Equilibrium, kinetic and thermodynamic studies on adsorption of cationic dyes from aqueous solutions using graphene oxide

Equilibrium, kinetic and thermodynamic studies on adsorption of cationic dyes from aqueous solutions using graphene oxide

Accepted Manuscript Title: Equilibrium, kinetic and thermodynamic studies on adsorption of cationic dyes from aqueous solutions using graphene oxide A...

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Accepted Manuscript Title: Equilibrium, kinetic and thermodynamic studies on adsorption of cationic dyes from aqueous solutions using graphene oxide Authors: Wojciech Konicki, Małgorzata Aleksandrzak, Ewa Mijowska PII: DOI: Reference:

S0263-8762(17)30179-X http://dx.doi.org/doi:10.1016/j.cherd.2017.03.036 CHERD 2634

To appear in: Received date: Revised date: Accepted date:

17-11-2016 27-3-2017 30-3-2017

Please cite this article as: Konicki, Wojciech, Aleksandrzak, Małgorzata, Mijowska, Ewa, Equilibrium, kinetic and thermodynamic studies on adsorption of cationic dyes from aqueous solutions using graphene oxide.Chemical Engineering Research and Design http://dx.doi.org/10.1016/j.cherd.2017.03.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Equilibrium, kinetic and thermodynamic studies on adsorption of cationic dyes from aqueous solutions using graphene oxide Wojciech Konickia,*, Małgorzata Aleksandrzakb, Ewa Mijowskab a

Department of Integrated Transport Technology and Environmental Protection, Maritime

University of Szczecin, H. Pobożnego St. 11, 70-507 Szczecin, Poland. b

Institute of Chemical and Environment Engineering, West Pomeranian University of

Technology, Pułaskiego St. 10, 70-322 Szczecin, Poland. *Corresponding author. E-mail address: [email protected] Fax: +48 91 48 09 643

Graphical abstract

Highlights 

Graphene oxide GO was used as an adsorbent.



Adsorption of two cationic dyes, Basic Yellow 28 and Basic Red 46 was studied.



Adsorption of dyes onto GO was favored at high pH.



Adsorption followed the pseudo-second-order kinetic model and Langmuir model.



Adsorption of dyes onto GO was spontaneous and endothermic in nature.

Abstract 1

The adsorption of cationic dyes Basic Yellow 28 (BY28) and Basic Red 46 (BR46) from an aqueous solution by graphene oxide (GO) as adsorbent is presented. Graphene oxide is prepared using a modified Hummers’ method and characterized by Fourier TransformInfrared Spectroscopy (FTIR), Thermogravimetric Analysis (TGA), Atomic Force Microscopy (AFM), Raman spectroscopy, Transmission Electron Microscopy (TEM) and zeta potential measurements. Adsorption properties of cationic dyes using GO were studied in different dye concentrations (10-50 mg L-1), pH of the solutions (3.0-11.0), and temperature range of 20-60OC. The Langmuir and Freundlich isotherm models were applied to fit the adsorption data. Equilibrium data fitted very well with the Langmuir isotherm model. The maximum monolayer adsorption capacity of BY28 and BR46 onto GO was 68.5 and 76.9 mg g-1, respectively. The experimental kinetics data were analyzed using pseudo-first-order, pseudo-second-order and intraparticle diffusion kinetic models. The kinetic studies showed that the adsorption data followed the pseudo-second-order kinetics model. In addition, various thermodynamic parameters, such as the Gibbs free energy (ΔGO), enthalpy (ΔHO), and entropy (ΔSO) were calculated and it was revealed that the adsorption of BY28 and BR46 was spontaneous and endothermic.

Keywords: Cationic dyes, Graphene oxide, Adsorption, Kinetics, Thermodynamics.

1. Introduction Many contaminants such as toxic organic and inorganic chemicals (Laohaprapanon et al., 2010; Prasse and Ternes, 2010), heavy metals (Zanin et al., 2017), and radioactive materials (Fang et al., 2016) are present in industrial wastewater. One of the major contaminants in the wastewater are synthetic dyes (Huang et al., 2011). They are extensively used in various branches of the textile industry, paper, leather, rubber, pharmaceutical, cosmetics, plastic, and food (Khodaie et al., 2013). The presence of even very low concentrations of dyes in water reduces light penetration through the water surface, precluding photosynthesis of the aqueous flora (Hu et al., 2013). Many of these dyes are also toxic and even mutagenic, teratogenic and carcinogenic and pose a serious threat to living organisms (Ratna and Padhi, 2012). Hence, the removal of dyes from aqueous solutions is a key environmental issue. To date, various methods are available for the removal of toxic pollutants from water and wastewater including ultrafiltration (Vinodhini and Sudha, 2016), ozonation (Yurteri and Gurol, 1987), ion exchange (Padmavathi et al., 2014), photocatalysis (Wang et al., 2014), 2

