Abstract

Hollow tubular structured kapok fibers (Ceiba pentandra) were coated with polyaniline (PANI) molecules using an in situ oxidative polymerization technique. The tubular morphology of the kapok fibers was retained after PANI coating. The Fourier transform infrared (FT-IR) spectrum of the PANI-coated kapok fibers illustrated the vibration modes associated with the presence of PANI molecules. The PANI-treated kapok fibers achieved complete wettability with water molecules (zero water contact angle) from initially being highly hydrophobic (contact angle = 120°). In the present work, the removal of contaminants such as methyl orange dye and Cu(II) from aqueous solution using polyaniline-coated kapok fibers was investigated. Isotherm studies show that the removal of methyl orange dye (R2 ≥ 0.959) and Cu(II) (R2 ≥ 0.972) using PANI-coated kapok fibers follow the Langmuir isotherm model with maximum sorption capacities determined to be 75.76 and 81.04 mg/g, respectively. Based from thermodynamic studies, the sorption of methyl orange dye and Cu(II) are endothermic, feasible and spontaneous. Furthermore, kinetic studies show that the both processes follow a pseudo-second-order model, implying that the rate-determining step is chemisorption.

INTRODUCTION

Natural fibers with hollow microtubular structure such as Platanus acerifolia seed fibers, populus seed fibers, milkweed fibers and kapok fibers have been the subject of various researches (Rengasamy et al. 2011; Likon et al. 2013; Siswoyo et al. 2014). This is because natural fibers are abundant, renewable, cheap and lightweight with high surface area-to-volume ratio. Natural fibers have been commonly used as substrate or template of different functional materials (Zheng et al. 2014; Agcaoili et al. 2017; Cao et al. 2017). However, free-standing or template-free functional materials have the tendency to agglomerate, which would minimize the surface contact area between the functional materials and aqueous environment. In order to minimize aggregation, functional material can be immobilized onto a substrate or template. This immobilization also results to the strategic distribution of the material over a specific surface area.

Kapok fibers, which are categorized as lignified organic seed fiber, are also characterized by its hollow structure with an oval cross-section, thin wall (0.8–1.0 μm in wall thickness and 8–10 μm in diameter), wax-covered smooth surface and porosity of more than 80% (Mwaikambo & Bisanda 1999; Chung et al. 2008; Xiang et al. 2013). Moreover, the fibers are considered to be excellent template material due to their natural microtubular structure. The excellent capacity of kapok fibers to separate oil, chemical oxygen demand (COD) and turbidity are attributed to its hydrophilic–oleophilic surface (Rahmah & Abdullah 2010). However, the hydrophobic characteristics of kapok limit the possible application as sorbent in aqueous media. In order to modify the surface characteristics, kapok fibers can be subjected to chemical or physical treatment.

Polyaniline (PANI), which is a type of conjugated polymer, has relatively good thermal and environmental stability, and high electrical conductivity (Anand et al. 1998; Gospodinova & Terlemezyan 1998; Eftekhari et al. 2017). Moreover, it is cost-effective and non-toxic. Emerladine salt, which is one of the useful forms of PANI, is characterized by its highly protonated form and relatively high conductivity, which can offer different possible applications such as fabrication of anti-electrostatic materials, conductive adhesive, conductive ink, and conductive paint (Xu et al. 2015). PANI molecules are rich in reactive nitrogen-based groups such as amino and imine groups, which becomes advantageous in serving as binding sites for sorption applications. However, application of pure PANI is limited due to its low surface polymer area. Modification of PANI into nanomaterials or composite materials helps overcome this limitation and improve other physical and chemical properties. PANI and PANI-incorporated based materials have been developed in the removal of various contaminants such as pigments (Ahmed et al. 2016; Laabd et al. 2016; Sharma et al. 2016) and heavy metals (Daraei et al. 2012; Li et al. 2014; Liu et al. 2016).

Copper (Cu), which is considered an inorganic pollutant, is generated by the pulp and paper, and metal plating industries (Volesky & Holan 1995). It is also found in fungicides, insecticides, and fertilizers (Volesky & Holan 1995). It is a well-known indispensable trace element that plays a vital role in enzymatic activities (Xie et al. 2017). However, high uptake of Cu can prove to be toxic that can cause various illnesses such as autism, tardive dyskinesia, hepatic and renal dysfunctions, gastrointestinal irritation, and hypertension (Pfeiffer & Mailloux 1987). Methyl orange (MO), which is a type of anionic azo dye, is characterized by its complexity of organic components, stable chemical quality, resistance to microbial degradation and high chromaticity (Xin et al. 2015). MO dye is commonly utilized by various food industries, pharmaceuticals, textiles, paper mills and printing shops (Hassandazeh-Tabrizi et al. 2016). The presence of dyes in water bodies leads to production of hazardous intermediates after undergoing oxidation, which are toxic to aquatic organisms (Li et al. 2017). In addition, coloring material in the water system reduces light penetration that decreases the photosynthetic activity of aquatic organisms (Mittal et al. 2007). Dyes contained in wastewater effluents have been determined to have caused health problems in humans due to their mutagenic and carcinogenic effects (Wang et al. 2005).

