Powder Technology 257 (2014) 141–148

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Powder Technology

j ourna l homepage: www.e lsev ie r .com/ locate /powtec

Understanding the adsorption property of graphene-oxidewith differentdegrees of oxidation levels

Sakthivel Thangavel, Gunasekaran Venugopal ⁎Nano Materials Research Lab, Department of Nanosciences and Technology, Karunya University, Coimbatore 641 114, Tamil Nadu, India

⁎ Corresponding author. Tel.: +91 98947 89648.E-mail addresses: gunasekaran@karunya.edu, pvsguna

(Dr. V. Gunasekaran).

http://dx.doi.org/10.1016/j.powtec.2014.02.0460032-5910/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 October 2013Received in revised form 20 January 2014Accepted 14 February 2014Available online 21 February 2014

Keywords:Various oxidationsAdsorptionZeta potentialEndothermicFunctional groups

In this paper, we report the systematic removal of synthetic dye compound from aqueous solution usinggraphene-oxide (GO) nanostructures as an adsorbent. The various levels of oxidized GO were used in thisstudy and their results were systematically compared. Highly oxidized GO revealed superior adsorption capacitythan the GO with lower degree of oxidization. In highly oxidized GO, the presence of more hydrophilic (sp3

hybridization) functional groups enhanced thedye adsorption. At ambient atmospheric condition, the adsorptionrates were increased with respect to the oxidation rate of GO which is due to its increase in negative chargemolecules in the hydrophilic functional groups. The adsorption property of GO was investigated by graduallyvarying the pH of solution, temperature and reaction time. Thermodynamic parameters were also calculatedusing Van't Hoff plot. The value of Gibbs free energy was found to be negative as the adsorption reaction wasspontaneous. The positive value of ΔH indicates that the adsorption process of all GO samples is purely basedon an endothermic process. The adsorbent was characterized by using XRD and the functional groups in GOwere characterized by using an FTIR spectrometer. Our results show a very simple and cost effective procedurefor removing the toxic and carcinogenic dyes from the waste water and their applications.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Synthetic dye stuff wastage has been highly harmful and toxic forhumans as well as the environment [1]. Every year synthetic dye stuffusage has been rapidly increasing in the industry. More than 20% ofdye effluent has been released to rivers or land, resulting in more num-ber of problems [2]. The synthetic dyes were produced from harmfulchemicals which encompass non-biodegradable aromatic functionalgroups [3]. Hence a cost effective and short time process is highly re-quired to handle this problem. Ozonation, electrochemical oxidation,photo-Fenton, semiconductor photocatalysis, chlorination, reverse os-mosis, anaerobic process and nano-filtration are some of the processesreported until now [4–7]. Hence out of these methods, adsorption isone of the simple and cost effective methods for removing dye wastagefrom waste water [8].

Carbon family materials are known for their tremendous adsorptioncapacity due to their high porosity and their electrostatic interactionwith adsorbate. Universally activated carbon has been extensively usedfor adsorption purpose. This carbon family not only eliminates harmfuldye, in addition it takes away antibiotics and non-biodegradable chemi-cal compounds similar to phenol [9]. A. Yu et al. reported that graphene-

@gmail.com

oxide (GO) can successfully remove the radioactive ravage from water[10]. In recent days, carbon nanotubes (CNT), graphene oxide and re-duced graphene-oxide (rGO) were used progressively for dye removalprocesses. Among all, GO has incredible adsorption property attributedto its high surface area and functional groups compared to others.

Graphene and graphene-oxide are used in a bunch of applicationssince the discovery of graphene. Graphene oxide, a single layer ofgraphene nanosheets functionalized by several oxygen-containinggroups has been synthesized via oxidation of graphite into graphiticoxide followed by exfoliation [11]. GO is the intermediate product andgraphene is the final product. A limited number of papers have been re-ported on GO adsorption by using different structures of GO such aspure GO, exfoliated GO, functionalized GO, GO composites, layered GO,rGO-based hydrogels, rGO and graphene [12–14]. GO has the abilityto adsorb non-biodegradable dye compounds like methylene green,methylene violet, orange G, rhodamine B, tetracycline (antibiotics),chromium, Cu2+, bis-phenol and acridine orange [15–20]. Excellentadsorption was observed in GO from all of the above reports. This re-markable adsorption was achieved owing to its high surface area andπ–π interaction on the surface of GO. The toxic biphenyl compoundwas removed due to the hydrogen bonding of graphene [21]. Comparedto graphene, GO has some striking properties such as hydrophilic na-ture, negative charge molecule and so on.

