Electronic Theses and Dissertations (PhDs)
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Item Preparation of nitrogen-doped multiwalled carbon nanotubes anchored 2D platinum dichalcogenides for application as hydrogen evolution reaction catalysts(University of the Witwatersrand, Johannesburg, 2024-09) Mxakaza, Lineo Florence; Moloto, Nosipho; Tetana, ZikhonaThe alkaline hydrogen evolution reaction (HER) (H2O + 2e − → H2 + 2OH−) is fast gaining traction as a sustainable hydrogen gas generation route but suffers from slow reaction kinetics because of the additional water dissociation step and large reaction overpotential. As such, the current state-of-the-art acidic medium Pt and Ru catalysts suffer from considerable loss of catalytic activity in an alkaline medium. We propose the development and use of platinum metal dichalcogenides for alkaline HER. Platinum dichalcogenides are 2D materials that offer the advantage of more exposed catalytic sites, show dramatic chalcogen-dependent electronic properties, and have a band gap (0.24 eV - 1.8 eV for PtS2 and PtSe2) thus extending the use of these materials to light-stimulated photo-electrochemical (PEC) HER. As such, PtS2 is reported to be a semiconductor, PtSe2 is semi-conductive/semi-metallic depending on the number of layers, and PtTe2 is metallic. The Pt-chalcogen covalent bond intensifies down the chalcogen group. Additionally, the interlayer interactions in Pt dichalcogenides are covalent, and just like the Pt-chalcogen bond, intensify as the chalcogen atom changes from sulphur to selenium to tellurium. This behaviour of Pt dichalcogenides results from the Pt bonding d orbitals and the chalcogen bonding p orbitals that are relatively close in energy than in other TMDs, and the difference in the energy becomes smaller and smaller down the chalcogen group. Herein, we report on the synthesis of PtSe2 and PtTe2 using the colloidal synthesis method for the first time and then applying them as electrocatalysts in alkaline HER. As mentioned, developing 2D materials results in band gap development, particularly in PtS2 and PtSe2. Following this, PtSe2 was explored as a photocathode in light-induced photo-electrochemical HER. Generally, semiconductors are poor electron transporters and one of the major requirements for an efficient PEC cathode is solar absorption, charge generation, and efficient charge separation. The charge separation properties of PtSe2 were improved by supporting this material on highly conductive, mechanically, and thermally stable nitrogen-doped multi-walled carbon nanotubes (N-MWCNTs). In Chapter 3, we report on the effect of varying selenium precursors from elemental selenium, sodium selenite to selenourea on the colloidal synthesis of PtSe2 in a mixture of oleylamine and oleic acid at 320 ℃. All the reactions resulted in the formation of PtSe2 although PtSe2 prepared from selenourea is amorphous, evidenced by relatively broader XRD peaks and a smaller crystallite size. HER activity of the three PtSe2 catalysts was evaluated in 1 M KOH at a scan rate of 5 mV/s and PtSe2 prepared from selenium exhibited the earliest onset potential of 46 mV, overpotential of 162 mV, and a smaller Tafel slope of 112 mVdec-1. This material exhibits the smallest resistance to electron transport and a high electrochemical surface area. We then explored the effect of altering tellurium precursor from elemental tellurium to tellurium tetrachloride, and sodium tellurite. Unlike the PtSe2 synthesis, different platinum tellurite phases, PtTe2, PtTe, and the mixed phase PtTe: PtTe2 were produced from Te, PtCl4, and sodium tellurite, respectively. Of the three, PtTe2 exhibited the highest alkaline HER activity with an onset potential of 29 mV, an overpotential of 107 mV, and a Tafel slope of 79 mVdec-1. In the same chapter, we compared the catalytic activity of PtSe2 (prepared from Se) and PtTe2 (prepared from Te) catalysts. We determined that PtTe2 has a high surface roughness and electrochemical surface, leading to relatively higher activity than PtSe2. However, PtTe2 is metallic and therefore does not have a band gap, which implies that it cannot be employed in light-stimulated catalysis reactions. In Chapter 4, we explored the use of PtSe2 as a light-stimulated PEC alkaline HER catalyst. We used