Removal of a Cationic Dye from Aqueous Solution Using Graphene

Dec 13, 2012 - The adsorption of the methyl green onto the GO nanosheets has been carried out at different experimental conditions such as adsorption ...
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Removal of a Cationic Dye from Aqueous Solution Using Graphene Oxide Nanosheets: Investigation of Adsorption Parameters Ponchami Sharma and Manash R. Das* Materials Science Division, CSIR-North East Institute of Science and Technology, Jorhat-785006, Assam, India S Supporting Information *

ABSTRACT: In this study, graphene oxide (GO) nanosheets have been used for the adsorption of methyl green, a cationic dye from aqueous solution. GO nanosheets consist of single layered graphite structure decorated with a number of oxygen containing functionalities such as carboxyl, epoxy, ketone, and hydroxyl groups which impart a negative charge density to it in aqueous solution at a wide range of pH. Thus, GO nanosheets can be predicted as a good adsorbent material for the adsorption of cationic species. The adsorption of the methyl green onto the GO nanosheets has been carried out at different experimental conditions such as adsorption kinetics, concentration of adsorbate, pH, and temperature. The kinetics of the adsorption data were analyzed using four kinetic models such as the pseudofirst-order model, pseudosecond-order model, intraparticle diffusion, and the Boyd model to understand the adsorption behavior of methyl green onto the GO nanosheets and the mechanism of adsorption. The kinetics of adsorption result shows that the adsorption maximum was reached at 60 min and follows the linear form of pseudosecond-order kinetics. The adsorption isotherm of adsorption of the methyl green onto the GO nanosheets has been investigated in the pH range of 4 to 9 at 25 °C. The equilibrium data were fitted well to the Langmuir model. Various thermodynamic parameters such as the Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) change were also evaluated. The negative value of ΔG indicates spontaneity of the adsorption process of the methyl green−GO system. The interaction of methyl green onto the GO nanosheets has been investigated by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy.

1. INTRODUCTION Pollution of water resources by various pollutants is nowadays a global environmental issue. Among the different types of water pollutants, dye represents a major polluting group.1,2 Recently the demand of dyes used in industries such as textiles, paper, plastics, leather, food, cosmetics, and so forth is increasing.1 The release of colored wastewater from these industries may present an eco-toxic hazard and introduce the potential danger of bioaccumulation.1 A range of treatment technologies such as trickling filters, active sludge, chemical coagulation, photodegradation, adsorption with different materials, membrane filtration, and so forth has been studied extensively for dye removal over the years.2,3 Though a number of processes are available for dye removal from aqueous system, adsorption is getting special interest from the researchers worldwide due to its high efficiency, cost effectiveness, and simple operation process.4 Therefore, there is always a tendency of the researchers to find new adsorbent materials which may give more efficient results in this regard. Carbon-based materials are widely used for the dye removal process.4−7 A recent literature review focused on the use of the graphene or graphene oxide (GO) materials as promising adsorbent materials for the removal of the dye molecule from the water.8−11 Graphene is made of single layer of carbon atoms which are closely packed into honeycomb twodimensional (2D) lattice.12,13 Graphite is the basic material © XXXX American Chemical Society

for preparation of individual graphene or GO nanosheets. Exfoliation of graphite oxide by ultrasonication results in single layered GO nanosheets. The large surface area, oxygen containing surface functionalities such as hydroxyl, carboxylic, carbonyl, and epoxide groups, and high water solubility makes GO a material of great interest in adsorption-based technologies as well as in other fields.14,15 The electrostatic charge−charge interaction of the graphene and GO with the adsorbate make them materials of choice for adsorption of charged species.9,10 Many research groups have reported the use of GO and graphene as adsorbent material for dye removal. The adsorption kinetics of acid red 3B onto expanded graphite was investigated by Peng and Gong,8 and they observed that the adsorption equilibrium time depends on the temperature. The adsorption equilibrium is reached at 2.5 h at 45 °C, whereas it takes 24 h at 5 °C. Both the pseudofirst-order and pseudosecond-order kinetic models were used to evaluate the kinetic parameter of adsorption of acid red 3B onto expanded graphite. Bradder et al.9 investigated the adsorption behavior of two cationic dyes namely methylene blue and malachite green onto the graphite and layered graphite oxide surfaces. The pseudosecond-order kinetic model was used to investigate the Received: September 17, 2012 Accepted: December 3, 2012