Synthesis of bulk amount of activated carbon is very costly; more-over sometimes activated carbon showsminimumadsorption property.

Table 1XRD parameters of GO samples with different degrees of oxidation.

Sample name Peak position ‘d’ value FWHM Intensity

Graphite 26.66 3.34 2.520 108G1 25.32 4.02 1.983 536G2 12.92 7.6 1.605 1936G3 11.64 8.3 1.081 2230G4 10.41 8.48 1.007 3623

142 S. Thangavel, G. Venugopal / Powder Technology 257 (2014) 141–148

In CNT, the adsorption equilibrium was achieved within 1 h; on con-trary in GO the equilibrium was reached spontaneously owing to π–πstacking. S.T. Yang et al. reported about the needs of systematic kineticdata and it is highly necessary in order to improve the efficacy of dyeremoval [15]. Still some issues are unresolved pertaining to GO adsorp-tion process. Therefore, we attempted to understand the adsorption ofGO by varying the degree of oxidization gradually while preparing GO.Consequently the effect of functional groups in the adsorption of dyewas studied using GO with various oxidation levels. So in this visionthe binding site between GO and dye molecules was studied usingMB as model dye with different pH and temperature. Kinetic data arealso given in this report.

2. Experimental

2.1. Materials and methods

Expandable graphite powder of size lesser than 25 μm, MB and 30%of H2O2 were procured from Rankem Chemicals (India). All reactionswere carried out using double distilled deionized (DD)water. Structuralcharacterization of the prepared adsorbatewas performedwith powderX-ray Diffractometer system (X-6000 Shimadzu). The functional groupsin GO were analyzed by an FTIR spectrometer [model Nicolet-6700].TheUV–vis spectroscopymeasurementswere done using a spectropho-tometer (JASCO V-60). Adsorption activity was carried out by using themodel dye MB (C14H14N3SO3Na).

2.2. Synthesis of graphene oxide with different degrees of oxidation level

The different degrees of oxidized GO were synthesized by usingmodified Hummers method [22,23]. Expandable graphite powder(2 g) was used as starting material. The same was stirred in 50 ml

10 20 30 40 50 60 70 80 902 Theta (deg)

Inte

nsit

y (a

.u)

25.41

12.66

11.7

10.41

Graphite

G1

G2

G3

G4

(b)

(a)

Fig. 1. (a) Powder X-ray diffraction patterns for pristine graphite andGO sampleswith dif-ferent degrees of oxidation levels (G1–G4). (b) Transmission electron microscopy imageof GO (G4).

of concentrated hydrochloric acid in a 500 ml beaker for 30 min and2 g of KMnO4. The solution was then stirred for another 5 to 8 h and90 ml of DD water was added to it. Then the reaction was terminatedby the addition of 5 ml of H2O2 (30%) solution. The obtained GOwas washed using 5% HCl aqueous solution and was followed byrepeated washing with deionized water until the pH of the solutionreaches neutral. After that 160 ml of DD water was added whichresults in the formation of precipitate. A uniform suspension ofGO was obtained after sonication. Similar process was followed forGO with different degrees of oxidation by adding 2 g, 4 g, 6 g and 8 gof KMnO4 and the samples were depicted as G1, G2, G3 and G4respectively.

2.3. Adsorption experiment

MB adsorption experiment was carried out in magnetic stirrer atconstant rate (500 rpm). 0.5 ml of known concentration of methyleneblue (MB) dye solution was taken and mixed with a particular amountof GO. At a regular time interval, 3 ml of dye was taken out from thesolution and centrifuged for 5 min at 3500 rpm. Supernatant free

1000 1200 1400 1600 1800 2000

G2

C-O-C

C-O

C=O

G1C-C

C-OHC-C

C-O-CC-OH C-C

Wavenumber (cm-1)

Tra

nsm

itta

nce

(%)

G4

G3

C-CC-OH

(a)

1000 1250 1500 1750 2000 2250 2500

G3

G2

G4(b)

G

G1

Raman shift (cm-1)

Inte

nsit

y (a

.u)

D

Fig. 2. (a) Fourier transform infrared spectra. (b) Raman spectra of GO samples withdifferent degrees of oxidation levels.

