Mechanistic aspects of enhanced congo red adsorption over graphene oxide in presence of methylene blue

Mechanistic aspects of enhanced congo red adsorption over graphene oxide in presence of methylene blue

Journal of Environmental Chemical Engineering 4 (2016) 3498–3511 Contents lists available at ScienceDirect Journal of Environmental Chemical Enginee...

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Journal of Environmental Chemical Engineering 4 (2016) 3498–3511

Contents lists available at ScienceDirect

Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece

Mechanistic aspects of enhanced congo red adsorption over graphene oxide in presence of methylene blue Deepak Kumar Padhia,b , K.M. Paridaa,c,* , S.K. Singha,b a Academy of Scientific and Innovative Research (AcSIR), Council of Scientific and Industrial Research, Anusandhan Bhawan, 2 Rafi Marg, New Delhi 110 001, India b Advanced Materials Technology Department, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, 751013 Odisha, India c Centre for Nano-Science and Nano-Technology, ITER, SOA University, Bhubaneswar, 751030 Odisha, India

A R T I C L E I N F O

Article history: Received 5 April 2016 Received in revised form 16 June 2016 Accepted 13 July 2016 Available online 15 July 2016 Keywords: Graphene oxide Electrostatic interaction pH Dye adsorption mechanism

A B S T R A C T

To elucidate the role of cationic dye for the enhanced adsorption of anionic dye over Graphene oxide(GO), we have made a novel approach to evaluate the adsorption capacity of GO for removal of Congo red (CR) from aqueous system in presence of Methylene blue (MB). Physicochemical and spectroscopic techniques have been used to assertion the interaction of dye molecules with the surface of GO. At pH = 2, GO possessed 96% adsorption capacity towards CR in presence of MB. The existing pH dependant electrostatic interaction mechanism between CR and MB dye has been explained for enhanced CR adsorption over GO surface. Fourier transform infrared spectroscopy and Raman spectrum of dye adsorbed GO also gives well support to evaluate the extent of electrostatic interaction between oxygen containing functional groups and functional moieties of dye resulting in good adsorption over GO surface. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction Graphene oxide (GO), a 2D-allotrope of carbon has stimulated significant interest in recent years due to its extraordinary physicochemical and mechanical properties [1–4]. In fact, the structure of graphite and graphene is almost clear, but the definite structure of GO hasn’t concluded yet as its structure depends on the synthesis process and the raw materials used. Generally, it is accepted that most of oxygen atoms on GO are present in the form of hydroxyl and epoxide functional groups on its basal planes and a few in the form of carbonyl and carboxyl groups at the sheet edges [5]. The presence of these oxygen bearing functional groups on the GO surface render it as hydrophilic and high negative charge density in aqueous medium. Through dispersion or stirring, GO exists as single layer and forms stable colloid in solution phase for which it can act as weak acid cation resin and adsorbs positively charged species like organic molecules [6], metal ions [7], polymer [8] and bio-molecules [9].

* Corresponding author at: Academy of Scientific and Innovative Research (AcSIR), Council of Scientific and Industrial Research, Anusandhan Bhawan, 2 Rafi Marg, New Delhi 110 001, India. E-mail addresses: [email protected], [email protected] (K.M. Parida). http://dx.doi.org/10.1016/j.jece.2016.07.011 2213-3437/ã 2016 Elsevier Ltd. All rights reserved.

As civilisation is developing, the use of several organic dyes is also increasing in various industries like cosmetics, food, textiles, paper etc. The wastes from these industries are toxic and carcinogenic in nature for which they badly influence the quality of water resulting in various undesirable consequences [10]. Hence, design of efficient, novel and cost effective materials is very important for dye removal by the process of adsorption, ion exchange, coagulation, precipitation, oxidation, filtration etc. [11– 13]. Among all the processes, adsorption is versatile and simple, easy and cost effective. In this regard many groups have reported the adsorption capacity of GO for the removal of various organic dyes. Wang et al. has reported GO possesses 351 and 248 mg g1 for Methylene blue (MB) and Malachite green (MG) respectively based on Langmuir isotherm which is much higher than activated carbon [14]. Sampath et al. investigated both cationic and anionic dye adsorption over GO and reduced graphene oxide (RGO) and explained all cationic dyes and anionic dyes can be efficiently adsorbed over GO and RGO, respectively [15]. Barkauskas et al. investigated the electrostatic interaction of NH2 and SO3 groups of Congo red (CR) dye molecule with the oxygen containing functional of GO in aqueous media [16]. Wang et al. reported the influence of temperature, pH, ionic strength, and dissolved organic matter (DOM) content on the adsorption capacity of GO for MB [17]. Das et al. have studied factors affecting the removal of MG using GO nano sheets [18]. As mixture of dyes is present in dye effluents, it is important to develop versatile adsorbents for