adsorption (Lee et al., 2016), electro-dialysis (Elmidaoui et al., 2001), chemical oxidation (Salem and El-maazawi, 2000), chemical precipitation and coagulation/flocculation (Fu and Wang, 2011). Among these, adsorption process is one of the most promising alternative techniques used for the removal of various contaminants including dyes, from water. Many adsorbents have been tested on the possibility to lower dye concentrations in aqueous solutions, such as fly ash (Gupta et al., 2000; Sun et al., 2010), saw dust (Garg et al., 2003), activated carbon (Singh et al., 2003), zeolite (Meshko et al., 2001), calcined animal bones (Haddad et al., 2013), mesoporous silica (Huang et al., 2011), magnetic nanomaterials (Wang et al., 2012; Qadri et al., 2009; Afkhami et al., 2010) and carbon nanotubes (Prola et al., 2013; Kuo et al., 2008; Yao et al., 2011). One of the candidate that can be used as an efficient adsorbent is graphene oxide. Graphene is an allotrope of carbon, whose structure is one-atom-thick planar sheets of sp2bonded carbon atoms that are densely packed in a honeycomb crystal lattice. Graphene oxide is similar to graphene, but presents large quantities of oxygen-containing functional groups in the forms of epoxy, hydroxyl, and carboxyl groups. The presence of all these functional groups on the graphene oxide makes it extremely hydrophilic. Therefore, graphene oxide can be easily dispersed in water and it can be used as an adsorbent in the aquatic environment. Considering the advantageous properties of graphene oxide, several authors have reported the use of graphene oxide as an adsorbent for the removal of dyes. Bradder et al. (2011) have synthesized graphene oxide by a modified Hummers-Offeman method as adsorbent for removal of dyes from water. Ramesha et al. (2011) have studied the adsorption of anionic and cationic dyes such as methylene blue, methyl violet, rhodamine B, and orange G from aqueous solutions by use of graphene and graphene oxide. Yang et al. (2011) have used graphene oxide for removal of methylene blue from aqueous solutions. Sun et al. (2012) have studied the removal of acridine orange from aqueous solution by graphene oxide. Liu et al. (2012) have studied the adsorption of methylene blue and methyl violet from an aqueous solution by three-dimensional graphene oxide. Yao et al. (2012) have synthesized Fe3O4/SiO2core/shell nanoparticles attached to graphene oxide as adsorbent for removal of methylene blue. Kyzas et al. (2013) have studied the adsorption of Reactive Black 5 onto graphene oxide. Sun et al. (2014) have prepared graphene oxide for removal of cationic red X-GRL. Hao et al. (2015) have prepared magnetic graphene oxide nanocomposite GO-Fe3O4 as adsorbent for removal of Chrysoidine Y from aqueous solution. All these above cited reports clearly indicate high potential of this material as an efficient dye adsorbent.

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In this work, the removal of two cationic dyes Basic Yellow 28 (BY28) and Basic Red 46 (BR46) from the aqueous solution were investigated using graphene oxide nanoparticles. The effects of main parameters, i.e., solution pH, temperature and initial dye concentration on the removal of dyes were studied. The kinetic data were analyzed using the pseudo-first-order, pseudo-second-order and intraparticle diffusion kinetic models. The experimental equilibrium adsorption data were fitted to the Langmuir and Freundlich isotherm models. Thermodynamic parameters, such as ΔGO, ΔHO and ΔSO, were also calculated.

2. Experimental 2.1. Materials and methods Graphene oxide was synthesized according to modified Hummers method (Marcano et al., 2010), previously reported elsewhere (Wojtoniszak et al., 2012). Briefy, concentrated sulfuric acid (98%) and orthophosphoric acid (90:10 mL) were added to a mixture of KMnO4 (4.5 g) and graphite (0.75 g) and then heated to 50OC and stirred for 24 h. The resulting mixture was poured into ice (100 mL) and H2O2 (30%, 1 mL) and then filtered using a polycarbonate membrane (Whatman, 0.2 µm pores). The solid material was washed three times with ethanol (200 ml) and 10% hydrochloric acid (200 ml) to remove the residual metal ions, and finally with distilled water until pH of the solution was 7. Next, the material was dried in air at 100OC for 24 h. Dyes were purchased from the Zachem Barwniki (Poland). Table 1 presents the characteristics of BY28 and BR46. The chemical structures of dyes are shown in Fig. 1. All solutions were prepared using deionized water.

