Impact of process parameters on removal of Congo red by graphene oxide from aqueous solution

Journal of Environmental Chemical Engineering 2 (2014) 260–272

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Impact of process parameters on removal of Congo red by graphene oxide from aqueous solution Sushanta Debnath a,b,*, Arjun Maity c,d, Kriveshini Pillay b,** a

Department of Chemical and Metallurgical Engineering, Tshwane University of Technology, Pretoria, South Africa Department of Applied Chemistry, University of Johannesberg, Johannesberg, South Africa c Smart Polymers Group, Materials Science and Manufacturing (MSM), Council for Scientific and Industrial Research (CSIR), Pretoria, South Africa d Department of Civil and Chemical Engineering, University of South Africa (UNISA), South Africa b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 16 October 2013 Accepted 23 December 2013

This study evaluated the performance of graphene oxide in the removal of Congo red (CR) dye from aqueous solution. The adsorbent, graphene oxide (GO), was prepared from graphite and was characterized using FTIR, SEM and XRD. Batch sorption studies were carried out to determine the effect of pH, contact time, initial concentration of CR and temperature on the adsorption of CR onto GO. Circumneutral pH was found to be favorable for the adsorption of CR onto GO. The equilibrium data fitted well with the Redlich–Peterson model and characterized by a Langmuir type isotherm. The kinetics of the adsorption data was analyzed using four kinetic models viz. pseudo-first-order, pseudo-secondorder, Elovich model and intra-particle diffusion models. The results from the kinetic studies indicated that the rate of adsorption follows a pseudo-first-order with respect to the CR solution concentration and that in general the order of data fit is pseudo-first-order > pseudo-second-order > Elovich equation. The kinetic parameters obtained from the kinetic studies suggested that the adsorption process is filmdiffusion-controlled. The results obtained from thermodynamic studies revealed that the adsorption process is endothermic in nature as well as the feasibility and spontaneity of CR adsorption onto GO. The values of DH8and DS8 of the adsorption process were 8.19 kJ mol1 and 0.10 kJ mol1, respectively. The low value of DH8 (<40 kJ mol1) indicated that adsorption process occurs mainly through a physical means. ß 2013 Elsevier Ltd. All rights reserved.

Keywords: Graphene oxide Congo red Kinetics Thermodynamics Isotherm Batch adsorber

Introduction Graphene, the building block of carbon nanotubes and graphite, has been gained an immense interest by researchers since its discovery in 2004 [1]. It has a two dimensional honey-comb like structure with a sp2 hybridized carbon lattice which has a flexible, porous structure and high chemical stability [2]. Oxidation of graphite produces graphite oxide (GrO) which produces exfoliated sheets of graphene oxide (GO) upon sonication [2]. GO contains a large amount of reactive oxygen atoms on the surface, resulting from the presence of epoxy, carboxyl and hydroxyl groups. The presence of these groups makes GO hydrophilic and thus it is easily used as reactant in aqueous solution [3]. Another advantage of

* Corresponding author at: Department of Applied Chemistry, University of Johannesberg, Johannesberg, South Africa. Tel.: +27 11 5596128; fax: +27 11 5596425. ** Corresponding author. Tel.: +27 11 5596128; fax: +27 11 5596425. E-mail addresses: [email protected] (S. Debnath), [email protected] (A. Maity), [email protected] (K. Pillay). 2213-3437/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jece.2013.12.018

using graphite oxide is its ability to adsorb number of chemicals onto the benzene ring by the strong p–p interaction [4]. GO finds applications in solar cells, nano-electronics, polymer composites, H2 production, intercalation materials, drug delivery, sensor, catalysis, photovoltaic flexible electronics, chemical sensors, liquid crystal devices, and as an indium tin-oxide (ITO) replacement [1,3,5–17]. The presence of large amount of reactive oxygen atoms on its surface makes it promising adsorbent for treatment of water or wastewater. GO and modified GOs have been used as adsorbent for the removal of reactive dyes [18–21], phosphates [22], tetracycline [3], microcystin-LR and microcystinRR [23], fluoride [24], and different metal ions like Zn(II), Pb(II), U(IV), Cu(II), As(III), As(V), Au(II) and Pd(II) [21,25–29]. GO has also been used as a detector and sensor for a number of gaseous pollutants like SO2, H2S, NH3 [30–35], etc. Congo red (CR) (disodium 4-amino-3-[[4-[4-[(1-amino4-sulfonatonaphthalen-2-yl)diazenyl] phenyl]phenyl]diazenyl]naphthalene-1-sulfonate) is a benzidine based anionic dye. It is generated from textiles, printing and dyeing, paper, rubber and plastics industry, among others. CR is known to metabolize into benzidine, which is a known human carcinogen [36]. Therefore, it