Sorption is one of the most practical treatment methods, especially if large volume of effluent is being considered. The process has several advantages such as economic feasibility, simplicity of design, ease of operation, and high efficiency without production of sludge and other harmful by products (Unnithan & Anirudhan 2001; Fu & Wang 2011). Previous studies utilized organic and natural sorbents for the removal of Cu(II) and MO, such as surfactant-modified silkworm exuviae (Chen et al. 2011), graphene oxide (Robati et al. 2016), goethite (Munagapati et al. 2017), polyacrylonitrile-coated kapok (Agcaoili et al. 2017), chemically-modified orange peel (Feng et al. 2009), peanut hull (Zhu et al. 2009), and tartaric acid-modified rice husk (Wong et al. 2003).

The main objective of the present work is to evaluate the polyaniline-coated (PANI-coated) kapok fibers in the removal of Cu(II) and MO from aqueous solution under static conditions. The kapok fibers were coated with PANI molecules using an oxidative-type in-situ technique. Specifically, this study aims to characterize the coated fibers using Fourier transform infrared (FT-IR) spectroscopy, water contact angle measurement and scanning electron microscopy (SEM). Equilibrium data were analyzed using isotherm models including Langmuir, Freundlich and Dubinin-Radushkevich equations. In addition, experimental data were fitted using kinetic equations and thermodynamic parameters, including ΔG°, ΔH° and ΔS°, were determined.

EXPERIMENTAL

Synthesis of polyaniline-coated kapok fibers

In 100 mL distilled water, 1.20 g NaClO2 (80% AR, Sigma Aldrich) was dissolved with 1.90 mL glacial CH3COOH (99.7%, AR, Macron Fine Chemical). Then, 1.5 g kapok fibers were immersed in the solution and the mixture was stirred at 700 rpm for 60 min at 90 °C. The treated fibers were washed with distilled water and dried at 60 °C.

The coating process involves dissolution of 2.0 g aniline (95%, AR, Loba Cheme) in 66.0 mL of 1.0 M HCl (37% fuming, Sigma Aldrich) and immersing 40 g kapok fibers in the solution. Then, mixture was stirred for 30 min in an ice bath. On the other hand, the oxidant solution was prepared by mixing 4.0 g (NH4)2S2O8 (98%, AR, HIMEDIA) in 16.0 mL of 1.0 M HCl placed in an ice bath. Then, the oxidant solution was added in a dropwise manner into the kapok-aniline mixture. Then, the mixture was stirred for an hour and left to stand for 16 h at 25 °C. The treated polyaniline-coated fibers were filtered and washed using distilled water until the supernatant became colorless. Then, the fibers were washed using ethanol (95%, AR, Ajaxfinechem), air dried for 24 h and dried in an oven at 60 °C for 24 h. The FT-IR spectra (Prestige-21 Shimadzu) of the kapok fibers were recorded with wavelength of 400–4,000 cm−1. Morphological characterization of kapok and modified kapok fibers was done using scanning electron microscope (JEOL JSM 5310).

Sorption experiment

Sorption experiments were carried out by adding 20 mL solution and a determined mass of PANI-coated kapok in a 125-mL Erlenmeyer flask that was agitated for 24 h. The effect of mass of PANI-coated kapok (20 to 50 mg) and pH of the solution (2.0 to 10.0) on the sorption capacity were determined. The adjustment of the pH solution was performed using 0.1 M HCl or NaOH.

Isotherm studies were performed where 30.0 mg PANI-coated kapok was added in 20.0 mL solution at different initial concentration (40 to 280 ppm) where aliquot was agitated for 24 h. The initial pH of the solution for Cu(II) and MO is pH 4.3 and 6.5, respectively. Then, the treated effluent was filtered and analyzed for residual contaminant. For the thermodynamics experiments, 30.0 mg PANI-coated kapok were mixed in a 20 mL solution and agitated for 8 h at 90 rpm under varying temperature (25 to 70 °C). For kinetic studies, a mixture of 30.0 mg PANI-coated kapok and 20.0 mL solution was agitated using a shaker bath at pre-determined time intervals (0 to 24 h).

The residual MO concentration was analyzed using ultraviolet-visible spectrophotometer (UV-Vis 1700 Shimadzu, Japan) at λ = 464 nm. The quantitative analysis of Cu(II) was performed using an ICP-OES Perkin Elmer DV 2000 series. The sorption capacity at any time t, qt was calculated using Equation (1): 
formula
(1)
where V (mL) refers to the total volume of the solution, M (g) is the adsorbent mass, C0 (mg/L) and Ct (mg/L) are the initial concentration and residual concentration of contaminant, respectively.

RESULTS AND DISCUSSION

Characterization of polyaniline-coated kapok fibers

Figure 1(a) shows the FT-IR spectrum of the polymer-coated sample, while Table 1 summarizes the peaks of the spectrum with the corresponding interpretations. The presence of the peaks at 1,578 and 1,489 cm−1 refers to the strong presence of quinoid and benzenoid rings, respectively (Trchova & Stejskal 2011). The peak at 1,300 cm−1 suggests the presence of amine group, which intermediates the two benzenoid rings as seen in the molecular structure of PANI emeraldine salt in Figure 2 (Trchova & Stejskal 2011). The peaks at 1,404 and 1,146 cm−1 show the presence of positively charged imine group (iminium), which intermediates a quinoid ring and benzenoid ring (Trchova & Stejskal 2011). The presence of iminium indicates that the PANI molecules are in the form of emeraldine salt, which is the protonated (doped) form of emeraldine base (Figure 1(b)). The downward peaks in the FT-IR spectrum such as 1,740, 1,420, 1,366, 1,231, 1,038 and 899 cm−1 are attributed to the different molecular vibrations of carbohydrate in the kapok fibers. Meanwhile, the peak at 1,632 cm−1 is attributed with the O-H bending of water molecules adsorbed in the kapok fibers.

