Surface Modification Effects on CNTs Adsorption of Methylene Blue and Phenol

This study compares the adsorption capacity of modified CNTs using acid and heat treatment. The CNTs were synthesized from acetone and ethanol as carbon sources, using floating catalyst chemical vapor deposition (FC-CVD) method. energy-dispersive X-ray spectroscopy (EDX) and Boehm method revealed the existence of oxygen functional group on the surface of CNTs. Heat modification increases the adsorption capacity of as-synthesized CNTs for methylene blue (MB) and phenol by approximately 76% and 50%, respectively. However, acid modification decreases the adsorption capacity. The equilibrium adsorption data fitted the Redlich-Peterson isotherm. For the adsorption kinetic study, the experimental data obeyed the pseudo-second-order model. Both modifications methods reduced the surface area and pore volume. The studies show that the adsorption of MB and phenol onto modified CNTs is much more influenced by their surface functional group than their surface area and pore volume.


Introduction
The effluent discharged from the textile industry contains mainly dyes.Dyes can cause allergic dermatitis and skin irritation [1].High amount of dyes releases into water surface causes abnormal coloration and has bad effect on the growth of bacteria and biological activity [2].Some are reported to be carcinogenic and mutagenic for aquatic organisms [3].Phenolic compounds are discharged from the coal tar, gasoline, plastic, rubber proofing, disinfectant, pharmaceutical and steel industries, domestic wastewaters, agricultural runoff, and chemical spills industries.The presence of phenolic compounds even at low concentration can cause unpleasant taste and odour.Adsorption is a promising method for wastewater treatment compared to the other methods such as precipitation and coagulation [4], chemical oxidation [5], sedimentation [6], filtration [7], osmosis, and ion exchange [8].The method is recognized for its high efficiency, low cost, simplicity, reusability of the adsorbent, and easy recovery.One of the most recent studied materials is carbon nanotubes (CNTs).
CNTs are made up of concentric rolled graphene sheets produced from laser ablation, chemical vapor deposition, and arc discharge.CNTs have been used as gas sensor [26], nanofiber-reinforcing composites [27], paper batteries [28], solar cells [29], and supercapacitors [30], due to their excellent electrical, electronic, and mechanical properties.Their high surface area, small diameter, various bulks and individual morphology plus their defected and easily functionalized surface are beneficial for CNTs to become a potential adsorbent for liquid adsorption.CNTs have been used as adsorbents for different types of pollutants such as inorganic pollutant (Cu(II) [31], Cr (VI) [32], and Zn(II) [33] and organic pollutant (methylene blue [16], natural organic matter (NOM) [34], and nitroaromatic compunds [24].
Surface functional groups play an important role in adsorption.Functional groups commonly found on the surface of as-prepared CNTs are carboxylic, lactonic, carbonyl, and hydroxyl [24,31,35,36].The quantity of the functional group on the external and internal surface of CNTs surface can be increased or be reduced by suitable surface treatment       [37].The functional groups were attributed to influence significantly the adsorption capacity of resorcinol on MWCNTs [25] and o-xylene and p-xylene on SWCNTs.Heat modification using argon is shown to be beneficial for producing high crystallinity and uniformity of carbon's surface [21,38].However, very few investigations were reported on the adsorption of organic pollutant onto heatmodified CNTs, except the negative effect of graphitized CNTs on adsorption of 1,2-dichlorobenzene [39].
In this study, heat-modified and acid-modified CNTs were used for the adsorption of methylene blue (MB) and phenol.The objective of this study was to compare the adsorption capacity of heat-modified and acid-modified CNTs.
The adsorption equilibrium was fitted by Langmuir, Freundlich, and Redlich-Peterson models.The adsorption kinetic was tested by pseudo-first-order kinetic, pseudo second-order kinetic and intraparticle diffusion model.

