Electrospun functionalized polyacrylonitrile–chitosan Bi-layer membranes for water filtration applications

TLDR: In this article, electrospun PAN nanofibrous membranes were functionalized with zinc oxide (ZnO) nanoparticles and coated with a layer of chitosan (Cs), in order to improve the mechanical properties and anti-bacterial and water filtration performance of the membranes.

Abstract: Water scarcity has become a global systemic risk, prompting the development of more efficient filtration technologies. Recently, increasing attention has been given to low cost membrane materials such as polyacrylonitrile (PAN) nanofibers for water filtration. In this study, electrospun PAN nanofibrous membranes were functionalized with zinc oxide (ZnO) nanoparticles and coated with a layer of electrospun chitosan (Cs), in order to improve the mechanical properties, and anti-bacterial and water filtration performance of the membranes. Morphological analysis revealed that the PAN/ZnO–Cs membranes featured a structural hierarchy comprising a layer of highly porous nanofibrous PAN membranes and a less fibrous and thinner layer of a Cs coating. Addition of the Cs layer increases the tensile strength and elastic modulus of the membranes. Results acquired from a water permeability test indicated that the bi-layer membranes possessed adequate transport properties for typical membrane applications. Furthermore, the additional Cs layer and ZnO nanoparticles significantly improved the heavy metal ion adsorption performance of the PAN membranes. Moreover, the efficiency of the PAN/ZnO–Cs membrane for bacteria filtration has a log reduction value 2 orders of magnitude higher than PAN membranes, while the efficiency of these membranes for antibacterial action (i.e. in terms of log reduction value) is 6 orders of magnitude higher than PAN membranes. These results indicate the PAN/ZnO–Cs membranes are structurally more stable than PAN membranes, better at bacteria removal during the filtration process and better at self-cleaning (i.e. membrane biofouling resistance) than PAN membranes, signifying the potential of these membranes for water filtration applications.

Figures

Figure 9. Time-kill curves of electrospun membranes against E. coli. (the argument in LRV is unitless because it is a ratio)

Figure 5.FTIR spectra of: (a) PAN powder, (b) pure chitosan powder, (c) pure ZnO powder, (d)

Figure 1. Schematic diagram of the structural hierarchy of the PAN/ZnO-Cs bi-layer membrane

Figure 6. X-ray diffraction patterns of: (a) PAN powder, (b) pure chitosan powder, (c) pure ZnO powder, (d) as-spun PAN nanofibers, (e) as-spun PAN/ZnO nanofibers, (f) as-spun PAN/Cs nanofibers and (g) as-spun PAN/Cs/ZnO nanofibers.

Figure 4. STEM micrographs of electrospun PAN/ZnO membranes. Scale bars represent 500 nm.

Figure 3. FESEM micrographs of a cross-section of an electrospun PAN/ZnO-Cs membrane.

Full text

This work is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License
Newcastle University ePrints - eprint.ncl.ac.uk
Makaremi M, Lim CX, Pasbakhsh P, Lee SM, Goh KL, Chang HK, Chan ES.
Electrospun functionalized polyacrylonitrile-chitosan Bi-layer membranes for
water filtration applications.
RSC Advances 2016, 6(59), 53882-53893.
Copyright:
This is the authors’ accepted manuscript of an article that was published in its final definitive form by
Royal Society of Chemistry, 2016.
DOI link to article:
http://dx.doi.org/10.1039/C6RA05942B
Date deposited:
17/11/2016
Embargo release date:
27 May 2017

1
Electrospun Functionalized Polyacrilonitrile-
Chitosan Bi-layer Membranes for Water Filtration
Applications
Maziyar Makaremi
a
, Pooria Pasbakhsh
a
*, Lee Sui Mae
b
, Chia Xin Lim
b
, K.L. Goh
c
,
Hengky Chang
d
a
Mechanical Engineering Discipline, School of Engineering, Monash University Malaysia,
47500 Selangor, Malaysia
b
School of Science, Monash University Malaysia, 47500 Selangor, Malaysia
c
School of Mechanical and Systems Engineering, Newcastle University, NE1 7RU, United
Kingdom
d
Nanotechnology and Materials Science, Biomedical Engineering Group, School of Engineering
(Manufacturing), Nanyang Polytechnic, Singapore, 569830, Singapore
KEYWORDS
Structural hierarchy, Mechanical properties, Zinc Oxide nanoparticles, Antibacterial, Heavy
metal adsorption

