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Abstract

Multi-walled carbon nanotubes (MWCNTs) encapsulated by polyaniline (PANI) were synthesized by in situ polymerization. Scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy and thermal gravimetric analysis (TGA) were used to characterize the synthesized composites (O-MWCNTs/PANI), and the surface area was calculated by the Brunauer–Emmett–Teller (BET) method. The removal capacity of alizarin yellow R (AYR) with O-MWCNTs/PANI was further investigated. Experiments were conducted to optimize the adsorption conditions, including contact time, pH, initial concentration of AYR and temperature. The results showed that the maximum adsorption capacity for AYR was 884.80 mg/g. The adsorption kinetics and the adsorption isotherm could be better described by the pseudo-second-order model and the Langmuir isotherm, respectively. Energy changes revealed that the adsorption process was exothermic and spontaneous in nature. Additionally, the O-MWCNTs/PANI showed higher adsorption capacity than pristine MWCNTs or PANI. Therefore, O-MWCNTs/PANI would be applied as an efficient adsorbent for the removal of dye from water.

Keywords

Multi-walled carbon nanotubes polyaniline alizarin yellow R adsorption azo dyes

Introduction

In recent years, a large quantity of pollutants containing dyes and pigments from the waste water pose a great threat to environmental safety. Dyes are categorized according to their chemical structure (mainly according to the structure of conjugated system) as azo, phthalocyanine, anthraquinone, indigo, nitro and nitroso methane dye (van der Zee and Villaverde, 2005). Among dyes, azo dyes characterized by the presence of one or more azo bonds (–N=N–) are the largest and most versatile class of dyes. It is reported that more than half of the annually produced amount of dyes (estimated for 1994 worldwide as one million tons) are azo dyes (Santos et al., 2003). These dyes have been widely used in various industries like food, textile, cosmetic, paper, paint, printing and pharmaceutical (Gholivand et al., 2015). In this work, alizarin yellow R (AYR), a mordant dye, which is applied in the dyeing of wool and nylon, was chosen as the absorbate, and its maximum adsorption wavelength is 370 nm (Salman et al., 2011).

Discharge of waste water containing dyes into natural ecosystems is aesthetically and environmentally unacceptable because of the strong persistent color and high biological oxygen demand loading (O’Neill et al., 1999). Some synthetic dyes possessing complex aromatic structure are carcinogenic for aquatic life and difficult to biodegrade (Fewson, 1988; Fu and Viraraghavan, 2001). Moreover, the dye residues can result in allergies and skin irritation in humans, via bio-accumulating in fauna and transferring to higher food chain (Bajc et al., 2011; Kooh et al., 2015). Therefore, the removal of dyes from industrial effluents becomes a major environmental challenge. So far, various biological treatments and physicochemical methods have been extensively investigated for removal of dyes from waste water, such as photocatalysis (Hoffmann et al., 1995; Rahman et al., 2013), chemical precipitation (Kadirvelu et al., 2003), electrochemical degradation (Rajkumar et al., 2007; Sirés and Brillas, 2012), ozonation (Mahmoodi, 2013), advanced oxidation processes (Guimaraes et al., 2012), membrane filtration (Unsal et al., 2015; Zularisam et al., 2006) and adsorption (Thommes and Cychosz, 2014; Yan et al., 2014). However, these techniques have their own limitations, including the excessive usage of chemicals, accumulation of secondary concentrated sludge, expensive plant requirements and high operational costs. Compared with the other techniques, adsorption has been found to be a superior technology for the dye-polluted water treatment, due to the high efficiency, relative simplicity of design, easier operation without harmful residues and the low operational cost (Wang et al., 2014). Nowadays, various materials as adsorbents, with high adsorption capacity, have been applied to remove dyes from water.

Carbon nanotubes (CNTs) have been proven as promising adsorbents for removing various environmental pollutants due to their unique quasi-one-dimensional hollow structure, high specific surface areas and good chemical and thermal stability (Tasis et al., 2006; Thommes and Cychosz, 2014). Moreover, their adsorption capacities and dispersibility could be increased by introduced different functional groups onto their surface, such as hydroxy (Chen et al., 2015), carboxyl group (Sang et al., 2012), polymers (Xie et al., 2005) and so on. Among which polyaniline (PANI) has attracted great attention in the world of research due to their well-behaved electrochemistry, easy protonation reversibility, excellent environmental stability, ease of doping, less energy requirement, good dispersion and abundant of amino (Konyushenko et al., 2006; Wei et al., 2014). Recently, multi-walled carbon nanotubes (MWCNTs) doped with PANI have been used to remove dyes from aqueous solutions (Ayad and El-Nasr, 2010).

