In this paper, poly(vinyl alcohol) (PVA) nanofiber was prepared and modified by diethylenetriamine (DETA) and ethylenediamine (EDA) in the presence of glutaraldehyde (GA). Dye removal ability of the modified nanofiber (PVA/DETA/EDA/GA) as a nanoadsorbent from water was studied. Fourier transform Infrared (FTIR) and scanning electron microscopy (SEM) were used to investigate the characteristics of the modified nanofiber. Direct Red 23 (DR23) and Direct Blue (DB78) were used. The effect of operational parameters such as pH, initial dye concentration, contact time, temperature and adsorbent dosage on dye removal was studied. The dye adsorption isotherms, kinetics and thermodynamics were investigated. The maximum dye adsorption capacity of the modified nanofiber was 370 and 400 mg/g for DR23 and DB78, respectively. Four isotherms, the Langmuir, the Freundlich, Tempkin and a modified Langmuir–Freundlich model were used. Dye adsorption on the modified nanofiber followed the Langmuir isotherm and pseudo-second kinetic order. Thermodynamic data showed that dye removal was a spontaneous, endothermic and physisorption process.

SYMBOL

     
  • 1/n

    adsorption intensity

  •  
  • B1 and KT

    the Tempkin constants

  •  
  • Ce

    the equilibrium dye concentration in solution (mg/L)

  •  
  • k1

    the pseudo-first order rate constant of adsorption (1/min)

  •  
  • k2

    the pseudo-second order rate constant of adsorption (g/mg min)

  •  
  • Ka

    the affinity constant for adsorption (L/mg)

  •  
  • KF

    the Freundlich constant

  •  
  • KL

    the Langmuir constant (L/g)

  •  
  • kp

    the rate constant of intraparticle diffusion (mg/g min0.5)

  •  
  • q

    the amount of dye adsorbed on adsorbent (mg/g)

  •  
  • qm

    the maximum adsorption capacity (mg/g)

  •  
  • qe

    the amount of dye adsorbed on adsorbent at equilibrium (mg/g)

  •  
  • (qe)Cal.

    the calculated qe

  •  
  • (qe)Exp.

    the experimental qe

  •  
  • qt

    the amount of dye adsorbed at time t (mg/g)

  •  
  • R

    the universal gas constant (8.314 J/mol K)

  •  
  • R2

    correlation coefficient value

  •  
  • T

    the absolute temperature (K)

  •  
  • SEM

    Scanning electron microscopy

  •  
  • FTIR

    Fourier transform infrared

  •  
  • Δ

    standard Gibbs free energy change

  •  
  • Δ

    standard enthalpy change

  •  
  • Δ

    standard entropy change

INTRODUCTION

Some azo dyes and their intermediates are toxic and carcinogenic. In addition, azo dyes are used more than other dyes. Thus, these compounds must be removed from colored wastewater. Several biological, chemical and physical methods (Mahmoodi & Shourijeh 2015, 2016) have been studied to remove dyes from effluents (Demirbas et al. 2002; Bulut & Aydin 2006; Mahmoodi et al. 2007; Amin 2008; Srinivasan & Viraraghavan 2010; Mahmoodi 2014, 2015; Revathi et al. 2014; Vesna et al. 2014; Ferrarini et al. 2016; Naghipour et al. 2016; Tian et al. 2016).

The adsorption process is one of the effective methods to remove azo dyes from water and wastewater (Khosla et al. 2013; Zodi et al. 2013). Nanomaterials as adsorbents have attracted considerable interest because of their large surface area, controllable surface properties, and pore structure. The adsorbents have developed to remove heavy metals and dyes due to the interaction of their surface functional groups with the target compounds (Lin et al. 2011). Thus, the numerous adsorption sites and large surface area are the most important factors to affect pollutant removal by adsorbents (Dural et al. 2011).

Electrospinning is a convenient, low-cast, effective, simple, and widely utilized method to prepare nanofibers as adsorbents. In addition, it has attracted considerable attention. The nanofiber with high stability, high specific surface area, and a large number of surface functional groups is expected to be a good adsorbent for pollutants via physical or chemical adsorption. A poly(vinyl alcohol) (PVA)/zinc oxide nanofiber was used as an adsorbent to remove U(VI), Cu(II) and Ni(II) from aqueous solution. The equilibrium data showed that the adsorption capacity for U(VI), Cu(II) and Ni(II) ions are 370.86, 162.48 and 94.43 mg/g, respectively (Hallaji et al. 2015). In other research, a hydrophilic PVA-co-ethylene nanofiber was prepared to remove heavy metal ions. The hydrophilic nanofiber was activated with cyanuric chloride and covalently linked to the activated nanofiber. The functionalized nanofiber showed high adsorption capability for heavy metal ions (Lu et al. 2014). PVA and thiol-functionalized PVA/SiO2 nanofibers were prepared by electrospinning and used to remove heavy metal ions from water. The results indicated that the functionalized composite nanofiber had greater capacity than the pure one (Wu et al. 2010).

A literature review showed that the surface modified PVA nanofiber by diethylenetriamine (DETA) and ethylenediamine (EDA) in the presence of glutaraldehyde (GA) was not used to remove dye from water. In this paper, the modified nanofiber (PVA/DETA/EDA/GA) was prepared and its dye removal ability as a nanoadsorbent was studied. The characteristics of the modified nanofibers were investigated by Fourier transform infrared (FTIR) and scanning electron microscopy (SEM). The effect of pH, contact time, adsorbent dosage, initial dye concentration and temperature on dye removal was investigated. In addition, the isotherms, kinetics and thermodynamics of dye adsorption were studied.

