Grantee Research Project Results

Final Report: Nanostructured Membranes for Filtration, Disinfection, and Remediation of Aqueous and Gaseous Systems

EPA Grant Number: GR832372
Title: Nanostructured Membranes for Filtration, Disinfection, and Remediation of Aqueous and Gaseous Systems
Investigators: Kit, Kevin , Davidson, P. Michael , Weiss, Jochen , Zivanovic, Svetlana
Institution: University of Tennessee , University of Massachusetts - Amherst
EPA Project Officer: Hahn, Intaek
Project Period: August 1, 2005 through July 31, 2008 (Extended to July 31, 2009)
Project Amount: $349,200
RFA: Greater Research Opportunities: Research in Nanoscale Science Engineering and Technology (2004) RFA Text | Recipients Lists
Research Category: Hazardous Waste/Remediation , Nanotechnology , Safer Chemicals

Objective:

The objective of this project was to develop electrospun nanofiber chitosan membranes that will have the ability to treat aqueous and gaseous environments by actions of filtration, disinfection, and metal binding. Chitosan is nontoxic and biodegradable, and has been shown to have beneficial antimicrobial and metal binding properties. These beneficial properties will be optimized in a nanofiber structure in which the surface area per mass is very high. The central hypothesis for the proposed research is that the degree to which these nanofiber chitosan membranes effectively filter contaminants, kill microbes, and bind harmful metals will be optimized by minimizing the size of the electrospun fibers and maximizing the available chitosan surface area. The project was originally divided into the following four tasks:

Determine processing-structure relationships for electrospun chitosan
Measure filtration properties of electrospun chitosan
Measure metal binding and physicochemical properties of electrospun chitosan
Measure antimicrobial properties of electrospun chitosan

However, we found it more logical to divide the project accordingly:

Determine processing-structure relationships for electrospun chitosan
Measure metal binding and antimicrobial properties in static conditions
Measure metal binding, antimicrobial, and filtration properties under dynamic flow conditions

In this way, we were able to determine the most effective fiber structures for metal binding and antimicrobial performance under near equilibrium conditions and then select the most effective for additional dynamic filtration studies.

Summary/Accomplishments (Outputs/Outcomes):

Project Activities

Processing-Structure Relationships

The goal of this task was to understand the relationship between electrospinning process variables and the structure of spun chitosan nanofibers. The processing conditions (chitosan molecular weight, polymer concentrations) that produced fibrous mats with a low frequency of defects was determined. Also the effects of these conditions on fiber diameter and actual fiber composition have been studied.

Electrospinning of pure chitosan was hindered by its high degree of inter and intra-chain hydrogen bonding and high solution viscosity. Addition of other synthetic polymers like PEO and PAAm greatly improved the spinnability of chitosan. We were able to obtain fairly uniform sized electrospun fibers by making blend solutions with small amounts of PEO (5%) in the blend solution using acetic acid as the solvent. Fiber formation and size was influenced by blend ratio of the two polymers, polymer concentration, polymer molecular weight and solvent. Heating of the polymer solution with hot air helped improve spinnability by reducing the bead like defects in the formed fibers and enabling the formation of fibers with as low as 5% PEO in the blend solution. Uniform bead-less fibers (as fine as 90 μm) were formed with 10% PEO in blend solution by spinning the solution at 70°C.

The advantage of heating the polymer solution during the spinning process was more pronounced for obtaining bead-less fibers of chitosan/PAAm blends. Room temperature spinning solutions were able to form beaded fibers at 75% chitosan in blend solution, however heating the solution to 70°C enabled us to spin HMWchitosan:PAAm (90:10) blend solutions with ~ 300 nm fiber diameter and < 2% bead density.

XPS analysis showed that the surface of PEO/chitosan fibers were somewhat depleted in chitosan content compared to the bulk composition. However, it was also found PAAm/chitosan fibers were enriched in chitosan content compared to the bulk composition

Metal binding and antimicrobial properties in static conditions

Chitosan blend nanofibers were highly effective in binding Cr(VI) metal ions and binding efficiency was a function of % chitosan in blend solution, molecular of weight, % DDA of chitosan and synthetic polymer used in blend solutions. The metal binding capacity in chitosan blend fibers is significantly higher than that observed for similar blend ratio chitosan/PEO blend films. A 93 µm thick Chitosan/PEO (90:10) blend film showed binding capacity of 0.44 mg chromium/g chitosan whereas the same blend ratio fibers showed 16 mg chromium/g chitosan binding capacity. Electrospun fibers exhibit greater binding capacity due to the high surface area to mass offered by the fibers compared to films.

