Abstract

In this work, the preparation and characterization of glutaraldehyde-crosslinked electrospun nanofibers of chitosan/poly(vinyl alcohol) (GCCPN) as a new adsorbent for tetracycline (TC) is reported. Electrospun nanofibers of chitosan/poly(vinyl alcohol) (PVA) were prepared by employing a 75:25 volumetric ratio of chitosan:PVA, voltage of 30 kV, collection distance of 10 cm, and injection flow rate of 2 mL/h. Then, the nanofibers were crosslinked via applying the glutaraldehyde on them for 3 h at 40 °C. The nanofibers were characterized using scanning electron microscopy, Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction. Uniform beadless nanofibers with minimum diameters of 3–11 and 6–18 nm were obtained before and after crosslinking, respectively. Then the applicability of the synthesized GCCPN for removal of TC from aqueous solutions was investigated. The response surface method was applied to evaluate the influence of pH (6–12), TC concentration (50–250 mg/L) and the adsorbent dose (0.05–0.25 g in 20 mL solution) on the adsorption characteristics of GCCPN. The maximum adsorption capacity was 102 mg/g. The adsorption kinetics was explained most effectively by the pseudo-second-order model. The adsorption data of TC on the GCCPN surface was explained well by the Langmuir isotherm model.

INTRODUCTION

Tetracycline, a well-known pharmaceutical antibiotic, is used extensively for the treatment and prevention of infectious diseases of human and animals, as well as feed additives employed to promote growth (Boxall et al. 2003). However, its residues have been detected worldwide in aquatic environments, including surface, ground and even drinking water (Nikolaou et al. 2007). Therefore, development of efficient and economical treatment techniques as well as improvement of present methods for removal of tetracycline (TC) from aqueous environments is urgently necessary (Liu et al. 2015). Different techniques have been investigated for the removal of TC. These include biodegradation, advanced oxidation, photocatalytic degradation and ozonation, ion-exchange, nanofiltration, membrane filtration, reverse osmosis and adsorption (Le-Minh et al. 2010). Membrane-based technologies are hampered by the problem of fouling, and advanced oxidation processes are limited by the formation of undesirable oxidation by-products, hindering their popularity in water treatment plants. Moreover, the costs of the chemical reagents and sludge disposal are usually high, and therefore they are an important concern in the chemical treatment options (Wang et al. 2016b).

Adsorption has been considered as an efficient method for the removal of TC from aqueous solutions (Wang et al. 2016a). A variety of different adsorbents including rice husk ash, porous synthetic resins, graphene oxide, activated carbon from potato peels, bamboo charcoal and clays have been used for the removal of TC (Fan et al. 2010; Gao et al. 2012; Chen et al. 2016a).

Chitosan, as a natural polymer, attracted significant attention due to its desirable properties, such as hydrophilicity, biocompatibility, and biodegradability, as well as its excellent adsorption capability (Li et al. 2016b). Researchers have used chitosan-based materials for adsorption of different pollutants from water. Some examples are: removal of heavy metals from aqueous solutions using chitosan (Mende et al. 2016), adsorption of nickel ions from water using chitosan and its triethylenetetramine derivative (Liao et al. 2016) and removal of chromium ions from water using chitosan/polycaprolactam nanofibrous filter paper (NFP) (Li et al. 2016b).

However, chitosan has a low specific surface area in the flake form. In addition, chitosan in the powder form cannot be easily separated from solution. These issues have imposed practical limitations on the industrial applicability of the chitosan as an adsorbent. Adsorbent materials should have small pore diameters and high surface area to achieve a high degree of adsorption. In addition, simple separation of the adsorbent from solution is a practically notable issue.

There has been much progress in the development of electrospinning techniques. Electrospinning has been introduced as a simple technique to produce ultrafine and morphologically controllable fibers. High specific surface area, high porosity and small pore size are some of the interesting characteristics of the fibers obtained by this method (Huang et al. 2003). A variety of different synthetic polymers such as poly(ethylene oxide) (Kriegel et al. 2009) and poly(vinyl alcohol) (Pakravan et al. 2011) have been used to generate electrospun nanofibers. NFP has been prepared using the electrospinning of chitosan/polycaprolactam (PA6), and the maximum value of chromium adsorption was reported as 114.7 mg/g, which was apparently greater than some previously reported chitosan-based sorbents (Li et al. 2016b).

