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Efficient removal of cationic dyes from colored wastewaters by dithiocarbamate-functionalized graphene oxide nanosheets: From synthesis to detailed kinetics studies
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Efficient removal of cationic dyes from colored wastewaters by dithiocarbamate-functionalized graphene oxide nanosheets: From synthesis to detailed kinetics studies Niyaz Mohammad Mahmoodi a,∗, Mina Ghezelbash a, Meisam Shabanian b, Fezzeh Aryanasab b, Mohammad Reza Saeb c a
Department of Environmental Research, Institute for Color Science and Technology, Tehran 1668814811, Iran Faculty of Chemistry and Petrochemical Engineering, Standard Research Institute (SRI), P.O. Box 31745-139, Karaj, Iran c Department of Resin and Additives, Institute for Color Science and Technology, Tehran 1668814811, Iran b
a r t i c l e
i n f o
Article history: Received 6 August 2017 Revised 10 September 2017 Accepted 11 October 2017 Available online xxx Keywords: Functionalization Graphene oxide nanosheet Dye removal Kinetic model Adsorption isotherm
a b s t r a c t In this work, dithiocarbamate-functionalized graphene oxide (GO-DTC) has been synthesized and applied in removal of cationic dyes; Basic Blue 41 (BB41) and Basic Red 46 (BR46). Morphology and chemical structure of the prepared GO-DTC were studied by SEM and FTIR analyses. Dye removal from wastewater solutions with variable concentration of dyes, pH, and GO-DTC dosage was evaluated. The experiments suggested a pseudo-second order kinetic model for dye adsorption onto GO-DTC, while dye adsorption isotherm data were found to fit Langmuir model. The adsorption efficiency of synthesized GO-DTC towards BB41 and BR46 was calculated to be 128.5 and 111 mg/g, respectively. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Dyes are the most commonly used types of organic materials in a wide variety of applications including cosmetic, textile, ink and leather industries, overall taking ∼60–70% of dye production worldwide. It is reported that ca. 12% of the synthetic textile dyes used every year are converted to wastewater, and 20% of the losses will enter the environment via effluents of wastewater treatment plants [1–6]. Introduction of dyes into the water resources causes serious threats to drinking water, aquatic life, and marine areas. In this regard, the mutagenicity and carcinogenicity of azo dyes and their derivatives are some significant examples. Therefore, removal of dyes from effluents has been extensively studied for many years. Although complete elimination of residual colors and organic dyes is a near-to-impossible task, a great deal of attempts has been dedicated to cleanup of effluents impregnated with such harmful chemicals. Until now, a wide variety of physical/chemical techniques such as photocatalysis, adsorption, etc. have been examined [7–11].
∗
Corresponding author. E-mail addresses: [email protected], [email protected] (N.M. Mahmoodi).
Adsorption has been frequently considered as a highly efficient approach to reduce the level of contamination in view of its simplicity, acceptable efficacy, and neutrality toward chemicals existing in the wastewater. Application of this technique in dye removal goes back to the early 20th century, where dye molecules were needed to be removed one by one in order to their hazardous impacts to the water resources. In recent years, however, development of procedures with reasonable cost and efficacy was of premier importance. From this perspective, application of natural materials and biosorbents from agricultural wastes received noticeable attention [12–16]. Chitosan (N-deacetylated derivative of chitin) is a bio-based materials [17]. Activated carbon and silica have been broadly applied in liquidphase purification processes because of their porous nature, which provides a large surface area. Nevertheless, inadequate adsorption capacity, slow kinetics of adsorption, low percent of the removed dyes, and insufficient potential for recycling placed serious limitation on the way use of conventional porous dye adsorbents. Nowadays, carbon-based porous structures with nano-scale planar or tubular structures are used as alternatives to conventional absorbents in removal of dyes from polluted environments [18–20]. Graphene-based materials have unique properties [21–26]. Application of graphene, single aromatic sheet of sp2 bonded carbon, has recently sparked a great excitement thanks to very large
https://doi.org/10.1016/j.jtice.2017.10.011 1876-1070/© 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: N.M. Mahmoodi et al., Efficient removal of cationic dyes from colored wastewaters by dithiocarbamatefunctionalized graphene oxide nanosheets: From synthesis to detailed kinetics studies, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.011
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specific surface area of this allotrope of carbon [27]. However, hydrophobic nature of graphene nanosheets brings a strong tendency to agglomeration in aqueous media. Exfoliation of graphite through oxidation with strong acids is a route to produce graphene oxide (GO) with acceptable stability in water [28]. There are some reports on application of GO as adsorbent in removal of dyes from aqueous solutions, in which the kinetics/efficiencies of dye adsorption on GO was expressed in terms of pH, temperature, contact time, stirring speed, and concentration of GO in the solution [29– 31]. Hybridization of GO with magnetic nanoparticles was also examined in recent years, which provides new platforms for wastewaters decontamination [32–34]. After dye pollutions and/or heavy metal ions are adsorbed on such hybrid nanoparticles, they can be simply detached from the solution by a magnet. Although GO is solely responsible for efficient adsorption of pollutants from wastewater, further modification of GO nanosheets with active functional groups accelerates the kinetics of adsorption. In this work, we synthesized and characterized dithiocarbamatefunctionalized graphene oxide (GO-DTC) as potential adsorbent for removal of Basic Blue 41 (BB41) and Basic Red 46 (BR46) cationic dyes from colored wastewater. The effects of pH, adsorbent dosage, and dye concentration on dye removal ability of GO-DTC were monitored through dye adsorption kinetic assessment. Pseudo-first and -second order kinetic models were examined in this work to explore the best model with the highest possible regression coefficient for assessing kinetics of adsorption. 2. Experimental 2.1. Materials Powder-like graphite (with diameter less than 150 μm and purity of 99.99% trace metals basis), potassium permanganate, sulphuric acid, hydrochloric acid, hydrogen peroxide, 3-aminopropyltriethoxysilane (APTES), sodium hydroxide and carbon disulfide (CS2 ) were commercial grades purchased from Merck and Sigma–Aldrich, and used without further purification. 