EXPERIMENTAL

Materials

PVA fiber (MW = 145,000 g/mol) was received from Sigma. DETA, EDA, hydrochloric acid, GA, sodium hydroxide and acetic acid were obtained from Merck. Direct Red 23 (DR23) and Direct Blue 78 (DB78) were used as model dyes (Table 1).

Table 1

The characteristics of dyes

Name DB78 DR23 
Molecular structure Azo class Azo class 
Molecular formula C42H25N7Na4O13S4 C35H25N7Na2O10S2 
Molecular weight 1,055.91 813.72 
Chemical structure   
Name DB78 DR23 
Molecular structure Azo class Azo class 
Molecular formula C42H25N7Na4O13S4 C35H25N7Na2O10S2 
Molecular weight 1,055.91 813.72 
Chemical structure   

Methods

Preparation of PVA/DETA/EDA/GA nanofiber

PVA solution (7 wt%) was provided by dissolving 0.7 g of PVA in 10 mL deionized water; after that, TETA (0.25, 0.5 wt%) and DEA (0.25, 0.5 wt%) were dispersed in PVA solution and then sonicated for 3 h at 90 °C. Next, the prepared solution was poured into a 10 mL plastic syringe with a 0.5 mm diameter capillary tip. A variable high voltage generator was used for the electrospinning process. The positive terminal of the generator was connected to the metallic syringe tip, while the negative terminal was connected to an aluminum foil. For making nanofibers, the experimental parameters of electrospinning are shown in Table 2. The nanofiber was crosslinked using GA vapor at 40 °C for 24 h, and heated at 40 °C for another 12 h to remove residual GA.

Table 2

Experimental parameters of electrospinning

PVA nanofiber (7%) DETA/EDA concentration (wt %) Voltage (kV) Flow rate (mL/h) Diameter of needle (mm) Distance (cm) 
Pure PVA No amine 17 0.5 16 
PVA/DETA/EDA 0.25 20 0.5 0.5 16 
PVA/DETA/EDA 0.5 22 0.5 0.5 16 
PVA nanofiber (7%) DETA/EDA concentration (wt %) Voltage (kV) Flow rate (mL/h) Diameter of needle (mm) Distance (cm) 
Pure PVA No amine 17 0.5 16 
PVA/DETA/EDA 0.25 20 0.5 0.5 16 
PVA/DETA/EDA 0.5 22 0.5 0.5 16 

Characterization of PVA/DETA/EDA/GA nanofiber

The functional groups of the prepared nanofiber were analyzed by FTIR spectrometer (ThermoNicolet NEXUS870 FTIR from Nicolet Instrument Corp., USA). The morphology of nanofiber was investigated using SEM (LEO1455VP and England).

Adsorption studies

Dye adsorption onto the modified nanofiber from water was studied at pH 2.1, temperature of 25 °C, contact time of 60 min and adsorbent dosage of 0.02 g. The pH of the solution was adjusted using HCl and/or NaOH. Dye concentration was measured at the maximum wavelength of the dyes (507 nm for DR23 and 604 nm for DB78) using a UV–vis spectrophotometer (CECILCE2021).

The effect of adsorbent dosage on the adsorption of dyes was studied with an initial dye concentration of 30 mg/L at 25 °C. The dye removal ability of the nanofiber (0.02 g) at different pH values (2.1, 3, 4, 5, 6, 7 and 8) was examined by contacting 250 mL of the dye solution with an initial dye concentration (30 mg/L) at room temperature (25 °C) for 60 min. Experiments were done with an initial dye concentration of 30 mg/L and 25 °C to study the effect of adsorption contact time. Nanofiber (0.02 g) was mixed in 250 mL of dye solution with concentrations varying in the range of 10–70 mg/L at three different temperatures (25, 35, 45 and 55 °C) to investigate the effect of initial dye concentration and temperature.

Several isotherm models are investigated in the literature. In this paper, the Langmuir, Freundlich and Tempkin models were used to study the adsorption isotherm. The Langmuir model assumes that adsorption occurs at homogeneous sites on the adsorbent with monolayer adsorption. The Langmuir equation is as (Langmuir 1918): 
formula
1
where qe is the amount of dye adsorbed at equilibrium time (mg/g), qm is the maximum amount of dye adsorbed on adsorbent (mg/g), Ce is the equilibrium concentration of dye in solution (mg/L), and KL is the Langmuir constant (L/mg).
The Freundlich model considers unequal available heterogeneous sites with different energies of adsorption. It is represented as (Freundlich 1906): 
formula
2
where KF is the Freundlich constant and n is the intensity of the adsorption constant for Freundlich.
In the Tempkin isotherm, the heat of adsorption decreases linearly with coverage. In addition, the adsorption is characterized by a uniform distribution of binding energies, up to some maximum binding energy (Foo & Hameed 2010). The Tempkin isotherm is as follows: 
formula
3

RESULTS AND DISCUSSION

Characterization

The reactions of PVA, DETA, EDA and GA to prepare the modified nanofiber are shown in Figure 1. PVA reacts with GA. Aldehyde reacts with alcohol to produce acetal. In addition, aldehyde reacts with amine, and the imine is produced.
Figure 1

The reactions of materials in the presence of GA to prepare the modified nanofiber.

