Adsorption of methylene blue from aqueous solution on modified ACFs by chemical vapor deposition

Chemical Engineering Journal 189–190 (2012) 168–174

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Adsorption of methylene blue from aqueous solution on modified ACFs by chemical vapor deposition Liping Wang a,b,∗ , Zhucheng Huang a , Mingyu Zhang c , Bin Chai a a b c

School of Minerals Processing and Bioengineering, Central South University, Changsha, Hunan 410083, China Department of Bioengineering and Environmental Science, Changsha University, Changsha, Hunan 410003, China State Key Laboratory for Powder Metallurgy, Central South University, Changsha, Hunan 410083, China

a r t i c l e

i n f o

Article history: Received 30 November 2011 Received in revised form 15 February 2012 Accepted 15 February 2012 Keywords: Modified ACFs Methylene blue Isothermal adsorption Thermodynamic Kinetics

a b s t r a c t The adsorption behavior of activated carbon fibers (ACFs) modified by chemical vapor deposition (CVD) method on methylene blue (MB) was investigated in details. The Brunauer–Emmet–Teller (BET) surface area analysis, scanning electron microscope (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR) analysis were employed to characterize the samples. The results showed that the pore size of the ACFs had a significant change after CVD modification. In addition, it was found that carbon nanotubes (CNTs) were well distributed on the surface of ACFs. FTIR analysis for modified ACFs after adsorption showed intensity changes of absorption peaks, in comparison to that before adsorption. The adsorption capacity at equilibrium increased with the increase of pH. Moreover, attempts were made to fit the isothermal data using Langmuir and Freundlich equations. The results manifested that the experimental data were well fitted by Freundlich equation. Furthermore, the adsorption kinetics was analyzed by pseudo-first-order, pseudo-second-order and intra-particle diffusion models. The results demonstrated that the kinetic data were well described by pseudo-second-order kinetic model. The adsorption rate was controlled by intra-particle diffusion and film diffusion and thermodynamic parameters including G, H and S were also determined. This showed that the adsorption process was a spontaneous, endothermic and increasing randomness process. The calculated Ea was 28.33 kJ/mol, indicating that the adsorption could be a physisorption process. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Dyes as an important organic material, are widely used in paper, textile, leather, dye synthesis, printing, foods and plastics industries. However, it has been found that many organic dyes are harmful to human beings, so the removal of dyes from wastewater has received considerable attention over the past decades. In recent years, a great deal of technologies for treating wastewater such as adsorption [1–3], catalytic oxidation method [4,5], traditional biological method [6], electrochemical process [7], membrane separation technology [1,8] and solvent extraction [9] have been developed. Among those schemes, adsorption as a highefficiency method has been widely applied to the treatment of dye wastewater [10–14], wherein, ACFs has become the high efficient adsorption materials since 1960s due to their high specific surface area, developed microporous structure, narrow pore size

distribution, great adsorption capacity, fast adsorption and desorption rate and easy regeneration [15–17]. However, the high surface area of ACFs is mainly contributed by their micropores [18,19], leading to the lower adsorption capacities of ACFs for some dyes due to their small pore sizes. In addition, it has been found that CNTs with high specific surface area and ␲–␲ interactions between bulk ␲ systems on CNTs surfaces and dye molecules with benzene rings perform stronger adsorbed capacity for dye molecules [20–24]. However, it is still a key problem on how to segregate tiny CNTs from the solution due to their limited volume and weight. In this paper, ACFs were modified using CVD method. CNTs were grown onto ACFs surface and some related experiments were conducted in details, in order to strengthen the adsorption ability of ACFs for dyes and improve segregation property. 2. Materials and methods 2.1. Materials

∗ Corresponding author at: School of Minerals Processing and Bioengineering, Central South University, Changsha, Hunan 410083, China. Tel.: +86 731 8887 7671; fax: +86 731 8887 7671. E-mail address: [email protected] (L. Wang). 1385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2012.02.049

Viscose-based ACFs mat was adopted in this experiment. Nitrogen, hydrogen and acetylene were used in our experiment with purity of 99.99%, 99.99% and 99.9%, respectively. Absolute ethyl