The structure of the material was characterized with FT-IR spectroscopy, acquired on the Nicolet 6700 FT-IR spectrometer. Thermogravimetric analysis was performed on the SDT Q600 Simultaneous TGA/DSC under an air flow of 100 mL/min and at a heating rate of 5OC/min. The morphology of the obtained materials was characterized via atomic force microscopy (Nanoscope V MultiMode 8, Bruker) and transmission electron microscopy (TEM, Tecnai F30). For AFM analysis, samples were deposited on silicon wafer with SiO 2 layer of 300 nm. The measurements were carried out in air under ambient conditions. Raman measurements were conducted on an In-Via Raman microscope (Renishaw) with excitation laser wavelengths of 514 nm. The zeta potentials of GO were determined at pH values in the range 1.7-12.2 by a Malvern Instrument Zetasizer 2000 at room temperature. Samples were 4

prepared identically to those of the adsorption experiments. To a solution of 200 mL of deionized water was added about 40 mg of GO. The flask with solution was agitated by magnetic stirrer. Before adding the adsorbent, the pH of the water was adjusted by the addition of small quantities of 0.1 M HCl or 0.1N NaOH solutions. Each experiment was performed two times and the average value was calculated. 2.2. Adsorption experiments Adsorption experiments were carried out in Erlenmeyer flask, where the dye solution (200 mL) with initial dye concentration (10-50 mg L-1) was placed. The experiments were conducted individually for BY28 and BR46. The flask with dye solution was sealed and placed in a temperature controlled shaking water bath (Grant OLS26 Aqua Pro, Grant Instruments Ltd) and agitated at a constant speed of 160 rpm. To observe the effect of temperature the experiments were carried out at three different temperatures, i.e., 20, 40 and 60OC. Before mixing with the adsorbent, various pH levels of the dye solution was adjusted by adding a few drops of diluted hydrochloric acid (0.1N HCl) or sodium hydroxide (0.1N NaOH). When the desired temperature was reached, about 40 mg of GO was added into flask. At the end of the equilibrium period 1 ml of aqueous sample was taken from the solution, and the liquid was separated from the adsorbent by centrifugation at 6000 rpm for 5 min. The determination of dye concentration was done spectrophotometrically at maximum absorbance λmax, through the use of a UV-visible spectrophotometer (Genesys 10S UV-Vis, Thermo Scientific). The amount of dye adsorbed at equilibrium qe (mg g-1) was calculated by following equation:

where CO (mg L-1) is the initial dye concentration, Ce (mg L-1) the dye concentration at equilibrium, V (L) the volume of the solution and m (g) is the mass of the adsorbent. The procedures of kinetic experiments were identical with those of equilibrium tests. At predetermined moments, aqueous samples were taken from the solution, the liquid was separated from the adsorbent and dye concentration in the solution was determined spectrophotometrically. The amount of dye adsorbed at time t qt (mg g-1) was calculated by following equation: 5

where Ct (mg L-1) is the dye concentration at any given time t. Each experiment was performed two times and the results are given as average values.

3. Results and discussion 3.1. Characterization of the adsorbent To investigate the oxidation process, FT-IR spectroscopy was carried out and results are presented in Fig. 2. In the spectrum of graphite the peak at 1115 cm-1 corresponds to a CC stretching vibration. This peak is also observed in the spectrum of graphene oxide. The absorption modes at 1265 cm-1, corresponding to C-O stretching vibration of carboxyl group, at 1408 cm-1, relating to COO- group, at 1731 cm-1, assigned to C=O stretching vibration of carboxyl group, and at 3420 cm-1, attributed to O-H stretching vibration of adsorbed water are observed in the both spectra. Several authors have also reported the presence of oxygen (Yadav et al., 2013; Jeon et al., 2015), oxygen functional groups (Haubner et al., 2010; Rao et al., 2015) and adsorbed water (Wei et al., 2013; Jeon et al., 2013) onto graphite. Furthermore, oxidation of graphite resulted in appearance of additional characteristic peaks corresponded to oxygen-containing functional groups. For instance, the peak at 1020 cm-1, is related to C-O stretching vibration of alkoxy group. Introduction of epoxy groups resulted in the absorption mode at 1180 cm-1 in the spectrum of graphene oxide. The band at 1350 cm-1 is attributed to C-OH stretching vibration. The mode at 1570 cm-1 is related to the asymmetric vibrations of carboxyl groups (Khanra et al., 2012; Larkin, 2011; Lazar et al., 2005; Xu et al., 2008).