Figure 1

The reactions of materials in the presence of GA to prepare the modified nanofiber.

The FTIR spectrum of the samples is shown in Figure 2. The peaks at 3,419.14, 2,922.21, 1,412.92, 1,353, 1,022.55 and 801 cm−1 in Figure 2(a) are the characteristic PVA absorption bands (Mahmoodi & Shourijeh 2015). The peaks at 1,415.07 and 3,426.93 cm−1 were attributed to the bending vibration band of the amine N–H group (Figure 2(b); Tahaei et al. 2008; Kampalanonwat & Supaphol 2011). The relative increase in the C=O band at approximately 1,734.1 cm−1 indicates that the aldehyde groups of GA did not completely react with the O–H groups of the PVA chain and the N–H groups of DETA and EDA. In addition, the C–O stretching in pure PVA is replaced by a broader absorption band 1,045.94 cm−1, which can be attributed to the ether (C–O) and the acetal ring (C–O–C) bands formed by the crosslinking reaction of PVA with GA (Figure 2(b)). Also, imine bands (–C=N–, 1,653 cm−1) were formed by the crosslinking reaction of amines with aldehyde (GA) (Figure 2(b)) (Mahmoodi & Shourijeh 2015). In addition, a peak at 2,855.5 cm−1 was observed due to the reaction of materials with GA to prepare the modified nanofiber (Figure 2(b)).
Figure 2

FTIR spectrum of nanofibers. (a) PVA nanofiber and (b) PVA/DETA/EDA/GA nanofiber.

Figure 2

FTIR spectrum of nanofibers. (a) PVA nanofiber and (b) PVA/DETA/EDA/GA nanofiber.

The SEM micrographs of PVA/DETA/EDA/GA nanofiber are shown in Figure 3. As can be seen, with a decrease in DETA/DEA concentration from 2.5 wt%, the average diameter of the electrospun nanofiber decreased considerably from 200 to 80 nm, respectively, and the perfect uniform fiber with the smallest diameter and without any drops and beads was obtained. The surface of the modified nanofiber exhibited similar morphologies compared to the PVA nanofiber mat, without any serious cracks or sign of degradation. In high conversions, adhesion among the nanofibers was found to decrease the effective surface area. In addition, it can be observed in Figure 3 that the nanofiber diameter increased by adding 0.5 wt% DETA/EDA compared with 0.25 wt% of DETA/EDA (Table 3).
Table 3

Comparing the average diameter of nanofibers

Nanofibers Pure PVA PVA/DETA/EDA/GA (0.5 wt%) PVA/DETA/EDA/GA (0.25%) 
Average diameter (nm) 170 220 155 
Nanofibers Pure PVA PVA/DETA/EDA/GA (0.5 wt%) PVA/DETA/EDA/GA (0.25%) 
Average diameter (nm) 170 220 155 
Figure 3

SEM micrographs. (a) and (b) PVA nanofiber, (c) and (d) PVA/DETA/EDA/GA nanofiber with 0.25 wt% of DETA/EDA and (e) and (f) PVA/DETA/EDA/GA nanofiber 0.5 wt% of DETA/EDA.

Figure 3

SEM micrographs. (a) and (b) PVA nanofiber, (c) and (d) PVA/DETA/EDA/GA nanofiber with 0.25 wt% of DETA/EDA and (e) and (f) PVA/DETA/EDA/GA nanofiber 0.5 wt% of DETA/EDA.

Adsorption studies

In this work, pure PVA/GA nanofiber and PVA/DETA/EDA/GA nanofiber were prepared and then applied to remove DR23 and DB78 from wastewater. The dye removal ability of two nanofibers was compared. The dye removal experiment was performed using 250 mL volume of 30 mg/L of dye solution with 0.02 g nanofiber. In the adsorption process, DR23 and DB78 molecules would adsorb onto the PVA nanofiber from their aqueous phase due to the van der Waals force interaction. Figure 4 shows that PVA/DETA/EDA/GA nanofiber has higher dye removal ability than pure PVA/GA nanofibers due to surface modification. Thus, PVA/DETA/EDA/GA nanofiber was used for further study.
Figure 4

Comparison of dye removal by pure PVA/GA nanofiber and PVA/DETA/EDA/GA nanofiber (Dye = 30 mg/L, V = 250 mL, pH = 2.1, t = 60 min and T = 25 °C).

Figure 4

Comparison of dye removal by pure PVA/GA nanofiber and PVA/DETA/EDA/GA nanofiber (Dye = 30 mg/L, V = 250 mL, pH = 2.1, t = 60 min and T = 25 °C).

The dye removal is dependent on the pH of the water because pH variation changes the ionization degree of the pollutant molecule and the adsorbent surface properties (Chowdhury et al. 2011; Liu et al. 2012). The effect of solution pH on the adsorption of DR23 and DB78 by the modified nanofiber is shown in Figure 5. The results show that the adsorption capacity increases when the pH decreases. The maximum adsorption of dyes occurred at pH = 2.1. At this pH, the amino functional group (–NH2) of the modified nanofiber protonates as –NH3+, and a strong electrostatic attraction occurs between the positively charged nanofiber (-NH3+) considering the ionization of functional groups and the anionic dye molecules. The number of positively charged sites of the nanofiber decreases at high pH values.
Figure 5

Effect of pH on dye removal by the modified nanofiber (Dye = 30 mg/L, V = 250 mL, t = 60 min and T = 25 °C).