S. Debnath et al. / Journal of Environmental Chemical Engineering 2 (2014) 260–272

is very important to remove residual Congo red from water sources before discharge to receiving water bodies. Treatment of wastewater containing dyes is conventionally done by several physicochemical and well as biological methods including coagulation-flocculation [37,38], advance oxidation, adsorption, ozonation [39], photo-chemical degradation, fungal decolorization [40], etc. However, adsorption is the most popular method among the aforementioned ones due to its low operational cost, low maintenance and simplicity [41]. Under the adsorption treatment method, carbon-based materials rank as the most widely used materials due to their stability and regenerability. Activated carbon is the most widely-used material among the carbon-based adsorbents, but its high cost has limited its usage on commercial scale. Another form of carbon based materials is the graphene based adsorbents, derived from graphite. Graphene is inexpensive, has a high adsorption capacity and thus is very promising adsorbent for different genre of pollutants [3,19,23,24,27–29,42–44,20,45,14,46–49]. The ‘‘wrinkly’’ surfaces of graphene interlock extremely well with the surrounding compositing material than other carbon based nano materials, which boosts the interfacial load transfer between graphene and the host material. As a planer sheet, graphene benefits from considerably more contact than the tube-shaped carbon nanotubes. Crack deflection processes are far more effective for two-dimensional sheets with a high aspect ratio such as graphene, as compared to one-dimensional nanotubes [50,51]. As an extension of the applications of GO, the present communication focuses on the synthesis and characterization of a highly oxidized graphene oxide. The prepared material was employed in the removal of CR from aqueous solution in a batch adsorption mode. The obtained experimental data were analyzed using appropriate kinetics, isotherm and thermodynamic models. A single stage batch adsorber is proposed and method is developed to predict the amount of GO required for various amount of effluent treated for different percentage of dye removal. Finally, a plausible mechanism for the removal of CR by GO is proposed.

Materials and methods The graphite flakes used for preparation of the graphene oxide was purchased from Sigma Aldrich (South Africa). The Congo red dye (chemical formula = C32H22N6Na2O6S2, CI = 22,120, FW = 696.7 and lmax = 488 nm) was also obtained from Sigma Aldrich (analytical grade). KMnO4, H2SO4 and NaNO3 required for oxidation of the graphite flakes were of analytical grade. The acid and base (HCl and NaOH) used for pH adjustment were of reagent grade. All other reagents were analytical grade. A stock solution of CR (1000 mg L1) was prepared by dissolving 1000 mg of the dye in distilled water and making up the volume to 1000 mL. The working CR solutions used in the experiments were prepared by diluting the stock solution to required concentration using deionized water. The instruments used for data collection and characterization of the adsorbent were (i) Orion 4 star pH meter (Thermo) for adjustment of pH of the solutions, (ii) Phillips X-ray diffractometer for the X-ray diffraction (XRD) analysis, (iii) attenuated total reflectance-Fo¨urier transform infra red (ATR-FTIR) spectrometer (Perkin Elmer Spectrum 100) for the spectra of GO, (iv) scanning electron microscope (SEM) (Tescan Vega, UK) for the image of the material for surface morphology, (v) Bruker Nanoscope AFM, (vi) PHI 5000 Versaprobe–Scanning ESCA Microprobe for XPS spectra and (vii) UV–vis spectrophotometer (Shimadzu: UV 1800) for colorimetric analysis of CR.

261

Preparation of GO Graphite oxide (GrO) was prepared using the modified Hummer’s method [52–54]. In brief, 5.0 g of natural graphite flakes was mixed with 2.5 g NaNO3 and 120 mL H2SO4 and stirred in a beaker on an ice-bath. 15.0 g KMnO4 was added into the mixture and mixed vigorously with temperature kept below 20 8C. The mixture was further stirred at room temperature overnight. Thereafter, another 15.0 g KMnO4 was added into the above mixture and stirred at room temperature for another 2 h to oxidize any remaining un-oxidized graphite. On addition of deionized water into the mixture, a yellowish paste was obtained and the temperature increased to about 98 8C with effervescence. The diluted suspension was stirred for another 4 h and then 50 mL of 30% H2O2 was added to the mixture. The mixture was subsequently washed with 100 mL 5% HCl and 30% H2O2 five times to purify it from the MnO2 and sulfates. Finally, a solid was obtained after drying at 60 8C under vacuum. Graphene oxide (GO) was prepared from the solid graphite oxide by sonicating the solid in double distilled water, until the solid was completely converted into stable colloidal form of exfoliated graphene oxide sheets. The colloidal graphene oxide (GO) was then precipitated, using 0.1 M NaOH solution to make it usable in adsorption studies. The solid precipitates of graphene oxide were filtered through a 0.45 mm nylon membrane filter to remove the liquid and then dried at 60 8C under vacuum. Adsorption experiments The effect of pH was studied by varying the solution pH from 2.0 to 10.0 using HCl or NaOH. Here, 50 mL aliquots of CR (concentration: 200.0 mg L1) solution was contacted with 0.05 g of GO in glass bottles (capacity: 100 mL), and placed into a thermostatic shaker water bath. The agitation speed and the time used were 150 (5) rpm and 24 h, respectively. As a control, similar experiment was also conducted at identical pH conditions without adding the adsorbent. After 24 h, the samples were withdrawn from glass bottles, centrifuged and analyzed for residual CR concentration using a visible spectrophotometer at lmax = 488 nm. The kinetic experiments for the CR adsorption were carried out at pH 7.0 (0.1). Here, 1.0 g of the adsorbent was added to 500 mL of the adsorbate solution (concentration: 200.0 mg L1) contained in a 1.0 L beaker at three different temperature (25, 35 and 45 8C (1)). For each temperature, the reaction mixture was stirred at a speed of 300 (5) rpm. Samples of 1.0 mL volume were withdrawn from the reaction mixture at a pre-fixed time interval, and centrifuged to separate out the adsorbent particles. For equilibrium isotherm, 0.05 g of GO was added to 100 mL glass bottles containing 50 mL of CR solution with varying concentration of 25.0–600.0 mg L1. The bottles with solution were agitated at a speed of 150 (5) rpm for 24 h in a thermostatic shaker water bath. Thereafter, the mixtures in the bottle were centrifuged and the supernatant solution was analyzed for residual CR concentration at lmax = 488 nm. The amount of CR adsorbed per unit mass of the adsorbent was calculated using Eq. (1): qe ¼