CNTs Preparation.
Two types of CNTs with different morphologies were produced by FC-CVD method using acetone and ethanol as carbon sources.The CNTs were synthesized using an apparatus consisting of a ceramic tube (50 mm OD, 40 mm ID, and 1 m long) located horizontally inside a furnace with two stages.The first stage was a quartile of a tube wrapped using heating tape in order to heat the catalyst up to 150 • C. The second stage was an electrical furnace (carbolite) equipped with silicon carbide heating element that could be heated up to 1200 • C, with 10 • C/min heating rate.
After the temperature at the second stage was maintained at 700 • C for about 1 hour, the temperature of the first stage (where ferrocene was located ∼500 mg) was raised to 150 • C. At this step, argon flow (75 mL/min) was stopped to avoid Fe nanoparticles being flown out of the first stage.Then after 3 minutes, hydrogen was bubbled into the carbon sources at flow rate of 150 and 100 mL/min for CNT-A and CNT-E, respectively.The first stage of the reactor was maintained at 150 • C during the reaction for 3-5 hours.After the synthesis, the CNTs were cooled to room temperature in argon flow.Finally, the as synthesized CNTs were heated by air oxidation at 350 • C for 1 hour to remove amorphous carbon [40].-E) and acetone (CNT-A) by acid and heat treatment are identified here as CNT-E-AM, CNT-A-AM, CNT-E-HM, and CNT-A-HM.

CNTs Characterization.
The CNTs were characterized using VP-SEM (LEO 1455), TEM (Philips HMG 400), EDX (LEO 1455), and HRTEM (PHILIPS, TECNAI 2).Thermal analysis was performed using TGA/SDTA 851 e (METTLER-TOLEDO) under maximum temperature of 1000 • C and heating rate of 5 • C/min by inserting a small amount of CNTs (∼10 mg) with an air flow rate of 10 mL/min.Samples were degassed prior to be used at 150 • C for 12 hour under vacuum.Specific total surface areas were calculated using the BET equation, whereas specific total pore volumes were evaluated from nitrogen uptake at a relative pressure (P/P 0 )

CNTs
Langmuir isotherm     of N 2 equal to 0.99.The Barret, Johner, and Halenda (BJH) method was used to determine the distributions of the mesopores [43].The zeta potential of CNTs was measured at pH 2-12 using a Zetasizer Nano Z (Malvern instrument).Seven measurements were made from each sample at each pH and the mean of zeta potential was determined as the point of isoelectric pH iep .
The functional groups on the surface of CNTs were detected by a Fourier transform infrared (FTIR) spectroscopy  from the assumption that NaHCO 3 reacts with carboxylic groups, Na 2 CO 3 reacts with carboxylic and lactonic groups, and NaOH reacts with carboxylic, lactonic, and phenolic groups.
2.5.Equilibrium Experiment. 100 mL of MB and phenol standard solution with predetermined initial concentration of 5, 10, 20, 30, 40, and 50 mg/L were put in the 250 mL conical flasks containing 0.05 g of CNTs.Those flasks with the mixture of CNTs and the adsorbates were completely wrapped with aluminum foil to prevent sunlight from causing color bleaching.The flasks were shaken using an orbital shaker, operated at 150 rpm at room temperature.The shaking process consummated for 72 hours.Then, the solution was centrifuged at 5000 rpm for 30 minutes.The clear supernatants were then decanted and analyzed using a UV-Vis spectrophotometer (Ultraspec 3100) at their maximum wavelength.
2.6.Kinetics Experiment.Kinetic experiments were performed at room temperature.CNTs (0.1 g) were introduced to 200 mL and 450 mL of MB and phenol solution, respectively, at optimum pH.Samples were taken at regular intervals and were analyzed using UV-Vis spectrophotometer.The quantity of the amount adsorbed at time t (min), q t (mmol/kg), was calculated from where C t is the concentration of solution at time t (mmol/L), C 0 is the initial concentration of solution at t = 0 (mmol/L), m is the CNTs weight (g), and V is the volume of solution (L).  2 shows the physical properties of CNTs before and after surface modification.After acid modification, the surface area decreased as compared to as synthesized CNTs.As observed from the EDX analysis (Table 3), there is a reduction of Fe particles on CNTs after surface modification.It was found that acid and heat modifications contribute to the elimination of catalyst particles.BET surface area also decreased for heat and acidmodified CNTs.This decrease maybe caused by the reduction of surface defect under high temperature as reported by Zhou et al. [45].As for acid-modified CNTs, the formation of functional groups after oxidation blocks the pore of CNTs, thus, decreasing their pore volume.Similar result was also found by Lu et al. [46].This observation is supported by FTIR result (shown in Figure 5), where many new oxygen functional groups were created after oxidation with HNO 3 .For CNT-E, the heat treatment reduces the functional groups and hence increasing the total pore volume.Similar result was also reported by Shen et al. [24] and Chin et al. [47].