2.2. Synthesis of dithiocarbamate-functionalized graphene oxide (GO-DTC) The modified Hummers method was applied in synthesis of GO from natural graphite powder [35]. The resulting material underwent ultrasonication for exfoliation of GO platelets in water and gaining GO nanosheets. Expandable graphite (1 g) was added to a 250 mL concentrated H2 SO4 (22 mL) and the resulting mixture was situated in an ice batch to maintain the temperature at 0 °C, followed by stirring for 30 min. KMnO4 (3 g) was then added drop-wise into the mixture under stirring maintaining the temperature below 20 °C. The resulting mixture was substantially stirred at 35 °C ± 3 °C for further 30 min. Then, 50 mL distilled water was poured into the reaction vessel and the mixture was heated up to 95 °C and kept at the assigned temperature for 15 min. The reaction was supposed to be terminated upon introduction of distilled water (150 mL) and 30% H2 O2 solution (15 mL). The resulting GO suspension was washed by recurrent centrifugation, first with 5% HCl aqueous solution, then with distilled water, until the pH of the solution became neutral. Finally, GO nanosheets were obtained by addition of 100 ml water to the GO precipitate and were subjected to ultrasound irradiation for 1 h with the aid of an ultrasonic bath, which assisted in exfoliation of GO nanosheets towards GO monolayer. The resulting GO in the form of fine powder was filtered, washed, and dried at 65 °C for 48 h (Fig. 1). GO–NH2 was prepared following the method described by Wu et al. with some modifications [36]. Firstly, 15 mL dry THF and 30 mL dry toluene were added to 1.5 g GO in a 100 mL two-necked
flask, and the mixture was dispersed by sonication. Then, 3.5 mmol of APTES was added to solution while stirring and the mixture was refluxed at 110 °C under N2 atmosphere for 24 h. The resultant was washed with toluene three times and then with dichloromethane to remove the residual APTES. The resulting GO–NH2 was dried in a vacuum oven at 40 °C for 24 h. 1.2 g GO–NH2 was poured into a reaction mixture containing 40 mL NaOH (0.1 M), 40 mL 2propanol and 15 mL CS2 . The resulting suspension was stirred for 6 h at ambient temperature. The powder-like product was filtered and washed with 2-propanol and dried at room temperature. 2.3. Characterization FTIR spectra were gathered using a Perkin-Elmer RXI spectrometer. The KBr pellet technique was used to probe changes in vibration transition frequencies of pristine and surface modified graphene oxide (GO) nanosheets in the wavenumber range of 40 0 0–40 0 cm−1 with a resolution of 4 cm−1 . Approximately 5 mg of the powder-like samples were incorporated by 100 mg KBr and pressed into pellets, then used for FTIR characterization. Morphology of synthesized GO was observed by a SEM (ZEISSULTRA55) equipped with field emission gun (FEG-SEM) working at an acceleration voltage of 15 kV. Prior to measurements, GO nanosheets were deposited on a micro-grid (200 mesh, EMS, Hatfield, PA, USA), followed by gold/palladium sputtering on a JEOL JFC-1100E ion sputter coater. 2.4. Adsorption procedure Adsorption was conducted in a batch system kept at room temperature (25 °C) for 60 min to attain equilibrium conditions. In order to investigate the decolorization process, three different variables, i.e., adsorbent dosage, initial dye concentration, and pH, were adopted. In a typical experiment, 250 mL dye solution with an initial concentration of 20 mg/L was taken and various adsorbent dosages of dyes (0.01–0.04 g) were introduced to the solution. In the course of adsorption reaction, samples were withdrawn at regular time intervals (5, 10, 15, 20, 30, 40, 50, 60 min) and the GO-DTC adsorbent nanosheets separated via centrifugation at 3500 rpm for 10 min. Then, the residual dye concentration in the sample was determined using UV–vis Perkin-Elmer Lambda 25 spectrophotometer at characteristic adsorption wavelength of 609 nm and 531 nm, assigned to BB41 and BR46, respectively. The dye removal efficiency was calculated using the following equation (Eq. (1)):
Removal efficiency(% ) = (C0 − Cr )/C0 × 100
(1)
where Co and Cr are the initial and residual concentration (mg/L) of dye in the solution, respectively. The amount of dye adsorbed into the GO-DTC at time t (qt , mg/g), was also obtained by (Eq. (2)):
qt = (C0 − Ct ) × V/m
(2)
where C0 (mg/L) is the initial concentration of dye, Ct is the concentration of dye at time t, in the solution, V is the volume of the solution (L) and m is the mass of GO-DTC (g). 3. Results and discussion 3.1. Characterization of the functionalized GO FTIR spectra of the synthesized GO, GO–NH2 , and GO-DTC are shown in Fig. 2. The band corresponding to FTIR spectrum of GO at 1579 cm−1 is attributable to aromatic C=C. In view of stretching vibrations of C=O of COOH at 1718 cm−1 on the edges of
Please cite this article as: N.M. Mahmoodi et al., Efficient removal of cationic dyes from colored wastewaters by dithiocarbamatefunctionalized graphene oxide nanosheets: From synthesis to detailed kinetics studies, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.011
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OH
Hummers method Ultrasonication
COOH
O
H3C
O O
H3C
COOH O
OH O
O
O Si
O
Si
O
Si
H2N
Si
CH3
O
NH2
(APTES)
Si O
O
O
COOH
GO
NH2
O
OH
OH
COOH
H2N
3
COOH
OH O
S
CS2 , NaOH
NH2
-
GO-NH2
S
S HN
O Si COOHO
OH O
O
O Si
O
O Si
S
O
Si O
O
OH
O
COOH
S
S -
S-
NH
N H
N H
S-
GO-DTC Fig. 1. Schematic representation of synthesis of dithiocarbamate-functionalized graphene oxide (GO-DTC).
grapheme sheets, it can be elucidated that carboxyl groups are introduced into GO sample [37]. The FTIR spectrum of GO–NH2 depicts stretching bands of aliphatic C–H at 2863 and 2932 cm−1 . The inclusion of dithiocarbamate was confirmed by a weak band at 1446 cm−1 (S–C–N stretching mode) in the spectrum of GO-DTC [38]. SEM micrograph of GO-DTC is shown in Fig. 3. The thickness of graphite sheets in its exfoliated state is expected to take values between 100 and 400 nm. SEM micrograph shows that graphite particles are expanded by oxidation and sonication to individual GO sheets.
3.2. Effect of operational parameters on dye removal The effects of the dosage of the GO-DTC on the removal efficiency of dye from aqueous solution were studied at the adsorbent dosage of 0.1–0.4 g by 20 mg/L of dye solution. As shown in Fig. 4, removal efficiency increased significantly upon increasing the dosage of the GO-DTC from 0.1 to 0.4 g for BB41–BR46, respectively. Increasing the dye removal efficiency with adsorbent dosages could be attributed to an increase in the adsorbent surface areas and available sites, which caused an increase in the dye removal efficiency [39–43].
