Faculty of Science (ETDs)
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Item The Taxonomy and Phylogeny of Varanopidae from the Middle Permian of outh Africa(University of the Witwatersrand, Johannesburg, 2024-08) Sibiya, Zoleka; Rubidge, Bruce; Benoit, JulienVaranopidae are the only pelycosaur-grade tetrapods from the Abrahamskraal Formation of the Beaufort Group. Although four varanopid species are described from the middle Permian of South Africa (SA), their validity has been questionable. Moreover, Ford and Benson recently proposed that varanopids belong to the Diapsida rather than Synapsida. Given this taxonomic and phylogenetic turmoil, this study i) describes two newly discovered and well-preserved varanopid specimens from the middle Permian of SA using CT scanning and ii) re-assesses the validity of the four species from the Karoo Basin in the light of new data from these specimens. The phylogeny of the group is addressed by updating two pre-existing cladistics matrices. The two new specimens exhibit a mosaic of features that overlap the diagnoses of several species of SA varanopids. In addition, some diagnostic traits of the four SA varanopid genera and species are found to be variable across ontogeny. For instance, dorsal osteoderms and the extent of ornamentation on the angular bone co-vary with size. It is therefore proposed that all varanopid specimens currently described from the middle Permian of South Africa belong to a single valid species, Heleosaurus scholtzi, with differences between specimens being accounted for by ontogenetic changes. The phylogenetic analysis supports Ford and Benson’s hypothesis but contra the prediction of this hypothesis, no differences in cranial fenestration during ontogeny are observed. Morphological characters preserved in the manus of the newly discovered specimen BP/1/8499 suggest that Heleosaurus was arboreal.Item Investigating the impact of the land reform policy on land use and land cover changes, in Ngaka Modiri Molema district of the North West province(University of the Witwatersrand, Johannesburg, 2024) Mmangoedi, Molebogeng Precious; Adam, ElhadiThe purpose of this study was to assess how land reform policies affected changes in land use and land cover in the province of North West's Ngaka Modiri Molema district municipality. The study employed remote sensing technologies to analyse changes in land use and land cover (LULC) resulting from the implementation of land reform programs between 1985 and 2015. The primary objective of the research was to systematically map Land Use and Land Cover types across five-year intervals spanning from 1985 to 2024, leveraging Landsat earth observation data in conjunction with a random forest classifier. These methodologies were employed to facilitate the identification of spatial patterns and trends associated with the implementation of land reform policies within the study area. Furthermore, the study utilized Landsat data and advanced change detection algorithms to quantitatively assess LULC changes over the specified timeframes. Through the application of spatial analysis techniques, the research aimed to elucidate the relationship between the implementation of land reform measures and corresponding shifts in LULC patterns across the research study area. The findings of the investigation indicated a noticeable expansion in built-up areas between the years 1985 and 2024 which was approximately 10.86%. This expansion was primarily attributed to the growth experienced by the municipality during this period. Additionally, more opportunities might have risen from the agricultural farming activities and also from the land reform policy being implemented. However, as the ownership changed due to land redistribution and more land was being acquired by black people through the land reform policy, agricultural farming decreased slightly throughout the years. The reduction was due to the factors that arose from inefficient policy implementation. The study also recommends that remote sensing techniques should be utilised to carry out studies to determine LULC changes that derive from land policies aiming at dealing with socio-economic factors and urbanisation. An incorporated agrarian reform sustainable programme has vast potential in cultivating the production of the projects, particularly if it involves packages in rural infrastructure, support services, and co-operatives. The major role of such an approach should be in the trainings conducted for the farmers, obtaining, and distributing