Electrospun nanofibrous polylactide/chitosan mats for the filtration of silver ions

Abstract

We developed biodegradable nanofibrous polylactide/chitosan membranes for the filtration of heavy metal ions. Polylactide and chitosan were first separately dissolved in trifluoroacetic acid. The solutions were then mixed and electrospun into nanofibrous membranes via an electrospinning process. The morphology of the spun nanofibers was examined using scanning electron microscopy. The average diameter of the electrospun nanofibers ranged from 419 to 695 nm. The metal removal capability of nanofibrous polylactide/chitosan membranes was measured and compared with that of bulk chitosan. The influence of various process conditions on metal removal capability was also examined. The experimental results suggested that the electrospun nanofibrous polylactide/chitosan membranes exhibit good heavy metal ion uptake capabilities. The metal removal capability of nanofibrous membranes increased with the initial metal ion concentrations and the pH value, while it decreased with the temperature and the filtering rate of the solutions. Furthermore, the electrospun membrane could be reused after the recovery process. The empirical results in this study suggested that electrospun nanofibrous polylactide/chitosan membranes can be good candidates for the removal of heavy metal ions.

Keywords

electrospinning nanofibrous membranes polylactide chitosan heavy metal removal

Ground water pollution causes irreparable damages globally to soil, plants, human, and animals, and spreads epidemics and chronic diseases. The contamination can be traced back industrially, domestically, agriculturally, and due to over-exploitation. Because of increased industrial and mining activities, a number of heavy metals end up in bodies of water. Heavy metals are natural components on Earth, and therefore cannot be degraded or destroyed. Despite the fact that some heavy metals are essential to maintain the metabolism of the human body, they are poisonous at higher concentrations. For example, the adverse effects of chronic exposure to silver are a permanent bluish-gray discoloration of the skin (argyria) or eyes (argyrosis).¹ Besides argyria and argyrosis, exposure to soluble silver compounds may produce other toxic effects, including liver and kidney damage, irritation of the eyes, skin, respiratory, and intestinal tract, and changes in blood cells. Furthermore, heavy metals are dangerous because they tend to bioaccumulate, that is, the concentration of a chemical increases in a biological organism over time, compared to the chemical's concentration in the environment. The removal of heavy metal ions has thus become one of the imminent issues for the ecosystem.

Currently the removal of such metals frequently depends upon precipitation of metal ions in the presence of flocculating agents, followed by the removal of the resulting sludge. Prior to discharge, the water and the precipitated metals need to be filtered through sand beds or other systems to remove the precipitate. However, such systems are quite expensive and can be problematic. Sand beds may become plugged, especially when organic materials are present, and result in biomass accumulation. To improve the heavy metal removal from contaminated water, various techniques have been under development, including chemical oxidation² and precipitation,³ ion exchange,⁴ reverse osmosis,⁵ electrochemical applications,⁶ biosorption,^7,8 adsorption,^9,10 and membrane separation.^11–16 Among these techniques, the membrane separation processes have been the most promising ones in the water purification industries; the processes are pressure driven and operate without heating, and thus are energetically lower than conventional thermal separation processes. This provides advantages in terms of cost competitiveness.

In this study, we developed biodegradable nanofibrous polylactide (PLA)/chitosan blend membranes via electrospinning for the removal of heavy metal ions. Among various degradable polymers, PLA is currently the most promising and popular ‘green’ ecofriendly material. It is an aliphatic polyester and biodegradable thermoplastic derived from renewable resources, such as corn starch, tapioca roots, chips, starch, or sugarcane. PLA has a crystallinity of around 37%, a glass transition temperature of 60–65℃, a melting temperature between 173℃ and 178℃, and can be easily processed by common processing methods, such as melt spinning and extrusion.¹⁷ Chitosan is a linear polysaccharide composed of randomly distributed β-(1-4)-linked D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit).¹⁸ It has been one of the most abundant and promising classes of the functional natural polymeric materials, exhibiting excellent biocompatibility, hydrophilicity, bioactivity, and non-toxicity properties. Chitosan is produced by the treatment of shrimp and other crustacean shells with the alkali sodium hydroxide. Chitosan can also be used in water treatment as a part of the filtration process to remove phosphorus, heavy minerals, and oils from the water. Based on the metal binding property of chitosan nanofibers,¹⁹ membranes made of chitosan nanofibers can be used as filters for the removal of heavy metal ions.

