Abstract
Multi-walled carbon nanotubes (MWCNTs) and their oxidized derivatives were used as adsorbents for the removal of methylene blue dye from aqueous solution. CNTs have consistent surface and distinct structure, thus they were selected as the novel adsorbents in this study. The site-energy distribution analysis could provide significant information for adsorption mechanisms. Therefore, this study concentrated on the site-sorption energy distribution analysis in combination with thermodynamic behavior of methylene blue adsorption on CNTs. The single point adsorption coefficient Kd was in order of MWCNTs-Ox>MWCNTs following the temperature 308 K>298 K. Surface area could not be the only single factor for methylene blue adsorption on CNTs, which suggested that morphology of CNTs, π-π electron donor-acceptor, electrostatic interaction and functional groups were also contributing factors. Thus, these all of interactions may drive the methylene blue adsorption. The thermodynamic parameters showed that the adsorption mechanism is spontaneous and exothermic. The site-energy distribution suggested that higher energy active sites would be engaged first, and consequently, the methylene blue molecules adsorb on the comparatively lower energy active sites of CNTs.
1. Introduction
The extensive consumption and then discharge of waste organic chemicals in numerous progressive industrial manufacturing and production processes are a major threat to water bodies [1]. Methylene blue [3,7-bis(dimethylamino)-phenothiain-5-ium chloride] is a cationic dye, which is usually used for dying textile substrates such as silk, cotton and wood, and it may be found in many industrial wastewaters [2]. Although methylene blue is not considered poisonous, it may stimulate some adverse effects specified as increased heart rate, vomiting, cyanosis, shock, diarrhea, tissue necrosis, quadriplegia and jaundice in human beings [3]. Therefore, methylene blue contaminants from wastewater should be eliminated before its release to the water stream. Methylene blue often serves as a good example for the removal studies of organic contaminants and dyes from aqueous solutions due to its known strong interaction and adsorption.
The removal of dyes from water media have been studied extensively through various techniques, including photocatalysis, membrane filtration, coagulation, adsorption and ionic exchange processes [4–8]. Nevertheless, the adsorption process is identified to be a simpler and more economical treatment method among these methods, due to its design simplicity, low cost, ease of process, insensitivity to smaller amounts and deadly pollutants of adverse substances [9–11]. Various adsorbents, such as biochar, graphene oxide and graphene, carbon nanotubes (CNTs) and activated carbon have been investigated rapidly for the adsorption process of methylene blue and other organics from aqueous media and solutions [12–18].
CNTs have achieved a great deal of attraction since their breakthrough by Wiles and Abrahamson [19] and rediscovery by Iijima [20]. CNTs and their modified derivatives have been used extensively as a prospective material in diverse applications due to their unique chemical and physical properties [21]. In the last few decades, the application of the CNTs for the adsorptive removal of inorganic and organic contaminants has been experimented widely. From a natural system perspective, the significant growth in use and production of CNTs promote health and environmental concerns, especially on discharge into the environment [22–25]. After exposure to the environment, these may undergo different environments causing biochemical and physical processes such as oxidation. The oxidation treatment creates hydrophilic surface functionalities and charges the surface, and it may also increase the mobility and colloidal stability of CNTs. A wide range of oxygen containing functional groups, such as hydroxyl (-OH), carbonyl (C=O) and carboxyl (-COOH) groups engraft into CNTs exposed surfaces preferentially at defect sites and open ends as a result of oxidation with oxidants or oxidizing acids. As a result, these individual CNTs may also persuade hazardous chemicals to biological or ecological receptors, and act potentially as a “Trojan horse” and thus possibly transfer harmful pollutants into places that they would not otherwise reach. Therefore, in circumstances of both active appraisal of environmental risk and engineered wastewater handling, it is crucially important to ameliorate the consequence of oxidation on sorption mechanisms of CNTs. Additionally, CNTs are effective role model adsorbents for investigation of adsorption interactions, since these possess uniform surfaces and defined structures [26]. Importantly, several production stages, the morphology of CNTs in impurity, physique, size and changes can be possessed, and can further allow novel insight to interpret the CNTs interaction involved in adsorption for essential pollutants and chemicals [27].
