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Comparison between polyvinyl alcohol (PVA) nanofiber and polyvinyl alcohol (PVA) nanofiber/hydroxyapatite (HA) for removal of Zn2+ ions from wastewater
⁎Corresponding author. maghrabyazza@yahoo.com (Azza El-Maghraby)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Nanocomposites in the form of continuous and aligned polyvinyl alcohol (PVA) nanofibers have been obtained by the electrospinning process. These fibers were used as matrix for hydroxyapatite (HA). PVA and PVA/HA nanofibers were fabricated and crosslinked with glutaraldehyde (GA). The hybrid of polymer with ceramics (HA) shows good properties of large surface area and good porosity that were demonstrated by different characterization techniques such as FTIR, SEM, XRD and EDX which illustrate the ratio of HA. The prepared nanofibrous membranes are used for removal of Zn2+ from simulated water. The potential of these synthesized sorbents to remove Zn2+ ions from aqueous solutions was investigated in batch experiments, where several parameters such as the sorbate/sorbent’s contact time, initial Zn2+ ions concentration and sorbent dosage were investigated. The equilibrium concentration of Zn2+ ions was identified by the atomic absorption spectrophotometry and the adsorption process was studied by the isotherm models such as Langmuir, Freundlich and Temkin. The Zn2+ adsorption process for PVA and PVA/HA nanofibrous membranes was fitted to Langmuir and Freundlich isotherm models respectively.
Keywords
Nanofibers
Electrospinning
Polyvinyl alcohol
Hydroxyapatite
Water treatment
Zn2+ removal
1 Introduction
Nowadays, pollution of water from the contamination of heavy metals is a severe environmental problem. Metals such as chromium, copper, iron, lead, silver, and zinc, which have high toxic and non-biodegradable properties, can cause problems to both the environment and the living organisms with extremely high levels (Abdouss et al., 2012; Freitas et al., 2008; Badmus et al., 2007). Many methods (such as adsorption, electroplating, ion exchange, membrane separation and precipitation) are being used to remove the ions of these metals from aqueous effluents. Among these, adsorption is commonly regarded as an effective and economical method for wastewater treatment (Illinois, n.d.).
Nano-scale materials can be designed to exhibit novel improvement of the physical and chemical properties and are designed to become a discipline as important as polymers are in chemistry (Kampalanonwat and Supaphol, 2010). More than 100 polymers, both synthetic and natural, have been successfully electrospun into nanofibers. Mostly from polymer solutions any polymers may be electrospun into nanofibers, provided that the polymer molecular weight is sufficiently high and the solvent can be evaporated in time during the jet transit period over a distance between the spinneret and the collector (Feng et al., 2013).
PVA is a synthetic water-soluble hydrophilic polymer. The basic properties of PVA are dependent on the degree of polymerization or on the degree of hydrolysis. It has been widely used in adhesives, emulsificantes, the textile and paper industry applications and in the attainment of amphiphilic membranes for enzyme immobilization where properties can be improved, such as mechanical, thermal and chemical stability. Applications of PVA are limited by its hydrophilicity, but chemical cross-linking improves its stability in aqueous media. Chemical cross-linking by glutaraldehyde (GA), a dialdehyde, during which the hydroxyl groups of the PVA and the aldehyde groups of GA react in the presence of a strong acid, has been well studied. Membranes of nanofibrous cross-linked PVA are being studied in filtration and membrane applications. Cross-linking is advantageous because it renders the nanofibers insoluble in all solvents and increases the mechanical properties of the membranes; insolubility is especially desirable for filtration applications.