agricultural resources and equipment to agrarian reform or beneficiaries of the land reform projects. Additionally, there should be an allowance for special grants which will be useful in supporting the government’s efforts.Item Spatiotemporal characteristics of surface water in Sua Pan, Botswana, using Earth Observation data: 1992–2022(University of the Witwatersrand, Johannesburg, 2024-10) Peplouw, Muchelene Tiara; Adam, Elhadi; Grab, StefanSurface water is a critical resource for sustaining both human and ecological health. However, climate change and human actions threaten its availability in semi-arid regions like Botswana. In addition, current research on monitoring and understanding surface water dynamics in Botswana lacks the application of remote sensing and machine learning. This highlights a crucial gap in knowledge that this study aims to address. This study investigates the spatiotemporal dynamics of land use/land cover (LULC) and surface water extent changes in Sua Pan, Botswana, from 1992 to 2022. Employing remote sensing, machine learning, and statistical techniques, the research offers valuable insights into the intricate relationships between land cover modifications, surface water variations, and climatic variables. Google Earth Engine (GEE) facilitated efficient analysis of Landsat imagery for LULC mapping. Random Forest (RF) effectively classified several land cover types within Sua Pan. To address the challenges of saline environments, a novel water index, the Saline Water Index (SWI), was developed specifically for Sua Pan. The McNemar statistical test compared the performance of SWI to established indices like the Modified Normalised Difference Water Index (MNDWI) and the Normalised Difference Salinity Index (NDSI). Surface water variations were analysed using homogeneity tests and the Mann-Kendall trend test. The relationships between hydro climatic data (rainfall, evapotranspiration, land surface temperature) retrieved from GEE and surface water area for both wet and dry seasons were evaluated using Pearson correlation coefficients and visualised by line and area graphs. Additionally, the influence of the El Niño Southern Oscillation (ENSO) on rainfall and surface water area was assessed using Analysis of Variance (ANOVA) to identify the specific ENSO phases that exert an influence. The findings demonstrate the effectiveness of GEE for LULC mapping with the RF algorithm, achieving moderate to high classification accuracy (65.2% - 90.69%) and Kappa coefficients (0.54 - 0.85). Surface water and bare area exhibited increasing trends (coefficients: 13.017 and 9.0609, respectively), whereas vegetation and salt hard pan showed decreasing trends (-16.786 and -5.3081, respectively). The newly developed SWI outperformed MNDWI and NDSI in detecting surface water, achieving the highest overall accuracy (94%) compared to MNDWI (64%) and NDSI (59%). The McNemar test confirmed no significant statistical difference between the SWI map and the validation dataset (p = 0.2673), while both MNDWI and NDSI maps showed significant differences (p < 0.0001). Utilising SWI, the study revealed that surface water was most prevalent in central and northeastern regions, with an average coverage of 33%. Seasonal homogeneity tests indicated a non-homogenous distribution of surface water area in wet seasons, with abrupt changes in 1994 and 2003. Conversely, dry seasons exhibited a homogenous distribution. The Mann-Kendall trend test identified a statistically significant (p-value = 0.01) but weak positive trend (tau = 0.329) for surface water areas in wet seasons. In contrast, the dry seasons displayed a non-significant (p-value = 0.734) and a very weak positive trend (tau = 0.043). Surface water area, rainfall, evapotranspiration, and temperature consistently increase during the wet seasons compared to the dry seasons. Notably, increased evapotranspiration significantly impacted surface water presence. ENSO exhibited no significant influence on either rainfall or surface water extent (p-value > 0.05 for both). These findings highlight the potential of earth observation data for real-time surface water monitoring in salt pans. The developed techniques offer valuable insights for policy decisions regarding environmental management and conservation efforts in Sua Pan. In addition, the study emphasises the importance of cost-effective approaches for water change assessment, particularly appropriate for under-resourced regions.Item A systematic study on the use of the sol-gel