Adsorption of methylene blue from aqueous solution onto viscose-based activated carbon fiber felts: Kinetics and equilibrium studies

Abstract

Two viscose-based activated carbon fiber felts (VACFF-1300 and VACFF-1600) with different specific surface areas and pore structures were prepared via two-step carbonization and steam activation and characterized by SEM observation, N₂ adsorption/desorption isotherms, Fourier-transform infrared, X-ray diffraction and X-ray photoelectron spectroscopy analysis. They were used as adsorbents for the removal of methylene blue dye from aqueous solution, and the adsorption equilibrium and kinetics were studied via batch adsorption experiments and the adsorption mechanisms were investigated. Results showed that the equilibrium data for methylene blue adsorption onto VACFF-1300 and VACFF-1600 fitted well to the Langmuir isotherm model, with maximum monolayer adsorption capacity of 256.1 mg/g and 325.8 mg/g, respectively. Besides, the adsorption kinetics study showed that the adsorption of methylene blue onto the two VACFF samples could be best described by the pseudo second-order model. Moreover, the intraparticle diffusion modelling showed that intraparticle diffusion is rate-controlling for both VACFF-1300 and VACFF-1600, and external diffusion is also a rate-controlling step for the latter.

Keywords

Activated carbon fiber felt viscose-based methylene blue adsorption equilibrium adsorption kinetics adsorption mechanism

Introduction

Dyes are widely used by textile industries to color their final products. With the rapid development of the textile industries, water pollution is becoming more and more serious. It is estimated that more than 10,000 tonnes of dyes are consumed in the worldwide textile industry per year and approximately 100 tonnes/year of dyes are discharged into water streams (Yagub et al., 2014). Various major problems concerning textile wastewater are high content of a variety of organic compounds and toxic substances, strongly colored, stable and difficult for biodegradation (Ai and Jiang, 2012; Hameed et al., 2007b). Thus, the treatment of textile wastewater containing various dyes has become an issue of worldwide concern due to its environmental and aesthetic problems. Methylene blue (MB) with hetero-polyaromatic structure, which presents a strong inhibitive function for biologic degradation and is very difficult to be degraded into small inorganic molecules by using common methods, is usually employed as the model dye to study adsorption properties of activated carbon (AC) materials (Ahmed and Dhedan, 2012; Hameed et al., 2007b; Li et al., 2013). In order to compare with other’s work, MB was also used as model dye in this study.

The main treatment technologies for efficient removal of dyes from dye-containing wastewater include biological treatment, ion-exchange, coagulation/flocculation, micro-electrolysis, Fenton process, advanced oxidation processes, ozonization, adsorption, membrane filtration, photocatalysis, as well as some combined techniques, and so on. All these methods can be divided into three categories: physical, chemical and biological, and comparative analyses of their advantages and disadvantages are conducted (Salleh et al., 2011). Among them, adsorption has found to be an efficient and economic process due to its simplicity of operation, low cost, versatility as well as high efficiency. Various absorbents such as montmorillonite (He et al., 2018), zeolite (Jamil et al., 2011), bentonite (Anirudhan and Ramachandran, 2015), kaolin (Mouni et al., 2018), vermiculite (Yu et al., 2015b), molecular sieves (Qun, 2012), AC (Cheng et al., 2018), carbon nanotubes (CNTs) (Loginov et al., 2014), graphene (Yu et al., 2015a) as well as graphene oxide (GO) (Yang et al., 2011) have been used for the removal of MB from aqueous solutions. Among the abovementioned adsorbents, AC is the most widely used adsorbent due to its large specific surface area, low density, chemical stability, suitability for large-scale production and the adjustability of the pore structures. However, the difficult separation of powdered activated carbon (PAC) and granular activated carbon (GAC) from the aqueous solutions is a big problem that restricts their practical application. Compared with PAC and GAC, activated carbon fiber (ACF), which is the third generation of activated carbon products, endows the advantage of shape diversity (cloth, felt, paper, honeycomb structures and corrugated sheets, etc.) while maintaining excellent adsorption property. Therefore, ACF has been widely used as an effective adsorbent in wastewater treatment due to its excellent adsorption property and easy separation from the aqueous solutions (Sun et al., 2014; Wang et al., 2012).

At present, the organic precursor fibers for preparing ACFs mainly include polyacrylonitrile fibers, pitch fibers, viscose fibers as well as phenolic fibers. Among them, viscose fiber has become the earliest organic precursor material for the preparation of ACF due to its low price and abundant resource. Besides, compared with ACFs prepared using other organic precursor fibers, viscose-based ACF is highly cost-effective due to its simplicity of obtaining larger specific surface area and adsorption capacity under the same carbonization and activation conditions. Thus, it is by far the most widely used and most studied ACF.