Figure 1

The (a) FT-IR spectrum of polyaniline-coated kapok fibers, (b) molecular structure of polyaniline emeraldine salt and contact angle of water on (c) kapok fibers and (d) polyaniline-coated kapok fibers.

Figure 1

The (a) FT-IR spectrum of polyaniline-coated kapok fibers, (b) molecular structure of polyaniline emeraldine salt and contact angle of water on (c) kapok fibers and (d) polyaniline-coated kapok fibers.

Table 1

Peaks of infrared spectrum of polyaniline-coated kapok fibers with corresponding associated modes of molecular vibration, structure and origin

Peak wave number (cm−1Assignments Origin 
(a) 1,740 C = O stretching Kapok fibers 
(b) 1,632 O-H bending from adsorbed water Kapok fibers 
(c) 1,578 quinoid (Q) ring-stretching Polyaniline 
(d) 1,489 benzenoid (B) ring-stretching Polyaniline 
(e) 1,458 C = C stretching of the aromatic rings Polyaniline 
(f) 1,420 HCH and OCH in-plane bending Kapok fibers 
(g) 1,404 C = N+ stretching in the Iminium group Polyaniline 
(h) 1,366 CH in-plane bending Kapok fiber 
(i) 1,339 ν (C-N) Polyaniline 
(j) 1,300 ν (C-N) of the secondary aromatic amine Polyaniline 
(k) 1,231 COH bending at C6 Kapok fibers 
(l) 1,180 N = Q = N/δ(C-H) Polyaniline 
(m) 1,146 Q = NH+-B (iminium group) or B-NH+•-B/δ(C-H) Polyaniline 
(n) 1,092 δ(C-H) Polyaniline 
(o) 1,038 C-C, C-OH, C-H ring and side group vibrations Kapok fibers 
(p) 899 COC, CCO and CCH deformation and stretching Kapok fibers 
Peak wave number (cm−1Assignments Origin 
(a) 1,740 C = O stretching Kapok fibers 
(b) 1,632 O-H bending from adsorbed water Kapok fibers 
(c) 1,578 quinoid (Q) ring-stretching Polyaniline 
(d) 1,489 benzenoid (B) ring-stretching Polyaniline 
(e) 1,458 C = C stretching of the aromatic rings Polyaniline 
(f) 1,420 HCH and OCH in-plane bending Kapok fibers 
(g) 1,404 C = N+ stretching in the Iminium group Polyaniline 
(h) 1,366 CH in-plane bending Kapok fiber 
(i) 1,339 ν (C-N) Polyaniline 
(j) 1,300 ν (C-N) of the secondary aromatic amine Polyaniline 
(k) 1,231 COH bending at C6 Kapok fibers 
(l) 1,180 N = Q = N/δ(C-H) Polyaniline 
(m) 1,146 Q = NH+-B (iminium group) or B-NH+•-B/δ(C-H) Polyaniline 
(n) 1,092 δ(C-H) Polyaniline 
(o) 1,038 C-C, C-OH, C-H ring and side group vibrations Kapok fibers 
(p) 899 COC, CCO and CCH deformation and stretching Kapok fibers 
Figure 2

SEM micrographs of (a and b) kapok fibers and (c and d) polyaniline-coated kapok fibers at 6,000 ×.

Figure 2

SEM micrographs of (a and b) kapok fibers and (c and d) polyaniline-coated kapok fibers at 6,000 ×.

The kapok fibers became hydrophilic after the treatment procedures as seen in Figure 1(c) and 1(d). The water contact angle of uncoated kapok fibers was around 120° (Figure 1(c)). Materials with water contact angle greater than 90° are considered hydrophobic. Hydrophobic materials repel water molecules on their surface, a condition that is not conducive for sorption in aqueous media. After the treatment processes, the water contact angle of the fibers reduced to zero (Figure 1(d)). Materials with a water contact angle less than 90° are considered hydrophilic. In addition, the zero value of the water contact angle implies that the fibers achieved complete wettability with water. The fibers need to have complete wettability with water to be best used for sorption purposes in aqueous media.

Figure 2 shows the SEM micrographs of kapok fibers before and after coating with PANI. The kapok fiber has micro-tubular structure, having a lumen in the middle. The diameter of the fiber is estimated to be about 19.98 μm (Figure 2(a)). After coating, most areas of the kapok fiber were evenly coated with PANI. The coated areas appeared to have coarser structures compared to that of the uncoated areas. The coating thickness is estimated to be about 2.38 μm. Interestingly, the tubular structure of the fibers was still observed after coating with PANI molecules.