A

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were used as received without any further purification. Deionized water was used throughout the whole experiment. 2.2. Preparation of GO. GO was prepared from powder graphite adopting the Hummers and Offeman method.22 Briefly, 1 g of graphite powder was added to 23 mL of concentrated H2SO4 in an ice bath. KMnO4 (3 g) was then added slowly with stirring and cooling to keep the temperature of the reaction mixture below 20 °C. The temperature of the reaction mixture was increased and maintained at 35 °C for 30 min. Then 46 mL of deionized water was added slowly to this mixture temperature was increased to 98 °C. After 15 min 140 mL of deionized water was added followed by 10 mL of 30 % H 2 O 2 solution. The solid product was separated by centrifugation (model: Hermle, Z36HK, Germany) and washed repeatedly with 5 % HCl solution and distilled water. The product was then filtered and washed with acetone to make it moisture free. The residue thus obtained was dried in an air oven at 65 °C overnight. The graphite oxide was suspended in water and exfoliated through ultrasonication (model: MM3010, OSCAR Ultrasonic, India) for 3 h to obtain GO nanosheets. Characterization of GO nanosheets thus obtained are reported in detail in our previous publications.23−25 2.3. Preparation of Dye Solution. The methyl green stock solution of 5 mM concentration was prepared by dissolving an accurately weighted amount of methyl green in deionized water. The pH of the stock dye solution was maintained at the same pH of the GO−water suspension for desired experiment. The desired concentrations of the solution were obtained by diluting the stock methyl green dye solution in accurate proportions to obtain different initial concentrations. 2.4. Adsorption Kinetics. The adsorption kinetics of methyl green (initial concentration: 0.2 mM) onto the GO− water suspension was carried out at pH 5 and at four different temperatures, namely, (18, 25, 30, and 35) °C, in aqueous medium. A sample of 2 mL of GO−water suspension (concentration: 10 g·L−1) was added in a round-bottom flask and mixed with 150 mL of water. The pH of the suspension was maintained at 5 by HCl or NaOH solution (0.01 N) using a pH meter (μ-pH system 362, Systronics, India) calibrated with standard buffer solutions. The buffer solutions were prepared by dissolving buffer tablets of pH 4, 7, and 9.2 (AR grade, Rankem, India) in 100 mL of deionized water at 20 °C. The GO-water suspension was then placed in a water bath over a magnetic stirrer integrated temperature compatible with PT 1000 temperature sensor (IKA, RCT basic safety control, Germany) and maintained at the desired temperature. The temperature of the reaction mixture was observed with the calibrated thermometer. 40 mL of dye solution was added to the suspension (concentration of the dye 1 mM, pH 5). Finally a balance amount of water was added to make the final volume of the reaction mixture 200 mL. The reaction mixture was withdrawn at 5 min intervals until 120 min and immediately frozen in an ice bath. The solution was separated from the precipitate by centrifuge at 12 000 rpm for 20 min. The residual concentration of the methyl green dye in the solution was determined at absorbance maxima, λmax = 626 nm by a UV−vis spectrophotometer (Simadzu UV-1800 spectrophotometer, Japan) calibrated with standard samples. The concentration of the methyl green after adsorption onto the GO nanosheets was determined from the standard calibration curve. The amount of methyl green adsorbed onto the GO nanosheets was determined by using the following equation