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removal of both cationic and anionic dyes. To the best of our knowledge no group have investigated the adsorption capacity of GO towards anionic dye in presence of cationic dye. Here in, we have studied the effect of MB presence on CR adsorption over GO. The role of MB for the enhancement of CR adsorption over GO surface is well explained on the basis of possible electrostatic interaction mechanism between the existing chemical structure of dye and GO, ultraviolet–visible spectroscopy analysis of dye solution, Fourier transform infrared spectroscopy (FTIR) and Raman analysis of dye adsorbed GO. 2. Experimental 2.1. Materials Natural graphite powder and potassium permanganate (KMnO4) were obtained from Sigma Aldrich chemicals; MB and CR were procured from Alfa Aesar. Sulphuric acid (H2SO4), Hydrochloric Acid (HCl) and Sodium nitrate (NaNO3) was procured from Finar Chemicals Limited, Ahmedabad. The chemicals were used as it is without any further purification.

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water to adjust the pH 6. Finally, the obtained product was dried at 80  C for 24 h. 2.2.1. Preparation of mixed dye solution The mixed dye solution of CR with MB i.e. CR + MB was prepared by mixing 100 ppm solution of CR and MB. 2.3. Adsorption experiment The adsorption experiment for all the dyes was carried out in 100 ml stopper conical flasks with constant stirring for 1 h in dark by taking 0.02 g of GO in 20 mL of 100 ppm dye solution. After completion of the adsorption, the resulting colloidal solution was centrifuged to separate the GO particles. Then the concentration of the dye molecule was measured by using ultraviolet–visible spectrophotometer (Varian Cary 100 spectrophotometer) at appropriate wave length. The absorbance was measured at lmax of 497 664, for CR and MB respectively. For kinetics measurement, samples were withdrawn at regular time intervals and analysed for dye concentration. The dye loaded adsorbent was further characterised by FTIR and Raman analysis.

2.2. Synthesis of GO 2.4. Analytical characterisation of GO GO was prepared by modified Hummers-offman method [19]. About, 0.5 g of graphite powder with 0.5 g of NaNO3 was suspended in 23 ml of concentrated H2SO4, taken in a round bottom flask. It was stirred for 15 min by a magnetic stirrer and then the container with the suspended particles was kept in an ice bath. About 4 g of KMnO4 was added slowly to obtain a purple-green colour solution. Then it was transferred to a water bath and the temperature was maintained around 40  C. It was stirred for 90 min, then 50 mL of deionised water was added and again stirred for 20 min. Then 20 mL of 30% H2O2 was added slowly to produce a golden- brown sol. 50 mL of deionised water was added to it and the resultant solution was centrifuged and washed several times with deionised

The prepared GO was characterized by Powder X-ray diffraction pattern (PXRD), FTIR spectroscopy, and Raman spectroscopy. The X’pert PRO PANalytical diffractometer (Netherlands) was used to record the PXRD patterns with automatic control. The analysis patterns were done with monochromatic Cu-Ka radiation from 2u = 10–70 with a scan rate of 2 min1. Raman spectra were recorded using Renishaw inVia Reflex (Serial No.- H33197). Here, Ar-ion was used as leaser source with wave length of 514 nm. FTIR spectra in absorbance mode were taken on a FTS 800 (Varian, Scimitar series (Australia)) in the range 4000–400 cm1 on a KBr wafer.

Fig. 1. XRD profile of GO.

