School of Chemistry (ETDs)
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Item Silver nanoparticle-modified cellulose nanocrystals for fouling control in membrane distillation(University of the Witwatersrand, Johannesburg, 2023-06) Mpala, Josephine Tshepiso; Nthunya, Lebea; Richards, Heidi; Etale, AnitaA global reduction in water resources and the growing demand for fresh water has motivated the quest for the development of sustainable water-augmenting technologies. Membrane distillation (MD) is envisaged as an attractive desalination technology, surpassing cost challenges faced by conventional desalination technologies. Yet, its industrial commercialization faces multiple limitations, including the production of low water fluxes, membrane wetting and membrane fouling. This study sought to investigate the performance of silver nanoparticles (AgNPs) embedded on cellulose nanocrystals (CNCs) (CNC-capped AgNPs) to lessen the impact of biofouling in MD. This was conducted through coating the polyvinylidene fluoride (PVDF) membrane with CNCcapped AgNPs. Prior to coating with CNC-capped AgNPs, PVDF membrane properties were improved (for MD suitability) through blending with polyvinylpyrrolidone (PVP) and functionalized carbon nanotubes (f-CNTs). The resulting membrane had an improved overall porosity, and a respective increase in surface roughness (75%) and mechanical strength (45%). Pristine CNC-capped AgNPs’ characterization presented stable AgNPs with minimal leaching. Transmission electron microscopy (TEM) micrographs revealed a uniform dispersion of spherically shaped AgNPs exhibiting 13.3 ± 3.4 nm average diameter. The presence of AgNPs on the surface of CNCs afforded excellent thermal stability and good anti-microbial activity, mainly against E. coli, P. aeruginosa, S. aureus, S. epidermis, and S. saprophyticus. Following membrane modification, preliminary anti-bacterial tests conducted on the CNC/AgNP-modified PVDF membrane revealed a 98.7%, 52.3%, 78.0%, 53.9% and 93.3% reduction of E. coli, P. aeruginosa, S. aureus, S. epidermis, and S. saprophyticus cells, respectively, demonstrating its ability to control biofouling. Although the CNC/AgNP-modified PVDF membrane exhibited improved membrane properties, such as high surface roughness, high liquid entry pressure (LEP), and good hydrophobicity, its performance in MD (with artificial seawater as the feed stream) was poor, producing the lowest average water flux (0.179 ± 0.0303 kg/m2 /hr) compared to the unmodified PVDF membrane (0.528 ± 0.0838kg/m2 /hr), mainly due to pore blockage. However, upon spiking the artificial seawater with a monoculture of G. Stearothermophilus, the CNC/AgNP-modified PVDF membrane displayed the most stable water flux while the unmodified PVDF membrane’s water flux decreased by 79.3% over the 24-hour (h) period. This was attributed to the formation of a biofouling layer on the PVDF membrane which was absent on the CNC/AgNP-modified PVDF membrane. The AgNPs on the surface of the membrane afforded minimal bacterial deposition during operation. These results ascertain the possibility of biofouling minimization in MD using CNC-capped AgNPs, contributing to MD’s body of work for its ultimate realization for up-scaling.Item Interfacial engineering of NbSe2 and TaSe2 to enhance their electrocatalytic activity for hydrogen production(University of the Witwatersrand, Johannesburg, 2023-07) Kolokoto, Tshwarela; Moloto, NosiphoThere has been a need to replace fossil fuels, develop sustainable energy systems, and alleviate the negative environmental effects. These effects can be alleviated by developing efficient processes such as water-splitting, which can produce hydrogen gas in an environmentally friendly manner and, in turn, use it as a clean fuel. However, this process requires an effective electrocatalyst comparable to Pt and cost-effective. Herein, we demonstrate that the electrocatalytic activity of NbSe2 and TaSe2 can be improved by metal inclusion using interfacial engineering for the hydrogen evolution reaction (HER). The readily synthesised NbSe2 was decorated with 20% wt. Ni, 20% wt. Pt, 10% wt. Pt / 10% wt. Ni using two synthetic methods, namely the ex-situ and in-situ methods. The ex-situ samples had higher HER activities than the in-situ samples. Pt/PtO2-NbSe2 (derived from Pt decorated NbSe2 using the ex-situ method) showed a significantly enhanced HER activity compared to bare