Journal of Molecular Liquids 196 (2014) 348–356

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Journal of Molecular Liquids

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

Modified nano-graphite/Fe3O4 composite as efficient adsorbent for theremoval of methyl violet from aqueous solution

Changzhen Li a, Yunhui Dong a,⁎, Juanjuan Yang a, Yueyun Li a,⁎, Congcong Huang b

a College of Chemical Engineering, Shandong University of Technology, Zibo 255049, PR Chinab Department of Chemistry, College of Science, Northeastern University, Shenyang 110819, PR China

⁎ Corresponding authors.E-mail addresses: dyh651118@126.com (Y. Dong), liy

http://dx.doi.org/10.1016/j.molliq.2014.04.0100167-7322/© 2014 The Authors. Published by Elsevier B.V

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 December 2013Received in revised form 8 March 2014Accepted 12 April 2014Available online 26 April 2014

Keywords:Nano-graphite/Fe3O4

AdsorptionMethyl violetThermodynamicsKinetics

Nano-graphite/Fe3O4 composite (NG/FC) synthesized through deposition–precipitation method, was developedfor the removal of methyl violet (MV) from aqueous solution. The parameters including pH, temperature andshaking time onMV removal efficiency were extensively investigated. The adsorption ofMV increased smoothlyin the pH range of 2.0–4.0, then remained at a higher level for the pH range of 4.0–10.0, but increased sharply atpH N 10.0, which demonstrated that the adsorption was affected strongly by pH. On the other hand, the adsorp-tion capacity was increased sharply with the temperature rising. The adsorption equilibrium time could bereached in 10 min. The adsorption thermodynamics and kinetics were also investigated. The equilibrium datacould fit the Langmuir isothermal model very well, and the thermodynamic analysis suggested that the adsorp-tion of MV on the nano-graphite/Fe3O4 composites is a spontaneous, physical and endothermic process. To gaindeep insight into the adsorption kinetics, both the pseudo-kinetic and particle diffusionmodels were examined.The results indicated the pseudo-second-order kinetic model fitted the experimental data better and the particlediffusion was proved to be the rate-determining step in the adsorption process of MV on NG/FC. The adsorbentexhibits excellent stability and remarkable regeneration ability as well.

© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/3.0/).

1. Introduction

Organic dyes, which are widely applied in textile, cosmetics, paper,and coloring industries, are one of the most serious industrial pollutionsources to the environment and drinking water. Removal of organicdyes from the effluents has become a significant issue in nowadays [1,2]. Waste effluent containing organic dye endangers not only the envi-ronment, but also human life. Methyl violet (MV) is one of the high bril-liance and color intensity cationic dyes [3], which can catch the attentionof both the public and the authorities with as low concentration as0.005 ppm [4]. Methyl violet absorbs and reflects sunlight into waterresulting in the interference on the photosynthesis of aquatic plants[3]. If the MV was inhaled, swallowed or absorbed through skin, it maycause respiratory tracks injury, vomiting, diarrhea, pain, headaches anddizziness [5], even mutagenic and carcinogenic [6]. Therefore, removalof MV from wastewater is quite necessary before emission.

Dye removal from aqueous solution has been extensively studied inthe past decade. Various treatment processes for removal dyes fromwastewaters, such as ozonation [7], coagulation [8], ultrafiltration [9],membrane filtration [10], chemical oxidation [11], electrochemical [12],photocatalytic degradation [13] and adsorption [14–16] have been

ueyun71@163.com (Y. Li).

. This is an open access article under

widely investigated, duringwhich adsorption is important in both scien-tific aspects and environmental applications [17]. Many different adsor-bents have been tested to remove MV from aqueous solutions, such aswaste materials [18,19], chitosan [20], agricultural waste [3], zeolitic[21], fly ash [22], hydrogels [23], perlite [24] and various carbonaceousadsorbents (e.g. activated carbon [25], graphite [16] and graphene [26,27],etc.). Among those different adsorbents, graphite [28,29] andgraphene [30,31] have been attended by plenty of researchers due tothe large theoretical specific surface area (2630 m2/g) [27]. However,the mainly limitation of MV removal by traditional adsorption processlies on the high cost and regeneration difficulties of the adsorbents[32]. Recently, the adsorption capability could be enhanced by the mod-ification of adsorbents via physical and chemical processes [2,33,34].Nano-graphite and nano-sized magnetic particles have caught more at-tentions in adsorption due to their unique electrochemical and structuralcharacteristics. The extraordinary magnetism and high specific surfacearea make it be a promising material in drug delivery, chemical, bio-chemical separation and environmental remediation [35–37] due tothe rapid adsorption rates, high adsorption capacities, and convenientmagnetic separation and recycle.