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3. Results and discussion 3.1. Physicochemical characteristics Fig. 1 shows the XRD pattern of as synthesized GO. The XRD pattern contains three peaks; a very strong peak 2u = 10.080 and two weak peaks at 2u = 19.900 and 42.300. The major peak at 10.080 corresponds to (002) inter layer spacing of 8.670A and indicates the incorporation of oxygen containing functional groups in between the layered GO structure. These three peaks are well matched with the XRD patterns of GO reported by others [14,20,21]. Further, the layered of structure of as synthesised GO was confirmed from TEM analysis and is shown in Fig. 2. Fig. 3 represents the Raman spectra of GO. The peak observed at 1351 cm1, 1590 cm1 corresponds to D and G band, respectively [5]. Two weak peaks at 2692 cm1 and 2928 cm1 can be assigned to 2D peaks [22]. The appearance of D band is caused by the presence of sp3 defects in GO and corresponds to the edge effect, disordering in atomic arrangement and charge puddles of GO for which it is slightly broader than the G band. The appearance of G band is associated with the plane vibration of sp2-carbon atoms of GO and caused due to doubly degenerated E2g phonon mode, while the 2D bands originates from second order Raman scattering process [23,24]. This evidence indicates that the oxygen containing functional groups are inserted into the skeleton of carbon network of graphite in the oxidation process and the original extended conjugated p-orbital of the carbon network was destroyed. 3.2. Adsorption of CR and MB over GO 3.2.1. Effect of contact time At room temperature, the time dependant behaviour of cationic (MB) and anionic (CR) dye adsorption over GO was performed by varying the contact time in the range of 10–60 min. The results are shown in Fig. 4. Due to large surface area of GO, its removal efficiency for cationic and anionic dyes increased with increasing contact time. The percentage adsorption for CR and MB dye was found to be 78% and 99%, respectively at 60 min.

Fig. 3. Raman spectra of GO.

3.2.2. Effect of adsorbent dose The adsorption percentage of cationic and anionic dyes increased as the amount of GO increases in the range 0.5–1 g/L. The use of GO as adsorbent showed that the adsorption is very fast and the equilibrium was attained in 1 h. No further change in equilibrium concentration was seen up to 2 h. Fig. 5 represents the percentage of adsorption of MB and CR dye over GO with varying its amount from 0.5–1.5 g/L. At adsorbent concentration 1 g/L, the adsorption percentage for MB increased up to 99%, while CR showed maximum 78%. Hence, 1 g/L of GO was chosen for subsequent adsorption experiments. 3.2.3. Adsorption kinetics In order to design the adsorption, the rate of adsorption of different dyes over GO surface is very important. The data obtained from experiment were applied to Pseudo-first-order [25] and

Fig. 2. TEM image of GO.

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Fig. 4. Removal efficiency of CR and MB over GO nanosheets in different time.

Fig. 5. Effect of adsorbent dose (GO) over CR and MB adsorption.

Pseudo-second order [26] kinetic model, which are well known to evaluate the adsorption kinetics. Ln (qe  qt) = Ln qe  k1t

(1)

t/qt = 1/k2qe2 + t/qe

(2)

The adsorption kinetics was modelled using the pseudo-first order and pseudo second order model, which is expressed in Eqs. (1) and (2), respectively. Where qe and qt are the amount of dyes adsorbed (mg/g) at equilibrium and time t (min) respectively,

k1 (min1) and k2 (mg g1 min1) are the rate constant of the pseudo-first-order and pseudo-second-order adsorption respectively. The adsorption kinetic plots are shown in Fig. 6. The kinetic parameters for two kinetic models and coefficients of MB and CR dyes were determined under similar conditions. It was found that (i) pseudo-second order reaction plots for MB and CR gives r2 values of 0.998 and 0.999, respectively (Fig. 6), which are closer to the unity for pseudo-second order model. From the above observation, both MB and CR dye adsorption over GO follow pseudo-second order kinetics.

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Fig. 6. Pseudo-second order model for MB and CR.