NbSe2. The Pt/PtO2-NbSe2 nanomaterial had the lowest overpotential, favourable kinetics and durability in an alkaline solution of 0.1 M KOH. The trend was as follows: Pt/PtO2-NbSe2 (Pt-decorated ex situ) > PtO-NbSe2 (Pt-decorated in-situ) > PtO/NiO-NbSe2 (Pt/Ni-decorated) > Ni/NiO-NbSe2 (Ni-decorated ex-situ) > Ni0.5Se/Ni(OH)2-NbSe2 (Ni-decorated in-situ) > NbSe2. In addition, NbSe2 was further decorated with 20% wt. Co using both the ex-situ and in-situ synthetic methods, and 10% wt. Pt / 10% wt. Co using the in-situ method. The ex-situ sample resulted in a higher HER activity compared to the in-situ samples. In particular, Co/Co3O4-NbSe2 nanomaterial (Co-decorated ex-situ) had the lowest overpotential, favourable kinetics and durability in an alkaline solution of 0.1 M KOH. The resultant trend was as follows: Co/Co3O4-NbSe2 (Co-decorated ex-situ) < Co3O4/CoSe2/PtO/PtO2-NbSe2 (Pt/Co-decorated in-situ) < Co3O4/CoSe2-NbSe2 (Co-decorated in-situ) < NbSe2. Consequently, the ex-situ method was the optimum synthetic method for forming NbSe2-based nanomaterials. TaSe2-based nanomaterials were formed similarly. TaSe2-based hybrids were formed by decorating TaSe2 with 20% wt. Ni, Co and Pt using the ex-situ method. The hybrid nanomaterials resulted in higher HER activities compared to pristine TaSe2 (i.e. Pt/PtO/PtO2-TaSe2 (Pt-decorated) > Ni/Ni(OH)2-TaSe2 (Ni-decorated) > Co/Co3O4-TaSe2 (Co-decorated) > TaSe2). Pt/PtO/PtO2-TaSe2 hybrid, in particular, resulted in the lowest overpotential under alkaline solutions (0.1 M KOH). Generally observed, was NbSe2-based electrocatalysts were better than TaSe2-based catalysts. In addition, the Pt-decorated ex-situ NbSe2 and Pt-decorated TaSe2 electrocatalysts were better than the model Pt/C catalyst, with the prior being the best overall. This is attributed to the basal sites of the NbSe2 and TaSe2. The ex-situ method was better than the in-situ method and this was due to the presence of metallic particles and the minimization of oxidation compared to the latter.Item Synthesis of carbon nanodots-peptide conjugates decorated with germanium for bioimaging(University of the Witwatersrand, Johannesburg, 2023-10) Machumele, Khanani Peggy; Makatini, Maya Mellisa; Maubane-Nkadimkeng, ManokoThe World Health Organization Global Cancer Observatory estimates that cancer caused 9.96 million deaths worldwide in 2020, making early detection crucial for diagnosis and treatment. Accurate identification of cancer plays a crucial role in the diagnosis and treatment process. It allows for customized and efficient therapies, minimizes unnecessary procedures and adverse effects, and improves the prognostic insights for patients and healthcare providers alike. The challenges in diagnosis include overdiagnosis, false positives/negative outcomes, and limited sensitivity. Advanced technologies are needed for better imaging accuracy and minimizing harm. This study aims to fabricate carbon dot-peptide conjugates to enhance bio-imaging capacity and selectivity. The peptides used are derived from the GKPILFF cell-penetrating peptide sequence and the RLRLRIGRR peptide, which is selective to cancerous cells. The Carbon dots were used to provide the photoluminescent properties required for bio-imaging of cancerous cells. Carbon dots (CDs) were synthesized using iso-ascorbic acid as the source of carbon using a microwave-assisted method. The nitrogen and germanium-modified carbon dots (Iso-N-Ge-CDs) demonstrated the highest photoluminescent properties compared to the unmodified CDs (Iso-CDs) and those with either N (Iso-N-CDs) or Ge (Iso-Ge-CDs). Photoluminescence emissions of longer wavelengths suitable for cell imaging were observed for the CDs, and the Iso-N-Ge-CDs demonstrated excitation-dependent emission wavelength behavior, pH sensitivity, and Fe3+ sensitivity. The 13 peptides derived from the peptide accelerating sequence GKPILFF and the cancer-selective peptide RLRLRIGRR were successfully synthesized. The peptides were characterized using Liquid Chromatography Mass Spectrometry (LCMS) and purified using preparative High-Pressure Liquid Chromatography (prep-HPLC). The secondary structure of the L-GKPILFF penetration acceleration peptide sequence (Pas) adopted a helical secondary structure. The D-GKPILFF derivative was found to adopt a random coil structure. These were confirmed using Nuclear Magnetic Resonance (NMR) techniques such as Total Correlation