) The MB adsorption on GO in the presence and absence of foreign ions was dependent on the initial solution pH and ClO− 4 concentration. The influence sequence of foreign cations on the adsorption of MB over the pH range from 2 to 12 was Li+ ≈ Na+ b K+. In addition, the − MB adsorption was greater in the presence of NO− 3 and ClO4 than that − − in the presence of Cl . MB removal in ClO4 was independent of the initial solution pH, and a synergistic effect may exist between GO and ClO− 4 in the MB removal process. (3) Electrostatic attraction played a dominant role in the removal of MB from aqueous solution by GO. The experimental adsorption capacity of MB on GO was 2255.35 mg/g. Moreover, MB adsorption followed pseudo-second-order kinetics and the Langmuir isotherms. Acknowledgements The financial support for this work from the National Natural Science Foundation of China under projects no. 51504176 and no. 51474167 is gratefully acknowledged. References [1] J.-L. Li, K.N. Kudin, M.J. McAllister, R.K. Prud'homme, I.A. Aksay, R. Car, Phys. Rev. Lett. 96 (2006) 176101. [2] L. Liu, S. Liu, Q. Zhang, C. Li, C. Bao, X. Liu, P. Xiao, J. Chem. Eng. Data 58 (2012) 209. [3] D.R. Dreyer, S. Park, C.W. Bielawski, R.S. Ruoff, Chem. Soc. Rev. 39 (2010) 228. [4] A.R. Ubbelohde, F.A. Lewis, Graphite and Its Crystal Compounds, Clarendon Press, 1960. [5] W. Peng, H. Li, Y. Hu, Y. Liu, S. Song, Mater. Res. Bull. 78 (2016) 119. [6] G.K. Ramesha, A.V. Kumara, H.B. Muralidhara, S. Sampath, J. Colloid Interf. Sci. 361 (2011) 270. [7] Y. Bian, Z.-Y. Bian, J.-X. Zhang, A.-Z. Ding, S.-L. Liu, H. Wang, Appl. Surf. Sci. 329 (2015) 269.

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