in situ colloidal synthesis to grow PtSe2 on the walls of N-MWCNTs to improve the overall electron transport properties of PtSe2. PtSe2 anchored on N-MWCNTs was also studied in the dark and under illumination using 1 sun (100 mW/cm2) to determine the influence of light on the HER catalytic activity of the hybrid materials. This study demonstrates that the light-stimulated HER activity of PtSe2 improves when minimal amounts of N-MWCNTs are incorporated in the PtSe2 sample matrix. This then leads to employing these materials as photocathodes in PEC HER.Item Microwave-assisted synthesis of palladium-based ferroalloy electrocatalysts for application in alkaline direct alcohol fuel cells(University of the Witwatersrand, Johannesburg, 2024-11) Ramashala, Kanyane Nonhlanhla Eugenia; Billing, Caren; Modibedi, R. Mmalewane; Ozoemena, Kenneth IkechukwuThis research work describes the study of Pd-based ferro-electrocatalysts for application towards direct ethanol fuel cells (DEFCs), direct ethylene glycol fuel cells (DEGFCs), direct glycerol fuel cells (DGFCs) and oxygen reduction reaction (ORR) operated in a basic environment. The initial part of the research was to explore the Pd-based monometallic and bimetallic (Pd/C and PdFe/C) by utilising varied methods such as the conventional sodium borohydride (NaBH4) and microwave-assisted technique (MW) towards the oxidation of glycerol (gly), intending to choose the best method viable for these catalysts. This study revealed that MW techniques tuned the physicochemical properties of Pd/C and PdFe/C by augmenting their crystallinity and defect. These led to improved electrocatalytic activities towards glycerol oxidation reaction (GOR) over NaBH4 technique. MW process as a powerful tool was further used in the entire study to synthesise bimetallic and trimetallic electrocatalysts in ethanol (EtOH), ethylene glycol (EG) and glycerol (Gly) oxidation reaction in an alkaline environment. The synthesised bimetallic catalysts studied in this research work were (PdFe/C, PdCo/C, and PdMn/C) at varied ratios of Pd: M (Pd2M/C (2:1) and PdM/C (1:1)). Amongst them all, Pd2Fe/C and PdFe/C were observed to be the most favourable catalysts towards all the alcohols, with the excellent specific activity of about, for EtOH (11.59 and 4.15 mA cm-2), EG (9.82 and 5.51 mA cm-2) and Gly (8.94 and 4.73 mA cm-2), respectively. The satisfactory performance exhibited by the PdFe/C electrocatalyst prompted the exploration of the second 3d transition metal (PdFeMn/C and PdFeCo/C), intending to investigate the synergistic behaviour between the non-noble metals and Pd. The XRD confirmed that these electrocatalysts are in a crystalline nature with a decrease in d spacing (from 0.2247 nm, PdFe/C to 0.2236 nm (PdFeMn/C)) after the insertion of Mn into PdFe/C. This was supported by the TEM images obtained for the PdFeMn/C catalyst with a particle size of sub 10 nm. The comparison studies towards EtOH, EG and Gly were investigated for all the electrocatalysts and there was a remarkable observation, which is dissimilar from the theoretical studies (DFT). Density Functional Theory (DFT) revealed that PdFeCo performed better in terms of Gibbs free energy, binding energy, and energy band gap than PdFeMn; however, the experimental studies favoring the performance of PdFeMn. The PdFeMn/C delivered the best electrochemical activities, including a superior electrochemical active surface area (ECSA), larger current densities and mass activity response, and less susceptibility to poisoning and high conductivity as compared to PdFe/C and PdFeCo/C electrocatalysts. Furthermore, the PdFeMn/C electrocatalyst exhibited remarkable electrochemical properties during the ORR (basic medium). Ultimately, the best two anode electrocatalysts (PdFe/C & PdFeMn/C) were explored and tested for the proof-of-concept in the two-electrode configuration with the micro-3D printed cell. The PdFeMn/C delivered improved µ-ethylene glycol fuel cell, µ-glycerol fuel cell, and µ-ethanol fuel cell activities with respective to high voltage and power density of 33.27 mW cm-2, 11.00 mW cm-2 and 45,80 mW cm-2 respectively, operated at 100 mV / s. These electrocatalysts have demonstrated promising results in advancing ADAFCs.Item Towards the development and determination of trace