Spectroscopy (TOCSY) and Rotating Frame Overhauser Enhancement Spectroscopy (ROESY) NMR. The CDs-peptide conjugates were successfully synthesized, and the confirmation of conjugation involved multiple methods, including UV-Vis and PL techniques. To the best of our knowledge, the thesis incorporates the first study to demonstrate long-range interactions through ROESY NMR. The NMR analysis indicated that the helical structure of the peptide could be affected after conjugation, leading to notable peak shifts. Since the helical structure is crucial for the peptide's bioactivity and stability enhancement, NMR spectra with fewer structural changes in the peptide region may improve its biological properties. The research contained valuable information for scientists aiming to design and characterize Carbon dot-peptide conjugates with enhanced permeability and selectivity that can effectively deliver materials into cytosolic space.Item Dissolution of non-functionalized and functionalized nanomaterials in simulated biological and environmental fluids(University of the Witwatersrand, Johannesburg, 2023-06) Mbanga, Odwa; Gulumian, Mary; Cukrowska, EwaThe incorporation of nanoparticles in consumer products is exponentially high, however, research into their behaviour in biological and environmental surroundings is still very limited. In the present study, the static system and the continuous flow-through dissolution protocols were utilized to evaluate and elucidate the dissolution behaviour of gold, silver, and titanium dioxide nanoparticles. The behaviour of these particles was studied in a range of artificial physiological fluids and environmental media, to obtain a more precise comprehension of how they would react in the human body and the environment. The biodurability and persistence were estimated by calculating the dissolution kinetics of the nanoparticles in artificial physiological fluids and environmental media. The details of the current research are described as follows: An investigation into the dissolution of non-functionalized and functionalized gold nanoparticles was conducted as the first component of the research, examining the effect of surface functionalization on dissolution. The study determined the dissolution rates of functionalized and non-functionalized gold nanoparticles. Dissolution was observed to be significantly higher in acidic media than in alkaline media. The nanoparticle surface modification, particle aggregation, and chemical composition of the simulated fluid significantly affected the dissolution rate. It was concluded that gold nanoparticles are biodurable and have the potential to cause long-term health effect as well as high environmental persistency. This work has been published in the Journal of Nanoparticle Research and is presented in this thesis as Paper 1. Silver nanoparticles were also included in this study because they have many applications and industrial purposes. Therefore, their risk assessment was also of utmost importance. The results indicated that silver nanoparticle solubility was influenced by the alkalinity and acidity of artificial media. Low pH values and high ionic strength encouraged silver nanoparticle dissolution and accelerated the dissolution rate. The agglomeration state and reactivity of the particles changed upon exposure to simulated fluids, though their shape remained the same. The fast dissolution rates in most fluids indicated that the release of silver ions would cause short-term effects. This work has been published in Toxicology Reports and has been presented in this thesis as Paper 2. Although titanium dioxide nanoparticles are insoluble and undergo negligible dissolution, it was of utmost importance to investigate their behaviour in biological and environmental surroundings. This is as a result of the incorporation of these particles in everyday consumer products, in the nanosized range which raises concerns about their safety. Therefore, in Paper 3 presented in this thesis the dissolution kinetics of titanium dioxide nanoparticles in simulated body fluids representative of the lungs, stomach, blood plasma and media representing the aquatic ecosystem were investigated to anticipate how they behave in vivo. This work has been published in Toxicology In Vitro and presented in this thesis as Paper 3. The results indicated that titanium dioxide nanoparticles were very insoluble, and their dissolution was