143S. Thangavel, G. Venugopal / Powder Technology 257 (2014) 141–148

solutionwas analyzed using anUV spectrometer at a wavelength of 664nm. The adsorption analyses were carried out by varying the followingparameters; the solution pHwas varied from 4 to 12. Secondly, the tem-perature was varied in the following manner i.e., 10 °C, 20 °C, 30 °C,40 °C and 50 °C. The dye adsorbed per unit weight of adsorbentwas cal-culated by using Eq. (1)

qt ¼C0−Ctð ÞV

Wð1Þ

where C0 is the initial concentration, Ct is the concentration at any time,W is the adsorbent amount and V is the volume of the solution. Theremoval efficiency was calculated with the help of Eq. (2)

Dye removal% ¼ C0−Ceð ÞVC0

� 100: ð2Þ

Here Ce is the equilibrium concentration.

3. Result and discussion

3.1. Structural characterization of graphene oxide

Structural property of different degrees of oxidized GO samples wasstudied by powder X-ray diffraction which is shown in Fig. 1(a). Thecharacteristic peaks were observed at around 2θ = 25.41°, 12.66°,

G1 G2 G3 G4

-24

-28

-32

-36

-40

Zet

a P

oten

tial

(m

V)

GO Sample

(a)

(b)

1.0 1.2 1.4 1.6 1.8 2.0

60

65

70

75

80

85

90

Rem

oval

eff

icie

ncy

%

[MB]x10-4M

Fig. 3. (a) Zeta potential of different degrees of oxygenated GO. (b) The plot of removalefficiency (G4) against different MB concentrations.

11.7° and 10.41° for G1, G2, G3 and G4 samples respectively. The inter-layer spacing also increased from3.43 Å to 8.48Å. Table 1 represents thevalues of 2θ, interplanar spacing (d), peak intensity and full width halfmaximum (FWHM). It was observed that the value of ‘d’ increaseddue to the addition of oxygenated functional groups, however, thevalue of FWHM decreased due to the higher presence of sp2/sp3 ratioin GO sheets. The molecular structure changes from sp2 to sp3 due tothe addition of oxygenated functional groups.

The as-prepared GO sample (G4) morphology was studied by usingtransmission electron microscopy (TEM) which is shown in Fig. 1(b).This figure clearly shows the sheet-like structure. Fig. 2(a) shows theFourier transform infrared spectroscopy (FTIR) spectra of different de-grees of oxidized GO. It was noted that the degree of oxidation levelhas been increased as of the following array: G1 b G2 b G3 b G4.When the degree of oxidation level was increased, more number offunctional groups were attached in basal plane of GO due to the oxida-tion process. We also analyzed as-prepared GO samples of different de-grees of oxidation level by using Raman spectroscopy. Fig. 2(b) showsRaman spectra of GO samples with different degrees of oxidation levelsin which two prominent peakswere observedwhich are correspondingto G-band and D-band. The intensity of the D peak exhibits about thedefect nature of GO. The dispersion property of the nanomaterial isstrongly influenced by zeta-potential which was analyzed for as-prepared GO samples. P. Leroy et al. reported that zeta-potential isone of the important parameters of ion adsorption and electrostaticinteraction between GO and dye [24]. Therefore surface charge ofGO at various oxidations was analyzed to determine the zeta poten-tial which is shown in Fig. 3(a). Here we used Smoluchowski ap-proximation theory for studying the zeta potential of GO because as-

(b)

(a)

20 40 60 80 100 120

70

75

80

85

90

95

100

% R

emov

al

Time (Sec)

GO4GO1GO3GO2

Fig. 4. (a) MB removal efficiency of GO samples of various oxidation levels. (b) Photogra-phy image of dye in water treated with GO samples with different degrees of oxidation[a) without GO, b) GO-1, c) GO-2, d) GO-3 and e) GO-4].

144 S. Thangavel, G. Venugopal / Powder Technology 257 (2014) 141–148

prepared GO shows sheet-like morphology [25,26]. The result showsthat the negative charges are increased in GO samples by the followingorder: G1 bG2 bG3 bG4. This also indicates that the adsorption efficien-cy has been increased from samples G1 to G4. Nowwe shall discuss theeffect of substrate (MB), catalyst concentration (GO), effect of solution(pH) and temperature on the adsorption process.

