Preparation and characterization of PAN nanofibers containing boehmite nanoparticles for the removal of microbial contaminants and cadmium ions from water

Water is the most vital substance as its lack or deficiency seriously threatens human life and health. Access to water without chemical contaminants and pathogens is necessary for human health. Bacterial infections such as cholera, dysentery, and typhoid account for the majority of common waterborne diseases, leading to millions of deaths all over the world. In particular, pathogenic bacteria existence, lack or excessive amounts of necessary ions, suspended particles, and water taste and smell are the dominant concerns related to potable water (Kumar & Pandit 2012). Also, heavy metals such as lead, copper, arsenic, mercury, and cadmium in drinking water have harmful effects on human health (Song et al. 2011).

Water disinfection methods include chemical and physical agents, radiation, and mechanical devices. Regarding bioenvironmental and economic issues, neither chemical agents nor radiation method can be recommended for water disinfection (Savage & Diallo 2005; Li et al. 2008; Botes & Eugene 2010). Nowadays, the absorption method has attracted a great deal of attention in the field of pollutant removal owing to both high-quality and cost-effectiveness. This procedure is largely affected by various parameters such as pollutant and absorbent interaction, surface area and particle size of the absorbent, pH, and contact duration (Kumar & Pandit 2012). Although filtration is known as the most conventional and practical method for biological pollutants' removal, it involves two serious challenges: (1) the high efficiency of pollutants' removal is obtained by a large pressure drop and (2) the aggregated biological residues are likely to pass through tiny pores, contaminating the filtrate (Fiksdal & Leiknes 2006; Li et al. 2009). Tremendous progress in nanoscale science and engineering has made a breakthrough in improving water filtration (Savage & Diallo 2005; Pendergast & Hoek 2011). The use of both natural and synthetic nano-materials such as chitosan, carbon nanotube, and fullerene, as well as metallic (e.g., Ag) and metallic oxides' nanoparticles ((NPs) e.g., Al₂O₃, TiO₂, ZnO, CuO), has been reported in many studies as antimicrobial NPs to filter water (Li et al. 2008; Botes & Eugene 2010). Mpenyana-Monyatsi et al. (2012), for example, showed the E. coli antibacterial characterization of all samples prepared by the precipitation of silver NPs on different beds such as zeolite, sand and cation, and anion resin. Moreover, Phong et al. (2009) documented 100% Escherichia coli (E. coli) and Bacillus subtilis bacteria removal by covering a flexible foam of polyurethane with silver NPs. Song et al. (2011) investigated the removal of heavy metal ions (Hg, Cd, Mn, and Cr) from aqueous solution using polyrhodanine-encapsulated γ-Fe₂O₃ NPs. They found that the magnetic polyrhodanine NPs are recyclable adsorbents and can be used to remove hazardous heavy metals from wastewater. Research works have also demonstrated alumina capability in decreasing or removing metal ions and viral pollutants (Hota et al. 2008; Brown & Sobsey 2009; Noordin & Liew 2010). Alumina is an umbrella term encompassing aluminum oxide (Al₂O₃), aluminum hydroxide, such as aluminum oxihydroxide (AlOOH), and aluminum trihydroxide (Al(OH)₃) (Noordin & Liew 2010). Being a relatively cheap, non-toxic, stable and chemical resistant, alumina has a broad range of applications, as a thermal insulator, a catalyst, a reinforcement agent, and a fire protector (Shackelford & Doremus 2008). Alumina NPs, due to the unique property of positively charged surface, can be regarded as a potentially more suitable filter. This is due to the fact that the electrostatic force of attraction is well established between positively charged NPs and negatively charged hydroxyl and phosphate groups in microorganisms (Pal et al. 2006; Li et al. 2009). Many industries including filtration can benefit from the exclusive properties of nanofibers. Nanofibers are usually produced using electrospinning which is a simple and low cost method. High surface area, tiny and interconnected pores, chemical activities, light, and ultrathin layers of nanofibers have noticeably improved filtration efficiency. In particular, high porosity, integrated pores, and functionalization of nanofibers have turned them into the top priority in the air (aerosol) and liquid filtration industry (Gibson et al. 2001; Kozlov et al. 2018; Cheng et al. 2019; Correa et al. 2019). Nanofibers' functionalization has been achieved by both metallic and inorganic NPs. The synergistic effect of organic–inorganic hybrid nanofibers has resulted in a vast range of applications due to some valuable properties (Cheng et al. 2019; Correa et al. 2019).