impurities in battery grade nickel sulphate(University of the Witwatersrand, Johannesburg, 2024-10) Mabowa, Mothepane Happy; Tshilongo, James; Chimuka, LukeThis study introduces innovative research focused on developing and optimizing advanced extraction techniques for refining nickel hydroxide from secondary material solutions. This precursor to nickel sulfate is effectively purified through impurity removal and precise determination, enhancing the final product to battery grade standards. The research addresses the extraction of metal hydroxide from secondary sources such as spent batteries and industrial waste, promoting recycling and reducing environmental impact. By refining analytical methodologies and improving impurity control, this study advances the sustainable production of high quality nickel sulfate essential for advanced rechargeable batteries. The key challenge addressed in this study is the presence of impurities in secondary material solutions, which complicates the process of refining nickel hydroxide and hinders the production of high-purity nickel sulfate suitable for battery applications. Existing methods for recovering nickel from secondary materials are often inefficient, leading to high impurity levels, low recovery rates, and significant environmental impacts. Current methods such as solvent extraction and precipitation often fail to achieve the desired purity levels for nickel sulfate, necessary for use in high-performance battery manufacturing. Furthermore, these methods can be costly, resource-intensive, and environmentally damaging. Analytical methods used to measure impurities also have limitations. The complex and saturated matrix of battery-grade solutions challenges accurate impurity determination, often necessitating indirect methods such as difference analysis from nickel sulfate, which may not fully capture all impurity types or their concentrations. To resolve these issues the study focuses on optimizing advanced extraction techniques from these secondary sources. The research includes: (1) investigating the effectiveness of S-Curve precipitation by varying parameters such as pH levels, nickel concentration, precipitate dosage, temperature, impurity concentration, and solubility products; (2) evaluating solvent extraction for copper removal prior to nickel precipitation; (3) validating various analytical techniques (FAAS, EDXRF, ICP-OES) for trace element analysis; (4) examining lime precipitation for the removal of iron, manganese, and copper; and (5) characterizing β-nickel hydroxide (Ni(OH)₂) using scanning electron microscopy (SEM), X-ray diffraction (XRD) patterns, Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The research employed a combination of precipitation methods, solvent extraction, and advanced analytical techniques. The S-curve precipitation of nickel hydroxide was optimised by varying pH levels, nickel concentration, and temperature. The study also examined lime precipitation as a method for impurity removal and used solvent extraction for copper removal before nickel recovery. Various solvents with different ratios were utilized at room temperature for copper extraction, and the 1:5 ratio of 5,8-diethyl-7-hydroxydodecan-6-oxime (LIX 63-70) proved to be effective. Analytical tools like FAAS, EDXRF, and ICP-OES were employed to validate the concentration of trace impurities, and techniques such as SEM, FTIR, XRD, and XPS were used to characterize the crystalline structure and purity of β-Ni(OH)₂. The first part of the work entailed devising a technique to extract base metals, specifically nickel, from the waste stream resulting from the nickel sulphide-fire assay waste. This study explores the recovery of nickel (Ni) through a combination of solvent extraction and precipitation techniques. The main objective is to develop an efficient process for separating Ni from copper (Cu) and iron (Fe) impurities, thereby optimising metal recovery at varying pH, concentration with addition of calcium hydroxide at 60˚C and contributing to the circular economy. The approach involves using LIX 63-70 for solvent extraction, which effectively loads Cu into the organic phase and allows for Ni liberation into the aqueous phase. Characterization of the S-curve precipitation process was carried out using various