The removal of BB41 and BR46 as a function of pH was studied in the range of 2.1–8.5. As shown in Fig. 5, the adsorption efficiency of both dyes experienced a little increase with the increase of the pH up to 8.5. When the pH increased, which was more favorable for the adsorption of cationic dyes, the dye removal increased. In this study, however, pH effect on the percentage of removal is not very noticeable. Therefore, the working pH was chosen to be 4.5, the natural pH of solution, at which there is no need for any excess material to change the pH of the solution. The effects of the initial concentration of the dye solutions on the adsorption were investigated at concentration of 20–50 ppm. As shown in Fig. 6, the initial concentration had an obvious effect on the dye removal efficiency, which varied from 42.64 and 11.44 for 50 ppm to 84.06 and 68.73 for 20 ppm for BB41 and BR46, respectively. The decreased dye removal efficiency for higher initial concentration of dye is due to the reduction of available active sites and saturation of them [40–42]. The effect of the contact time on adsorption efficiency of GODTC was investigated during 0–60 min. As shown in Fig. 6, removal of BB41 and BR46 by the GO-DTC were achieved after a sufficient contact time under the given experimental conditions. According to Fig. 4, about 70% of BB41 and BR46 were removed during the first 20 min of contact time. At the beginning period of the adsorption, the adsorption of dye could mainly happen on the external
Please cite this article as: N.M. Mahmoodi et al., Efficient removal of cationic dyes from colored wastewaters by dithiocarbamatefunctionalized graphene oxide nanosheets: From synthesis to detailed kinetics studies, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.011
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surface of the GO-DTC and a large number of vacant surface sites were available to speed up the rate of adsorption. By increasing the contact time more amount of dye was adsorbed onto the external surface of the GO-DTC, and when the outer surface was saturated, then dye would need to diffuse into the pore structure to be adsorbed by the interior layers of GO-DTC. As a result, the adsorption rate would become relatively slow [39,43]. 3.3. Adsorption kinetics
Fig. 2. The FTIR spectra of the synthesized nanomaterials.
Fig. 3. SEM image of the prepared GO-DTC.
In order to pattern diffusion of dyes into the adsorbent particles and providing some more insights into the reaction pathways, it is essential to choose an appropriate model for the analysis of data. The adsorption kinetics of dye onto the GO-DTC was investigated via pseudo-first order, pseudo-second order, Elovich and intraparticle diffusion models. Properties and rate constants of kinetic models at different adsorbent dosages are represented in Table 1. To indicate the most appropriate model on account of goodness of fit to experimental data, values of R-squared (R2 ) and the difference between experimental and calculated quantities of qe must be considered. In case of the pseudo-first order kinetic model, the overall adsorption rate is directly proportional to the driving force, which is the difference between the initial and equilibrium concentrations of the dye. From data in Table 1, it is apparent that the relatively low value of R2 makes evidence the weaker ability of the pseudo-first order model to representation of actual data. By contrast, the higher values of R2 together with relatively low difference between the theoretical and experimental adsorption capacities for the pseudo-second order model are indicative of reliability of this model for description of the kinetics of adsorption. Elovich equation has also been applied to describe second-order kinetics. It is to be mentioned that actual solid surfaces are supposed to be energetically heterogeneous. From the R2 values, it can be concluded that adsorption process does not follow pseudo-second order kinetics. The pseudo-second order and Elovich kinetic models could not identify the diffusion mechanism; thus, kinetic results were then analyzed by using the intraparticle diffusion model. The intraparticle diffusion model based on the Weber and Morris theory verifies that the regression of qt vs. t1/2 is linear with the intercept C. Therefore, if the value of C is zero the adsorption process is solely controlled by the intraparticle diffusion. In this study, the graph of qt versus t1/2 was plotted. It was found that the regression was linear with non-zero C value, proving that the adsorption follows intraparticle diffusion in addition to pseudo-second order kinetic model. The C values give an idea about the thickness of the boundary layer [40–42,44]. In addition, Table 1 indicates the experimental qe ((qe )exp ) values and the calculated ones ((qe )cal ).
Fig. 4. The effect of adsorbent dosage (g) on dye removal by GO-DTC from single system (a) BB41, (b) BR46 (Dye: 20 mg/L, pH: 4.5, V: 250 mL, temperature (25 °C) and t: 60 min).
Please cite this article as: N.M. Mahmoodi et al., Efficient removal of cationic dyes from colored wastewaters by dithiocarbamatefunctionalized graphene oxide nanosheets: From synthesis to detailed kinetics studies, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.011
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Fig. 5. The effect of pH on dye removal by GO-DTC from single system: (a) BB41, (b) BR46 (Dye: 20 mg/L, adsorbent: 0.04 g, V: 250 mL, temperature (25 °C) and t: 60 min).
Fig. 6. The effect of dye concentration (mg/L) on dye removal by GO-DTC from single system: (a) BB41 and (b) BR46 (adsorbent: 0.04 g, pH: 4.5, V: 250 mL, temperature (25 °C) and t: 60 min). Table 1 Linearized kinetic coefficients for dye adsorption onto GO-DTC at different adsorbent dosages.
Dye
BB41
BR46
Adsorbent (g)
0.0100 0.0200 0.0300 0.0400 0.0100 0.0200 0.0300 0.0400
qe (exp) [mg/g]
128 119 114 105 111 101 94 86
Pseudo-first order
Pseudo-second order
Intraparticle diffusion
Elovich
log (qe - qt ) = log(qe ) k1/2.303t
(t/qt ) = 1/(k2 qe 2 ) + (1/qe )t
qt = kdiff t1/2 + C
qt = 1/β ln(αβ ) + 1/β ln(t)
qe (cal) [mg/g]
k1
126 98 92 83 116 116 67 65
0.0590 0.0603 0.0514 0.0541 0.0390 0.0806 0.0500 0.0490
R2
0.9572 0.9823 0.9709 0.9787 0.9368 0.9699 0.9258 0.9449
R2
qe (cal) [mg/g]
k2
156 135 130 119 172 137 106 97
0.0 0 04 0.0 0 08 0.0 0 07 0.0 0 09 0.0 0 01 0.0 0 04 0.0010 0.0010
3.4. Adsorption isotherms Adsorption isotherm helps in determination of mechanism of process, properties of the adsorbent and the adsorbate–adsorbent interactions. Thus, the correlation of equilibrium data is essential for adsorption data interpretation and prediction. The adsorption isotherms were determined by contacting 250 mL of dye solution with initial dye concentration of 20 ppm using mechanical stirring at room temperature (25 °C) for 60 min at a constant stirring speed of 200 rpm, but at different adsorbent dosages (0.01–0.04 g). The adsorption data of GO-DTC are analyzed by fitting them into Langmuir, Freundlich, Tempkin and Dubinin–Radushkevich
0.9885 0.9964 0.9934 0.9975 0.8668 0.9755 0.9888 0.9917
kdiff [mg/g min1/2 ]
C [mg/g]
16 14 14 13 14 14 12 10
9 18 13 14 6 3 13 10
R2
β [mg/g
α [g/mg]
R2
min] 0.9814 0.9291 0.9486 0.9401 0.9764 0.9454 0.8891 0.9218
0.0301 0.0392 0.0380 0.0418 0.0296 0.0312 0.0458 0.0498
25 46 31 32 10 15 28 23
0.9830 0.9927 0.9869 0.9966 0.9028 0.9717 0.9101 0.9582
(D–R) isotherm models. These models were examined and the results are shown in Table 2. The Langmuir model assumes that the number of adsorption sites is fixed, a monolayer surface is formed, there is no interaction between the adsorbate molecules and also the surface of adsorbent is homogenous [40–42]. The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor or equilibrium parameter, RL . The values of RL are represented in Table 2. Depending on the value of RL , the adsorption process could be evaluated as irreversible (RL = 0), favorable (0˂RL ˂1), linear (RL = 1) and unfavorable (RL ˃1). The values of RL for BB41 and BR46 were obtained to be 0.05502 and 0.16334, respectively, indicating the favorable
Please cite this article as: N.M. Mahmoodi et al., Efficient removal of cationic dyes from colored wastewaters by dithiocarbamatefunctionalized graphene oxide nanosheets: From synthesis to detailed kinetics studies, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.011
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Fig. 7. The mechanism of dye adsorption by GO-DTC (a) BB41 and (b) BR46.