The objective of this work is to investigate the adsorption behavior between the PANI-modified MWCNT composites and AYR. Fourier transform infrared (FT-IR) spectroscopy, thermal gravimetric analysis (TGA), scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET) were used to characterize the properties of the materials, and the role of some adsorption conditions such as contact time, initial pH values and solution temperature was studied. The adsorption isotherms, adsorption kinetics, adsorption thermodynamics were also investigated. Compared with the pristine MWCNTs or PANI, MWCNTs modified by PANI composites exhibited higher adsorption efficiency.

Experimental

Materials and reagents

MWCNTs (specific surface area 80–140 m²/g, diameter 20–40 nm), the purity of which was more than 97%, were purchased from Shenzhen Nanotech Port Co., Ltd. Aniline monomer and AYR were supplied by Tianjin Guangfu Technology Development Co., Ltd. Ammonium persulfate was obtained from Tianjin Hengxing Chemical Reagent Co., Ltd. All chemicals were of analytical reagent grade and used without further purification, and all the solutions were prepared by deionized water.

Preparation of oxide MWCNTs

Five hundred milligrams of raw MWCNTs were ultrasonically treated with 50.0 mL of nitric acid for 10 min. Then the mixture suspension was stirred by magnetic and refluxed at 120℃ for 24 h, which produced carboxyl groups at the defect sites of MWCNTs and thus improved the solubility of the MWCNTs in hydrochloric acid (HCl) solution. After cooling to 20℃, the product was filtered through a 0.45 µm PTFE membrane and washed with deionized water until the pH was neutral, then dried under a vacuum at 60℃ for 24 h.

Preparation of PANI grafted O-MWCNT (O-MWCNTs/PANI)

The oxide MWCNTs (O-MWCNTs)/PANI composites were synthesized by in situ chemical oxidation polymerization. First, 2.5 mL of aniline monomer was dissolved in 50.0 mL; 0.6 mol/L HCl solution and 400 mg of O-MWCNTs were added into the solution and ultrasonicated for 30 min at room temperature to obtain uniform dispersion of the O-MWCNTs, then transferred into a 250 mL flask with an ice-bath. Second, a solution containing 5.7 g ammonium persulfate, an initiator for the polymerization reaction of aniline monomer, and 25.0 mL deionized water was slowly added dropwise into the suspension with constant magnetic stirring at a reaction temperature of 0℃–5℃ for an additional 3 h. Then, the resulting green suspension was filtered and rinsed several times with HCl, deionized water and acetone, respectively. Finally, the O-MWCNTs/PANI were dried under a vacuum at 60℃ for 24 h. Neat PANI was synthesized via the same steps without the O-MWCNTs, and the PANI coated MWCNTs (MWCNTs/PANI) was prepared following the same steps with the additions of raw MWCNTs. Figure 1 shows the process for the modification of the MWCNTs.

Figure 1.

Schematic diagram of the modification of the MWCNTs.

Characterization methods

Microstructure and morphology of the absorbents were measured by using a SEM (MIRA3 TESCAN). FT-IR spectroscopy was conducted on a Nicolet 6700 FT-IR spectrometer. Thermogravimetric analysis (TGA) was recorded on a SDT Q600 V8.0 Build 95 thermal analyzer in the temperature range of 25℃–600℃ with a heating rate of 10℃/min in argon media. The specific surface area of materials was determined by a BET method on a JW-BK132F apparatus at 77 K by N₂ adsorption–desorption isotherms.

Adsorption studies

A series experiments were conducted to study the influence of crucial parameters like contact time, pH, temperature and initial dye concentration. The absorbance (at 370 nm) of AYR solution was measured by using a UV/VIS spectrophotometer. Prior to the measurement, a calibration curve, to estimate the concentration of AYR, was plotted over a range of standard AYR solutions.