Different techniques used for alumina nanofibers' production, such as electrospinning, sol–gel, and mercury-mediated and chemical vapor deposition in flame, have been extensively reported (Panda & Ramakrishna 2007; Teoh et al. 2007; Yang et al. 2009; Noordin & Liew 2010). Furthermore, filtration performance in removing bacterial pollutants and heavy metal ions has been evaluated in many studies (Hota et al. 2008; Brown & Sobsey 2009; Li et al. 2009; Noordin & Liew 2010). Despite numerous studies conducted on filters made of alumina nanofibers, there are few investigations on using alumina NPs-containing nanofibers (polymer nanofiber/ceramic). Boehmite (AlOOH) or hydrated alumina (aluminum hydroxide) is an intermediate produced in the alumina synthesis procedure with many hydroxyl groups attached on the surface, helping it to be largely used as an absorbent and a catalyst (Vatanpour et al. 2012; Karger-Kocsis & Lendvai 2018). The results obtained by the study of Li et al. (2009) on alumina nanofibers confirmed the viral aerosol agent removal, indicating better efficiency and less pressure drop, as compared to high-efficiency particulate air (HEPA) filters. Hota et al. (2008) also electrospun boehmite NPs-containing nylon 6 and polycaprolactone nanofibers, showing cadmium adsorption in both hydrophobic and hydrophilic polymers. The large bacterial removal of boehmite NPs can encourage scientists to develop filtration methods. The above-mentioned studies have shown that nanofibers and boehmite NPs are superior to conventional membranes and chemical disinfectants (e.g., chlorine) due to their high surface area, as well as the ability to eliminate bacteria and heavy metals from water without formation of toxic by-products. Therefore, the objective of this paper is to investigate the filtration and bacterial removal efficiency of electrospun PAN/boehmite nanofibers on carbon-coated polyurethane foam as the substrate, for removal of E. coli and cadmium ion in water. The effect of boehmite NPs' concentration and the electrospinning duration of PAN/boehmite nanofibers on their performance were evaluated.

Polyacrylonitrile (PAN) with an average molecular weight of 70,000 g/mol was obtained from Polyacryl Co., Isfahan (Iran). Dimethyl formamide (DMF) was purchased from Merck as the solvent for the PAN. E. coli (ATCC 35218), as a kind of Gram-negative bacteria, S. aureus (ATCC 29213) as a kind of Gram-positive bacteria, were used for antibacterial evaluation. Aluminum isopropoxide (Al [OCH (CH₃)₂]₃, 98%) and cadmium chloride were supplied by Merck (Germany) and used as the precursors for the synthesis of boehmite and the Cd (II) ions' solution, respectively. Deionized water was used throughout the experiments.

PAN/boehmite nanofibers were fabricated by using the electrospinning method. Figure 1 shows the schematic of the electrospinning setup. It is described here: first, different amounts of boehmite NPs (5, 10, 30, and 50% wt) were dispersed in DMF under ultrasonic stirring for 20 min and at a power of 70 W, 40 kHz. Next, the accurately weighed PAN (12% wt) was added to the above suspension and the polymer solution was magnetically stirred for 24 hours at room temperature.