Effect of initial solution pH and mass of PANI-coated kapok

Mass of sorbent and initial pH of the solution are two essential parameters that would affect the operating costs of the wastewater treatment system. Based on Figure 3(a), increasing the mass of PANI-coated kapok from 20 to 30 mg led to an increase in sorption capacity from 50.27 to 57.85 mg/g, respectively. A higher mass of 30 mg would imply larger surface area and binding sites available to remove MO molecules from the solution. However, further increasing the mass of PANI-coated kapok to 50 mg resulted in a slight decrease in sorption capacity. This shows that aggregation of the PANI-coated kapok fibers may have occurred at higher dosages that would lead to lower surface area available for sorption and greater length for diffusional path of MO molecules (Jafari et al. 2018). In Figure 3(b), results illustrate that acidic solution (pH < 7.0) has better sorption capacity in comparison to basic solution. An acidic pH would indicate positively-charged surface of the sorbent that is due to the polaron/bipolaron sites (Zheng et al. 2012). The positively charged surface of the PANI-coated kapok would electrostatically interact with the negatively-charged sulfonate group (-SO3Na) present in MO dye (Haitham et al. 2014). Basic pH would imply that the emeraldine salt of PANI would be in its neutral form wherein the binding sites become deactivated and results in lower sorption capacities.

Figure 3

Effect of (a) mass of sorbent and (b) pH on the sorption capacity of PANI-coated kapok in the removal of MO.

Figure 3

Effect of (a) mass of sorbent and (b) pH on the sorption capacity of PANI-coated kapok in the removal of MO.

Sorption isotherms

Isotherm models are utilized to measure the sorption capacity of a sorbent in removing a certain contaminant. The equilibrium data were evaluated using isotherm models, such as Langmuir, Freundlich and Dubinin-Radushkevich (D-R) equations.

The Langmuir model assumes the formation of a monolayer sorption occurring on the sorbent surface where the binding sites are described as energetically homogenous (Zawani et al. 2009). Moreover, only one sorbate molecule can occupy an active site and there are no interactions occurring between sorbate molecules (Labidi et al. 2016). The linear form of the Langmuir model is given as Equation (2): 
formula
(2)
where qe is the equilibrium quantity of adsorbate onto the adsorbent (mg/g), KL is the Langmuir constant, and qm is the maximum sorption capacity (mg/g) required to form a monolayer (Langmuir 1918).
The empirical Freundlich model assumes that a fraction of increase in the occupied sorption sites would result to a logarithmic increase in the sorption enthalpy, where the sorbent surface is heterogeneous in nature where physical forces are involved in the multilayer sorption (Freundlich 1906). The linear form of the Freundlich isotherm is provided as Equation (3): 
formula
(3)
where KF is the Freundlich sorption coefficient that refer to the sorption capacity (mg/g) and 1/n is an empirical exponent referring to the sorption intensity (L/mg).
The D-R model was initially applied to describe the isotherm of sub-critical vapors in microporous solids, where van der Waals forces mainly govern the sorption system (Dubinin & Radushkevich 1947). The linear D-R equation is given as Equation (4): 
formula
(4)
where qD-R refers to the theoretical D-R saturation capacity (mg/g), β is the D-R constant (mol2/kJ2) and ɛ is Polanyi potential (kJ/mol). The Polanyi potential can be computed using Equation (5): 
formula
(5)
where R is the gas constant (8.3145 J/mol•K) and T is operating temperature (K). The mean free energy of sorption, E (kJ/mol) can be compute from the D-R constant as Equation (6): 
formula
(6)

The calculated isotherm parameters derived from Langmuir, Freundlich and D-R models are listed in Table 2. Based on the correlation coefficient values, Langmuir (0.972 ≥ R2 ≥ 0.959) model best represents the sorption of MO and Cu(II) using the PANI-coated kapok system when compared to Freundlich (0.966 ≥ R2 ≥ 0.910) and D-R (0.873 ≥ R2 ≥ 0.699) models. This indicates that sorption occurs in a homogenous distribution of MO and Cu(II) onto the binding sites of the PANI-coated kapok. Moreover, this indicates that only one kind of interaction occurs between sorbate and sorption site. Likewise, this indicates that once a sorption site is occupied no further sorption occurs. The maximum sorption capacity derived from the Langmuir isotherm was determined to be 75.76 mg/g for MO and 81.04 mg/g for Cu(II). Based on the values, Cu(II) is favorably remoed over MO by PANI-coated kapok. This is due to the larger ionic micelles of MO with an average molecular size of 26.14 Å when compared to the hydrated ionic radius of Cu(II) of 0.87 Å (Hameed et al. 2008), which implies that Cu(II) can easily diffuse from the bulk of solution onto the sorbent surface when compared to MO.

Table 2

Isotherm parameters derived from for the sorption of MO and Cu(II) onto the PANI-coated kapok fibers

Isotherms Parameters Adsorbent
 
Methyl orange Cu(II) 
Langmuir qm (mg/g) 75.76 81.04 
KL (mL/mg) 0.087 0.015 
R2 0.959 0.972 
Freundlich KF (mg/g) 21.83 3.37 
4.15 1.77 
R2 0.910 0.966 
D-R qD-R (mg/g) 60.70 51.45 
E (kJ/mol) 1,210.02 135.72 
R2 0.699 0.873 
Isotherms Parameters Adsorbent
 
Methyl orange Cu(II) 
Langmuir qm (mg/g) 75.76 81.04 
KL (mL/mg) 0.087 0.015 
R2 0.959 0.972 
Freundlich KF (mg/g) 21.83 3.37 
4.15 1.77 
R2 0.910 0.966 
D-R qD-R (mg/g) 60.70 51.45 
E (kJ/mol) 1,210.02 135.72 
R2 0.699 0.873 

Figure 4(a) and 4(b) show the theoretical curves generated by the Langmuir, Freundlich and D-R isotherm model plotted against the experimental data of MO and Cu(II) sorption. A good agreement was observed between the theoretical curve generated by the Langmuir model and experimental data, which validates that Langmuir best describes the sorption of MO and Cu(II) using PANI-coated kapok.