However, the mechanical drawbacks and solubility problems of chitosan impose serious limitations on the applicability of the electrospinning methods for production of chitosan fibers. To overcome these aforementioned problems, the blends of chitosan with different polymers such as poly(ethylene oxide) (PEO), poly(vinyl alcohol) (PVA), poly(lactic acid), poly(caprolactone), cellulose and nylon-6 have been proposed by researchers (Miya et al. 1980). By using PEO, a remarkable positive deviation in the zero shear viscosity of chitosan/PEO blends was observed, causing strong hydrogen bonding between functional groups of chitosan and PEO chains which resulted in an electrospinnable blend (Kriegel et al. 2009). Among these polymers, PVA is the most used polymer for blending of chitosan due to its non-toxic nature, water solubility, and suitable fiber characteristics (Lin et al. 2006).

The aim in this work was to use chitosan-based nanofibers for adsorption of TC from aqueous solutions. Nanofibers of chitosan/PVA were synthesized using electrospinning technique and crosslinked via reaction with glutaraldehyde. The adsorption behaviour of the crosslinked nanofibers for the removal of TC from aqueous solutions was studied.

MATERIALS AND METHODS

Materials

Chitosan (medium molecular weight), tetracycline hydrochloride (99%, ACS grade), acetic acid and glutaraldehyde solution grade I (25% in water), were purchased from Sigma-Aldrich Chemical Co. PVA (molecular weight = 120,000) was obtained from Samchun Chemical Co., Ltd (South Korea). Deionized water was used to prepare all aqueous solutions.

Measurement of viscosity

The viscometry method was used to measure the molecular weights of the purchased chitosan powder and the hydrolyzed chitosan. Three different solutions of chitosan (0.001, 0.0005 and 0.00025 g/L) were prepared in an aqueous solvent system containing NaCl 0.2 M and acetic acid 0.1 M. The dropping times, through an Ubbelohde capillary tube, were measured separately for solvent (t0) and for chitosan solution (t1). Equation (1) was used to calculate the intrinsic viscosity, as extrapolated at zero concentration. This equation could estimate the reduced viscosity, ηred.  
formula
(1)
where C is the concentration of chitosan.

Based on the Mark–Howink equation in which [ɳ] = using 1.8 × 103 g/mol and 0.93, respectively, for Km and a as the constants for the used solvent system (Homayoni et al. 2009), the molecular weight (Mv) of the used chitosan was obtained to be 3.97 × 105 g/mol.

Preparation of glutaraldehyde-crosslinked chitosan/PVA nanofibers

Electrospinning solutions were prepared by dissolving PVA (1.5 g) in 10 mL aqueous acetic acid solution (90% v/v). The solutions were stirred at 70 °C until the PVA was completely dissolved. Separately, 0.6 g hydrolyzed chitosan was dissolved in another 10 mL aqueous acetic acid solution (90% v/v). After cooling the PVA solution to room temperature, these two solutions were mixed for 24 h. The prepared mixture was loaded into a 1 mL syringe with a needle tip (23 gauge, 0.337 mm inner diameter) and, using a micro-syringe pump, the flow rate of 1.5 mL/h was achieved. Voltage of 22 kV was applied and collection distance of 15 cm was used for the aluminum foil collector from the needle tip. A view of the electrospinning apparatus is presented in Figure 1. The influence of four parameters, flow rate of solution (1–2 mL/h), voltage (15–30 kV), distance between needle and collector (10–20 cm), and the chitosan/PVA volumetric ratio (25:75, 50:50, 75:25, 100:0), on electrospinning was investigated based on the variation in the adsorption of TC.

Figure 1

Schematic view (a) and main view (b) of electrospinning apparatuses.

Figure 1

Schematic view (a) and main view (b) of electrospinning apparatuses.