To study the kinetic adsorption, the flask fitted with 20.0 mL dye solution of initial concentration of 100.0 mg/L, and 5.0 mg of adsorbent was shaken for different contact time (0 to 50 min) at 25℃ in a shaking thermostatic bath. The concentration of dye solution was determined at different contact time. Each data point was acquired from an individual flask. The dye adsorption capacities of adsorbent were calculated according to the following equation

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

(1) where q_e (mg/g) is the adsorption capacity of absorbent. C₀ and C_e (mg/L) are the initial and equilibrium concentrations of dye in the supernatant after separation, respectively. V(L) is the volume of the solution, and m(g) is the weight of adsorbent.

The effect of solution pH on dye removal was investigated by mixing 5.0 mg of O-MWCNTs/PANI to a dye solution of 200.0 mg/L in separate experiments at different pH levels (6–10). The pH was adjusted using HCl (0.10 mol/L) and NaOH (0.10 mol/L) solutions. Agitation was done for 2 h, and then the final dye concentrations were measured.

For adsorption isotherm, 20.0 mL of dye solutions of different concentrations (100.0–350.0 mg/L) were continuously shaken with 5.0 mg of adsorbent for 2 h to obtain complete equilibrium at 25℃, 35℃ and 45℃, respectively. Then, the residual concentration of AYR in the supernatant was tested.

Adsorption kinetics

The adsorption kinetics is one of the most important parameters for evaluating the adsorption efficiency. Three principal kinetic models, the pseudo-first-order, pseudo-second-order and intraparticle diffusion model (Robati, 2013), were used in this study to interpret the kinetic experimental rate, and predict the rate-limiting step.

The pseudo-first-order equation is expressed as

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

(2) where q_e (mg/g) and q_t (mg/g) are the adsorption capacity at equilibrium and any time t (min), respectively. k₁ (min⁻¹) is the pseudo-first-order rate constant.

The pseudo-second-order reaction is based on the weight of absorbate adsorbed on the adsorbent and is represented as the following form

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

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

The intraparticle diffusion model is expressed as

q_{t} = K_{3} t^{0.5} + C

(4) where K₃ (mg/g·min^0.5) is the intraparticle diffusion rate constant and C (mg/g) is a constant for any experiment.

Adsorption isotherm

A series of experiments was carried out to study the adsorption behavior of AYR on O-MWCNTs/PANI. The most common adsorption models were used to analyze the experimental data, namely Langmuir, Freundlich isotherms and Dubinin–Radushkevich isotherm (Dada, 2012; Garbovskiy, 2016). The Langmuir adsorption isotherm presumes that monolayer adsorption takes place at specific homogeneous sites of the adsorbent. The model of Langmuir isotherms was applied for the adsorption equilibrium as follows

\frac{C_{e}}{q_{e}} = \frac{1}{b \times q_{m}} + \frac{C_{e}}{q_{m}}

(5) where q_e (mg/g) is the amount of dye adsorbed by per unit mass of absorbent at equilibrium. C_e (mg/L) is the equilibrium concentration of absorbate. q_m (mg/g) is the maximum adsorption capacity, and b (L/mg) is the constant of Langmuir related to the affinity between the absorbent and the absorbate.

The Freundlich isotherm is a fairly satisfactory empirical equation which can be used for multilayer adsorption that involves heterogeneous surface energy systems.

This isotherm is generally given by

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

(6) where k_f (L/mg) is physical constant which indicates of adsorption capacity and n is an indicator of the favorability of the adsorption process. The adsorption process is considered favorable if 1 < n < 10.

The Dubinin–Radushkevich isotherm applied to distinguish the physical and chemical adsorption is expressed as

\ln q_{e} = \ln q_{m} - β ɛ^{2}

(7) where β (mol²/KJ²) is a constant and ɛ is the Polanyi potential which can be calculated from the following equation

ɛ = RT \ln (1 + \frac{1}{Ce})

(8)

The mean free energy E (KJ/mol) can be computed using the following relationship

E = \frac{1}{\sqrt{2 β}}

(9)

If the value of E is less than 8 KJ/mol, the adsorption process is physical in nature. If it is higher than 8 KJ/mol, the adsorption process is chemical.