Thermogravimetric analysis is a useful tool to examine thermal stability of the materials and perform quantitative analysis of the sample during its combustion. Fig. 3 presents TGA (left graph) and DTG (right graph) curves of graphite and graphene oxide as a result of the nanomaterials mass loss during heating in air. As shown in Fig. 3, graphene oxide exhibits mass loss at three temperature ranges. The first one, at approximately 150OC, corresponds to the combustion of thermally labile oxygen-containing functional groups such as hydroxyl and epoxy groups (Tapas et al., 2012). The second mass loss, in range of 200260OC is related to decomposition of more stable functional moieties (e.g. C-O) (Xiea et al., 6

2015). The mass loss at 500-550OC is assigned to the combustion of carbon skeleton (Song et al., 2014), which is also observed in the graphite TGA curve at the range of 700-900OC. Basing on TGA analysis it was calculated that graphene oxide contains 71 wt.% of oxygenfunctional groups (Jeong et al., 2009; Wilson et al., 2009).

A morphology of graphene oxide was analyzed with TEM and AFM microscopic tools. TEM images of GO are presented in Figure 4. Graphene oxide shows a sheet-like morphology with wrinkles and ripples. The lateral size of GO is in the range of 0.3-3.3 µm. Fig. 5 presents topography (left panel) and height profiles (right panel) of graphite and graphene oxide acquired with atomic force microscopy. The samples were deposited on oxidized Si substrates with 300 nm thickness of SiO2 layer. Basing on the height profiles of the samples, the thickness of graphite considerably decreased after its oxidation to roughly 23 nm. It was reported that the thickness of a single-layer graphene on a SiO2/Si substrate with approximately 1 nm rms roughness, would be 0.8-1.2 nm. The enhanced thickness of monolayer graphene may be attributed to the interaction between the sample and the tip (Gupta et al., 2006; Nemes-Incze et al., 2008). In bi- or few-layered graphene the thickness arises with additional layer. According to X-ray diffraction analysis (data presented elsewhere (Wojtoniszak et al., 2012)) and basing on Bragg’s law, the interlayer distance in the prepared graphene oxide was calculated to 0.85 nm. Therefore the obtained GO is composed of two and three layers (Soldano et al., 2010). In order to characterize the vibronic properties of graphene oxide Raman spectroscopy was used. Fig. 6 presents Raman spectra of starting graphite and graphene oxide. In the spectrum of graphite three peaks are observed: (1) D band at 1354 cm-1, (2) G band at 1583 cm-1 and (3) 2D band at 2725 cm-1. The first band is a breathing mode of A1g symmetry involving phonons near the K zone boundary (Wilson et al., 2009). The G mode originates from the in-plane vibration of sp2 carbon atoms and it is a doubly degenerate phonon mode (E2g symmetry) at the Brillouin zone center (Li et al., 2007). The 2D band comes from a two phonon double resonance Raman process (Ferrari et al., 2000). After oxidation treatment the each band underwent significant changes. G band became broader and up-shifted to 1611 cm1

, which may be associated to the isolated double bonds resonating at higher frequencies (Ni

et al., 2008). The higher intensity of D band in Raman response of graphene oxide is related to the presence of certain fraction of sp3 carbon atoms due to the amorphization of graphite during the oxidation process (Ferrari, 2007). A thickness of graphene may be determined 7

basing on the shape and the position of 2D band, what was reported by Ferrari et al. (2006). They plotted the evolution of 2D band as a function of layers in the single-, bi- and multilayer graphene. Briefly, the 2D band of a bilayer graphene has 4 components: 2D1B, 2D1A, 2D2A and 2D2B. Two of them, 2D1A and 2D2A, have higher relative intensities (Ni et al., 2008). These components are clearly seen in the spectrum of graphene oxide what confirms the formation of bilayer graphene oxide (right panel in Fig. 5).

The zeta potentials of GO were measured at pH in the range 1.7-12.2, and were negative over the entire pH range studied, varying from -15.5 mV to -34.9 mV (Fig. 11).