The formed chitosan/PVA blend nanofiber was kept at room temperature (24 ± 2 °C) for 2 min before being peeled off from the aluminum foil. The collected nanofibers on the foils were pasted on the wall of desiccators, and then a Petri dish containing glutaraldehyde was placed at the bottom of the desiccator for time intervals of 1, 3, 5 and 7 h at 40 °C to optimize the degree of the crosslinking of nanofibers. After each time interval, the samples were weighed and then kept in distilled water for 72 h. The samples were removed form the distilled water, dried and weighed again. Each experiment was repeated three times.

Instrument setups used for characterization of the prepared nanofibers

The morphology of nanofibers was examined using scanning electron microscopy (SEM) (TESCAN-VEGA3 SBU-Easy Probe model made in the Czech Republic). Nanofibers' diameter distribution was calculated using the Image J software, from the SEM images. To obtain more reliable data, using microstructure measurement software, 60 points of each sample were analyzed, and the average diameters were calculated.

By using Fourier transmission infrared (FTIR) spectroscopy, FTIR spectra (Tensor 27 model made in Germany) of the chitosan powder, PVA crystal, chitosan/PVA blend nanofibrous mats and crosslinked chitosan/PVA blend nanofibrous mats were recorded at room temperature using a Perkin Elmer 65 FTIR-ATR instrument. The spectra of the samples were recorded over a wavenumber range of 600 to 4,000 cm−1.

The X-ray diffraction (XRD) patterns of the nanofibers were obtained using an X-ray diffractometer (Philips, The Netherlands) with a back monochromatic and a Cu anticathode. The scanning range was 5° < 2θ > 85° at room temperature (25 °C).

Tetracycline adsorption experiments

Adsorption experiments were performed in batch mode; typically a pre-determined dose (g) of glutaraldehyde-crosslinked chitosan/PVA nanofibers (GCCPN) as the adsorbent was added to a 20 mL aqueous solution containing a known concentration of TC. A 1,000 mg/L solution of TC in deionized water was used as the stock solution. These experiments were performed at room temperature (24 ± 2 °C) with the shaking rate of 120 rpm.

The adsorption capacity qt (mg/g) of the GCCPN for TC at each time was calculated using the following equation:  
formula
(2)
where C0 and Ct (mg/L) are the concentration of TC at time zero (initial concentration) and time t, respectively, W is the weight of the adsorbent used (g), and V is the volume (L) of TC solution. A spectrophotometer (Rayleeih UV-2601) was used for determination of the TC concentration. The absorbance of the TC solutions were recorded at the wavelength of 268 nm.

Experimental design employed to study the influence of operating variables on adsorption

The effects of the initial pH of solution (A: 6, 9, 12), the initial concentration of TC (B: 50, 150, 250 mg/L) and the dose of adsorbent (C: 0.05, 0.15, 0.25 g) on the adsorption of TC from water were investigated using a central composite experimental design. The quadratic model was used to represent the response surface for the selected factors.

The designed experimental model included 27 experimental trials for adsorption parameters, with each experiment being conducted in duplicate. The TC removal efficiency (%) was chosen as response and then the vectors of variables (X) were considered. Single, double and triple coefficients (β) and the corresponding responses (Y) were used to develop an appropriate model. A quadratic model that can be fitted is defined by Equation (3):  
formula
(3)
where Y is the predicted response, β0 is the intercept parameter, βi is the linear coefficient, βii is the quadratic coefficient, βij is the interaction coefficient, and Xi and Xj are the coded values of the independent factors.

Analysis of variance was used to obtain the interaction between the variables and the responses (Ghafari et al. 2009) with 95% confidence interval (p-value less than 0.05 shows the significance of considered parameter).

Adsorption isotherms

The batch experiments of adsorption were performed by adding 0.15 g of the GCCPN into 20 mL of TC solutions containing different concentrations of TC (50 to 250 mg/L) at room temperature (24 ± 2 °C). Equilibrium data were analyzed via Langmuir, Freundlich and Temkin isotherm models of adsorption.