qi  qr m

(1)

where qe is the adsorbed amount of CR per unit mass of the adsorbent (mg g1), qi and qr are initial and residual amount (mg) of CR, respectively, added and remained in solution. ‘m’ is the mass (g) of the adsorbent added for the experiments. The effect of the ionic strength was evaluated by varying the concentration of the ionic solutions (NaCl or NaHCO3) from 0.1– 2.0 M with 50 mL of 100 mg L1 dye solution and 0.05 g of GO.

S. Debnath et al. / Journal of Environmental Chemical Engineering 2 (2014) 260–272

262

The pHzpc of the material was determined according to the method described by Babic et al. [55]. Here, a series of 0.1 M NaCl solutions were prepared at different initial pH varying from 2.0 to 12.0. A fixed amount of the adsorbent (0.05 g) was then added into 100 mL glass bottles containing 50 mL of the prepared NaCl solution. The mixtures were agitated at (300  10 rpm) using an orbital shaker until the pH readings of two successive measurements were equal. Results and discussion Characterization The point of zero charge (pHpzc) of the adsorbent was determined by the pH drift method. Fig. 1 shows a plot of pH drift (DpH) against initial pH. The pHpzc value obtained for GO was 7.46. Below the pHzpc the surface of GO is positively charged and thus effective in removing negatively charged species from aqueous solution while at pH values above the pHzpc the GO surface is negatively charged. Fig. 2 represents the SEM image of the prepared GO. It is observed that the surface of the adsorbent has irregular morphology which is porous in nature. The bright points in the image indicate the developed charge on the surface. Fig. 3 shows the FTIR image of synthesized graphene oxide. The absorption peaks at 1733 and 1620 cm1 indicate the stretching vibration of C5 5O and the C5 5C vibration of oxidized graphite flakes, respectively. Peaks at 1397, 1223 and 1049 cm1 are assigned for carboxyl (–O5 5C–OH), epoxy C–O and alkoxy C–O stretching vibrations, respectively. The broad peak around 3400 cm1 is assigned for the vibrational frequencies of absorbed –OH groups. The strong peak at 1397 cm1 (carboxyl O5 5C–O) group is the active group responsible for the strong interaction between the negatively charged (D-SO3) (D = dye molecule) groups of the dye molecule at pH < pHzpc. Similar findings for the peak positions of GO were also found by other researchers [22,56]. The XRD pattern of GO is shown in Fig. 4. Here, the main diffraction peak appears at 2u = 12.48 with a full width half maximum (FWHM) at 0.82. The interlayer distance between the graphene inter layer sheets are calculated to be 0.97 nm. Similar results have reported by other authors [57–59]. The characteristic peaks of natural graphite at 2u = 26.28, 44.88 and 558 are not found in the XRD pattern of GO, suggesting that the synthesized sample is free of contamination with unoxidized graphite. 1 1.5

Fig. 2. SEM image of graphene oxide.

Fig. 5 shows the AFM image and the surface mapping of GO. The ‘‘wrinkly’’ surface of the material was demonstrated by tappingmode AFM. The average distance between two peaks is 324 nm. It was found that the thickness of the GO sheet is 78.58 nm. The GO sheet thickness increases as a result of the bulky carbonyl, epoxy, and carboxyl groups on graphene oxide sheets [60]. XPS analysis of the surface composition of GO shows presence of 66, 33 and 1% of C, O and Si, respectively (Fig. 6a). In graphite the elemental composition of C, O and Si are 85%, 12% and 2%, respectively (Fig. 6b). This confirms that GO sheets are produced via incorporation of oxygen atoms in graphite during Hummer’s method of oxidation. The Si contamination may be from the preparation vessels. The deconvoluted XPS C1s (Fig. 6c) and O1s (Fig. 6d) core spectra of GO show well resolved peaks of carbons and oxygen of different chemical environments. The peaks at different binding energies (eV) in the C1s spectra are 284.5, 286.3 and 288 correspond to the aromatic carbon (as of graphite), C–O bonds of epoxy or hydroxyl groups and C5 5O groups of carboxylic groups, respectively [61]. The O1s spectra of GO shows two peaks at 531.5 and 533.2 eV which corresponds to the C5 5O and the O–H groups in GO, respectively [62].

1 1.0

Δ pH

0 0.5

0 0.0 2

4

6

8

10

12 2

-0 0.5

-1 1.0

Initial pH -1 1.5 Fig. 1. Determination of pHpzc of GO.

Fig. 3. FTIR image of GO.

S. Debnath et al. / Journal of Environmental Chemical Engineering 2 (2014) 260–272

5

addition of the adsorbent. In particular, the color of the dye changes to blue (pH 3.0–5.0) and in the range of pH 10.0–12.0, it changes to red in color (which is different from the original red color of the dye). Color and intensity change of the dye with change in pH alone may attribute to the molecular structural change of the dye at that pH range [63,64]. The form of the dye with lmax = 488 nm is prevalent in the pH range 6.0–9.0. A similar result was reported for the adsorption of CR on montmorrilonite [64].