synthetic method for lithium manganese oxide-based cathode materials(University of the Witwatersrand, Johannesburg, 2024-09) Muntswu, Zwivhuya; Billing, Caren; Ferg, Ernst E.; Billing, David G.This dissertation investigated the synthesis of two lithium manganese oxide-based cathode materials (Li1.03Mn1.97O4 and LiAl0.4Mn1.6O4) using the sol-gel method and probing the phase transitions during the synthesis. The sol-gel synthetic method involved dissolving stoichiometric amounts of lithium nitrate, manganese nitrate hydrate, and citric acid in distilled water forming an aqueous solution. The starting precursor materials were dried at 140 °C which formed a crystalline phase of -Aqua-S-citrato (2-)-manganese(II) with an orthorhombic crystal system and P222 space group. The thermal behaviour of the precursor was explored to understand the effects of calcination/annealing temperatures. Thermal analysis of precursors prepared using nitrate salts with a 1:1 total metal ion to citric acid ratio displayed thermal stability to temperatures higher than 380 °C with the formation of a final metal oxide after 70% mass loss due to the decomposition of the organic and nitrate materials. However, when increasing the concentration of the complexing agent, an increase in material decomposition due to an increase in organic material is seen. The precursor materials prepared with a lower complexing agent concentration result in materials that have thermal instability when exposed to high temperatures. Thermal analysis of Li1.03Mn1.97O4 and LiAl0.4Mn1.6O4 prepared using acetate salts as starting materials shows material decomposition at high temperature of ~600 °C Calcining both undoped and Al-doped nitrate precursors at moderate temperatures (380 °C to 500 °C) resulted in the formation of Li1.03Mn1.97O4 and LiAl0.4Mn1.6O4 with a pure cubic spinel structure and an Fd-3m space group, however, increasing the calcining temperature to 800 °C for the undoped nitrate-based precursor revealed an impurity phase formation relating to dilithium manganese oxide with a monoclinic crystal system. On the other hand, calcining acetate-based precursors at moderate temperatures (380 °C to 500°C) results in metal oxides with low crystallinity compared to metal oxides prepared with nitrate-based precursors. Calcining acetate-based precursors at 800 °C was more favourable since they form the desired metal oxides without any impurities which might imply structural phase stability at high temperatures. The local and average crystallographic structures (via PDF and XRD respectively) of various nitrate-based metal oxides were investigated, where a good agreement between collected data and a calculated structural model revealed the formation of a cubic spinel structure of space group Fd-3m. Li1.03Mn1.97O4 and LiAl0.4Mn1.6O4 metal oxides were achieved from calcining precursors at moderate temperatures of 380 °C and 450 °C. The PDF high r-value signal displays a good fit which confirms to the average structure data information where the r-value signal which correspond to the local structure refinements have a minor discrepancy when fitted with a cubic spinel of space group Fd-3m.Item Diversity and Abundance of Arthropods on Conventional Sugarcane under Field Conditions in South Africa(University of the Witwatersrand, Johannesburg, 2024-09) Smith, Roshay; Malinga, Lawrence; Bouwer, GustavInsect diversity and abundance are often the base for formulating strategies that involve the appropriate application of pest control methods, considering the ecosystem services provided by insects. Therefore, the aim of this study was to provide recent baseline data on the diversity and abundance of insects in conventional sugarcane based on two sugarcane fields in KwaZulu-Natal. Three sampling methods, namely pitfall, sticky and water pan traps, were used to sample insects in rain-fed and irrigated sugarcane in Gingindlovu and Pongola from March to October 2022. This study collected 12 493 insects belonging to 14 insect orders and 88 families in rain-fed sugarcane and 22 309 insects belonging to 14 orders and 94 families in irrigated sugarcane. Significant differences in the diversity indices were found between the sampling methods and the sampling periods. This study provides recent baseline data on the diversity and abundance of insects in sugarcane.Item Double-diffusive convection in rotating fluids under gravity modulation(University of the Witwatersrand, Johannesburg, 2024-09) Mathunyane, Alfred Ntobeng; Duba, C. Thama; Mason, D.P.This