In this work, a recyclable and efficient adsorption capacity adsorbentnano-graphite/Fe3O4 composite (NG/FC) is demonstrated to the removalof MV through experiment and used several traditional models to simu-late the experimental data. The parameters, such as pH, temperature

the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Fig. 1. The synthesis and application of the nano-graphite/Fe3O4 composite.

349C. Li et al. / Journal of Molecular Liquids 196 (2014) 348–356

and shaking time, are extensively investigated. Moreover, the adsorptionisotherm of MV on NG/FC is simulated by using the Langmuir andFreundlich equations. The adsorption kinetics of MV on NG/FC is also ex-plored. The experimental results are correlated with the mathematicalmodel for a more in-depth understanding.

2. Experiment

2.1. Synthesis of sample

Methyl violet (MV), FeCl3·6H2O, CH3COONa (NaAc) and hydrazinehydrate (HHA) were purchased from Tianjin Damo chemical reagentfactory. Nano-graphite (NG) (1–100 nm) was derived from a graphitemine in Pingdu City (Shandong province, China). The MV solution wasprepared by dissolving MV in a certain volume of double distilledwater. The adsorbents were synthesized through deposition–

4000 3000 2000 1000

70

80

90

100

% tr

ansm

itta

nce

Wavenumbers(cm-1)

nano-graphite nano-graphite/Fe

3O

4 composite3421.0

2970

.229

21.8 17

16.3

1588.51214.6 10

88.0

878.

4

574.

545

1.5

Fig. 2. FTIR spectrum of the nano-graphite and nano-graphite/Fe3O4 composite.

precipitation method, during which the graphite powder was diffusedin deionizer water by ultrasonication. Subsequently, FeCl3·6H2O, NaAcand HHAwere dissolved in the solution at room temperature. The solu-tion was mixed in a water bathing vibrator at 353.15 K for 30 min,followed by cooling down at room temperature. The particles were sep-arated from the liquid phase by filtration and the residueswere washed3 times with methyl alcohol. The nano-graphite/Fe3O4 composite (NG/FC) was obtained after dried in vacuum oven.

2.2. Structure characterization

The nano-graphite (NG) and nano-graphite/Fe3O4 composites (NG/FC) are characterized by scanning electron microscopy (SEM)(Multimode NS3a, America), high resolution transmission electronmicroscope(HRTEM), X-ray diffraction(XRD), and Fourier transform in-frared (FTIR) (Nicolet 5700, America) in pressed KBr pellets (spectralresolution: 1 cm−1, scanning times: 150), BET and BJH models.

2.3. Adsorption experiments

The adsorption of MV on NG and NG/FC was performed by batchtechniques under ambient conditions. The whole process was tolerantto air. Briefly, 0.01 g of adsorbentswas added into 25mLofMV solutionsin different initial concentrations under stirred. The sampleswere sepa-rated from the solution through magnetic separation. The effect of pHand adsorption time was screened in detail. The desired pH of the sus-pensions in each tube was adjusted by using 0.1 mol/L HCl or NaOH so-lutions. The adsorption isotherms which indicate the MV adsorptionbehavior, was investigated at 298.15, 308.15 and 318.15 K, respectively.The residual concentration of dye was calculated by using a UV–visspectrophotometer (UV2550, DAOJIN) at λmax = 584 nm. The amountof MV adsorbed per unit mass of the adsorbent was evaluated by themass balance equation as below:

qe ¼c0−ceð ÞV

mð1Þ

where qe (mg·g−1) is the amount adsorbed per gram of adsorbent, C0and Ce are the initial and equilibrium concentrations of MV in the

350 C. Li et al. / Journal of Molecular Liquids 196 (2014) 348–356

solution (mg·mL−1), respectively, m is the mass of the adsorbent (g),and V (mL) is the initial volume of the MV solution. The synthesis andapplication of the adsorbent were shown in Fig. 1.

2.4. Adsorption kinetic

To insight into the mechanism of the adsorption process, two kindsof classical kinetic models were applied to analyze the experimentaldata.