The Langmuir model explains the monolayer adsorption of the adsorbate (TC) onto the homogeneous structure of the adsorbent (GCCPN). This model is expressed by Equation (4) (Kurniawan et al. 2012):  
formula
(4)
where qmax is the maximum adsorption capacity (g/g), qe is the adsorption capacity of equilibrium (g/g), Ce is the TC concentration (g/L) at equilibrium, and KL is the constant of the isotherm (L/g).
The Freundlich isotherm model of adsorption can be applied to non-ideal sorption and also multilayer sorption on heterogeneous surfaces, and is expressed using Equation (5) (Finocchio et al. 2010):  
formula
(5)
where KF (mg/g) and n are constants of the Freundlich isotherm, indicating the adsorption capacity of GCCPN and the affinity of TC to the surface of the GCCPN, respectively.
The Temkin isotherm, considering the interactions between TC and adsorbent, assumes that the free energy of the sorption as a function of the surface coverage can be shown as Equation (6) (Papurello et al. 2016):  
formula
(6)
where bT is the constant of the Temkin isotherm, AT is the constant of equilibrium binding corresponding to the maximum energy of binding (L/mg), T is temperature (K) and R is the ideal gas constant.

Adsorption kinetics

In this research, the adsorption data were investigated using pseudo-first-order, pseudo-second-order and the intraparticle diffusion kinetics models. The pseudo-first-order kinetic equation is given as Equation (7) (Li et al. 2003):  
formula
(7)

In this model, qt is the amount of TC molecules removed at time t (mg/g), qe is the adsorption capacity at equilibrium (mg/g), k1 is the pseudo-first-order rate constant (1/min), and t is the contact time (min).

In the pseudo-second-order model, chemical sorption on the surface is the rate-limiting step, which gives high rates at the beginning of the adsorption process, due to high unoccupied sorption sites on the surface of the adsorbent. The pseudo-second-order model assumes that the removal of adsorbate (TC) from solution is due to physicochemical interactions between the two phases. This kinetic model is shown in Equation (8) (Wang et al. 2007).  
formula
(8)
It turns to a linear form in Equation (9):  
formula
(9)
where k2 (g/mg.min) is the pseudo-second-order rate constant of adsorption. The intraparticle diffusion model describes diffusion-controlled adsorption, in which the diffusion rate of TC towards GCCPN controls the rate of adsorption. In this model the equilibrium capacity can be presented by Equation (10):  
formula
(10)
where k3 is the rate constant of the intraparticle diffusion model (mg/g.min0.5), and L is the intercept (mg/g) (Hameed& Daud 2008).

RESULTS AND DISCUSSION

Determination of nanofiber diameters and morphology

All samples were studied using SEM to determine their morphology and diameter distribution. Fibers were not formed for samples with 100% chitosan (chitosan/PVA volumetric ratio of 100:0 which was used first). Figure 2 shows the nanofibers formed using different ratios of chitosan/PVA used for electrospinning. The smallest diameters of the electrospun fibers were obtained at the 50:50 volumetric ratio of chitosan/PVA. Chitosan is a cationic polysaccharide containing amino groups which are ionizable in the acidic or neutral solutions. There might be higher charge density on the surface of the chitosan-based jet ejected from electrospinning due to the ionic polyelectrolyte property of chitosan. It has been reported that the conductivity of polymer solution is increased in the presence of cationic and anionic polyelectrolytes. Increasing the charges carried by the ejected jet from electrospinning can cause higher elongation forces on the jet. Therefore, higher charge density of the jet can produce thinner fibers. By increasing the chitosan content in the electrospinning solution to 50:50, beads were generated in the formed mats (Figure 2(b)). The nanofiber diameter in webs was reduced, and more beads were generated by increasing the chitosan ratio of the electrospinning solution to 75:25, as shown in Figure 2(c). This might be due to the repulsive forces between ionic groups of the chitosan preventing the formation of continuous nanofibers (Homayoni et al. 2009; Charernsriwilaiwat et al. 2010). This reduction in the nanofiber sizes can cause an apparent enhancement in the adsorption capacity of the synthesized adsorbent. However, the reduction of the nanofiber sizes may be surmounted by the reduction of the free amine groups on the surface of the polymer due to the chitosan–glutaraldehyde interaction (Monteiro & Airoldi 1999). On the other hand, high degree of porosity and open structure of the adsorbent, shown in the SEM images, can enhance the possibility of penetration of target compounds into nanofiber mats and availability of the functional groups on the surface of nanofibers.