A novel class of materials called organic-inorganic hybrids would combine properties of organic polymers with ceramics such as hydroxyapatite (HA). Hybrids would combine properties of organic polymers with ceramics. These different components can be mixed at length scales ranging from nanometer to micrometer, in virtually any ratio leading to the so-called hybrid organic-inorganic materials (Fonseca et al., 2006). Hydroxyapatite is an ideal material for long-term containment of contaminants because of its high sorption capacity for actinides and heavy metals, low water solubility, high stability under reducing and oxidizing conditions, availability, and low cost. It was conducted in adsorption of a wide variety of metals (e.g., Cr, Co, Cu, Cd, Zn, Ni, Pu, Pb, As, Sb, U, and V). The sorption takes place through ionic exchange reaction, surface complexation with phosphate, calcium and hydroxyl groups and/or co-precipitation of new partially soluble phases (Mobasherpour et al., 2011).
The aim of this work was to prepare and characterize the cross linked polyvinyl alcohol nanofibers and nanofibrous composite of both polyvinyl alcohol/hydroxyapatite nanofibers and compare between them by the removal of the contaminant Zn2+ ion from aqueous solution. The thermodynamics of this adsorption process of Zn2+ from the simulated water have been studied by three isotherm models such as Langmuir, Freundlich and Temkin.
2 Materials and methods
2.1 Materials
Polyvinyl alcohol “Mwt = 72.000 g/mol”, Calcium nitrate tetrahydrate (Ca(NO3)2·4H2O) (98%) and Phosphoric acid, (H3PO4) (85%) were obtained from Sigma Aldrich. Glutaraldehyde 25% aq. Solution was purchased from Alfa Aesar, “A Johnson Matthey Company”. Zinc nitrate, Zn(NO3)2·6H2O, M = 297.48 g/mol, was obtained from Fluka. All chemicals were used without further purification.
2.2 Methods
2.2.1 Preparation of hydroxyapatite (HA)
The HA Powder was synthesized by the co-precipitation process of 1:2 Molar ratio of Ca(NO3)·4H2O and H3PO4 of the same volume. The HA particles were precipitated upon the addition of NH4OH till pH equal to 10. After the precipitation the solution was aged in a water bath for 24 h. The precipitation was filtered and then dried at 100 °C for 24 h (Scalera et al., 2013) (Fig. 1).
2.2.2 Preparation of polyvinyl alcohol (PVA) nanofibers
Different concentrations (5, 8, 10) wt% of PVA water solutions were prepared by stirring at 90 °C for 2 h. The prepared solutions of PVA solutions have been cross linked with glutaraldehyde (GA) by the addition of 0.25% glutaraldehyde for cross linking reaction by the stirring for 2 h at 60 °C. The prepared solutions were electrospun to form polymeric nanofibers. The electrospinning apparatus (NANONC, Germany) consisted of high voltage supply, a syringe infusion pump and a ground electrode (an Aluminum foil on a stainless steel collector). The polymeric solutions were loaded into a plastic syringe (10 mL), and the applied voltage was “15 and 30” kV. The solution was constantly supplied using a syringe pump at a flow rate of 0.5 mL/h and the spinning distance between the needle and the ground electrode was 20 cm.
2.2.3 Preparation of cross-linked PVA nanofiber/hydroxyapatite nanocomposite
By the characterization of the polymeric nanofibers, we choose the 10 wt% of PVA water solution due to the smallest fiber diameters and the highest surface area. The hydroxyapatite (1 wt%) added to the 10% PVA water solution by the stirring for 1 h at 90 °C. The addition of 0.25% GA is for cross linking reaction between the polymeric chains by the stirring for 2 h at 60 °C (Wang and Hsieh, 2010; Figueiredo et al., 2008). The polymeric solutions were loaded into a plastic syringe (10 mL) and allowed for electrospinning with the conditions as previously mentioned.
2.2.4 Zn2+ adsorption by the prepared nanofibrous membranes
The study of the adsorption of metal ions (Zn2+) from an aqueous solution was investigated by the batch method. The metal ion solutions were prepared with different concentrations from 100 to 500 mg/L. Dried samples of nanofibrous membranes (PVA and PVA/HA) were used as adsorbent materials. The membranes were weighed 50 mg in aqueous solutions (100 mL) which were agitated at a constant speed of 300 rpm at room temperature (Wang and Hsieh, 2010). The concentration of Zn2+ ion has been determined by Atomic Absorption spectrometry (AAS).