3.2.4. Adsorption isotherms Adsorption isotherm studies was carried out to get the maximum mixed dye adsorption capacity of the GO adsorbent by following Langmuir model (Eq. (3)) and Freundlich model (Eq. (4)) [17] 1/qe = 1/qm + 1/bqmce

ln qe = ln kf + 1/n (lnce)

(3)

(4)

By following this equation the maximum absorption capacity (qm) and Freundlich constant (Kf) were obtained. Fig. 7(a) & (b) shows the isotherm plots for mixed dye adsorption on GO. The absorption data of mixed dye by GO fit the Freundlich model nicely (Fig. 7(b)). In this case the correlation coefficient (R) is 0.992 and Kf value of Freundlich was found to be 467.3 mg/g (L/mg)1/n. Here, the n value is 14.1 which is giving the indication of the favourite state of the absorption process [17]. Because, it is well known that when n is larger than 2, the adsorbent is considered as a good one. Therefore, GO is an excellent adsorbent of mixed dyes.

3.2.5. Thermodynamic parameters Thermodynamic parameters studies were incorporated for better understanding the nature of the adsorption process. Gibbs energy change (DG ), enthalpy change (DH ), and entropy change (DS ) of the adsorption of the mixed dyes onto the GO were determined by using the following equations [18] Kd = qe/ce

(5)

DGo = RT ln Kd

(6)

ln Kd = DS/R  DH/RT

(7)

where, Kd is the distribution coefficient, T is the temperature, and R is the gas constant, respectively. DH and DS are calculated from the slope and intercept of van’t Hoff plots of ln Kd vs T (Fig. 8) and the calculated thermodynamic parameters are presented in Table 1. Futher, the adsorption process is endothermic which was concluded form the calculated DH value i.e 16.342 KJ mol1.The

Fig. 7. Langmuir (a) & Freundlich (b) plot for the adsorption of CR + MB onto GO.

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Fig. 8. Vant Hoff plot for adsorption of CR + MB onto GO.

Table 1 Thermodynamic Parameters for Adsorption of MB + CR onto GO. Temperature ( C)

DG (KJ mol1)

20 25 30 35

2.786 3.242 3.431 3.923

DH (KJ mol1)

DS (KJ mol1)

16.342

0.067

3.3. Mechanism of adsorption

activation energy, Ea, is calculated by using the Arrhenius equation [18] ln k = ln A  Ea/RT

where, R is the universal gas constant, T is the temperature in Kelvin and A is the Arrhenius pre-exponential factor. In this case, the activation energy for the adsorption of mixed dyes was found to be 32.73 kJ mol1 (calculated from the slope ln k vs 1/T). As the activation energy (Ea) for this adsorption process is lower (<42 KJ mol1) value, indicates physical adsorption [18] (Fig. 9).

(8)

3.3.1. CR dye adsorption over GO CR is a secondary di-azo dye. Its structure is highly sensitive towards the pH of the solution. According to the pH variation of the solution, its chemical structure changes which lead to develop different colour. Barkauskas et al. [16] have proposed the existing

Fig. 9. Plot of lnk vs 1/T for determination of activation energy for adsorption of CR + MB onto GO.

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CR structure in aqueous media at several pH which is represented in Fig. 10. Structure I and structure II in Fig. 10 shows the chemical structure CR molecule at pH > 5 and pH < 3 respectively while structure III, IV, V and VI represents the ammonium–azonium tautomerism of CR dye in acidic solution. Generally, CR monomer (pH  8) shows maximum absorbance band at 488–500 nm and two absorption band at UV region [27]. In our case, the maximum absorbance for CR (pH = 2) was found at 497 nm which can be assigned to the existence of anionic monomer (Fig. 10). In addition to that two absorption bands at 288 and 264 nm in the UV region can be ascribed to the benzoic and naphthalene rings, respectively. The presence of two functional groups i.e. two sulfonic acid groups (Fig. 10(I)) and two primary amine groups make it dipolar in aqueous solution. But in acidic condition, hydrophobic property of CR minimises due to partially self association and formation of ammonium–azonium tautomerism of CR monomer. This leads to change of colour from red to blue and shifting of main absorption band towards higher wavelength [16]. In this case (pH = 2), the main absorption band is red shifted to 608 nm confirms its tautomerism state which is more favourable to interact with the MB+ monomer in mix condition. Hence, regarding to the existing chemical structure CR monomer, the adsorption mechanism over GO at basic and neutral pH can be explained as follows. At basic, the mechanism of interaction between CR molecule and GO sheets is proposed as follows,