Figure 2

The SEM images (2,000× magnification) together with diameter distribution for prepared electrospun nanofibers with volumetric ratios equal to (a) 25:75, (b) 50:50, (c) 75:25, and (d) 100:0 of chitosan/PVA used for adsorption of tetracycline.

Figure 2

The SEM images (2,000× magnification) together with diameter distribution for prepared electrospun nanofibers with volumetric ratios equal to (a) 25:75, (b) 50:50, (c) 75:25, and (d) 100:0 of chitosan/PVA used for adsorption of tetracycline.

Figure 3 shows the effects of the crosslinking degree on the weight loss of the synthesized blend due to solubility in the water. Figure 3 also shows the formation of the highly crosslinked chitosan/PVA blend, after 3 h, significantly hindered the solubility of the blend; thus there was no notable weight loss of sample after 3 h.

Figure 3

The SEM images (2,000× magnification) of (a) chitosan/PVA nanofibers before crosslinking and (b) glutaraldehyde-crosslinked nanofibers of chitosan/PVA. (c) The chart of sample weight loss versus crosslink time.

Figure 3

The SEM images (2,000× magnification) of (a) chitosan/PVA nanofibers before crosslinking and (b) glutaraldehyde-crosslinked nanofibers of chitosan/PVA. (c) The chart of sample weight loss versus crosslink time.

FTIR spectroscopic analysis

The FTIR spectra of the chitosan powder, PVA crystals, chitosan/PVA nanofibers and GCCPN were recorded at room temperature. The obtained spectra are shown in Figure 4.

Figure 4

FTIR spectra of chitosan powder, PVA crystals, nanofibers of chitosan/PVA, and glutaraldehyde-crosslinked chitosan/PVA nanofibers.

Figure 4

FTIR spectra of chitosan powder, PVA crystals, nanofibers of chitosan/PVA, and glutaraldehyde-crosslinked chitosan/PVA nanofibers.

The presence of double absorption bands centered at about 3,300 and 3,370 cm−1confirms the presence of N-H groups in the chitosan structure. These double absorption bands are in overlapping condition with a broad band of the hydroxyl groups' stretching vibration. After crosslinking reaction between aldehyde groups of glutaraldehyde and some amine groups of chitosan, the imine groups might be formed. This conversion can be seen by the presence of a peak at about 1,660 cm−1 in the FTIR spectrum of GCCPN. Another confirmation of the presence of the glutaraldehyde moiety in the structure of the final GCCPN blend is the appearance of an unsaturated carbon-carbon double band absorption peak at about 1,590 cm−1 consistent with condensation of glutaraldehyde.

FTIR spectrum characteristic absorption bands of –OH groups at 3,421 and 1,096 cm−1 are consistent with pure PVA structure. The absorption bands of PVA at 2,939 and 1,419 cm−1 could be attributed to the stretching vibration and the bending vibration of C–H, respectively (Liu et al. 2011). Characteristic absorbance bands at 940 and 1,116 cm−1 are associated with saccharine, while the strong band at 1,651 cm−1 is attributed to amine groups for chitosan (Kumbar et al. 2002; 2003). Obviously, a broad absorption band of –OH in the range of 3,421–3,443 cm−1 covered the absorption bands of free amine groups in the GCCPN blend. Figure 4 shows that the existence of the relevant functional groups of both PVA and chitosan in the GCCPN blend is obvious. However, crosslinking of chitosan and PVA using the same crosslinking agent may give the same functional groups for all different prepared GCCPN.

XRD characterization of nanofibers

The XRD patterns of nanofibers (Figure 5) were characterized by a very distinct peak at around 20°. The average size of grains was obtained from the XRD pattern using Scherrer's equation (Shah et al. 2012):  
formula
(11)
where D is the distance between crystalline pages, k is the dimensionless shape factor (0.94), λ is the X-ray wavelength, and β stands for the full-width at the half maximum of the X-ray. Figure 5 shows that both chitosan and PVA had a sharp peak at the 2θ = 20°; also, PVA had a smaller peak at about 2θ = 40°. After the crosslinking process, the peaks at about 2θ = 40° were broad and short, indicating the lower degree of crystallinity of the crosslinked chitosan/PVA blend. The lower crystallinity of the crosslinked chitosan/PVA blend is an advantageous parameter for its applicability for adsorption of TC, because the polymers in crystalline forms have higher strength, thus lowering the structural flexibility for interaction with adsorbate molecules.
Figure 5

XRD results of the crystallinity of pure chitosan (a), pure PVA (b), nanofibers of chitosan/PVA (c), and glutaraldehyde-crosslinked chitosan/PVA nanofibers (d).