A 1 mL syringe fitted with a metallic needle having an inner diameter of 0.7 mm was used to extrude the polymer solution. The electrospun fibers were collected as a non-woven web on a cylinder whose surface was covered with carbon-coated polyurethane foam; this was done to not only supply appropriate mechanical properties, but also to allow pleating filter production. The electrospinning conditions of the polymer solutions were adjusted based on the production of bead-free and uniform nanofibers. The flow rate and electrospinning distance were fixed at 0.35 mL/h and 15 cm, respectively. The applied voltage of polymer solutions is provided in Table 1. The collector rotation speed and the traverse speed of the pump were 75 rpm and 4 cm/min, respectively, in order to uniformly coat nanofibers on the carbon-coated polyurethane foam. Electrospinng was performed for 2, 4, and 6 hours at ambient temperature (25 °C).

X-ray diffraction (XRD), which was performed by Xpert MPD with CuKα radiation (λ = 1.54 Å) at a range of 2θ = 10–100°, made it possible to characterize phase purity and the average particle size of the synthesized boehmite. Fourier transform infrared (FTIR) spectroscopy studies were also performed on the samples using a Bomem MB – Series 100 spectrometer (Canada), in the range of 400–4,000 cm⁻¹, by employing the KBr disk method. Nanofibers' morphology investigation was conducted by a Hitachi-S-4160 field emission scanning electron microscope (FE-SEM) and Philips EM 208S transmission electron microscope ((TEM) The Netherlands). The nanofibers' diameter was obtained from an average of 100 measurements. The viscosity of PAN/boehmite solutions was measured using a Brookfield DVII + Pro viscometer (USA) at 25 °C.

The XRD analysis result of boehmite nanoparticles (NPs) is shown in Figure 2; it was also compared with the available XRD data in the literature (Santos et al. 2009). The XRD pattern of the analyzed sample proved it to be an excellent mat for the poorly crystallized boehmite (pseudoboehmite). The three angles reflection in 2θ = 10–40° differed slightly in broadening and significantly in their intensity in the case of the poorly and well-crystallized boehmite. For the pseudoboehmite, notably, the lower intensity, broadening of the reflection, and a higher d value at 2θ = 14.6°, which corresponded to the (020) plane, were observed, as reported in the literature (Santos et al. 2009). Thus, XRD data confirmed the successful synthesis of the poorly crystallized boehmite NPs (pseudoboehmite). According to the Debye-Scherrer equation, the average particle size was approximately 12.34 nm.

Figure 3(a) shows the FTIR spectra of boehmite NPs. The strong and well-resolved bands at 3,305 and 3,081 cm⁻¹ characterized a well-crystallized boehmite; it could be attributed to (Al)O-H asymmetrical and symmetrical stretching vibrations, respectively. Al-O-H asymmetrical and symmetrical stretching of boehmite corresponded to the bands at 1,154 and 1,074 cm⁻¹, respectively. The peaks at 739 and 603 cm⁻¹ were due to the OH torsional mode of boehmite. The FTIR analysis results were consistent with the XRD results, thereby confirming that the crystalline phase of the prepared NPs was boehmite (Boumaza et al. 2009; Dabbagh & Shahraki 2013).

To prove the presence of boehmite NPs within the nanofibers, the FTIR and TEM analyses of PAN and PAN/boehmite nanofibers were performed and results are depicted in Figures 3 and 4. The nanofibers' TEM image shows the presence of a large number of boehmite NPs that are evenly distributed in the prepared PAN nanofibers. The TEM image also shows that the boehmite NPs have a rod shape structure. The FTIR spectrum of PAN (Figure 3(b)) showed characteristic absorption bands such as C-H stretching vibrations at 2,943 cm⁻¹ and C-N stretching vibrations at 2,247 cm⁻¹. The absorption band at 1,453 cm⁻¹ was associated with the C-H bending vibration (Foroozmehr et al. 2015). According to Figure 3(c), the characteristic absorption bands of boehmite (484, 742, 1,077, 1,170, 3,092, and 3,309 cm⁻¹) were present in the FTIR spectra of PAN/boehmite nanofibers, thereby confirming that boehmite NPs had been successfully incorporated in PAN nanofibers. Furthermore, lack of shift or change in absorption wavenumbers indicated the absence of the chemical interaction between PAN and boehmite NPs in the produced nanofibers.

