Theses and Dissertations (Chemical Engineering)

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Exploring dosage forms for release of oleic acid as a mosquito larvicide
Dzingai, Pethile (University of Pretoria, 2025)
Anopheles arabiensis is one of the primary malaria vectors in Southern Africa. Larviciding is a potential supplementary intervention that could help to reduce malaria transmission. Oleic acid, a non-toxic food additive, showed larvicidal activity against 3rd and 4th instar of the species corresponding to LC50 = 13  21 ppm and LC90 = 31  666 ppm after 48 h. This project explored controlled-release dosage form for oleic acid. Systems explored include microcapsules, films, oleogels, conventional emulsions and Pickering emulsions with zein, a plant protein derived from corn, as the excipient. Zein is insoluble in water but dissolves in aqueous ethanol. Such solutions provided the starting point for the preparation of the various dosage forms. Laboratory experiments suggest that Pickering emulsions represented the best delivery option. Pickering emulsions containing 30 wt.% oleic acid formulated with 3.0 wt.% zein particles as stabilisers showed the best formulation stability. Effective larviciding, with mortality approaching 100%, was achieved when this formulation was high shear mixed into water to a level of 250 ppm oleic acid. Unfortunately, it was not possible to dilute these Pickering emulsions using just mild agitation as would be necessary under real conditions. This is because stability was lost due to the pH of the natural water being too close to the isoelectric point of zein. This problem was avoided using oleic acid emulsion stabilised by sorbitan mono-oleate. However, in this case it took a long time for lethal oleic acid concentrations to develop some distance from the application point. This delay is attributed to the low solubility of oleic acid monomers and the diffusion-limited transport of aggregates forming in water. In conclusion, the study highlights the potential of using zein for the controlled release of oleic acid as a mosquito larvicide in emulsified form with oleic acid.
Co-existing environmental metallic and organic contaminants in the aquatic environment : detection, quantification, and risk assessment
Qhubu, Mpho (University of Pretoria, 2026-02-14)
Urban-industrial wastewater systems are increasingly burdened by the simultaneous presence of metallic and non-metallic organic contaminants, presenting complex risks to aquatic ecosystems and human health. In rapidly urbanising areas such as Johannesburg, industrial expansion and growing wastewater flows amplifies chemical pollution, while current regulatory and treatment systems remain insufficiently equipped to manage emerging contaminants at large. This study investigated the detection, quantification, transformation, and ecological and human health risks associated with co-existing potentially toxic elements (PTEs) and contaminants of emerging concern (CECs) across multiple industrial effluent discharge points and wastewater treatment plants (WWTPs) operated under Johannesburg Water. Through this multidisciplinary approach, the research aimed to establish an Integrated Regulatory and Monitoring Framework (IRMF) to support risk-based environmental governance and enhanced protection of urban water resources. Sampling was conducted at industrial effluents consisting of key industrial sectors such as sweet manufacturing, metal processing, food and meat production, personal care product (PCP) manufacturing, along WWTP influent and effluent streams over three sampling monitoring cycles. Key physicochemical properties such as pH, electrical conductivity (EC), chemical oxygen demand (COD), and dissolved oxygen (DO) were evaluated to assess wastewater quality dynamics across industrial discharge points and WWTP treatment stages. Elevated COD and EC values in sweet manufacturing, processed meat, and personal-care product (PCP) industries indicated organic and inorganic pollution loads entering WWTP systems with insufficient pre-treatment. DO concentrations at many industrial effluents were frequently <2 mg/L, reflecting high microbial oxygen demand and suppressed oxygen availability essential for aquatic health. Metallic elements were quantified using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Suspected organic compounds were extracted using offline solid-phase extraction and profiled using Ultra-Performance Liquid Chromatography coupled with Quadrupole Time-of-Flight Mass Spectrometry (UPLC-QTOF-MS) using mass spectrum fragmentation ions and accurate mass-to-charge