2
ABSTRACT
Water scarcity has become one of the global systemic risks, prompting the development
of more efficient filtration technology. Recently, increased attention has been given to low cost
membrane materials such as polyacrylonitrile (PAN) nanofibrous mesh for water filtration. In
this study, electrospun PAN nanofibrous membranes were functionalized with zinc oxide (ZnO)
nanoparticles and coated with a layer of electrospun chitosan (Cs), in order to improve the
mechanical properties, and anti-bacterial and water filtration performance of the membrane.
Morphological analysis revealed that the PAN/ZnO-Cs membrane featured a structural hierarchy
comprising a layer of highly porous structure of nanofibrous PAN membranes and a less fibrous
and thinner layer of Cs coating. Addition of the Cs layer increases the tensile strength and elastic
modulus of the membranes. Results acquired from water permeability test indicated that the bi-
layer membranes possessed adequate transport properties for typical membrane applications. The
additional Cs layer and ZnO nanoparticles significantly improved the heavy metal ion adsorption
performance of the PAN membranes. The efficiency of the membrane for bacteria filtration,
parameterizes by the log reduction value, for PAN/ZnO-Cs membranes is 2 orders of magnitude
higher than PAN membranes; the efficiency of the membrane for antibacterial action (i.e. in
terms of log reduction value) for the former is 6 orders of magnitude higher than the latter. These
results indicate the PAN/ZnO-Cs membranes are structurally more stable than PAN membranes,
better at bacteria removal during the filtration process and better at self-cleaning (i.e. membrane
biofouling resistance) than PAN membranes, signifying the potential of these membranes for
water filtration applications.

3
INTRODUCTION
The demand for clean drinking water is fundamental for human life and it has sparked
immense interest in production of high efficient filtration devices which employ advanced
functional nanosized materials such as nanofibers. Electrospinning, a simple and cost effective
method, has been extensively used to produce nanofibrous filtration membranes with small pore
size, large pore volumes and excellent mechanical stability [1,2]. Nanofibrous filtration
membranes are made of randomly laid nano-sized fibers which can effectively filter out particles
by the size exclusion mechanism [3]. Furthermore, incorporation of specific functionality on the
surface of these membranes extends their ability to perform a one step removal of
microorganisms and chemical compounds along with sized-based separation [4].
Electrospun polyacrilonitrile (PAN) nanofibrous membranes find applications in many
areas such as electrically conductive nanofibers, wound dressing, biocatalyst, tissue scaffolding,
and drug delivery systems [5]. High chemical resistance, thermal stability and wetability of PAN
nanofibers [6] have led to their extensive use as ultrafiltration [7] and nanofiltration [8]
membranes. However, incorporation of functional chemicals and polymers is required in order to
enhance their performance for heavy metals adsorption and removal of microorganisms.
Chitosan (Cs) is the derivate of chitin (the second most abundant biological
polysaccharide). Chitosan has received a considerable attention due to its unique properties such
as biodegradability and non-toxicity as well as heavy metal ion adsorption and antibacterial
performance. Owing to these exceptional properties, chitosan finds applications in many areas
such as food preservation, wound dressing, drug delivery and biosensors [9–13]. Furthermore,
Several studies have been done on inherent antimicrobial character of chitosan exerted towards
bacteria and viruses for water filtration application [14,15]. It has been reported that
electrospinnability of chitosan has been restricted due to its polycationic nature in solution, rigid
chemical structure and specific inter and intra-molecular interactions [16]. Therefore, addition of
a synthetic biodegradable polymer such as poly(vinyl alcohol) (PVA) [17] or poly(ethylene
oxide) (PEO) [18] to the chitosan solution can improve the electrospinnability of chitosan,
leading to formation of nanofibrous membranes containing high degree of chitosan nanofibers.