kinetics of adsorption of methylene blue and malachite green onto the graphite and graphite oxide. The two well-known isotherm models, namely, Langmuir16 and Freundlich17 models, were also used for the adsorption isotherm study. The adsorption equilibrium of methylene blue and malachite green on graphite was reached at around (40 to 60) min, whereas it takes only around 20 min in case of graphite oxide. The surface acidity and oxygen functionalities present in graphite oxide play an important role in enhancing its cationic dye adsorption capacity. It is also shown that the adsorption capacity of the methylene blue and malachite green onto the layered graphite oxide is more than that of graphite. Ramesha et al.10 investigated the adsorption of a number of cationic and anionic dye molecules such as methylene blue, methyl violet, rhodamine B, and orange G onto graphene and GO in aqueous medium. They have presented the kinetics, isotherm, and effect of pH on the extent of dye removal from water in their study. Their investigation also reveals that the kinetics and isotherm follow pseudosecond-order kinetics and Langmuir model, respectively. The cationic dyes were better adsorbed on the GO surface, while the anionic dyes were better adsorbed in the graphene surface. It is concluded that the charge−charge interaction is responsible for this adsorption behavior. In case of the cationic dyes, charge transfer from negatively charged GO to the dye molecules takes place, while in case of the anionic dyes, charge transfer takes place from negatively charged dye molecules to graphene species. Liu et al.11 investigated the effect of pH, contact time, temperature, and adsorbent dosage for adsorption of methylene blue onto graphene surface. Like the other researchers, they also found a kinetic and isotherm process to follow pseudosecond-order and Langmuir models, respectively. The adsorption capacity increases with pH which may be due to the formation of more functional groups on graphene that increases their adsorption capacity. The negative value of Gibbs free energy indicates the spontaneity of the process. Similar results were also independently presented by Zhang et al.18 and Yang et al.19 for the adsorption of methylene blue on to the GO surfaces. Fan et al.20,21 investigated the adsorption of methylene blue and methyl blue on the magnetic chitosan/GO composites. The adsorption processes follow the pseudosecond-order kinetics and Langmuir isotherm model. They found that the adsorption of methylene blue on the magnetic chitosan/GO composites increases with the increase of pH of the medium.20 On the other hand, the adsorption density of methyl blue on the same adsorbent (magnetic chitosan/GO composites) increases in the pH range 4.5 to 6.5; after that adsorption density decreases.21 However, so far to our knowledge, detailed investigation of adsorption of methyl green dye molecule onto graphene and GO nanosheets based on kinetics, equilibrium, and thermodynamic parameters has not been reported yet. Here, we report the adsorption of methyl green, a cationic dye onto the GO nanosheets in aqueous medium with special emphasis on kinetics of adsorption, thermodynamic parameters, and effects of pH and temperature on the adsorption process.

2. EXPERIMENTAL SECTION 2.1. Materials. Graphite powder ( 99 %, NICE-Chemical, India), NaOH (99 %, Qualigens, India), and methyl green (Loba Chemie, India) B

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(1)