Figure 5

XRD results of the crystallinity of pure chitosan (a), pure PVA (b), nanofibers of chitosan/PVA (c), and glutaraldehyde-crosslinked chitosan/PVA nanofibers (d).

RSM analysis of adsorption conditions

After synthesis of the GCCPN blend, its capability for TC adsorption from aqueous solution was examined. Primary experiments showed that the adsorption rate of the TC molecules by the prepared GCCPN is fast during the first 30 h of the contact time. After this period, the adsorption rate is gradually reduced. It can be concluded that the equilibrium is reached within 48 h. Therefore, all experiments were performed within this time period. Based on the central composite design of Statgraphics Centurion XVII software, 27 runs of the adsorption experiments were conducted for the removal of TC by the prepared GCCPN adsorbent. In these experiments, the effects of three important factors on the adsorption efficiency of TC were investigated: initial pH of the solution (A), the initial concentration of TC (B) and the dose of adsorbent (C). The details of the factor levels together with the modelled and observed removal efficiency values are given in Table 1. During these 27 runs of the adsorption experiments the removal efficiency response of TC varies between 34% and 97%.

Table 1

Central composite design of three variables along with observed and modelled values

Run no. pH Initial concentration of tetracycline (mg/L) Dose of adsorbent (g) Removal efficiency (%)
 
Observed Predicted 
50 0.05 52.06 53.47 
12 150 0.15 97.69 97.60 
150 0.05 60.04 58.01 
150 0.25 34.99 34.37 
12 150 0.05 69.85 70.64 
250 0.05 89.02 89.83 
250 0.25 89.02 92.60 
150 0.15 53.78 54.90 
12 250 0.05 90.45 86.50 
10 150 0.15 73.1 71.72 
11 12 50 0.05 39.89 35.99 
12 50 0.15 53.34 55.04 
13 50 0.05 58.21 59.35 
14 12 250 0.25 95.02 94.89 
15 250 0.15 61.73 59.56 
16 50 0.15 65.44 64.33 
17 250 0.25 53.09 55.05 
18 12 250 0.15 70.88 63.12 
19 250 0.05 40.76 42.41 
20 12 150 0.25 45.79 47.47 
21 150 0.25 39.67 42.95 
22 50 0.25 68.88 71.18 
23 50 0.25 66.89 67.94 
24 150 0.05 45.64 45.58 
25 250 0.15 75.99 77.77 
26 12 50 0.05 86.34 85.78 
27 12 50 0.25 98.56 106.1 
Run no. pH Initial concentration of tetracycline (mg/L) Dose of adsorbent (g) Removal efficiency (%)
 
Observed Predicted 
50 0.05 52.06 53.47 
12 150 0.15 97.69 97.60 
150 0.05 60.04 58.01 
150 0.25 34.99 34.37 
12 150 0.05 69.85 70.64 
250 0.05 89.02 89.83 
250 0.25 89.02 92.60 
150 0.15 53.78 54.90 
12 250 0.05 90.45 86.50 
10 150 0.15 73.1 71.72 
11 12 50 0.05 39.89 35.99 
12 50 0.15 53.34 55.04 
13 50 0.05 58.21 59.35 
14 12 250 0.25 95.02 94.89 
15 250 0.15 61.73 59.56 
16 50 0.15 65.44 64.33 
17 250 0.25 53.09 55.05 
18 12 250 0.15 70.88 63.12 
19 250 0.05 40.76 42.41 
20 12 150 0.25 45.79 47.47 
21 150 0.25 39.67 42.95 
22 50 0.25 68.88 71.18 
23 50 0.25 66.89 67.94 
24 150 0.05 45.64 45.58 
25 250 0.15 75.99 77.77 
26 12 50 0.05 86.34 85.78 
27 12 50 0.25 98.56 106.1 
Table 1 shows the experimental data from the 27 runs and the modelled removal efficiency achieved by response surface method (RSM) as referred to in Equation (12). The coefficients were calculated by the regression method. Figure 6 shows each of the estimated effects and their interactions. By removing the insignificant parameters such as AB, AC, and BC, the obtained relationships between the chosen parameters and the removal efficiencies of TC can be shown by Equation (12):  
formula
(12)