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Ni(NO3 )2 was loaded by soaking the viscose-based ACFs in 0.1 mol/L Ni(NO3 )2 solution. Then, the treated ACFs were dried in a vacuum oven at 75 ◦ C to evaporate ethanol solvent for 12 h. After that, the decomposition and reduction of Ni(NO3 )2 on the dried ACFs was carried out at 550 ◦ C under hydrogen atmosphere. With 99.9% acetylene as carbon source, 99.99% hydrogen as diluent gas and nitrogen as inert protection gas, CNTs were grown onto the surface of ACFs in a quartz flow reactor by a home-made tube furnace. The C2 H2 /H2 mixture gases were introduced at 550 ◦ C for a time. In the end, the reactor was cooled down to the room temperature under nitrogen atmosphere.

modification are shown in Fig. 2a and b. Fig. 2c is the HRTEM of modified ACFs. The smooth surface of unmodified ACFs is clearly observed in Fig. 2a, and the morphology of modified ACFs with rough surface and uniformly distributed CNTs are displayed in Fig. 2b. The presence of CNTs on ACFs surface is inferred according to Fig. 2c. Fig. 3a illustrates the pore distribution of ACFs and modified ACFs. The micropore volumes of ACFs before and after modification were 0.4978 cm3 /g and 0.4923 cm3 /g, respectively. However, according to BJH, the mesopore volumes of ACFs before and after modification were 0.031 cm3 /g and 0.174 cm3 /g, respectively. It was obvious that micropores and mesopores dominate the pore for the modified ACFs, while micropores play a predominant role for ACFs. The result indicated that the modification process resulted in the transformation of pore distribution, leading to the increase of the average pore size, which would be in favor of the adsorption of MB. The infrared spectrums of samples are depicted in Fig. 3b. The absorption peaks at 3400 cm−1 , 3436 cm−1 and 3472 cm−1 were corresponding to the stretch vibration of NH2 and NH. The peak at 3741 cm−1 ascribed to the existence of OH. Comparing with ACFs, the absorption intensity of the peak at 3436 cm−1 obviously decreased for modified ACFs, indicating NH2 and NH participated in chemical reactions in the modification process. FTIR spectrum of modified ACFs after adsorption showed some changes of band relative to that before adsorption. The absorption intensity of OH reduced mainly because OH was converted to H+ and H+ was subsequently exchanged with MB [25].

2.3. Characterization methods

3.2. Effect of initial pH

The specific surface area was determined by automatic specific surface analyzer (QUANTACHROME AUTOSORB-1), and pore size distribution was gained by automated pore size analyzer (QUDRASORB SI). FEI Nova Nano SEM230 was used to characterize the morphology of samples. The internal structure was observed by TEM (JEM-3010). Zeta potentials were measured with dynamic potential and particle size analyzer (Coulter Delsa440sx) by adjusting pH. MB concentrations were determined by spectrometry at the wavelength of maximum absorbance at 665 nm using UV-759 ultraviolet and visible spectrophotometer.

It was found by many studies [10,26,27] that pH was an important effect factor on the adsorption properties. The effect of pH on MB adsorption capacity of modified ACFs was studied and the results are illustrated in Fig. 4a. It can be seen from Fig. 4a that the adsorption capacity of MB increased with pH increasing. The adsorption capacity rapidly increased from 288.07 to 295.72 mg/g when the pH of the MB solution increased from 2 to 3. Then, the adsorption capacity slightly increased from 295.72 to 297.93 mg/g when the pH of the MB solution increased from 3 to 9. Zeta potential of modified ACFs could explain this phenomenon. As shown in Fig. 4b, pHPZC , the point of zero charge of modified ACFs, was found to be 2.65. When pH > pHPZC , the modified ACFs surface had negative charges which inclined to adsorb cations in solution. While pH < pHPZC , the modified ACFs surface had positive charges which inclined to adsorb anions [28]. MB molecules produced cations in aqueous solution, and thus the adsorption capacity greatly increased when the pH of solution is above pHPZC . Adsorption mechanism of modified ACFs mainly included as follows: the first was cation exchange, the second was ␲–␲ interactions between bulk ␲ systems on CNTs surfaces and benzene rings of dye molecules and the third was electrostatic force between modified ACFs and MB molecules. When pH < pHPZC , positive charges were produced on the modified ACFs and there was electrostatic repulsive force between modified ACFs and MB molecules. Therefore, the adsorption capacity reduced to minimum when pH was lower than 2, while when pH > pHPZC , the adsorption capacity increased because the modified ACFs surface was negatively charged and thus the electrostatic attraction force between the modified ACFs surface and MB molecules was enhanced.