analytical techniques. The precipitation of Ni(OH)₂ was optimised at pH 6.5, as evidenced by X-ray diffraction (XRD) patterns, Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The results show that Ni(OH)₂ precipitates in a crystalline β-phase, with XPS confirming the successful precipitation and minimal presence of Cu and Fe impurities at pH of 6.5 at 60˚C. Notably, the study also identifies the presence of Fe and Ca impurities at pH 2.5, as indicated by scanning electron microscopy (SEM), energy-dispersive X ray spectroscopy (EDX), and XRD analyses. The study addresses a critical research gap by providing a detailed assessment of the separation process for Ni from complex waste streams. It demonstrates the efficacy of 5,8-diethyl-7-hydroxydodecan-6-oxime (LIX 63-70) in selectively extracting Cu and reveals the influence of pH on the purity of Ni(OH)₂ precipitates. The process also involves significant lime consumption for neutralising the feed solution, with about 71% used to adjust the solution to pH 2.0, highlighting the importance of optimising reagent usage. The research presents a successful method for recovering Ni from fire assay waste in separating value-added metals from impurities. The findings contribute to advancements in metal recycling and repurposing, supporting the development of sustainable waste management practices and the promotion of a circular economy. Paper II evaluates the accuracy and reliability of elemental analysis in synthetic cathode liquor using Energy Dispersive X-ray Fluorescence (EDXRF) and Flame Atomic Absorption Spectroscopy (FAAS) with both factory default settings and after internal calibration and compares these results with those obtained from Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The research aims to test the performance of EDXRF and FAAS for identifying and quantifying elements such as calcium (Ca), sodium (Na), cobalt (Co), iron (Fe), nickel (Ni), copper (Cu), arsenic (As), selenium (Se), antimony (Sb), and bismuth (Bi). The investigations into the impact of these parameters on the variations in absorbance for the targeted impurities guarantee satisfactory linearity and recovery. The recovery was quantified by comparing the concentration of elements in spiking samples and certified reference materials (CRMs) to known quantities, while the sensitivity of each method was assessed by the limits of detection (LOD). Linearity was assessed by constructing calibration curves at a variety of concentrations and calculating the coefficient of determination (R²) to guarantee precise results at varying concentration levels. Initial EDXRF results using default settings showed substantial inconsistencies, particularly with Ca, where measured values invariably showed 0 mg/L despite actual concentrations ranging from 0 to 0.15 mg/L, and Ni, where measured concentrations varied between 493,327 and 529,280 mg/L compared to the true value of 120,000 mg/L. After calibration, EDXRF displayed better accuracy for Co, Fe, and Cu but experienced limits with light elements like Se and Sb due to high LOD. FAAS demonstrated effective results for Co, Cu, Fe, and Mg but encountered limits, particularly in detecting low amounts of metals like Na. FAAS readings for Na demonstrated high variability with a standard deviation (SD) of 505.24 mg/L and a relative standard deviation (RSD) of 23.39%. Furthermore, differences in FAAS measurements for Ca, Fe, and Ni were seen, with fluctuations in standard deviation (SD) and relative standard deviation (RSD) suggesting a certain level of inconsistency. The ICP-OES results confirms the accuracy of FAAS by closely aligning with its measurements for elements such as Co and Ni. The precision of FAAS is further demonstrated by the low standard deviations (8.08 mg/L for Co and 4 mg/L for Ni) of ICP-OES results (e.g., Co: 990 mg/L, Ni: 126 mg/L). This validation underscores the dependability of FAAS to these components due to selection of FAAS for its cost-effectiveness and broad applicability in industrial analysis. Evaluation of numerous methods is crucial for a thorough evaluation of elemental analysis accuracy, as evidenced by the comparison with ICP-OES. In addition, it is crucial to distinguish between the discourse on analytical methods and recovery metrics, as recovery