Electronic Theses and Dissertations (Masters)
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Item High entropy spinel oxides and iron-cobalt based electrocatalysts for rechargeable zinc-air batteries(University of the Witwatersrand, Johannesburg, 2024-08) Mongwe, Agnes Monosi; Ozoemena, Kenneth Ikechukwu; Haruna, Aderemi B.The development of effective and stable rechargeable zinc-air batteries (RZABs) using noble-metal free bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) has been a key challenge to its practical applications. This MSc research work strategically investigated some synthetic methods aimed at tuning the physicochemistry and electrochemical properties of two electrocatalysts (i.e., noble-metal free high entropy spinel oxide (HESOx) ((CoCuFeMnNi)3O4) and spinel Fe2CoO4 for rechargeable zinc-air batteries). For HESOx, a simple and reproducible Pechini method was used to synthesize a homogeneous nanosized electrocatalyst HESOx-550. The HESOx-550 was thereafter supported on onion-like carbon (OLC) in (1) an acidic environment to produce HESOx-550/OLCAT (where AT stands for acid-treated) and (2) a nonacidic environment to produce HESOx-550/OLC. The effects of the different synthesis environments on these three samples were thoroughly investigated using different analytical techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, thermogravimetric analysis (TGA) and Nitrogen Gas Adsorption analysis. The Nitrogen Gas Adsorption analysis results show that the HESOx-550/OLCAT has the largest surface area and more volume. The electron paramagnetic resonance (EPR) and O1s XPS data consistently proved that HESOx-550/OLCAT has improved oxygen vacancies which are essential in improving conductivity and offering abundant reaction sites. The HESOx-550/OLCAT shows the best bifunctional ORR and OER electrocatalytic performance with a bifunctionality index (ΔE) of 0.70 V in 1 M KOH. In addition, the RZAB air electrode with HESOx 550/OLCAT exhibits high areal capacity (60 mAh cm-2) and areal energy density (73.2 mWh cm-2) with a long-term cycle stability over 112 h in 6.0 M KOH and 0.2 M zinc acetate. The HESOx-550/OLCAT RZAB shows better electrochemical performance than 10wt.% Pt/C- IrO2 when cycled over 315 h under 27% depth of discharge condition. For Fe2CoO4, iron cobalt-based electrocatalysts on Vulcan carbon support were synthesized using a simple reduction method to produce two composites (FeCo-Fe2CoO4/CAnnealed), and (FeCo Fe2CoO4/CMicrowave). The physicochemical analytical methods such as XRD, XPS, Raman, TGA and Nitrogen Gas Adsorption analysis were used to investigate the samples. The electrochemical analysis showed that the FeCo-Fe2CoO4/CAnn had a very low “bifunctionality index” (ΔE) of 0.76 V and the FeCo-Fe2CoO4/CAnn air cathode RZAB demonstrated good stability for over 50 h under harsh DOD conditions (35.2%). The assembled RZABs have areal energy densities of 48.4 mWhcm-2 and 60.5 mWhcm-2 which are higher than the minimum recommended areal energy density of 35 mWhcm-2 (and better than most electrocatalysts reported in the literature). This study has significant contributions to the progress of practical applications of RZABs.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 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 Biocatalytic oxidative conversion of valencene to nootkatone mediated by lipoxygenase and cytochrome P450(University of the Witwatersrand, Johannesburg, 2024-03) Raboya, Christopher; Ngwira, Kennedy; Brady, DeanNootkatone (NK) is an oxygen-containing sesquiterpene with a significant grapefruit aroma and plays an important role in the flavour and fragrance industry. The natural production of NK through extraction produces trace amounts and is therefore not a viable option to meet industrial needs. The chemical synthesis often utilises reagents harmful to the environment. The purpose of this research was to explore the use of crude lipoxygenase (LOX) enzyme extracted from soya beans, commercial mutants of CYP450 as well as laccase enzymes for the conversion of valencene (VL), the aroma components of citrus fruits to NK. For the LOX reactions, a conversion of 28.79% (mol/mol) was obtained when the reactions were performed with LOX only. The best conversion of 74.46 % was realised when FeSO4.7H2O and MnSO4 were added to the reaction. In the temperature studies, 70 ˚C was shown to be the optimal temperature for the conversion. In