12.4 425

Intensity (a (a.u) u)

4

3

2

25.005

37.925 42 2.665 47.205

Effect of ionic strength on adsorption of CR by GO 64.135

1

0 10

263

20

30

40

50

60

70 0

2θ (degre ee) Fig. 4. XRD pattern of GO.

But in the XPS spectra of pristine graphite (Fig. 6 e and f) both in C1s and O1s spectra, presence of a single peak is observed, which corresponds to the aromatic carbon for C1s and O–H for O1s only in the respective individual plots. Effect of pH on decoloration of CR by GO The initial pH values of the solution affect on the adsorptive removal of the dye solution from its aqueous solution. The initial pH of the solution also affects the structural stability of the dye molecules. Fig. 7 shows the effect of pH on adsorption of CR by GO. The percentage of adsorption is between 98.25 and 99.32 in the pH range 6.0–10.0 with maximum removal at pH 7.0. In the range of pH 3.0–5.0 and 10.0–12.0 there was apparent decoloration of the dye, which was observed from the blank experiment without

The effect of ionic strength on the adsorption of CR by GO was observed by varying the concentration of NaCl and NaHCO3. NaCl and NaHCO3 are used as additives in the dyeing bath for enhancing the dyeing efficiency. The concentration of NaCl and NaHCO3 were varied from 0.01 M to 2.0 M. It is observed that dye removal improves with increase in NaCl and NaHCO3 concentrations (Fig. 8). This might be due to the fact that at higher concentration of both the salts, the ionic strength of the CR solution is maintained at a constant value and the adsorption is high. But, at low concentration of the salts, the ions, Na+, Cl, and HCO3 compete for the available adsorption sites and hence reduce the adsorption of CR. Kinetics of adsorption of CR by GO Fig. 9(a) and (b) illustrates rate of adsorption of CR (qt, mg g1) at a specific reaction time (t, min) at pHi 7.0 (0.1) by GO for initial CR concentration of (a) 100 and (b) 200 mg L1, respectively. The kinetic data was analyzed by the equations (Eqs. (2)–(4)) shown below using the non-linear method of analysis using the Origin 8.0 or Graph Pad Prism 5 software. qt ¼ qe ð1  ek1 t Þ t Pseudo-second-order model : qt ¼ ð1=k2 q2e Þ þ ðt=qe Þ Pseudo-first-order model :

Elovich model :

qt ¼

Fig. 5. AFM image and the surface mapping of GO.

1

b

lnðabÞ þ

1

b

lnðtÞ

(2) (3)

(4)

264

S. Debnath et al. / Journal of Environmental Chemical Engineering 2 (2014) 260–272

Fig. 6. XPS surface survey of (a) GO and (b) graphite after 3 cycles and the C1s core level spectra of (c) GO and (e) graphite, and the O1s core level spectra of (d) GO and (f) graphite.

where qt and qe are the adsorption capacity (mg g1), respectively, at time t (min) and at equilibrium; k1 (min1) and k2 (mg L1 min1) are the pseudo-first-order and pseudo-secondorder rate constants, a (mg g1 min1) is the initial adsorption rate and b (g mg1) is the desorption constants. The estimated kinetic parameters with regression coefficients (R2) and the statistical error (x2) are shown in Table 1.Based on the

values of the linear regression coefficient (R2), it could be suggested that the present adsorption kinetic data is well described by the pseudo-first-order model (Eq. (2)) than the other two models analyzed. The order of data fit, in general, is pseudo-firstorder > pseudo-second-order > Elovich equation. The rate constants (k1 or k2) were increased with increasing temperature indicating the adsorption of CR is endothermic in nature.

S. Debnath et al. / Journal of Environmental Chemical Engineering 2 (2014) 260–272 100

100

80

90

With GO Without GO Removal Percentage

Removal percentage

265

60

40

20

80

NaCl NaHCO3

70

60 0 2

4

6

8

10

12

50

Initial pH

0.0

0.5

Fig. 7. Effect of pH on adsorption of CR in presence and in absence of GO.

1.5

2.0

-1

Fig. 8. Effect of ionic strength on adsorption of CR by GO.

Intraparticle diffusion parameters The overall solute adsorption onto a solid surface may be controlled either by one or more steps, i.e., boundary-layer (film) or external diffusion, pore diffusion, surface diffusion and adsorption onto the pore surface or a combination of several steps. Generally, an adsorption process is diffusion controlled if the rate is dependent upon the rate of diffusion of adsorbate toward the adsorbent. According to Weber and Morris [20], if the rate-limiting step is intra-particle diffusion, then the amount adsorbed (qt, mg g1) at any time t (min) should be directly proportional to the square root of contact time t, which is defined mathematically as (Eq. (5)), qt ¼ kid t0:5 þ C

