Abstract
The lack of clean water that is safe for household usage is a major problem, which affects most communities in South Africa. Most people in rural communities depend on water that is contaminated with pathogenic strains of bacteria such as Bacillus cereus (B. cereus) and Escherichia coli (E. coli).
Nanocomposite membrane filtration has been widely considered as one of the most promising options for the removal of pathogenic bacteria. In this study, graphitic carbon nitride (g-C3N4) was prepared through impregnation methods using melamine. Selenium nanoparticles (SeNPs) and Se/HPEI were prepared using a chemical reduction method where L-ascorbic acid was used as a reducing agent. Se/g-C3N4/HPEI nanocomposite was also prepared in a one pot synthesis through the combination of chemical reduction and impregnation methods. The prepared nanocomposite was incorporated into a polyethersulfone (PES) matrix through phase inversion method. The synthesized nanoparticles, nanocomposite and membranes were characterized using transmission electron microscopy (TEM), scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX), X-ray diffraction (XRD), selective area energy diffraction (SAED), contact angles, water intake capacity, water Flux and filtration studies. The antibacterial properties of the membranes against E. coli and B. cereus were investigated with the disc diffusion assay, growth rate inhibition and Colilert-18 procedure.
The TEM images of SeNPs and Se/HPEI revealed spherical and rod like structures, respectively. They further showed that the size and shape of the nanoparticles were affected by the reaction temperature and concentration of the precursor. TEM micrographs of the Se/g-C3N4/HPEI nanocomposite showed a typical rippled 2D flat layered structure with dispersed Se nanorods with an average diameter of 37.60 nm. XRD analysis was in agreement with the data documented in the JCPDS file no 01-086-2246 for SeNPs and JCPDS file no 00-066-0813 for g-C3N4. The hybrid membranes exhibited lower pure water flux (40.28 - 26.53 Lm-2 h-1), improved
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hydrophilicity (63.21°- 35.54°) and higher bacteria rejection (28.03 -100%) as compared to the pristine PES (127.36 Lm-2 h-1, 67.75° and 19.67%).
The modified nanocomposite membranes displayed improved antibacterial properties against B. cereus and E. coli achieving an inhibition of growth rate of approximately 100% at the highest concentration (0.9%). It was also found that the modified nanocomposite membrane has an inhibition zone range from 6-12 nm against the two bacteria strains tested. The SEM of the modified membrane results show that the morphology of the B. cereus and E. coli was damaged and lost integrity of membrane, this was due to the bactericidal effect of the modified membrane.