limited in all simulated fluids. Acidic media such as the synthetic stomach fluids were most successful in dissolving the particles, while alkaline media had lower dissolution. High ionic strength seawater also had a higher dissolution rate than freshwater. The dissolution rates of the particles were low, and their half-times were long. The results indicated that these particles could potentially cause health issues in the long term, as well as remain unchanged in the environment. This work has been published in Toxicology In Vitro and presented in this thesis as Paper 3. The last component of the research compared the dissolution kinetics of gold, silver and titanium dioxide nanoparticles through the use of the continuous flow-through system. The findings indicated that titanium dioxide nanoparticles were the most biodurable and persistent, followed by gold and silver nanoparticles. Therefore, it was suggested that product developers should use the OECD's guidelines for testing before releasing their product to the market to ensure its safety. This work has been published in Nanomaterials MDPI and presented in this thesis as Paper 4.Item Carbon nitride-based catalysts for thermal carbon monoxide oxidation: Does phase matter?(University of the Witwatersrand, Johannesburg, 2023-06) Mohamed, Ahmed Gamal Abdelmoneim; Ozoemena, Kenneth Ikechukwu; Abdullah, Aboubakr M.; Eid, KamelCarbon monoxide (CO) has a poisonous effect on all living organisms as it binds to the hemoglobin of blood cells, preventing oxygen uptake. Thus, the conversion of CO to less dangerous gas such as CO2 is an essential process. This work presented the utilization of carbon nitrides (C3Nx) in different phases (βgC3N4, βC3N5, βC3N6) for thermal carbon monoxide (CO) oxidation. Herein, gC3N4, C3N5, and C3N6 were prepared by pyrolysis of their amine precursors, which were doped with Fe by two distinct methods; mechanical mixing (Fe/C3Nx-M) and polymerization (Fe/C3Nx-P). The controlled preparation of Fe/gC3N4-P allowed the formation of hierarchical porous structures with high surface area (219 m2/g) compared to the Fe/gC3N4-M (77 m2/g). This enabled the ease of reactants diffusion, enhanced the electron transfer, and maximized the atomic utilization. Accordingly, Fe/gC3N4-P (T100= 245 °C) presented higher catalytic activity than Fe/gC3N4-M (T100= 291 °C). In addition, bimetallic FeTi/gC3N4-P and trimetallic FeTiCu/gC3N4-P catalysts achieved the complete conversion of carbon monoxide (CO) at lower temperatures; 175 and 147 °C, respectively, which was attributed to the enhanced reducibility, and synergistic effect of Ti and Cu. Besides, FeTi/gC3N4-P and FeTiCu/gC3N4-P showed higher catalytic activity than Pd/C commercial catalyst (T100= 198 °C). In addition, the trimetallic FeTiCu/gC3N4-P showed a stable catalytic behavior without any deactivation for more than ten hours. This study showed that the C3Nx phases worked successfully in the thermal catalytic CO oxidation. However, the gC3N4 phase is the most active one when doped with metal(s), as it offered higher crystallinity, graphitization, and thermal stability than C3N5 and C3N6. This study also paves the way for the utilization of gC3N4 as a support for different metals to be used efficiently in various thermal catalytic applications, not only CO Oxidation.Item Tailored Fabrication of MXene/Chitosan Nanocomposites as Efficient Adsorbents for Heavy Metals Removal from Wastewater(University of the Witwatersrand, Johannesburg, 2023-08) Ibrahim, Yassmin Ahmed Ismail; Eid, Kamel; Ozoemena, Kenneth IkechukwuMXene (Ti3C2Tx) has been extensively utilized in water purification systems, including toxic metal ions removal, owing to the unique layered structure and abundant oxygen surface groups. However, challenges such as aggregation and solubility of Ti3C2Tx nanosheets in water have prompted the need for innovative strategies. In this study, we introduce a i3C2Tx-incorporated chitosan matrix (MX/CS) adsorbent designed to address solubility concerns during water treatment. MX/CS adsorbents are tested towards the capture of “cadmium” (Cd 2+) and “Zinc” (Zn2+) ions in aqueous solutions at varied pH values (i.e., acid, neutral and alkaline), initial ions concentrations (25, 50 and 100 ppm), and varied Ti3C2Tx loading (i.e., 1, 5 and 10), to study the optimization adsorption parameters. In addition, the Ti3C2Tx nanosheets were activated/alkalinized at ratio (2:1, i.e., 