One of which is pseudo-first-order model [38]:

ln qe−qtð Þ ¼ lnqe−k1t ð2Þ

where qe is the amount of adsorbedMV at equilibrium time and k1 is therate constant of the pseudo-first-order model.

The other one is pseudo-second-order model [39]:

tqt

¼ 1k2qe

2 þtqt

ð3Þ

and k2 is the rate constant of the pseudo-second-order model.

2.5. Desorption and recycle

Themixture of 0.01 g of NG/FC in 25mL of MV solutionswas shakenfor 30 min at room temperature. The adsorbents were separated andwashed with ethanol till colorless. Then the NG/FC was collected by amagnet and reused for dye removal after vacuum drying. The cycles ofadsorption–desorption processes were successively conducted 20times. The recovery of the adsorbents was obtained from the followingequation:

n% ¼ m1

0:01g� 100% ð4Þ

where m1 (g) is the mass of the recycled adsorbents.

3. Results and discussions

3.1. Surface properties and morphology

The FTIR spectrum of the NG and NG/FC is shown in Fig. 2. Thepeak at 3421.0 cm−1 is attributed to the stretching vibrations of

2 4 6 80.00

0.05

0.10

0.15

0.20

0.25

Pore

Vol

ume

dv/d

w (

cm3 /

g,ST

P)

Pore W

0.0 0.2 0.4 0.6 0.80

100

200

300

400

500

600 adsorptiondesorption

Qua

ntit

y A

dsor

bed(

STP

)

R

(a)

Fig. 3. N2 adsorption–desorption isotherms (inset) and corresponding BJH pore-size distributiobution was calculated from the desorption branch of the isotherm.

dissociative \OH on the surface of the sample. The peaks ranged from2921.8 to 2970.2 cm−1 are corresponding to the stretching vibrationsof C\H. The peak at 1588.5 cm−1 is because of the stretching vibrationsof C_O bonds and the peaks at 878.4–1214.0 cm−1may be attributed tothe asymmetric stretching modes of C_C bonds. There is an obviouspeak in Fig. 2 at 1716.3 cm−1, which demonstrates the presence of car-bon–oxygen double bonds on the surface of graphite. However, after areaction with HHA, these bonds were restored to \OH; thus the peakof \OH from the spectrum of NG/MC is stronger than that of NG, butthere are no big changes to the characteristic peak of C_O bond. Thepeaks at 451.5 and 574.5 cm−1 are the characteristic peaks of C\Feand Fe\O, respectively, which demonstrate the good integration be-tween NG and Fe3O4.

The corresponding Barrett, Joyner and Halenda (BJH) pore-size dis-tribution curves of the samples are displayed in Fig. 3 and the insetsare the N2 adsorption–desorption isotherms, both of which indicatethe 3D intersection of solid porous materials [40]. The average poresize of the adsorbent is 3.7 nm with a wide size distribution indicatingthat the micropores are dominated in the total pore volume of the NG.The specific surface areas of NG and NG/FC are 603.52 m2/g and292.63 m2/g, respectively. There are two reasons for the differences inthe surface areas between the two materials. One is that some micro-pores of NG/FC were occupied by Fe3O4 particle during the syntheticstep. The other one is that some parts of the composite powders ofNG/FC were aggregated after Fe3O4 particle was incorporated to thecomposite.

In order to demonstrate the hypothesis above, the morphology ofthe two powders are characterized by SEM shown in Fig. 4. Comparedto NG in Fig. 4(a), the pores of NG/FC in Fig. 4(b) are filled with Fe3O4

particles. The enlarged imagine in Fig. 4(f) displays the details of Fe3O4

particles, which proves the intercalation of Fe3O4 particles into NG/FCand straightforward explains the reason why NG/FC exhibits a relativesmaller surface area compare with NG.

The structural morphology of the nano-graphite is analyzed by highresolution transmission electron microscope (HRTEM) shown in Fig. 5.The thickness of graphene sheets measured using HRTEM lattice imag-ing, clearly as we can see, came out to be about 2–5 layers.

The XRD pattern of NG/FC is displayed in Fig. 6, the diffraction peaksof the planes at 2θ= 26.58°are the characteristic peaks of C. In addition,the diffraction peaks of the planes at 2θ = 35.795°, 44.257°, 57.467°,and 62.52° are the characteristic peaks of Fe3O4, which suggest thatFe3O4 has been well loaded on the nano-graphite.