3.2. Effect of substrate and catalyst concentration on adsorption process

To find the most excellent concentration of the dye, MB concentra-tion was changed from 1.2 × 10−4 M to 2 × 10−4 M at constant ad-sorbent (G4) concentration (8 mgL−1). As seen in Fig. 3(b) the dyeadsorption increases with the increment of dye concentration. How-ever beyond 1.4 × 10−4 M, there was no significant change in dyeadsorption as the available surface area of the sample remainsunchanged (8 mgL−1). Therefore 1.4 × 10−4 M was considered tobe the most favorable concentration for dye adsorption and thesame was used for all adsorption experiments in this work. Fig. 4(a)represents the plot between time and removal efficiency of GOwith different degrees of oxidation. It was found that removal efficiencyincreases gradually from GO samples G1 to G4. It is mainly attributed tothe positive charge of MB and GO negative charge. As we increased theoxidation degree of GO, the amount of negative charges increasesresulting in higher adsorption rate in G4. This increases the negativecharges from low oxidation state to high oxidation state of GO whichwas confirmed by zeta potential studies shown in Fig. 3(a).

Fig. 4(b) shows the photography images of MB dye solutions beforeand after the adsorption experiment. It is evidently seen from GO sam-ples that G2, G3 and G4 have excellent adsorption ability as there isno significant difference in color found among them (i.e. all exhibited97 ± 2% removal efficiency).

0 20 40 60 80 1000

50

100

150

200

250

300

350

400 (G1)

q t (m

g/g)

q t (m

g/g)

Time/min

0 20 40 60 80 100Time/min

5710698

0

100

200

300

400

500

(G3)

5897610

(c)

(a)

Fig. 5. Effect of pH (5–10) on adsorption of MB by

3.3. Effect of solution pH and temperature on adsorption process

For all the GO samples, adsorption test was performed by using dif-ferent pH. L. Fiu et al. reported that 3-dimensional GO showed negativecharges even at high pH [29]. The adsorption efficiency was enhancedas we increased the pH from 5 to 10 by adding NaOH or HCl. As per lit-erature, when the pH is decreased to the value of 1 or 2, more numberof H+ ions were released from carboxyl group on GO surface. Due tothis, the amount of negative charges was decreased in GO surfaceresulting in the depletion in MB adsorption [18]. Though MB is a cat-ionic dye and GO has negative charge, there will be an increase in neg-ativity of GO as we increase the pH. As a result, the anionic and cationicinteractions were further improved by increasing the pH which isshown in Fig. 5. Fig. 5(a) shows the capacity of least adsorption ofdye by G1 sample at low pH. With the increase of pH, a huge amountof dye intake was observed (i.e. amount of adsorption (qt) increasedfrom 153 mg g−1 to 374 mg g−1). Fig. 5(b) represents the capacityof dye adsorption of GO (G2) with different pH ranges. At pH 5, thedye adsorption efficiency was observed at 290 mg g−1, while atpH 10, dye adsorption efficiencywas raised up to 453mg g−1. Similarly,samples G3 and G4 also show their adsorption efficiency of dye whichwas increased from 370 mg g−1 to 459 mg g−1 and 420 mg g−1 to470 mg g−1 respectively. Our observation shows that the dye removalproperty of all GO samples increased with respect to their oxidationlevels. Hence the adsorption efficiency of dye by GO samples wasfound to be in the order of G1 b G2 b G3 b G4. The adsorption equilibri-um was achieved within 20 min for high oxidation state and the oppo-site was observed for low oxidation state of GO. This is due to the factthat the highly oxidized GO contains a larger number of functionalgroups which increase adsorption sites than the lowly oxidized GO.From these results, it is noted that large dye adsorption was observedeven at low oxidation state at high pH. Our results compare well with

q t (m

g/g)

q t (m

g/g)

0 20 40 60 80 100Time/min

0 20 40 60 80 100Time/min

0

100

200

300

400

500(G2)

7581096

0

100

200

300

400

500 (G4)

7659108

(d)

(b)

GO with different degrees of oxidation levels.

Table 2Comparisons of adsorption capacity of organic dyes on GO reported in literature andpresent study.

Adsorbent Adsorption efficiency (%) Adsorbate Reference

GO 93 Methylene green [26]GO 99 and 98.8 MB and MV [16]GO 98.6 MB [10]GO-1 90 MB Present studyGO-2 95 MB Present studyGO-3 97 MB Present studyGO-4 99.8 MB Present study

Table 3Gibbs free energy for adsorption of MB onto GO with different degrees of oxidation.

Sample name 10 °C 20 °C 30 °C 40 °C 50 °C

G1 −4.472 −5.983 −13.468 −20.352 −25.149G2 −5.272 −5.991 −16.101 −20.452 −27.311G3 −5.297 −6.023 −16.266 −21.915 −27.436G4 −5.394 −6.192 −16.382 −21.982 −27.477

145S. Thangavel, G. Venugopal / Powder Technology 257 (2014) 141–148

recent reports given in Table 2 [15,27]; GO with low oxidation nearlyshows the same efficiency of MB removal.