where Co is the initial concentration of methyl green (mM), Ct is the concentration of methyl green at time t (mM), V is the total volume of the suspension (L), and m is the mass of adsorbent (g). 2.5. Adsorption Isotherm. The batch adsorption isotherm experiments were carried out at different initial pH values, namely, 4, 5, 6, 7, and 9, at 18 °C. A sample of 0.2 mL of GO suspension (concentration: 10 g·L−1) was mixed with 10 mL of water in a screw-capped glass tube with the help of a vortex mixer. The desired pH of the suspension was adjusted by adding either NaOH or HCl solution (0.01/0.001 N). The balance amount of water was added to make the final volume of the suspension 20 mL. Then the required amount of methyl green dye solution (stock solution 5 mM) was added and shacked. After reaching the equilibrium time (60 min), the reaction mixture were transferred to an ice bath to freeze the reaction. Then, dye solutions were separated from the mixture by centrifuge (12 000 rpm for 20 min) and analyzed by UV−vis spectroscopy. 2.6. DRIFT Spectroscopy. To understand the mechanism of adsorption of methyl green onto the GO nanosheet, diffuse reflectance infrared Fourier transform (DRIFT) spectra of the GO was recorded before and after adsorption of the dye molecule. For DRIFT spectroscopic studies, 1 mM methyl green was equilibrated with 100 mg·L−1 of GO at pH 5 and 25 °C maintaining total volume of the solution 100 mL following the same procedure adopted for adsorption. The suspension was centrifuged, and the residue was dried in a vacuum desiccator over fused calcium chloride. The DRIFT spectra was recorded with a IR Affinity-1, Shimadzu, Japan FTIR spectrophotometer using a Shimadzu DRS-8000 DRIFT accessory and IRsolution software. In all cases, the spectra were recorded with 200 scanning and 4 cm−1 spectral resolution.

Figure 1. Adsorption density vs time curve for adsorption of methyl green onto GO nanosheets at different temperature. Initial concentration of methyl green: 0.2 mM, concentration of GO suspension: 100 mg·L−1, pH 5, total volume 200 mL. The data points are the average of triplicate experiments, and an error bar is shown for reference.

qe were evaluated from Figure S1 (shown in Supporting Information) and are presented in Table 1. The R2 value of the plot is found to be in the range of 0.920 to 0.951. It is also observed from the estimated qe values to be much lower than that of the experimental values [deviation: (89.44 to 81.35) %]. The linear form of pseudosecond-order kinetics is presented by the equation27 t /qt = 1/k 2qe 2 + 1/qet

where k2 is the pseudosecond-order adsorption rate constant. The values of k2 and qe were estimated from the slope and intercept of plots of t/qt versus t (Figure 2) and presented in Table 1. It is observed that the R2 value (0.999) is better than the pseudofirst-order kinetic model. The estimated k2 value is found to be much closer to that of the experimental concentration data. The percentage of deviation is in the range of (0.00 to 1.13) %. The nonlinear form of pseudosecond-order model is given in eq 428

3. RESULTS AND DISCUSSION 3.1. Adsorption Kinetics of Methyl Green onto the GO Nanosheets. Adsorption kinetics is an important step to investigate the adsorption process. Kinetics of adsorption of methyl green onto the GO nanosheets in aqueous medium at pH 5 and at four different temperatures ((18, 25, 30, and 35) °C) is presented in Figure 1. It is observed from Figure 1 that the kinetic equilibrium is reached at 60 min and the adsorption of methyl green onto GO nanosheets increases with the increase of the temperature. After attaining equilibrium, the adsorption density gradually decreases in all temperatures which indicate that the desorption of the dye molecule from the adsorbent surface starts after equilibrium is reached. This may be due to the fact that the adsorption process is governed by physical adsorption and the dye molecules tend to revert back to the solution after attaining equilibrium. The process of physical adsorption is discussed later. The kinetic parameters of the adsorption of the methyl green onto the GO nanosheets were evaluated by using the pseudofirst-order and pseudosecond-order kinetic models. The Lagergren pseudofirst-order kinetic model is expressed as26 log(qe − qt ) = log qe − (k1t /2.303)

(3)

qt = k nlqe 2t /(k nlqet + 1)

(4)

where knl is the rate constant of the pseudosecond-order kinetic equation in nonlinear form and qt and qe are the adsorption densities of methyl green at time t and at equilibrium, respectively. The calculated values of the qe and knl are from Figure S2 (shown in the Supporting Information) and are presented in Table 1. The R2 value is found to be very poor (0.77 to 0.90). It is observed that the corrected correlation coefficient (R2) for the linear regression plots of the pseudosecond-order model is better than the pseudofirstorder kinetics, and the qe,cal values approach more closely to qe,exp values compared to those calculated from the pseudofirstorder model and pseudosecond-order nonlinear model, suggesting that the adsorption kinetics follows the linear pseudosecond-order model. Similar results were obtained by other researchers also for the adsorption of dye molecules onto graphene and GO nanosheets.8−11,18−21 The intraparticle diffusion model was used to understand the transport of adsorbate from the exterior surface to the pores of adsorbent. It gives the information about the steps involved in adsorption process, which is described by eq 5.29