(5)

where C is the boundary layer thickness and kid (mg g1 min1/2) is the intra-particle diffusion rate constant. C is a function of boundary layer effect. If the Weber–Morris plot of qt versus t0.5 gives a single straight line, passing through the origin, the particle diffusion would be the rate controlling step [65,66]. However, if the plot of the data produces multi-linear portions, then two or more steps influence the sorption process. Fig. 10 represents the plots of mass of CR adsorbed (qt, mg g1) per unit mass of GO versus t0.5 (min0.5). It can be seen that the plot

a

comprises of two linear portions – a sharp first linear portion is due to the boundary-layer (film) diffusion and the second linear one due to pore diffusion [45]. The non-linearity of plots over a range of square root time (t0.5, min0.5) indicates the multistage adsorption of solute. Extrapolation of the linear portions of the plots back to the y-axis gives the intercept, providing the measure of the boundary-layer thickness (C). The deviation of the straight line from the origin might be due to the difference in rate of mass transfer in the initial and the final stages of adsorption. Further, such deviation of straight line from the origin has indicated that the pore diffusion is not the rate-limiting step. The values for kid1, kid2 and C were computed from the respective slopes and intercept of the plots and are summarized in Table 1. As seen from Table 1, kid1 values are higher than those of kid2 suggesting that film diffusion is more prevalent than pore diffusion in the present adsorption process. In order to confirm the above, diffusion coefficients (DP, cm2 s1 and DF, cm2 s1) were calculated using the following equation (Eqs. (6) and (7)) [45,14]:

DP ¼

0:03r02 t 0:5

(6)

b 200

100

150

25 oC 35 oC 45 oC Pseudo 1st order model fit Pseudo 2nd order model fit Elovich model fit

50

0

0

100

200

300

time (min)

400

qt (mg.g-1)

q t (mg.g -1 )

1.0

Ionic strength (mol.L )

25 oC 35 oC 45 oC Pseudo 1st order model fit Pseudo 2nd order model fit Elovich model fit

100

50

0

0

100

200

300

400

time (min)

Fig. 9. The plot of CR adsorption capacity (qt, mg g1) versus contact time (t, min) by GO at CR initial concentration of (a) 100 mg L1 and (b) 200 mg L1.

S. Debnath et al. / Journal of Environmental Chemical Engineering 2 (2014) 260–272

266

Table 1 The kinetic parameters evaluated for Congo red adsorption by GO at initial pH 7.0 (0.1). Initial conc.

100 mg L1

Temperature

25 8C

35 8C

45 8C

25 8C

35 8C

45 8C

0.06 100.8 0.98 4.876

0.09 104.3 0.98 4.595

1.52 108.6 0.98 4.608

0.11 197.43 1.00 3.881

0.15 196.81 1.00 2.951

9.97 200.38 0.99 3.085

2.94 164.8 0.98 5.856

6.52 136.6 0.97 5.871

14.02 121.0 0.96 6.092

4.02 253.05 0.98 4.869

7.01 238.09 0.99 4.246

10.01 230.75 0.98 7.489

3.17 3.67 0.94 8.862

5.01 4.04 0.94 7.977

12.03 2.31 0.96 6.096

8.62 1.96 0.99 6.789

Pseudo-first-order k1  101 (min1) qe (mg g1) R2

x2 Pseudo-second-order k2  105 (mg L1 min1) qe (mg g1) R2

x2

200 mg L1

Elovich equation

a (mg g1 min1) b  102 (g mg1) x2

2.23 3.36 0.93 10.02

Intraparticle diffusion kid1 (mg g1 min0.5) kid2 (mg g1 min0.5) C R21 R22 DP  108 (cm2 s1) DF  108 (cm2 s1)

7.58 2.57 51.11 0.95 0.93 2.63 2.20

R2

DF ¼

8.02 1.44 72.14 0.95 0.89 3.18 2.66

0:23r o dC s C L t 0:5

8.96 0.42 90.90 0.98 0.98 4.48 3.76

(7)

where ‘r0’ is the average radius of the adsorbent particle (1.17  102 cm), t0.5 (min) is the time required to complete the half of the adsorption, d is the film thickness (103 cm) [46], Cs and CL are the concentrations (mg g1) of adsorbate in solid and liquid phase at t = t and t = 0, respectively. According to Michelsen et al. [81], if the calculated intra-particle diffusion coefficient (DP) value lies in the range 1011–1013 cm2 s1, then the intra-particle diffusion is the rate determining step while, if the calculated film diffusion co-efficient (DF, cm2 s1) value lies in the range 106– 108, then the boundary-layer (film) diffusion is the rate limiting step. In this study, the calculated DF-values were found to be in the order of 108 cm2 s1 (Table 1), which is in between 106 and 108 cm2 s1, suggesting that CR adsorption reaction with GO is a

15.36 6.12 87.30 0.99 0.81 4.17 3.48

qt (mg.g-1)

200

150

100

50

0

0

5

10

15

20

time 0.5 (min 0.5) Fig. 10. The Weber–Morris plot of the kinetic data for CR adsorption by GO from aqueous solution at different temperatures and concentrations.