Most of the viscose-based ACFs reported in the published papers are usually carbonized and activated in a sealed box or tubular high-temperature furnace under a programmed temperature (Bhati et al., 2013, 2014; Plens et al., 2015). Their activation time is usually longer than 1 h and the production is intermittent The activation time is long and it is intermittent batch preparation. In this study, continuous production with a production rate of 1.0–1.8 m/min was realized via two modified semi-open high-temperature furnaces, and a novel two-step carbonization method was adopted. After low-temperature carbonization, short-time high-temperature carbonization is carried out before steam activation to further reduce carbon residue and tar generated due to decomposition of carbon in the carbonized material, which will block the pore formation (Su et al., 2012). Besides, a small amount of oxygen, which is added during steam activation process in the semi-open furnace, can increase the reaction rate several folds (Su and Wang, 2007). In this way, the pore formation was significantly promoted; thus, the activation time could be greatly shortened. Viscose-based activated carbon fiber felts (VACFFs) with different specific surface area and pore structure were prepared and used as adsorbents, and the adsorption behavior of MB onto them were investigated and their adsorption equilibrium and kinetics were studied.

Experimental

Preparation of viscose-based ACF felts

Firstly, viscose fibers with a length of 50–80 mm were used as the raw material and four-passage needle-punching processing technology was adopted to fabricate the nonwoven felts. Viscose fiber felt with a thickness of 5.2 mm and a gram weight of 500 g/m² was obtained via adjusting the input and output speeds of each needle-punching machine and their needling density. Secondly, the prepared viscose fiber felt was employed as precursor to prepare VACFF by using a steam activation method. After being impregnated by diammonium phosphate with a concentration of 7.5 wt% for 1 h, the viscose fiber felt was squeezed to a pick up ratio of 200% and dried at 110°C before being placed in a modified high-temperature furnace for low-temperature carbonization at 300°C for 30 min. Then, the carbonized felt was taken out and placed in the other modified high-temperature furnace to carry out high-temperature carbonization at 800°C for 5 min in the back heat zone and steam activation at 900°C for 5–7 min by inletting water vapor in the front heat zone. VACFF samples with different specific surface area and pore structure were prepared by regulating activation time and amount of activating gases through adjusting the conveying speed and the flow rate of water vapor. The detailed preparation process is illustrated in Figure 1.

Figure 1.

Schematic illustration of preparation steps of VACFF samples.

VACFF-1300 activated under a water vapor flow of 110 m³/h for 5 min and VACFF-1600 activated under a water vapor flow of 125 m³/h for 7 min were employed in this study. The suffix numbers give the approximate Brunauer–Emmett–Teller (BET) surface area of VACFF in m²/g. The yield of VACFF-1300 and VACFF-1600 was measured to be 18.1% and 15.3%, respectively. The two VACFF samples were cut into small pieces and washed several times with deionized water, dried at 100°C for 24 h and stored for further studies.

Characterization of VACFFs

The surface morphology of the two VACFF samples was observed using an LEO 1530 EP field emission scanning electron microscope (LEO Electron Microscopy Group, Germany).

Textural characterization of the two VACFF samples was carried out by nitrogen adsorption at 77 K using an ASAP 2020 automatic physisorption analyzer (Micromeritics Instrument Corp., USA). The specific surface area (S_BET) was calculated by the BET equation, the total pore volume (V_p) was determined at P/P₀ = 0.98, the desorption average pore diameter (4V/A by BET) (L) was determined by the Barrett–Joyner–Halenda method, and the pore size distributions were calculated by the density functional theory (DFT) method.

An X-ray diffraction (XRD) analysis of the two VACFF samples was carried out on a Rigaku SmartLab 9 kW diffractometer (Rigaku, Japan) equipped with Cu Kα X-ray source in the 2θ angle ranging from 5° to 80° incremented by a 0.02° step.

Fourier-transform infrared (FTIR) spectroscopy measurements were conducted via potassium bromide tableting technique using a Nicolet 6700 FTIR Spectrometer (Thermo Scientific, USA) over the wave range from 4000 to 400 cm⁻¹.

X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo ESCALAB 250Xi (Thermo Scientific Ltd, USA) spectrometer, using a monochromatic Al Kα source (1486.6 eV) at constant pass energy of 50 eV and steps of 0.1 eV. The pressure in the analysis chamber was maintained below 1 × 10⁻⁹ Torr for data acquisition and the spectra were calibrated by C 1s (284.8 eV) before determining the distribution of surface functional groups.

The pH at the point zero charge (pH_PZC) of the two VACFF samples was measured using the batch equilibrium method (Altenor et al., 2009). For this purpose, 50 mL of a 0.01 M NaCl solution was placed in 125 mL glass conical beaker. Then, the pH was adjusted to successive initial values between 2 and 12, by using either NaOH or HCl, and then the 0.15 g adsorbent (VACFF-1300 or VACFF-1600) was added into each beaker. After the beaker was shaken at 120 r/min for 48 h in a water bath oscillator at room temperature, the final pH (pH_final) was measured with a pH/CON 510 Benchtop meter (OAKTON, USA) and plotted against the initial pH (pH_initial). The pH at which the curve crosses the line pH_final = pH_initial is taken as the pH_PZC of the two VACFF samples.