10 12 14 16

nano-graphite nano-graphite/Fe

3O

4

idth (nm)

1.0

elative Pressure (P/Po)

0.0 0.2 0.4 0.6 0.8 1.0

50

100

150

200

250 (b)adsorptiondesorption

n curve of nano-graphite (a) and nano-graphite/Fe3O4 (b) composite, the pore-size distri-

Fig. 4. SEM images of nano-graphite (a, c, e) and nano-graphite/Fe3O4 composite (b, d, f).

Fig. 5. High resolution TEM images of nano-graphite.

351C. Li et al. / Journal of Molecular Liquids 196 (2014) 348–356

3.2. Effect of pH and dye concentration

The pH value of the dye solution plays an important role in thewhole adsorption process, especially on the adsorption capacity. Mostof the dyemolecules exist in ionic form in the solution, and the solubilitydepends on the degree of dissociation. The adsorption level is signifi-cantly affected by surface charge of the adsorbent, which is highly de-pendent on the pH value of the solution [41].

Fig. 7 shows the effects of pH on the adsorption ofMV onNG/FC. Theadsorption of MV increased smoothly in the pH range of 2.0–4.0,remained at a higher level for pHat 4.0–10.0, and then increased sharplyat pH values higher than 10.0. MV is an alkaline and cationic triphenyl-methane dyes (shows in Fig. 1), which is readily to be adsorbed throughπ–π stacking and ionic interaction. At lower pH value (b4.0), the disso-ciation degree of the\OH on the surface of the adsorbent is restrainedbecause the positively charged adsorbent surface leads to repulsion be-tween the adsorbent and the cationic dye MV. So the adsorption occursonly through the π–π stacking which results in a lower adsorption

10 20 30 40 50 60 70 80250

300

350

400

450

500

550

Lin

(Cps

)

2-Theta-Scale

Fig. 6. XRD images of nano-graphite/Fe3O4 composite.

0 2 4 6 8 10 12

100

120

140

160

180

200

q e(m

g/g)

pH

0.06mg/ml0.07mg/ml0.08mg/ml

Fig. 7. Effect of pH on the adsorption of MV on nano-graphite/Fe3O4 composite.

352 C. Li et al. / Journal of Molecular Liquids 196 (2014) 348–356

efficiency.With the pH rising, the inhibitory action to the dissociation of\OH becomes weaker and the negative charges are increased, which isin favor to the interaction between cationic dye MV and negativelycharged adsorbent. The π–π stacking also enhances the adsorption.

N+

++

+

++

++

MVMV

N+

MV

N+

stacking

--

-

--

MV

N+MV

Electrosta

(a) pH<4.0 (b) 4.0<p

Fig. 8. Proposed mechanism for the MV remo

Zhao et al. [31] got similar results in the investigation of the effect ofpH on the adsorption of naphthalene and 1-naphthol to sulfonatedgraphene. All of factors contribute to explain the increase of adsorptioncapacity with the pH increasing. It is noteworthy that the solubility ofdyes decreases at pH N 10.0, which accelerates the precipitation onthe surface of the adsorbents resulting in the sharply increase of adsorp-tion capacity for the dye. The mechanism of the adsorption capacity atdifferent pH is illustrated in Fig. 8.

The effects of dye concentration are also showed in Fig. 7. It is obvi-ous that the adsorption of MV increases with the dye concentration ris-ing, which suggests that high concentrations of MV favor to theformation of precipitate on theNG/FC surface due to the limited solubil-ity of MV [42].

Considering that leaching of metal ions from nano-graphite/Fe3O4

composite into the treated water is undesirable, we further performeda leaching test in the aqueous solution at different pH values to evaluatethe stability of NG/FC. Fig. 9 shows the percentage of leached Fe at differ-ent pH values and shaking for 2 h,which shows that the leaching of Fe isnegligible at pH over 3.0 and enhances significantly when pH is lowerthan 3.0. A similar trend was also reported for leaching of iron ionsfrom Fe3O4-based nanoparticles [43,44]. These results imply that adsor-bents are unstable at very low pH values. Therefore, we advise that NG/FC should be not used at very low pH values.

3.3. Effect of shaking time

The adsorption ofMV on the NG/FC is quite quickly and achieves theadsorption equilibrium in less than 10 min as shown in Fig. 10. Theremoval amounts of MV on NG/FC maintain in the same level evenextending the shaking time. Noteworthy, the adsorption percentage(Adsorption% ¼ c0−ce

c0� 100%) was calculated to be 98.9% at c(MV) of

0.04 mg/mL. On the basis of these results, NG/FC was proved to be anefficient organic dye scavenger possessing high adsorption capacity.