3.2. Effect of initial dye concentration on adsorption kinetics The effect of the initial dye concentration on adsorption of the BY28 and BR46 onto GO at pH 7.0 and 20OC is shown in Fig. 7. The rate of adsorption of dyes was rapid initially and decreased gradually with the increasing in contact time until the equilibrium was reached.

Fig. 7 clearly shows that the equilibrium adsorption capacity increases with an increase of the initial concentration of dyes from 25.0 to 57.2 mg g-1 and from 36.0 to 71.0 mg g-1 for BY28 and BR46, respectively. The initial dye concentration provides an important driving force to overcome all the mass transfer resistances of the dye between the aqueous and solid phases. Hence, a higher initial concentration of dye will enhance the adsorption proces. To examine the underlying mechanism of the adsorption process, three kinetic models including the pseudo-first-order, pseudo-second-order and intraparticle diffusion kinetic models, are used to test experimental kinetic data. The pseudo-first-order equation is given by:

where k1 (min-1) is the pseudo-first-order rate constant adsorption and t (min) time. The values of k1 and qe were calculated from the linear plots of ln(qe-qt) versus t. The pseudo-secondorder model can be expressed as:

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where k2 (g mg-1 min-1) is the pseudo-second-order rate constant adsorption. Values of k2 and qe were calculated from the slope and intercept of the linear plots obtained by graphical representation of t/qt versus t. Since neither the pseudo-first-order nor the pseudo-second-order model can identify the diffusion mechanism, the kinetic results were analyzed by the intraparticle diffusion model. The intraparticle diffusion model was proposed as (Weber and Morris, 1963):

where C (mg g-1) is the constant related to the thickness of the boundary layer and kp (mg g−1 min−0.5) is the intraparticle diffusion rate constant, which can be evaluated from the slope of the linear plot of qt versus t0.5. The linear plots of pseudo-second-order kinetics model are shown in Fig. 8. Kinetic constants obtained by the linear regression and the correlation coefficients R2 for the models are listed in Table 2. The R2 values obtained for both dyes indicated better fit to pseudosecond-order model than pseudo-first-order model. Additionally, the results show good agreement between the experimental and the calculated qe values of the pseudo-second-order model. These results suggested that the experimental kinetic data for BY28 and BR46 adsorption onto GO followed the pseudo-second-order model better than the pseudo-firstorder model. Fig. 9 shows the plot of the intraparticle diffusion model for the adsorption of BY28 and BR46 onto GO. According to Eq. (5), a plot of qt versus t0.5 should be a straight line. If this plot passes through the origin, then intraparticle diffusion is the rate controlling step. As shown in Fig. 9 the plots of qt against t0.5 were multi-linear and there were two different portions, indicating the different stages in adsorption. The first part of the line (dotted line) was attributed to the diffusion of dye through the solution to the external surface of adsorbent (external mass transfer). The second part of the line (solid line) described the gradual adsorption stage, where intraparticle diffusion was rate limiting. The lines do not pass through the origin, what indicates that the intraparticle diffusion is involved in the adsorption process but not the only rate-controlling step. A similar type of plot was reported previously by Ma et al. (2014), Wang et al. (2014) and Hao et al. (2015). The values of C were helpful in determining the boundary thickness: a larger C value corresponded to a greater boundary layer diffusion effect. The C values increased from 20.7 to 42.1 mg g-1 and from 32.0 to 56.8

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mg g-1 with the initial concentrations (10-50 mg L-1) for BY28 and BR46, respectively. The results of this study demonstrate that increase of the initial concentrations promotes the boundary layer diffusion effect.

3.3. Adsorption isotherms The adsorption equilibrium isotherm is important for describing how the adsorbate molecules distribute between the liquid and the solid phases, the adsorbent, when the adsorption process reaches an equilibrium state. This study employed the Langmuir and Freundlich models to describe the equilibrium adsorption. A basic assumption of the Langmuir theory is that sorption takes place at specific homogeneous sites within the adsorbent. Compared to the Langmuir isotherm, the Freundlich model is generally found to be better suited for characterizing multilayer adsorption process. The linear form of Langmuir isotherm model is given by the following equation (Langmuir, 1918):

where QO (mg g-1) is the monolayer adsorption capacity and b (L mg-1) is a constant related to the energy of adsorption. The values of QO and b were calculated from the slope and intercept of the linear plot Ce/qe versus Ce. The essential characteristics of Langmuir isotherm can be expressed in terms of dimensionless equilibrium parameter (RL), which is defined by the following equation:

where b (L mg-1) is the Langmuir constant and CO (mg L-1) is the highest initial concentration of the adsorbate. The value of RL indicates the type of the isotherm to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1) or irreversible (RL = 0). A linear form of the Freundlich expression is represented by the following equation (Freundlich, 1906):

where KF (mg g-1(L mg-1)1/n) and n are Freundlich constants, which represent adsorption capacity and adsorption strength, respectively. The values of KF and n were calculated from 10

the slope and intercept of the linear plot ln qe versus ln Ce. The value of n ranging from 1 to 10 indicated that the adsorption process is favourable. The fitting results of both Langmuir and Freundlich models are shown in Fig. 10 and the adsorption parameters along with correlation coefficients R2 are given in Table 3. The values of Freundlich constant n were found to be less than 10 and the values of RL were in the range of 0-1, indicating that the adsorption process of BY28 and BR46 onto GO is favourable. The correlation coefficient values for Langmuir isotherm were greater than for Freundlich isotherm. It showed that Langmuir isotherm gave better fits than Freundlich isotherm, so it illustrated that the adsorption of BY28 and BR46 onto GO was a monolayer adsorption. The values of the maximum adsorption capacity QO were 68.5 and 76.9 mg g-1 for BY28 and BR46, respectively. Table 4 lists the comparison of maximum monolayer adsorption capacity of BY28 and BR46 dyes onto various adsorbents.

3.5. Effect of initial pH The pH is one of the most important factors controlling the adsorption of dye onto adsorbent. The effect of initial solution pH on adsorption capacity at equilibrium of BY28 and BR46 onto GO was investigated in the range of pH values from 3.0 to 11.0 at the fixed dyes concentration of 30 mg L-1 and 20OC (Fig. 11).

As can be seen in Fig. 11, when the initial pH of the dye solution increased from 3.0 to 11.0 for BY28 and BR46, the adsorption capacity at equilibrium increased from 40.3 to 64.5 mg g-1 and from 44.9 to 81.4 mg g-1, respectively. Additionally, Fig. 11 shows the effect of the initial pH on zeta potentials of GO. When the initial pH increased from 1.7 to 12.2, the zeta potential of GO decreased from -15.5 to -34.9 mV, and it was negative over the whole studied pH range. A negative charge on GO surface over the whole pH range is due to the presence of the abundant oxygen-containing functional groups onto GO surface. BY28 and BR46 are cationic dyes which dissociate to the methyl sulphate anion (CH3SO4-) and the positively charged cationic dye, where the positive charge is localized on a single nitrogen atom. At basic pH the oxygen-containing functional groups (carboxylic -COOH and hydroxylic -OH) are deprotonated to anionic form (-COO- and –O-) and generates electrostatic attraction force with dyes cations (Fig. 12a). Therefore, the adsorption of BY28 and BR46 onto GO surface is more favorable at higher pH. However, in acidic pH, oxygencontaining functional groups are protonated to the cationic form (-COOH2+ and –OH2+), 11

causing a decrease in the number of negatively charged sites, which in turn does not favor the adsorption of positively charged dyes molecules. Thus, the amount of BY28 and BR46 adsorbed on the GO tended to decrease with the decrease of pH, which can be attributed to the electrostatic repulsion between the positively charged sites on the surface and the positively charged dyes molecules as well as a decrease in the number of negatively charged sites on the surface of adsorbent. Simultaneously, the dyes can be adsorbed on the surface of GO by hydrogen bond and π-π interactions. Hydrogen bonds can occur between hydroxyl or carboxyl surface groups of GO and the aromatic rings and nitrogen atoms in the dyes molecules. Additionally, considering that at acidic medium electron π-rich regions bind hydronium ions (MorenoCastilla, 2004), hydrogen bonds can be formed additionally in such a way that hydronium ion acts as the proton donor for O-H-π hydrogen bonds since π-electrons of benzene molecules are the proton acceptors (Leszczynski and Shukla, 2012). The organic molecules with C=C double bonds or benzene rings, containing π electrons can form π-π bonds with graphene. BY28 and BR46 have two benzene rings and contain π electrons. Thus, these π electrons can interact with π electrons of benzene rings of GO by means of π-π electron coupling (Fig. 12b).

3.6. Effect of temperature The influence of temperature on the adsorption capacity of BY28 and BR46 onto GO was determined at 20, 40 and 60OC and it is shown in Fig. 13. It was observed that the adsorption equilibrium of BY28 and BR46 increased from 48.5 to 53.1 mg g-1 and from 62.0 to 66.1 mg g-1 respectively, with increase of the temperature from 20 to 60OC. This behaviour indicates that the adsorption reaction is endothermic.