2.2.5 Isotherms of the adsorption process
Langmuir, Freundlich and Temkin models are three isotherm models which explain the adsorption of adsorbate onto homogeneous and heterogeneous active sites of the adsorbent, respectively. Further to this, the initial concentration of metal ion Zn2+ was changed from 100 to 500 mg/L and the Zn2+ concentrations were determined by the atomic absorption spectrometry after a certain time (t). The Langmuir model of isotherm stated that “the surface catalysis is usually preceded by chemisorption, and the kinetics of surface reactions due to the monolayer adsorption” (Figueiredo et al., 2008). The model is valid for monolayer adsorption onto a surface containing a finite number of identical sites, while Temkin model considered the effects of some indirect adsorbate/adsorbent interactions on adsorption isotherms and suggested that because of these interactions of the heat of adsorption, the molecules in the layer would decrease linearly with coverage. The form of the Langmuir isotherm can be represented by the following Equation:
In contrast to the homogeneous Langmuir isotherm, the Freundlich adsorption isotherm is an empirical model, which can be used for evaluating the heterogeneous binding system of Zn2+ onto the nanofiber surfaces.
The Freundlich equation (Eq. (2)) is expressed as
The Temkin isotherm has been used in the following form (Haider and Park, 2009):
The adsorption data can be analyzed according to Eq. (4). A plot of qe versus ln Ce enables the determination of the isotherm constants A and B. The constant B is related to the heat of adsorption.
2.2.6 Characterization of the prepared materials
Fourier Transmission–IR Spectrophotometer (FTIR-8400S, Shimadzu, Japan) with a resolution of 2 cm−1 was used to explore the change in the functional groups before and after functionalization of the nanofibers with HA. The samples (5 mg) were mulled with dry potassium bromide (KBr) (20 mg) crystals at room temperature. The spectrum was recorded in the wave number range of 4000–350 cm−1 with a resolution of 2 cm−1. To analyze the crystalline structure of the PVA powder, the electrospun (ES) PVA fiber and the ES-PVA/HA fibers, X-ray diffraction was performed using X-ray 7000 Shimadzu-Japan at room temperature. The Bragg angle (2θ) in the range from 10° to 80° was measured to determine the degree of crystallinity of the HA. The X-ray source is a Cu target generated at 30 kV and 30 mA with a scan speed 4 deg/min. The size and morphology of the electrospun fibers were investigated by the field scanning electron microscopy (SEM (JEOL JSM 6360LA, Japan), with an Electron Dispersive X-ray unit (EDX).
3 Results and discussion
PVA nanofibrous membrane and PVA/HA nanocomposites were obtained by electrospinning of PVA water solution with GA and electrospinning of PVA water solution with HA in the presence of GA. The removal of Zn2+ from simulated water was studied.
3.1 Characterization of the prepared hydroxyapatite
The SEM micrograph, the FT-IR analysis and the XRD pattern of the porous HA templates are shown in Fig. 2A, B and C, respectively. The dense agglomerated grains of HA were observed in the SEM micrographs of the calcined HA templates. The micrographs of the clusters of HA particles have a homogenous size. The average size of the clusters was mean ± standard deviation (218 ± 75 nm) for the co-precipitation procedure of [Ca(NO3)2·4H2O + H3PO4] (Dawid, 2001).
The functional groups of the HA have been studied by the FT-IR as shown in the following. the band observed at 3417 cm−1 was attributed to the hydroxyl group (OH) stretching (Lan et al., 2015). The bands at 1629 cm−1, 1660 cm−1 were attributed to V2 bending mode of the water molecules that are adsorbed on the surface (Guzm, 2005; Rehman and Bonfield, 1997). A weak band of CO32− was detected in the region around 1567.78 cm−1. This band indicates or confirms the minor amount of carbonate substitution. The vibrations of the O—P—O bonds corresponding to the absorption band of PO43− appeared at 580 cm−1 indicate the bending mode of PO43−. The peak at 976.89 cm−1 corresponds to symmetric stretching mode of PO43−. The most intensive band that appeared at 1060 cm−1 corresponds to the asymmetric stretching vibrations of the P—O (Lak et al., 2008).