adsorption. From the values of the coefficient of determinant (R2 ) it can be concluded that the adsorption isotherm followed Langmuir model. The Freundlich isotherm is based on the assumption that there exists multilayer adsorption with interaction between adsorbed
molecules and predicts that dye bounds on the adsorbent surface increase upon increase of dye concentration in the solution [40– 42]. When the value of 1/n is unity, the adsorption is linear; when the value is lower than unity, the adsorption process is chemical;
Please cite this article as: N.M. Mahmoodi et al., Efficient removal of cationic dyes from colored wastewaters by dithiocarbamatefunctionalized graphene oxide nanosheets: From synthesis to detailed kinetics studies, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.011
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Table 2 Linearized isotherm coefficients for dye adsorption onto GO-DTC at different adsorbent dosages. Isotherm
Equation
Plot
Parameters
Unit
Value BB41
BR46
Langmuir
Ce /qe = (1/Qm KL ) + Ce /Qm
Ce /qe vs. Ce
Qm KL R2 RL 1/n KF R2 B1 KT R2 Qs β × 10−7 R2 E = (1/(2β )0.5 )
mg/g L/mg – – – L/mg – – L/mg – mg/g – – kJ/mol
137 0.8588 0.9969 0.0550 0.1248 90 0.9760 14 425 0.9664 124 4 0.8176 1.1180
112 0.2561 0.9964 0.1633 0.2733 52 0.9958 27 4 0.9905 112 20 0.9253 0.50 0 0
RL = (1)/(1 + KL C0 ) Freundlich
log qe = log KF + (1/n) log Ce
log qe vs. log Ce
Temkin
qe = B1 ln KT + B1 ln Ce
qe vs. ln Ce
D-R
Ln qe = ln Qs _βε 2
lnqe vs. ε 2
ε = RT ln ((1) + (1/Ce ))
when the value is bigger than unity, the process is favorable physical; when the value is close to zero, the adsorbent is of heterogeneous surface. In this study the value of 1/n is lower than unity for both dyes and it means that adsorption process is chemical [45]. Tempkin equation suggests that due to indirect adsorbate– adsorbent interaction, when the dye molecules cover the surface, the heat of adsorption of all the molecules in the layer would decrease linearly [40–42]. The Dubinin–Radushkevich (D–R) sorption isotherm is more general than the Langmuir isotherm, as its derivation is not based on ideal assumptions such as equivalent sorption sites, absence of resistance between adsorbed and incoming particles and surface homogeneity on microscopic level. The D–R equation can include the effect of temperature as well as the properties of both adsorbent and adsorbate. The formula for mean sorption energy is as follows: 0.5
E = ( 1/2β )
(3)
The value of E gives reliable information for predicting the mean adsorption of energy [45]. Low free energy value E of 1.118 and 0.5 kJ/mol, respectively assigned to BB41 and BR46, obtained in this study indicate that the mechanism of dye adsorption by GO-DTC was observed as physiosorption process due to electrostatic or Van der Waal’s attractions. The molecules of dyes (BB41 and BR46) with positive charges attach themselves to the surface of the synthesized nanomaterial. The mechanism of dye adsorption is indicated in Fig. 7. 3.5. Recycling studies Adsorbent recycling is one of the most important factors for wastewater treatment feasibility. The recycled adsorbent was reused for three runs (81%, 80% and 80% for BB41 and 63%, 63% and 61% for BR46). It indicated that adsorption capacity of the synthesized nanomaterial did not decrease considerably. 4. Conclusion In this paper, the dithiocarbamate-functionalized graphene oxide (GO-DTC) nanosheet was synthesized and used as an adsorbent for removal of cationic dyes (Basic Blue 41 (BB41) and Basic Red 46 (BR46)) from colored wastewater. The dye adsorption kinetics and isotherm followed the pseudo-second order model and the Langmuir model, respectively. Dye removal efficiency increased with adsorbent dosage, which was attributed to an increase in the adsorbent surface area and available sites. Dye removal decreased
at higher initial dye concentration due to reduction of available active sites and saturation of them on the adsorbent. When the outer surface of the GO-DTC was saturated, then dye would need to diffuse into the pore structure to be adsorbed by the interior of the adsorbent. Thus, the adsorption rate would become relatively slow. The results of this research provoked that the prepared GO-DTC nanosheets can be successfully applied as a vigorous adsorbent in removal of cationic dyes from colored wastewater. References [1] Mahmoodi NM. Synthesis of magnetic carbon nanotube and photocatalytic dye degradation ability. Environ Monit Assess 2014;186:5595–604. [2] Mahmoodi NM. Photodegradation of dyes using multiwalled carbon nanotube and ferrous ion. J Environ Eng 2013;139:1368–74. [3] Mahmoodi NM. Photocatalytic degradation of dyes using carbon nanotube and titania nanoparticle. Water Air Soil Pollut 2013;224:1612. [4] Prevost B, Goulet M, Lucas FS, Joyeux M, Moulin L, Wurtzer S. Viral persistence in surface and drinking water: Suitability of PCR pre-treatment with intercalating dyes. Water Res 2016;91:68–76. [5] Mahmoodi NM. Binary catalyst system dye degradation using photocatalysis. Fibers Polym 2014;15:273–80. [6] Mahmoodi NM. Manganese ferrite nanoparticle: Synthesis, characterization and photocatalytic dye