Preparation of MB solutions

MB (Figure 2) were acquired from Fisher Scientific Co., Ltd (USA) and used as received. A stock solution of 1000 mg/L was prepared by dissolving accurately weighed samples of dye in a liter of distilled water. The experimental solutions were prepared by diluting stock solution of MB with distilled water to the desired concentration. The pH value of each solution was adjusted to be 6.0 ± 0.1 by adding 0.1 M HCl and/or 0.1 M NaOH and measured with a pH/CON 510 Benchtop meter (OAKTON, USA).

Figure 2.

The chemical structure of MB.

Batch equilibrium studies

The influences of initial dye concentration (100–500 mg/L, step size: 50 mg/L) on the removal of MB were studied in a batch mode. In each adsorption experiment, the 50 mL samples of MB solution with known concentration were put in a 125 mL glass conical beaker, and then the 25 mg adsorbent (VACFF-1300 or VACFF-1600) was added into the beaker. The beaker was shaken for 72 h in a thermostatic water bath oscillator at 30 ± 0.5°C to reach equilibrium. Then, 5 mL of the solution was withdrawn and centrifuged at 7000 r/min for 6 min to precipitate the small amount of chalked VACFF dispersed in the solution during long time shaking. The concentration of MB in the solution was obtained by analyzing the supernatant using a UV–Visible spectrophotometer (Thermo scientific Evolution 600) at 665 nm. The adsorption capacity at equilibrium, q_e (mg/g) and removal efficiency, R (%) were calculated using the following equations

q_{e} = (\frac{C_{0} - C_{e}}{W}) \times V

(1)

R (%) = (\frac{C_{0} - C_{e}}{C_{0}}) \times 100

(2)

where C₀ and C_e are initial and equilibrium concentrations of MB (mg/L), respectively, W was the mass of adsorbent (g) and V was the volume of the solution (L).

Batch kinetic studies

The effect of contact time on MB removal was conducted by adding 25 mg of adsorbent (VACFF-1300 and VACFF-1600) into 50 mL solution with MB concentration of 200 mg/L in a 125 mL glass conical beaker. The concentration of MB in the solution was measured at predetermined time intervals using the abovementioned method and the amount of adsorbed MB at time t, q_t (mg/g), was calculated by

q_{t} = (\frac{C_{0} - C_{t}}{W}) \times V

(3)

where C₀ is the initial MB concentration (mg/L), C_t is the MB concentration at time t (mg/L).

Mathematical modeling

Isotherm models

Equilibrium data for MB adsorption onto VACFF samples were fitted by the two most commonly used isotherm models, the Freundlich and Langmuir isotherm models (Ai and Jiang, 2012).

The Freundlich isotherm is an empirical equation based on an exponential distribution of adsorption sites and energies. It is expressed as

\ln q_{e} = \ln k_{F} + \frac{1}{n} \ln C_{e}

(4)

where q_e is the adsorption capacity at equilibrium (mg/g), C_e is the equilibrium concentration of MB (mg/L) and k_F [(mg/g)/(L/mg)ⁿ] and n are Freundlich constants related to adsorption capacity and adsorption intensity, respectively, with k_F representing the relative adsorption capacity of the adsorbent and n representing the degree of dependence of adsorption on the C_e of MB (Zamani and Tabrizi, 2014).

The Langmuir isotherm assumes a surface with homogeneous binding sites, equivalent sorption energies and no interaction between adsorbed species. It is expressed as

\frac{C_{e}}{q_{e}} = \frac{C_{e}}{q_{max}} + \frac{1}{q_{max} k_{L}}

(5)

where q_e is the adsorption capacity at equilibrium (mg/g), C_e is the equilibrium concentration of MB (mg/L) and q_max (mg/g) and k_L (L/mg) are Langmuir constants related to adsorption capacity and rate of adsorption, respectively, with q_max representing the maximum monolayer coverage of adsorbent with adsorbate and k_L representing the enthalpy of the adsorption (Zamani and Tabrizi, 2014).

Kinetic models

In order to investigate the mechanism of adsorption, four kinetic models, namely, pseudo-first-order, pseudo-second-order, Elovich equation and Intraparticle diffusion models, were analyzed.

The pseudo-first-order equation is expressed as follows

log (q_{e} - q_{t}) = \log q_{e} - \frac{k_{1}}{2.303} t

(6)

where k₁ is the rate constant of adsorption (1/min).

The pseudo-second-order model is represented by the following linear form

\frac{t}{q_{t}} = \frac{1}{k_{2} q_{e}^{2}} + \frac{t}{q_{e}}

(7)

where k₂ is the pseudo second-order rate constant of adsorption [g/(mg·min)].