4
Electrospun composite chitosan membranes with high antibacterial and adsorption
properties have been extensively studied for water filtration applications. In most cases, chitosan
has been used as a coating layer on the surface of a non-woven membrane of a synthetic polymer
with high mechanical stability. For instance, Desai et al. [14] fabricated nanofibrous filter media
by electrospinning of chitosan/PEO blend solutions onto a spunbonded non-woven
polypropylene substrate. Results indicated 2–3 log reduction in bacterial colonization of
Escherichia coli after 6 hours of contact time. In another study, Cooper et al. [19] reported that
chitosan/polycaprolactone(PCL) fibrous membranes significantly reduced Staphylococcus
aureus bacterial colonization compared to PCL fibrous membranes. Furthermore, Haider et al.
[20] investigated the heavy metal ion adsorption performance of chitosan nanofibrous
membranes. Results indicated the highly effective performance of the membranes in adsorption
of toxic metal ions such as Cu(II) and Pb(II) .
Recently, there have been several reports on the incorporation of metallic and metal oxide
materials into electrospun polymers for improving the antibacterial performance [21–23]. These
inorganic materials have attracted particular interest due to their stability in harsh processes and
minimal toxicity [24]. For instance, Dasari et al. [25] reported on the fabrication of electrospun
polylactic acid (PLA) membranes embedded with Ag and Cu. Antibacterial action of the
membranes were characterized using Saccharomyces cerevisiae bacteria; the findings reported
that the membrane has an appreciable effect on the bacterial, reducing the bacteria population by
85%. Metal oxides, such as ZnO, possess bactericidal properties and can inhibit both gram-
positive and gram-negative bacteria [26,27]. Hwang et al. [28] evaluated the antibacterial action
of electrospun ZnO/TiO
2
composite fibrous membrane and found that the presence of ZnO
nanoparticles on the surface of the nanofibres exhibited appreciable antibacterial effects against
gram-negative Escherichia coli and gram-positive Staphylococcus aureus. Moreover, since ZnO
can facilitate a high amount of surface active sites for adsorption of heavy metal ions from an
aqueous solution, it is a promising candidate for the removal of contaminants from the
environment [29]. For instance, ZnO nano-sheets that have been prepared via a hydrothermal
approach exhibit good sorption capacity for Pb
2+
due to presence of surface hydroxyl groups (Ma
et al. [30]).

Frequently asked questions

Q1. What have the authors contributed in "Electrospun functionalized polyacrilonitrile- chitosan bi-layer membranes for water filtration applications" ?

In this paper, the incorporation of specific functionality on the surface of these membranes extends their ability to perform a one step removal of microorganisms and chemical compounds along with sized-based separation. 

Q2. What future works have the authors mentioned in the paper "Electrospun functionalized polyacrilonitrile- chitosan bi-layer membranes for water filtration applications" ?

Furthermore, results acquired from water permeability test indicated that the prepared membranes possessed adequate transport properties for typical membrane applications, which signifies the potential of these membranes to be used for water filtration applications, although further studies required in order to fabricate these membranes in an industrial scale. 

Q3. What bacteria were used for inoculating the "Influent" water sample?

Gram negative bacteria Escherichia coli (E. coli ATCC 29522) and Gram positivebacteria Enterococcus faecalis (E. faecalis ATCC 29212) were used for inoculation of the "influent" water sample. 

Q4. Which solvent was purchased from R&M Chemicals?

The spinning solvent dimethyl sulfoxide (DMSO) was purchased from Fischer Scientific while acetic acid was purchased from R&M Chemicals. 

Q5. What is the effect of the chitosan layer on the microbial cell membrane?

On one hand, ZnO nanoparticles are potent antimicrobial agents that are able to kill microbial cells and on the other hand, the amidoxime groups on the chitosan layer possess antimicrobial functionality through their ionic inter-actions with negatively charged surface of bacteria. 

Q6. What is the adsorption efficiency of electrospun chitosan membranes?