degradation ability. Desalination Water Treat 2015;53:84–90. [7] Yao L, Zhang L, Wang R, Chou S, Dong Z. A new integrated approach for dye removal from wastewater by polyoxometalates functionalized membranes. J Hazard Mater 2016;301:462–70. [8] Mahmoodi NM, Hayati B, Arami M, Mazaheri F. Single and binary system dye removal from colored textile wastewater by a dendrimer as a polymeric nanoarchitecture: equilibrium and kinetics. J Chem Eng Data 2010;55:4660–8. [9] Santos SCR, Boaventura RAR. Adsorption of cationic and anionic azo dyes on sepiolite clay: equilibrium and kinetic studies in batch mode. J Environ Chem Eng 2016;4:1473–83. [10] Mahmoodi NM, Najafi F. Synthesis, amine functionalization and dye removal ability of titania/silica nano-hybrid. Microporous Mesoporous Mater 2012;156:153–60. [11] Mahmoodi NM, Hayati B, Arami M. Kinetic, equilibrium and thermodynamic studies of ternary system dye removal using a biopolymer. Ind Crops Prod 2012;35:295–301. [12] Zietzschmann F, Stützer C, Jekel M. Granular activated carbon adsorption of organic micro-pollutants in drinking water and treated wastewater – aligning breakthrough curves and capacities. Water Res 2016;92:180–7. [13] Mahmoodi NM. Surface modification of magnetic nanoparticle and dye removal from ternary systems. J Ind Eng Chem 2015;27:251–9. [14] Tan KL, Hameed BH. Insight into the adsorption kinetics models for the removal of contaminants from aqueous solutions. J Taiwan Inst Chem Eng 2017;74:25–48. [15] Mahmoodi NM. Dendrimer functionalized nanoarchitecture: synthesis and binary system dye removal. J Taiwan Inst Chem Eng 2014;45:2008–20. [16] Baccar R, Blánquez P, Bouzid J, Feki M, Attiya H, Sarrà M. Modeling of adsorption isotherms and kinetics of a tannery dye onto an activated carbon prepared from an agricultural by-product. Fuel Proces Technol 2013;106:408–15. [17] Ranjbar-Mohammadi M, Arami M, Bahrami H, Mazaheri F, Mahmoodi NM. Grafting of chitosan as a biopolymer onto wool fabric using anhydride bridge and its antibacterial property. Colloids Surf B Biointerfaces 2010;76:397–403. [18] Saleh TA. Nanocomposite of carbon nanotubes/silica nanoparticles and their use for adsorption of Pb(II): from surface properties to sorption mechanism. Desalination Water Treat 2016;57:10730–44.
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Journal of the Taiwan Institute of Chemical Engineers 0 0 0 (2017) 1–8
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Efficient removal of cationic dyes from colored wastewaters by dithiocarbamate-functionalized graphene oxide nanosheets: From synthesis to detailed kinetics studies Niyaz Mohammad Mahmoodi a,∗, Mina Ghezelbash a, Meisam Shabanian b, Fezzeh Aryanasab b, Mohammad Reza Saeb c a
Department of Environmental Research, Institute for Color Science and Technology, Tehran 1668814811, Iran Faculty of Chemistry and Petrochemical Engineering, Standard Research Institute (SRI), P.O. Box 31745-139, Karaj, Iran c Department of Resin and Additives, Institute for Color Science and Technology, Tehran 1668814811, Iran b
a r t i c l e
i n f o
Article history: Received 6 August 2017 Revised 10 September 2017 Accepted 11 October 2017 Available online xxx Keywords: Functionalization Graphene oxide nanosheet Dye removal Kinetic model Adsorption isotherm
a b s t r a c t In this work, dithiocarbamate-functionalized graphene oxide (GO-DTC) has been synthesized and applied in removal of cationic dyes; Basic Blue 41 (BB41) and Basic Red 46 (BR46). Morphology and chemical structure of the prepared GO-DTC were studied by SEM and FTIR analyses. Dye removal from wastewater solutions with variable concentration of dyes, pH, and GO-DTC dosage was evaluated. The experiments suggested a pseudo-second order kinetic model for dye adsorption onto GO-DTC, while dye adsorption isotherm data were found to fit Langmuir model. The adsorption efficiency of synthesized GO-DTC towards BB41 and BR46 was calculated to be 128.5 and 111 mg/g, respectively. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Dyes are the most commonly used types of organic materials in a wide variety of applications including cosmetic, textile, ink and leather industries, overall taking ∼60–70% of dye production worldwide. It is reported that ca. 12% of the synthetic textile dyes used every year are converted to wastewater, and 20% of the losses will enter the environment via effluents of wastewater treatment plants [1–6]. Introduction of dyes into the water resources causes serious threats to drinking water, aquatic life, and marine areas. In this regard, the mutagenicity and carcinogenicity of azo dyes and their derivatives are some significant examples. Therefore, removal of dyes from effluents has been extensively studied for many years. Although complete elimination of residual colors and organic dyes is a near-to-impossible task, a great deal of attempts has been dedicated to cleanup of effluents impregnated with such harmful chemicals. Until now, a wide variety of physical/chemical techniques such as photocatalysis, adsorption, etc. have been examined [7–11].
∗
Corresponding author. E-mail addresses: [email protected], [email protected] (N.M. Mahmoodi).