3.4. Adsorption kinetic

The liner plot of ln(qe− qt) vs. t for the pseudo-first-ordermodel andtqtvs. t for the pseudo-second-order model is shown in Fig. 11(a) & (b).

The constants of the two models with the correlation coefficients areshown in Table. 1. Based on the results above, the pseudo-second-order kinetic model fits the adsorption process better obviously. Theqe ismore approximate to the experimental data and the correlation co-efficients of the two initial MV concentrations are 0.9965 and 0.9985,which are very close to 1. In Table. 1, the rate constant of the pseudo-second-order model (k2) is tiny (0.0033 and 0.0040, respectively) indi-cating that the adsorption equilibrium can occur in a short time as theresult shown in Fig. 11.

stacking

--

MV

N+

N+

MV

N+

--

-

--

--

MV

N+MV

N+

Precipitationtic interaction

H<10.0 (c) pH> 10.0

val on nano-graphite/Fe3O4 composite.

0 1 2 3 4 5 6 7 80

20

40

60

80Ir

on le

ache

d(%

)

pH

Fig. 9. The percentage of leached Fe at different pH values.

0 10 20 30 40 50 60-2

-1

0

1

2

3

4

5 0.06mg/mL 0.07mg/mL

ln (

q e-q t)

t (min)

(a)

0 10 20 30 40 50 60

0.0

0.1

0.2

0.3

0.4

0.5

0.06mg/mL 0.07mg/mL

t/qt (

min

mg

g-1)

t (min)

(b)

Fig. 11. Pseudo-first-order (a) and pseudo-second-order (b) kinetic plots.

353C. Li et al. / Journal of Molecular Liquids 196 (2014) 348–356

3.5. Intra-particle diffusion model

To further evaluate the controlling step of the adsorption process ofMV onto NG/FC, we used the intra-particle diffusionmodel according tothe method reported by Weber and Morris [35]. This model can beexpressed by the following equation:

qt ¼ kt1=2 þ C ð5Þ

where k is the rate constant of the intra-particle and C is the intercept. Itis clear that the plot of qt vs. t1/2 should be linear according to Eq. (5). If aplot of qt vs. t1/2 gave a straight line, the adsorption process should in-volve the intra-particle diffusion, and if this line passed through the or-igin, the particle diffusion would be the controlling step [36].

Several factors controlling the adsorption rate have been clarifiedbefore [45–47]. The intra-particle diffusion kinetic for adsorption ofMV onto NG/FC is shown in Fig. 12, which can be divided into threestages. The first stage was a rapid adsorption process occurred withinfirst 5 min which is attributed to the external surface adsorption. TheMVmolecules diffused through the bulk solution to the external surfaceof the adsorbent or the boundary layer diffusion of MV molecules. Thesecond stagewas a gradual adsorption process, where intra-particle dif-fusion rate was rate controlling. Thereafter, the adsorption of MV onto

0 10 20 30 40 50 60

30

60

90

120

150

q t (m

g/g)

Time (min)

0.06mg/mL0.07mg/mL

Fig. 10. Effect of shaking time on the adsorption of MV onto nano-graphite/Fe3O4

composite.

NG/FC became very slow and stable, approaching an equilibrium stageand maximum adsorption, which was similar to the results reportedby Wang's group [48].

3.6. Adsorption isotherm

The adsorption isotherm is important to both theoretical and practi-cal applications. To optimize the adsorption system, themost appropri-ate correlations of the equilibrium data is quite necessary. Equilibriumisotherm equations are used to describe the experimental sorptiondata. The parameters, which were obtained from different models, pro-vided important information to the adsorption mechanisms, surfaceproperties and affinities of the adsorbent.

In this study, three different temperatures at pH= 8.0 were used todescribe the adsorption isotherms shown in Fig. 13.

Table 1The obtained constants of pseudo-first-order and pseudo-second-order kinetic modelswith correlation coefficients.

C0(mg/mL) Pseudo-first-orderkinetic model

Pseudo-second-orderkinetic model

qe(mg/g) k1(min−1) R qe(mg/g) k2(g·min/mg) R

0.06 48.022 0.089 0.890 139.082 0.0033 0.99650.07 41.436 0.092 0.775 139.664 0.0040 0.9985

0.0 1.0x10-2 2.0x10-2 3.0x10-2

3.0x101

6.0x101

9.0x101

1.2x102

1.5x102

298.15 K 318.15 K 338.15 K

q e (m

g/g)

Ce (mg/ml)

Fig. 13. Adsorption isotherms of MV on nano-graphite/Fe3O4 composite at three differenttemperatures.