study employs the method of normal modes and linear stability analysis to investigate double-diffusive convection in a horizontally layered, rotating fluid, specifically focusing on its application to oceanic dynamics. Double diffusive convection arises when opposing gradients of salinity and temperature interact within a fluid, a phenomenon known as thermohaline convection, and it is crucial for the understanding of ocean circulation and its role in climate change. With the increasing mass of water due to glaciers melting, fluid pressure variations occur, leading to slight fluctuations in gravity. We conduct both stationary and oscillatory stability analyses to determine the onset of double-diffusive convection under gravity modulation. Our analysis reveals that time-dependent periodic modulation of gravitational fields can stabilize or destabilize thermohaline convection for both stationary and oscillatory convection, with amplitude stabilizing and frequency destabilizing. The wavenumber in the y- direction also affects convection in the equatorial regions. This wavenumber exhibits destabilizing effects for large values and stabilizing effects for small values for both stationary and oscillatory convection. Rotation along with gravity modulation tends to destabilize the system for both stationary and oscillatory convection. The key difference between stationary and oscillatory convection is that oscillatory convection exhibits large values of the Rayleigh number, thus susceptible to overstability while stationary convection tends to have relatively smaller Rayleigh numbers and thus more stable. This research provides insights into the complex interplay between gravity modulation and thermohaline convection, contributing to our understanding of ocean dynamics and their implications for climate change.Item Palaeoclimate Reconstruction Using Charcoal from a Mid-Holocene Stratum 4b, Wonderwerk Cave, South Africa(University of the Witwatersrand, Johannesburg, 2024-06-18) Hlophe, Busisiwe; Bamford, MarionCaves are often sources for palaeoenvironmental information in the dry interior regions of southern Africa. The Wonderwerk Cave, located in Northern Cape, is a national heritage site with a well-preserved, nearly complete record of the local Holocene LSA techno-complexes and past environmental indicators. Hence, it has the potential to provide valuable insights into past human behaviour as well as the local palaeoclimate and vegetation. This study examined wood charcoal from stratum 4b, Excavation 1 of the Wonderwerk Cave. The aim was to evaluate the taxonomic composition and the physiognomy of the charcoal to understand the environment around the cave 6.9-5.9 ka. The study also investigated the modern uses of the vegetation to infer how humans may have utilized the woody plants around them during that period. The results indicate the landscape cover included woody plants adapted to dry conditions and summer rainfall. However, the archaeological taxa identified also included two species that thrive in moist environments, Halleria lucida and Olinia ventosa, suggesting fluctuations in climate. The taxonomic composition also suggests an environment similar to the modern bushveld found near the cave. The low conductivity capacity as well as vulnerability and mesomorphy indices of the woods support the implication that the vegetation was adapted to low water availability or aridity. The cave's inhabitants likely used the identified plants for firewood, magical and medicinal purposes, and more between 6.9 and 5.9 ka.Item Synthesis and electrochemical properties of high-entropy spinel oxides, cobalt atomic clusters and zinc oxide as electrode materials for rechargeable zinc-air batteries(University of the Witwatersrand, Johannesburg, 2024-07) Gaolatlhe, Lesego; Ozoemena, Kenneth IkechukwuThis thesis investigated cathode and anode electrode materials for application in rechargeable zinc-air battery (RZAB). Two types of cathode materials were strategically studied in RZAB applications: (a) cobalt carbon composites of (i) cobalt atomic clusters (Co AC@CBPDC) and (ii) cobalt nanoparticles (Co NP@CBPDC), and (b) high-entropy spinel oxide (HESOx, containing five transition metals – Cu, Mn, Fe, Ni, and Co). The activities of these materials toward oxygen reduction reaction (ORR and oxygen evolution reaction (OER) were investigated in both half- and full-cell configurations as a proof-of-concept in RZAB cells in alkaline electrolyte. Considering that conventional zinc plate has several