Electrospinning is a simple and effective nanofabrication method for preparing nanofibrous membranes with diameters ranging from 5 to 500 nm or higher, which are 10²–10⁴ times smaller than those prepared by the traditional method of solution or melt spinning. It also has the advantages of fabricating nanofibers from a huge range of materials.²⁰ However, the electrospinning of pure chitosan from its aqueous solution remains a challenge, mainly due to its high viscosity and strong hydrogen bonds. Alternatives have been proposed to produce chitosan-loaded mats. Su et al.²¹ developed chitosan/poly(ethylene oxide) blend nanofibers via electrospinning and examined the influence of metal ions on the morphology and integrity of electrospun chitosan nanofibers. They found that the presence of 0.4–1.6% NaCl in the blend solutions produced nanofibers with accompanying re-crystallized inorganic salts, and the addition of appropriate amounts of CaCl₂ or FeCl₃ resulted in the beneficial effect on defect-free nanofibers. Haider and Park²² prepared chitosan nanofibers by employing the electrospinning techniques followed by the neutralization of electrospinning with potassium carbonate. Their spun nanofibers exhibited good adsorption of Cu(II) and Pb(II) ions from aqueous solutions. Sencadas et al.²³ investigated the parameters affecting electrospun chitosan fiber size distribution and morphology. Mats with uniform fibers of submicron diameters without beads could be obtained by employing the appropriate solvent and processing conditions. Despite the fact that these previous studies successfully developed nanofibrous membranes for heavy metal removal, the fabrication of these membranes requires a complex process.

In this study, we developed biodegradable nanofibrous PLA/chitosan blend membranes via electrospinning for the removal of heavy metal ions. To prepare the nanofibers, PLA and chitosan were first separately dissolved in trifluoroacetic acid (TFA). The solutions were then mixed and electrospun into nanofibrous membranes. The morphology of the spun nanofibers was examined using scanning electron microscopy. The metal removal capability of nanofibrous PLA/chitosan membranes was measured and compared with that of bulk chitosan. The influence of various process conditions on metal removal capability was investigated. The recovery of the fabricated membrane for metal ion removal was also examined.

Materials and method

Materials

The PLA used was commercially available with a melt flow rate of 35 g/10 min (Ingeo Biopolymer, Nature Works, USA). Chitosan and TFA were purchased from Sigma-Aldrich (Saint Louis, MO, USA). For the heavy metal ion removal test, silver nitrate (AgNO₃) and nitric acid were also purchased from Sigma-Aldrich (Saint Louis, MO, USA).

Electrospinning

The electrospinning setup utilized in this study consisted of a syringe and needle (the internal diameter is 0.42 mm), a ground electrode, an aluminum sheet, and a high-voltage supply. The needle was connected to the high-voltage supply, which could generate positive direct current (DC) voltages and current up to 35 kV and 4.16 mA/125 W, respectively. For the electrospinning of nanofibrous membranes, predetermined weight percentages of PLA (28, 30, and 32% wt) were first dissolved in TFA under sonication for 30 minutes, and then mixed by a magnetic stirrer for 12 hours, while chitosan was also dissolved separately in TFA at three different weight concentrations of 5%, 6%, and 7% by weight with sonication and magnetic mixing. The PLA and chitosan solutions were then mixed together at the ratio of 1:1 for 30 minutes before electrospinning. During spinning, the solutions were delivered by a syringe pump with a volumetric flow rate of 0.72, 0.84, or 0.96 mL/h. The distance between the needle tip and the ground electrode was 16, 18, and 20 cm. The positive voltage applied to polymer solutions was set to 15 kV. Table 1 lists the test trials and the processing parameters. In addition, nanofibrous membrane of virgin PLA was also prepared as a blank reference. All electrospinning experiments were carried out at room temperature.