Figure 4

Plot of experimental data points against the theoretical isotherm curves generated by the Langmuir, Freundlich and D-R models for (a) MO and (b) Cu(II) sorption and kinetic plots using (c) pseudo-first-order and (d) pseudo-second-order equation for the removal of MO and Cu(II) onto PANI-coated kapok.

Figure 4

Plot of experimental data points against the theoretical isotherm curves generated by the Langmuir, Freundlich and D-R models for (a) MO and (b) Cu(II) sorption and kinetic plots using (c) pseudo-first-order and (d) pseudo-second-order equation for the removal of MO and Cu(II) onto PANI-coated kapok.

Sorption thermodynamics

In the sorption system, thermodynamics parameters (ΔG°, ΔH°, ΔS°) will help define the nature of the removal of MO and Cu(II) using PANI-coated kapok. The parameters can be calculated using Equations (7) and (8): 
formula
(7)
 
formula
(8)
where T is the solution temperature (K), R is the universal gas constant (8.314 J/mol•K) and KD is the Langmuir equilibrium constant, ΔG° refers to the Gibbs free energy (kJ/mol), ΔH° refers to the enthalpy (kJ/mol) and ΔS° refers to the entropy (kJ/mol•K).

Based from Table 3, the negative values of ΔG° under all temperature range studied imply that the sorption of MO and Cu(II) using PANI-coated kapok is feasible and spontaneous. In both sorption of MO and Cu(II), the values of ΔG° become more negative as temperature was increased, which indicates that sorption was more spontaneous and favored at higher temperature. The magnitude of ΔG° would serve as one of the criteria to determine between physisorption and chemisorption. When ΔG° values ranges from −20 to 0 kJ/mol, this refers to physisorption while ΔG° values from −80 to −400 kJ/mol would indicate chemisorption (Yu et al. 2001). Results show that sorption of MO and Cu(II) using PANI-coated kapok is governed by physical sorption. The values of ΔH° for the removal of MO and Cu(II) are positive, which indicate that the sorption process is endothermic. Moreover, the positive values of ΔS° for MO and Cu(II) suggest that sorption process lead to increased randomness at the solid-solution interface.

Table 3

Thermodynamic parameters for the sorption of MO and Cu(II) onto the PANI-coated kapok

Contaminant Temperature (K) Thermodynamic parameters
 
ΔG° (kJ/mol) ΔH° (kJ/mol) ΔS° (kJ/mol•K) 
Methyl orange 298 −10.11 8.96 0.064 
323 −11.54 
343 −13.01 
Cu(II) 298 −11.32 1.32 0.042 
323 −12.37 
343 −13.23 
Contaminant Temperature (K) Thermodynamic parameters
 
ΔG° (kJ/mol) ΔH° (kJ/mol) ΔS° (kJ/mol•K) 
Methyl orange 298 −10.11 8.96 0.064 
323 −11.54 
343 −13.01 
Cu(II) 298 −11.32 1.32 0.042 
323 −12.37 
343 −13.23 

Sorption kinetics

The kinetics of sorption, most importantly the kinetic rate constant, can be utilized in the design of the sorption system. Among the kinetic equations, the pseudo-first-order and pseudo-second-order equation of MO and Cu(II) were investigated. The pseudo-first-order model, otherwise known as the Lagergren equation, is based on interfacial kinetics. It is provided in Equation (9) (Lagergren 1898): 
formula
(9)
The pseudo-second-order model describes sorption occurring where one sorbate can interact with more than one sorption site. The linear form of the model is given as Equation (10) (Ho & McKay 1999; Ho 2006): 
formula
(10)
where k2 is the pseudo-second-order rate constant (g/mg•min).

Table 4 shows the kinetic parameters for sorption of MO and Cu(II) onto the PANI-coated kapok. It was observed that the values of R2 for MO and Cu(II) sorption using the pseudo-second-order model are 0.990 and 0.999, respectively. These R2 values indicate that the pseudo-second-order model best fit the experimental data derived from removal of MO and Cu(II) using PANI-coated kapok. Furthermore, the theoretical values of qe generated by the pseudo-second-order model for MO and Cu(II) are closer to the experimental values. This is further validated by Figure 4(d), where the experimental data points are in good agreement with the theoretical line generated by the pseudo-second-order equation. The said fitness to the pseudo-second-order model implies that the rate determining step for the sorption of MO and Cu(II) is chemisorption. The kinetic rate constant derived from the pseudo-second-order model for Cu(II) has a greater value than MO due to the smaller hydrated ionic radius of Cu(II) where it can diffuse faster and results in better sorption capacity.