Results and Discussion
Figures 2(a) and 2(b) show N 2 adsorption isotherms for both adsorbent at 77 K. Acid-modified CNTs have a higher adsorption of N 2 than heat-modified CNTs.A steep increase of N 2 adsorption is observed below (P/P 0 ) < 0.1,  3, suggesting the presence of micropores.Heat-modified CNTs exhibit a type IV adsorption isotherm according to the IUPAC classification.A clear hysteresis indicating the production of mesopores.Modified CNT-A did not have high N 2 uptakes compared to as synthesized samples.CNT-E-AM has the highest N 2 uptakes, followed by as synthesized CNT-E, then CNT-E-HM.Based on pore size distribution in Figure 4, there are two major peaks at around 2-4 nm and 10 nm, and these two peaks decreased for modified CNT-A.However, for modified CNT-E, the volume decreased for pore size of 2-4 nm but no changes occurred for pore size of 10 nm.

Functional Group Analysis.
Figure 5 presents the surface functional groups on the CNTs.After surface modification, CNTs became more hydrophilic and possessed more active functional groups hydroxyl (-OH), carboxylic acids and phenolic groups (O-H), and carbonyl groups (>C=O) at 3445, 1735, and 1400 cm −1 , respectively [48].On the other hand, hydroxyl functional group is found to be reduced due to the decomposition of surface oxygen functional group under heat modification [49].The increase of basic properties as quantified from the Boehm method for heat-modified CNTs is caused by basic groups for instance, pyrones and chromenes [50] and also electron-rich oxygen-free sites located on the carbon basal planes [51].CNTs after acid modification had improved hydrophilic properties, making them more dispersed in water [52].This improvement is due to the addition of oxygen functional groups as shown in Table 4.The total acidity increased approximately 44% and 40% for CNT-A and CNT-E, respectively.However, CNTs become more hydrophobic after heat treatment as indicated by their reduction in carboxyl functional group.Their total acidity decreased 40.3% and 19.26% for CNT-A and CNT-E, respectively.Heat modification reduces the oxygen functional groups on CNTs [53], activated carbon fiber [54], and graphite edge surface [55], as supported by the FTIR result shown in Figure 5.

Zeta Potential Analysis. Zeta potential of as synthesized
CNTs and modified CNTs are shown in Figure 6.As the pH of solution increases, the zeta potential decreases.Under acid modification, the surface of CNT-A and CNT-E became acidic, that is, 5.9 and 4.2, respectively.Zeta potential of CNTs becomes more negative after oxidation, consistent with the results from Kuo [31] and Li et al. [56].The pH iep for CNT-A-HM and CNT-E-HM was 8 and 8.6, respectively, indicating the basic characteristics of both surfaces.The pH iep of heat-treated CNTs is affected by the increment of basic sites (Table 4).Liu et al. [57], Karanfil, and Kilduff [58] reported that CNTs and activated carbon show basic characteristics, and reduction in polarity after heat treatment.CNT-A (as synthesized), CNT-A-AM, CNT-A-HM, CNT-E (as synthesized) CNT-E-AM, and CNT-E-HM, respectively.The optimum pH for adsorption of phenol for assynthesized CNT-A is at pH = 8 and for as-synthesized CNT-E is at pH = 4. Adsorption capacity is high at acidic environment due to dispersion interaction [38].At this pH range, phenol is considered as neutral molecule.Above pH of 9.99, phenolate (anionic species) will dominate the solution, causing the repulsion interaction between negatively charged surfaces of CNT based on the zeta potential analyzer results (from −10 to −60 mV) as shown in Figure 6.This condition explains the decreasing of phenol being adsorbed as it approaches basic environment.Besides, the presence of OH − ions on the adsorbents reduces the phenolate ions uptake [21,39,40].

Adsorption
Isotherm.The equilibrium experimental data for adsorbed MB and phenol on modified CNTs were fitted using the adsorption isotherm equations, namely, Langmuir, Freundlich, and Redlich-Peterson.In this study, the best fit isotherm models to the experimental data were determined using the value of coefficient of determination, R 2 [59] q e,cal − q e,exp 2 q e,cal − q e,exp 2 + q e,cal − q e,exp 2 , where q e,cal is the equilibrium capacity obtained from isotherm model, q e,exp is the equilibrium capacity obtained from experiment, and q e,exp is the average of q e,exp .
Besides the value of R 2 , the applicability of equilibrium models was verified through the sum of squares error (SSE, %) [60]: where N is the number of data.