The XRD analysis of the prepared HA shows the sharp narrow peaks indicated by 2θ from 25° to 32°. The crystalline peaks were observed at 2θ = 25.8°, 31.8°, 39.8°, 46.7°, 49.5° and 51.5°, corresponding to HA. All the peaks of the prepared powders correspond to that of the standard HA that was reported in the ASTM card (JCPDS No. 74-0566); however, the peaks of the produced HA had different intensity than the standard one (Mohandes et al., 2014; Sobczak et al., 2009). These sharp narrow peaks are indicated that the prepared HA is a crystalline sample.
3.2 Characterization of polyvinyl alcohol (PVA) nanofibers
Fig. 3 shows the representative surface morphologies of electrospun PVA nanofiber membranes with different concentrations of PVA/H2O solutions (5, 8, 10) wt%. Scanning electron microscopic evaluation revealed the formation of solid, uniform, nanofibers with random orientation. The massive increase in an electrostatic repulsion that present in a high concentration tends to decrease the fiber diameters and induce an increase of the surface area (Adzila et al., 2013; Ting et al., 2011; Rajzer et al., 2012). The 10% PVA nanofiber diameters are (231.5 ± 80 nm) which are the smallest fiber diameters if compared with (5, 8)% PVA solutions. For concentration (10% PVA/H2O solution) a series of experiments were carried out. The solution was pushed out from the syringe tip with constant flow rate of 0.5 mL/h and the applied voltage was varied between “15 and 30” kV with constant distance between tip and the collector 20 cm. The mean average diameter of the fibers was “15 and 30” kV as follows: (231.5 ± 80 nm) and (172.5 ± 45 nm) respectively.
3.3 Characterization of the cross-linked PVA nanofibers
Fig. 4A shows the mechanism of the crosslinking reaction of GA with PVA nanofibers. The strategy used to investigate the acid-free reaction between PVA and GA to form the A and B reversible structures (Zhang et al., 2013). The crosslinking ‘contracted’ nanofibers within the nanofiber mats, produce a denser structure with closely packed nanofiber membranes (Liao et al., 2010) that is shown in Fig. 4B.
3.4 Characterization of nanocomposite PVA/HA nanofibrous membrane
The encapsulation of nano-HA in PVA nanofibers was characterized by SEM that is shown in Fig. 5. The spherical HA particles were mostly oriented along the axial direction inside individual fibers, whereas a few were perpendicular to axial direction extruding the fibers (Song et al., 2012). The elemental composition of nanofibers determined by EDX reveals the components (Ca, P) (3.69, 2.01) respectively that are encapsulated within fibers.
The FT-IR analysis of the prepared membranes of PVA nanofibers and nanocomposite of PVA/HA nanofibrous membranes is shown in Fig. 6. The bands at 1316, 3348 cm−1 are associated with C—O—H stretching of PVA and O—H bonding in PVA. The band at 1150 cm−1 was (C—O—C) formed through the crosslinking between OH of a typical PVA structure and —C— of GA molecule. Furthermore, the strengthening of the band at 1720 cm−1 is indicative of C⚌O bond which is characteristic of GA (Shaikh et al., 2012). The band at 2850 cm−1 appeared on both PVA nanofibers and nanofiber composite, which revealed to (O⚌C—H) of the GA structure (Figueiredo et al., 2008; Shaikh et al., 2012).
3.5 Removal of Zn2+ from the wastewater
Figs. 7 and 8 show the removal percentage of Zn2+ from the simulated water by using nanofibrous (PVA, PVA/HA) membranes. Different concentrations of Zn2+ solutions (100, 200, 300, 400 and 500) ppm have been studied as a function in time until 2 h.