rates are more closely associated with preconcentration techniques than with the analytical methods themselves. This work aims to fill a significant research need by emphasising the need for internal calibration for EDXRF and the necessity of using several analytical methods in conjunction to obtain dependable results. It stresses the strengths and limits of each method, providing a complete approach to enhancing analytical accuracy in industrial applications. The study in Paper III investigates the characterisation and retrieval of β-Ni(OH)₂ from fire assay waste using chemical precipitation. Various analytical methods are used to confirm the successful synthesis and purity of the molecule. Nickel hydroxide (Ni(OH)₂) is a functionally diverse chemical with a broad spectrum of uses. A hexagonal crystalline structure of β-Ni(OH)₂ is confirmed by X-ray diffraction (XRD) analysis, therefore validating the successful precipitation procedure. Fourier-transform infrared spectroscopy (FTIR) spectra provide additional evidence for the presence of nickel hydroxide by displaying distinct peaks associated with υ(OH) and υ(NiO) bonds. The X-ray photoelectron spectroscopy (XPS) analysis reveals the significant Ni²⁺ oxidation peak, which confirms the successful precipitation at a pH of 6.5. Additionally, XPS analysis detects the presence of contaminants such as chlorine and calcium in the waste matrix. Scanning electron microscopy (SEM) shows layered granules with a predominantly transparent brucite analogue crystalline phase, typical of β-Ni(OH)₂. It also exposes rough textures and uneven aggregation, indicating increased oxide concentrations on the Ni surface. The presence of nickel (Ni) and oxygen (O), as well as calcium (Ca) impurities arising from the chemical precipitation process, is confirmed by energy-dispersive X-ray spectroscopy (EDX). An investigation of particle size distribution indicates an average particle size of 2.0 µm. The results of Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) indicate a reduction in Ni concentrations, with recorded values of 62.7 g/L in the pregnant leach solution, 0.8 g/L in the precursor solution, and 0.501 g/L in the solid precipitate (cake). The copper loading efficiency is measured to be 79%, accompanied by a nickel loss of 9.73% and a nickel recovery rate of around 90.27%. This effective separation process demonstrates a cost-efficient and environmentally responsible method for recycling nickel from acidic chloride media, underlining the broader potential for nickel reuse in industrial processes. This study conducts a comparative analysis of nickel oxide (NiO) that is derived from fire assay nickel sulphide (FA-NiS) and produced through chemical precipitation and sol-gel methods. The focus is on the structural, morphological, and sensing properties of the NiO. This research is significant in that it is the first to report on the application of NiO synthesised from waste materials for volatile organic compound (VOC) sensing. The primary goal is to clarify the distinctions in properties between NiO obtained through these methods and evaluate their suitability for environmental sensing applications. Nickel was initially extracted from the raffinate using 5,8-diethyl-7-hydroxydodecan-6-oxime. Subsequently, nickel hydroxide (Ni(OH)₂) was precipitated with lime (Ca(OH)₂) at pH levels of 2.5 and 6.5. The hydroxide was subsequently transformed into NiO through a thermal treatment process. The presence of nickel and oxygen at pH 6.5, as well as iron, nickel, and oxygen at pH 2.5, was verified through the use of scanning electron microscopy (SEM)-energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). In both the sol-gel and chemical precipitation procedures, the X-ray diffraction (XRD) analysis demonstrated a cubic crystal structure with high average crystal sizes of 39-41 nm. The sol-gel process resulted in homogenous spherical particles, as evidenced by SEM imaging, whereas chemical precipitation resulted in aggregated layered grains. It is important to note that NiO precipitated at pH 2.5 exhibited coalesced hexagonal particles with a substantial amount of nickel and iron. The transition from nickel hydroxide to nickel oxide is essential because NiO is highly effective for VOC sensing due to its semiconductor