18.81 2.35 156.21 0.99 0.95 6.28 5.34

film (boundary-layer) diffusion phenomenon. This further confirms the observations found from the Weber Morris plot. Adsorption isotherm for removal of CR by GO Fig. 11 shows the isotherm equilibrium data points obtained for adsorption of CR by GO at temperatures 25, 35, and 45 8C with maximum fluctuation of 1 8C. To evaluate the nature of the adsorption reaction, the data was analyzed with the non-linear fit methods on the Graph Pad Prism 5 software using Eqs. (11)–(13) [49] shown below: Langmuir model : Freundlich model :

qe ¼

qm K a C e 1 þ K aCe ð1=nÞ

qe ¼ K F Ce

RedlichPeterson model :

25 oC 100mg/L 35 oC 100mg/L 45 oC 100mg/L 25 oC 200mg/L 35 oC 200mg/L 45 oC 200mg/L

17.69 3.64 131.90 0.99 0.95 5.14 4.36

6.74 1.91 0.98 12.64

qe ¼

AC e 1 þ BCeg

(11) (12) (13)

where qe has usual significance and given elsewhere, qm the monolayer adsorption capacity (mg g1), Ce is the equilibrium solute concentration (mg L1), Ka, KF, A and B are the Langmuir, Freundlich and Redlich–Peterson (R–P) isotherm constants, respectively. The non-linear fits of the adsorption data for CR on GO are also shown also in Fig. 11. The related isotherm parameters with regression coefficients (R2) and statistical error chi-square (x2) obtained from the non-linear plots is summarized in Table 2. Based on the regression coefficient (R2) and the statistical error chisquare (x2) values, it can be concluded that the experimental data is best described by the three-parameter R–P isotherm (Eq. (13)) (0.97  R2  0.99 and x2 = 29.96–42.87) and, the goodness of data fits was fairly well with the two parametric models viz. the Langmuir (Eq. (11)) (R2 = 0.96–0.99 and x2 = 27.93–37.25) for all the studied range of temperatures. The goodness of data fit with the R–P model (Eq. (13)) was more close to the Langmuir model (Eq. (11)) than that of the Freundlich model (Eq. (12)) with the ‘g’ value closer to 1.0, which in turn reduces the R–P equation to Langmuir equation. The ‘g’-values of the R–P model (Eq. (13)) was

S. Debnath et al. / Journal of Environmental Chemical Engineering 2 (2014) 260–272

qe (mg.g-1)

600

Table 2 Parameters for adsorption isotherm analysis for the adsorption of Congo red on GO at initial pH 7.0 (0.1). Model

400

25 oC 35 oC 45 oC Langmuir model fit Freundlich model fit Redlich Peterson model fit

200

0

267

0

200

400

Langmuir Ka (L mg1) qm (mg g1) R2

x2 Freundlich Kf (mg g1) n R2

600

Ce (mg.L-1)

x2

Fig. 11. The equilibrium data (points) on CR adsorption from aqueous solution by GO at 25, 35, and 45 8C with non-linear fits of isotherm model equations.

increased from 0.90 to 1.08 with increasing the reaction temperature from 15 to 45 8C, respectively. The ‘g’-values (Table 3) of the R–P model isotherm suggested Langmuir type monolayer adsorption increases with increasing temperature on the CR adsorption reaction onto GO surface. Thus, this indicates the surface transformation from slight heterogeneous to more homogeneous sites with increasing temperature on the reaction and that becomes increasingly monolayer type with increasing temperature. Evaluation of adsorption energy

Redlich–Peterson A (L g1) B (L mg1) g R2

x2

lnqe ¼ lnq0m  K DR e2

(15)

e ¼ RTlnð1 þ 1=C e Þ

(16)

where e is the Polanyi potential (Eq. (16)), q0m is the D–R adsorption capacity (mol kg1), and KDR is a constant related to adsorption energy (mol2 kJ2). The q0m and KDR parameters were evaluated from the intercepts and slopes of the plots of ln(qe) versus e2 (Fig. 12). The mean free energy of adsorption (EDR) is the free energy change when one mole of ion is transferred to the surface of the adsorbent from infinity in the solution [51,52] and, it was calculated using the following equation (Eq. (17)), (17)

The calculated D–R equation parameters and mean free energies evaluated are given in Table 3. The magnitude of EDR is useful for determining the type of reaction where by values of EDR between 8.0 and 16.0 kJ mol1 describes the adsorption reaction that occur by electrostatic [51,52] mechanism. The EDR values obtained in this study suggested that CR adsorption on GO followed electrostatic mechanism with some other inherent mechanisms lying within.

25 8C

35 8C

45 8C

0.0829 546.1 0.98 28.08

0.0994 557.5 0.99 27.93

0.1075 571.5 0.97 37.25

137.8 4.28 0.83 92.06

149.5 4.39 0.86 86.27

170.7 4.75 0.90 77.11

37.90 0.0702 1.08 0.98 30.78

68.78 0.1211 0.9005 0.99 29.96

124.4 0.2030 0.9886 0.98 42.87

Table 3 Dubinin–Radushkevich model parameters for evaluation of adsorption energy of CR adsorption by GO. Temperature

25 8C

35 8C

45 8C

KDR (mol2 kJ2) EDR (kJ mol1) q0m (mol kg1) R2

3.731  109 11.57 3.86 0.9937 0.0886

3.077  109 12.74 3.01 0.9848 0.1375

2.111  109 15.39 1.79 0.9741 0.1797

x2

The equilibrium data shown (as points) in Fig. 12 were analyzed by Dubinin–Raduskevich (D–R) equation [51,52] (Eq. (15)) in order to determine the adsorption energy of the adsorption process.