Adsorption thermodynamics

Thermodynamic studies were conducted to explicate the mechanism of the adsorption (Chowdhury et al., 2011). Different thermodynamic parameters such as standard free energy ( $Δ G^{Θ}$ ), enthalpy ( $Δ H^{Θ}$ ) and entropy ( $Δ S^{Θ}$ ) for the adsorption of AYR on O-MWCNTs/PANI were estimated using the following Van Hoff thermodynamic equations (Hoff et al., 1996)

Δ G^{Θ} = - RT \ln K_{c}

(10)

- RT \ln K_{c} = Δ H^{Θ} - T Δ S^{Θ}

(11)

\ln K_{c} = \frac{Δ S^{Θ}}{R} - \frac{Δ H^{Θ}}{RT}

(12) where K_c is the equilibrium distribution coefficient for the adsorption process calculated by the follow equation

K_{c} = \frac{C_{0} - C_{e}}{C_{e}}

(13)

R (8.314 J/mol·K) is the gas constant, and T (K) is the absolute temperature. The values $Δ H^{Θ}$ and $Δ S^{Θ}$ were determined from the slope and intercept of the plot $\ln K_{c}$ versus $1 / T$ , respectively.

Results and discussion

Characterization of absorbent

The uniform deposition of PANI on the MWCNTs is demonstrated by SEM, which shows the typical core–shell structure of coated MWCNTs (Figure 2). It can be clearly observed that the surface of O-MWCNTs/PANI (Figure 2(b)) is smoother than the surface of pure PANI (Figure 2(c)). It may explain why the adsorption capacity of O-MWCNTs/PANI is higher than that of pure PANI. Compared with the O-MWCNTs (Figure 2(a)), we conclude that the aniline monomer is uniformly polymerized on the surface of the O-MWCNTs and forms a shell of tubular composite.

Figure 2.

SEM images of (a) O-MWCNTs; (b) O-MWCNTs/PANI and (c) PANI.

FT-IR spectra of O-MWCNTs, O-MWCNTs/PANI and pure PANI were showed in Figure 3 in the range of 500–4000 cm⁻¹. The characteristic stretching vibrations of υ(OH) and υ(C=O) at 3440 cm⁻¹ and 1752 cm⁻¹ appear in FT-IR spectrum of O-MWCNTs, which would be an important active site for adsorption of aniline monomer onto O-MWCNTs for the further in situ polymerization reaction (Feng et al., 2003). For the O-MWCNTs/PANI and pure PANI, the peak at 1290 cm⁻¹ is due to the stretching vibrations υ(C–N) of the amide (Zhang et al., 2004). The peaks at about 3400 cm⁻¹ are attributed to the stretching vibration of the secondary amine υ(–NH–). In additional, the comparison of the FT-IR spectra of O-MWCNTs with O-MWCNTs/PANI and pure PANI nanocomposite illustrates that new bands are appeared at 1466 cm⁻¹ and 1560 cm⁻¹ which are assigned to C=C stretching vibrations of the quinoid and benzenoid rings on the PANI chain, respectively (Wu and Lin, 2006). It indicates that PANI was covalently attached to the O-MWCNTs. The functional groups can increase the adsorption capacity of material by providing more adsorption sites. Further, the peaks in the pure PANI at 1107 cm⁻¹ and 1239 cm⁻¹ are attributed to the Ar-N-Ar and –N=Q=N– stretch (Q is the quinoid ring), respectively. However, these two peaks in the O-MWCNTs/PANI shift to 1103 cm⁻¹ and 1235 cm⁻¹, the red-shift indicating the favorable interaction between the PANI and O-MWCNTs. In other words, with the existence of O-MWCNTs, the ability of electron delocalization on the PANI chain was strengthened. It is another reason which verifies that the adsorption capacity of O-MWCNTs/PANI would be better than that of pure PANI. These peaks, illustrating the successful coating of the PANI on O-MWCNTs, are the typical peaks of PANI.

Figure 3.

The FT-IR spectra of O-MWCNTs, O-MWCNTs/PANI and PANI.

The TGA, identifying the thermal stability of materials and the amount of polymers grafted onto the MWCNTs, was conducted on a thermal analyzer in the temperature region of 25℃–600℃. In Figure 4, the TGA curves of the O-MWCNTs, O-MWCNTs/PANI and PANI are presented. The weight loss below 200℃ should be owing to the deintercalation of water in external surface and internal pores or cavities. The PANI decomposed almost entirely at about 470℃, illustrating that about 35.1 wt% of PANI was loaded on the surface of O-MWCNTs. Moreover, the BET-specific surface areas of MWCNTs, O-MWCNTs/PANI and PANI are 90.749, 59.623 and 59.203 m²/g, respectively.