ratios (m/z). A total of thirteen PTEs (Ag, Al, As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, V, and Zn) were detected across industrial effluents (IE), wastewater treatment plant influents (PI), and effluents (PE). Ag, Al, As, Cd, Cr, Pb, and Zn frequently exceeded regulatory thresholds across various industrial sectors and wastewater treatment stages, indicating widespread usage, incomplete removal, and potential transformation during treatment processes. The lower ambient temperatures recorded during Batches 2 and 3 likely influenced metal speciation, mobility, and wastewater treatment dynamics. Colder conditions can enhance the partitioning of metals onto suspended solids and sludge particles, thereby reducing their dissolved fraction in the aqueous phase. This behaviour explains the observed decreases in measured environmental concentrations (MECs) for certain potentially toxic elements (PTEs) during these sampling batches. The identified compounds were octinoxate, aspartame, 1,2-benzenedicarboxylic acid decyl octyl ester, propiconazole, triclosan, oxybenzone, polyethylene glycol (PEG), arachidonic acid, and chlorophyll-alpha (α). Measured environmental concentrations (MECs) were evaluated against predicted no-effect concentrations (PNECs) to calculate hazard and risk quotients (HQs/RQs) for key aquatic trophic levels (algae, crustaceans, fish). Risk Quotient (RQ) analysis across algae, crustaceans, and fish revealed consistently high ecological risks (RQ ≥ 1) for metals including Cr, Cu, Zn, Fe, and Pb in most of the industrial effluent samples, with extremely elevated values, exceeding 10,000 for Cu and Zn, particularly at IE-3 and IE-5 which are the beverages and food industry. Despite undergoing wastewater treatment, many metals retained high RQs in effluent streams, demonstrating that transformation through WWTPs does not guarantee detoxification, especially at PI and PE points receiving direct industrial effluents. Triclosan, propiconazole, oxybenzone, and octinoxate exhibited high risk levels, with RQ > 1 at the detected sampling points for both acute and chronic toxicity assessments, indicating potential ecological concern. Semi-quantitative concentrations resulted in RQ exceedances for key personal-care and fungicide compounds, signalling a meaningful risk to aquatic primary producers, invertebrates, and fish. Non-carcinogenic hazard analysis through ingestion and dermal pathways showed extremely high exposure risks for all age groups, especially for infants and children with Pb, Cr, Ni, and Al presenting the most severe threats (HQ ≥ 10,000). These risks intensified during colder conditions (Batches 2 and 3), likely due to reduced dilution and increased pollutant concentrations in winter. The findings informed the development of an Integrated Regulatory and Monitoring Framework (IRMF) designed to align analytical detection with risk-based management and compliance monitoring across industrial tiers. This framework supports the formulation of targeted discharge controls, prioritization of contaminants of emerging concern (CECs), re-evaluation of South African legislation regarding CECs, and balancing of industrial pre-treatment and municipal wastewater regulations in South Africa’s rapidly urbanizing contexts. Core framework components include sector-specific industrial pre-treatment requirements based on contaminant profiles, utilisation of passive and real-time sensor systems for continuous detection, development of a national CEC Observatory to enable shared data interpretation and hotspot mapping, and coordinated regulatory oversight across municipalities, research institutions, and environmental authorities. Overall, this study represents comprehensive analysis of metallic and organic CEC co-existing in South Africa’s wastewater network. The findings demonstrate that current treatment systems remain inadequate for protecting aquatic ecosystems and safeguarding public health from persistent toxicants. By integrating scientific evidence with policy innovation, the IRMF proposed in this study provides a vital roadmap to support South Africa’s transition toward proactive, risk-based regulation of emerging contaminants. Ultimately, the study’s outcomes align strongly with United Nations Sustainable Development Goals (SDGs), particularly SDG 6 (Clean Water and Sanitation), by enabling risk-based monitoring and improved control of harmful pollutants, while also advancing SDG 3 (Good Health and Well-Being) and SDG 12 (Responsible Consumption and Production) through enhanced industrial accountability and environmental protection.