The adsorption ability and phenomena of CNTs has been well reviewed and documented elsewhere. However, a gap is present in our perceptive of CNTs oxidation and the path in which oxidation may increase the adsorption behavior and the capability of CNTs. Another significant motivation for this work is that the relatively simple chemical and physical composition of CNTs (comparative to naturally occurring carbonaceous material or activated carbon sorbents) make these materials a superior substrate for a mechanistic study direction of sorption capacities and affinities with different surface functional groups based on carbon. We concentrated on multi-walled CNTs (MWCNTs) instead of single-walled CNTs (SWCNTs) for two main reasons: (1) MWCNTs are being used in greater quantities and thus their presence in an environment is expected to be in higher concentrations; and (2) MWCNTs are comparatively less expensive to produce, hence it is more probable that a potential use will be found in the near future in large scale industrial applications. The goals of this work were as follows: (1) to inquire about the methylene blue adsorption interaction on MWCNTs and oxidized MWCNTs; and (2) to inquire about the thermodynamic adsorption mechanism of methylene blue on MWCNTs in combination with site-sorption energy distribution analysis. Both of these objectives aim to specify significant data on adsorption mechanisms and to evaluate the environmental consequence of oxidized CNTs.
2. Materials and methods
2.1. Adsorbents and adsorbates
All chemicals used in this work were of analytical grade. MWCNTs, 10–20 nm in diameter, 5–15 μm in length with a purity >95%, were purchased from Sun Nanotechn (China). Nitric acid 65% and hydrochloric acid 35–38% were obtained from Sigma Aldrich (China). Methylene blue was obtained from Sigma Aldrich with purity >90% and used for sorption experiments. The chemical properties of methylene blue dye are available widely in literature, thus not discussed here.
2.2. Oxidation of MWCNTs
Several researchers have shown that treatment with strong oxidizing agents (e.g., HNO3, HF/HNO3, HNO3/H2SO4, KMnO4, H2O2, or NaClO) greatly raises the sorption ability of MWCNTs [28–30]. A mixture of concentrated HNO3 and H2SO4 was selected to oxidize MWCNTs and these acids create hydrophilic and charged surface functional groups such as carboxyl content [31, 32]. In brief, MWCNTs were oxidized with an HNO3/H2SO4 mixture [1:3 (v/v)] [31]. An MWCNT sample (5 g) was immersed in 400 ml of HNO3/H2SO4 solution (e.g., concentrated HNO3 and H2SO4 were mixed at a ratio of 1/3). The mixture of MWCNTs and acid was then heated at 70°C for 8 h without stirring and accumulated on a 0.45 μm polycarbonate membrane. The residual MWCNTs were thoroughly washed with ultrapure water until neutral pH and then centrifuged at 8000 rpm for 10 min. The residue, MWCNTs-Ox, was dried at 70°C for 24 h. The oxidized MWCNTs samples are denoted MWCNTs-Ox.
2.3. Sample characterization
The characteristics of CNTs were analyzed with a CHN analyzer (MicroCube, Germany), X-ray photo-electron spectroscopy (XPS) analyzer, Brunauer-Emmet-Teller (BET) and Fourier-transform infrared (FT-IR) spectroscopy (Varian 640-IR, USA). For BET-N2, surface area measurements were obtained from N2 at 77 K. CHNS elemental characterization was performed by taking 2 mg samples for analysis in the Micro Cube elemental analyzer (Germany). The temperature of the reduction tube and the combustion tube were 850°C and 1150°C. For O mode analysis, only the combustion tube was used at 1150°C. All analyses were performed in duplicate. With the XPS, the pressure ranges were set as fast entry chamber 2×10-6 mbar, preparation chamber 4×10-8 mbar and sample analysis chamber 4×10-9 mbar. For normal non-charging samples, these settings for high transmission were used for analysis at 90° “electron take off angle”. The analyzer slit width was set for 0.8 mm. Data analysis and data acquisition were performed with MULTIPAK software. For the FT-IR absorbance spectra, CNT samples were prepared in KBr pellets (0.5%) and analyzed using a Varian-IR spectrometer. In FT-IR a diffuse-reflectance sampling accessory (between 400 cm-1 and 4000 cm-1) was used with an average of 100 scans with resolution of 4 cm-1. The bulk characterization, surface characterization, surface area, pore volume and FT-IR spectra are listed in Table 1 and Figure 1, respectively.