properties. The study highlights the importance of utilising NiO in the detection of volatile organic compounds (VOCs) and the impact of each synthesis method on the material's sensory capabilities. Using certified reference materials, analytical methodologies, such as inductively coupled plasma optical emission spectrometry (ICP-OES) and X-ray fluorescence (XRF), demonstrated high-purity NiO (approximately 75%) with a low relative standard deviation (RSD <0.05%) and 90% recovery. The CRM AMIS 56 and SARM 33 were analysed alongside the samples to ensure reliable results were reproducible. Even at the lowest concentration of 1.5 ppm, NiO derived from fire assay waste demonstrated unambiguous sensing responses at 25˚C and 150˚C, with recovery times of 80 and 120 seconds, respectively. The potential of NiO from fire assay waste as an intriguing candidate for VOC sensing applications under ambient conditions was indicated by the highest response (Rg/Ra = 1.198 for 45 ppm ethanol) observed at 150˚C. The findings in paper IV highlight the suitability of nickel oxide synthesized from different methods for environmental sensing applications, particularly in volatile organic compounds detection. In Paper V, the focus is on the removal and characterization of impurities from pregnant nickel solutions at various pH levels, with an emphasis on enhancing nickel recovery and sustainable resource management. Lime is used as a precipitation agent to target impurities such as iron, lead, tin, manganese, and copper. The study employs inductively coupled plasma optical emission spectrometry (ICP-OES) to quantify and characterize these impurities. The objectives include improving analytical approaches for detecting trace contaminants, evaluating ICP-OES reliability for quality control, and assessing precipitation efficiency across different pH levels. Results reveal successful Fe3+ precipitation within the pH range of 2.0-3, alongside efficient manganese and copper precipitation at pH 5.5-6 and 4-6, respectively, aligning with established behaviours. The findings emphasise the significance of pH control for optimizing impurity removal from pregnant nickel solution, offering insights for enhancing nickel recovery processes in industrial settings. ICP-OES, supported by standard solutions and certified reference materials (CRMs), demonstrated exceptional linearity with correlation coefficients above 0.9995. The method showed high sensitivity, with detection limits and recoveries of CRM samples consistently within 10%. The study found that precipitation efficiency varies significantly with pH. Nickel (Ni) exhibited reduced precipitation at pH 2.02, with substantial precipitation occurring only at pH 6.5. Manganese (Mn) began precipitating at pH 2, achieving a peak removal efficiency of 98% at pH 6. Copper (Cu) precipitation started at pH 4, with a maximum efficiency of 99.3% between pH 4 and 6. Iron (Fe³⁺) was efficiently removed at pH 2.0-3.0. Significant variations in contaminant concentrations were observed, influenced by pH and precipitation agents. Fe³⁺ was removed with 100% efficiency at pH 2.5, while Cu precipitation was highly effective (99.3%) between pH 4 and 6. The decrease in Ni concentration at pH 2.02 was attributed to interactions with other metals rather than direct Ni precipitation. SEM revealed the morphology of the precipitates, showing a cauliflower-like structure for Ni(OH)₂ at pH 6.5 and the EDX confirmed the elemental composition of the precipitates, including Fe, Cu, Ni, Sn, Si, Al, Cl, Ca, and hydroxyl groups (OH), highlighting the presence of impurities precipitated at pH 2.5. This research highlights the effectiveness of ICP OES and EDX in trace impurity analysis and provides insights into optimizing precipitation processes, contributing to better recycling strategies and quality control in nickel processing and battery-grade materials. The study found that β Ni(OH)₂ precipitated optimally at pH 6.5, with a recovery rate of approximately 90.27% and minimal copper (79% loading efficiency) and iron impurities. Lime precipitation effectively removed Fe³⁺ at pH 2.5 and Cu between pH 4 and 6, with high removal efficiencies. Analytical methods such as EDXRF and FAAS, when calibrated, provided accurate results for trace elements, though discrepancies