2MX:OH/CS), where more negative-ions sites are provided, thus, enhancing the preferential sorption for heavy metal ions in terms of high adsorption capacities, and kinetics than the non-activated samples (MX-10/CS). Freundlich isotherms are predominated for the Cd2+ and Zn2+ ions adsorption on both adsorbents. A selectivity study reveals that Zn2+ ions got adsorbed faster on the adsorbents than Cd2+ ions because of its low atomic radii and electronegativity. Finally, the adsorbents will be generated and prepared for additional adsorption cycles to test their stability. The second part of this work is to present the novel fabrication of multifunctional hydrophobic polymeric foam MX nanocomposites for large-scale ultrafast wastewater treatment. Likewise, the foam nanocomposite will be tested for the adoption of multi-ions solution over wide pH rage to demonstrate the applicability of the novel adsorbent for large-scale applications. Overall, this research contributes to the advancement of water treatment technologies by enhancing the stability of MXene-based adsorbents and introducing an innovative fabrication method for hydrophobic polymeric foam MX nanocomposites. The outcomes demonstrate the applicability of these novel adsorbents for efficient and scalable water purification solutions.Item The synthesis of aryl benzamides as potential HIV-1 non-nucleoside reverse transcriptase inhibitors (NNRTIs)(University of the Witwatersrand, Johannesburg, 2023-07) Mohasoa, Likhopotso Cecilia; Zimuwandeyi, Memory; Bode, Moira L.Dihydro-alkoxybenzyloxopyrimidines are heteroaryl-containing compounds that have previously been shown to exhibit excellent activity against HIV-1 reverse transcriptase (RT) enzyme. In our own laboratory, 2-chloro-N-(6-(piperidin-1-yl)pyridin-2-yl)benzamide was identified as a compound with activity against wild-type HIV-1. Using these two structural types as a guide, as part of our ongoing studies to search for anti-HIV therapeutic agents that target the RT enzyme, a library of arylbenzamide compounds bearing a pyrimidine ring as a central core was synthesized. These compounds contained an oxygen linker to allow flexible rotation of the molecule in the RT active site, with the aim of achieving activity against wild-type and mutant HIV-1. As a starting point, in order to first identify a suitable synthetic method and then apply it for our target novel compounds, four different carboxylic acids and two classes of amines were tested. Amidation reactions were carried out on unsubstituted benzoic acid, 3-methoxybenzoic acid, 3-hydroxybenzoic acid, and 3-((2,6-dichloropyrimidin-4-yl)oxy)benzoic acid. In this last case, the 3-hydroxybenzoic acid moiety had already been linked to the pyrimidinyl core in order to test which order of reaction worked best: linking followed by amidation, or the reverse. Reaction of these benzoic acid derivatives with anilines and aminopyridines gave the resulting benzamides in 22-99% yields after optimization. When triethylamine was used as a base in amidation reactions involving 2-amino-3-bromopyridine, 2-amino 5-bromopyridine and 2-amino-5-methylpyridine, diacylation was favoured, while when pyridine was used, monoacylation predominated. The reactions to link benzoic acid derivatives to the pyrimidinyl core were carried out by displacement of chlorine on 2,4,5-trichloropyrimidine. The displacement of the first chloride was tested using three types of nucleophiles. The first nucleophile was methyl 3-hydroxybenzoate, effectively a protected benzoic acid, which afforded methyl 3-((2,6-dichloropyrimidin-4-yl)oxy)benzoate in 81% yield. Problems with subsequent hydrolysis of the ester made this route impractical. The second nucleophile was 3-hydroxybenzoic acid which provided 3-((2,6-dichloropyrimidin-4-yl)oxy)benzoic acid in 81% yield. The third nucleophile was N-(5-bromopyridin-2-yl)-3-hydroxybenzamide, where amidation had already been performed, which transformed into the desired compound N-(5-bromopyridin-2-yl)-3- ((2,6-dichloropyrimidin-4-yl)oxy)benzamide in 28%. The low yield obtained from reaction of the amidated nucleophile identified the most promising route to be linking of 3-hydroxybenzoic acid to 2,4,5-trichloropyrimidine first, followed by amidation. After the successful displacement of the first chlorine atom, two of the resulting analogues 3-((2,6-dichloropyrimidin-4-yl)oxy)-N-(p-tolyl)benzamide and