To evaluate the effect of temperature on adsorption process of BY28 and BR46 onto GO, the thermodynamic parameters such as change in free energy (ΔGO), enthalpy (ΔHO) and entropy (ΔSO) were determined using following equations (Karagoz et al., 2008):

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where T (K) is the solution temperature, Ka is the adsorption equilibrium constant. Enthalpy (HO) and entropy (SO) were calculated from the slope and intercept from the plot of ln qe/Ce versus 1/T (Fig. 14). The value of Gibbs free energy (GO) was calculated using Eq. 11.

Table 5 contains the values of thermodynamic parameters for the BY28 and BR46. The positive values of ΔHO confirmed that the adsorption proces of BY28 and BR46 onto GO was endothermic. In addition, the positive values of entropy ΔSO, indicating the increased randomness at the solid-solution interface during the adsorption of dyes onto GO. The GO values for the BY28 and BR46 were negative in the whole tested temperature range, confirming that the adsorption of dyes onto GO was spontaneous and thermodynamically favorable. With the increase of temperature, the values of GO for both dyes were more negative, which suggested that the equilibrium capacity was increased. This observation is in agreement with the results observed for the adsorption of methylene blue onto magnetic graphene-carbon nanotube composite (Wang et al., 2014), adsorption of methylene blue onto graphene (Liu et al., 2012a) and adsorption of cationic Red X-GRL onto graphene oxide (Sun et al., 2014). Simultaneously, physisorption and chemisorption can be classified, to a certain extent, by the magnitude of enthalpy ΔHO and Gibbs free energy GO. Typically, the adsorption enthalpy of physisorption is lower than 40 kJ mol-1, while that of chemisorptions may reach value between 40 and 120 kJ mol-1 (Chatterjee and Woo, 2009). Whereas, the change in free energy for physisorption is between -20 and 0 kJ mol-1 and for chemisorption is in a range of 80 to -400 kJ mol-1 (Crini and Badot, 2010). In our study, the values of ΔHO and GO for both dyes were in the range of physisorption. Therefore, the values of ΔHO and GO suggest that adsorption of dyes onto GO was driven by a physisorption process.

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4. Conclusions In summary graphene oxide (GO) was used as an adsorbent for removal of two cationic dyes Basic Yellow 28 (BY28) and Basic Red 46 (BR46) from the aqueous solution. The removal efficiency of both dyes was strongly dependent of pH. The dyes adsorption capacity increased with the increase of pH, and the maximum removal was attained at pH 11 (64.5 mg g-1 for BY28 and 81.4 mg g-1 for BR46). The adsorption followed the pseudosecond-order kinetics model and the equilibrium data fitted well to the Langmuir isotherm model. The maximum monolayer adsorption capacity was 68.5 and 76.9 mg g-1 for BY28 and BR46, respectively. The results show that the intraparticle diffusion is not the only ratelimiting step, but also other kinetic models may control the rate of adsorption of BY28 and BR46 onto GO. The thermodynamic analysis indicated that the adsorption proces of both dyes onto GO was endothermic and spontaneous. The values of ΔHO and GO suggested that the adsorption of BY28 and BR46 onto GO was a physisorption process. The results of this work showed that GO may be used as a very effective adsorbent in removal of cationic dyes from aqueous solutions.

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21

Fig. 1 - Three-dimensional molecular structure of BY28 and BR46 (black - carbon atoms, white - hydrogen atoms, red - oxygen atoms, blue - nitrogen atoms and yellow sulfur atoms).

22

Fig. 2 - FTIR spectra of graphite (A) and graphene oxide (B).

23

Fig. 3 - TGA (left graph) and DTG (right graph) curves of graphite and graphene oxide.

24

Fig. 4 - TEM images of graphene oxide.

25

Fig. 5 - AFM images (left panel) and height profiles (right panel) of graphite (a) and graphene oxide (b).

26

Fig. 6 - Raman spectra of graphite (A) and graphene oxide (B).

27

Fig. 7 - The effect of initial dye concentration on adsorption capacity of the BY28 and BR46 onto GO. Experimental conditions: T=20OC, pH=7. 28

Fig. 8 - Pseudo-second-order kinetics of adsorption BY28 and BR46 onto GO. Experimental conditions: T=20OC, pH=7. 29

Fig. 9 - Intraparticle diffusion model of adsorption BY28 and BR46 onto GO. Experimental conditions: T=20OC, pH=7. 30

Fig. 10 - Langmuir (a) and Freundlich (b) isotherms for BY28 and BR46 adsorption onto GO at 20OC.