The Elovich model is described by the following equation

q_{t} = \frac{1}{β} \ln α β + \frac{1}{β} \ln t

(8)

where

α

is the initial adsorption rate [mg/(g·min)] and

β

is the desorption constant (g/mg).

Intraparticle diffusion model is described using the following equation

q_{t} = k_{int} t^{0.5} + C

(9)

where k_int is the rate constant of intraparticle diffusion, [mg/(g·min^0.5)], C is the intercept of the linear plot of q_t against t^0.5, which is proportional to the thickness of the boundary layer and can be used to judge whether intraparticle diffusion is the only rate-controlling step or not.

Results and discussion

Surface morphology

The SEM observation results of the two prepared VACFF samples are shown in Figure 3. It can be seen from Figure 3 that the surfaces of VACFF-1300 and VACFF-1600 are both rich in grooves arranged in parallel along the axial direction and there is no clear difference between them with regard to appearance.

Figure 3.

SEM images of VACFF samples: (a) VACFF-1300; (b) VACFF-1600.

Textural characteristics

The N₂ adsorption isotherms for the two VACFF samples are given in Figure 4. It was found that both VACFF samples exhibit a type I adsorption isotherm. The pore volume of VACFF-1300 was filled below a relative pressure of about 0.1, and kept almost constant at higher relative pressure, indicating that it was highly microporous. For VACFF-1600, the filling of the pore volume took place at a high relative pressure as well as below a relative pressure of about 0.1, which suggests that it contained both mesopores and micropores within its structures. The existence of the more significant adsorption hysteresis of VACFF-1600 also confirmed this.

Figure 4.

The N₂ adsorption isotherms for VACFF samples.

The detailed porous texture parameters of the two VACFF are listed in Table 1. As shown in Table 1, the BET-specific surface area of VACFF-1300 and VACFF-1600 are 1284 and 1614 m²/g, respectively. They both have a highly microporous structure with an average pore diameter less than 2 nm and a total pore volume of 0.54 and 0.72 cm³/g, respectively. The DFT analysis shows that the micropore ratio of VACFF-1300 and VACFF-1600 are 97.3% and 89.4%, respectively, and VACFF-1600 has much larger portion of mesopores than the VACFF-1300. This is further confirmed by the pore size distribution as shown in Figure 5.

Figure 5.

The pore size distribution of VACFF samples.

Table 1.

Porous texture parameters of VACFF samples.

Adsorbent	S_BET (m²/g)	V_p (cm³/g)	L (nm)	DFT analysis
Adsorbent	S_BET (m²/g)	V_p (cm³/g)	L (nm)	Micropore (%)	Mesopore (%)	Macropore (%)
VACFF-1300	1284	0.54	1.67	97.3	1.90	0.87
VACFF-1600	1614	0.72	1.77	89.4	9.90	0.89

VACFF: viscose-based activated carbon fiber felt; DFT: density functional theory.

During carbonization, the random arrangement of aromatic rings in the internal structure results in cracks, thus forming pores, but the pore volume and specific surface area are still small. During steam activation at 900°C, the low-density non-crystalline region is burnt off to produce micropores. Then, the activating gases, water vapor mixed with a small amount of air, attack the carbon atoms in the aromatic ring structure to form numerous pores, gaps and cracks, thus increasing the pore volume and specific surface area (Su et al., 2012). Compared with VACFF-1300, higher degree of activation was achieved for VACFF-1600 with increased activation time and amount of activating gases; thus, the porous development of VACFF-1600 was promoted and a much higher pore volume and specific surface area were obtained.

XRD analysis

Figure 6 shows the XRD patterns of the two different VACFF samples. It can be observed from Figure 6 that there are two diffraction peaks appeared near 2θ = 24° and 2θ = 44° for each VACFF sample, indicating the presence of graphite-like crystallites. The strong diffraction peak at 24° shows a carbon layer (002) structure, while the relatively weak diffraction peak at 44° is also a carbon layer (101) structure. Both diffraction peaks are broad, which proves the disordered graphite structure of the two VACFF samples. There is little variation in the positions (2θ) of the two VACFF samples in the diffraction peak. The interlayer spacing of main characteristic crystal (d₀₀₂) is calculated by Bragg equation and the grain size is calculated by Scherrer equation using full width at half maximum (FWHM) and 2θ. The results of XRD analysis are listed in Table 2. It can be seen from Table 2 that interlayer spacing (d₀₀₂) of VACFF-1300 and VACFF-1600 is 0.3689 nm and 0.3821 nm, respectively, which is larger than 0.3354 nm of ideal graphite crystallites. The larger the L_c value, the thicker the graphite-like crystal is; the larger the L_c/d value, the denser the structure arrangement is. Compared with VACFF-1300, VACFF-1600 has larger interlayer spacing (d₀₀₂) and L_c value but smaller L_c/d value, indicating that the graphite-like crystal within VACFF-1600 is thicker and formed a relatively looser structure. This can be used to explain why VACFF-1600 has a high pore volume from a microstructural perspective.