EDR ¼ ð2K DR Þ1=2

Temperature

(Eq. (20)) [67,68]:

DG ¼ RTlnK c

(20)

From thermodynamics, the Gibbs free energy change is also related to enthalpy change and entropy change at constant temperature by the following equation (Eq. (21)):

DG ¼ DH  T DS

(21)

The values of DH8 and DS8 were calculated from the slope and intercept of the linear plots (R2 = 0.97) of DG8 versus T (Fig. 13). The slope and the intercept of the plot give the DS8 and the DH8 values, respectively. The values obtained are given in Table 4. Negative values of DG8 increases with increasing temperature indicate the feasibility and spontaneity of the CR adsorption process on GO. The positive enthalpy change (DH8) values for the CR adsorption reaction (Table 4) indicate the endothermic nature of the present reaction. Low positive enthalpy change (DH8 < 40 kJ mol1) also indicated the physical sorption of CR onto GO surface. The positive entropy change (DS8) for this reaction (Table 4) has also indicated the increase in number of species at the solid–liquid interface and, hence the randomness in the interface which is presumably due to the release of aqua molecules when the aquated CR is adsorbed on the surface of the adsorbent. Confirmation of the favorability of the adsorption process

Thermodynamic parameters for removal of CR by GO The thermodynamic parameters such as change in Gibbs free energy (DG8), enthalpy (DH8), and entropy (DS8) were estimated to evaluate the feasibility and nature of the adsorption reaction. The Gibbs free energy change of the process is related to the equilibrium constant ðK c ¼ ð1000qe =C e ÞÞ by the following equation

To confirm the favorability of the CR adsorption by GO, the separation factor (RL) was calculated using the following equation (22) [69,70]: RL ¼

1 1 þ KaCo

(22)

S. Debnath et al. / Journal of Environmental Chemical Engineering 2 (2014) 260–272

268

25

7

ln(qe)

-ΔGo (kJ.mol-1)

24

6

o

25 C 35 oC 45 oC

5

4

3 5.0×10 08

23 22 21

1.0×10

09

1.5×10

20 295

09

300

305

310

315

320

T (K)

ε2 Fig. 12. D-R isotherm plot for CR adsorption by GO from aqueous solution at 25, 35 and 45 8C.

Fig. 13. The plots of DG8 (kJ mol1) versus T (K) for estimation of thermodynamic parameters of CR adsorption by GO.

Table 4 Thermodynamic parameters for adsorption of CR by GO.

0.20 o

298 K

308 K

318 K

DG8 (kJ mol1) DH8 (kJ mol1) DS8 (kJ mol1)

21.63 8.19 0.10

22.57

23.63

where ‘RL’ is a dimensionless separation factor indicating the shape of the isotherm, Ka is the Langmuir constant and Co is the initial adsorbate concentration. The isotherm is (i) unfavorable when RL > 1, (ii) linear when RL = 1, (iii) favorable when RL < 1, and (iv) irreversible when RL = 0. The plotted RL-values range 0–0.2 for all the concentration and temperature ranges studied (Fig. 14), which indicate that CR adsorption is favorable on GO surface.

25 C o 35 C o 45 C

0.15

RL

Temperature

0.10

0.05

0.00

0

200

400

6 600

800

1000

-1

Initial conc. (mg.L )

Comparison of some reported adsorbents for adsorption of CR The maximum adsorption capacity of GO for CR, based on the Langmuir adsorption capacity value (qm) was compared with those of other adsorbents reported in literature. It was found that the adsorption capacity of GO is lower than hydroxyapatite/chitosan composite reported by Hou et al. [71], but higher than many other adsorbents (Table 5). In general, the adsorption capacity of GO is comparable with the nano-dimensional adsorbents, but are much higher than adsorption capacities of low-cost adsorbents. This indicates that GO as a low cost material could be employed as potential adsorbents for the removal of toxic dyes like CR from aqueous solution.

Fig. 14. Variation of separation factor (RL) with change in initial CR concentration.

Designing single stage batch adsorber from equilibrium data Adsorption isotherm data are useful in predicting the design of single stage batch adsorption systems [61,78–80]. Designing of batch adsorber model is essential to extrapolate the findings of the lab-bench scale study to a large scale one which in turn could be utilized in designing an industrial wastewater treatment system. A schematic diagram of the batch adsorber model is given in Fig. 15.

Table 5 Comparison of adsorption capacity with other reported adsorbents. Adsorbent

pH

Langmuir adsorption capacity (mg g1)

Ref.

Eucalyptus wood saw dust CaCl2 modified bentonite Hydroxyapatite/chitosan composite Cattail root Hollow Zn-Fe2O4 nanospheres Iron oxide alumina nanocomposites Jute stick powder Maghemite nano-particles Anlinipropylsilica xerogel GO

7.0 5–9 No optimal pH 7.0 (0.1) 6.0 7.0 6.0 5.9 5.0 8.0

31.25 227.27 769 34.59 16.58 498 35.7 208.33 22.62 571.5

[59] [65] [71] [72] [73] [74] [75] [76] [77] Present study

S. Debnath et al. / Journal of Environmental Chemical Engineering 2 (2014) 260–272

269

80

95% 90% 80% 70%

M (kg)

60

40

20

0 Fig. 15. Batch adsorber process diagram for removal of CR by GO.

for reducing the final concentration to 70–95% for different volume of the dye solution is shown in Fig. 16. The design procedure for a single stage batch adsorber is outlined. The mass of the adsorbent required for removal of the dye from given volume and concentration of the solution could be calculated from the plot. For instance, if 20 m3 dye solution has to be treated, the required mass of GO to decrease the initial dye concentration from 100 mg L1 to final concentration of 5, 10, 20 and 30 mg L1 are 14.88, 14.09, 12.53 and 10.96 kg, respectively. The initial concentration of the influent dye was taken as 100 mg L1 at 35 8C. The amount of the GO required for reducing the final concentration to 5.0 mg L1 for different volume of the dye solution is shown in Fig. 17. For instance, to purify 10 m3 of initial concentration of 25, 50, 100 and 200 mg L1 of the dye, required amount of GO are 17.33, 35.25, 70.49 and 140.96 kg, respectively.