Adsorption has been frequently considered as a highly efficient approach to reduce the level of contamination in view of its simplicity, acceptable efficacy, and neutrality toward chemicals existing in the wastewater. Application of this technique in dye removal goes back to the early 20th century, where dye molecules were needed to be removed one by one in order to their hazardous impacts to the water resources. In recent years, however, development of procedures with reasonable cost and efficacy was of premier importance. From this perspective, application of natural materials and biosorbents from agricultural wastes received noticeable attention [12–16]. Chitosan (N-deacetylated derivative of chitin) is a bio-based materials [17]. Activated carbon and silica have been broadly applied in liquidphase purification processes because of their porous nature, which provides a large surface area. Nevertheless, inadequate adsorption capacity, slow kinetics of adsorption, low percent of the removed dyes, and insufficient potential for recycling placed serious limitation on the way use of conventional porous dye adsorbents. Nowadays, carbon-based porous structures with nano-scale planar or tubular structures are used as alternatives to conventional absorbents in removal of dyes from polluted environments [18–20]. Graphene-based materials have unique properties [21–26]. Application of graphene, single aromatic sheet of sp2 bonded carbon, has recently sparked a great excitement thanks to very large
https://doi.org/10.1016/j.jtice.2017.10.011 1876-1070/© 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: N.M. Mahmoodi et al., Efficient removal of cationic dyes from colored wastewaters by dithiocarbamatefunctionalized graphene oxide nanosheets: From synthesis to detailed kinetics studies, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.011
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specific surface area of this allotrope of carbon [27]. However, hydrophobic nature of graphene nanosheets brings a strong tendency to agglomeration in aqueous media. Exfoliation of graphite through oxidation with strong acids is a route to produce graphene oxide (GO) with acceptable stability in water [28]. There are some reports on application of GO as adsorbent in removal of dyes from aqueous solutions, in which the kinetics/efficiencies of dye adsorption on GO was expressed in terms of pH, temperature, contact time, stirring speed, and concentration of GO in the solution [29– 31]. Hybridization of GO with magnetic nanoparticles was also examined in recent years, which provides new platforms for wastewaters decontamination [32–34]. After dye pollutions and/or heavy metal ions are adsorbed on such hybrid nanoparticles, they can be simply detached from the solution by a magnet. Although GO is solely responsible for efficient adsorption of pollutants from wastewater, further modification of GO nanosheets with active functional groups accelerates the kinetics of adsorption. In this work, we synthesized and characterized dithiocarbamatefunctionalized graphene oxide (GO-DTC) as potential adsorbent for removal of Basic Blue 41 (BB41) and Basic Red 46 (BR46) cationic dyes from colored wastewater. The effects of pH, adsorbent dosage, and dye concentration on dye removal ability of GO-DTC were monitored through dye adsorption kinetic assessment. Pseudo-first and -second order kinetic models were examined in this work to explore the best model with the highest possible regression coefficient for assessing kinetics of adsorption. 2. Experimental 2.1. Materials Powder-like graphite (with diameter less than 150 μm and purity of 99.99% trace metals basis), potassium permanganate, sulphuric acid, hydrochloric acid, hydrogen peroxide, 3-aminopropyltriethoxysilane (APTES), sodium hydroxide and carbon disulfide (CS2 ) were commercial grades purchased from Merck and Sigma–Aldrich, and used without further purification. 2.2. Synthesis of dithiocarbamate-functionalized graphene oxide (GO-DTC) The modified Hummers method was applied in synthesis of GO from natural graphite powder [35]. The resulting material underwent ultrasonication for exfoliation of GO platelets in water and gaining GO nanosheets. Expandable graphite (1 g) was added to a 250 mL concentrated H2 SO4 (22 mL) and the resulting mixture was situated in an ice batch to maintain the temperature at 0 °C, followed by stirring for 30 min. KMnO4 (3 g) was then added drop-wise into the mixture under stirring maintaining the temperature below 20 °C. The resulting mixture was substantially stirred at 35 °C ± 3 °C for further 30 min. Then, 50 mL distilled water was poured into the reaction vessel and the mixture was heated up to 95 °C and kept at the assigned temperature for 15 min. The reaction was supposed to be terminated upon introduction of distilled water (150 mL) and 30% H2 O2 solution (15 mL). The resulting GO suspension was washed by recurrent centrifugation, first with 5% HCl aqueous solution, then with distilled water, until the pH of the solution became neutral. Finally, GO nanosheets were obtained by addition of 100 ml water to the GO precipitate and were subjected to ultrasound irradiation for 1 h with the aid of an ultrasonic bath, which assisted in exfoliation of GO nanosheets towards GO monolayer. The resulting GO in the form of fine powder was filtered, washed, and dried at 65 °C for 48 h (Fig. 1). GO–NH2 was prepared following the method described by Wu et al. with some modifications [36]. Firstly, 15 mL dry THF and 30 mL dry toluene were added to 1.5 g GO in a 100 mL two-necked
flask, and the mixture was dispersed by sonication. Then, 3.5 mmol of APTES was added to solution while stirring and the mixture was refluxed at 110 °C under N2 atmosphere for 24 h. The resultant was washed with toluene three times and then with dichloromethane to remove the residual APTES. The resulting GO–NH2 was dried in a vacuum oven at 40 °C for 24 h. 1.2 g GO–NH2 was poured into a reaction mixture containing 40 mL NaOH (0.1 M), 40 mL 2propanol and 15 mL CS2 . The resulting suspension was stirred for 6 h at ambient temperature. The powder-like product was filtered and washed with 2-propanol and dried at room temperature. 2.3. Characterization FTIR spectra were gathered using a Perkin-Elmer RXI spectrometer. The KBr pellet technique was used to probe changes in vibration transition frequencies of pristine and surface modified graphene oxide (GO) nanosheets in the wavenumber range of 40 0 0–40 0 cm−1 with a resolution of 4 cm−1 . Approximately 5 mg of the powder-like samples were incorporated by 100 mg KBr and pressed into pellets, then used for FTIR characterization. Morphology of synthesized GO was observed by a SEM (ZEISSULTRA55) equipped with field emission gun (FEG-SEM) working at an acceleration voltage of 15 kV. Prior to measurements, GO nanosheets were deposited on a micro-grid (200 mesh, EMS, Hatfield, PA, USA), followed by gold/palladium sputtering on a JEOL JFC-1100E ion sputter coater. 2.4. Adsorption procedure Adsorption was conducted in a batch system kept at room temperature (25 °C) for 60 min to attain equilibrium conditions. In order to investigate the decolorization process, three different variables, i.e., adsorbent dosage, initial dye concentration, and pH, were adopted. In a typical experiment, 250 mL dye solution with an initial concentration of 20 mg/L was taken and various adsorbent dosages of dyes (0.01–0.04 g) were introduced to the solution. In the course of adsorption reaction, samples were withdrawn at regular time intervals (5, 10, 15, 20, 30, 40, 50, 60 min) and the GO-DTC adsorbent nanosheets separated via centrifugation at 3500 rpm for 10 min. Then, the residual dye concentration in the sample was determined using UV–vis Perkin-Elmer Lambda 25 spectrophotometer at characteristic adsorption wavelength of 609 nm and 531 nm, assigned to BB41 and BR46, respectively. The dye removal efficiency was calculated using the following equation (Eq. (1)):
Removal efficiency(% ) = (C0 − Cr )/C0 × 100
(1)
where Co and Cr are the initial and residual concentration (mg/L) of dye in the solution, respectively. The amount of dye adsorbed into the GO-DTC at time t (qt , mg/g), was also obtained by (Eq. (2)):
qt = (C0 − Ct ) × V/m
(2)
where C0 (mg/L) is the initial concentration of dye, Ct is the concentration of dye at time t, in the solution, V is the volume of the solution (L) and m is the mass of GO-DTC (g). 3. Results and discussion 3.1. Characterization of the functionalized GO FTIR spectra of the synthesized GO, GO–NH2 , and GO-DTC are shown in Fig. 2. The band corresponding to FTIR spectrum of GO at 1579 cm−1 is attributable to aromatic C=C. In view of stretching vibrations of C=O of COOH at 1718 cm−1 on the edges of
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OH
Hummers method Ultrasonication
COOH
O
H3C
O O
H3C
COOH O
OH O
O
O Si
O
Si
O
Si
H2N
Si
CH3
O
NH2
(APTES)
Si O
O
O
COOH
GO
NH2
O
OH
OH
COOH
H2N
3
COOH
OH O
S
CS2 , NaOH
NH2
-
GO-NH2
S
S HN
O Si COOHO
OH O
O
O Si
O
O Si
S
O
Si O
O
OH
O
COOH
S
S -
S-
NH
N H
N H
S-
GO-DTC Fig. 1. Schematic representation of synthesis of dithiocarbamate-functionalized graphene oxide (GO-DTC).