Figure 6.

XRD patterns of VACFF samples.

Table 2.

The results of XRD analysis of VACFF samples.

Adsorbent	2θ (°)	d₀₀₂ (nm)	FWHM (°)	β₀₀₂ (rad)	L_c (nm)	L_c/d
VACFF-1300	24.1	0.3689	9.056	0.158	0.8870	2.404
VACFF-1600	23.3	0.3821	8.998	0.157	0.8914	2.333

FWHM: full width at half maximum.

Surface chemistry

The FTIR spectra of two VACFF samples are shown in Figure 7. It can be seen from Figure 7 that the spectra of the two VACFF samples are nearly the same; they both showed –OH stretching vibration at 3435 cm⁻¹ for VACFF-1300 and 3436 cm⁻¹ for VACFF-1600 due to the existence of surface hydroxyl groups and chemisorbed water. The band at 1635 cm⁻¹ for both samples is attributed to the stretching vibration of –C=O.

Figure 7.

FTIR spectra of VACFF samples.

In order to further identify the elemental constitution and surface functional groups of the two VACFF samples, XPS analyses were carried out. The XPS survey spectra are presented in Figure 8. The XPS survey spectra of two VACFF samples indicated that the predominant surface elements were carbon and oxygen with the significant C 1s and O 1s peaks at binding energy around 285 eV and 533 eV, respectively. Weak N 1s peak at 400.9 eV was also observed, confirming the existence of small amount of N-containing functional groups. According to the area simulating curve, the atomic ratios of O/C, N/C and (O + N)/C are estimated and listed in Table 3. As expected, VACFF-1600 prepared with increased activation time and amount of activating gases displayed higher oxygen content (O/C%) than VACFF-1300, increased by 0.63%. These results revealed that increasing degree of activation could effectively increase the O-containing functional groups on the surface of VACFF.

Figure 8.

XPS survey spectra of VACFF samples.

Table 3.

The atomic ratio of VACFF samples.

Adsorbent	O/C (%)	N/C (%)	(O+ N)/C (%)
VACFF-1300	8.86	2.07	10.93
VACFF-1600	9.49	2.20	11.69

For the sake of a better understanding of the types and relative contents of the O-containing functional groups in the two VACFF samples, the major C 1s peak was deconvoluted into five individual curves from the following groups (Figure 9): (A) graphitized carbon at 284.8 eV; (B) C–O in hydroxyl groups, phenol, alcohol or ether around 286.2 eV, (C) C=O in carbonyl groups around 287 eV, (D) O–C=O in carboxyl or ester groups around 288 eV and (E) π–π* transitions in the aromatic carbon at 290.5 eV. According to the area integration curves, the relative contents of each component are calculated and listed in Table 4.

Figure 9.

High resolution C 1s spectra of VACFF samples and fitting curves: (a) VACFF-1300 and (b) VACFF-1600.

Table 4.

Deconvolution results of C 1s spectra of VACFF samples.

Peak	VACFF-1300			VACFF-1600
Peak	Position (eV)	FWHM (eV)	Relative contents (%)	Position (eV)	FWHM (eV)	Relative contents (%)	Assignment
A	284.79	1.12	72.69	284.78	1.17	73.68	C–C
B	286.18	1.1	10.82	286.18	1.17	11.63	C–O
C	287.12	1.26	6.51	287.12	1.36	5.74	C=O
D	288.71	1.52	4.99	288.71	2.13	6.22	O–C=O
E	290.56	2.48	4.99	290.58	1.69	2.73	π–π*

VACFF: viscose-based activated carbon fiber felt; FWHM: full width at half maximum.

It can be seen from Table 4 that the majority of the O-containing functional groups were acidic groups such as carboxyl, lactone and phenolic groups. Therefore, the ACF was acidic. This was further confirmed by the pH at the point zero charge (pH_PZC) of the two VACFF samples, which were determined to be 5.09 and 5.14 for VACFF-1300 and VACFF-1600, respectively, as shown in Figure 10. Therefore, effective adsorption sites can be provided by these O-containing functional groups on VACFF surface.

Figure 10.

The pH_PZC of VACFF samples.

Figure 11.

Effect of initial MB concentrations on their removal by VACFF samples (C₀=100–500 mg/L, dosage = 25 mg/50 mL, equilibrium time = 72 h, pH = 6.0 ± 0.1, temperature = 30 ± 0.5°C).