Table 1.

Process parameters used in the experiments

Test	Weight percentage of polylactide (%)	Weight percentage of chitosan (%)	Flow rate (ml/h)	Distance (cm)	Porosity (%)	Average fiber diameter (nm)
1	28	5	0.72	16	88.6	544
2	28	6	0.84	18	88.5	421
3	28	7	0.96	20	88.3	419
4	30	5	0.84	20	87.5	613
5	30	6	0.96	16	86.2	485
6	30	7	0.72	18	86.6	453
7	32	5	0.96	18	84.6	695
8	32	6	0.72	20	85.7	665
9	32	7	0.84	16	85.5	575

Weight percentages of polylactide and chitosan are the percentages of the materials in the solutions. The nanofibrous PLA/chitosan membranes prepared using the bold values showed the best removal capability of heavy metal ions.

Characterization of nanofibers

The morphology of electrospun PLA/chitosan nanofibrous membranes was observed on a scanning electron microscope (SEM) (field emission scanning electron microscope (FESEM); Jeol Model JSM-7500F, Japan) after gold coating. The diameter distributions and the average diameter of the nanofibers were obtained based on the diameters of 90 (30 fibers each for three SEM photos) randomly selected fibers.

A Fourier transform infrared (FTIR) spectrometry was employed to examine the spectra of electrospun PLA/chitosan membranes. The FTIR analysis was conducted on a Bruker Tensor 27 spectrometer at a resolution of 4 cm⁻¹ and 32 scans. Membrane samples were pressed as KBr discs, and spectra were recorded over the 400–4000 cm⁻¹ range.

Metal removal capability of the polylactide/chitosan nanofibrous membranes

The metal removal capability of the nanofibrous PLA/chitosan membranes was studied using the device shown schematically in Figure 1. The membranes used for the test have a diameter of 40 mm and a thickness of 0.15 mm. A predetermined amount of solutions (50 mL) containing different concentrations of silver ions was placed at the top of the setup. With the application of vacuum pressure, the solutions were drawn to pass through the nanofibrous membranes. The filtering rates through the membranes could be adjusted by the throttle value. All tests were carried out at room temperature. After the heavy metal ions were taken up by the membranes, the solutions were collected by the bottle at the bottom for measurement of the residual metal ion concentrations. The concentrations were measured by an inductively coupled plasma inductively coupled plasma-optical emission spectroscope (ICP-OES) Model Varian 710-ES. All samples were analyzed in triplicate. The uptake capacity (UC) and the adsorptivity of metal ions by the PLA/chitosan membrane were calculated by the difference of metal ion concentrations in the collected solutions before and after the metal removal experiment:

UC = C_{0} - C

(1)

Adsorptivity = \frac{C_{0} - C}{C_{0}} \times 100 (%)

(2)

where C₀ and C are the concentration of metal ions before and the after the metal removal experiment, respectively, in mg/L. The metal removal capability was expressed in terms of adsorbed silver weight (mg) by multiplying the UC by the volume of the test solutions (mL).

Figure 1.

Schematic of the device used for the adsorption tests.

Comparison of bulk chitosan and chitosan nanofibers

Electrospun nanofibrous PLA/chitosan membrane was cut into a disk with a diameter of 40 mm. The disk was then placed in a glass containing 50 mL of heavy metal solution with Ag(I) ions at the concentration of 10 mg/L. The solution with the disk was mixed by a magnetic stirrer for various hours. To have a comparison, bulk chitosan powder was employed as a control for heavy metal uptake. Chitosan powder weighing 24 mg was added to 50 mL of heavy metal solution with Ag(I) ions at initial concentrations of 10 mg/L. The solutions were also mixed by a magnetic stirrer for various hours. Afterwards, the concentrations of residual metal ions in both the disk group and the control group were measured.