At initial stage of the reaction, qo = 0 and at equilibrium, Ct = Ce and qt = qe. (Ce and qe are the concentration of the dye in solution and on adsorbent in equilibrium, respectively.) Eq. (23) reduces to, V(Co  Ce) = Mqe (24)

Adsorption of CR on GO is Langmuir monolayer type. Consequently, the value of the equilibrium adsorption capacity value (qe) from Langmuir equation may be substituted in Eq. (24) to find out the process parameters of adsorption of CR by GO. Fig. 16 shows the series of plots for adsorption of CR onto GO derived from Eq. (2). The initial concentration of the influent dye was taken as 100 mg L1 at 35 8C. The amount of the GO required

Proposed mechanism for removal of CR by GO It is well know that the obtained GO contains several oxofunctional groups such as carboxylic group (–COOH), epoxy group

COOH O

O

O OH COOH

NH2

+

O O

OH

100

Fig. 16. Adsorbent mass against volume of effluent treated for various percentage of dye removal.

(23)

M Co  Ce ¼ V qe

50

V (m3)

V (m3) is the volume of the solution in a batch adsorber system with an initial dye (CR) concentration of Co (g m3) and the concentration of dye in the system at any time ‘t’ is Ct (g m3). If, the mass of the adsorbent (GO) in the adsorber is M (kg) and the solute loading of the dye at any time ‘t’ on the adsorbent changes to qt (g kg1) from the initial solute loading of qo (g kg1), then according to the mass balance (Eq. (23)), VðC o  C t Þ ¼ Mðqt  qo Þ

0

NH2 N N

N N

SO3Na

SO3Na

COOH COOH

COOH

NH2 O N N

SO3Na

O

N N

SO3Na

O

O OH

COOH

COOH

NH2

O COOH

OH

COOH COOH

Scheme 1. p–p interaction between GO basal plane and aromatic ring of CR molecule.

270

S. Debnath et al. / Journal of Environmental Chemical Engineering 2 (2014) 260–272

500

Conclusion

400

10m3 20m3 50m3 75m3 100m3 150m3 200m3 250m3

M (kg)

300

200

100

0

0

50

100

150

200

250

300

Co (g.m-3) Fig. 17. Adsorbent mass against initial concentration of CR removal at various volume of treated effluent (V).

The present study investigates the removal efficiency of Congo red using graphene oxide (GO) as adsorbent from aqueous solution. The removal efficiency of CR is dependent of temperature positively. The optimum pH found for the adsorption process is 7.0. The time required to reach the equilibrium is about 300 min, whereas, 90% adsorption is completed within 200 min. Adsorption kinetics analysis reveals that the adsorption is governed by pseudo-first-order model (R2  0.99). Intraparticle diffusion analysis indicated that the process may be controlled by more than one mode of diffusion controlled mechanism; with pore diffusion is the prevalent one. The adsorption process followed the Langmuir isotherm as well as Redlich Peterson isotherm with ‘g’ value close to unity. The adsorption process is a spontaneous, endothermic and entropically favorable. The activation energy for the adsorption of CR by GO is substantially low, indicating favorability of the adsorption, which is confirmed from the calculated RL values. Finally, the adsorbents were compared with other reported adsorbents and a single stage batch adsorber has been designed and found that GO can be used effectively in removing Congo red from aqueous solutions as well as waste water.

References and hydroxyl group (–OH) as confirmed by ATR-FTIR and XPS studies during the preparation of GO using modified Hummers method. It is also noted that at pH < 7.8, GO exists anionic form in aqueous solution. This indicates that most of the carboxylic groups (–COOH) are present in carboxylate form (–COO) in aqueous medium. CR is anionic dye, which will exist as negatively charged ions (–O3S–CR–NH2) in solution. Therefore, the adsorption of CR onto the GO can be attributed to the following plausible mechanisms: (1) hydrogen bonding interaction between the amino group of CR and the negatively charged oxygen atom of GO (Eq. (1)), (2) hydrogen bonding interaction between –OH group of GO and negatively charged oxygen atom of CR (Eq. (26)), (3) nucleophilic reaction on the epoxy group by attacking negatively charged oxygen atom of GO (Eq. (27)), (4) p–p interaction between the aromatic ring of the dye molecules and the GO basal planes (Scheme 1). GOCOO þ NH2 DSO3  ! GOCOO    NH2 DSO3  ðhydrogenbondingÞ

(25)

NH2 DSO3  þ HOGO ! NH2 DSO3     HOGO ðhydrogenbondingÞ

(26)

H H O

O H+

HN

D

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