grapheme sheets, it can be elucidated that carboxyl groups are introduced into GO sample [37]. The FTIR spectrum of GO–NH2 depicts stretching bands of aliphatic C–H at 2863 and 2932 cm−1 . The inclusion of dithiocarbamate was confirmed by a weak band at 1446 cm−1 (S–C–N stretching mode) in the spectrum of GO-DTC [38]. SEM micrograph of GO-DTC is shown in Fig. 3. The thickness of graphite sheets in its exfoliated state is expected to take values between 100 and 400 nm. SEM micrograph shows that graphite particles are expanded by oxidation and sonication to individual GO sheets.
3.2. Effect of operational parameters on dye removal The effects of the dosage of the GO-DTC on the removal efficiency of dye from aqueous solution were studied at the adsorbent dosage of 0.1–0.4 g by 20 mg/L of dye solution. As shown in Fig. 4, removal efficiency increased significantly upon increasing the dosage of the GO-DTC from 0.1 to 0.4 g for BB41–BR46, respectively. Increasing the dye removal efficiency with adsorbent dosages could be attributed to an increase in the adsorbent surface areas and available sites, which caused an increase in the dye removal efficiency [39–43].
The removal of BB41 and BR46 as a function of pH was studied in the range of 2.1–8.5. As shown in Fig. 5, the adsorption efficiency of both dyes experienced a little increase with the increase of the pH up to 8.5. When the pH increased, which was more favorable for the adsorption of cationic dyes, the dye removal increased. In this study, however, pH effect on the percentage of removal is not very noticeable. Therefore, the working pH was chosen to be 4.5, the natural pH of solution, at which there is no need for any excess material to change the pH of the solution. The effects of the initial concentration of the dye solutions on the adsorption were investigated at concentration of 20–50 ppm. As shown in Fig. 6, the initial concentration had an obvious effect on the dye removal efficiency, which varied from 42.64 and 11.44 for 50 ppm to 84.06 and 68.73 for 20 ppm for BB41 and BR46, respectively. The decreased dye removal efficiency for higher initial concentration of dye is due to the reduction of available active sites and saturation of them [40–42]. The effect of the contact time on adsorption efficiency of GODTC was investigated during 0–60 min. As shown in Fig. 6, removal of BB41 and BR46 by the GO-DTC were achieved after a sufficient contact time under the given experimental conditions. According to Fig. 4, about 70% of BB41 and BR46 were removed during the first 20 min of contact time. At the beginning period of the adsorption, the adsorption of dye could mainly happen on the external
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surface of the GO-DTC and a large number of vacant surface sites were available to speed up the rate of adsorption. By increasing the contact time more amount of dye was adsorbed onto the external surface of the GO-DTC, and when the outer surface was saturated, then dye would need to diffuse into the pore structure to be adsorbed by the interior layers of GO-DTC. As a result, the adsorption rate would become relatively slow [39,43]. 3.3. Adsorption kinetics
Fig. 2. The FTIR spectra of the synthesized nanomaterials.
Fig. 3. SEM image of the prepared GO-DTC.
In order to pattern diffusion of dyes into the adsorbent particles and providing some more insights into the reaction pathways, it is essential to choose an appropriate model for the analysis of data. The adsorption kinetics of dye onto the GO-DTC was investigated via pseudo-first order, pseudo-second order, Elovich and intraparticle diffusion models. Properties and rate constants of kinetic models at different adsorbent dosages are represented in Table 1. To indicate the most appropriate model on account of goodness of fit to experimental data, values of R-squared (R2 ) and the difference between experimental and calculated quantities of qe must be considered. In case of the pseudo-first order kinetic model, the overall adsorption rate is directly proportional to the driving force, which is the difference between the initial and equilibrium concentrations of the dye. From data in Table 1, it is apparent that the relatively low value of R2 makes evidence the weaker ability of the pseudo-first order model to representation of actual data. By contrast, the higher values of R2 together with relatively low difference between the theoretical and experimental adsorption capacities for the pseudo-second order model are indicative of reliability of this model for description of the kinetics of adsorption. Elovich equation has also been applied to describe second-order kinetics. It is to be mentioned that actual solid surfaces are supposed to be energetically heterogeneous. From the R2 values, it can be concluded that adsorption process does not follow pseudo-second order kinetics. The pseudo-second order and Elovich kinetic models could not identify the diffusion mechanism; thus, kinetic results were then analyzed by using the intraparticle diffusion model. The intraparticle diffusion model based on the Weber and Morris theory verifies that the regression of qt vs. t1/2 is linear with the intercept C. Therefore, if the value of C is zero the adsorption process is solely controlled by the intraparticle diffusion. In this study, the graph of qt versus t1/2 was plotted. It was found that the regression was linear with non-zero C value, proving that the adsorption follows intraparticle diffusion in addition to pseudo-second order kinetic model. The C values give an idea about the thickness of the boundary layer [40–42,44]. In addition, Table 1 indicates the experimental qe ((qe )exp ) values and the calculated ones ((qe )cal ).
Fig. 4. The effect of adsorbent dosage (g) on dye removal by GO-DTC from single system (a) BB41, (b) BR46 (Dye: 20 mg/L, pH: 4.5, V: 250 mL, temperature (25 °C) and t: 60 min).
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5
Fig. 5. The effect of pH on dye removal by GO-DTC from single system: (a) BB41, (b) BR46 (Dye: 20 mg/L, adsorbent: 0.04 g, V: 250 mL, temperature (25 °C) and t: 60 min).
Fig. 6. The effect of dye concentration (mg/L) on dye removal by GO-DTC from single system: (a) BB41 and (b) BR46 (adsorbent: 0.04 g, pH: 4.5, V: 250 mL, temperature (25 °C) and t: 60 min). Table 1 Linearized kinetic coefficients for dye adsorption onto GO-DTC at different adsorbent dosages.