Influence of initial heavy metal ions concentrations

The influence of initial metal ion concentration on the metal removal capability of the membranes was examined. Solutions with various initial concentrations of silver ion (from 2 to 10 mg/L) were prepared. After filtering, the metal removal capability of the membranes was calculated.

Effect of filtering rate on the metal removal capability

The influence of the filtering rate of the membranes on the metal removal capability was examined. The experiments were performed with Ag(I) ions at initial concentrations of 10 mg/L. The filtering rate through the membranes was adjusted by the throttle value to range between 0.86 and 60 L/h. The concentrations of the adsorbed metal ions on the membranes were measured and calculated after the filtering process.

Influence of the temperature and pH value of the solution

The influences of solution temperature and pH value on the metal removal capability of nanofibrous membranes were also determined. Solutions containing 10 mg/L of silver ions with different temperatures and pH values were prepared. They were then filtered using the PLA/chitosan membranes. After filtering, the residual heavy metal ions were measured to determine the adsorptivity and the metal removal capability of the membranes.

Recovery of the nanofibrous membrane

The recovery study was performed with Ag(I) ions with an initial concentration of 10 mg/L. Nitric acid solution (50 mL) was passed through the used membranes at a rate of 0.86 L/h to wash out the adsorbed metal ions on the membranes. This is followed by the wash of the nanofibrous membranes with deionized water for neutralization and recondition. To examine the reusability of the recovered PLA/chitosan membranes, 50 mL of Ag(I) ion solution was passed through the recovered membranes at a filtering rate of 0.86 L/h. The recovery efficiency was calculated according to the following equation:

RE = \frac{U C_{c}}{U C_{o}} \times 100 (%)

(3)

where RE is the recovery rate (%), UC_o is the uptake of metal ions by the fresh membrane in mg/L, and UC_c is the uptake of metal ions after the recovery procedure in mg/L.

Results and discussion

The low mechanical properties of chitosan have hindered their broad utility for applications.¹⁸ With the addition of PLA materials, the mechanical strength of electrospun chitosan nanofibers can be enhanced. Furthermore, by adopting different processing parameters as listed in Table 1, nanofibers of various morphologies could be successfully fabricated. Figure 2 shows the SEM micrographs of the electrospun PLA/chitosan nanofibers under magnification of 5000×, while Figure 3 shows the diameter distributions of these fibers. The diameters of the spun nanofibers ranged between 419 and 695 nm, and the porosity of the nanofibrous matrix was high. Furthermore, the FTIR spectroscopy analysis was performed to verify the chitosan in the electrospun nanofibrous membranes. Figure 4 displays the measured spectra of virgin PLA nanofibers and PLA/chitosan blend nanofibers. The absorption peaks of chitosan were observed in electrospun PLA/chitosan nanofibers. In the FTIR spectrum of the chitosan-loaded PLA membrane, the vibration peaks at 1650 and 1320 cm⁻¹ of amide was enhanced with the addition of chitosan. The absorbance at the region of 1260 cm⁻¹ could be ascribed to saccharide groups of chitosan.²⁴ The results of FTIR spectra confirmed that the chitosan was successfully embedded in the PLA matrix.

Figure 2.

Scanning electron microscope images of nanofibers prepared using different processing conditions (test conditions 1– 9 in Table 1).

Figure 3.

Diameter distributions of electrospun nanofibers prepared using test conditions 1–9 in Table 1.

Figure 4.

Fourier transform infrared spectra of electrospun nanofibrous membranes.