addition, we observed that vegetable oils provided sufficient unsaturated fatty acids to facilitate the conversion of VL to NK with sunflower oil being the best. In exploring the potential of LOX to oxidise other organic molecules, caryophyllene was oxidised to novel caryophyllene oxide, and styrene was oxidised to benzoic acid, 1-phenylethane-1,2-diol, and 2-hydroxyl-2-phenylethyl benzoate. This is the first time that such oxidations are reported, and this underlines the potential of LOX in biotransformation and organic synthesis. For CYP450 reactions, the best conversion of 16.70 % was obtained using a variant sourced from Prozomix. Evaluation of parameters such as temperature, pH (7.0 to 7.5), using buffer solutions should be explored to optimise the activity of the enzyme. Laccase from Novoprime Base 268 showed no activity for the conversion of VL to NK despite making use of mediators. Therefore, we should explore changing the reaction conditions, varying the pH of the reaction, buffer strength or mediator and laccases from other sources such as Cerrena unicolor and Trametes versicolor, as well as a fresh batch of laccase from Trichoderma (Merck) should be assessed in the conversion of VL to NK.Item Silver nanoparticle-modified cellulose nanocrystals for fouling control in membrane distillation(University of the Witwatersrand, Johannesburg, 2023-06) Mpala, Josephine Tshepiso; Nthunya, Lebea; Richards, Heidi; Etale, AnitaA global reduction in water resources and the growing demand for fresh water has motivated the quest for the development of sustainable water-augmenting technologies. Membrane distillation (MD) is envisaged as an attractive desalination technology, surpassing cost challenges faced by conventional desalination technologies. Yet, its industrial commercialization faces multiple limitations, including the production of low water fluxes, membrane wetting and membrane fouling. This study sought to investigate the performance of silver nanoparticles (AgNPs) embedded on cellulose nanocrystals (CNCs) (CNC-capped AgNPs) to lessen the impact of biofouling in MD. This was conducted through coating the polyvinylidene fluoride (PVDF) membrane with CNCcapped AgNPs. Prior to coating with CNC-capped AgNPs, PVDF membrane properties were improved (for MD suitability) through blending with polyvinylpyrrolidone (PVP) and functionalized carbon nanotubes (f-CNTs). The resulting membrane had an improved overall porosity, and a respective increase in surface roughness (75%) and mechanical strength (45%). Pristine CNC-capped AgNPs’ characterization presented stable AgNPs with minimal leaching. Transmission electron microscopy (TEM) micrographs revealed a uniform dispersion of spherically shaped AgNPs exhibiting 13.3 ± 3.4 nm average diameter. The presence of AgNPs on the surface of CNCs afforded excellent thermal stability and good anti-microbial activity, mainly against E. coli, P. aeruginosa, S. aureus, S. epidermis, and S. saprophyticus. Following membrane modification, preliminary anti-bacterial tests conducted on the CNC/AgNP-modified PVDF membrane revealed a 98.7%, 52.3%, 78.0%, 53.9% and 93.3% reduction of E. coli, P. aeruginosa, S. aureus, S. epidermis, and S. saprophyticus cells, respectively, demonstrating its ability to control biofouling. Although the CNC/AgNP-modified PVDF membrane exhibited improved membrane properties, such as high surface roughness, high liquid entry pressure (LEP), and good hydrophobicity, its performance in MD (with artificial seawater as the feed stream) was poor, producing the lowest average water flux (0.179 ± 0.0303 kg/m2 /hr) compared to the unmodified PVDF membrane (0.528 ± 0.0838kg/m2 /hr), mainly due to pore blockage. However, upon spiking the artificial seawater with a monoculture of G. Stearothermophilus, the CNC/AgNP-modified PVDF membrane displayed the most stable water flux while the unmodified PVDF membrane’s water flux decreased by 79.3% over the 24-hour (h) period. This was attributed to the formation of a biofouling layer on the PVDF membrane which was absent on the CNC/AgNP-modified PVDF membrane. The AgNPs on the surface of the membrane afforded minimal bacterial deposition during operation. These results ascertain the possibility of biofouling minimization in MD using CNC-capped AgNPs, contributing to MD’s body of work for its ultimate realization for up-scaling.Item Carbon nitride-based catalysts for thermal carbon monoxide oxidation: Does phase matter?