The response surface quadratic model presented a high correlation coefficient (R2 = 0.98) accounting for 98.03% of the variability in the adsorption. Such a high R2 value indicated the goodness of the fit of the model to the actual data and the high statistical significance of the model. As a result, a suitable agreement between the experimental and predicted values of TC adsorption is observed in Table 1.

Figure 6

Standard Pareto chart of the adsorption parameters of tetracycline on GCCPN.

Figure 6

Standard Pareto chart of the adsorption parameters of tetracycline on GCCPN.

Figure 6 shows that pH, the initial concentration of TC and adsorbent dosage had significant effects on the adsorption process of the model. Increasing the initial pH of the solution from 6 to 12 has an adverse effect on the adsorption percentage of TC. The GCCPN adsorbent and TC molecules both have similar functional groups, thus the reduction in the adsorption percentage may be due to the repulsive electrical interaction between active sites of the adsorbent and functional groups of TC at pH value higher than 6. The distribution of TC species in the aqueous solution and the surface properties of sorbent are strongly pH-dependent. Tetracycline is an amphoteric molecule which has three acidic dissociation constants (3.30, 7.68 and 9.68). It can exist in cationic (H3TC+) (which is dominant below pH 3.30), zwitterion (H2TC) (which is dominant between pH 3.30 and 7.68), and anionic (HTC and TC2−) (which is dominant above pH 7.68) forms. It is likely that the removal of TC by GCCPN adsorbent is reduced by the repulsive force between negative charges of the TC molecules and adsorbent active sites. In this study, high adsorption was achieved at pH 6, which was chosen as the optimum pH value. Conversely, the initial concentration of TC and dosage of adsorbent had maximum and minimum amounts in their ranges, respectively. The optimum values for the initial concentration of TC and the dosage of adsorbent were found to be 100 mg/L and 0.25 g, respectively.

Isotherms of adsorption

The adsorption isotherm provides important information about the mechanism of the adsorption process. The adsorption isotherm of GCCPN was obtained with TC concentrations ranging from 50 to 250 mg/L at 25 °C. Three different isotherm models (Langmuir, Freundlich and Temkin isotherms), were used to analyze the experimental data (Min et al. 2004). The model simulation parameters for each model are listed in Table 2. The best regressions (R2 = 0.996) was observed for the Langmuir isotherm, indicating the formation of a monolayer of TC on the surface of the GCCPN. The maximum adsorption capacity of the chitosan/PVA nanofibers for adsorbing TC as obtained by nonlinear curve fitting was found to be 102 milligrams of TC per gram of adsorbent.

Table 2

Isotherm parameters for TC adsorption by the glutaraldehyde-crosslinked chitosan/PVA nanofibers

Langmuir Freundlich Temkin 
qm (mg/g) 153.846 KF (L/g) 151.896 AT (L/g) 76.077 
KL (L/mg) −0.903 n  58.48 bT (J/mol) 124.613 
R2  0.996 R2  0.882 R2  0.938 
Langmuir Freundlich Temkin 
qm (mg/g) 153.846 KF (L/g) 151.896 AT (L/g) 76.077 
KL (L/mg) −0.903 n  58.48 bT (J/mol) 124.613 
R2  0.996 R2  0.882 R2  0.938 

Table 3 compares the adsorption capacity of the GCCPN adsorbent with those of the previously reported adsorbents for the removal of TC from the aqueous environment. GCCPN adsorbent not only has simple synthesis route and degradable structure but also has notable adsorption capacity.

