Inclusion of nano-silver compounds in RO membranes as solutions to fouling by microbes and natural organic matter during seawater desalination
Date
2023-08
Authors
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Journal ISSN
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Publisher
University of the Witwatersrand, Johannesburg
Abstract
The access to safe and potable water has become a salient discussion for governments across the globe. This is due to pronounced levels of the decline in volumes of available freshwater. Attributions to this phenomenon are mainly climate change, eutrophication, discharge of untreated effluent, heightened irrigation, and industrialization. Currently exploited freshwater sources are rivers, lakes, dams, glaciers, and aquifers. However, inconsistent rainfall patterns have rendered some of these sources as ‘stressed’, which is exacerbated by exponential population growth and misallocation of available freshwater. In hindsight, seawater was identified as a possible source of potable water. However, the high levels of salinity and miscellaneous contaminants (i.e., pathogens and natural organic matter) necessitates treatment of seawater prior its usage. Therefore, the purpose of this work is to develop rugged polyamide thin film nanocomposite (TFN) reverse osmosis (RO) membranes with antifouling properties for seawater desalination. TFN were fabricated by the inclusion of silver-based (i.e., silver sulfide) nanoparticles during interfacial polymerization of the polyamide active layer. Silver compounds are known to have superior antibacterial and photocatalytic properties, due to plasmonic and photo absorption properties. For this reason, silver oxide (Ag2O), silver sulfide (Ag2S), and silver chloride (AgCl) nanoparticles (NPs) were colloidally synthesized. These were then characterized and evaluated in photocatalytic and antibacterial applications. Cytotoxicity studies were also done to determine which of these NPs pose less risk to human health. The consolidation of data from these applications advised which of these NPs would be suitable for incorporation into the polyamide layer to produce fouling resistant TFN. Microscopic analysis depicted well-defined shapes, with average sizes of 23.0±5.7 (Ag2O), 30.6±7.4 (Ag2S), and 10.6±7.2 nm (AgCl). X-ray diffraction determined Ag2O, Ag2S, and AgCl NPs to have cubic, monoclinic, and cubic lattices, respectively. Optical spectroscopy determined Ag2O, Ag2S, and AgCl NPs to have band gap energies of 2.97, 3.11, and 3.05 eV, respectively. These observations inferred that crystalline NPs that exhibit surface plasmon resonance (SPR) in the visible region were successfully synthesized. SPR is a desired characteristic for photocatalysts, and indeed Ag2O, Ag2S, and AgCl NPs achieved humic acid degradation (HA) efficiencies of 86.2, 88.1, and 76.5%, respectively. In antibacterial studies, the broth micro-dilution method indicated that the minimum inhibitory concentration (MIC) values against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) for Ag2O, Ag2S and AgCl NPs were 0.03125, 0.125, and 0.125 mg/mL, respectively. The well-diffusion tests showed that Ag2O NPs had the largest zones of inhibition (ZOI), followed by Ag2S, then AgCl NPs. These observations demonstrated the concentration-dependent mitigation of bacterial cell proliferation. The NPs were further tested for cytotoxicity against human embryotic kidney 293 (HEK 293) cells. It was found that the cytotoxic concentration that rendered 50 % viability (CC50) were 0.0302, 0.3606 and 0.3419, and were obtained for Ag2O, Ag2S and AgCl NPs, respectively. This data implied that Ag2O NPs were the most toxic, while Ag2S and AgCl NPs were least toxic. In light of the above, Ag2S NPs were selected to be incorporated into TFN RO membranes. TFN RO membranes were fabricated by the addition of three different concentrations of Ag2S NPs in the aqueous phase to form the active polyamide (PA) layer on a polysulphone (PSF) support, namely 20, 30, and 50 mg. Fourier transform infrared (FTIR) spectroscopy detected vibrational peaks at 1659 cm-1 (amide I C=O stretch), 1542 cm-1 (amide II C-N stretch) 1481 cm-1 (C-H bend), 1385 cm-1 (C-O stretch), 1242 cm-1 (C-N stretch), and 779cm-1 (aromatic C-H and C=C wagging). The presence of aromatic and amide functional groups corroborated the formation of the TFN active layer, which is responsible for RO filtration of dissolved ions in water. Moreover, atomic force microscopy (AFM) revealed that average surface roughness decreased with increased Ag2S NP loading. TFN loaded with 20, 30, and 50 mg Ag2S NPs recorded water contact angles (WCA) of 54.1, 45.4, and 43.3°, respectively. The WCA of thin film composite membranes (TFC) without Ag2S NPs was recorded to be 73.5°. This demonstrated that the inclusion of Ag2S NPs increased surface hydrophilicity. In addition, salt rejection and water flux were higher for 30 mg loaded TFN (98 % and 32.7 L/m2h) compared to those of TFC (97% and 24.8 L/m2h). The bacterial growth inhibition was observed to be significantly high for 30 mg loaded TFN (80 %) compared to that of TFC (38 %). These observations indicate that the inclusion of Ag2S NPs significantly enhanced the performance of RO membranes and cost effectiveness of desalination.
Description
A Thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy, to the Faculty of Science, School of Chemistry, University of the Witwatersrand, Johannesburg, 2023.
Keywords
Silver-based nanoparticles, Anti-fouling, Reverse osmosis membranes, Polyamide, Desalination, UCTD
Citation
Nchoe, Obakeng Boikanyo. (2023). Inclusion of nano-silver compounds in RO membranes as solutions to fouling by microbes and natural organic matter during seawater desalination. [PhD thesis, University of the Witwatersrand, Johannesburg]. https://hdl.handle.net/10539/42447