Fiber diameter and chitosan content on the metal removal capability

The metal removal capability of the electrospun membranes prepared by various process conditions was measured and is shown in Figure 5. Clearly the PLA/chitosan nanofibers electrospun using Condition 6 exhibited the highest adsorptions toward silver ions. This might be attributed to the fact that the nanofibers of Condition 6 contained a higher percentage of chitosan (7 %wt as listed in Table 1). The fabricated membrane thus showed superior metal ion adsorptivity. Generally, a smaller fiber diameter distribution leads to greater surface areas of the nanofibers and provides abundant surface areas for the adsorption interaction of chitosan and the metal ions. Despite the nanofibers prepared using Condition 6 exhibited greater fiber diameter distribution (453 nm, see Table 1) than those prepared using Conditions 2 and 3 (421 and 419 nm, respectively), they still showed the highest metal removal capability. This might be because the influence of chitosan content in the fibers outweighed that of fiber diameter. Since the nanofibrous PLA/chitosan membranes prepared using Condition 6 showed the best removal capability of heavy metal ions, they were thus selected for all subsequent experiments.

Figure 5.

Uptake capacity of electrospun nanofibrous membranes prepared by different process conditions.

Bulk chitosan versus PLA/chitosan membranes

The metal removal capability of nanofibrous PLA/chitosan membranes was compared with that of bulk chitosan. Bulk chitosan powder (24 mg) was dissolved in the solution (50 mL) containing metal ions as a control. The membranes were prepared using the processing Condition 6 in Table 1 for the tests. Both groups (bulk material group and electrospun membrane group) contained the same weight of chitosan. Figure 6 shows the measured metal ion UC of bulk chitosan and the nanofibrous PLA/chitosan membranes subjected to different soak times. Electrospun nanofibrous PLA/chitosan membranes exhibited inferior metal removal capability to the bulk chitosan when the soak time was less than 1 hour, after which the nanofibers showed superior metal adsorptivity compared with the bulk chitosan. This is due to the fact that when the soak time is short, due to the capillary effect, the aqueous solution may not be able to completely penetrate into the nanofibrous membranes. Thus, the interaction between the chitosan and metal ions is not enough to uptake the metals. The metal uptake of the PLA/chitosan membranes is inferior to that of bulk chitosan. As the soak time is increased, the interaction increases as well. Nanofibers, due to their increased surface areas, provide abundant binding sites for the removal of silver ions. The metal removal capabilities thus exceed those of bulk chitosan particles at long soak times.

Figure 6.

Metal ion uptake capacity of the bulk chitosan and nanofibrous polylactide/chitosan membranes.

Effect of initial ion concentrations on metal removal capability

The effect of initial ion concentrations on the removal capability of heavy metal ions for the fabricated PLA/chitosan membranes was investigated. Five different initial silver ion concentrations, namely 2, 4, 6, 8, and 10 mg/L, were used for the tests. The filtering rate used was 0.86 L/h. Figure 7 shows the effect of the initial metal ion concentration on the adsorption capability. Clearly, the metal removal capability of PLA/chitosan membrane increased with the initial Ag(I) ion concentrations.

Figure 7.

Effect of initial ion concentrations on the uptake capacity of nanofibrous membranes.

At the initial ion concentration of 2 mg/L, the adsorptivity of silver ions was 30.1%, which was lower than those at higher initial concentrations. The adsorptivity increased somewhat as the initial ion concentrations reached 4 mg/L and above. The highest metal uptake and the adsorptivity values were 0.74 (Ag/Chitosan) (mg/g) and 41%, respectively, at 8 mg/L of initial silver ion concentration. During electrospinning, the chitosan might be completely encapsulated by the PLA matrix and did not have contact with the heavy metal ions in the filtering process. This might explain why the adsorptivity of the electrospun fibers is lower than that of previous study.¹⁹ Nevertheless, the PLA/chitosan membranes developed in this study could effectively remove the heavy metal ions within a short period of time (ranging from 3 to 209 s for the flow rates of 60 to 0.86 L/h, respectively). This would provide advantages in terms of lower cost and higher throughput metal ion filtration.