N-(4-bromophenyl)-3-((2,6-dichloropyrimidin 4-yl)oxy)benzamide were functionalized with sulfur and nitrogen nucleophiles by displacement of a second chlorine atom. Ethanethiol proved to be highly nucleophilic, leading to pyrimidine C-O bond cleavage and sulfur disubstitution, while the nitrogen ucleophiles propylamine and piperidine afforded their corresponding derivatives in good yields without breaking the carbon-oxygen bond. The newly coupled propyl compound was further derivatized by means of hydrolysis with sodium hydroxide to yield the desired novel 3-((6-hydroxy-2-(propylamino)pyrimidin-4-yl)oxy)-N-(p tolyl)benzamide or 3-((6-oxo-2-(propylamino)-1,6-dihydropyrimidin-4-yl)oxy)-N-(p-tolyl)benzamide compound.Item Biophysical studies of metal chelate binding by HSA: Towards an understanding of metallodrug transport(University of the Witwatersrand, Johannesburg, 2023) Sookai, Sheldon; Munro, OrdeHuman serum albumin (HSA) is the most abundant blood protein, transporting many exogenous compounds including clinically deployed and investigational drugs that are generally organic in nature. HSA may largely influence the pharmacokinetics and pharmacodynamics of these drugs. Therefore, studying their interactions with HSA is vital in progressing drug development. In this thesis we present work on the synthesis and characterisation of five Schiff base bis(pyrrolide-imine) ligands that were metalated with either Au(III) (Chapters 2 and 3) or Pt(II) (Chapters 4 and 5). One of the ligands H2L1 was further metalated with Ni(II) and Pd(II) (Chapter 6). In Chapters 2 and 3 focus on a patented class of anti-cancer bis(pyrrolide-imine) Au(III) Schiff base chelates. Three Au(III) chelates were synthesized in Chapter 2 and underwent National Cancer Institute (NCI)-60 cytotoxic screening. Among them, AuL1 and AuL3 underwent full-five dose testing and recorded GI50 values of 7.3 µM and 11.5 µM, and IC50 values of 15.7 µM and 30.9 µM, respectively. AuL1 was tested further and found to be an interfacial poison of topoisomerase II at 0.5–5 µM and a catalytic inhibitor at 50 µM. In Chapter 3, two chiral tetradentate cyclohexane-1,2-diamine-bridged bis(pyrrole-imine) Au(III) complexes were reported, both of which were found to be cytotoxic in the NCI-60 screen. The chiral Au(III) chelates had a different mode of action compared to AuL1. Hierarchical cluster analysis suggest that their mode of action is similar to that of taxol. All five Au(III) chelates bound to HSA with moderate affinity (104–105 M–1) and minimally perturbed the structure of the protein. This highlights the potential for the Au(III) complexes to be transported by the HSA-mediated pathway. Chapters 4 and 5 focused on the synthesis of novel and previously reported Pt(II) Schiff base chelates to spectroscopically and computationally study their interaction with HSA and elucidate if the chelates could act as theranostic agents. It was found that switching the linking bis(imine) carbon linkage altered the binding affinity of the complex. However, the Pt(II) ion ensured that all three Pt(II) chelates preferred binding to Sudlow’s site II of HSA. The data was corroborated by molecular docking simulations and ONIOM calculations. Only 2 was found to be cytotoxic when irradiated with UV light but was found to act as a photosensitizer rather than a theranostic agent. Chapter 6 investigated the influence of d8 metal ions (Ni(II), Pd(II) and Pt(II) within the same ligand scaffold (H2PrPyrr) binding to HAS, which was investigated by steady state fluorescence quenching. The affinity constants, Ka, ranged from -3.5 -103 M−1 to-1- 106 M–1 at 37 C, following the order Pd(PrPyrr) > Pt(PrPyrr) > Ni(PrPyrr) >H2PrPyrr. The Pd(II) chelate was prone to hydrolysis and had a unique binding mode which we attribute to the unusually high binding affinity. The complexes uptake is enthalpically driven, hinging mainly on London dispersion forces. In summation, twelve metal complexes were successfully synthesized, of which 11 bound to HSA with a moderate binding affinity. The Au(III) chelates preferred Sudlow’s site I, while the Pt(II) chelates preferred Sudlow’s site II. Overall, the metal complexes bound fully intact to HSA.Item Defect–engineered lithium titanate anode materials for lithium–ion batteries(University of the Witwatersrand, Johannesburg, 2023-10) Podile, Seromo; Haruna, Aderemi Bashiru; Ozoemena, Kenneth IkechukwuEnergy is one of the aspects that plays