Effect of initial MB concentration on the removal of MB

Adsorption isotherms are usually determined under equilibrium conditions. A series of equilibrium adsorption experiments for MB dye were carried out at different initial concentrations (100–500 mg/L, step size: 50 mg/L). Figure 6 shows the effect of the initial MB concentrations on the removal efficiency and adsorption capacity at equilibrium (q_e) of the two VACFF samples. As can be seen from Figure 11, with an increase in the initial MB concentration from 100 to 500 mg/L, the q_e of VACFF-1300 and VACFF-1600 increased significantly at initial stage and reached a dynamic equilibrium gradually, while the removal efficiency decreased exponentially. This is because the driving force for the adsorption of MB onto VACFF is mainly the concentration gradient of MB molecules. The higher the initial concentration of MB, the larger the driving force provided, which will finally improve the amount of MB molecules adsorbed. However, due to the limited adsorption sites of VACFF, once it reached saturation, no more MB molecules will be adsorbed, even under larger adsorption force. Thus, the q_e of both VACFF samples maintained as nearly unchanged after the initial MB concentration exceeded 300 mg/L and the removal efficiency continued to decline.

It is evident that the VACFF is efficient to adsorb MB dye from aqueous solution, and the process attained equilibrium gradually. In order to further clarify the adsorption mechanism of the two VACFF samples, the adsorption isotherms and adsorption kinetics as well as intraparticle diffusion were analyzed next.

Adsorption isotherms

Adsorption isotherms of the two VACFF samples were analyzed according to the linear form of Freundlich and Langmuir models using equations (4) and (5), respectively as shown in Figure 12. The values of the constants of the two isotherms equations and the relative coefficients (R²) were determined by linear regression and listed in Table 5. The lower R² of the Freundlich equation suggests that the adsorption of MB by the two VACFF samples do not follow the Freundlich isotherm model. The much higher R² close to 1 of the Langmuir equation suggests that the Langmuir isotherm model yields a somewhat better fit than the Freundlich isotherm model. As also illustrated in Table 5, the value of 1/n is 0.0669 and 0.0732 for ACFF-1300 and ACFF-1600, respectively, and they are both below 1, which indicates a normal Langmuir isotherm (Hameed et al., 2007b). Moreover, the calculated q_max values (256.13 mg/g for ACFF-1300 and 325.83 mg/g for ACFF-1600) are very close to the experimental values obtained from Figure 12, suggesting that the Langmuir equation can be used to fit the experimental adsorption data well. Conformation of the experimental data into Langmuir isotherm model indicates that a monolayer coverage of MB has taken place on the homogeneous surface of the two VACFF samples. Similar observation was reported by the adsorption of MB onto bamboo AC (Hameed et al., 2007b) and by the adsorption of direct dyes on AC prepared from sawdust (Malik, 2004) and by the adsorption of Congo red dye on AC prepared from coir pith (Namasivayam and Kavitha, 2002).

Figure 12.

Adsorption isotherms for the adsorption of MB onto VACFF samples: (a) Freundlich model; (b) Langmuir model (C₀=100–500 mg/L, dosage = 25 mg/50 mL, equilibrium time = 72 h, pH = 6.0 ± 0.1, temperature = 30 ± 0.5°C).

Table 5.

Freundlich and Langmuir adsorption isotherm constants for MB adsorbed by VACFF adsorbents.

Adsorbent	Freundlich			Langmuir
Adsorbent	1/n	k_F [(mg/g)/(L/mg)ⁿ]	R ²	q_max (mg/g)	k_L (L/mg)	R ²
VACFF-1300	0.0669	183.69	0.5725	256.13	1.5485	0.9989
VACFF-1600	0.0732	224.71	0.8433	325.83	3.7627	0.9996

VACFF: viscose-based activated carbon fiber felt.

The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless equilibrium parameter (R_L) (Hameed et al., 2007a; Wu et al., 2014), which is defined by

R_{L} = \frac{1}{1 + k_{L} C_{0 max}}

(10)

where k_L is the Langmuir constant (L/mg) and C_0max is the highest initial dye concentration (mg/L). The value of R_L indicates the type of the isotherm to be either unfavorable (R_L > 1), linear (R_L = 1), favorable (0 < R_L < 1) or irreversible (R_L = 0). Value of R_L was found to be 0.0013 and 0.0005 for VACFF-1300 and VACFF-1600, respectively. This further confirmed that the both the VACFF samples are favorable for adsorption of MB dye under conditions used in this study.

In order to evaluate the adsorption capacity of MB adsorbed by the two VACFF samples, the adsorption capacity of several common adsorbents reported in other literatures were compared with this study. The theoretical maximum adsorption capacity (q_max) of various adsorbents is listed in Table 6. Although such a comparison may not be completely comprehensive, it can be seen that the use of VACFF can be an effective way to remove MB from aqueous solution. It can also be seen that VACFF-1600 with larger pore volume and specific surface area is more competitive to other counterparts.

Table 6.

Maximum adsorption capacity (q_max calculated from Langmuir model) of MB adsorbed by various adsorbents.