(2)

where k1 is the pseudofirst-order rate constant and qe and qt are the adsorption capacity of dye molecules onto GO at equilibrium and at time t, respectively. The values of k1 and C

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Table 1. Kinetic Model Parameters for Adsorption of Methyl Green onto the GO Nanosheetsa T/°C models pseudofirst-order

pseudosecond-order (linear)

pseudosecond-order (nonlinear)

a

parameters

18 °C

25 °C

30 °C

35 °C

qexp/mmol·g−1 k1/min−1 qe,cal/mmol·g−1 SD u R2 k2/L·mol−1·min−1 qe,cal/mmol·g−1 SD u R2 k2n,l/mmol·L−1·min−1 qe,cal/mmol·g−1 SD u R2

1.771 0.038 0.327 0.400 0.121 0.920 0.289 1.792 0.046 0.013 0.999 0.414 1.732 0.066 0.198 0.765

1.773 0.038 0.241 0.505 0.179 0.938 0.310 1.806 0.074 0.026 0.999 0.848 1.757 0.052 0.018 0.866

1.799 0.033 0.251 0.470 0.157 0.944 0.490 1.784 0.046 0.016 0.999 0.961 1.771 0.063 0.021 0.849

1.800 0.030 0.193 0.479 0.160 0.951 0.548 1.800 0.000 0.000 0.999 0.450 1.771 0.064 0.021 0.896

SD: Standard deviation = [(qcal − qexp)/(n − 2)]1/2, u: standard uncertainty = SD/√n, n: number of data points in the set.

Figure 2. Plot of t/qt vs time for adsorption of methyl green onto GO nanosheets.

qt = kit 1/2 + C

Figure 3. Plot of qt vs t1/2 for the adsorption of methyl green onto GO nanosheets.

(5)

The plot of Bt vs time is called a Boyd plot (Figure 4), which is employed to distinguish between external transport (film diffusion) and intraparticle diffusion. If the plot gives a straight line passing through the origin, then the adsorption process is governed by an intraparticle diffusion mechanism; otherwise they are governed by film diffusion or external mass transport. The curves of the Boyd model plot do not pass through the origin which indicates that external mass transport mainly governs the rate-limiting process of adsorption of methyl green onto GO nanosheets. Ai et al.31 reported similar results for adsorption of methylene blue onto activated carbon, cobalt ferrite, and alginate composite beads. 3.2. Thermodynamic Parameters. The thermodynamic parameters provide in-depth information about the energetic changes associated with adsorption process. The thermodynamic parameters, namely, the standard Gibbs energy change (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°) of the adsorption of the methyl green onto the GO nanosheets were determined by using the following equations31

where ki is the intraparticle diffusion rate constant and C is a constant. Though intraparticle diffusion curves give good agreement to the linear fitting (R2 = 0.96 to 0.97) they do not pass through the origin (Figure 3) which implies that intraparticle diffusion is not the rate-controlling step of the adsorption process. Further, the Boyd kinetic model was used to determine the actual rate-controlling step involved in the methyl green adsorption onto the GO nanosheets. The Boyd kinetics equation is30 F = 1 − [(6/π 2)exp(−Bt )]

(6)

where F is the fraction of solute adsorbed at different times t and parameter B is a mathematical function of F and is given by

F = qt /qe

(7)

In the substitution of eq 7 in eq 6, the kinetic expression can be represented as Bt = −0.4977 − ln(1 − F )

Kd = qe /Ce

(8) D

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3.3. Adsorption Isotherm. To investigate the nature of electrostatic interaction of the dye molecule with GO nanosheets at different experimental conditions, batch adsorption isotherm experiments were carried out in five different pH values of 4, 5, 6, 7, and 9 and at 25 °C. The adsorption density of the methyl green onto the GO nanosheets against the equilibrium concentration of the methyl green at different pH values of the medium and at 25 °C is presented in Figure 5. In this study, the equilibrium adsorption

Figure 4. Plot of Bt vs time for the adsorption of methyl green onto GO nanosheets.