Fig. 1. The molecular structure of MB.

alcohol and 65–68% HNO3 were also used. All chemicals used in this study were of analytical-laboratory grade. Molecule structure of MB is shown in Fig. 1. It has a maximum visible absorbance at a wavelength of 665 nm. 2.2. Modifying methods

2.4. Adsorption experimental Batch adsorption experiments were performed using 150 mL glass bottles with addition of prescribed modified ACFs and 100 mL MB solution with different concentrations. The glass bottles were sealed and placed in a bathing constant temperature vibrator. Then, the glass bottles were shaken for a time at a rotating speed of 120 r/min. 2 M HCl or 0.5 M NaOH was used to adjust the pH value of the solution, which was measured with a pH meter. The amount of MB adsorption at equilibrium qe (mg/g) was calculated by the following equation: qe =

(C0 − Ce )V m

(1)

where C0 and Ce (mg/L) are the liquid-phase concentrations of dye at initial and equilibrium, respectively; V (L) is the volume of the solution and m (g) is the mass of adsorbent. 3. Results and discussion

3.3. Adsorption isotherm 3.1. Characterizations The specific surface area of modified ACFs was high and reached 435.78 m2 /g. The SEM images of ACFs before and after

The adsorption isotherms of MB on modified ACFs at 298, 308, 318, 328 and 338 K are shown in Fig. 5, respectively. As could be seen in Fig. 5, the equilibrium adsorption capacity increased

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Fig. 3. The pore size distribution and FTIR: (a) the pore size distribution and (b) FTIR.

with the increase of equilibrium MB concentrations at the range of experimental concentration. This could be due to the increase in the driving force from the concentration gradient when the initial MB concentrations increased, which gave rise to the increased adsorption capacity. Fig. 5 also shows that the adsorption capacity increased with the increase of temperature, indicating that the adsorption of MB onto modified ACFs was endothermic. Adsorption isotherm models are commonly used for the description of adsorption behavior and Langmuir and Freundlich adsorption models were widely applied [10,12,29]. The Langmuir model assumes that the adsorption is supposed to be a monolayer adsorption and there is no interaction between the adsorbate molecules. Langmuir isotherm equation is expressed as follows: 1 1 Ce = + Ce qe q0 KL q0

(2)

where Ce (mg/L) is the equilibrium concentration; qe (mg/g) is the adsorption capacity, and q0 and KL are the Langmuir constants related to adsorption capacity and rate of adsorption, respectively. The Freundlich isotherm model postulates that the surface of adsorbent is heterogenous and adsorption belongs to polymolecular layer adsorption. The linear form of the Freundlich equation is Fig. 2. SEM and TEM of ACFs: (a) SEM before modification; (b) SEM after modification and (c) TEM after modification.

ln qe = ln KF +

1 ln Ce n

(3)

where qe is the adsorption capacity at equilibrium (mg/g); Ce is the equilibrium concentration of the MB (mg/L). KF and n are Freundlich constants. The derived Langmuir and Freundlich isotherm models based on the experimental data in Fig. 5 are shown in Fig. 6. From Fig. 6a,

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Fig. 4. Effect of the pH on the adsorption capacities of modified ACFs for MB (a) and Zeta potential of modified ACFs (b). Adsorption experiment-MB concentration: 300 mg/L; adsorbent dose: 0.1 g/100 mL; temperature: 35 ◦ C; equilibrium time: 300 min.