Adsorbent	Experimental conditions				References
Adsorbent	Dose (g/L)	Temp (K)	C₀ (mg/L)	q_max (mg/g)	References
VACFF-1300	0.5	303	100–500	256.13	This study
VACFF-1600	0.5	303	100–500	325.83	This study
Bamboo-based AC	1	303	100–500	454.20	(Hameed et al., 2007b)
Agricultural wastes-based AC	0.5	303	50–450	398.19	(Ahmed and Dhedan, 2012)
Raw AC	2	303	200–600	178.25	(Cheng et al., 2018)
Fe-Ce-AC	2	303	200–600	255.76	(Cheng et al., 2018)
AC	0.5	293	40–120	270.27	(Li et al., 2013)
CNTs	0.5	293	40–120	188.68	(Li et al., 2013)
GO	0.5	293	40–120	243.90	(Li et al., 2013)
Graphene	0.5	293	20–120	153.85	(Liu et al., 2012b)

VACFF: viscose-based activated carbon fiber felt; AC: activated carbon; GO: graphene oxide; CNT: carbon nanotube.

Adsorption kinetics

In order to evaluate the effectiveness of the two VACFF adsorbents, the plots of the amount of MB adsorbed, q_t, as a function of the contact time at an initial MB concentration of 200 mg/L is presented in Figure 13. It can be seen from Figure 13 that the initial adsorption stage is rapid for both VACFF-1300 and VACFF-1600, which is due to the adsorption of the molecules on the external surface of them. The following stage is a slow adsorption process, which is attributed to the slow diffusion of the dye molecules into the porous structure of the adsorbent since the majority of the available external active sites have already been occupied at initial stage (Li et al., 2013). Compared with VACFF-1300, the adsorption rate of VACFF-1600 is much faster due to its larger specific surface area as well as larger mesopore ratio (Table 1).

Figure 13.

Effect of contact time on the amount of MB adsorbed by VACFF samples (C₀=200 mg/L, dosage = 25 mg/50 mL, equilibrium time = 50 h, pH = 6.0 ± 0.1, temperature = 30 ± 0.5°C).

Aiming to further understand the detailed characteristics of the adsorption process, adsorption kinetics of MB onto the two VACFF samples were analyzed according to the pseudo-first-order, pseudo-second-order and Elovich kinetic models using equations (6) to (8), respectively, as shown in Figure 14. The kinetic parameters and the relative coefficients (R²) were determined by linear regression and listed in Table 7.

Figure 14.

Adsorption kinetics for the adsorption of MB onto VACFF samples: (a) pseudo-first-order model; (b) pseudo-second-order model; (c) Elovich model.

Table 7.

Adsorption kinetics parameters of MB adsorption onto the two VACFF samples.

Kinetic model	Parameters	Values
Kinetic model	Parameters	VACFF-1300	VACFF-1600
Pseudo-first-order model	q_e (mg/g)	271.28	118.78
	k₁ (1/min)	0.0014	0.0018
	R ²	0.8555	0.9559
Pseudo-second-order model	q_e (mg/g)	262.74	321.32
	k₂ [g/(mg·min)] $\times$ 10^–6	7.3095	44.1899
	R ²	0.9747	0.9998
Elovich model	$α$ [mg/(g·min)]	1.7602	48.1469
	$β$ (g/mg)	0.0197	0.0246
	R ²	0.9471	0.9373

VACFF: viscose-based activated carbon fiber felt.

It can be observed that compared with the pseudo-first-order and Elovich models, the R² of the pseudo-second-order model are larger for both VACFF-1300 and VACFF-1600 adsorbents and reached a value of 0.9747 and 0.9998, respectively. Besides, the calculated q_e values obtained from pseudo-second-order equation also appeared to agree very well with the experimental q_e data, which is 253.39 mg/g for VACFF-1300 and 325.06 mg/g for VACFF-1600. Based on q_e values and R², the pseudo-second-order kinetic model was chosen as more appropriate for the kinetic modelling of MB adsorbed by the two adsorbents. The pseudo-second-order model assumes that two reactions are occurring in the adsorption process, the first one is fast and reaches equilibrium quickly, and the second is a slower reaction that can continue for long time periods (Zamani and Tabrizi, 2014). Besides, the much larger k₂ values also confirmed the much faster adsorption rate of VACFF-1600 than VACFF-1300.

Intraparticle diffusion analysis

The adsorption of adsorbate from solution by solid porous adsorbent is usually carried out according to three consecutive mass transport steps (Hameed et al., 2007b; Kannan and Sundaram, 2001): (a) the adsorbate migrates through the boundary layer via film diffusion onto the external surface of the adsorbent, i.e. external diffusion; (b) the adsorbate moves from the external surface into the interior pores via pore diffusion, i.e. intraparticle diffusion; (c) the adsorbate is adsorbed onto the active sites at the interior porous surface of the adsorbent. Due to the high porosity of the two VACFF samples (Table 1), intraparticle diffusion was expected in the adsorption process. In order to further clarify the rate-controlling step, the adsorption process of MB onto the two VACFF samples was analyzed by the intraparticle diffusion model using equation (9), as shown in Figure 15. The corresponding fitting parameters and the relative coefficients (R²) were determined by linear regression and listed in Table 8.