ΔGo = −RT ln Kd

(10)

o

o

ln Kd = ΔS /R − ΔH /RT

(11)

where Kd is the distribution coefficient, T is the temperature, and R is the gas constant, respectively. ΔH° and ΔS° are calculated from the slope and intercept of van't Hoff plots of ln Kd vs T−1 (shown in Figure S3, Supporting Information). The calculated values of thermodynamic parameters of the methyl green−GO system are given in Table 2. The negative ΔG°

Figure 5. Plot of adsorption capacity (qe) vs equilibrium concentration for adsorption of methyl green onto GO nanosheets [inset: plot of maximum adsorption capacity (Qo) vs pH (Qo,cal: calculated maximum adsorption capacity, Qo,obs: observed maximum adsorption capacity)]. The temperature was 18 °C. The data points are the average of triplicate experiments, and an error bar is shown for reference.

Table 2. Thermodynamic Parameters for Adsorption of Methyl Green onto GO Nanosheets temperature °C 18 25 30 35

ΔG°

ΔH° −1

KJ·mol

−2.733 −2.856 −3.183 −3.622

ΔS° −1

KJ·mol

12.337

data are analyzed using the Langmuir isotherm model.16,31 The Langmuir isotherm theory is based on the assumption of monolayer coverage of adsorbate over a homogeneous adsorbent surface. The Langmuir isotherm is represented as

KJ·mol−1·K−1 0.051

Ce/qe = 1/Q oKL + (1/Q o)Ce

where KL is the Langmuir constant related to the energy of adsorption and Qo is the Langmuir monolayer adsorption capacity. The values of Qo and KL are calculated from the slope and intercept of the linear plot of Ce/qe against Ce. The Langmuir parameters are shown in Table 3. The correlation coefficient of the isotherm is relatively high (0.936 to 0.989), which indicates that the Langmuir model is suitable for describing the adsorption equilibrium of methyl green onto GO nanosheets. This observation is akin to the observations of a number of literature data.8−11,18−21 It is observed from Figures

values at different temperatures indicate the spontaneous nature of the adsorption of methyl green onto the GO nanosheets. The calculated ΔH° value from van't Hoff plots was found to be 12.337 KJ·mol−1, which indicates the adsorption process is endothermic.30 Activation energy gives information about the adsorption mechanism. A low activation energy (< 42 KJ·mol−1) value indicates physical adsorption, while high activation energy indicates chemical adsorption. The activation energy, Ea, is calculated by using the Arrhenius equation32

ln k = ln A − Ea /RT

(13)

Table 3. Parameters of the Langmuir Isotherm Model for the Adsorption of Methyl Green onto the GO Nanosheetsa

(12)

where A is the Arrhenius pre-exponential factor, R is the universal gas constant, and T is the temperature in Kelvin. The activation energy for the adsorption of methyl green onto GO nanosheets was found to be 24.301 kJ·mol−1, as calculated from the slope of the plot of ln k vs 1/T, which indicates the process is governed by physical adsorption (Figure S4 in the Supporting Information). He et al.32 reported that cationic dye adsorption onto natural attapulgite is due to the physical adsorption process with the lower value of activation energy (8.300 kJ·mol−1 for red X-GRL and 17.683 kJ·mol−1 for brilliant blue).