Table 4

Estimated kinetic parameters for sorption of MO and Cu(II) onto the PANI-coated kapok

Contaminant qexp Pseudo-first-order
 
Pseudo-second-order
 
qe k1 R2 qe k2 R2 
Methyl orange 28.43 17.35 0.0074 0.8341 29.37 0.0005 0.9900 
Cu(II) 88.53 11.89 0.0024 0.7537 86.74 0.0011 0.9990 
Contaminant qexp Pseudo-first-order
 
Pseudo-second-order
 
qe k1 R2 qe k2 R2 
Methyl orange 28.43 17.35 0.0074 0.8341 29.37 0.0005 0.9900 
Cu(II) 88.53 11.89 0.0024 0.7537 86.74 0.0011 0.9990 

Where k1 is the pseudo-first-order rate constant (min−1).

Table 5 presents the sorption capacity of other sorbents derived from previous studies that were utilized in the removal of MO and Cu(II). When compared to other sorbents, the sorption capacity of the PANI-coated kapok is satisfactory when compared to sorbents of previous works. This indicates a strong potential for the application of PANI-coated kapok in the removal of MO and Cu(II) from wastewater effluents.

Table 5

Methyl orange and Cu(II) sorption capacities of different sorbents

Sorbate Sorbent Sorption capacity (mg/g) Reference 
Methyl orange PANI-coated kapok fiber 75.76 This study 
Polyacrylonitrile-coated kapok fiber 34.72 Agcaoili et al. (2017)  
Surfactant-modified silkworm exuviae 87.03 Chen et al. (2011)  
Graphene oxide 16.83 Robati et al. (2016)  
Goethite 55.00 Munagapati et al. (2017)  
Chitosan beads 73.00 Munagapati et al. (2017)  
Cu(II) PANI-coated kapok fiber 81.04 This study 
Polyacrylonitrile-coated kapok fiber 90.09 Agcaoili et al. (2017)  
Chemically-modified orange peel 72.73 Feng et al. (2009)  
Peanut hull 21.25 Zhu et al. (2009)  
Tartaric acid-modified rice husk 29.00 Wong et al. (2003)  
Sorbate Sorbent Sorption capacity (mg/g) Reference 
Methyl orange PANI-coated kapok fiber 75.76 This study 
Polyacrylonitrile-coated kapok fiber 34.72 Agcaoili et al. (2017)  
Surfactant-modified silkworm exuviae 87.03 Chen et al. (2011)  
Graphene oxide 16.83 Robati et al. (2016)  
Goethite 55.00 Munagapati et al. (2017)  
Chitosan beads 73.00 Munagapati et al. (2017)  
Cu(II) PANI-coated kapok fiber 81.04 This study 
Polyacrylonitrile-coated kapok fiber 90.09 Agcaoili et al. (2017)  
Chemically-modified orange peel 72.73 Feng et al. (2009)  
Peanut hull 21.25 Zhu et al. (2009)  
Tartaric acid-modified rice husk 29.00 Wong et al. (2003)  

Sorption mechanism of MO and Cu(II)

The nitrogen atom of the iminium/imine group, located beside the quinoid ring is potentially reactive, thus it is possible binding sites of various substances. The said nitrogen atom has attached hydrogen atom when PANI is protonated (Figure 5(a)) and has no attached hydrogen atom when deprotonated (Figure 5(b)). The attached hydrogen atom of the emeraldine salt is loosely bound, where various substances can potentially replace the bound hydrogen atom and bind with the reactive nitrogen atom.

Figure 5

Molecular structure of polyaniline (a) emeraldine salt (protonated) and (b) emeraldine base (deprotonated).

Figure 5

Molecular structure of polyaniline (a) emeraldine salt (protonated) and (b) emeraldine base (deprotonated).

It is assumed that MO anions established bonds with the nitrogen atom of the iminium group of PANI-coated kapok. Their corresponding charges, the negative charge for MO and positive charge for the nitrogen atoms of the PANI-coated kapok, formed electrostatic attraction as bonds that led to their reaction. On the other hand, a covalent bond is formed between Cu(II) and nitrogen atom of the iminium group, which has two lone pair of electrons that could lead to formation of amide copper complexes.

The values of pseudo-second-order rate constant (k2) for MO and Cu(II) ions using the PANI-coated kapok fibers are smaller when compared to the rate constants of previous studies (Feng et al. 2009; Zhu et al. 2009; Chen et al. 2011; Agcaoili et al. 2017). The low values could be attributed to the slow reactions between contaminants and nitrogen atoms due to the displacement of the attached hydrogen atom before the reaction can proceed.

CONCLUSION

Kapok fibers were coated with PANI molecules using an oxidative-type in-situ technique. Infrared spectrum shows vibrational modes associated with the presence of PANI molecules distributed onto the surface of kapok. After the coating process, the water contact angle of the fibers dropped from 120° (highly hydrophobic) to 0° (hydrophilic). Results show that the tubular structure of the kapok fibers was retained after the coating process. In the present study, the capacities of PANI-coated fibers to adsorb MO and Cu(II) were determined to be 75.76 and 81.04 mg/g using the Langmuir isotherm model. Based from the thermodynamic studies, both MO and Cu(II) sorption were evaluated to be spontaneous, feasible and endothermic. Meanwhile, the kinetic data were determined to best fit the pseudo-second-order model, implying that chemisorption is the rate-determining step for the removal of MO and Cu(II) using PANI-coated kapok.