171

Fig. 6. Langmuir (a) and Freundlich (b) isotherms for MB dye adsorption onto modified ACFs at different temperatures.

favorable (0 < RL < 1), or (4) irreversible (RL = 0), and is expressed as follows: the calculated parameters from the regressive analysis of Langmuir isotherm including q0 and KL are given in Table 1. Moreover, studies [10,27,29] demonstrated that another important parameter, RL , namely the separation factor, can also be used to evaluate the type of the adsorption as (1) unfavorable (RL > 1), (2) linear (RL = 1), (3)

RL =

1 1 + KL C0

(4)

where KL is the Langmuir constant and C0 (mg/L) is the highest dye concentration. The values of RL are calculated and listed in Table 1. suggests that Langmuir model closely fitted the experimental data due to R2 ≥ 0.95. The equilibrium adsorption capacity gradually increased from 478 to 521 mg/g when the temperature increased from 298 to 338 K. It suggested that the elevated temperature was favorable to adsorption. KL increased firstly and then decreased with raising temperature. RL values were less than 1 and greater than zero, suggesting a favorable adsorption [10]. Freundlich adsorption isotherm is shown in Fig. 6b and the calculated n and KF by regression analysis are listed in Table 1. Freundlich model well fitted the experimental results with R2 > 0.97. The values of KF increased with the increase of temperature, indicating the increase of temperature was beneficial to adsorption, which was in accordance with the results predicted by Langmuir model. 3.4. Adsorption kinetics

Fig. 5. Adsorption isotherms of MB onto modified ACFs at different temperatures. Adsorption experiments-pH: 7; adsorbent mass: 0.1 g/100 mL; equilibrium time: 300 min.

The adsorption rate is an important parameter in the adsorption process. Therefore, adsorption kinetics was investigated in this paper. The effect of contact time on the adsorption capacity is presented in Fig. 7a. Fig. 7a shows that the adsorption rate was rapid in the initial 60 min, and then progressively became slower

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Table 1 Isotherm parameters for removal of MB by modified ACFs at different temperatures. Isotherms

Parameters

Langmuir

q0 (mg/g) KL (L/mg) KL (L/mol) R2 RL KF [mg/g (L/mg)1/n ] n R2

Freundlich

T (K) 298

308

318

328

338

478 0.375 1.402 × 105 0.9500 0.00881 120 1.46 0.9957

485 0.433 1.619 × 105 0.9835 0.00764 136 1.44 0.9875

485 0.561 2.098 × 105 0.9902 0.00591 161 1.42 0.9804

490 0.682 2.550 × 105 0.9779 0.00486 189 1.42 0.9759

521 0.662 2.475 × 105 0.9725 0.00501 200 1.39 0.9868

and slower with extending the contact time. This is likely because there were a great deal of adsorption active sites on the surface of modified ACFs during the initial time, causing the big diffusion driving force of MB molecules. However, when the contact time extended, the diffusion driving force of MB molecules diminished. On the other hand, the repulsive forces between MB molecules in the solution and MB molecules on the surface of modified ACFs are enhanced, thus the adsorption rate was lessened. After 300 min, the adsorption equilibrium was practically reached. In order to make adsorption mechanism more clear, the adsorption data were fitted using the pseudo-first kinetic, second-order kinetic and the intra-particle diffusion models. The pseudo-firstorder kinetic model presumes the rate of adsorbate occupying activate sites is proportion to the unoccupied activate sites. The pseudo-first-order kinetic model is represented by: ln(qe − qt ) = ln qe − k1 t

(5)

where qe (mg/g) is the equilibrium adsorption capacity, qt (mg/g) is the adsorption capacity at time t, and k1 (min−1 ) is the rate constant of the pseudo-first-order equation. The value of k1 can be calculated from the plot lg(qe − qt ) against t. The pseudo-first-order kinetic

curves are plotted in Fig. 7b. Table 2 lists the relevant parameters obtained by regression analysis. Table 2 shows that the correlation coefficients are bigger than 0.94 and there is a well linear relationship between lg(qe − qt ) and t. The pseudo-second-order kinetic model is as follows: 1 t t = + qt qe k2 q2e