Table 3

Comparison of different adsorbents for the removal of tetracycline from water

Adsorbent Adsorption capacity Reference 
Amino functionalized Fe3O4 on coordination complex modified polyoxometalates nanoparticles 191.86 mg/g Ou et al. (2016)  
Macroporous polystyrene/graphene oxide composite monolith 197.9 mg/g Chen et al. (2016b)  
Activated carbon produced by KOH activation of tyre pyrolysis char 356 mg/g Acosta et al. (2016)  
Amino-Fe(III) functionalized mesoporous silica 112.3 mg/g Zhang et al. (2015)  
Rice husk ash as an agricultural waste 8.37 mg/g Chen et al. (2016a)  
Carbonized pomelo peel composite with aniline for electro-adsorption of tetracycline 92.32% Li et al. (2016a)  
Glutaraldehyde crosslinked nanofibers of chitosan/PVA 102 mg/g This study 
Adsorbent Adsorption capacity Reference 
Amino functionalized Fe3O4 on coordination complex modified polyoxometalates nanoparticles 191.86 mg/g Ou et al. (2016)  
Macroporous polystyrene/graphene oxide composite monolith 197.9 mg/g Chen et al. (2016b)  
Activated carbon produced by KOH activation of tyre pyrolysis char 356 mg/g Acosta et al. (2016)  
Amino-Fe(III) functionalized mesoporous silica 112.3 mg/g Zhang et al. (2015)  
Rice husk ash as an agricultural waste 8.37 mg/g Chen et al. (2016a)  
Carbonized pomelo peel composite with aniline for electro-adsorption of tetracycline 92.32% Li et al. (2016a)  
Glutaraldehyde crosslinked nanofibers of chitosan/PVA 102 mg/g This study 

Kinetics of adsorption

The kinetics parameters of adsorption give important information to design and model the adsorption process (Liu et al. 2015). Intraparticle diffusion, pseudo-first-order and pseudo-second-order kinetic models were used to examine the kinetics of the adsorption process. The results of fittings for kinetic models are shown in Table 4. Analysis of the correlation coefficients R2 of the applied kinetic models showed that R2 values obtained for the pseudo-second model were equal to 0.99, indicating the chemical nature of the adsorption process. Thus, it was concluded that chemical adsorption is the controlling step of the adsorption process (Yao et al. 2014). The presence of different functional groups such as amine, imine, and hydroxide in the structure of the adsorbent provides suitable active sites for chemical interactions with TC molecules.

Table 4

Characteristics of the kinetic models for TC adsorption by glutaraldehyde-crosslinked chitosan/PVA nanofibers

Intraparticle diffusion model Pseudo-second-order Pseudo-first-order 
k3 mg g−1 min−0.5 18.507 k2 g mg−1 min−1 0.00024 k1 min−1 0.0328 
L mg g−1 2.41 qe mg g−1 217.39 qe mg g−1 9.60 
R2  0.506 R2  0.973 R2  0.986 
Intraparticle diffusion model Pseudo-second-order Pseudo-first-order 
k3 mg g−1 min−0.5 18.507 k2 g mg−1 min−1 0.00024 k1 min−1 0.0328 
L mg g−1 2.41 qe mg g−1 217.39 qe mg g−1 9.60 
R2  0.506 R2  0.973 R2  0.986 

CONCLUSIONS

Synthesized chitosan/PVA nanofibers showed notable capability for adsorption of the TC from aqueous solutions. It was found that volumetric ratio of the chitosan/PVA has significant effect on the adsorption capacity of the chitosan/PVA nanofibers. The optimum TC removal efficiency was achieved using nanofibers obtained by chitosan/PVA volumetric ratio of 75:25. Investigation of the adsorption process using the RSM showed that the initial concentration of TC, pH and adsorbent dosage had significant effects on the adsorption capacity. The maximum capacity for adsorption of TC was 102 mg/g. The adsorption isotherm obeys the Langmuir model, confirming the monolayer adsorption of the TC on the surface of the adsorbent. The kinetic data are in agreement with the pseudo-second-order model, indicating the chemical nature of the TC adsorption.

ACKNOWLEDGEMENTS

This research was financially supported by Iran National Science Foundation (project number 87046/16).

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