Eq. (9) shows the ionisation of CR in water at pH = 8. Eq. (10) represents the possible van der walls attraction between negatively charged CR molecule (CR-SO3) and layered GO sheets. In the adsorption, a electrostatic repulsion exits between CR-SO3 molecules and acidic functional groups of GO. Probably, only van der Waals interaction plays a major role in the interaction between CR and GO molecule. Ramesh et al. has reported the same observation for exfoliated GO/Orange G system [15]. Therefore, the resultant percentage of adsorption is low compared to CR dye adsorption at acidic condition (pH = 2). The percentage of 100 ppm CR dye adsorption over GO (1 g/L) is 56% at pH = 8 (Fig. 11). The percentage of CR dye adsorption over GO is gradually increases with lowering the pH and shows highest at pH = 2 i.e. 78%. The possible interaction mechanism between CR ion and GO nano sheets in acidic condition can be explained as follows, From the existing dipolar ion structure of CR monomer (Fig. 10) in the acidic condition, it can be speculated that the protonated amino or azo nitrogens having positive charge can be adsorbed over highly negatively charged GO surface and is represented below in Eqs. (11) and (12), respectively. COONH3 GO COO + NH3+ (CR) ! GO

(11)

CR-SO3Na ! CR-SO3 + Na+

N+ = N (CR) ! GO COO N¼N GO COO + 

(12)

(9)

GO ! Van der walls attraction

Fig. 10. Chemical structure of CR in aqueous media at PH > 5 and PH < 3.

CR-SO3

(10)

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Fig. 11. Adsorption of CR dye over GO at pH = 2, 4, 6 and 8.

But the electrostatic force of attraction between protonated amino or azo nitrogens and  COO of GO is obvious but not absolute due to the simultaneous electrostatic repulsion of  SO3 groups. Thus, the percentage of CR adsorption minimises over GO and lies at 78%. 3.3.2. MB dye adsorption over GO We found GO shows high efficiency for adsorption of MB compared to CR (Fig. 3). This is due to its surface chemistry, sp2/sp3 coexisting structure, water solubility and fluorescence quenching property. XRD pattern shows the layered structure of GO and FT-IR also exhibits the presence of  COOH,  OH, and >O functional groups. Among them, OH and >O functional groups are present in the basal plane and  COOH groups at the edge of GO. These oxygen containing functional groups make GO hydrophilic in nature for which water molecule can easily intercalate in between the GO layers. In aqueous medium, GO forms a stable colloidal suspension and acts as weak cation exchange resin giving rise to the formation of highly negatively charged surface. MB is a cationic dye and exists as positively charged ions in aqueous medium (Fig. 12). So, MB+ ions can easily accumulate on to the negative surface of GO by strong electrostatic force of attraction. The interaction mechanism is proposed as follows, COOCS GO COO + CS+ (MB) ! GO

Fig. 12. Chemical structure of Methylene blue in aqueous medium.

(13)

Eq. (13) represents the electrostatic force of attraction between  COO groups of layered GO sheets and the existing cationic forms of MB in aqueous solution. 3.3.3. CR dye adsorption over GO in presence of MB As discussed earlier, due to highly negative charge surface of GO, the resultant adsorption of MB dye is dominant over CR dye. When we introduce MB to CR dye solution at pH = 2, the CR and MB dyes lost their original colour and gave rise to a new colour due to charge separation between them. After mixing, there is a great enhancement on the percentage of CR dye adsorption compared to their neat adsorption under similar conditions. But there is no effective change in the percentage of adsorption of MB. This is because of the possible selective electrostatic attraction of cationic sites of mixed dye molecule with the highly negative charged surface of GO. In mixed condition, at pH = 2, the adsorption of CR over GO increased from 78% up to 96% in presence of MB (Fig. 13). The enhancement of percentage of adsorption of anionic dyes in presence of cationic dye depends how firmly the anionic dyes can bind with the cationic dye molecules in the adsorption. In mixed condition, the charge separation between CR and MB molecule will certainly depend on different parameters like their concentration, pH and temperature. Then, it is possible to explain the existence of much suitable chemical structure of MB+ and CR ion which favour a good attraction between them compared to other mixed dyes. The UV–vis absorption spectra of CR and MB are also well agreed with our speculation (Fig. 14). It can be seen that the maximum absorbance of CR and MB was found at lmax = 497 and 664 nm, respectively. But in the UV–vis absorption spectra CR + MB solution, the red shifting of lmax value of CR from 497 to 608 nm is due to its highly acidic condition. The broadening of main absorption band of MB at 664 nm clearly suggests the charge transfer between CR and MB+. A gradual decrease in absorbance intensity for MB + CR over GO is clearly observed from Fig. 15.