0.0 5.0x10-3 1.0x10-2 1.5x10-22.0x10-22.5x10-23.0x10-23.5x10-2

0.0

5.0x10-5

1.0x10-4

1.5x10-4

2.0x10-4

2.5x10-4

298.15 K 318.15 K 338.15 K

Ce/

q e (g

/mL

)

Ce (mg/ml)

(a)

-4.0 -3.5 -3.0 -2.5 -2.0 -1.5

1.4

1.6

1.8

2.0

2.2

298.15 K318.15 K338.15 K

Log

q e (m

g/g)

LogCe (mg/mL)

(b)

Fig. 14. Langmuir (a) and Freundlich (b) isotherms for MV adsorption on nano-graphite/Fe3O4 composite at three different temperatures.

354 C. Li et al. / Journal of Molecular Liquids 196 (2014) 348–356

The Langmuir and Freundlich models are adopted to correlate theexperimental data [45,46]. The Langmuir isothermmodel can be repre-sented by the following equation:

qe ¼bqemaxce1þ bce

: ð6Þ

Eq. (6) can be expressed in linear form:

ceqe

¼ 1bqemax

þ ceqemax

ð7Þ

where qemax(mg/g), is themaximum adsorption capacity amount ofMVat complete monolayer coverage, and b is the constant related to theheat of adsorption.

The results indicated that higher temperature did favor to the ad-sorption of MV on NG/FC, whereas this behavior was diminished atlower temperature.

The Freundlich isotherm model represents properly the adsorptiondata at low and intermediate concentrations on heterogeneous surfacesand it is usually expressed as follows [45]:

qe ¼ K Fcne : ð8Þ

0 1 2 3 4 5 6 7 820

40

60

80

100

120

140

0.06mg/mL 0.07mg/mL

q t (m

g/g)

t1/2

Fig. 12. Intra-particle diffusion kinetic for adsorption of MV onto nano-graphite/Fe3O4

composite.

Eq. (8) can be expressed in linear form:

logqe ¼ logK F þ n logce ð9Þ

where KF represents the adsorption capacitywhen dye equilibrium con-centration equals to 1, and n represents the degree of dependence of ad-sorption with equilibrium concentration [47].

The experimental data of MV adsorption (Fig. 13) were regressivelyanalyzedwith the Langmuir and Freundlich isothermmodels (Fig. 14a &b) with the relative values calculated from the twomodels in Table 2. Itcan be concluded that Langmuirmodel coincideswith the experimentaldata better than Freundlich model, which indicates that the Langmuirisotherm with complete monolayer coverage of the adsorbent particlescan fit the experimental data very well [49].

3.7. Thermodynamic studies

The thermodynamic parameters (ΔH0, ΔS0 and ΔG0) for MV adsorp-tion on NG/FC can be calculated from the temperature dependent

Table 2The parameters for Langmuir and Freundlich isotherms at different temperatures.

T(K) Langmuir Freundlich

qemax(mg/g) b(g/mL) R R KF(mg/g) n(g/mL) R

298.15 K 144.7178 0.17 × 104 0.99957 394.82 0.22115 0.57307318.15 K 147.4926 0.28 × 104 0.99976 416.73 0.23813 0.65621338.15 K 151.5152 0.29 × 104 0.99976 436.97 0.26178 0.68235

0.0029 0.0030 0.0031 0.0032 0.0033 0.0034

9.5

10.0

10.5

11.0

11.5 0.05mg/mL 0.06mg/mL 0.07mg/mL

lnK

d

1/T (K)

Fig. 15. Effect of temperature on the distribution coefficients of MV onto nano-graphite/Fe3O4 composite at different initial MV solution concentrations.

2 4 6 8 10 120

20

40

60

80

100

n%

pH

Fig. 16. The recovery of nano-graphite/Fe3O4 composite at different pH.

