Properties evaluation of electrospun alginate-based nanofibrous membranes for filtration applications
Mokhena, Teboho Clement
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This study entails the electrospinning of a sodium alginate (SA) natural polymer with the aid of an electrospinnable synthetic polymer, polyethylene oxide (PEO), in order to develop nanofibrous-based membranes for wastewater treatment. The two most abundant natural polymers (i.e. cellulose and chitosan) were also incorporated in order to improve the selectivity and antifouling of the membrane. In order to improve the antibacterial activity of the membrane, chitosan was used to synthesize silver nanoparticles (AgNPs). The membranes were characterized under different conditions, depending on their intended wastewater treatment application. The electrospinnability of the alginate/PEO blend was dependent on the storage time, with 10 days being the minimum duration to obtain smooth defect-free nanofibres. The adsorption capacity of the electrospun alginate nanofibres was studied under different concentrations, pH, and temperature using copper (Cu2+) as a model heavy metal. The maximum adsorption was approximately 15.6 mg g-1 at 25 °C and a pH of 4, and it maintained a reasonable metal adsorption over 5 recycling intervals. The adsorption capacity for chromium of the membrane based on the electrospun alginate nanofibres coated with cellulose nanowhiskers was 78 mg g-1 at a pH of 11. Complete removal of nanoparticles and a high retention of oil/water (>98%) was obtained. This was as a result of the pore size, the functional groups, and the inherited hydrophilic character of the membrane. In the case of the antibacterial alginate electrospun nanofibre membranes, the fibres were coated with silver nanoparticles and the susceptibility of gram negative and gram positive bacteria was also investigated using diffusion and kinetic methods. Spherical silver nanoparticles were obtained after 12 hours of heating at 95 °C using chitosan as a capping and reducing agent. The composite membrane was potent to gram negative and gram positive bacteria due to the presence of the AgNPs. The complexation between alginate and chitosan illustrated the possibility of the controlled release of AgNPs into wastewater streams. Further analyses were carried out on the silver nanoparticles containing chitosan as selective barrier in a three-tier membrane. The electrospun nanofibres (middle layer) was double-crosslinked by using calcium and glutaraldehyde in order to reduce the swelling behaviour of the alginate in an aqueous medium. The membrane displayed a larger than 98% rejection of nanoparticles (10-35 nm) and a larger than 93% retention of oil emulsions retention. It was found that the presence of AgNPs in the barrier layer did not affect the membrane performance.