short-comings as an anode for RZAB, a new material, polydopamine-derived carbon-coated zinc oxide (ZnO@PDA-DC), was also synthesised and applied in RZAB as a possible alternative anode to the popular zinc plate. First, Co AC@CBPDC and Co NP@CBPDC were prepared using the metal-organic framework (MOF) route through the microwave-assisted solvothermal method and acid treatment. From the XRD results, the spectra showed dominant {111} and {200} phases, characteristic of metallic cobalt with a face-centred cubic (fcc). There were trace amounts of CoO observed indicating the coexistence of Co/CoO. From TEM imaging, Co AC@CBPDC was highly defective with a visible porous carbon structure than its counterpart (Co NP@CBPDC) and showed dispersed atomic clusters. BET data showed that Co AC@CBPDC had a higher surface area (144.8 m2/g) than the Co NP@CBPDC (33.25 m2/g). The improved physicochemical merits of the Co AC@CBPDC allowed for better ORR and OER activities than the Co NP@CBPDC in terms of low halfway potential (E1/2), onset potential (Eonset), overpotential at 10 mA/cm2 (ƞ10), potential gap (∆E) between the overpotential of OER and the halfway potential, and a higher kinetic current density (jk). The enhanced electrochemistry of the Co AC@CBPDC was attributed to the defects created by the acid treatment. As proof of real-life applicability, the Co AC@CBPDC electrocatalyst delivered an excellent air cathode in a parallel plate RZAB cell with notable OCV (1.23 V), peak power density (49.9 mW/cm2), a real energy density (477 mAh/cm2), long-term stability for 210 h, enhanced voltage retention, Coulombic efficiency (ca. 100 %) and DOD (51.3%), comparable to literature. In addition, an all-solid-state RZAB based on the Co AC@CBPDC catalyst gave a higher and constant OCV (1.73 V) at varied bending angles (0 – 180 degrees) and excellent stability. Second, new HESOx materials were prepared via the Pechini method at two different annealing temperatures of 500 and 750 oC (abbreviated herein as HESOx-500 and HESOx-750). P-XRD results showed that these are inverse spinel oxides, with {311} as the dominant phase. HR-TEM images proved that they are single nanocrystalline materials. XRD and BET data showed that the HESOx-500 is smaller in size, more porous, and has a higher surface area than its counterpart (HESOx-750). HESOx-500 showed superior ORR performance with an onset potential of 0.93 V and a E1/2 of 0.88 mV. The OER performance also showed improved ƞ10 compared to IrO2 with an overpotential of 340 mV at a current density of 10 mA/cm2, and a 45 ± 5.0 mV/dec Tafel slope, above the performance of IrO2 (66 ± 6.1 V/dec). The ∆E of HESOx-500 was 0.69 V. The material was further tested as a cathode material in a RZAB cell. The optimised RZAB cell showed remarkable performance with a theoretical potential of 1.67 V and long-term stability of 375 h at 10 mA/cm2. The performance was attributed to the high-entropy compositional design with a high number of surface oxygen vacancies and different metal oxidation states. Finally, having dealt with the issue of bifunctionality in RZAB, a new ZnO@C anode material was also considered. The ZnO@PDA-DC (where PDA-DC means polydopamine-derived carbon) was used due to its ability to form Zn2+ pathways. Electrochemical potentiodynamic polarisation tests were performed to understand and compare the corrosion inhibition effects in an alkaline medium (6 M KOH). The ZnO@PDA-DC showed better corrosion inhibition properties than the zinc plate and other samples: low corrosion current (icorr = 0.107 uA/cm2) and corrosion potential (Ecorr = 1.077 V), and a mixed inhibition effect, indicating reduced hydrogen evolution reaction and zinc dissolution. Due to the excellent corrosion inhibition properties of the ZnO@PDA-DC, it was then evaluated in the RZAB cell. The shallow galvanostatic charge-discharge cycle stability at 2 mA/cm2 was able to maintain 150 h in a RZAB at a voltage gap of 0.76 V to 0.80 V. The results demonstrated that enhanced rechargeability is possible with ZnO@PDA-DC for RZAB.Item Synthesis of platinum-based electrocatalysts using nitrogen doped onion-like carbon and WS2 composites as the support for electrooxidation of ethanol in direct alcohol fuel cells(University of the Witwatersrand, Johannesburg, 2024-10) Bila, Laercia Rose; Gqoba, Siziwe; Maubane-Nkadimeng, Manoko S.The study reports on the synthesis of onion-like carbons (OLCs)/tungsten disulfide (WS2) composites as catalyst support for direct alcohol fuel cells (DAFC). OLCs were synthesized using