Figure 4.

The TGA curves of O-MWCNTs, O-MWCNTs/PANI and PANI.

Effect of pH

The pH of the solution, an important controlling parameter in the adsorption process, could affect aqueous chemistry and surface binding sites of the adsorbents. Therefore, in order to study the effect of initial pH on the removal of AYR, experiments were conducted in the pH range of 6.7–10.0. It can be seen from Figure 5 that the quantity of adsorption increased with increasing the pH value from 6.7 to 8.5 and then decreased when the pH value increased from 8.5 to 10.0, that is, the optimal adsorption ability exhibited at pH = 8.45.

Figure 5.

Effect of pH on the adsorption of AYR by O-MWCNTs/PANI at 25℃.

This trend can be explained by considering the structure of the dye and the absorbent. Research shows that PANI could selectively adsorb dyes in different forms, that is, protonated PANI emeraldine salts (ES) could preferentially adsorb anionic dyes and deprotonated PANI emeraldine base (EB) for cationic, respectively (Ayad and El-Nasr, 2010). Figure 6 shows the different forms of PANI and their conversion mechanism. Compared with the PANI-ES, the PANI-EB is easier to selectively adsorb AYR, owing to the electrostatic interactions between the functional groups in dye and PANI-EB. With the increase of the pH value, the PANI-ES was gradually transformed into PANI-EB which contributed to increasing the adsorption capacity, that is, at pH < 8.5, the quantity of adsorption increased with the pH value increasing. Meanwhile, AYR containing azo group was converted into AYR with quinone group, which became anionic dye (Figure 7). Therefore, at pH > 8.5, the quantity of adsorption decreased because the negatively charged PANI-EB provided fewer effective sites for adsorption, owing to the increase of the repulsive forces. Thus, the removal of AYR from aqueous solutions should be conducted at the pH 8.5.

Figure 6.

Conversion mechanism between the protonated PANI emeraldine salts (ES) and the deprotonated PANI emeraldine base (EB).

Figure 7.

Conversion mechanism between the AYR containing azo group and the AYR with quinone group.

Adsorption kinetics

The study of contact time for the adsorptive removal of AYR dye from aqueous solutions is essential, because it is important to determine the time duration required to obtain complete equilibrium, and the results are shown in Figure 8(a). As can be seen from Figure 8(a), when the contact time was increased, the amount of adsorbed dye was drastically increased during the first 5 min, which illustrated that the O-MWCNTs/PANI could rapidly adsorb AYR. After the first 5 min, the quantity of adsorption gradually increased between 5 and 50 min and no longer changed, which indicated the equilibrium reached, and the quantity of adsorption was at the maximum value. Therefore, all experiments’ agitation time were set for 120 min.

Figure 8.

(a) Effect of time on the adsorption of AYR onto O-MWCNTs/PANI; (b) Pseudo-second-order kinetic for the adsorption AYR onto O-MWCNTs/PANI and (c) The fitting line of intraparticle diffusion model.

To understand more about the adsorption process, such as the mechanism of adsorption, chemical reaction, diffusion control and mass transfer, which represent the efficiency of the process, the kinetics data were fitted into three kinetics models, and the parameters of the three kinetic equations were shown in Table 1. It was found that the correlation coefficient (R²) for the pseudo-second-order adsorption rate equation had high value (R² = 0.9981), which was higher than the R² value (R² = 0.3897) of the pseudo-first-order adsorption rate equation and the R² value (R² = 0.6961) of the intraparticle diffusion model. Furthermore, a very large difference existed between experimental adsorption capacity (q_e = 396.50 mg/g) and calculated adsorption capacity (q_m = 99.16 mg/g) for the pseudo-first-order model, while the calculated q_m value for the pseudo-second-order model was 395.26 mg/g, which was agreed very well with the experimental data 396.50 mg/g. Figure 8(c) shows the fitting lines of intraparticle diffusion model. There was no point passing through the origin indicated that the intraparticle diffusion was not the rate-controlling step. Therefore, the adsorption kinetic study of AYR onto O-MWCNTs/PANI fitted well with the pseudo-second-order adsorption rate equation. Based on the parameters of kinetic model, a conclusion was obtained, that is, the adsorption of AYR onto O-MWCNTs/PANI was chemisorptions mechanism with an electrostatic attraction.