Chitosan blend nanofibers showed 2-3 log reduction in E-coli K-12 micro-organism and reduction efficiency was a function of % chitosan in blend solution. This value is similar to ones obtained for 35 µm thick films of chitosan:PEO blends with similar blend ratios, but the mass of chitosan in films was up to 10 times higher than that in the fibers. It was found that binding capacity increased with a reduction in fiber size and with increased degree of deacetylation of the chitosan.

Dynamic flow studies

Nanofibrous filter media using chitosan based electrospun nanofibers were successfully fabricated by electrospinning onto spunbonded PP non-woven substrates. Dynamic metal binding efficiencies using as little 0.5 gsm of nanofibrous filter media showed promising results (binding capacity up to 35 mg chromium/ g chitosan) for commercial applicability of these filters. The nanofibrous filter media however was unable to achieve desired anti-microbial effectiveness because of the slow reaction between the protonated amine in chitosan and negative components of the bacterial cell wall. Air and water filtration efficiencies of the nanofibrous filter media measured using aerosol and PS beads suspended in water respectively showed high efficiencies which correlated with the fibrous media size and shape. However the nanofibrous layer lacked mechanical strength to with stand pressure applied during the PS bead filtration which affected the results.

Technical Details

Processing-Structure Relationships

Materials

Chitosan with two different molecular weights was used. Chitosan of molecular weight Mv = 1400 kDa (HMW) with varying degree of deactylation (DDA) i.e. 80% DDA, 70% DDA, and 67% DDA was used as received from Primex Inc. Chitosan of lower molecular weight Mw = 100 kDa (LMW) and 83% degree of deactylation was used as received from Sigma. Polyethylene oxide (PEO, 900 kDa) and polyacrylamide (PAA, 5000 kDa) were used as received from Scientific Polymer Inc.

Electrospinning

The electrospinning apparatus consisted of a metered flow pump (Harvard Apparatus Pump II), a high D.C voltage supply (Gamma High Voltage Research, Inc. Model HV ES 30P/DAM), and aluminum foils as targets for fiber collection. Processing conditions were as follows; solution flow rate of 0.08 ml/min, applied voltage of 30 kV, and tip-target distance of 10 cm. An air assisted heating unit was designed similar to one described by Chun and coworkers¹ to heat the polymer solution while it was being ejected through the needle by passing hot air around the needle at flow rates up to 75 ft³/hr, and temperatures ranging from room temperature (25°C) to 70°C.

Effect of Solvents and Spinning Solution temperature

Electrospinning of neither HMW nor LMW chitosan at varying concentrations in varying strengths of acetic acid (10% - 90%), hydrochloric acid (0.03N – 0.5N) and trifluroacetic acid (50%) resulted in fiber formation, even when spun at higher temperatures (up to 70°C) as shown in Figure 1. Amongst the three solvents tried initially, further studies were carried out with acetic acid as it was seen as the most promising candidate based on the shape of particles along with appearance of fibrils and the desire to stay away from more toxic solvents like TFA.

Figure 1 SEM images of the pure electrospun chitosan samples 1.4 wt% HMW chitosan spun from 50% acetic acid (left), 5 wt% LMW chitosan spun from 90% Acetic Acid (right).

Table 1 Summary of processing condition for electrospinning of pure chitosan

Type of Chitosan (Molecular weight)	Solvent	Spinning Solution Temperature (°C)	Polymer Concentration
HMW Chitosan (Mv - 1400 kDa)	0.03N HCl	25, 40, 70	0.6 – 1.5 wt%
	0.1N HCl	25, 40, 70	0.1 - 2 wt%
	0.5N HCl	25, 40, 70	1.5 wt%
	50% TFA	25, 40, 70	1.5 wt%
	90% AA	25, 40, 70	1.2 wt% + 1.5 wt% Urea
	90% AA	25, 40, 70	1.5 wt%
LMW Chitosan (Mv - 100 kDa)	0.1N HCl	25, 40, 70	1.7 wt%
	90% AA	25, 40, 70	5 wt% with addition of salt
	30% AA	25, 40, 70	6 wt% with addition of salt
Hydrolyzed Chitosan (Mv – 300kDa)	80% AA	25	5 wt %
Hydrolyzed Chitosan (Mv – 80kDa)	90% AA	25	4 wt%
Hydrolyzed Chitosan (Mv – 20kDa)	80% AA	25	5 wt %, 6 wt%