waste engine oil over a flame pyrolysis (FP) method. The pristine OLCs (p-OLCs) were functionalized and purified using nitric acid (HNO3). The functionalized OLCs (F-OLCs) were further doped with nitrogen using melamine to increase the electronic properties of the OLCs. WS2 was synthesized using the colloidal method and oleylamine was used as the capping agent. Pt/p-OLCs, Pt/F-OLCs, and Pt/N-OLCs were synthesized using a reflux method where ethylene glycol was the reducing agent. Finally, WS2/N-OLCs were synthesized using the colloidal method and then Pt was dispersed on WS2/N-OLCs to form Pt/WS2/N-OLCs. High-resolution transmission electron microscopy showed the presence of onion-like rings in the OLCs and the quasi-spherical morphology, while a flower-like morphology was observed for WS2. Powder X-ray diffraction revealed that the synthesized WS2 had traces of WO3 due to the oxidation of WS2 which introduces WO3 impurities. Energy Dispersive X-ray Spectroscopy revealed that the OLCs derived from waste engine oil present some impurities that were attributed to the motor wear as well as the fuel. When Pt was loaded onto the WS2/N OLCs composite, the WS2 lost its original nanoflower morphology, which was attributed to the presence ethylene glycol used as a reducing agent. X-ray photon spectroscopy confirmed the successful synthesis of the Pt electrocatalysts. Cyclic voltammetry was used to determine the oxidation of ethanol and the current density of the synthesized electrocatalysts. Interestingly, the Pt/p-OLCs electrocatalyst had a higher current density compared to Pt/F-OLCs and Pt/N-OLCs. This was attributed to metal impurities found in p-OLCs, which were reduced during the purification process. The Pt/WS2/N-OLCs electrocatalyst showed higher current density compared to Pt/WS2 but this was low compared to Pt/N-OLCs. The data reveals that the addition WS2 shows a co-catalyst behaviour, rather than a support.Item Optically stimulated luminescence dating of Kalundu and Urewe tradition ceramics(University of the Witwatersrand, Johannesburg, 2024-03) Haupt, Rachel Xenia; Schoeman, Maria; Evans, MaryOptically stimulated luminescence (OSL) dating is a method of providing the direct age of artefacts. While radiocarbon and seriation dating provide indispensable insight into archaeological sites, the direct dating of artefacts is beneficial in entangled contexts. The Lydenburg Heads Site is significant to the beginning of the Early Farming Communities (EFCs) sequence within the Mpumalanga province. The site has been occupied multiple times, as can be seen from the presence of the two major ceramic traditions of the age, Urewe and Kalundu. The site was originally excavated and analysed by Evers (1982) in the 1970s, with a reanalysis of the ceramic assemblage by Whitelaw (1996) and organic residue analysis on the ceramics by Becher (2021). The use of OSL dating on twelve ceramic sherds allowed for new insights into the chronological intricacies within the study site. To determine the age of the ceramics, the OSL quartz dating technique was used. The adjustments to the technique involved the use of a less destructive means of sample extraction. A slightly altered version of the standard means of sample extraction was used to create a comparison and allow the dating of the ceramics to be reliable. The minimal destruction technique (MET) combined with the bulk sampling proved useful to the dating of the ceramics. The use of previously excavated ceramics meant that some aspects of age determination required estimation and analysis. The major obstacles from such were the water content, the depth of burial, and the lack of in situ soil samples. In light of the elements of ambiguity for the site, the OSL dating considered these variations and how they affected the age. The Urewe tradition ceramics were determined to be in 6th and 8th century AD. The finding creates the alignment with the range of the radiocarbon ages done within previous work and the assumptions made by Evers (1982) and Whitelaw (1996). The Kalundu tradition ceramics ages were determined to be between the 7th and 10th century AD, conflicting with previous assumptions on the occupation. The result is the possibility the ceramic assemblages could be considered to be contemporaneous. The work in this thesis has, in part, been presented at the Luminescence and Electron Spin Resonance Dating conference in Copenhagen (LED2023) and at the Association of Southern African Professional Archaeologists 2024 Biennial Meeting (ASAPA 2024).