Figure 5

Effect of pH on dye removal by the modified nanofiber (Dye = 30 mg/L, V = 250 mL, t = 60 min and T = 25 °C).

Experiments were done at different initial dye concentrations ranging from 10 to 70 mg/L (Figure 6). The dye removal decreases with an increase in the initial dye concentration because of the saturation of adsorption sites on the adsorbent surface (Zou et al. 2013).
Figure 6

Effect of dye concentration on dye removal by the modified nanofiber. (a) DB78 and (b) DR23 (V = 250 mL, t = 60 min, T = 25 °C and pH = 2.1).

Figure 6

Effect of dye concentration on dye removal by the modified nanofiber. (a) DB78 and (b) DR23 (V = 250 mL, t = 60 min, T = 25 °C and pH = 2.1).

Temperature can affected the dye removal ability of the adsorbent, because it changes the adsorption capacity of the adsorbent (Kahraman et al. 2012; Yagub et al. 2014). In order to investigate the effect of temperature on the dye adsorption capacity of the modified nanofiber, the experiments were performed at different temperatures (25–55 °C) (Figure 7). The experimental results show that the adsorption capacity increases with increasing temperature. It indicates that the adsorption of DR23 and DB78 on the modified nanofiber is endothermic in nature.
Figure 7

Effect of temperature on dye removal by the modified nanofiber. (a) DB78 and (b) DR23 (Dye = 30 mg/L, V = 250 mL, t = 60 min and pH = 2.1).

Figure 7

Effect of temperature on dye removal by the modified nanofiber. (a) DB78 and (b) DR23 (Dye = 30 mg/L, V = 250 mL, t = 60 min and pH = 2.1).

The effect of nanofiber dosage on dye removal is shown in Figure 8. The data indicated that dye adsorption increased with adsorbent dosage since there were more adsorption sites against a constant amount of dye molecule. Based on Figure 8, the amount of 0.02 g was considered as the optimum amount of adsorbent and used for further study.
Figure 8

Effect of adsorbent dosage on dye removal by the modified nanofiber. (a) DB78 and (b) DR23 (Dye = 30 mg/L, V = 250 mL, t = 60 min, T = 25 °C and pH = 2.1).

Figure 8

Effect of adsorbent dosage on dye removal by the modified nanofiber. (a) DB78 and (b) DR23 (Dye = 30 mg/L, V = 250 mL, t = 60 min, T = 25 °C and pH = 2.1).

In Figure 9, the effect of the adsorption time of dye removal from the solutions by the modified nanofiber can be observed. The adsorption was initially quite rapid during the first 5 min and then slowed down, and reached equilibrium. Rapid adsorption in the initial minutes is due to a great number of vacant adsorption sites existing on the surface of the nanofiber (Mahmoodi & Shourijeh 2016).
Figure 9

The effect of adsorption time of dye removal by the modified nanofiber (Dye = 30 mg/L, V = 250 mL, T = 25 °C and pH = 2.1).

Figure 9

The effect of adsorption time of dye removal by the modified nanofiber (Dye = 30 mg/L, V = 250 mL, T = 25 °C and pH = 2.1).

Adsorption isotherm

The interaction of the adsorbent and adsorbate molecule is studied using an adsorption isotherm. In addition, isotherm plays an important role to understand the adsorption mechanism.

The values of isotherm constants are shown in Table 4. As can be seen from the R2 values of DB78 and DR23, the dye removal isotherm can be approximated as the Langmuir model (Table 4). It confirms that the one layer adsorption of DB78 and DR23 takes place at specific homogeneous sites of the modified nanofiber surface.

Table 4

Isotherm constants for dye adsorption on the modified nanofiber at different dye concentrations

Dye Isotherm
 
Langmuir
 
Freundlich
 
Tempkin
 
R2 qm (mg/g) KL (L/g) R2 1/n KF R2 B1 KT 
DR23 0.99 370 0.66 0.96 0.28 368.12 0.97 59 378 
DB78 0.99 400 1.25 0.90 0.25 487.07 0.91 60 1,779 
Dye Isotherm
 
Langmuir
 
Freundlich
 
Tempkin
 
R2 qm (mg/g) KL (L/g) R2 1/n KF R2 B1 KT 
DR23 0.99 370 0.66 0.96 0.28 368.12 0.97 59 378 
DB78 0.99 400 1.25 0.90 0.25 487.07 0.91 60 1,779 