The effect of temperature in the process of adsorption of MB byvarious oxidized GO was analyzed by changing the experimental tem-perature from 10 °C to 50 °C which is presented in Fig. 6. Temperaturehas a major role in adsorption process. While increasing the tempera-ture, the diffusion rate was decreased and consequently the solutionviscosity was reduced [28]. The thermodynamic parameter for MBadsorption by GO was obtained by using a universal.

ΔG0 ¼ −RT � lnK: ð3Þ

Here ΔG is Gibbs free energy, R is universal gas constant (8.314 mol−1

K−1), T is the temperature in Kelvin and lnK is equilibrium constant.This ΔG has three possibilities: if ΔG = −Ve, the reaction is spontane-ous; if ΔG = +Ve, the reaction is non-spontaneous; and if ΔG = 0,the reaction is in equilibrium. The values of ΔG for GO with differentdegrees of oxidation level were calculated and are given in Table 3.

0 20 40 60 80 1000

100

200

300

400

500

q t (m

g/g)

q t (m

g/g)

Time (Sec)

0 20 40 60 80 100Time (Sec)

50°C

30°C

10°C

40°C

20°C

(G1)

0

100

200

300

400

500

(G3)

40°C

10°C

20°C

40°C

50°C

(c)

(a)

Fig. 6. Effect of temperature (10 °C, 20 °C, 30 °C, 40 °C, and 50 °C) o

Other important thermodynamic parameters are entropy and en-thalpy changes of the adsorption of MB onto GO with different degreesof oxidation. The entropy and enthalpy describe about the reaction na-ture of either endothermic or exothermic and available energy for dyeadsorption respectively.

lnK ¼ ΔS0

R−ΔH0

RTð4Þ

where ΔS0 represents the entropy change and ΔH0 symbolizes the cor-responding enthalpy change of the system which were obtained fromthe slope and intercept of Van't Hoff plot [30,31]. The Van't Hoff ploton adsorption of MB by GO with different degrees of oxidation levelsis presented in Fig. 7 and their values are given in Table 4. The valuesof ΔH0 are 23.611, 7.7320, 7.1632 and 5.404 for G1, G2, G3 and G4respectively. This ΔH0 indicates that the adsorption process of all GOsamples is purely based on an endothermic process.

3.4. Equilibrium adsorption isotherm

The single layer adsorption and multi-layer adsorption mechanicson the surface of the adsorbent can be analyzed by Langmuir isotherm

q t (m

g/g)

q t (m

g/g)

0 20 40 60 80 100Time (Sec)

0 20 40 60 80 100Time (Sec)

0

100

200

300

400

500

30°C

10°C

30°C

50°C

40°C

(G2)

0

100

200

300

400

500

30°C

50°C

10°C

20°C

40°C

(G4)

(d)

(b)

n adsorption of MB by GO with different degrees of oxidation.

0.0031 0.0032 0.0033 0.0034 0.00355.2

5.4

5.6

5.8

6.0

6.2

6.4

6.6

In K

In K

In K

In K

1/T0.0031 0.0032 0.0033 0.0034 0.0035

1/T

0.0031 0.0032 0.0033 0.0034 0.00351/T

0.0031 0.0032 0.0033 0.0034 0.00351/T

(G1)

6.40

6.45

6.50

6.55

6.60

6.65

6.70 (G4)

6.1

6.2

6.3

6.4

6.5

6.6 (G2)

6.30

6.35

6.40

6.45

6.50

6.55

6.60

6.65

6.70

6.75

(G3)(c) (d)

(b)(a)

Fig. 7. Van't Hoff plot on adsorption of MB by GO samples with different oxidation levels.

146 S. Thangavel, G. Venugopal / Powder Technology 257 (2014) 141–148

and Freundlich isotherm respectively. To determine the mechanics ofMB adsorption by various degrees of oxidized GO samples [32], the ad-sorption isothermwas fitted by both Langmuir isotherm and Freundlichisotherm for all GO samples. If the adsorption obeys the Langmuirmodel, Ce/qe vs qe should be a linear plot. The Langmuir adsorptionequation is given in Eq. (5).