(6)

where k2 [g/(mg min)] is the rate constant of the pseudo-secondorder equation. The pseudo-second-order kinetic model can be applied if the plot of t/qt against t is linear. The pseudo-second-order kinetic curves are plotted in Fig. 7c. The regression parameters are also calculated and listed in Table 2 demonstrates that the values of R2 are larger than 0.9990, indicating that the adsorption better fitted the pseudo-second order kinetic model. This was because the values of R2 were greater and the predicted equilibrium adsorption capacities were more similar to the experimental data than those predicted by the pseudo-first-order kinetic model. Normally, the adsorption process of porous media is very complex and one step or a few steps may decide the adsorption rate and adsorption capacity. The adsorption experimental data were

Fig. 7. The effect of contact time on the adsorption capacity (a); pseudo-first order adsorption kinetics (b); pseudo-second order adsorption kinetics (c) and intra-particle diffusion adsorption kinetics (d). Adsorption experiments-pH: 7; initial concentration: 300 mg/L; adsorbent mass: 0.1 g/100 mL.

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Table 2 Kinetic parameters for removal of MB by modified ACFs at different temperatures. Kinetic models

Parameters

Pseudo-first order

qexp (mg/g) qe (mg/g) k1 (min−1 ) R2 qe (mg/g) k2 /[g/(mg min)] R2 kt C R2

Pseudo-second order

Intraparticle diffusion model

T (K) 308

318

328

338

296.62 175.56 0.0278 0.9724 308.65 0.000336 0.9996 1.215 276.99 0.8410

297.58 129.80 0.0257 0.9437 307.69 0.000398 0.9995 0.735 285.54 0.9207

297.94 120.06 0.0293 0.9454 306.75 0.000490 0.9996 0.430 290.92 0.8946

298.15 86.66 0.0319 0.9801 302.11 0.000949 0.9999 0.364 292.36 0.6562

Table 3 Thermodynamic parameters for MB adsorption onto modified ACFs. T (K)

G (kJ/mol)

H (kJ/mol)

S [J/(mol K)]

R2

308 318 328 338

−30.72 −32.40 −33.95 −34.90

12.52 12.52 12.52 12.52

140.9 140.9 140.9 140.9

0.9785 0.9785 0.9785 0.9785

treated by the intra-particle diffusion model to elucidate the diffusion mechanism, which is expressed as: qt = kt t 1/2 + C

(7)

[mg/(g min0.5 )]

where kt is the intra-particle diffusion rate constant, C is a constant related to the thickness of boundary. The plots of qt against t0.5 are drawn according to the intra-particle diffusion model. It is well known that when the regression is linear and the plot passes through the origin, the intra-particle diffusion is the only rate-controlling step; in contrast when the regression is linear but the plot does not pass through the origin, then the intraparticle diffusion is not the only rate-controlling step [26,30]. The regression is not linear, suggesting that one step or a few steps may control the adsorption rate. Fig. 7d depicts the plots of qt against t0.5 with the intra-particle diffusion model. From Fig. 7d, the regression is multistage linear. The first stage expressed external boundary diffusion and the second stage was intra-particle diffusion. The regression parameters are also presented in Table 2. Table 2 shows that the adsorption is controlled by intra-particle diffusion to some extent because of low R2 values. Furthermore, the second stage line does not pass through the origin, indicating the adsorption process was controlled by not only intra-particle diffusion, but also film diffusion. The values of C increased from 276.99 to 292.36 with the temperature increasing from 308 to 338 K, suggesting the increase of boundary layer thickness.