Acid hydrolysis of chitosan was done to further reduce the molecular weight of HMW chitosan following a procedure similar to that of Liu et.al², and chitosan with varying molecular weights (300 kDa, 80 kDa, 20 kDa) was obtained. Electrospinning of these different molecular weights of chitosan also did not result in fiber formation. In order to reduce the amount of inter- and intra-chain hydrogen bonding in chitosan, urea was added which has been shown to disrupt hydrogen bonding in other polysaccharides³. Salt (NaCl) was also added to the electrospinning solution, as it is known that addition of salt helps increase solution conductivity and improve spinnability of polymer solutions^4,5. Table 1 presents a summary of processing conditions that were studied, none of which resulted in fiber formation, instead forming electrosprayed particles.

Electrospinning of Chitosan/PEO blends

Chitosan, when blended with as little as 10% PEO, resulted in the formation of non-woven mats of fibers. Figure 2 shows SEM images of HMW chitosan blended with HMW PEO with increasing % PEO in the blend. It can be seen with increasing % PEO, fiber diameter increases and number of bead defects is reduced.

Figure 3 shows a plot of fiber diameter of electrospun fibers vs. % PEO in the blend solutions. From the fiber diameter data it can be seen that with increasing % PEO, fiber diameter increases which could be due to higher concentration of polymer in solution with increased PEO content. The concentration of polymers in solution was determined by studying solubility of polymer blends at different blend ratios and optimizing them so as to be able to form solutions which could form a stable jet which would lead to formation of fibers. At constant polymer concentration, viscosity of solution decreases with increased PEO content and reduced strength of acetic acid. Fibers formed using high molecular weight chitosan are thinner compared to those obtained using low molecular weight chitosan; this can be attributed to higher solution concentration of low molecular chitosan blends.

Figure 2 SEM images of HMW chitosan: HMW PEO blend fibers (a) 1.33 wt% HMW chitosan: HMW PEO (90:10) (b) 1.6 wt% HMW chitosan: HMW PEO (75:25) (c) 2.00 wt% HMW chitosan: HMW PEO (50:50).

Figure 3 Fiber Diameter v/s % PEO (Error bars represent standard deviation (n=60), letters indicate difference at p<0.05, wt% in parentheses indicate total polymer wt% in solution)

Effect of Spinning Solution Temperature

Solutions were electrospun at higher temperatures by blowing hot air around the feed needle. Figure 4 shows SEM micrographs of electrospun fibers of 1.33 wt% HMW Chitosan: HMW PEO (95:5) electrospun by blowing air at 25 ft³/hr at room temperature (25°C), 40°C and 71°C. It can be seen that as temperature increases, less defective fibers are obtained as indicated by lower bead density values at higher temperatures as shown in Figure 5. The increased temperature reduces the solution viscosity and the flowing air helps increase the spin-draw ratio, both of which together lead to further stretching of the unstable polymer jet during the whipping motion and aiding in the formation of beadless fibers. As air flow and temperature (71°C) are increased, there would be faster evaporation of solvent which would lead to increase in concentration of polymer solution exiting the syringe and formation of slightly thicker fibers. The effect of air flow rate on fiber formation was also studied. Increasing air flow from 25 ft³/hr to 75 ft³/hr did not have a significant effect on bead density compared to air temperature but increased air flow at higher temperatures led to a slight increase in fiber diameter.

Electrospinning of Chitosan/PAAm blends

Figure 6 summarizes the effect of blend ratios and spinning solution temperature on fiber formation. It can be seen that with increasing temperature, the fiber diameter increases slightly and the bead density decreases. SEM images of electrospun solutions containing 95% chitosan (Figure 6a) shows poor fiber formation at room temperature, and very few fibers are collected on the target. As the temperature is increased (Figures 6bc) fiber formation is improved with bead less fibers formed at 70°C. When chitosan

Figure 4 SEM images of 1.33 wt% HMW chitosan: HMW PEO (95:5) blend fibers at different spinning solution temperature and constant air flow rate of 25 ft³/hr