Table 1.

Kinetic parameters for the adsorption of AYR.

Model	Parameter	Parameter value
First-order kinetic	q_e (mg/g)	99.159
	K₁ (min⁻¹)	0.0779
	R ²	0.3897
Second-order kinetic	q_e (mg/g)	395.26
	K₂ (g·mg⁻¹ min⁻¹)	0.00242
	R ²	0.9981
Intraparticle diffusion	C (mg/g)	352.32
	K₃ (mg/(g·min^0.5))	4.989
	R ²	0.6961

Adsorption isotherm

Isotherm parameters and the curve-fitting lines of the Langmuir, Freundlich and Dubinin–Radushkevich isotherms were illustrated in Table 2 and Figure 9, respectively. Obviously, the Langmuir equation with higher coefficients (R² = 0.9988–0.9996) than the Freundlich equation (R² = 0.8015–0.9197), which indicated that the experimental data could be better explained by the Langmuir isotherm. Meanwhile, the values of q_m obtained from Langmuir isotherm was fitted well with the experimental adsorption capacity, but the q_m calculated from the Freundlich isotherm did not agree with the experimental value. Langmuir isotherm is more suitable to explain the monolayer adsorption manner of the adsorbate on the adsorbent. This may be due to the homogeneous surface of the modified nanoparticles with all energetically identical adsorption sites. As seen from Table 2, the values of E (E = 14.14, 13.61, 11.62 KJ/mol for 298 K, 308 K, 318 K, respectively) suggested that the adsorption process was chemical adsorption.

Table 2.

Fitting results with Langmuir, Freundlich and Dubinin–Radushkevich isotherm for AYR.

Model	Parameter	Parameter value
Model	Parameter	25℃	35℃	45℃
Langmuir isotherm	q_m (mg/g)	840.336	826.446	781.250
	b (L/mg)	0.504	0.445	0.621
	R ²	0.9988	0.9988	0.9996
Freundlich isotherm	1/n	0.108	0.121	0.140
	k (L/mg)	505.496	469.393	414.905
	R ²	0.8015	0.9197	0.8418
Dubinin–Radushkevich isotherm	q_m (mg/g)	864.308	827.814	822.813
	Β (mol²/KJ²)	0.0025	0.0027	0.0037
	E (KJ/mol)	14.14	13.61	11.62
	R ²	0.8288	0.9817	0.9747

Figure 9.

(a) Adsorption isotherms of AYR; (b) Langmuir isotherms for the adsorption of AYR by O-MWCNTs/PANI and (c) Dubinin–Radushkevich isotherms for the adsorption of AYR onto O-MWCNTs/PANI.

Adsorption thermodynamics

Physical adsorption and chemical adsorption are the two typical types of the adsorption on solids, but the differentiation between them is not clear. The thermodynamics parameters (Table 3) about energy changes provide useful information of the adsorption process. Compared with the chemical adsorption, physical adsorption is nonspecific, whose energy variation is usually smaller than that of chemical adsorption (Gha1edi et al., 2011). As seen from Table 3, the negative value of

Δ H^{Θ}

confirmed the adsorption of AYR onto the O-MWCNTs/PANI was an exothermic reaction, that is, lower temperature was conducive to the adsorption of AYR onto O-MWCNTs/PANI.

Table 3.

Thermodynamic parameters for the adsorption of AYR onto O-MWCNTs/PANI.

ΔH^Θ (kJ/moL)	ΔS^Θ (J/K·moL)	T (K)	ΔG^Θ (kJ/moL)	R ²
−6.403	−14.898	298	−1.964	0.99986
		308	−1.814
		318	−1.665

Moreover, negative $Δ G^{Θ}$ at all temperatures suggested that the adsorption process of AYR onto the active adsorption sites offered by the O-MWCNTs/PANI was spontaneous in nature. Negative $Δ S^{Θ}$ reflected the affinity of the O-MWCNTs/PANI for the dye and the decrease of randomness at the solid/liquid interface occurring in the internal structure during adsorption process.