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Fig. 13. The adsorption percentage of CR, MB and MB+ CR over GO.

Fig. 14. UV–visible absorbance spectra of MB, CR and MB + CR.

In addition to that, the colour of the mixed dye solution becomes almost clear in 60 min which can be seen in naked eyes (Fig. 11(b)). This observation indicates the complete adsorption of both anionic (CR) and cationic (MB) dyes over GO surface in 60 min during the adsorption. Moreover, Rao et al. has observed that the interaction of molecules containing the electron withdrawing and electron donating functional groups of dye molecules with graphene causes the charge separation between them and leads to change in the electronic structure of grapheme [25]. Hence, the possible interaction between dye molecule and oxygen containing

functional groups can be explained on the basis of FT-IR and Raman analyses of dye adsorbed GO. 3.3.3.1. FTIR analysis of CR +MB adsorbed GO. FTIR analysis has been carried out to evaluate the possible electrostatic interaction between oxygen containing functional group with corresponding functional groups of both CR and MB dyes at individual and in also mixed condition. The FTIR spectra of synthesised GO and CR + MB adsorbed GO is presented in Fig. 16.

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Fig. 15. The full spectra of UV–vis absorbance vs. time for MB + CR.

Fig. 16. FT-IR spectra of GO and CR + MB adsorbed GO.

In the FTIR spectra of GO, the peak at 1730 and 1624 cm1 correspond to the C¼O vibration and aromatic C¼C respectively. The peak at 1397, 1220, 1048 cm1 arise due to the C O stretching of carboxy, phenolic and epoxy grops, respectively [29]. In this addition, the strong broad peak observed at 3298 cm1 for the stretching vibration of hydroxyl group [30]. But in the FTIR spectra of GO  CR + MB (Fig. 16), there is significantly decrease in the intensity of peak at 1730 and 1220 cm1 which clearly suggests

interaction of  COOH and phenolic OH group of GO with the functional moieties of dye molecule. Das et al. and Haubner et al. have reported the decrease in the stretching band intensity of C¼O due to adsorption of MB and MG over GO, respectively [18,31]. Hence, we can propose that MB molecule is adsorbed first from the coexisting CR and MB solution. As MB is a cationic dye, it can easily accumulate over negatively charged surface of GO which is due to the presence of COOH group at its edge. The significant decrease

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Fig. 17. (a)–(c) show the structure of CR, MB and Possible interaction between CR and MB respectively.

in the phenolic  OH peak intensity may be due to the bond formation between CS+ ion of MB dye with the  OH group of GO through hydrogen bonding. Das et al. reported similar observation for ¼N+ immonium ion of the MG with  OH group of GO [18]. In addition to that, the decrease of aromatic C¼C peak intensity suggests the aggregation of dye molecules over the sketal carbon network of GO. On the based upon FTIR study of CR + MB dye adsorbed GO, the possible interaction between one of ammonium–azonium tautomerism of CR monomer and MB+ ion and their coexisting interaction with GO in aqueous media is presented in Figs. 17 and 18 respectively. Moreover from Fig. 13, it can be explained that, MB+ dye contains ‘S’ and ‘N’ atom in one ring where ‘S’ atom is electron deficient and ‘N’ atom has a lone pair on it. Probably, the lone pair over ‘N’ attacks the protonated NH2 group of CR molecule in the mixed solution. Then, this attachment facilitates the electrostatic attraction of CS+ of MB with the  COO groups of GO. That’s why the extent of adsorption of CR dye molecule dramatically increased compared to neat adsorption.