a central role in moving society forward since it is one of the most important agenda of global economic and energy forums. There is an urgent need to move to clean energy given the environmental and the health benefits resulting from implementing energy systems that utilize green energy. The proposed energy sources in these systems are primarily natural (e.g. wind and solar), which means they are beyond human control and would work better if coupled with energy storage devices (ESDs). From this emerge the importance of energy storage systems (ESSs) which mostly perform based on the materials utilized to assemble the devices. In this study, we seek to enhance the power and energy densities of two of the prominent energy storage systems, namely lithium-ion batteries (LIBs) and lithium-ion capacitors (LICs), using modified commercial lithium titanate (LTO) materials as anodes. The materials consist of the pristine LTO (LTO-p), LTO coated with cerium fluoride (LTO-CeF3) and dry and wet irradiated pristine LTO (LTO-p-md and LTO-p-mw) and LTO coated with cerium fluoride (LTO-CeF3-md and LTO-CeF3-mw). Microwave irradiation was used to study the possible defect that the radiation can bring to the materials and possibly use the microwave effect to improve their electrochemistry. The effects of the coated layer of CeF3 and microwave irradiation on the structure of the commercial LTO were extensively studied using powder X-ray diffractometer (XRD), Raman spectroscopy, transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and Brunauer-Emmett-Teller (BET) theory. Structural investigations of TEM micrographs revealed CeF3 was present on the surface of the coated LTO materials. Further analysis did show that some portion of the CeF3 coating layer was co-doped into the LTO nanostructures. The findings from XRD and XPS analyses showed that co doping promoted a mixed state of Ti3+ and Ti4+ resulting from charge compensation when Ce3+ and F possibly substituted Ti4+ and O2-. This mixed state of titanium ions allowed the materials to have high electric conductivity than the pristine LTO (LTO-p). The spectrographs obtained from XPS analysis also showed that LTO microwave irradiated materials without coating experienced the same mixed states, which may have originated from oxygen vacancies that allowed for charge compensation when some of the LTO-p Ti4+ had to turn to Ti3+. The HRTEM analysis demonstrated changes to the lattice planes spacing of the modified LTO materials and these changes supported the (111) lattice plane shifts observed in the XRD analysis. The electrochemistry of the lithium-ion battery was studied on all the materials using cycling voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques. From GCD technique, the results indicated that the modified materials had higher specific capacities than LTO-p at all rates under rate capability studies. All as prepared materials were stable with coulombic efficiency of almost 100% for 100 cycles using 1 C, with modified materials surpassing LTO-p specific capacity. Cycling at a high rate (5 C) for 1000 cycles, saw LTO-md, LTO-mw, LTO-CeF3 and LTO-CeF3-md having high-capacity degradation after 500 cycles. It was also observed that initial capacities were retained up to the 1000th cycle for LTO-CeF3-mw and LTO-p. With an aim of comparing LTO-p and LTO-CeF3 half cells, these two materials were further studied for lithium-ion capacitors (LICs). This choice was based on LTO-CeF3 having a better charge-discharge profile, high specific capacity and coulombic efficiency than LTO-p. The improvements resulted from the coating layer (leveraging the high ionic conductivity and chemically inert properties of CeF3) and the observed defects seen on the XPS studies created by the coating synthesis process. The LTO-CeF3 anode was paired with a carbon black cathode to construct a full LIC cell. The assembled cell produced a maximal energy density of 107.4 Wh kg 1 with an accompanying power density of 2000 W kg-1, and it yielded a power density of 10 000 W kg -1 with an energy density of 61.8 Wh kg -1.Item Inclusion of nano-silver compounds in RO membranes as solutions to fouling by microbes and natural organic matter during seawater desalination(University of the Witwatersrand, Johannesburg, 2023-08) Nchoe, Obakeng Boikanyo; Moloto, Nosipho; Sikhwivhilu, Keneiloe; Tetyana, PhumlaniThe 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.