ACKNOWLEDGEMENTS

This work was supported by a grant from Department of Science and Technology and the University of the Philippines System under the Office of the Vice-President for Academic Affairs under the Emerging Inter-Disciplinary Research Program (OVPAA-EIDR C06-035). The authors wish to thank Mr Kennette Arguelles for the assistance in the preparation of the manuscript.

CONFLICT OF INTEREST

The authors declare that they do not have any conflict of interest.

REFERENCES

REFERENCES
Agcaoili
A. R.
,
Herrera
M. U.
,
Futalan
C. M.
&
Balela
M. D. L.
2017
Fabrication of polyacrylonitrile-coated kapok hollow microtubes for adsorption of methyl orange and Cu(II) ions in aqueous solution
.
Journal of the Taiwan Institute of Chemical Engineers
78
,
359
369
.
Ahmed
S. M.
,
El-Dib
F. I.
,
El-Gendy
N. S.
,
Sayed
W. M.
&
El-Khodary
M.
2016
A kinetic study for the removal of anionic sulphonated dye from aqueous solution using nanopolyaniline and Baker's yeast
.
Arabian Journal of Chemistry
9
,
S1721
S1728
.
Anand
J.
,
Palaniappan
S.
&
Sathyanarayana
D. N.
1998
Conducting polyaniline blends and composites
.
Progress in Polymer Science
23
(
6
),
993
1018
.
Chung
B. Y.
,
Cho
J. Y.
,
Lee
M. H.
,
Wi
S. G.
,
Kim
J. H.
&
Kim
J. S.
2008
Adsorption of heavy metal ions onto chemically oxidized Ceiba pentandra (L.) Gaertn. (Kapok) fibers
.
Journal of Applied Biological Chemistry
51
(
1
),
28
35
.
Daraei
P.
,
Madaeni
S. S.
,
Ghaemi
N.
,
Salehi
E.
,
Khadivi
M. A.
,
Moradian
R.
&
Astinchap
B.
2012
Novel polyethersulfone nanocomposite membrane prepared by PANI/Fe3O4 nanoparticles with enhanced performance for Cu(II) removal from water
.
Journal of Membrane Science
415–416
,
250
259
.
Dubinin
M. M.
&
Radushkevich
L. V.
1947
The equation of the characteristic curve of the activated charcoal
.
Proceedings of the USSR Academy of Sciences Physical Chemistry
55
,
331
337
.
Eftekhari
A.
,
Li
L.
&
Yang
Y.
2017
Polyaniline supercapacitors
.
Journal of Power Sources
347
,
86
107
.
Feng
N.
,
Guo
X.
&
Lian
S.
2009
Adsorption study of copper (II) by chemically modified orange peel
.
Journal of Hazardous Materials
164
(
2–3
),
1286
1292
.
Freundlich
H. M. F.
1906
Over the adsorption in solution
.
The Journal of Physical Chemistry
57
,
385
470
.
Fu
F.
&
Wang
Q.
2011
Removal of heavy metal ions from wastewaters: a review
.
Journal of Environmental Management
92
,
407
418
.
Gospodinova
N.
&
Terlemezyan
L.
1998
Conducting polymers prepared by oxidative polymerization: polyaniline
.
Progress in Polymer Science
23
(
8
),
1443
1484
.
Haitham
K.
,
Razak
S.
&
Nawi
M. A.
2014
Kinetics and isotherm studies of methyl orange adsorption by a highly recyclable immobilized polyaniline on a glass plate
.
Arabian Journal of Chemistry
https://doi.org/10.1016/j.arabjc.2014.10.010. In press
.
Hameed
B. H.
,
Tan
I. A. W.
&
Ahmad
A. L.
2008
Adsorption isotherm, kinetic modeling and mechanism of 2,4,6-trichlorophenol on coconut-husk based activated carbon
.
Chemical Engineering Journal
144
(
2
),
235
244
.
Hassandazeh-Tabrizi
S. A.
,
Motlagh
M. M.
&
Salahshour
S.
2016
Synthesis of ZnO/CuO nanocomposite immobilized on γ-Al2O3 and application for removal of methyl orange
.
Applied Surface Science
384
,
237
243
.
Ho
Y. S.
2006
Review of second-order models for adsorption systems
.
Journal of Hazardous Materials
136
(
3
),
681
689
.
Ho
Y. S.
&
McKay
G.
1999
Pseudo-second order model for sorption processes
.
Process Biochemistry
34
,
451
465
.
Laabd
M.
,
Ait Ahsaine
H.
,
El Jaouhari
A.
,
Bakiz
B.
,
Bazzaoui
M.
,
Ezahri
M.
,
Albourine
A.
&
Benlhachemi
A.
2016
Congo red removal by PANi/Bi2WO6 nanocomposites: kinetic, equilibrium and thermodynamic studies
.
Journal of Environmental Chemical Engineering
4
(
3
),
3096
3105
.
Labidi
A.
,
Salaberria
A. M.
,
Fernandes
S. C. M.
,
Labidi
J.
&
Abderrabba
M.
2016
Adsorption of copper on chitin-based materials: kinetic and thermodynamics studies
.
Journal of the Taiwan Institute of Chemical Engineers
65
,
140
148
.