80

85

90

Ads

orpt

ion%

355C. Li et al. / Journal of Molecular Liquids 196 (2014) 348–356

adsorption. The values of enthalpy (ΔH0) and entropy (ΔS0) can be cal-culated from the slope and intercept of the plot of lnKd vs. 1/T (Fig. 15)via applying the following equations:

Kd ¼ C0−Ceq

Ceq� Vm

ð10Þ

lnKd ¼ ΔS0

R−ΔH0

RTð11Þ

where V is the volume (mL) and m is the mass of adsorbent in the sys-tem (g). The Gibbs free energy (ΔG0) of specific sorption was calculatedfrom the well-known equation:

ΔG0 ¼ ΔH0−TΔS0: ð12Þ

The thermodynamic parameters (ΔH0, ΔS0, ΔG0) calculated fromtemperature dependence are tabulated in Table 3. The positive valueof standard enthalpy change (ΔH0) indicated that the adsorption pro-cess was endothermic, which was also demonstrated by the adsorptionisotherms at three different temperatures. One possible explanation forthis positive enthalpy was that MV was dissolved well in water, whilethe mass transfer resistance of MV moving to the surface of the adsor-bent had to be overcome before the adsorption on NG/FC which neededenergy. Because this energy exceeds the exothermic of zwitterion com-bination in the solid surface, high temperature was propitious to pro-mote the reaction. The Gibbs free energy change (ΔG0) was negative,as expected for a spontaneous process under the conditions applied.The value ofΔG0 becamemore negativewith increasing of temperature,which indicated more efficient sorption at high temperature. The posi-tive value of entropy change (ΔS0) suggested the affinity of NG/FC to-ward MV in aqueous solutions and might suggest some structurechanges on the NG/FC. Similar results have been previously published[27,50]. The result of MV sorption on NG/FC was a spontaneous and en-dothermic process.

Table 3The thermodynamic data of MV adsorption onto nano-graphite/Fe3O4 composite at differ-ent MV initial solution concentrations.

C0 (mg/mL) ΔH0 (kJ/mol) ΔS0 (J/mol K) ΔG0 (kJ/mol)

303.15 K 323.15 K 343.15 K

0.05 19.20 154.01 −26.71 −29.79 −32.870.06 13.09 128.12 −25.11 −27.67 −30.240.07 5.50 94.62 −22.71 −24.60 −26.49

3.8. Recycle of the adsorbents and desorption

The regeneration ability and stability of the adsorbent are two im-portant factors influencing the practical application of the adsorbent.Adsorbent should possess sufficiently high stability to avoid the compo-nent leaching from the adsorbents to environment, which might causesecondary pollution to the water environment. The stability of the NG/FC was tested by detecting the content of the leached Fe [51]. In thiswork, there were no Fe ions detected during the adsorption process atpH N 7.0, which indicated the high stability of the composite.

Fig. 16 shows the retrieve experiment at 298.15 K in different pHvalues. As shown in Fig. 13, the recovery of the adsorbent was low atpH b 7.0. The recovery ratio maintained at a high level of 99.8% withthe pH range of 7.0–10.0, and then decreased with the pH increasing.When pH N 10.0, the precipitation of MV covers the surface of the ad-sorbent causing the recovery slightly down. On the other hand, the sys-tem was becoming acidic and the Fe3O4 particles were dissolved atlower pH values, which lowered the magnetism and recovery perfor-mance of the adsorbent.

The cyclic adsorption–desorption tests are also carried out to evalu-ate the regeneration abilitywith results shown in Fig. 17. The adsorptionrate was basically maintained at a level higher than 82% for each cycleafter desorption with 20 cycles, which proved the high regenerationability of the NG/FC.

0 2 4 6 8 10 12 14 16 18 2070

75

cycles

Fig. 17. Adsorption rate of MV on the nano-graphite/Fe3O4 composite in 20 cycles of ad-sorption–desorption.

356 C. Li et al. / Journal of Molecular Liquids 196 (2014) 348–356

4. Conclusions

In summary, the graphite-based adsorbent NG/FC was successfullysynthesized through deposition–precipitation method. The pore struc-ture analysis by nitrogen isotherm data and SEM image showed thatthe Fe3O4 particles were successfully intercalated into the interior ofNG. The adsorption capacities ofMVonNG/FC are enhancedwith the in-creasing of the pH value, dye content and temperature of the solution.The adsorption equilibrium could be reached within 10 min by usingNG/FC.Moreover, the Langmuir isothermmodel fitted the experimentaldata very well, and the high stability and remarkable regeneration abil-ity indicated that the NG/FC is a promising adsorbent for the removal ofMV from waste water.

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