The FE-SEM microstructures and the diameter distribution graph of electrospun PAN nanofibers with different boehmite NPs loadings in a 12% wt PAN polymer solution are shown in Figure 5. The rise of the nanofibers' diameter, as shown in Table 1 and Figure 5, was caused by an increase in the polymer solution viscosity due to the addition of boehmite NPs. Increasing viscosity by increasing the concentration of nanoparticles can be attributed to the adsorption and immobilization of a large amount of polymer by nanoparticles (Polymer properties database 2015). The rise in boehmite content from 0 to 50% wt led to an increase in the average diameter of PAN/boehmite nanofibers from 191 to 322 nm. The size of boehmite NPs was measured to be 68 and 237 nm for boehmite loadings of 10 and 50% wt, respectively, as can be seen from FE-SEM provided in Figure 6. These results implied further agglomeration of boehmite NPs on the surface of nanofibers containing 50% wt boehmite.

The permeation performance of the nanofiber layers was evaluated by measuring the air permeability and pure water flux (Figure 7). The variations in the air permeability of PAN/boehmite nanofiber layers with different boehmite content and electrospinning duration are depicted in Figure 7(a). It could be inferred from the results that air permeability was significantly enhanced by increasing boehmite content up to 30% wt; however, increasing boehmite content to 50% wt dramatically reduced air permeability. Based on Darcy's equation, increasing air permeability is associated with a decrease in pressure drop (Gibson et al. 2001). It was clear that when nanofibers' diameter was increased, the inter nanofiber space (pore size) was enlarged too (Abuzade et al. 2012). Therefore, air permeability enhancement of samples could be attributed to the increase in the pore size of nanofiber layers. The clogged pores caused by boehmite NPs' accumulation reduced the air permeability of nanofibers containing 50% wt boehmite. It is worth noting that the thickness of the nanofiber mat was increased with electrospinning duration, resulting in the reduced pore size and air permeability reduction (Kang et al. 2015). Based on the literature, more porosity and larger pores could cause higher air permeability and fluxes (Kaur et al. 2012). Figure 7(b) shows the pure water flux of different PAN/boehmite nanofiber layers. As results show, pure water flux of nanofiber layers is high and was decreased by increasing the electrospinning durations, but did not significantly change by increasing boehmite loading.

The particles' removal efficiency was measured on PAN50 nanofibers layer, electrospun for 6 hours. As shown in Table 2, the sample has a particles' separation efficiency of over 97% for particles larger than 0.3 μm. Particles' removal efficiency was increased with increasing particle size.

Antibacterial test results of PAN/boehmite nanofibers against E. coli with different boehmite concentrations and electrospinning durations are provided in Table 3. The bacterial removal efficiency of PAN/boehmite nanofibers was dramatically enhanced by the increase in boehmite content up to 10% wt. Nevertheless, further increase in boehmite concentration from 10 to 30 and 50% wt had no much effect on antibacterial properties. Moreover, the bacterial removal efficiency of samples was improved as the electrospinning time enhanced, which could be regarded as a direct result of the increased nanofibers and boehmite in layers. As stated in the Introduction, the bacterial removal mechanism may be due to the electrostatic force between negatively charged microorganisms (E. coli) and positively charged boehmite NPs of the PAN/boehmite nanofibers (Pal et al. 2006; Li et al. 2009). To summarize, nanofiber layers of PAN/boehmite with 50% wt boehmite content exhibited higher air permeability (larger pore sizes and lower pressure drop), better bacterial removal efficiency, suitable pure water flux, and high particles' removal efficiency.