Langmuir Isotherm.
Langmuir isotherm [61] assumes that the single adsorbate binds to a single site on the adsorbent, and all the surface sites on the adsorbents have the same affinity for the adsorbate.The equation is where Q m is the amount of adsorbate adsorbed per unit weight of adsorbent in forming a complete monolayer on the adsorbent's surface, q e is the amount of adsorbate adsorbed per unit weight of adsorbent at equilibrium concentration, C e , and K L is the Langmuir constant.
The essential characteristics of the Langmuir isotherm can be described by a separation factor, defined by the following equation [62]: where R L is the dimensionless equilibrium parameter, and C 0 is the initial adsorbate concentration.Tables 5 and 6 summarize the adsorption isotherms plot in Figure 8. Heat-treated CNTs have the highest adsorption capacity for both systems.Adsorption process was favorable based on the separation factor, R L between 0.0009 and 0.3850.[63] is based on the distribution of adsorbate between the adsorbent and aqueous phases at equilibrium.

Freundlich Isotherm. The Freundlich model
The basic Freundlich equation is where K F is the overall adsorption capacity, and 1/n is the heterogeneity factor that indicates the strength of bond energy between adsorbate and adsorbent.
CNT-E-HM has the highest adsorption capacity for MB and phenol.Based on 1/n values for both adsorbates, surface of CNT-A-HM is the most heterogeneous since its value is close to 0 [64].[65] represents the adsorption equilibrium over a wide concentration range of adsorbate.The adsorbate concentration at equilibrium condition is computed as follows:

Redlich-Peterson Isotherm. Redlich and Peterson model
where β R , K R , and b R are constant parameters, normally, less than unity.This equation reduces to a linear isotherm at low surface coverage.In addition, at high adsorbate concentration, this equation will be equal to the Freundlich isotherm and when β R = 1, it will be equal to the Langmuir isotherm.The Langmuir and Redlich-Peterson isotherm fitted the experimental data for MB and phenol adsorption onto all adsorbents tested, respectively, as they have the highest R 2 and lowest SSE value.

Adsorption Kinetics.
In the liquid phase adsorption process, the kinetic study is usually conducted to identify the kinetic reaction between the adsorbent and the adsorbate as well as the time required to achieve the maximum adsorption amount.

Pseudo-First-Order Kinetic.
The pseudo-first-order kinetic model is given as [66] Integrating this equation for the boundary conditions t = 0 to t = t and q = 0 to q = q t gives ln q e − q = ln q e − k 1 t, ( 9 ) where q t is the amounts of adsorbate adsorbed (mg/g) at time t (min), and k 1 is the rate constant of pseudo-first-orderadsorption (min −1 ).The validity of the model is checked by linearizing the plot of ln(q e − q t ) versus t where its slope is the rate constant of pseudo-first order adsorption.The values of k 1 and q e at different initial concentration are presented in Tables 7, 8, 9, 10, 11, 12, 13, and 14.The show that the data only abides the model for the first 50 mins.According to Ho and McKay (1999) [67], the firstorder kinetic model is generally applicable only over the initial stage of the adsorption processes.
3.5.2.Pseudo-Second-Order Kinetic.This model assumes that the differences between the average solid phase concentration at time t (min), q t (mmol/kg), and the equilibrium concentration, q e (mmol/kg), is the driving force for adsorption and the overall adsorption rate is proportional to the square of the driving force [67].The pseudo-second-order equation based on adsorption equilibrium capacity is expressed as [68] Rearranging the variables in (10) gives Taking into account the boundary conditions t = 0 to t = t and q = 0 to q = q t , the integrated linear form of (11) can be rearranged to obtain(12) where k 2 is pseudo-second-order constant (kg/mmol/min) to be used to calculate the initial adsorption rate as below The values of q e , k 2 , and h obtained from this rate model at different concentration are given in Tables 7 to 14.The high R 2 value indicates that the experimental data fit the pseudosecond-order model.In conclusion, chemical adsorption might be the rate-limiting step, by either valent forces, through sharing of electrons between adsorbent and sorbate, or covalent forces, through the exchange of electrons between the parties involved [69].The similar result also found using CNTs as adsorbents [16,31,70].The pseudo-second-order constant is the highest for phenol-CNT-A which is 0.019151 × kg/mmol • min, indicating the fastest mobility of phenol because the film resistance is small as shown by the boundary layer thickness, C i from intraparticle diffusion model, 3.762 mmol/kg [71].The molecular size of phenol is also smaller compared to MB as shown in Table 1.Based on the study of Lu et al. [72], smaller molecules diffused faster than the bigger ones.The rate constants of the pseudo-second-order model (k 2 ) decreased as the initial concentration of MB and phenol in adsorption systems increased.The same phenomenon was also reported by Kuo et al. [70], and Hameed and Rahman [73].At lower concentration, the competition for the adsorption surface sites is lower compared to higher concentration.