(University of the Witwatersrand, Johannesburg, 2023-06) Mohamed, Ahmed Gamal Abdelmoneim; Ozoemena, Kenneth Ikechukwu; Abdullah, Aboubakr M.; Eid, KamelCarbon monoxide (CO) has a poisonous effect on all living organisms as it binds to the hemoglobin of blood cells, preventing oxygen uptake. Thus, the conversion of CO to less dangerous gas such as CO2 is an essential process. This work presented the utilization of carbon nitrides (C3Nx) in different phases (βgC3N4, βC3N5, βC3N6) for thermal carbon monoxide (CO) oxidation. Herein, gC3N4, C3N5, and C3N6 were prepared by pyrolysis of their amine precursors, which were doped with Fe by two distinct methods; mechanical mixing (Fe/C3Nx-M) and polymerization (Fe/C3Nx-P). The controlled preparation of Fe/gC3N4-P allowed the formation of hierarchical porous structures with high surface area (219 m2/g) compared to the Fe/gC3N4-M (77 m2/g). This enabled the ease of reactants diffusion, enhanced the electron transfer, and maximized the atomic utilization. Accordingly, Fe/gC3N4-P (T100= 245 °C) presented higher catalytic activity than Fe/gC3N4-M (T100= 291 °C). In addition, bimetallic FeTi/gC3N4-P and trimetallic FeTiCu/gC3N4-P catalysts achieved the complete conversion of carbon monoxide (CO) at lower temperatures; 175 and 147 °C, respectively, which was attributed to the enhanced reducibility, and synergistic effect of Ti and Cu. Besides, FeTi/gC3N4-P and FeTiCu/gC3N4-P showed higher catalytic activity than Pd/C commercial catalyst (T100= 198 °C). In addition, the trimetallic FeTiCu/gC3N4-P showed a stable catalytic behavior without any deactivation for more than ten hours. This study showed that the C3Nx phases worked successfully in the thermal catalytic CO oxidation. However, the gC3N4 phase is the most active one when doped with metal(s), as it offered higher crystallinity, graphitization, and thermal stability than C3N5 and C3N6. This study also paves the way for the utilization of gC3N4 as a support for different metals to be used efficiently in various thermal catalytic applications, not only CO Oxidation.Item Defect–engineered lithium titanate anode materials for lithium–ion batteries(University of the Witwatersrand, Johannesburg, 2023-10) Podile, Seromo; Haruna, Aderemi Bashiru; Ozoemena, Kenneth IkechukwuEnergy is one of the aspects that plays a central role in moving society forward since it is one of the most important agenda of global economic and energy forums. There is an urgent need to move to clean energy given the environmental and the health benefits resulting from implementing energy systems that utilize green energy. The proposed energy sources in these systems are primarily natural (e.g. wind and solar), which means they are beyond human control and would work better if coupled with energy storage devices (ESDs). From this emerge the importance of energy storage systems (ESSs) which mostly perform based on the materials utilized to assemble the devices. In this study, we seek to enhance the power and energy densities of two of the prominent energy storage systems, namely lithium-ion batteries (LIBs) and lithium-ion capacitors (LICs), using modified commercial lithium titanate (LTO) materials as anodes. The materials consist of the pristine LTO (LTO-p), LTO coated with cerium fluoride (LTO-CeF3) and dry and wet irradiated pristine LTO (LTO-p-md and LTO-p-mw) and LTO coated with cerium fluoride (LTO-CeF3-md and LTO-CeF3-mw). Microwave irradiation was used to study the possible defect that the radiation can bring to the materials and possibly use the microwave effect to improve their electrochemistry. The effects of the coated layer of CeF3 and microwave irradiation on the structure of the commercial LTO were extensively studied using powder X-ray diffractometer (XRD), Raman spectroscopy, transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and Brunauer-Emmett-Teller (BET) theory. Structural investigations of TEM micrographs revealed CeF3 was present on the surface of the coated LTO materials. Further analysis did show that some portion of the CeF3 coating layer was co-doped into the LTO nanostructures. The findings from XRD and XPS analyses showed that co doping promoted a mixed state of Ti3+ and Ti4+ resulting from charge compensation when Ce3+ and F possibly substituted Ti4+ and O2-. This mixed state of titanium ions allowed the materials to have high electric conductivity than the pristine LTO (LTO-p). The spectrographs obtained from XPS analysis also showed that LTO microwave irradiated materials without coating experienced the same mixed states, which may have originated from oxygen vacancies that allowed for charge compensation when some of the LTO-p Ti4+ had to turn to Ti3+. The HRTEM analysis demonstrated changes to the lattice planes spacing of the modified LTO materials and these changes supported the (111) lattice plane shifts observed in the XRD analysis. The electrochemistry of the lithium-ion battery was studied on all the materials using cycling voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques. From GCD technique, the results indicated that the modified materials had higher specific capacities than LTO-p at all rates under rate capability studies. All as prepared materials were stable with coulombic efficiency of almost 100% for 100 cycles using 1 C, with modified materials surpassing LTO-p specific capacity. Cycling at a high rate (5 C) for 1000 cycles, saw LTO-md, LTO-mw, LTO-CeF3 and LTO-CeF3-md having high-capacity degradation after 500 cycles. It was also observed that initial capacities were retained up to the 1000th cycle for LTO-CeF3-mw and LTO-p. With an aim of comparing LTO-p and LTO-CeF3 half cells, these two materials were further studied for lithium-ion capacitors (LICs). This choice was based on LTO-CeF3 having a better charge-discharge profile, high specific capacity and coulombic efficiency than LTO-p. The improvements resulted from the coating layer (leveraging the high ionic conductivity and chemically inert properties of CeF3) and the observed defects seen on the XPS studies created by the coating synthesis process. The LTO-CeF3 anode was paired with a carbon black cathode to construct a full LIC cell. The assembled cell produced a maximal energy density of 107.4 Wh kg 1 with an accompanying power density of 2000 W kg-1, and it yielded a power density of 10 000 W kg -1 with an energy density of 61.8 Wh kg -1.Item Reconstructing Late Quaternary environmental change on the southern Cape coast of South Africa: A 30,000-year geochemical record from Pearly Beach(University of the Witwatersrand, Johannesburg, 2023-11) Mey, Rachel Tess; Humphries, Marc; Quick, LynneThe southern Cape coast of South Africa has undergone complex and dynamic climate changes due to its position at the boundary of major atmospheric and oceanic systems. However, climate changes that occurred in the region during the Last Glacial Maximum (30,000 – 18,000 cal yr BP) and the deglaciation period that followed (18,000 – 11,000) are still not fully understood. Climatic change along the southern Cape coast is of archaeological, cultural and environmental significance given the ecological diversity and hominin fossil, richness of the region. Moreover, predictions of future climate change and water scarcity management issues in the southern Cape make it vital to understand and response to future climate scenarios. To address this gap, this study investigated the geochemical composition of a 5.42 m sediment record (KSV 1) extracted from a freshwater coastal wetland near Pearly Beach on the southern Cape coast. The wetland, situated in the winter rainfall zone, is influenced by the southern westerly storm track, the South Atlantic anticyclone, and Agulhas-Benguela interactions, making it an ideal location to investigate changes to large-scale atmospheric and oceanic circulation systems. Using a combination of inorganic elemental (X-ray fluorescence, XRF) and stable carbon isotope (δ13C) analysis, this study presents a 30,000 year record of palaeoenvironemental change from Pearly Beach and considers these in relation to broad-scale climatic perturbations that occurred in southern Africa over this period. A chronology for the core was derived from radiocarbon dating of 8 bulk sediment samples. Sedimentation rates over the record averaged 0.02 cm/yr, giving the XRF analyses which were conducted at 2 cm intervals, an average temporal resolution of ~ 110 years. Sediments deposited 30,000 – 15,000 cal yr BP consist primarily of quartz-rich (60 – 90% SiO2) silt and fine sand with relatively low quantities of clay material. An increase in the relative proportion of Al2O3 from ~15,000 cal yr BP, coupled with a rise in sedimentation rate, marks an increase in the accumulation of fine-grained material within the wetland that is associated with a rise in global sea levels following