Figure 5 Bead density of various HMW chitosan: HMW PEO blend fibers at different spinning solution temperature and constant air flow of 25 ft³/hr (Error bars represent standard deviation (n=3), letters indicate significant difference at p<0.05)

content was reduced to 90% (Figures 6def) with increasing spinning temperature, the transformation from beaded fibers to uniform bead free fibers is seen. Further reduction to 75% chitosan in the blend leads to formation of bead free fibers at room temperature (Figure 6g). As spinning temperature is increased (Figures 6hi), an increase in fiber diameter is seen. At higher temperature there is a faster evaporation of solvent leading to faster drying of the charged jet. Renekar et.al have observed that spinning highly volatile polymer solutions leads to formation of polymer skin on the outside of the jet, subsequently leading to formation of a flat ribbon like structure.⁶ SEM images of the Chitosan/PAAm fibers formed at high temperature also show some flat ribbon shaped fibers which would contribute to the apparent increase in fiber diameter at higher temperatures.

Figures 7 and 8 show quantitatively the fiber diameter and bead density data, respectively, of electrospun chitosan/PAAm solutions at different blend ratios and increasing spinning solution temperatures.

Figure 6 SEM images of 1.4 wt% HMWChitosan: PAAm blends at different blend ratios and hot air blown at 25ft³/hr at different temperatures (fig 6a,6b,6c are HMWChitosan: PAAm (95:05) blend ratio, 6d,6e,6f are HMWChitosan: PAAm (90:10) blend ratio, and 6g,6h,6i are HMWChitosan: PAAm (75:25) blend ratio fibers)

Figure 7 Fiber diameter of 1.4 wt% HMW chitosan: HMW PAAm blend fibers at different air temperature. (Error bars represent standard deviation (n=60), letters indicate significant difference at p<0.05)

Figure 8 Bead density of 1.4 wt% HMW chitosan: HMW PAAm blend fibers at different air temperature. (Error bars represent standard deviation (n=3), letters indicate significant difference at p<0.05)

XPS Results

The surface chemistry of the electrospun fibers was characterized using a Thermo Scientific K-alpha X-ray photoelectron spectrophotometer (XPS) with an Al K-alpha (1480 eV) monochromatic source and 400 micron spot size. Surface atomic composition (as atomic %) of C, N, and O were used to determine chitosan surface content is terms of weight fraction. Due to the measured atomic fractions of pure chitosan film differing from the atomic percentages determined from the molecular structure, our results can only be reported as upper and lower bounds. Calculations based on the atomic fractions in the molecular stucture give a lower bound while those based on the measured atomic fractions of pure chitosan film give an upper bound on chitosan weight fraction.

Figure 9 shows the calculated surface chitosan wt% vs. the wt % of chitosan in blend solution. The location of the upper and lower bounds indicate that the spun blend fiber surfaces are depleted somewhat in chitosan compared to composition of the spinning solutions.

Figure 9 Calculated surface chitosan wt% vs. chitosan wt% in solution.

Similar analysis of the chitosan/PAAm blend fibers is shown in Figure 10. The surface chitosan content decreases with increasing fiber diameter for all blend ratios; similar to what has been observed for chitosan/PEO blends. However the surface chitosan content is in closer agreement with original chitosan concentration in solution then what was observed for chitosan/PEO blends. The location of the upper and lower bounds indicate that the surface chisosan content is at least equal to (and possibly greater than) the chitosan composition in the spinning solutions. It was also found that changing degree of deacytelation of chitosan (67, 70, 80 %) did not have any effect on surface chitosan content.

Figure 10 Surface chitosan wt % for chitosan:PAAm blend solutions with increasing % chitosan in solution.

Metal Binding and Antimicrobial Properties in Static Conditions

Metal binding - Chitosan/PEO Blends

Figure 11 shows the amount of chromium bound (mg Cr per g chitosan) for blend fibers with different chitosan: PEO blend ratios. HMW chitosan: PEO (90:10) blends show the highest amount of Cr bound per g chitosan. It can be observed that metal binding is strongly related to the % chitosan in the blend solution, and molecular weight of chitosan. With decreasing % chitosan in blend fiber, the binding capacity of the fibers decreased.

The binding capacity of blends made using high molecular weight chitosan was seen to be higher. High molecular weight chitosan would offer higher number of available –NH₃⁺ sites for metal binding for similar surface area fibers due to longer chain lengths and higher degree of protonation⁷. (XPS results in have also shown higher surface nitrogen concentration for HMW blends compared to LMW blends at same blend ratio). The high binding capacity observed for HMW chitosan fibers could also be result of thinner fibers providing higher surface area compared to LMW chitosan blends. To rule out the possibility of association of PEO with the metal ions an electrospun PEO fiber mat was also tested for metal binding and the results showed no binding.