Lagergren
S. Y.
1898
About the theory of so-called adsorption of soluble substances
.
Kungliga Svenska Vetenskapsakademiens Handlingar
24
(
4
),
1
39
.
Langmuir
I.
1918
The adsorption of gases on plane surfaces of glass, mica and platinum
.
Journal of the American Chemical Society
40
(
9
),
1361
1403
.
Likon
M.
,
Remškar
M.
,
Ducman
V.
&
Švegl
F.
2013
Populus seed fibers as a natural source for production of oil super absorbents
.
Journal of Environmental Management
114
,
158
167
.
Liu
Y.
,
Chen
L.
,
Li
Y.
,
Wang
P.
&
Dong
Y.
2016
Synthesis of magnetic polyaniline/graphene oxide composites and their application in the efficient removal of Cu(II) from aqueous solutions
.
Journal of Environmental Chemical Engineering
4
(
1
),
825
834
.
Mittal
A.
,
Malviya
A.
,
Kaur
D.
,
Mittal
J.
&
Kurup
L.
2007
Studies on the adsorption kinetics and isotherms for the removal and recovery of methyl orange from wastewaters using waste materials
.
Journal of Hazardous Materials
148
(
1–2
),
229
240
.
Mwaikambo
L. Y.
&
Bisanda
E. T. N.
1999
The performance of cotton-kapok fabric-polyester composites
.
Polymer Testing
18
(
3
),
181
198
.
Pfeiffer
C. C.
&
Mailloux
R.
1987
Excess copper as a factor in human diseases
.
Journal of Orthomolecular Medicine
2
(
3
),
171
182
.
Rahmah
A. U.
&
Abdullah
M. A.
2010
Evaluation of Malaysian Ceiba pentandra (L.) Gaertn. for oily water filtration using factorial design
.
Desalination
266
(
1–3
),
51
55
.
Robati
D.
,
Mirza
B.
,
Rajabi
M.
,
Moradi
O.
,
Tyagi
I.
,
Agarwal
S.
&
Gupta
V. K.
2016
Removal of hazardous dyes-BR and methyl orange using graphene oxide as an adsorbent from aqueous phase
.
Chemical Engineering Journal
284
,
687
697
.
Siswoyo
E.
,
Endo
N.
,
Mihara
Y.
&
Tanaka
S.
2014
Agar-encapsulated adsorbent based on leaf if Platanus sp. to adsorb cadmium ion in water
.
Water Science and Technology
70
(
1
),
89
94
.
Trchova
M.
&
Stejskal
J.
2011
Polyaniline: the infrared spectroscopy of conducting polymer nanotubes
.
Pure and Applied Chemistry
83
(
10
),
1803
1817
.
Volesky
B.
&
Holan
Z. R.
1995
Biosorption of heavy metals
.
Biotechnology Progress
11
(
3
),
235
250
.
Wang
S.
,
Boyjoo
Y.
,
Choueib
A.
&
Zhu
Z. H.
2005
Removal of dyes from aqueous solution using fly ash and red mud
.
Water Research
39
(
1
),
129
138
.
Wong
K. K.
,
Lee
C. K.
,
Low
K. S.
&
Haron
M. J.
2003
Removal of Cu and Pb by tartaric acid modified rice husk from aqueous solution
.
Chemosphere
50
(
1
),
23
28
.
Xiang
H. F.
,
Wang
D.
,
Liu
H. C.
,
Zhao
N.
&
Xu
J.
2013
Investigation on sound absorption properties of kapok fibers
.
Chinese Journal of Polymer Science
31
(
3
),
521
529
.
Xie
X.
,
Deng
R.
,
Pang
Y.
,
Bai
Y.
,
Zheng
W.
&
Zhou
Y.
2017
Adsorption of copper(II) by sulfur microparticles
.
Chemical Engineering Journal
314
,
434
442
.
Xin
Q.
,
Fu
J.
,
Chen
Z.
,
Liu
S.
,
Yan
Y.
,
Zhang
J.
&
Xu
Q.
2015
Polypyrrole nanofibers as a high-efficient adsorbent for the removal methyl orange from aqueous solution
.
Journal of Environmental Chemical Engineering
3
(
3
),
1637
1647
.
Xu
C.
,
Chen
H.
&
Jiang
F.
2015
Adsorption of perflourooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) on polyaniline nanotubes
.
Colloids and Surfaces A: Physicochemical and Engineering Aspects
479
,
60
67
.
Yu
Y.
,
Zhuang
Y. Y.
&
Wang
Z. H.
2001
Adsorption of water-soluble dye onto functionalized resin
.
Journal of Colloid and Interface Science
242
(
2
),
288
293
.
Zawani
Z.
,
Luqman
C. A.
&
Thomas
S. Y. C.
2009
Equilibrium, kinetics and thermodynamic studies adsorption of Remazol Black 5 on the palm kernel shell activated carbon (PKS-AC)
.
European Journal of Scientific Research
37
(
1
),
63
71
.
Zheng
Y.
,
Liu
Y.
&
Wang
A.
2012
A kapok fiber oriented polyaniline for removal of sulfonated dyes
.
Industrial & Engineering Chemistry Research
51
,
10079
10087
.
Zhu
C. S.
,
Wang
L. P.
&
Chen
W. B.
2009
Removal of Cu(II) from aqueous solution by agricultural by-product: Peanut hull
.
Journal of Hazardous Materials
168
(
2–3
),
739
746
.