Bulk and surface elemental characterization of multi-walled carbon nanotubes (MWCNTs).
| Sample | Elemental composition (%)a | Surface elemental composition (%)b | BET SAcm2 g-1 | PVdcm3 g-1 | |||||
|---|---|---|---|---|---|---|---|---|---|
| C | H | O | N | C | O | N | |||
| MWCNTs | 96.3 | 2.3 | 1.16 | 0.2 | 97 | 2 | 0 | 148 | 0.24 |
| MWCNTs-Ox | 93.1 | 2.1 | 3.7 | 1.1 | 94.7 | 5.3 | 0 | 98 | 0.12 |
aElemental analyzer.
bXPS.
cBET: Brunauer-Emmet-Teller method.
dPore volume (obtained at P/P0 by BET).
Fourier-transform infrared (FT-IR) spectra of carbon nanotubes (CNTs).
2.4. Batch equilibrium experiments
Adsorption isotherms were obtained using a batch equilibration technique. In brief, for the adsorption experiment, about exact weights of CNTs (1±3 mg) were mixed with 4 ml of varying concentrations of methylene blue dye (1–50 mg l-1) solution, at two different temperatures 25±1°C and 35±1°C and in the presence of 0.2 m NaCl to adjust ionic strength and 200 mg l-1 NaN3 to restrain microbial activities. Prepared samples and blank solutions (without prepared sorbate) in the controlled environment were mixed properly for 24 h on a reciprocating shaker. Calibration control and the blank solution without sorbent were performed in duplicate. After the initial evaluation of experimentation, further isotherm studies were carried out on MWCNTs and MWCNTs-Ox in the presence of methylene blue dye. To obtain the equilibrium and accuracy, samples were kept in the dark for 24 h in a rotary shaker. The experimental parameters temperature and rotation were under constant observation during the mentioned time period. Later, all samples were directed to centrifuge at 2500 rpm for 10 min and concentrations of the supernatants were evaluated on a UV-vis spectrophotometer. The adsorption capacity of tested CNTs at the equilibrium state was calculated as follows:
where qe (mg g-1) is the sorbate adsorption capacity, V (l) is the volume of solution, W(g) is the weight of adsorbent and C0 and Ce (mg l-1) are the initial and equilibrium concentrations of sorbate, respectively.
2.5. Quantification
The concentrations of sorbate in the supernatants were quantified using a UV-vis spectrophotometer (UV-2401, Shimadzu, Japan) at a wavelength of 665 nm. Sorbed amounts of methylene blue dye on tested MWCNTs were calculated as the difference between the initial and final aqueous solution concentrations. A difference of <3% was observed in the initial and final equilibrium concentrations of sorbate in blank.
2.6. Mathematical modeling
2.6.1. Isotherm models:
The equilibrium adsorption isotherm is important in adsorption study design. Generally, the adsorption isotherm identifies the interaction in between adsorbates and adsorbents and thus is significant in optimum usage of adsorbents [33]. Several isotherm equations are present to describe the sorption phenomena, and two important equations were selected for the modeling of sorption data: the Freundlich and Langmuir isotherm equations. The adsorption isotherms were fitted using the Freundlich and Langmuir models with Sigma Plot 10.0.
The Freundlich adsorption isotherm was used to identify the surface adsorption with heterogeneous energy distribution. The Freundlich isotherm equation is an empirical equation and given in logarithmic form as [7]:
Where kF is a measure of the adsorption ability and 1/n is an indicator of adsorption intensiveness. The kF and 1/n can be obtained from the linear plot of ln qe vs. ln Ce. Additionally, the sorption non-linearity could be compared easily on the basis of non-linearity factor n. In addition, thermodynamics and site-sorption energy analysis based on the Freundlich model are available widely in literature.