The effect of degree of deacetylation on the metal binding capacity of chitosan/PEO (90:10) blend fibers was also studied. Solutions of 1.33 wt% HMW chitosan of varying degrees of deacetylation (80% DDA, 70% DDA and 67%DDA) with PEO in 75% acetic acid were electrospun to form non-woven mats. The metal binding capacity as shown in Figure 12 was highest at 80% DDA as expected because of the increase in number of available –NH₃⁺ sites for metal binding, and there was no significant statistical difference between the 67% (7.35 mg chromium/g chitosan) and

Figure 11 Metal binding of chitosan/PEO blend fibers at different % of chitosan in solution (Error bars represent standard deviation (n=3), letters indicate significant difference at p<0.05)

70% DDA (4.44 mg chromium/ g chitosan) chitosan/PEO blend fibers. The slight increase at 67% DDA (45.5 nm) could be due to thinner fibers formed compared to 70% DDA (62.4 nm) resulting in increased surface area. The metal binding capacity in chitosan blend fibers is significantly higher than that observed for similar blend ratio chitosan/PEO blend films⁸. A 93 µm thick LMW Chitosan/PEO (90:10) blend film showed binding capacity of 0.44 mg chromium/g chitosan. Electrospun fibers exhibit greater binding capacity due to the high surface area to mass offered by the fibers compared to films.

Metal Binding - Chitosan/PAAm Blends

Figure 13 shows the Cr (VI) binding capacity vs. % chitosan in blend of chitosan/PAAm blend nanofibers. The results once again show that the blends containing higher molecular weight chitosan and higher % chitosan in blend fiber show greater binding capacity. Figure 14 shows the Cr (VI) binding capacity vs. fiber diameter for 1.4 wt% HMW Chitosan:PAAm (90:10) blend fibers formed by spinning the solution at varying temperatures. Also plotted on the secondary axis is the surface nitrogen atom % vs the fiber diameter. It can be seen that with increasing fiber diameter the surface nitrogen content does not change, hence the binding capacity remain unaffected by change in diameter.

Figure 12 Metal binding of HMW chitosan: HMW PEO (90:10) blend fibers at different DDA (Error bars represent standard deviation (n=3), letters indicate significant difference at p<0.05).

Figure 13 Metal binding of chitosan/PAAm blend fibers at varying % chitosan in blend fiber (Error bars represent standard deviation (Error bars represent standard deviation (n=3), letters indicate significant difference at p<0.05, n=3)

Figure 14 Cr (VI) binding capacity (primary axis), surface nitrogen atom % (secondary axis) vs. fiber diameter for 1.4 wt% HMW Chitosan:PAAm (90:10) blend fibers formed by spinning the solution at varying temperatures. (Error bars represent standard deviation (n=3), letters indicate significant difference at p<0.05, n=3)

Anti-microbial Properties of Chitosan/PEO Blends

In anti-microbial studies usually the reduction in microbial activity is reported on log basis however since all our analysis so far has been based on weight of chitosan we have also plotted data based on reduction in bacteria divided by weight of film. Figure 15 shows a plot of effect of % chitosan in blend fiber and molecular weight of chitosan in blend fiber on the anti-microbial effectiveness of chitosan/PEO blend fibers. As the weight of all the fiber mats studied was not same, the weight of the fiber mats is being plotted on the secondary y-axis to

References:

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Journal Articles:

No journal articles submitted with this report: View all 13 publications for this project

Supplemental Keywords:

Sustainable Industry/Business, RFA, Scientific Discipline, Waste, Water, Remediation, Technology for Sustainable Environment, Sustainable Environment, Engineering, Chemistry, & Physics, Chemistry and Materials Science, New/Innovative technologies, Physics, Environmental Engineering, metal removal, detoxification, membrane technology, membranes, ultrafiltration, pollution prevention, aquifer remediation design, innovative technologies, industrial wastewater, environmental sustainability, nanocatalysts, contaminated aquifers, groundwater contamination, remediation technologies, antimicrobial nanostructured membranes, environmentally applicable nanoparticles, transition metal carbides, groundwater remediation, nanotechnology, in situ remediation, electrospun nanofiber chitosan membranes, membrane-based nanostructured metals, environmental chemistry, disinfection, metal binding

Progress and Final Reports:

Original Abstract

The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.