Figure 15.

Intraparticle diffusion modelling for the adsorption of MB onto VACFF samples.

Table 8.

Intraparticle diffusion model constants for MB adsorbed by VACFF adsorbents.

Adsorbent	k_int [mg/(g·min^0.5)]	C (mg/g)	R ²
VACFF-1300	4.5157	19.11	0.9943
VACFF-1600 (stage 1)	4.4632	20.35	0.9929
VACFF-1600 (stage 2)	1.8552	225.69	0.8666

VACFF: viscose-based activated carbon fiber felt.

It can be seen from Figure 15 and Table 8 that the experimental data of VACFF-1300 showed a very high linear relative coefficients R² of 0.9943, confirming the existence of intraparticle diffusion and indicating that intraparticle diffusion is rate-controlling. Moreover, the relatively larger k_int values suggest a larger intraparticle diffusion rate. Besides, the much lower C value suggests that the influence of the boundary layer on the adsorption of MB onto VACFF-1300 is small and the external diffusion is not a rate-controlling step. However, the experimental data of VACFF-1600 showed a multistage linear line. According to Table 8, the C value was 20.35 mg/g at the first stage and it increased to 225.69 mg/g at the second stage, which indicated that the boundary layer had a significant influence on the adsorption of MB on the VACFF-1600. Thus, the capacity of VACFF-1600 was exhausted at the second stage and the diffusion became more difficult, resulting in slower adsorption confirmed by the much smaller k_int values. The first stage can be attributed to the boundary layer diffusion of MB molecules, i.e. external diffusion, while the second stage can be attributed to the intraparticle diffusion (Liu et al., 2012b). In other words, the adsorption of MB on VACFF-1600 is thought to have taken place probably via external adsorption until the surface functional sites are fully occupied (stage 1); thereafter, MB molecules diffuse into the pores of VACFF-1600 for further adsorption (stage 2). The R² of the linear fitted line at the first stage and second stage is 0.9929 and 0.8666, respectively, and both lines in the plot did not pass through the origin, indicating that both external diffusion and intraparticle diffusion are the rate-controlling steps for the adsorption of MB on the VACFF-1600.

Adsorption mechanism

As previously confirmed by FTIR and XPS results, there are plenty of O-containing groups such as carboxyl, lactone and phenolic groups on the surface of the two VACFF samples. The pH_PZC is determined to be 5.09 for VACFF-1300 and 5.14 for VACFF-1600, respectively. Thus, under the working pH of 6.0, the surfaces of the two VACFF samples are negatively charged due to deprotonated acidic groups. The oxygen atoms have a large affinity to positively charged molecules and cations because of strong electrostatic interactions. MB is a typical cationic dye; thus, electrostatic interactions are clearly involved in MB adsorption onto VACFF. On the other hand, the chemical structure of MB has rich aromatic rings and cationic atoms, which are favorable for adsorbing on the surface of VACFF through π–π stacking and ionic interaction (Liu et al., 2012a). Similar conclusions were drawn that the adsorption of MB on the graphene adsorbents is mainly due to both electrostatic interaction and π–π stacking interaction (Li et al., 2013; Zamani and Tabrizi, 2014).

Conclusions

In summary, the two VACFFs prepared via steam activation are both favorable for adsorption of MB from aqueous solution. The equilibrium data for MB adsorption onto VACFF-1300 and VACFF-1600 fitted well to the Langmuir isotherm model, with maximum monolayer adsorption capacity of 256.13 mg/g and 325.83 mg/g, respectively, indicating that a monolayer coverage of MB has taken place on the homogeneous surface of the two VACFF samples. The adsorption kinetics studies showed that the adsorption of MB onto two VACFF samples could be best described by the pseudo-second-order model, indicating a two-stage adsorption process and VACFF-1600 showed a much faster adsorption rate than VACFF-1300. The intraparticle diffusion analysis showed that due to different pore size distributions, the intraparticle diffusion is the only rate-controlling step for the adsorption of MB on VACFF-1300, while both external diffusion and intraparticle diffusion are the rate-controlling steps for the adsorption of MB on the VACFF-1600. Adsorption mechanism study reveals that electrostatic interaction, π–π stacking and ionic interaction are involved in MB adsorption onto VACFF.

Footnotes

Declaration of Conflicting Interests

The author(s) declare(s) that there is no conflict of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was financially supported by National Key R&D Program of China (Grant No. 2018YFC0810303 and Grant No. 2016YFC0204204), National Natural Science Foundation of China (Grant No. 51702167), “Innovative Talent Funds” supported project of Nantong University (Grant No. CXZR201503), 2017 Graduate student scientific research innovation project of Jiangsu Province (Grant No. KYCX17-1927) and 2018 Jiangsu college students' innovation and entrepreneurship training project (Grant No. 201810304025Z).

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