The assumptions based on Langmuir isotherm accounts for the fact that sorption occurs at a particular homogeneous surface available inside the adsorbent, and forms monolayer sorption without interactions among the solutes. This model assumes uniform energies of sorption onto the surface and no transmigration of sorbate in the surface plane. The linearized Langmuir isotherm equation is represented as follows [7]:
where b is the Langmuir coefficient or equilibrium constant related to the affinity of binding sites (L mg-1) and Qm is the maximum adsorption capacity and comprises a practical restricted adsorption ability when the surface is fully covered with dye molecules that serve in the comparability of adsorption performance. Qm and b can be measured from the slope and intercept of the straight lines of plot Ce/qe vs. Ce.
Because the number of parameters used in the two models were not the same, the coefficient of determination (r2) could not be compared directly. The adjusted radj2 was calculated and compared:
where m is the number of data points and p is the number of parameters in the fitting equation.
2.6.2. Thermodynamic and site sorption energy analysis:
Thermodynamics: The study on temperature dependence of adsorption could provide valuable information regarding the energetic changes during the adsorption process. The adsorption isotherms of methylene blue dye on MWCNTs at 288 K and 298 K were obtained to determine the thermodynamic parameters. The adsorption coefficient K0 was defined as:
The standard Gibbs free energy change (ΔG0), standard enthalpy change (ΔH0), and standard entropy change (ΔS0) were determined from K0 by the following equations:
Rearrangement of Eqs. (5) and (6) yields:
The ΔH0 and ΔS0 were obtained from the slope and intercept of the linear plot of ln K0 against 1/T. R is the universal gas constant (8.314 kJ mol-1K-1), and T is the temperature (K).
Site energy distribution: The relationship between the adsorption energy and the equilibrium aqueous solute concentration can be described as below:
where E* (kJ mol-1) is the site adsorption energy of absorbent for adsorbate. Combining Eqs. (1) and (9) yields:
F(E*) was derived from the Freundlich model as:
Differentiating Eq. (10) with respect to E* and combining it with Eq. (11) yields:
Eq. (12) was used for site adsorption energy distribution analysis for adsorption data.
3. Results and discussion
3.1. CNTs characteristics
The bulk and surface characteristics of the MWCNTs and their oxidized derivatives are shown in Table 1. The oxidation greatly changed the surface properties of MWCNTs suggesting that there were some new functional groups introduced. MWCNTs aerated with HNO3:H2SO4 demonstrated greater surface oxygen based on XPS quantitative measurements.
The total extent of surface oxygen increased from 2 to 5% upon oxidation treatment. The bulk composition also changed compared to their parent MWCNTs showing an increase in N and O contents, while there was a slight decrease in C content. These two different analysis results may be attributed to opposite measuring sides for CNTs. Elemental analysis represents bulk makeup of the total mass whereas XPS measurement provides surface quantification. The higher O content in oxidized MWCNTs suggests that functional groups are generally located around the outer surface of CNT aggregates.
The values of the surface area measurement and pore structure of MWCNTs and MWCNTs-Ox are listed in Table 1. The surface area for the MWCNTs was found to be 148 m2g-1. The surface area of the oxidized MWCNTs (oxidized with H2SO4:HNO3) was lower than that of their parent MWCNTs and was found to be 98 m2g-1. This suggests that oxidation with H2SO4:HNO3 was very effective and strong enough to damage some of the MWCNTs and to make most of the amorphous carbon vanish. The formation of some new surface functional groups (C=O, -COOH and -OH) might be anticipated, which reduced the adsorption of nitrogen by available active sites [34]. Also, it is well-known that oxidation primarily occurs at defective sides of CNTs. These newly introduced functional groups on the surface of CNTs will have a positive affinity for adsorption interaction.
FT-IR is a well-used qualitative technique for the evaluation of functional groups or characterization of chemically modified CNTs. The FT-IR spectra of MWCNTs and MWCNTs-Ox are shown in Figure 1. The absorption peaks at 3420 cm-1 are attributed to the -OH stretching vibration anticipated to the presence of chemisorbed water and surface hydroxyl groups. The peaks around 1630 cm-1 are assigned to C=C skeletal stretching [35]. The bands near to 1100 cm-1 correspond to the C-O vibration of the respective oxygen-containing functional groups [36]. The hydrophilic nature of these functional groups gives CNTs an easy dispersion in water.