Fig. 7 shows the removal percent of Zn2+ by using PVA nanofibrous (50 mg) membranes with constant stirring rate 300 rpm. The adsorption amount increases sharply up to 15 min and then still constant (100% removal percent) until 2 h. However, the removal percent of Zn2+ by using (PVA/HA) nanofiber membranes with the same conditions is variable with different Zn2+ concentrations as shown in Fig. 8.
The adsorption amount increases sharply up to 20 min then it seems constant up to (15, 45, 50, 55)% removal percent and the large adsorption amount with 500 ppm simulated solution. The initial increase might be due to high surface area and inter or intra nanofibrous pores that present in nanofibrous membranes (Song et al., 2012). The removal % of Zn2+ in case of nanocomposite was less than PVA nanofibrous membrane. Unfortunately, the hydroxyapatite effect negatively on the adsorption capacity of the composite may be due to the penetration of the particles of HA inside the chains of PVA crosslinked polymeric structure (Roland, 2014), so decrease the surface area of the nanocomposite and also blocking of the active groups composite that has been shown in the following Fig. 9.
3.6 Studying the adsorption isotherms
Fig. 10 shows the kinetic isotherms of Langmuir, Freundlich and Temkin respectively for adsorption of Zn2+ by (PVA) nanofiber membrane, while Fig. 11 shows the same kinetic isotherms for adsorption of Zn2+ by (PVA/1% HA) nanofiber membrane. Table 1 summarizes the results of the isotherm constants for the three different equilibrium isotherms tested.
| Isotherm | Parameters | ||
|---|---|---|---|
| PVA (NF) | PVA/1% HA | ||
| Langmuir model | |||
| R2 | 0.8149 | 0.1354 | |
| KL | −5 ∗ 104 | 0.595 | |
| aL | −120 | −3.16 ∗ 10−3 | |
| Freundlich model | |||
| R2 | 3 ∗ 10−5 | 0.795 | |
| Kf | 1.7 ∗ 105 | 8.46 ∗ 10−5 | |
| 1/nf | −0.0032 | 1.9125 | |
| nf | −312.5 | 0.522 | |
| Temkin model | |||
| R2 | 0.646 | 0.1766 | |
| B | −208.37 | 304.98 | |
| A | 1 | 0.020 | |
On the basis of the correlation coefficients (R2), for the Langmuir model (R2 = 0.8149), it was evident experimental data fit for the linear Langmuir isotherm equation model for adsorption of Zn2+ by PVA nanofiber membranes, while the PVA/HA nanofibers, the data fitting for Freundlich and Temkin models R2 = (0.866, 0.751) respectively. The results show that the adsorption in case of PVA nanofibers takes place as a monolayer homogenous adsorption with the nanofibrous surface, but in case of addition of HA to PVA nanofibers the adsorption of Zn2+ takes place as a heterogeneous adsorption.
4 Conclusions
In this work, we successfully fabricated polymeric nanofibrous (PVA and PVA/HA) composites. The comparison study of the fiber diameters of the different concentrations (5, 8, 10) wt% indicated that the 10% PVA nanofiber diameters have the smallest fiber diameters (231.5 ± 80 nm) if compared with (5, 8) wt% PVA solutions. Potential applications based on such fibers specifically their use as adsorbent in wastewater treatment have been realized. After the crosslinking of the polymeric nanofibers (PVA and PVA/HA) with GA, they were used for removal of Zn2+ from a simulated water. The adsorption process was studied by the isotherm models such as Langmuir, Freundlich and Temkin. The Zn2+ adsorption process with PVA and PVA/HA nanofibrous membranes was fitted to Langmuir and Freundlich isotherm models respectively. Therefore, the prepared PVA/HA nanofiber composite would be a promising material that can be used in the removal of heavy metals.
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