Ce

qe¼ Ce

qmþ 1

bqmð5Þ

where Ce is the equilibrium concentration and qm is the maximumamount of dye adsorbed by constant adsorbent. The calculated valuesof adsorption isothermal constants are given in Table 5. Freundlich ad-sorption can be explained by using Eq. (6).

lnqe ¼ lnK f þ1n f

� �lnCe ð6Þ

where Kf and 1/nf are constants and the factor 1/nf is related to thecapacity of the adsorption [33]. Figs. 8 and 9 show the isotherm plotsin which the data were well fitted with the Langmuir model and

Table 4Thermodynamic parameters for adsorption of MB onto GO with different degrees ofoxidation.

Sample name R2 lnK ΔS0 (J mol−1) ΔH0 (K J mol−1)

G1 0.9976 6.671 127.2 23.611G2 0.9995 6.723 78.56 7.7320G3 0.9924 6.829 78.23 7.1632G4 0.9995 6.8561 72.33 5.404

Freundlich model for all the GO samples. The corresponding constants(Kf and 1/nf) and their values are given in Table 5.

4. Conclusion

The GO adsorption property with different degrees of oxidationlevels was presented in this paper in a well arranged manner. Partiallyand fully oxidized GO has shown 99% of dye removal. The XRD patternand FTIR spectra revealed the stepwise addition of functional groupsin the GO samples from G1 to G4. The adsorption property was foundto be increased from G1 to G4 at ambient atmospheric conditionand it was strongly influenced by solution pH and temperature. It wasfound that when the solution pH was increased from 5 to 10, theadsorption property was increased. Due to the endothermic reaction,the adsorption efficiency was increased at high temperature.

Acknowledgments

The author (T.S.) is so thankful to the management of KarunyaUniversity (KU) for providing Silver Jubilee Fellowship (SJF) scholar-ship to carry out the research work. Also, we extend our sincere

Table 5Adsorption isotherm constants of GO samples with different degrees of oxidation.

Sample name Langmuir isotherm Freundlich isotherm

R2 1/q 1/qm R2 Kf 1/nf

G1 0.998 2.23 3.16 0.989 0.021 0.04G2 0.999 0.16 0.16 0.989 0.024 0.02G3 0.998 0.12 0.12 0.997 0.246 0.003G4 0.999 1.75 1.75 0.994 0.0225 0.0029

0.20 0.21 0.22 0.23 0.24 0.25 0.260.046

0.048

0.050

0.052

0.054

0.056

0.058

0.060

Ce/

qe/g

L-1

Ce/

qe/g

L-1

Ce/

qe/g

L-1

Ce/

qe/g

L-1

Ce/mg L-1 Ce/mg L-1

Ce/mg L-1 Ce/mg L-1

5 10 15 20 25 30 350.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

6.6 6.7 6.8 6.9 7.0 7.10.144

0.146

0.148

0.150

0.152

0.154

0.156

5.6 5.7 5.8 5.9 6.0 6.1

0.134

0.136

0.138

0.140

0.142

0.144

0.146

0.148

0.150

0.152

(G1) (G2)

(G4)(G3)(c) (d)

(a) (b)

Fig. 8. Langmuir adsorption isotherms of MB in different oxidation levels of GO.

-1.2 -1.0 -0.8 -0.6 -0.4 -0.21.66

1.68

1.70

1.72

1.74

1.76

1.78

1.80

1.82

(G3)

-1.180 -1.175 -1.170 -1.165 -1.160 -1.155 -1.1501.80

1.82

1.84

1.86

1.88 (G4)

-0.70 -0.68 -0.66 -0.64 -0.62 -0.60 -0.581.60

1.65

1.70

1.75

1.80

1.85

log

q elo

g q e

log

q elo

g q e

log Ce log Ce

log Ce log Ce

(G1)

-1.1 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4

1.60

1.62

1.64

1.66

1.68

1.70

1.72

(G2)

(c) (d)

(a) (b)

Fig. 9. Freundlich adsorption isotherms of MB in different oxidation levels of GO.

147S. Thangavel, G. Venugopal / Powder Technology 257 (2014) 141–148

148 S. Thangavel, G. Venugopal / Powder Technology 257 (2014) 141–148

thanks to Mr. A. Raja and Mr. M.B.S. Pravin at the Center for Researchin Nanotechnology (CRN) at Karunya University for the timely helpin doing sample characterization. Authors also extend their sincerethanks to DST-Nanomission, New Delhi, for the financial supportextended toM.Tech Nanotechnology program at Karunya University,Coimbatore, India.

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