Journal of Nanomaterials
where C i is the intercept, and k p is the intraparticle diffusion rate constant (mg/g min 1/2 ), evaluated from the slope of the linearized plot of q t versus t 1/2 .The regression plot of q t versus t 0.5 in Figures 9(b) and 10(b) indicates linearity for all of the adsorbents tested, but it does not pass through the origin.This suggested that intra- particle diffusion was involved in adsorption, but it was not the only rate-controlling step.

Comparison of Adsorption Capacity.
The maximum adsorption capacity of MB and phenol onto acid-modified CNTs decreased 3-9% as compared to as synthesized CNTs.
For MB, there is an ionic repulsion between the CNTs modified with HNO 3 and MB [75].The production of acidic oxygen functional groups on the CNTs surface (refer to Table 4) extracts the electrons from the π band of the carbon.As a result, the interaction between the MB molecules and the CNTs is reduced.The same result obtained for the adsorption of MB onto acid-modified commercial activated carbons by Wang et al. [76] and Tan et al. [77].
The CNTs surface contains more acidic functional group with a decrease of hydroxide groups [76].As for phenol, the increase of surface oxygen group, especially carboxylic group, makes the surface of acid-modified CNTs more hydrophilic.Through H bonding, the formation of water clusters negatively affects the accessibility and affinity of phenol, therefore, reducing the adsorption capacity [78,79].
The more basic surface of the heat-treated CNTs promotes the high adsorption capacity of MB and phenol.For MB, the dispersive interactions between the π electrons on the surface of the basic carbon and the free electrons of the cationic dye molecule present in the aromatic rings and multiple bonds enhancing their adsorption [80].As for phenol, adsorption mechanism of "donor-acceptor complex" contributes to its higher adsorption, where phenol acts as electron acceptor and basic surface of carbon acts as donor [66].The adsorption capacities of MB and phenol on CNTs are as follows: Heat-treated CNTs > As synthesized CNTs > Acid-treated CNTs.(15) Based on Tables 15 and 16, the adsorption capacities obtained in this study are comparable to various adsorbent and activated carbon, suggesting the potential usage of CNTs for wastewater treatment.

Comparison of Adsorption of MB and Phenol.
In general, MB and phenol can adsorb well onto these types of CNTs.In MB system, the adsorption is enhanced by electrostatic interaction.As for phenol, the adsorption is dominated by π-π dispersion [81].The molecular size of MB and phenol is small enough to enter the pore size of CNTs which are 3.28 and 2.42 nm for CNT-A and CNT-E, respectively.Furthermore, both the adsorbates structures are planar, which is beneficial for face-face conformation [82].
The MB adsorption onto the adsorbents is influenced by the mesopores whereas phenol adsorption is enhanced by the micropores [83].Based on the N 2 adsorption isotherm as indicated in Figures 2(a) and 2(b), CNTs contain more mesopores.Therefore, the MB adsorption onto both adsorbents is higher than phenol adsorption.This work demonstrates that CNTs are suitable adsorbents for bigger molecular size adsorbate like MB compared to phenol as shown in Table 1.