Qo,obs pH 4 5 6 7 9

mmol·g 4.11 5.22 6.11 6.44 7.55

Qo,cal −1

mmol·g 4.821 5.496 6.167 6.268 7.613

KL −1

g·mmol−1

SD

u

R2

2.361 7.968 28.256 11.527 4.376

0.319 0.198 0.090 0.156 0.094

0.106 0.066 0.030 0.052 0.031

0.989 0.957 0.946 0.972 0.936

a SD: standard deviation = [(Qo,cal − Qo,exp)/(n − 2)]1/2, u: standard uncertainty, n: number of data points in the set.

E

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5 and 6 that the adsorption density and removal efficiency of the methyl green increase with the increase of pH of the

Figure 7. Plot of adsorption capacity (qe) vs equilibrium concentration for adsorption of methyl green onto GO nanosheets at four different temperatures and pH 5. The data points are the average of triplicate experiments, and an error bar is shown for reference.

Figure 6. Removal efficiency of methyl green onto GO nanosheets at different pH values and a temperature of 18 °C.

suspension. This is due to the interaction between the −COOH group of GO nanosheets and positively charged nitrogen moiety of methyl green dye molecule. GO nanosheets remain negatively charged in a wide range of pH 2 to 11. The positively charged nitrogen moiety of methyl green dye molecule interacts with the negatively charge GO nanosheets with the increase of the pH of the suspension. Thus, with an increase in pH, the charge−charge interaction of the dye molecule and GO nanosheets increases, resulting in high adsorption capacity. Ramesha et al.10 also reported a similar pH dependency of cationic dye removal efficiency of the GO nanosheets. In their study, they found that the removal efficiency of methylene blue increases from < 50 % to > 95 % with the increase of pH from 2 to 10.10 The effect of temperature on adsorption capacity was also evaluated by performing an adsorption isotherm of methyl green onto the GO nanosheets at four different temperatures, namely, (18, 25, 30, and 35) °C, at pH 5 (shown in Figure 7). It is observed that the adsorption density of methyl green onto the GO nanosheets increases with the increase of temperature. The adsorption process of methyl green onto the GO nanosheets was also characterized by Vermeulan criteria31,33 associated with the Langmuir isotherm. The Vermeulan criteria can be expressed in terms of a dimensionless constant separation factor RL that is given by

RL = 1/(1 + KLCi)

Figure 8. Separation factor RL versus initial concentrations Co for the adsorption of methyl green onto GO nanosheets.

Table 4. Comparative Adsorption Efficiency of Methyl Green on Different Adsorbents adsorbent NiFe2O4−carbon nanotube composites carbon nanotube montmorillonite cross-linked amphoteric starch graphene oxide

(14)

The value of RL indicates the adsorption process to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL = 0).31 From Figure 8, it is clear that the values of RL lie between 0 and 1, which indicates that the adsorption is a favorable process. The comparative adsorption efficiency of the methyl green onto the different adsorbents in aqueous medium is presented in Table 4.34−36 It is observed that GO is the most efficient adsorbent (93 % adsorption of methyl green) compared to the other reported adsorbents such as carbon nanotube, montmorillonite, NiFe2O4−carbon nanotube composites, cross-linked amphoteric starch, and so forth. This may be due to the fact that, in the case of GO nanosheets, the main driving force for adsorption is electrostatic attraction9,10 of the oppositely charged species along with π−π interaction37 which

adsorption efficiency (%)

reference

56

34

60 67 84 93

34 35 36 present study

is comparatively stronger than the π−π interaction, van der Waals interaction, and hydrogen bonding alone. The π−π interaction is responsible for the adsorption onto carbon nanotubes37 and also van der Waals interaction and hydrogen bonding which generally act as the force of interaction in case of other adsorbents like clay.38 3.4. Mechanism of Adsorption. DRIFT spectra of the GO nanosheets before and after adsorption of methyl green were recorded to understand the mechanism of adsorption process of methyl green on the GO nanosheets (Figure 9). The DRIFT spectra of GO exhibit a band at 1734 cm−1 which is F