Influence of filtering rates

The influence of filtering rate through the PLA/chitosan membranes on the uptake of silver ions was investigated for the kinetic study. As shown in Figure 8, at a lower filtering of 0.86 L/h, the metal uptake of the membranes reached the maximum. After that, the metal uptake decreased significantly with the filtering rate of the solutions through the membranes. The adsorption of the heavy metal ions onto the PLA/chitosan membranes is mainly performed by a chemical process involving valence forces via sharing or exchanging electrons. In this study, the higher filtering rates led to shorter interaction time between the heavy metal ions and chitosan in the nanofibers. The metal removal capability of electrospun PLA/chitosan membranes decreased accordingly.

Figure 8.

Influence of filtering rate through the polylactide/chitosan membranes on the uptake of Ag(I) ions.

Temperature and pH values of solutions

The influence of filtering temperature on the metal removal capability of silver ions was examined. As shown in Figure 9, the metal uptake and adsorptivity decreased gradually with the temperature of up to 35℃, after which the UC dropped significantly. This might be due to the fact that the adsorption of metal heavy ions is an exothermal process. With the increase of the temperature, the adsorption of metal ions on the nanofibers will be reduced. The metal removal capability of PLA/chitosan nanofibers thus decreased with the temperature.

Figure 9.

Influence of solution temperature on the metal ion uptake capacity of the nanofibrous membranes.

The effect of pH values on the uptake of silver ions by the chitosan/PLA membranes was investigated. Silver nitrate (AgNO₃) of four pH values, 1, 3, 5, and 7, were prepared for the test. The measured result in Figure 10 suggested that the metal removal capability of nanofibers increased with the pH value. This can be explained by the fact that in an acidic environment the abundant H⁺ ions will compete with the silver ion and adsorb to the membranes. The metal removal capability of electrospun membranes decreased accordingly.

Figure 10.

Effect of pH value on the uptake capacity of the nanofibrous membranes.

Recovery of nanofibrous chitosan/PLA membranes

The recovery of the fabricated nanofibrous membranes for the removal of silver ions was investigated. To recover the metal binding property, the used chitosan/PLA membrane was washed with nitric acid solution at a concentration of 3% (0.68 mol/L). When the acidic solution passed through the membrane, the adsorbed metal ions were substituted with abundant H⁺ ions. After being washed by the nitric acid, the membranes were washed several times with deionized water to neutralize the membranes. After the recovery process, the metal removal capability of the nanofibrous membrane was evaluated. The experimental result in Figure 11 suggests that the metal removal capability of the recovered membrane decreased with the number of reuses. Nevertheless, even after four times of recovery, the PLA/chitosan membrane still maintained up to 78% of silver ion uptake efficiency. The PLA/chitosan membranes developed in this study showed good reusability for the removal of heavy metal ions. All these suggest that the biodegradable nanofibrous PLA/chitosan membranes can be used for filtration of heavy metal ions from contaminated water.

Figure 11.

Removal efficiency of metal ions in recovered nanofibrous membranes.

Conclusions

In this study, we developed biodegradable nanofibrous PLA/chitosan membranes for the removal of heavy metal ions. PLA and chitosan were first separately dissolved in TFA. The solutions were then mixed and electrospun into nanofibrous membranes via an electrospinning process. The morphology of as-spun nanofibers was examined by scanning electron microscopy. The average diameter of electrospun nanofibers ranged from 419 to 695 nm. The adsorption capability of heavy metal ions on nanofibrous PLA/chitosan membranes was measured and compared with that of bulk chitosan. The influence of various process conditions on the metal removal capability was also examined. The experimental results suggest that the electrospun nanofibrous PLA/chitosan membranes exhibit good silver ion removal capability. The metal removal capability of nanofibrous membranes increased with the initial metal ion concentrations and the pH value, while it decreased with the temperature and the filtering rate of the solutions. Furthermore, the electrospun membrane could be reused after the recovery process. The empirical results in this study suggest that electrospun nanofibrous PLA/chitosan membranes are good candidates for the removal of heavy metal ions.

Footnotes

Funding

This work was supported by the Ministry of Science and Technology of Taiwan (NSC102-2221-E-182-071-MY3) and Chang Gung Memorial Hospital (CMRPD2C0131).

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