also shows a red shift to 1588 cm1. The shifting of “G” band position in raman spectral data confirms that the charge transfer occurs from MB, CR and MB + CR molecules to GO. Sampath et al. and Rao et al. have reported similar observation for exfoliated GO and grapheme, repectively [15,28]. The ID/IG value of GO-MB, GOCR and GO-CR + MB are 0.783, 0.752 and 0.814, respectively and those values are less than that of GO i.e. 0.845. This indicates the extent of interaction of dye molecule over GO surface. In case of GO-MB, MB+ ion can be easily attracted by the negatively charged surface of GO and hence, lower the ID/IG value. Though, NH2+ group of CR is attracted on GO surface, the presence of SO3 on it causes repulsion, hence the ID/IG value decreases even more than that of ID/IG value of GO-MB. In case of GO-MB + CR, the ID/IG value is 0.814, more than GO-MB and GO-CR.This may be due to strong interaction of GO with both MB and CR. From this raman spectral data, it is speculated that in mixed dye (MB + CR), MB+ ions dominant over CR ions and first MB+ ions get attached over the surface of GO followed by CR ions (Fig. 15). So in this case, the repulsion is minimum and causing great enhancement of resultant adsorption of CR dye molecules.

3.3.3.2. Raman analysis of CR+ MB adsorbed GO. GO forms composite with dye molecules i.e. GO-MB, GO-CR and GOMB + CR due to strong electrostatic force of interaction taking place between them during the process of adsorption. Therefore, we found shifting in the “G” band position and decreasing in the value of ID/IG (Table 2). The “G” band position of neat GO is 1590 cm1, while MB and CR adsorbed GO shows a red shift to 1587 cm1 and blue shift to 1593 cm1 respectively (Fig. 19). However, MB + CR adsorbed GO

3.4. Reusability study Reusability study should was out for GO adsorbent by using folllowing eluent such as water (H2O), ethanol (C2H5OH), hydrochloric acid (HCl) and mixture of HCl & C2H5OH for desorption of dye molecules from GO surface. Desorption rate followed an order as: water < ethanol < Hcl < HCl & C2H5OH. Almost 91% of mixed dyes could be desorbed by using 0.1 M HCl and C2H5OH (2:1) as eluent. After successful desorption of mixed dyes from the surface

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Fig. 18. Schematics showing mechanism of interaction of CR + MB over GO.

Table 2 The intensity ratio of “D” and “G” band (ID/IG) and Variation in the G band position of GO in presence of MB, CR and MB + CR after adsorption. Material

Position of “G” band (cm1)

ID/IG

GO GO-MB GO-CR GO-MB + CR

1590 1587 1593 1588

0.845 0.783 0.752 0.814

of GO, reusability study was carried out. It was observed that up to 3rd cycle, GO showed adsorption efficiency of about 80% towards the mixture of MB and CR (Fig. 20). 4. Conclusions The percentage of CR dye adsorption over GO is dramatically increased in presence of MB dye which can be possibly explained on

the basis following points i.e. (1) Possible electrostatic interaction arising between the adsorbate (GO) and adsorbent (CR, MB and CR + MB), (2) Surface modification of GO by cationic dyes from negative charge to positive charge for facilitating the better interaction towards the anionic dye molecule, (3) the existence of strong electrostatic force of attraction between the existing pH dependent chemical structure of both CR and MB dyes in aqueous solution. We found GO Exhibits 96% removal efficiency for CR in presence of MB. The adsorption isotherm of adsorption of CR + MB onto GO were fitted well to the Freundlich isotherm model, giving the indication of the favourite state of the absorption process on the heterogeneous surface of GO. Several thermodynamic parameters such as Gibbs energy change (DG ), enthalpy change (DH ), and entropy change (DS ) of the adsorption were evaluated. Further, the negative value of DG indicates that the adsorption of CR + MB onto GO is spontaneous. Hence, GO can be used as an effective adsorbent for mixed dye removal from an industrial effluent containing mixed organic dyes. Definitely, our results can put more efforts and interests on graphene-based materials for removal of mixed dye from aqueous solution than any other activated carbon-based materials.

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Fig. 19. Raman spectra of CR, MB and CR + MB adsorbed GO.

Fig. 20. Reusability study of GO for the adsorption of CR + MB.

Acknowledgement We are very much thankful to Prof. B. K. Mishra, Director, Institute of Minerals & Materials Technology (IMMT), Bhubaneswar, Odisha, India, for his keen interest, constant encouragement and kind permission to publish this paper.

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