deglaciation. A distinct shift to higher δ13C values around this time points to a change in vegetation community, with an increase in contribution from C4 drought-tolerant plants. The presence of calcium carbonate in sediments provides a strong indicator for marine intrusions at site and a pronounced increase in marine influence is inferred from ~8,500 cal yr BP, corresponding with the stabilisation of sea levels near to present-day levels. Variations in elements typically associated with heavyminerals (titanium and zirconium) are used to infer changes in depositional energy at the site. Enrichments in Ti and Zr suggest a period of enhanced transport energy between 18,800 and 14,500 cal yr BP. This is coupled with an increase in the Si/Al ratio and linked to enhanced aeolian activity. An increase in wind intensity during this period is attributed to the intensification in the Southern Hemisphere westerly winds at this time. This corresponds with available fossil pollen records, which suggest an increase in moisture at Pearly Beach 18,500 to 15,000 cal yr BP that was possibly linked to a slowdown in the Atlantic Meridional Overturning Circulation (AMOC) and build-up of heat in the southeast Atlantic. It is possible that a build-up of heat in the southeast Atlantic brought on by a slowdown in the AMOC increased moisture uptake in the frontal storms, resulting in stronger low-pressure systems and intensified storms, as reflected in the KSV-1 record. Comparisons with other records from South Africa, including Mfabeni, Cold Air Cave and Cango Caves, indicate that the central and eastern parts of the country experienced a cooling event ~18,500 and 14,500 cal yr BP. New geochemical data from Pearly Beach suggests that this cooling may have been linked to intensification of westerly winds at this time. An apparent lag effect in the warming of South Africa after deglaciation may be attributed to the enhanced influence of westerly winds during the glacial-interglacial period, which pushed cold air masses across the interior of the country. The complex interplay between local sea level changes, westerly wind dynamics, and regional climatic conditions underscores the significance of the geochemical record from Pearly Beach in understanding past environmental changes and their broader implications for southern Africa.Item Geochemical Investigation Into Holocene Palaeoenvironmental Change Along The Southern Cape Coast, South Africa(University of the Witwatersrand, Johannesburg, 2023-06) Dyubele, Viwe; Quick, Lynne; Humphries, MarcClimatic conditions across southern Africa are affected by the complex interaction of different atmospheric and oceanic circulation systems, the understanding of which is important to predicting future climate change. Palaeoenvironmental reconstruction is an essential tool to understand long-term environmental change and the response of ecosystems to such changes. This study examines the geochemical composition of a sediment core (WR1-1) extracted from a freshwater wetland located near Plettenberg Bay on the southern Cape coast. The wetland is located ~4 m above present sea level and positioned ~500 m from the modern coast. Situated within the year-round rainfall zone, the site is influenced by tropical easterly flow and the southern westerlies. Elemental and stable isotope geochemistry are used to reconstruct the palaeoenvironmental change at the site over the last ~8000 cal yr BP. Variations in CaO/Al2O3, Sr/Al2O3 and δ13C indicate that marine conditions dominated from 7300 to 6400 cal yr BP. Marine influence at the site decreased dramatically from ~6300 cal yr BP, as the system transitioned to a freshwater back-barrier wetland. Enrichments in SiO2/Al2O3 and Zr/Al2O3 track changes in depositional energy and suggest that the period 3800 – 3200 cal yr BP was associated with increased aeolian activity. This is interpreted to reflect increased aridity and is consistent with geochemical and pollen records from nearby sites at Eilandvlei and Voëlvlei. This suggests that a shift to more arid conditions during this time was a broad feature of the climate in the year-round rainfall zone of South Africa. The timing of this event corresponds with a marked decrease in Antarctic sea ice and pronounced aridity along the east coast of South Africa, suggesting that mid to late Holocene aridity in the YRZ was likely driven by declines in moisture from both westerly and easterly wind systems.