3.2. Adsorption of methylene blue
Adsorption isotherms of methylene blue on MWCNTs and MWCNTs-Ox are presented in Figure 2 and the fitting parameters of the Freundlich model and the Langmuir model are summarized in Table 2. Figure 2 compares the adsorption isotherms of methylene blue on CNTs as a function of the temperature on the unit mass surface area (the left side), whereas the right side comparison is based on unit surface area. The adsorption data was analyzed with Freundlich isotherm (FM) and Langmuir isotherm (LM) models.
Adsorption isotherms of methylene blue on multi-walled carbon nanotubes (MWCNTs) at two different temperatures. Circles (• and ○) represent MWCNTs and oxidized MWCNTs (MWCNTs-Ox), respectively: (A) solid phase concentration on unit mass basis; (B) solid phase concentration on unit surface area basis.
Fitting parameters for methylene blue dye adsorption on multi-walled carbon nanotubes (MWCNTs) and oxidized MWCNTs (MWCNTs-Ox) at two different temperatures.
| Temperature | Sorbents | Freundlich model | Langmuir model | Log Kd (l g-1) | K′ | |||||
|---|---|---|---|---|---|---|---|---|---|---|
| KF (l g-1)n | n | R2adj | Q0 (mg g-1) | KL (l g-1) | R2adj | 0.01 Cs | 0.1 Cs | |||
| 298 K | MWCNTs | 12.19 | 0.39 | 0.970 | 7.61 | 8.19 | 0.991 | 0.86 | 0.06 | -1.30 |
| MWCNTs-Ox | 16.93 | 0.40 | 0.977 | 11.07 | 7.80 | 0.984 | 1.16 | 0.25 | -0.82 | |
| 308 K | MWCNTs | 22.47 | 0.37 | 0.993 | 18.92 | 4.97 | 0.953 | 1.38 | 0.34 | -0.8 |
| MWCNTs-Ox | 17.11 | 0.31 | 0.980 | 18.77 | 2.82 | 0.956 | 1.73 | 0.11 | -0.29 | |
K′ is the adsorption coefficient per unit surface area, K′=K/Asurf.
The interaction effects can be easily estimated and tested by analysis of variance (ANOVA). The purpose of the ANOVA is to investigate which adsorption parameters significantly affect the performance characteristic and the contribution of each parameter on the adsorption efficiency. In addition, the Fisher’s ratio can also be used to determine which parameters have a significant effect on the performance characteristic. The F>Prob in ANOVA confirms that the adsorption of methylene blue was a significant factor. The sum of the squares used to estimate factors affect and Fisher’s F ratios (defined as the ratio of mean square effect and the mean square error) are also represented in Table 3.
Analysis of variance (ANOVA) for adsorption of methylene blue.
| Temperature | Sorbents | D F | Sum of squares | Mean square | F-value | Prob>F | |
|---|---|---|---|---|---|---|---|
| 298 K | MWCNTs | Regression | 1 | 3019.60 | 3019.60 | 428.50 | <0.0001 |
| Residual | 12 | 84.56 | 7.04 | ||||
| Total | 13 | 3104.16 | 238.78 | ||||
| MWCNTs-Ox | Regression | 1 | 5822.46 | 5822.46 | 552.19 | <0.0001 | |
| Residual | 12 | 126.52 | 10.54 | ||||
| Total | 13 | 5948.99 | 457.61 | ||||
| 308 K | MWCNTs | Regression | 1 | 9292.69 | 9292.69 | 2102.74 | <0.0001 |
| Residual | 12 | 53.03 | 4.41 | ||||
| Total | 13 | 9345.72 | 718.90 | ||||
| MWCNTs-Ox | Regression | 1 | 3593.23 | 3593.23 | 654.53 | <0.0001 | |
| Residual | 12 | 65.87 | 5.48 | ||||
| Total | 13 | 3659.11 | 281.47 |
MWCNTs, multi-walled carbon nanotubes; MWCNTs-Ox, oxidized multi-walled carbon nanotubes.