Conclusion
A comparative study for different surface modifications of CNTs has been studied.The pseudo-second-order-kinetic model equation is the best to describe adsorption of MB and phenol on CNTs.Intraparticle diffusion was also identified to be one of the rate-controlling factors.By considering R 2 and SSE values, Langmuir and Redlich-Peterson isotherm fitted the experimental data for MB and phenol adsorption onto all adsorbents tested, respectively.Both surface modifications reduced the surface area of CNTs.The MB and phenol adsorption isotherms at room temperature show that the acid-modified CNTs have the lowest adsorption capacity, resulting from reduction in their surface area and the existence of abundant of surface oxygen functional groups.However, heat-treated CNTs have the highest adsorption capacity for MB and phenol, contributing by the basicity surface, in spite of their low surface area.

Figure 7 :Figure 8 :
Figure 7: Effect of pH on (a) MB and (b) phenol adsorption capacity onto CNTs: ¤ CNT-E-HM; • CNT-E (as synthesized); AE CNT-E-AM; × CNT-A (as synthesized); £ CNT-A-HM; CNT-A-AM.Each point is the average value of triplicate samples.Error bars represent the range.

3. 1 .
Characterization of the CNTs.Figures 1(a) and 1(b) show the TEM images of the CNT-A-HM and CNT-E-HM respectively.No obvious physical change after surface modification is observed.Table

3. 4 .
Adsorption Equilibrium 3.4.1.Optimum pH.Figures 7(a) and 7(b) show the effect of pH on adsorption of MB and Phenol onto modified CNT-A and CNT-E at initial concentration of 10 mg/L.For CNT-MB system, all modified CNTs have optimum pH of 10 which is caused by electrostatic interaction between the negative charge of their surface and positive charge of MB.The zeta potentials are at pH of 7.8, 5.9, 8, 8.6, 4.2, and 8.6, for

3. 5 . 3 .
Intraparticle Diffusion Kinetic.The intraparticle diffusion model to elucidate the diffusion mechanism is originally developed by Weber and Morris[74]

Figure 10 :
Figure 10: Plot of (a) pseudo-first-order-kinetic model, (b) pseudo-second-order-kinetic model, and (c) intraparticle diffusion model for the adsorption of 50 mg/L of phenol onto CNTs.¤ CNT-E-HM; Å CNT-E (as synthesized); + CNT-E-AM; × CNT-A (as synthesized); £ CNT-A-HM; CNT-A-AM.Each point is the average value of triplicate samples.Error bars represent the range.

Table 1 :
Structures and characteristics of MB and phenol.

Table 2 :
Physical properties of CNTs.

Table 5 :
The coefficients of the Langmuir, Freundlich, and Redlich-Peterson isotherms for MB system.

Table 6 :
The coefficients of the Langmuir, Freundlich, and Redlich-Peterson isotherms for phenol system. (a)

Table 7 :
Comparison of the first-order, second-order and intraparticle diffusion rate constants values for MB adsorption on CNT-A-AM at different initial concentration.

Table 8 :
Comparison of the first-order, second-order, and intraparticle diffusion rate constants values for MB adsorption on CNT-A-HM at different initial concentration.
was conducted by adding 0.05 g of CNTs into a 100 mL flask containing 50 mL of the following 0.1 M solutions: NaHCO 3 , Na 2 CO 3 , NaOH, and HCl.The flask was sealed and shaken at 25 • C for 48 h, and then filtered through a 0.45 μm filter paper.The filtrate (10 mL) was pipetted and mixed with 0.1 M HCl.The excess acid was titrated with 0.1 M NaOH.The quantities of acidity of various types were determined

Table 9 :
Comparison of the first-order, second-order, and intraparticle diffusion rate constants values for phenol adsorption on CNT-A-AM at different initial concentration.

Table 10 :
Comparison of the first-order, second-order, and intraparticle diffusion rate constants values for phenol adsorption on CNT-A-HM at different initial concentration.

Table 11 :
Comparison of the first-order, second-order, and intraparticle diffusion rate constants values for MB adsorption CNT-E-AM at different initial concentration.

Table 12 :
Comparison of the first-order, second-order, and intraparticle diffusion rate constants values for MB adsorption on CNT-E-HM at different initial concentration.

Table 13 :
Comparison of the first-order, second-order, and intraparticle diffusion rate constants values for phenol adsorption on CNT-E-AM at different initial concentration.

Table 14 :
Comparison of the first-order, second-order, and intraparticle diffusion rate constants values for phenol adsorption on CNT-E-HM at different initial concentration.