A modified Langmuir–Freundlich (MLF) can be used to explain the adsorption of pollutants (silver ions) onto adsorbents (halloysite nanotubes) (Kiani 2014). This isotherm is the only isotherm that can model the pH-dependent adsorption effect. In addition, it has relatively fewer complex and a lesser number of parameters compared to the other models such as the surface complex model (Chen et al. 2008). The Langmuir–Freundlich isotherm is as (Inyang et al. 2014): 
formula
4
qm is the maximum adsorption capacity (mg/g), with the value of qm determined by measuring isotherm data at the low pH value of 2.1 (Jeppu et al. 2010). The total six unknowns (five Ka and one n) were estimated with MATLAB Optimizer. The value of n at different pH values has a constant value because n is a material property. In this study, we used different initial dye concentrations (10, 20, 30, 40, 50, and 60 mg/L) at a constant pH value (2.1, 3, 4, 6 and 8) to obtain the dye equilibrium concentration.
When Ka is expressed as a function of pH, the MLF isotherm was written as Equation (5). 
formula
5
To study the relationship between Ka and pH value, we draw logKa vs. pH. This plot is shown in Figure 10. It shows the linear relationship between logKa for both dyes and pH value as explained in Equations (6) and (7) for adsorption of DR23 and DB78. The R2 value was 0.990 and 0.962 in DR23 and DB78, respectively. Jeppu and Clement showed the linear relationship between logKa vs. pH (Jeppu & Clement 2012). The estimated parameters of the MLF model are given in Table 5 for dye adsorption on the modified nanofiber. When the pH value decreases, the amount of logKa increases. At lower pH values, the surface sites become more positively charged, so the surface will adsorb more negatively charged ions (anionic dye molecules) (Jeppu & Clement 2012). The n value shows the heterogeneity of the system. The n value for our system is close to 1, thus our system has a homogenous material. 
formula
6
 
formula
7
Table 5

The MLF parameters for dye adsorption on the modified nanofiber

Dye pH qm (mg/g) n Ka (L/mg) 
DR23 2.1 337.83 0.666 
337.83 0.318 
337.83 0.203 
337.83 0.093 
337.83 0.031 
DB78 2.1 406.504 0.736 1.848 
406.504 0.736 0.809 
406.504 0.736 0.218 
406.504 0.736 0.065 
406.504 0.736 0.026 
Dye pH qm (mg/g) n Ka (L/mg) 
DR23 2.1 337.83 0.666 
337.83 0.318 
337.83 0.203 
337.83 0.093 
337.83 0.031 
DB78 2.1 406.504 0.736 1.848 
406.504 0.736 0.809 
406.504 0.736 0.218 
406.504 0.736 0.065 
406.504 0.736 0.026 
Figure 10

Affinity constant Ka for dye adsorption on the modified nanofiber. (a) DB78 and (b) DR23.

Figure 10

Affinity constant Ka for dye adsorption on the modified nanofiber. (a) DB78 and (b) DR23.

The best fit data were those obtained by minimizing the error function, root mean square error, as defined in Equation (8). 
formula
8
where m is the number of data at each pH value. The experimental data and the predicted results of adsorbent capacity (q) vs. dye concentration are shown in Figure 11. The R2 values of fitting were 0.93, 0.94, 0.97, 0.95 and 0.92 at pH 2.1, 3, 4, 6 and 8 for DR23, and the R2 values of fitting were 0.95, 0.93, 0.93, 0.89 and 0.91 at pH 2.1, 3, 4, 6 and 8 for DB78, respectively. The R2 values indicated the MLF model was able to closely match the experimental data.
Figure 11

Adsorbent capacity (q) vs. dye concentration for different pH values (Experimental data (closed symbols) and the MLF model predictions (dashed lines)). (a) DR23 and (b) DB78.

Figure 11

Adsorbent capacity (q) vs. dye concentration for different pH values (Experimental data (closed symbols) and the MLF model predictions (dashed lines)). (a) DR23 and (b) DB78.

Dye adsorption kinetics

Operating conditions for the full-scale batch process choose the kinetics of dye adsorption onto adsorbent materials because the adsorption kinetics show the solute adsorption rate, and obviously this rate controls the residence time of the adsorbate at the solution interface. When designing the adsorption system, rate control for the residence time of the adsorbate at the solution interface is most important (Yagub et al. 2014). In addition, the adsorption kinetics were obtained by fitting of pseudo-first order, pseudo-second order and intraparticle diffusion. The equations of the adsorption kinetics are shown in Table 6.

Table 6

Adsorption kinetic models

Adsorption kinetic Equation 
Pseudo-first order log(qe–qt) = log(qe)–(k1/2.303)t 
Pseudo-second order  
Intra particle diffusion  
Adsorption kinetic Equation 
Pseudo-first order log(qe–qt) = log(qe)–(k1/2.303)t 
Pseudo-second order  
Intra particle diffusion  

The results of the adsorption kinetics experiments show that the pseudo-second order models fitted well to the experimental data for all the adsorbent doses with the coefficients of determination (R2) (Table 7). The calculated values of qe from the pseudo-second order models were approximately equal to the experimental values of qe.

Table 7

Kinetics constants for dye adsorption on the modified nanofiber at different dye concentrations

Dye (mg/L) (qe)Exp (mg/g) Pseudo-first order
 
Pseudo-second order
 
Intraparticle diffusion
 
R2 (qe)Cal (mg/g) k1 (1/min) R2 (qe)Cal (mg/g) k2 (g/mg/min) R2 I kp (mol/g min1/2
 DR23          
10 120.98 0.98 19.49 0.053 0.99 121.95 0.0080 0.96 98.32 3.05 
30 276.29 0.88 38.90 0.061 0.99 277.77 0.0044 0.94 237.14 5.33 
50 304.92 0.98 31.65 0.054 0.99 303.03 0.0057 0.94 270.93 4.73 
70 355.76 0.95 194.98 0.95 0.99 370.37 0.0062 0.95 154.8 27.82 
 DB78          
10 123.31 0.92 36.30 0.105 0.99 125 0.91 0.91 95.45 4.07 
30 328.7 0.99 54.95 0.057 0.99 333.3 0.95 0.95 270.88 8.06 
50 437.71 0.97 41.68 0.028 0.99 434.78 0.97 0.97 385.62 6.32 
70 397.11 0.99 120.22 0.0099 0.99 400 0. 81 0.81 234.19 17.86 
Dye (mg/L) (qe)Exp (mg/g) Pseudo-first order
 