More specifically, the amine group (—NH2) as one of chitosan organic compounds, is known to be very effective in removing heavy metals via chelating cationic metal ions and/or adsorbing anionic metal species through electrostatic interactions with protonated amino groups (—NH3 +) or via hydrogen bonding [45]. 

Q7. What is the role of ZnO in the removal of contaminants from the environment?

since ZnO can facilitate a high amount of surface active sites for adsorption of heavy metal ions from an aqueous solution, it is a promising candidate for the removal of contaminants from the environment [29]. 

Q8. What is the common use of chitosan?

In most cases, chitosan has been used as a coating layer on the surface of a non-woven membrane of a synthetic polymer with high mechanical stability. 

Q9. How much reduction in bacteria concentration after 8 hours of contact?

Electrospun PAN/ZnO membranes caused 6 orders of magnitude reduction in bacteria concentration after 8 hours of contact time (more than 3 orders of magnitude occurred after the first 3 hours of the contact). 

Q10. What are the tensile strengths of PAN?

tensile strengths of electrospun PAN and PAN/Cs membranes incorporated with ZnO nanoparticles are respectively 34% and 43%, lower than PAN membranes; the elastic moduli of these ZnO nanoparticles incorporated membranes are respectively 18 and 28% lower than PAN membranes. 

Q11. Why does the chitosan layer decrease the permeate flux of electrospun?

This could be due to low surface porosity of electrospun chitosan layer compared to electrospun PAN membrane which provides fewer accessible pores for water, resulting in reduction in permeate flux. 

Q12. What could be attributed to the reduced rigidity of the PAN membranes?

This could be attributed to the inhomogeneity and reduced rigidity of the PAN fibers due to ZnO agglomeration, implicating the possible formation of additional pathways for bacteria which facilitated their movement through the membrane. 

Q13. What is the structure of the PAN/ZnO-Cs membrane?

Morphological analysis revealed that the PAN/ZnO-Cs membrane featured a structural hierarchycomprising a layer of highly porous structure of nanofibrous PAN membranes and a less fibrousand thinner layer of Cs coating. 

Q14. What was the XRD method used to determine the structure of the electrospun membranes?

To investigate the crystalline structure of the electrospun samples, X-ray diffraction(XRD) of both powder and membrane samples was carried out using the D8 Discover X-raydiffractometer (Bruker, Germany) with nickel-filtered Cu (K) radiation. 

Q15. How long was the filter holder autoclaved?

Each membrane with thickness ranging from 0.042 to 0.072 mm was cut into circle with 25 mm diameter and sterilized under UV light for 30 minutes, while filter holder (syringe filter holder 25 mm, Sartorius) was autoclaved for 15min at 121 C. 

Q16. What was the effect of the incorporated nanoparticles on the fiber diameter of PAN?

incorporated ZnO (5 w/w %) nanoparticles did not affect the average fiber diameter of PAN, but formed minor agglomeration and clusters along the PAN fibers axis, which disturbed the rigidity of some of fibers (Figure 2b, f). 

Q17. What is the role of chitosan in the biodegradable membrane industry?

addition of a synthetic biodegradable polymer such as poly(vinyl alcohol) (PVA) [17] or poly(ethylene oxide) (PEO) [18] to the chitosan solution can improve the electrospinnability of chitosan, leading to formation of nanofibrous membranes containing high degree of chitosan nanofibers. 

Q18. What are the main reasons for the use of electrospun polymers?

there have been several reports on the incorporation of metallic and metal oxide materials into electrospun polymers for improving the antibacterial performance [21–23]. 

Q19. What mechanism is likely to explain the antibacterial activity of PAN/ZnO membrane?

Of these mechanisms, the most likely explanation for the antibacterial activity of PAN/ZnO membrane is the presence of active radical oxygen species of superoxide anion (O2 -) and their strong oxidizing interactions with bacterial cells. 

Q20. What is the effect of the chitosan layer on the water flux?

it can be seen from Table 2 that addition of the chitosan layer dramatically decreased the permeate flux (i.e. the volume flowing through the membrane per unit area per unit time) of electrospun PAN membranes by 97%.