Dye
BB41
BR46
Adsorbent (g)
0.0100 0.0200 0.0300 0.0400 0.0100 0.0200 0.0300 0.0400
qe (exp) [mg/g]
128 119 114 105 111 101 94 86
Pseudo-first order
Pseudo-second order
Intraparticle diffusion
Elovich
log (qe - qt ) = log(qe ) k1/2.303t
(t/qt ) = 1/(k2 qe 2 ) + (1/qe )t
qt = kdiff t1/2 + C
qt = 1/β ln(αβ ) + 1/β ln(t)
qe (cal) [mg/g]
k1
126 98 92 83 116 116 67 65
0.0590 0.0603 0.0514 0.0541 0.0390 0.0806 0.0500 0.0490
R2
0.9572 0.9823 0.9709 0.9787 0.9368 0.9699 0.9258 0.9449
R2
qe (cal) [mg/g]
k2
156 135 130 119 172 137 106 97
0.0 0 04 0.0 0 08 0.0 0 07 0.0 0 09 0.0 0 01 0.0 0 04 0.0010 0.0010
3.4. Adsorption isotherms Adsorption isotherm helps in determination of mechanism of process, properties of the adsorbent and the adsorbate–adsorbent interactions. Thus, the correlation of equilibrium data is essential for adsorption data interpretation and prediction. The adsorption isotherms were determined by contacting 250 mL of dye solution with initial dye concentration of 20 ppm using mechanical stirring at room temperature (25 °C) for 60 min at a constant stirring speed of 200 rpm, but at different adsorbent dosages (0.01–0.04 g). The adsorption data of GO-DTC are analyzed by fitting them into Langmuir, Freundlich, Tempkin and Dubinin–Radushkevich
0.9885 0.9964 0.9934 0.9975 0.8668 0.9755 0.9888 0.9917
kdiff [mg/g min1/2 ]
C [mg/g]
16 14 14 13 14 14 12 10
9 18 13 14 6 3 13 10
R2
β [mg/g
α [g/mg]
R2
min] 0.9814 0.9291 0.9486 0.9401 0.9764 0.9454 0.8891 0.9218
0.0301 0.0392 0.0380 0.0418 0.0296 0.0312 0.0458 0.0498
25 46 31 32 10 15 28 23
0.9830 0.9927 0.9869 0.9966 0.9028 0.9717 0.9101 0.9582
(D–R) isotherm models. These models were examined and the results are shown in Table 2. The Langmuir model assumes that the number of adsorption sites is fixed, a monolayer surface is formed, there is no interaction between the adsorbate molecules and also the surface of adsorbent is homogenous [40–42]. The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor or equilibrium parameter, RL . The values of RL are represented in Table 2. Depending on the value of RL , the adsorption process could be evaluated as irreversible (RL = 0), favorable (0˂RL ˂1), linear (RL = 1) and unfavorable (RL ˃1). The values of RL for BB41 and BR46 were obtained to be 0.05502 and 0.16334, respectively, indicating the favorable
Please cite this article as: N.M. Mahmoodi et al., Efficient removal of cationic dyes from colored wastewaters by dithiocarbamatefunctionalized graphene oxide nanosheets: From synthesis to detailed kinetics studies, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.011
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Fig. 7. The mechanism of dye adsorption by GO-DTC (a) BB41 and (b) BR46.
adsorption. From the values of the coefficient of determinant (R2 ) it can be concluded that the adsorption isotherm followed Langmuir model. The Freundlich isotherm is based on the assumption that there exists multilayer adsorption with interaction between adsorbed
molecules and predicts that dye bounds on the adsorbent surface increase upon increase of dye concentration in the solution [40– 42]. When the value of 1/n is unity, the adsorption is linear; when the value is lower than unity, the adsorption process is chemical;
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Table 2 Linearized isotherm coefficients for dye adsorption onto GO-DTC at different adsorbent dosages. Isotherm
Equation
Plot
Parameters
Unit
Value BB41
BR46
Langmuir
Ce /qe = (1/Qm KL ) + Ce /Qm
Ce /qe vs. Ce
Qm KL R2 RL 1/n KF R2 B1 KT R2 Qs β × 10−7 R2 E = (1/(2β )0.5 )
mg/g L/mg – – – L/mg – – L/mg – mg/g – – kJ/mol
137 0.8588 0.9969 0.0550 0.1248 90 0.9760 14 425 0.9664 124 4 0.8176 1.1180
112 0.2561 0.9964 0.1633 0.2733 52 0.9958 27 4 0.9905 112 20 0.9253 0.50 0 0
RL = (1)/(1 + KL C0 ) Freundlich
log qe = log KF + (1/n) log Ce
log qe vs. log Ce
Temkin
qe = B1 ln KT + B1 ln Ce
qe vs. ln Ce
D-R
Ln qe = ln Qs _βε 2
lnqe vs. ε 2
ε = RT ln ((1) + (1/Ce ))
when the value is bigger than unity, the process is favorable physical; when the value is close to zero, the adsorbent is of heterogeneous surface. In this study the value of 1/n is lower than unity for both dyes and it means that adsorption process is chemical [45]. Tempkin equation suggests that due to indirect adsorbate– adsorbent interaction, when the dye molecules cover the surface, the heat of adsorption of all the molecules in the layer would decrease linearly [40–42]. The Dubinin–Radushkevich (D–R) sorption isotherm is more general than the Langmuir isotherm, as its derivation is not based on ideal assumptions such as equivalent sorption sites, absence of resistance between adsorbed and incoming particles and surface homogeneity on microscopic level. The D–R equation can include the effect of temperature as well as the properties of both adsorbent and adsorbate. The formula for mean sorption energy is as follows: 0.5
E = ( 1/2β )
(3)
The value of E gives reliable information for predicting the mean adsorption of energy [45]. Low free energy value E of 1.118 and 0.5 kJ/mol, respectively assigned to BB41 and BR46, obtained in this study indicate that the mechanism of dye adsorption by GO-DTC was observed as physiosorption process due to electrostatic or Van der Waal’s attractions. The molecules of dyes (BB41 and BR46) with positive charges attach themselves to the surface of the synthesized nanomaterial. The mechanism of dye adsorption is indicated in Fig. 7. 3.5. Recycling studies Adsorbent recycling is one of the most important factors for wastewater treatment feasibility. The recycled adsorbent was reused for three runs (81%, 80% and 80% for BB41 and 63%, 63% and 61% for BR46). It indicated that adsorption capacity of the synthesized nanomaterial did not decrease considerably. 4. Conclusion In this paper, the dithiocarbamate-functionalized graphene oxide (GO-DTC) nanosheet was synthesized and used as an adsorbent for removal of cationic dyes (Basic Blue 41 (BB41) and Basic Red 46 (BR46)) from colored wastewater. The dye adsorption kinetics and isotherm followed the pseudo-second order model and the Langmuir model, respectively. Dye removal efficiency increased with adsorbent dosage, which was attributed to an increase in the adsorbent surface area and available sites. Dye removal decreased
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Please cite this article as: N.M. Mahmoodi et al., Efficient removal of cationic dyes from colored wastewaters by dithiocarbamatefunctionalized graphene oxide nanosheets: From synthesis to detailed kinetics studies, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.011