The decrease from −30.72 to −34.90 kJ/mol in the values of G with the increase of temperature from 308 to 338 K indicated that the adsorption process became more favorable at higher temperatures. The calculated H value of 12.52 kJ/mol indicated that the adsorption reaction was endothermic [10,31]. S was determined as 140.9 J/(mol K), which was due to the displacement of previously adsorbed solvent molecules by the adsorbate species [10], resulting in the increasing randomness at the solid–solution interface during the adsorption process. The changed Arrhenius equation was used to depict the relation between the rate constants k2 of the pseudo-second-order model and the temperatures, as follows: ln k2 = ln k0 −

Ea RT

(10)

3.5. Thermodynamic studies Thermodynamic studies are used to illustrate whether the adsorption process was spontaneous or not. Thermodynamic parameters include G (kJ/mol), H (kJ/mol) and S [J/(mol K)], which can be calculated by the following formulas: G = −RT ln KL

(8)

G = H − T S

(9)

where R is the gas constant, 8.314 J/(mol K), T (K) is the absolute temperature, KL (L/mol) is the Langmuir constants. Fig. 8a shows the plot of G against T. H and S are obtained by linear regression analysis according to Eq. (9). Thermodynamic parameters are calculated and listed in Table 3. Table 3 shows the values of G are negative, suggesting the adsorption process was spontaneous.

Fig. 8. Thermodynamic regression for MB adsorption onto modified ACFs.

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where k2 [g/(mg min)] is the rate constants of the pseudo-secondorder model, k0 is the constant, Ea (kJ/mol) is the activation energy, T (K) is the temperature and R is the gas constant [8.314 J/(mol K)]. The regression curve is plotted in Fig. 8b with ln k2 against 1/T. The activation energy was determined as 28.33 kJ/mol by the slope of the plot. The activation energy of adsorption was less than 40 kJ/mol, implying the adsorption was a fast physisorption and the adsorption could rapidly completed [10]. 4. Conclusions In the present paper, it could be observed that pore size distribution of the modified ACFs had a significant change and the CNTs were well-distributed on the surface of ACFs. Cation exchange was considered to be one of the adsorption mechanisms. Results showed the adsorption capacity of modified ACFs at equilibrium increased with the increase of pH. Langmuir and Freundlich model well fitted experimental data. The adsorption process well fitted pseudo-second order kinetic and was controlled by intra-particle diffusion to some extent. Moreover, the negative values of G indicated that the adsorption process was spontaneous. H was 12.52 kJ/mol, indicating that the adsorption reaction was endothermic. S was 140.9 J/(mol K), indicative of the increasing randomness at the solid–solution interface during the adsorption process. Ea was less than 40 kJ/mol, implying the adsorption was a fast physisorption. Furthermore, the adsorption experimental results showed that the modified ACFs performed high adsorption capacity and could be a promising high efficient adsorbent. Acknowledgements This project was financially supported by the Nation Natural Science Foundation of China (Grant No. 50802115), the Science and Technology Plan Project of Hunan Province (2010FJ4075) and the Science Foundation of Changsha University (CDJJ-10010205). References [1] H.-C. Chiu, C.-H. Liu, S.-C. Chen, S.-Y. Suen, Adsorptive removal of anionic dye by inorganic–organic hybrid anion-exchange membranes, J. Membr. Sci. 337 (2009) 282–290. [2] Ö. Gerc¸el, H.F. Gerc¸el, A.S. Koparal, Ü.B. Ögütveren, Removal of disperse dye from aqueous solution by novel adsorbent prepared from biomass plant material, J. Hazard. Mater. 160 (2008) 668–674. [3] S.D. Khattri, M.K. Singh, Removal of malachite green from dye wastewater using neem sawdust by adsorption, J. Hazard. Mater. 167 (2009) 1089–1094. [4] C. Bradu, L. Frunza, N. Mihalche, S.-M. Avramescu, M. Neata, I. Udrea, Removal of Reactive Black 5 azo dye from aqueous solutions by catalytic oxidation using CuO/Al2 O3 and NiO/Al2 O3 , Appl. Catal. B: Environ. 96 (2010) 548–556. [5] M. Qamar, M.A. Gondal, K. Hayat, Z.H. Yamani, K. Al-Hooshani, Laser-induced removal of a dye C.I. Acid Red 87 using n-type WO3 semiconductor catalyst, J. Hazard. Mater. 170 (2009) 584–589. [6] T. Li, J.T. Guthrie, Colour removal from aqueous solutions of metal-complex azo dyes using bacterial cells of Shewanella strain J18 143, Bioresource Technol. 101 (2010) 4291–4295.

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