Sorption capacities of the tested MWCNTs were compared with sorption coefficient log Kd values. The single-point (concentration based) sorption coefficients (log Kd) preceded the order of 35>25°C with MWCNTs-Ox>MWCNTs. Among different sorption combinations, the higher methylene blue adsorption on MWCNTs-Ox with lower surface area indicates that there were some additional driving factors that play important roles in adsorption. The CNTs structural morphology such as pore volume, surface area or “orderliness” could not only the single factor altering the adsorption of methylene blue, as suggested by the conflict in surface properties, found among the two CNTs. However, these phenomena also suggest that sorption to CNTs cannot only be attributed to surface area, but the other additional binding mechanisms such as π-π electron-donor acceptor interaction and H-bonding were also contributing factors. Additional interaction mechanisms should be taken into account as well. Ma et al. [37] reported the excellent adsorption affinity of methylene blue due to oxygen containing functional groups on CNTs surface. Wang et al. [38] revealed that π-π stacking was the main driving force responsible for the MWCNT-dye interaction.
3.3. Adsorption mechanisms
The anticipated adsorption ability of organic chemicals on CNTs may not be straight and various potential interactions between CNTs and organics have been proposed [26]. Generally, π-π electron-donor acceptor, hydrophobic interaction, hydrogen bonding, electrostatic and covalent interactions, and van der Waals forces are accountable for the sorption of organic chemicals on CNTs and might act simultaneously or individually [39]. The surface area could not be the single factor to evaluate the adsorption ability of the CNTs here. The oxidized CNTs had a lower surface area compared to their corresponding CNTs. The highly hydrophobic side wall of CNTs makes them highly π electron rich for sp2 carbons. Methylene blue may form hydrophobic interactions with the side wall of CNTs through π-π electron coupling. Thus, methylene blue may interact with the side wall, i.e., on the axis direction of CNTs. The π-π interaction specifically involves in between the bulk π-system of CNTs and dye molecules with C=C or benzene rings. Methylene blue is a cationic dye which has C-C double bonds and contains π-electrons, which makes them positive in aqueous media. These electrons can easily form interactions with the electrons of benzene rings on CNT surfaces through π-π electron coupling [40]. Therefore, the electrostatic attraction also benefits methylene blue adsorption by CNTs. The newly formed functional groups such as -COOH and -OH are the primary groups contributing to the hydrogen bond formation between the CNTs and dye molecules [41–43]. A schematic presentation of the CNTs and dye interaction is shown in Figure 3.
Schematic illustration of typical adsorption sites and the possible interaction between multi-walled carbon nanotubes (MWCNTs) and methylene blue dye: (A) axis direction; (B) electrostatic attraction and (C) π-π stacking.
3.4. Adsorption thermodynamics and site-sorption energy
The intrinsic energetic changes involved during the adsorption interaction can be obtained from thermodynamic parameters. The temperature can affect the adsorption equilibrium in two different ways: (1) generally, a higher temperature increases the rate of diffusion of the adsorbate molecules through the solution to the internal and external surface; and (2) it may alter the equilibrium adsorption ability of the adsorbent for the specific adsorbate. The adsorption isotherms of methylene blue dye were obtained at 298 and 308 K to find out the effect of thermodynamic parameters. As stated above, the higher temperature greatly changed the methylene blue dye adsorption.
As presented in Figure 4, ΔG0 values were negative, which suggests that the adsorption mechanism is spontaneous and directly proportional to the temperature. The higher temperature favored the adsorption with a high dispersion of methylene blue onto CNTs. The driving force of the adsorption process will become stronger with more negative values of ΔG0. This suggests that the methylene blue dye molecules would occupy firstly the higher energy adsorption sites, and subsequent methylene blue dye molecules have to locate on comparably lower energy sites of CNTs. This is consistent with thermodynamics proof for increased adsorption of methylene blue dye on oxidized CNTs.
Plots of standard Gibbs free energy change (ΔG0) for adsorption of methylene blue at 298 K and 318 K. Circles (● and ○) represent multi-walled carbon nanotubes (MWCNTs) and oxidized MWCNTs (MWCNTs-Ox), respectively.