Contrast experiments

The contrast experiment was investigated by fitting the flask with 20.0 mL dye solution of initial concentration of 50.0 mg/L and 5.0 mg of MWCNTs, O-MWCNTs, MWCNTs/PANI, O-MWCNTs/PANI, respectively. For another contrast experiment, 20.0 mL of dye solutions of 200.0 mg/L were continuously shaken with 5.0 mg of PANI and O-MWCNTs/PANI. It can be seen from Figure 10(a), because of the higher dispersity and more emerging active sites of the O-MWCNTs/PANI, the adsorption capacity of which was higher than those of the other three absorbents. Although the BET-specific surface area of O-MWCNTs/PANI was lower than MWCNTs, the ability of the electron delocalization contributed to the more efficient adsorption and higher adsorption capacity. Meanwhile, due to the lower surface area of O-MWCNTs, the adsorption capacity of which was lower than that of MWCNTs (Vuković et al., 2010). In Figure 10(b), the adsorption capacity of the PANI (566.8 mg/g) and the O-MWCNTs/PANI (715.8 mg/g) was presented. Obviously, owing to the existence of O-MWCNTs, the ability of the electron delocalization on the PANI chain was strengthen, and the adsorption capacity of O-MWCNTs/PANI was superior to that of PANI. A comparison of the present O-MWCNTs/PANI with some other absorbents is shown in Table 4, which indicated that the adsorption capacity of O-MWCNTs/PANI is higher than the other potential adsorbents (Gul et al., 2016; Gupta et al., 2005; Salman et al., 2011; Zolgharnein et al., 2014).

Figure 10.

(a) Contrast adsorption experiments at the initial concentration of 50.0 mg/L: (1) r-MWCNTs, (2) O-MWCNTs, (3) MWCNTs/PANI and (4) O-MWCNTs/PANI and (b) Contrast adsorption experiments at the initial concentration of 200.0 mg/L: (1) PANI and (2) O-MWCNTs/PANI.

Table 4.

The adsorption capacities of several adsorbents for AYR.

Materials	Dyes	Qm (mg/g)	Ref
Acid-treated wood charcoal	AYR	14.93	Salman et al. (2011)
Nano γ-alumina	AYR	39.0	Zolgharnein et al. (2014)
Fe₃O₄ supported chitosan-graphene oxide composite (Fe₃O₄©-GO)	AYR	14.82	Gul et al. (2016)
Municipal solid waste incinerator bottom ash	AYR	23.84	Gupta et al. (2005)
O-MWCNTs/PANI	AYR	884.80	Present

AYR: alizarin yellow R; O-MWCNTs/PANI: oxide-multi-walled carbon nanotubes/polyaniline.

Conclusion

The O-MWCNTs/PANI were successfully obtained and fully characterized. The adsorption of AYR onto the O-MWCNTs/PANI was investigated, and the result showed that O-MWCNTs/PANI could effectively remove AYR from aqueous media. The O-MWCNTs/PANI exhibited a high adsorption capacity of 884.80 mg/g for AYR at an initial concentration of 300 mg/L. The adsorption kinetics showed that the experimental data could be better explained by the pseudo-second-order kinetics model. Adsorption isotherm processes indicated that the Langmuir isotherm was good for explaining the AYR adsorption. Furthermore, thermodynamics parameters revealed that the adsorption process was exothermic and spontaneous in nature. Hence, it can be concluded that O-MWCNTs/PANI are an effectively absorbent for removing AYR from water. However, for large-scale adaptation, the cost effectiveness of the adsorbent needs to be further reduced.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts 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 supported by the National Natural Science Foundation of China (No. 21571191 and No. 21471163) and Provincial Natural Science Foundation of Hunan (2016JJ1023).

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Multi-walled carbon nanotubes modified by polyaniline for the removal of alizarin yellow R from aqueous solutions

Abstract

Keywords

Introduction

Experimental

Materials and reagents

Preparation of oxide MWCNTs

Preparation of PANI grafted O-MWCNT (O-MWCNTs/PANI)

Characterization methods

Adsorption studies

Adsorption kinetics

Adsorption isotherm

Adsorption thermodynamics

Results and discussion

Characterization of absorbent

Effect of pH

Adsorption kinetics

Adsorption isotherm

Adsorption thermodynamics

Contrast experiments

Conclusion

Footnotes

Declaration of Conflicting Interests

Funding

References