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the involvement of the −OH group in the dye adsorption. The −OH groups may be bind with the =N+ immonium ion of the methyl green dye molecule through hydrogen bonding which also contributes toward adsorption of the dye molecules onto GO nanosheets. This type of electrostatic interaction for the adsorption of cationic dye molecules onto GO nanosheets is also reported by Bradder et al.,9 Ramesha et al.,10 and Haunber et al.40 The possibility of π−π interaction between methyl green and GO nanosheets may also be taken into account in the adsorption process. Methyl green has CC double bonds which contain π-electrons. These π-electrons can easily interact with the πelectrons of the GO nanosheets which will enhance the amount of adsorption. Such π−π interaction of cationic dye molecule with GO nanosheets is also reported by Li et al.37

4. CONCLUSIONS This study shows that GO nanosheets can be successfully utilized for the removal of dye from aqueous solution by the adsorption process with a removal efficiency > 90 %. The process is governed by physical adsorption which is mainly due to electrostatic interaction of oppositely charged adsorbate− adsorbent species along with the π−π interaction. The removal efficiency is dependent on pH and temperature of the medium. Based on this study, the optimum conditions for effective removal of methyl green by adsorption onto GO nanosheets can be tuned up.

Figure 9. FTIR spectra of methyl green (MG), graphene oxide before dye adsorption (GO), and after dye adsorption (MG-GO).

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assigned to ν(CO) of the −COOH group.9,10,39,40 The bands at (1604 and 1435) cm−1 appear due to ν(CC) of aromatic rings and C−O−H bending of phenolic groups, respectively.39,41 The bands at (1227 and 1061) cm−1 are assigned to ν(C−O) stretching of phenolic and epoxy groups, respectively.9,39,41 The DRIFT spectra of methyl green show a band at 1586 cm−1 which is due to the =N+ immonium ion.42 The bands at (1485 and 1173) cm−1 are attributed to vibrations of the heterocycle skeleton of the dye molecule.42 The band at 1173 cm−1 is assigned to C−H3 bending vibrations.42 It is observed from Figure 9 (MG-GO spectra) that the intensity of the band at 1734 cm−1 decreases considerably after adsorption of methyl green dye molecule onto the GO nanosheets which may be due to electrostatic interaction between −COOH group of GO nanosheets and positively charged nitrogen moiety of methyl green dye molecule. Haubner et al.40 observed the decreases of band stretching of the ν(CO) at 1735 cm−1 after adsorption of methylene blue onto the GO nanosheets. The −COOH groups exist at the edge of the GO nanosheets resulting in a negatively charged surface or acidic surface.9 Therefore, −COO− ions is present in the GO−water suspension. Methyl green is a cationic dye; it remains as positively charged ion in solution. Hence, the adsorption of methyl green onto GO nanosheets may be attributed to electrostatic attraction which may be schematically represented as in Scheme 1. Also the band at 1227 cm−1 due to phenolic group of the DRIFT spectra of GO nanosheets significantly decreases after the adsorption of methyl green which indicates

ASSOCIATED CONTENT

S Supporting Information *

Pseudofirst-order and pseudosecond-order nonlinear kinetic models, van't Hoff plot, and plot of the determination of the activation energy. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-9957178399. Fax: +91-376-2370011. E-mail: [email protected] (M. R. Das). Funding

P.S. acknowledges CSIR, New Delhi for a SRF grant. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to the Director, CSIR-NEIST, Jorhat for his interest in carrying out this work. The authors also thank Dr. D. K. Dutta, Chief Scientist, Materials Science Division, CSIR-NEIST for providing the UV−visible spectrophotometer facility.

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REFERENCES

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Scheme 1. Proposed Electrostatic Interaction of Methyl Green and GO

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