Pseudo-second order
 
Intraparticle diffusion
 
R2 (qe)Cal (mg/g) k1 (1/min) R2 (qe)Cal (mg/g) k2 (g/mg/min) R2 I kp (mol/g min1/2
 DR23          
10 120.98 0.98 19.49 0.053 0.99 121.95 0.0080 0.96 98.32 3.05 
30 276.29 0.88 38.90 0.061 0.99 277.77 0.0044 0.94 237.14 5.33 
50 304.92 0.98 31.65 0.054 0.99 303.03 0.0057 0.94 270.93 4.73 
70 355.76 0.95 194.98 0.95 0.99 370.37 0.0062 0.95 154.8 27.82 
 DB78          
10 123.31 0.92 36.30 0.105 0.99 125 0.91 0.91 95.45 4.07 
30 328.7 0.99 54.95 0.057 0.99 333.3 0.95 0.95 270.88 8.06 
50 437.71 0.97 41.68 0.028 0.99 434.78 0.97 0.97 385.62 6.32 
70 397.11 0.99 120.22 0.0099 0.99 400 0. 81 0.81 234.19 17.86 

Adsorption thermodynamic

The dye removal was evaluated by Gibbs free energy (ΔG0, kJ/mol), enthalpy (ΔH0, kJ/mol) and entropy (ΔS0, kJ/mol K) changes. The thermodynamic parameter ΔG0 was estimated using the parameters obtained in the best fit of the isotherm according to Equation (9), and ΔH0 and ΔS0 were determined by the van 't Hoff plot, adjusting data to Equation (10) and obtaining a slope ΔH0/R and interception ΔS0/R (Johir et al. 2016). 
formula
9
 
formula
10
where KL is the Langmuir constant obtained from the isotherms that showed a better fit, T is the absolute temperature (K) and R is the universal gas constant (8.314 × 103 kJ mol−1 K−1). An adsorption thermodynamics study was conducted to determine the different thermodynamic parameters involved in the adsorption process. From the batch adsorption isotherms at pH 2.1 and different temperatures (25, 35, 45 and 55 °C), the thermodynamic parameters, Langmuir constant (KL), Gibbs free energy (ΔG0), change in enthalpy (ΔH0), and change in entropy (ΔS0) were calculated and are shown in Table 8. The ΔG0 value is negative for DR23 and DB78 due to the spontaneity of dye adsorption by the modified nanofiber. The positive values of ΔH0 and positive values of ΔS0 proved the endothermic nature of the dye adsorption and the increased randomness of the adsorbent–solution interface during the adsorption process, respectively (Pohndorf et al. 2016).
Table 8

Thermodynamic parameters for dye adsorption on PVA/DETA/EDA/GA nanofiber

KL
 
ΔH° (kJ/mol) ΔS° (kJ/mol) ΔG (kJ/mol)
 
25 °C 35 °C 45 °C 55 °C 25 °C 35 °C 45 °C 55 °C 
DR23 
 14 26 46 36.66 4.210 0.0167 −0.7666 −0.9336 −1.1006 −1.2676 
DB78 
 35.71 48.07 100 180.01 5.437 0.0217 −1.0296 −1.2466 −1.4636 −1.6806 
KL
 
ΔH° (kJ/mol) ΔS° (kJ/mol) ΔG (kJ/mol)
 
25 °C 35 °C 45 °C 55 °C 25 °C 35 °C 45 °C 55 °C 
DR23 
 14 26 46 36.66 4.210 0.0167 −0.7666 −0.9336 −1.1006 −1.2676 
DB78 
 35.71 48.07 100 180.01 5.437 0.0217 −1.0296 −1.2466 −1.4636 −1.6806 

Comparison of pollutant removal ability of PVA/DETA/EDA/GA nanofiber with other adsorbents

The maximum adsorption capacity of PVA/DETA/EDA/GA nanofiber for DB78 and DR23 with other adsorbents is presented in Table 9. It could be seen that PVA/DETA/EDA/GA nanofiber has a higher adsorption capacity in comparison to the other adsorbents.

Table 9

Comparing the maximum adsorption capacity of PVA/DETA/EDA/GA nanofiber for adsorption dye with other previously prepared adsorbents