The plots of standard enthalpy change (ΔH0) and standard entropy change (ΔS0) are presented in Figure 5. The negative values of ΔH0 confirm the exothermic nature of the adsorption process. To eliminate the randomness at the solid-solution system, ΔS0 was used. The positive values of ΔS0 may be interrelated to the extent of hydration of cationic dye molecules. The restructuring or reorientation of water closely to methylene blue dye molecules is contrary in terms of entropy, because it interrupts the water molecules and inflicts a new and more consistent structure on the surrounding water molecules. The ΔS0 values speculate the affinity of the CNTs for the methylene blue and indicate some structural changes in dye and CNT system. Thus, the positive ΔS0 may be assigned to increased disorderliness due to the reason that the dye molecules surrounded by water molecules decreased and therefore the level of exemption of water molecules increased.
In general, the energy distribution on the energy axis (E*) is related to the attraction of the sorbate molecules along with the sorbents active surface. It is obvious that E* decreases in particular with the increase in qe, The higher energy active sites are quicker and ready to interact and absorb with methylene blue molecules, therefore the remaining methylene blue dye molecules have to take the lower energy active sites of the CNTs (Figure 6). In addition, the site adsorption energy E* decreases with the increase in temperature, implying that a higher temperature is more encouraging for the adsorption of methylene blue on CNTs, which is consistent with the changes in ΔG0 as discussed above.
Plots of standard enthalpy change (ΔH0) and standard entropy change (ΔS0) of methylene blue adsorption on multi-walled carbon nanotubes (MWCNTs). Symbols (● and ○) and (▾ and Δ) represent MWCNTs and oxidized MWCNTs (MWCNTs-Ox). Closed and open symbols represent temperatures 298 K and 318 K, respectively.
Plots of site energy distribution of methylene blue dye adsorption. E* is the net energy and F(E*) is site energy distribution. Symbols (● and ○) and (▾ and Δ) represent multi-walled carbon nanotubes (MWCNTs) and oxidized MWCNTs (MWCNTs-Ox). Closed and open symbols represent temperatures 298 K and 318 K, respectively.
4. Conclusion
MWCNTs and their oxidized derivatives were used to determine the adsorption ability for methylene blue with a specific focus on the highly assorted adsorption energy dispersion on the surface of CNTs. The Freundlich and Langmuir models were used to analyze the isotherms and the Freundlich model was shown to provide the best fitting. The surface area was not the only single factor for methylene blue adsorption on CNTs. Additional factors and interactions also contributed in adsorption drive such as H-bonds, hydrophobic interactions, CNTs morphology and functional groups. The thermodynamic parameters showed that the adsorption system is spontaneous and exothermic. The site-energy distribution analysis suggested that the high energy active sites are quicker to adsorb methylene blue molecules; thus, the remaining methylene blue molecules would adsorb on the low energy active sites of CNTs. Thermodynamic adsorption behavior in combination with site-sorption energy distribution analysis could provide better understanding of the sorption system. The study is expected to facilitate a better evaluation and understanding of the environmental behavior of CNTs and methylene blue.
About the authors
Abdul Ghaffar received his BE degree in Textile Engineering from the College of Textile and Polymer Engineering, Pakistan in 2009. Currently, he is enrolled as an MS student in Environmental Engineering at the Kunming University of Science and Technology. His research interests focus on fabrication, application and industrialization of nanomaterials as environmental protection materials and wastewater treatment.
Muhammad Naeem Younis graduated from the University of Punjab, Lahore, Pakistan with a degree in Chemical Engineering, after which he went on to study for an MS degree in Chemical Engineering at the King Fahd University of Petroleum and Minerals (KFUPM) in 2012. While at KFUPM, his research focused on the development, characterization and evaluation of catalysts for petrochemicals and related processes.
Acknowledgments
We wish to express thanks to Dr. Abdul Hannan, (SKMCH, Lahore) for his assistance during the draft of the manuscript. Also, we would like to thank the Department of Chemical Engineering, King Fahd University of Petroleum and Minerals, for their help in XPS analysis and FT-IR results.
Conflict of interests: The authors declare that there are no competing interests.
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