were noted for certain elements. The advancements in extraction and purification techniques, coupled with improved analytical methods and the novel application of NiO in VOC sensing, contribute significantly to the field of nickel recovery and processing. This research supports sustainable recycling practices and enhances the practical utility of recovered nickel, advancing both industrial applications and waste management strategies. Overall, this thesis contributes to advancing the understanding of impurity removal processes in nickel recovery and underscores the importance of precise control and characterization techniques in industrial applications.Item Application of oxidative enzymes in membrane systems for the bioremediation of triazines in wastewater(University of the Witwatersrand, Johannesburg, 2024-10) Lesaoana, Mahadi; Richards, Heidi L.; Brady, DeanThe prevalence of herbicidal pollutants present in various environmental matrices have become a global concern. The discharge and accumulation of s-triazine agrochemicals in effluents remains a major challenge, threatening the quality of freshwater resources. These are newly identified recalcitrant contaminants of concern (CECs) with complex structures, and inadvertent exposure poses deleterious ecological risks and human health-related adverse effects. Unfortunately, they have shown resistance to conventional treatment strategies, hence their persistence in wastewater treatment plant (WWTP) effluents and water bodies. Therefore, there is an urgent need for the exploration of alternative technologies for the effective eradication of such contaminants from water samples. The bioconversion of such micropollutants using oxidative enzymes like laccase is a promising research avenue, providing a sustainable, economically and ecologically benign strategy. The current research examined the potential of a hybrid biocatalytic membrane system to degrade common s-triazine agrochemical herbicides in aqueous solutions. Specifically, the use of Novoprime base 268 laccase coupled with hollow fibre polyethersulfone (PES) membranes was investigated for the bioremediation of atrazine (ATZ), ametryn (AMT), simazine (SMZ) , prometon (PMT) and terbuthylazine (TERB) in wastewater. In batch-mode reactions, major operating parameters (i.e. pH and temperature profiles, enzyme dosage and contact time) were varied for the laccase-assisted catalysis of s-triazine compounds. Optimised conditions provided highest removal efficiencies (> 88.9%) at pH 5.0, combined with a temperature of 25°C and 1.0 mg L-1 solution concentration after 24h reaction time. Through the addition of redox mediators viz. 2,2-azino-bis-3-ethylbenzothiazoline-6-sulphonic acid (ABTS), violuric acid (VA), vanillin (VA), syringaldehyde (SRA) and acetosyringone (ASR) recalcitrant triazine degradation was enhanced by 10 to 20 % at 1.50 mm. Subsequently, the performance of a standalone continuous flow-mode membrane system was evaluated firstly, using a bed adsorption column only operated under various conditions. The efficiencies were compared to batch-mode enzymatic experiments. The adsorption of triazines by PES was only weakly influenced by pH, and the optimum removal was attained at pH 5.0 (5.0 mg L-1), 2.35 g bed mass (14.0 cm height) and 24h column operation time. The overall removal percentages were 72.6%, 75.2%, 71.4%, 67.4%, and 68.2% for ATZ, AMT, SMZ, PMT and TERB, respectively. Although the results indicated satisfactory performances by both systems, their performance is limited when used as separate units (continuous membrane vs laccase reactor). A biocatalytic membrane system was achieved by integrating laccase into the dynamic packed-bed membrane column. Relevant process control design parameters of the fixed-bed biocatalytic column were carefully evaluated and recorded an optimum of 93.2 % removal efficiency as observed at a feed flow rate 2.0 mL min-1, at a bed height of 14.0 cm using an atrazine influent concentration of 5.0 mg L-1. Equilibrium dynamics of the breakthrough modelling were best fitted by Thomas model. Results attained demonstrated selectivity for triazines in matrix-matched real river water samples with remarkable recyclability after six successive operational cycles. This reflects the potential workability of