Absorbent (nanofibers) Application   Pollutants Adsorption capacity (mg/g) Reference 
PVA/DETA/EDA/GA Adsorption of dye   DB78 400 This work 
DR23 370 
PVA/chitosan Adsorption of dye   Direct Red 80 151 Mahmoodi & Shourijeh (2015)  
Direct Red 81 95 
Reactive Red 180 114 
PVA/SiO2 Adsorption of dye M-NH2 Indigo Carmine 287 Teng et al. (2011)  
M-CD 495 
PVA/ZnO Adsorption of metal ions   U(VI) 370.86 Hallaji et al. (2015)  
Cu(II) 162.48 
Ni(II) 94.43 
PVP/ZnO/SnO2 Adsorption of dye  Congo Red 90.8 Chen et al. (2015)  
chitosan/PVA/zerovalent iron Adsorption of metal ions   Arsenic (III) 142.9 Chauhan et al. (2014)  
Arsenic (V) 200.0 
HTCC/PVA Adsorption of a non-enveloped mammalian virus  Pathogens 55.730 Mi & Heldt (2014)  
Chitosan/PVA/zeolite (composite) Adsorption of metal ions  Chromium (VI) 450 Habiba et al. (2017)  
Fe3O4/SiO2/APTES/PVA Adsorption of metal ions  Uranium (VI) 68.96 Mirzabe & Keshtkar (2015)  
pgf-PVA/SiO2 Adsorption of metals ions   Manganese(II) 234.7 Islam et al. (2015)  
Nickel(II) 55,730 
Chitosan Adsorption of metals ions  Chromium (VI) 131.5 Li et al. (2015)  
PVA/MOF Adsorption of metals ions PVA/Sb-TBC Pb(II) 92.27 Shooto et al. (2016)  
PVA/Sr-TBC 58.85 
PVA/La-TBC 92.27 
PVA/TEOS/APTES Adsorption of metal ions  Uranium (VI) 168.1 Keshtkar et al. (2013)  
PVA-co-PE Adsorption of metal ions  Bilirubin 110 Wang et al. (2016)  
PVA/TEOS/APTES Adsorption of metal ions  Cadmium(II) 327.3 Irani et al. (2012)  
Chitosan/TiO2 Adsorption of metal ions   Cu(II) 715 Razzaz et al. (2016)  
Pb(II) 570 
Co(II) 510 
Ni(II) 45 
PVA/TETA/GA Adsorption of dye   Direct Red 80 118 Mahmoodi & Shourijeh (2016)  
Direct Red 81 177 
Reactive Red 180 181 
Absorbent (nanofibers) Application   Pollutants Adsorption capacity (mg/g) Reference 
PVA/DETA/EDA/GA Adsorption of dye   DB78 400 This work 
DR23 370 
PVA/chitosan Adsorption of dye   Direct Red 80 151 Mahmoodi & Shourijeh (2015)  
Direct Red 81 95 
Reactive Red 180 114 
PVA/SiO2 Adsorption of dye M-NH2 Indigo Carmine 287 Teng et al. (2011)  
M-CD 495 
PVA/ZnO Adsorption of metal ions   U(VI) 370.86 Hallaji et al. (2015)  
Cu(II) 162.48 
Ni(II) 94.43 
PVP/ZnO/SnO2 Adsorption of dye  Congo Red 90.8 Chen et al. (2015)  
chitosan/PVA/zerovalent iron Adsorption of metal ions   Arsenic (III) 142.9 Chauhan et al. (2014)  
Arsenic (V) 200.0 
HTCC/PVA Adsorption of a non-enveloped mammalian virus  Pathogens 55.730 Mi & Heldt (2014)  
Chitosan/PVA/zeolite (composite) Adsorption of metal ions  Chromium (VI) 450 Habiba et al. (2017)  
Fe3O4/SiO2/APTES/PVA Adsorption of metal ions  Uranium (VI) 68.96 Mirzabe & Keshtkar (2015)  
pgf-PVA/SiO2 Adsorption of metals ions   Manganese(II) 234.7 Islam et al. (2015)  
Nickel(II) 55,730 
Chitosan Adsorption of metals ions  Chromium (VI) 131.5 Li et al. (2015)  
PVA/MOF Adsorption of metals ions PVA/Sb-TBC Pb(II) 92.27 Shooto et al. (2016)  
PVA/Sr-TBC 58.85 
PVA/La-TBC 92.27 
PVA/TEOS/APTES Adsorption of metal ions  Uranium (VI) 168.1 Keshtkar et al. (2013)  
PVA-co-PE Adsorption of metal ions  Bilirubin 110 Wang et al. (2016)  
PVA/TEOS/APTES Adsorption of metal ions  Cadmium(II) 327.3 Irani et al. (2012)  
Chitosan/TiO2 Adsorption of metal ions   Cu(II) 715 Razzaz et al. (2016)  
Pb(II) 570 
Co(II) 510 
Ni(II) 45 
PVA/TETA/GA Adsorption of dye   Direct Red 80 118 Mahmoodi & Shourijeh (2016)  
Direct Red 81 177 
Reactive Red 180 181 

CONCLUSIONS

In this paper, modification of nanofiber was done using different amines such as DETA and EDA in the presence of GA and its ability to remove dye from colored wastewater was investigated. DR23 and DB78 were used. The characteristics of the modified nanofiber were studied by FTIR and SEM. The adsorption data satisfactorily fitted to the Langmuir adsorption isotherm and pseudo-second kinetic order. Maximum dye adsorption capacity was 400 mg/g for DB78 and 370 mg/g for DR23. The satisfactory fit to the Langmuir adsorption isotherm suggests that the adsorption sites were homogeneous with monolayer adsorption coverage. Furthermore, the specific adsorption capacity of PVA/DETA/EDA/GA nanofiber increased when the initial dye concentration increased. The thermodynamic parameters, ΔG0 and ΔH0 were negative and positive, respectively, showing that the adsorption process was spontaneous and endothermic.