The reuse cycles of bacterial removal efficiency were repeated four times for the PAN50 nanofibers electrospun for 6 hours and the results are shown in Figure 8(a). It should be noted that the 500 mL of E. coli suspended in distilled water passed through the sample four consecutive times and its bacterial removal efficiency measured after each stage, and finally, the SEM image of the filter surface was taken, as shown in Figure 8(b). According to the results, it can be concluded that by passing water from the PAN50 nanofibers more than once, the bacterial removal efficiency increases from 99.7% to 99.99%. These results show that at least four times water passage through the sample is required to reach 4-log removal of bacteria. FE-SEM micrographs (Figure 8(b) and 8(c)) of the sample surface before and after antibacterial filtration indicate that E. coli bacteria is captured by the PAN/boehmite nanofibers layer accompanied by no structural deformation of the nanofibers layer during filtration. To get a better idea of the spectrum of bacterial removal efficiency, the optimized sample by E. coli (PAN50 nanofibers electrospun for 6 hours) was tested against Gram-positive bacteria (S. aureus) as representative. Bacterial removal efficiency of the sample mentioned against S. aureus of 99.39%, was obtained, which indicates the ability of the resulting nanofibers to remove Gram-positive bacteria in addition to the Gram-negative bacteria.

The Cd (II) ion adsorption values of PAN50 nanofibers with the maximum antibacterial activity, along with pure PAN nanofibers and carbon-coated polyurethane foam, are shown in Table 4. The results show that PAN/boehmite nanofibers had a cadmium adsorptivity of about 74% by loading 50% wt boehmite NPs. The adsorption capacity of carbon-coated polyurethane foam, PAN, and PAN/boehmite nanofibers was obtained to be 0.16, 0.20, and 0.23 mg/g, respectively, after the 60 min contact with 20 mL of the 5 ppm Cd(II) ions solution at pH = 4. Results obtained in the literature have revealed that the main mechanism of Cd (II) sorption is ion exchange; thus, carboxyl and hydroxyl functional groups could be mainly responsible for the sorption of Cd (II) (Iqbal et al. 2009). Hence, the cadmium absorption capability of PAN/boehmite nanofibers could be attributed to the hydroxyl groups of boehmite NPs. Hota et al. (2008) also reported the sorption capacity of 0.21 and 0.20 mg/g for nylon-boehmite and PCL-boehmite nanofibers containing 50% boehmite, as compared to the results obtained in the current study (0.23 mg/g).

In this paper, the sol–gel process and the electrospinning method, as simple and cheap methods, are used to prepare PAN/boehmite nanofibers. Prepared nanofibers are characterized and are used for the removal of microbial contaminants and cadmium ions from water by using absorption as a high-quality and cost-effective method.

Both XRD and FTIR results confirmed the successful synthesis of pseudoboehmite NPs with an average particle size of about 12.34 nm. TEM observation proved that boehmite NPs were uniformly dispersed in the PAN nanofibers. The nanofibers' diameter rose due to the increase in boehmite content, which was accompanied by increased air permeability, but the pure water flux did not change significantly. Boehmite-containing nanofiber layers were investigated to demonstrate E. coli bacteria removal capability through electrostatic attraction. The antibacterial property of PAN nanofiber layer was found to be directly proportional to boehmite content and the electrospinning time. A layer composed of PAN nanofibers containing 50% wt boehmite was produced through a 6-hour electrospinning process on carbon-coated polyurethane foam, and was almost better than 5, 10, and 30% wt nanofiber-containing layers in bacterial removal. Permeability of such a sample, compared to pure PAN nanofibers layer revealed that not only was bacteria removal improved to 99.99% in the presence of 50% wt boehmite, but also high permeability, structural stability, and rejecting micro particles efficiently was observed. The results also confirmed that boehmite NP-containing nanofibers were effective in cadmium (II) ions' removal in water. Based on the results of this study, it seems that electrospun PAN/boehmite layers can be recommended as a potential filter for production of safe water.