Electronic Theses and Dissertations (PhDs)
Permanent URI for this collectionhttps://hdl.handle.net/10539/38002
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Item Studies on the chemistry and biochemistry of gold(III) carboxamide pincer chelates(University of the Witwatersrand, Johannesburg, 2024-06) Razuwika, Rufaro; Nowakowska, Monika; Mathura, adhnaCancer, a group of diseases characterised by the uncontrollable growth of abnormal or mutated cells within an organ, is a global concern. Metallodrugs have emerged as promising solutions to this pandemic, leading to intense research on different metal complexes. In this study, gold(III) carboxamide pincer complexes were evaluated as potential chemotherapeutic agents. The novel NNN-type carboxamide pincer molecules (ligands) effectively stabilising the gold(III) metal centre. The strong σ-donor properties of both the anionic and pyridine N groups further enhanced this stability. Ligands 1a-1f exhibited atropisomerism, a common feature in drug discovery, and containing special heterocycles such as quinolones, indazole, benzophenone, and phenanthroline, which are particularly relevant in drug development. Atropisomerism, however, was lost upon metalation of the ligands. Three complexes, 2d, 2e, and 2f, were successfully synthesised and isolated. Complex 2d was subjected to biochemical property testing and in vitro analysis due to its superior stability and solubility compared to 2e (poor stability) and 2f (poor solubility in the buffer solution used in the study). Speciation studies, combined with computational studies, suggested that 2d exists as a neutral complex under physiological conditions. This inert complex demonstrated stability against the reducing agent glutathione, indicating resilience to reduction under physiological conditions. DNA spectroscopic titration studies revealed that 2d exhibited intensive interaction with ct-DNA, with binding constants Ka1 = 1.48 x109 M-1 and Ka2 = 6.59 x105 M-1. This interaction resulted in a notable increase in the DNA melting point by 4 °C and an enhancement in viscosity in a dose-responsive manner. The DNA titrations, melting point, and viscosity studies suggested a dual binding mode of 2d to ct-DNA, involving base binding with a nearly equal preference for A, T, G, and C bases, and groove binding. Complex 2d exhibited a high affinity towards the transport protein HSA (Ka values were 1.57 x104 M-1), suggesting that it can be transported in the body by means of the HSA-mediated pathway, enhancing its efficacy and stability. In comparison to its affinity towards DNA, there is a significant difference allowing for the successful transfer of 2d from HSA to DNA. The poor solubility of complex 2d in aqueous environments may have hindered its cellular uptake, but binding to HSA could mitigate this, ensuring minimal interference with its cytotoxicity towards different cancer cell lines. MTT studies demonstrated that 2d has comparable cytotoxicity towards the breast cancer cell line MCF-7 with an IC50 of 9 µM. The IC50 for HT-29 was, however, too high to measure accurately (>100 µM). In conclusion, complex 2d exhibits promising anticancer properties based on its DNA binding studies and cytotoxicity evaluations. This suggests that this class of compounds can be applied in cancer treatments, with potential modifications to compounds 2e and 2f to improve their solubility and stability.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 Structural Characterization of Bimetal-Phosphate Based Solid-State Electrolytes: A PXRD, PDF and XAS Study(University of the Witwatersrand, Johannesburg, 2024) Nkala, Gugulethu Charmaine; Billing, David G.; Billing, Caren; Vila, Fernando D.; Forbes, Roy P.In this work, NASICON-type lithium titanium phosphate (LiTi2(PO4)3, LTP) was synthesized following the conventional solid-state reaction methodology. Single and double-doped formulations of LTP were made, with the primary objective of improving the room-temperature ionic conductivity, for their application as potential solid-state electrolytes for all-solid-state Li ion batteries. The primary characterization technique applied was ambient-temperature powder X-ray diffraction (PXRD) at both laboratory and synchrotron experimental conditions. The Rietveld refinement approach was used to determine the qualitative and quantitative phase compositions of each sample, revealing the rhombohedral (R-3c, space group #167) main phase, with phosphate-based secondary phases. Total scattering data, through the pair distribution function (PDF) was applied, revealing lattice site preference during the substitution of Ti with Al, Sn and Dy at the 12c site. Further analysis through small-box modelling indicated the local structure deviation below 10 Å, from rhombohedral (R-3c) to monoclinic (P21/n, space group #14). The application of experimental X-ray absorption spectroscopy (XAS) revealed a stable 4+ oxidation state for Ti regardless of doping. However, the extended X-ray absorption fine structure (EXAFS) data showed that the replacement of Ti with Sn results in heavy disorder and subsequent changes in the PO4 tetrahedra, corroborating the findings from Raman spectroscopy. Theoretical XAS spectra were computed using FEFF, providing insights into the origins of experimentally observed XAS features from first-principles. Applying electrochemical impedance spectroscopy (EIS) to assess the ambient-temperature ionic conductivity, co-doped systems showed an improvement in the conductivity. The application of characterization techniques at various length scales has been demonstrated to provide insights into the mechanisms governing the performance of the solid-state electrolytes.