REFERENCES

REFERENCES
Chauhan
D.
Dwivedi
J.
Sankararamakrishnan
N.
2014
Novel chitosan/PVA/zerovalent iron biopolymeric nanofibers with enhanced arsenic removal applications
.
Environ. Sci. Pollut. Res.
21, 9430
9442
.
Chen
C. L.
Hu
J.
Xu
D.
Tan
X. L.
Meng
Y. D.
Wang
X. K.
2008
Surface complexation modeling of Sr (II) and Eu (III) adsorption onto oxidized multiwall carbon nanotubes
.
J. Colloid Interface Sci.
323
,
33
41
.
Chen
X.
Zhang
F.
Wang
Q.
Han
X.
Li
X.
Liu
J.
Lin
H.
Qu
F.
2015
The synthesis of ZnO/SnO2 porous nanofibers for dye adsorption and degradation
.
Dalton Transactions
44, 3034
3042
.
Chowdhury
S.
Chakraborty
S.
Saha
P.
2011
Biosorption of Basic Green 4 from aqueous solution by Ananas comosus (pineapple) leaf powder
.
Colloids Surfaces B Biointerfaces
84
,
520
527
.
Ferrarini
F.
Bonetto
L. R.
Crespo
J. S.
Giovanela
M.
2016
Removal of Congo red dye from aqueous solutions using a halloysite-magnetite-based composite
.
Water Sci. Technol.
73
,
2132
2142
.
Freundlich
H.
1906
Over the adsorption in solution
.
J. Phys. Chem.
57
,
385
470
.
Habiba
U.
Siddique
T. A.
Joo
T. C.
Salleh
A.
Ang
B. C.
Afifi
A. M.
2017
Alcohol/Zeolite composite for removal of methyl orange, congo red and chromium (VI) by flocculation/adsorption
.
Carbohydr. Polym.
157
,
1568
1576
.
Islam
M. S.
Rahaman
M. S.
Yeum
J. H.
2015
Phosphine-functionalized electrospun poly(vinyl alcohol)/silica nanofibers as highly effective adsorbent for removal of aqueous manganese and nickel ions
.
Colloids Surfaces A: Physicochem. Eng. Aspects
48
,
49
18
.
Kampalanonwat
P.
Supaphol
P.
2011
Preparation and adsorption behavior of aminated electrospun polyacrylonitrile nanofibers mats for heavy metal ion removal
.
Appl. Mater. Interface
12
,
3619
3627
.
Khosla
E.
Kaur
S.
Dave
P. N.
2013
Tea waste as adsorbent for ionic dyes
.
Desalin. Water Treat.
13
,
1
10
.
Mahmoodi
N. M.
Limaee
N. Y.
Arami
M.
Borhany
S.
Mohammad-Taheri
M.
2007
Nanophotocatalysis using nanoparticles of titania: Mineralization and finite element modelling of Solophenyl dye decolorization
.
J. Photochem. Photobiol. A: Chem.
189
,
1
6
.
Mahmoodi
N. M.
Mokhtari-Shourijeh
Z.
2015
Preparation of PVA-chitosan blend nanofiber and its dye removal ability from colored wastewater
.
Fibers Polym.
161
,
861
869
.
Mi
X.
Heldt
C. L.
2014
Adsorption of a non-enveloped mammalian virus to functionalized nanofibers
.
Colloids Surfaces B: Biointerfaces
121
,
319
324
.
Razzaz
A.
Ghorban
S.
Hosayni
L.
Irani
M.
Aliabadi
M.
2016
Chitosan nanofibers functionalized by TiO2 nanoparticles for the removal of heavy metal ions
.
J. Taiwan Inst. Chem. Eng.
58
,
333
343
.
Revathi
G.
Ramalingam
S.
Subramaniam
P.
2014
Assessment of the adsorption kinetics and equilibrium for the potential removal of direct yellow – 12 dye using Jatropha curcus L. activated carbon
.
Chem. Sci. Trans.
3
,
93
106
.
Shooto
N. D.
Dikio
C. W.
Wankasi
D.
Sikhwivhilu
L. M.
Mtunzi
F. M.
Dikio
E. D.
2016
Novel PVA/MOF nanofibres: fabrication, evaluation and adsorption of lead ions from aqueous solution
.
Nanoscale Res. Let.
11
,
414
.
Srinivasan
A.
Viraraghavan
T.
2010
Decolorization of dye wastewaters by bio sorbents: a review J
.
Environ. Manage.
91
,
1915
1929
.
Tahaei
P.
Abdouss
M.
Edrissi
M.
Shoushtari
A. M.
Zargaran
M.
2008
Preparation of chelating fibrous polymer by different diamines and study on their physical and chemical properties
.
Mater. Sci. Eng. Technol.
39
,
839
844
.
Wang
W.
Zhang
H.
Mengying
Z. Z.
Wang
L. Y.
Liu
Q.
Chen
Y.
Li
M.
Wang
D.
2016
Amine-functionalized PVA-co-PE nanofibrous membrane as affinity membrane with high adsorption capacity for bilirubin
.
Colloids Surfaces B: Biointerfaces
.
Yagub
M. T.
Sen
T.
Afroze
K. S.
Ang
H. M.
2014
Dye and its removal from aqueous solution by adsorption: a review
.
Adv. Colloid Interface Sci.
209
,
172
184
.
Zodi
S.
Merzouk
B.
Potier
O.
Lapicque
F.
Leclerc
P.
2013
Direct red 81 dye removal by a continuous flow electrocoagulation/flotation reactor
.
Sep. Purif. Technol.
108
,
215
222
.