31

Fig. 11 - The effect of initial pH of dye solution on adsorption capacity of the BY28 and BR46. Experimental conditions: CO,BY28 and BR46=30 mg L-1, T=20OC.

32

33

Fig. 12 - Scheme of the BY28 and BR46 adsorption onto GO.

34

Fig. 13 – Effect of temperature on adsorption of the BY28 and BR46 onto GO. Experimental conditions: CO,BY28 and BR46=30 mg L-1, pH=7. 35

Fig. 14 - Van’t Hoff plot for the adsorption of the BY28 and BR46 onto GO.

36

Table 1 - Characteristics of dyes.

Dye

Chemical formula

Molecular weight (g mol-1)

λmax (nm)

Type

BY28

C21H27N3O5S

433.52

450

Cationic dye

BR46

C18H21N6O4S

417.51

530

Cationic dye

37

Table 2 - Comparison of the pseudo-first-order, pseudo-second-order and the intraparticle diffusion models for different initial concentrations of BY28 and BR46 at 20OC. Pseudo-first-order Pseudo-second-order Intraparticle diffusion model model model Co qe,exp Dye k1 qe,cal k2 qe,cal kp C 2 R2 R R2 -1 −1 (mg (mg (mg (g mg (mg (mg g (mg (min-1) L-1) g-1) g-1) min-1) g-1) min−0.5) g-1)

BY28

BR46

10

25.0

0.0205

11.6

0.8993

0.00516

25.6

0.9999

0.2832

20.7

0.8505

20

40.6

0.0212

23.1

0.9627

0.00243

42.0

0.9997

0.6766

30.6

0.8665

30

48.5

0.0249

32.9

0.9749

0.00191

50.5

0.9995

0.8563

35.9

0.8987

40

54.5

0.0221

34.2

0.9759

0.00163

56.8

0.9996

0.9308

40.7

0.9169

50

57.2

0.0231

36.4

0.9846

0.00157

59.5

0.9994

1.0319

42.1

0.8527

10

36.0

0.0225

13.3

0.8847

0.00579

36.6

0.9999

0.2671

32.0

0.8586

20

54.0

0.0231

26.7

0.9429

0.00248

55.6

0.9998

0.6122

45.0

0.8382

30

62.0

0.0201

31.9

0.9413

0.00180

63.7

0.9997

0.8573

49.0

0.8985

40

66.5

0.0198

35.0

0.9478

0.00161

68.5

0.9996

0.9441

52.2

0.9274

50

71.0

0.0250

41.9

0.9806

0.00160

73.5

0.9997

0.9705

56.8

0.8575

38

Table 3 - Langmuir and Freundlich parameters for the adsorption of the BY28 and BR46 onto GO at 20OC. Dye Isotherm parameters Langmuir isotherm

BY28

BR46

QO (mg g-1)

68.5

76.9

b (L mg-1)

0.127

0.265

RL

0.136

0.070

R2

0.9989

0.9981

15.0 2.63

28.4 3.79

0.9829

0.9793

Freundlich isotherm KF [(mg g-1)(L mg-1)1/n] n R2

39

Table 4 - Comparison of the maximum monolayer adsorption of BY28 and BR46 onto various adsorbents. Adsorbent Amberlite XAD-4 Clinoptilolite GO Boron waste HMCN

Adsorbate BY28 BY28 BY28 BY28 BY28

QO (mg g-1) 8,7-14,9 52,9-59,6 68.5 75.0 909.1

Princess tree leaf Moroccan crude clay GO Activated carbon

BR46

43.1

Ref. Yener et al., (2006) Yener et al., (2006) This study Olgun and Atar, (2009) Konicki et al., (2015) Deniz and Saygideger, (2011)

BR46 BR46 BR46

54.0 76.9 714.28

Karim et al., (2009) This study Duc et al. (2012)

40

Table 5 - Thermodynamic parameters for the adsorption of the BY28 and BR46 onto GO. Dye concentration DHO DSO DGO at temperature (OC) Dye R2 (J mol-1 K1) (mg L-1) (kJ mol-1) (kJ mol-1) 20 40 60 BY28 30 2.74 16.5 0.9810 -2.12 -2.41 -2.79 20 40 60 BR46 30 2.25 18.2 0.9926 -3.07 -3.45 -3.79

41