the integrated system for extended enzymatic reactions evaluated under robust experimental conditions. As a benchmarking exercise, cost-analysis studies showed comparable projected scalability of our configuration at 1200 m3/d capacity at an estimated total cost of R7.036 mil.Item Energy storage properties of carbon onion-carbon nanofibre composites containing transition metal compounds(University of the Witwatersrand, Johannesburg, 2022) Khawula, Tobile Nokuphiwa Yollanda; Ozoemena, K. I.The quest for electrical energy storage has been a key driver for researchers to come up with more effective means of storing this form of energy due to the intermittent nature of renewable energy sources. Several countries have swiftly adopted the transformative potential of renewables, in particular solar energy, while others have delayed the implementation due to complex policies surrounding renewable energy projects. A way forward would be innovative regulatory approaches that encourage the pairing of solar systems with other generation technologies, and with storage, to offer a “round the clock” supply. Rechargeable batteries and supercapacitors are widely employed energy storage systems. A rechargeable battery system offers high energy density, with lithium-ion batteries (LIBs) being the most widely used. For some applications, it is imperative that energy is delivered at a much faster rate. This characteristic feature is known as power density, and supercapacitors have proven to be much better than batteries in this case. The large-scale commercialization and adoption of a supercapacitor are hindered by its low energy density. The electrode material is a major determinant of the success of supercapacitors. Generally, these are supported on high surface area carbon materials. This study focused on the development of electrospun polyacrylonitrile (PAN) fibres embedded with onion- like carbon (OLC) and iron (II) phthalocyanine (FePc) particles, and encapsulation of the fibres with Molybdenum disulphide (MoS2). Furthermore, composite fibres were either integrated with manganese (III) oxide (Mn2O3) or engineered with defects for enhanced performance in symmetric supercapacitors. The synthesis of electrode materials was divided into four phases; In the first phase (1), OLC nanoparticles were embedded in electrospun PAN fibres and decorated with the Mn2O3 and evaluated as supercapacitor electrode materials. For enhanced interfacial electrochemistry and overall capacitance, the electrode material in (1) was encapsulated with MoS2 in phase (2). In phase (3) FePc embedded in the PAN electrospun fibres were evaluated for supercapacitor applications. Limited specific capacitance and poor cycling stability were observed, thus suggesting integrating OLC and further encapsulation with MoS2, in phase (4). The morphology of the fibres was vii engineered with defects in the form of Fe2+ vacancies to maximize the electrochemical reactions of the OLC/MoS2 fibre composite. The electrochemical properties of the fibre composite materials were investigated and OLC/Mn2O3-CNF exhibited a specific capacitance, energy and power density of electrodes were 200 F g-1, 44.63 Wh kg-1 and 3 235 W kg-1, respectively with excellent capacitance retention. While the MoS2 encapsulated and Mn2O3 decorated fibre composite, OLC/MoS2@Mn2O3 displayed a specific capacitance, energy and power density of 348 Fg-1 18.42 Wh kg-1 and 5 095 W kg-1, respectively. It is pertinent to note that the capacitance of the electrodes was retained throughout the 5 000 cycles of the charge-discharge test. Upon thermal treatment at 600 °C, FePc-PAN transformed into FeN4-CMF and exhibited a specific capacitance, energy and power density of 147 F g-1, 12.48 Wh kg-1 and 4 320 W kg-1, respectively. The vacancy-rich (FeN4)d-OLC- CNF@MoS2 composite obtained by the removal of Fe2+ atoms, showed a specific capacitance, energy density and power density of 481 F g-1, 76 Wh kg-1 5833 W kg-1, respectively. This study underscores strategic processes that can be adapted in the design, synthesis and optimization of supercapacitors-based electrodes for enhanced performance.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 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 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.