Potential toxicity of emerging chemicals of concern used in sanitisers and disinfectants on aquatic organisms : case study of the freshwater snail, Physa acuta
Motaung, Lenah Kagiso (University of Pretoria, 2026-02-06)
Aquatic contamination is regarded as a significant global concern. However, sediments are the ultimate sinks of particle-bound contaminants. Thus, storing harmful compounds leading to long-term ecological harm and risks to benthic organisms. This study adopted a tiered approach to assess the ecological impact of chemicals commonly found in disinfection and sanitisation products by incorporating screening protocols, statistical modelling, and organism-level experimentation. An initial PBT (Persistence, Bioaccumulation, Toxicity) screening process was used to identify chemicals of potential environmental concern based on their physicochemical properties, environmental behaviour, and documented toxicological profiles. Quaternary ammonium compounds (QACs) were amongst the chemicals of concern, and this is due to their wide use and applications in disinfectants and other products. Secondly, their effects, along with other micro chemicals and nano particles like titanium dioxide, remain poorly understood, especially on invertebrates, which have been neglected in toxicity studies. The compounds that met the set prioritisation criteria, the two QACs, benzalkonium chloride (BAC) and Didecyldimethyl ammonium chloride (DDAC) and the titanium dioxide nanoparticles (nTiO2), were subsequently assessed using insilico Species Sensitivity Distributions (SSDs) constructed from EC50, LC50, and NOEC datasets to estimate HC5 values, identify sensitive taxa, and derive protective concentrations where feasible for Ecological Risk Assessment (ERA). The SSD based ERA was compiled from data extracted from the ECOTOX database and peer-reviewed journals using targeted key words. The hazardous 5th percentile (HC5) was calculated and the PNEC determined from the HC5 from NOEC SSDs to determine the Risk Quotient (RQ) for each chemical. The HC5 values derived from BAC EC50 and LC50 SSD were low at 0.001 µg L-1 and 3.33 µg L-1, respectively, while the HC5 for NOEC was 0.032 µg L-1. Higher values were observed for DDAC with the HC5 recorded as 19.12 µg L-1.and 0.528 µg L-1 for LC50 and NOEC. The HC5 values for nTiO₂ endpoints were comparatively higher and falling within the milligram range, 4 050.4 and 347.4 µg L-1 for LC50 and EC50. A consistent trend across all three chemicals was observed whereby daphnids were identified as the most sensitive species for at least one of the evaluated endpoints for each chemical. The analysis also highlighted the data gaps and under representation of many classes of organisms in ecotoxicity studies, more so sediment dwelling organisms, especially molluscs. Thus, the toxicity studies were conducted on Physa Acuta snails to determine the effects on the apical endpoints, survival, and reproduction, furthermore, investigate the inhibition of glutathione S-transferase (GST) antioxidant enzyme. The results of the 96-h acute exposure studies results showed survival of P. Acuta snails to fall in the range of 96.6% and 100% across the treatment groups with exposure concentrations of up to 800 µg kg −1 and 800 µg L-1 in the sediment and overlaying water, respectively. There were no significant differences in the number of capsules laid by snails over the exposure time compared to the control group that was not exposed to the any of the three chemicals. On the other hand, the number of eggs laid per capsule over the 96 hours was significantly affected. The reproduction effects of the snails in ascending order were: nTiO2 < DDAC < BAC compared to the control. Additionally, results from the biomarker assays revealed significant inhibition of glutathione S-transferase (GST) antioxidant enzyme across all three test groups and across all concentrations compared to the controls. Snails exposed to DDAC exhibited the lowest GST inhibition, except at a concentration of 400 µg kg-1, while those exposed to nTiO2 showed the highest inhibition. These GST test results reveal significant adverse effects at the molecular level that are not immediately apparent at the organism level. This highlights the importance of studying the molecular effects of chemicals, even during short-term exposure periods. Secondly, chronic exposure studies are recommended to better understand the toxicity mechanism of the three chemicals over more real-life environmental scenarios.
Development of PVDF-based protective coatings : applications for PVC architectural textiles
Sonnendecker, Anya (University of Pretoria, 2026-01-10)
South Africa, endowed with the world’s largest known fluorspar reserves, has historically exported its mineral resources as raw materials, leaving substantial economic potential untapped. Recognising this, the country’s beneficiation strategy encourages the addition of value within its own borders. One key opportunity is the domestic production of fluorochemical products—particularly those derived from fluorspar—which could yield far greater revenue than the export of raw material. Within this context, the local development and application of polyvinylidene fluoride (PVDF)-based coatings offer promising avenues for both the textile industry and broader economic growth. This Thesis focuses on the formulation of PVDF-based coatings suitable for PVC-coated polyester (PES) architectural textiles. These textiles, widely used in flexible architectural structures, suffer from material degradation under prolonged environmental exposure, predominantly due to plasticiser migration and ultraviolet (UV) damage. Introducing a PVDF topcoat, known for its excellent UV resistance and durability, can significantly enhance the service life of these fabrics. While PVDF-coated textiles are available internationally, their high cost limits local adoption. This research addresses this challenge by developing PVDF coating formulations that can be integrated into existing dip-coating production lines with minimal capital outlay, thereby reducing reliance on expensive imports and creating a competitive local alternative. The central aim of this research was to identify at least two economically viable PVDF-based coatings that improve flexibility, adhesion, and overall longevity without requiring complex infrastructure upgrades. Conventional lacquers and copolymer-based PVDF approaches often require additional adhesive layers or specialised processing capabilities not readily available in underdeveloped regions. To overcome these barriers, the research explored two main strategies: Plasticiser-Enhanced PVDF Coatings: Various plasticisers were investigated to improve the inherent rigidity of PVDF, thereby achieving greater coating flexibility. Intensive characterisation using FTIR, DSC, SEM, and DMTA analyses guided the selection of plasticisers that could reduce PVDF’s melting and crystallisation temperatures and improve flexibility. However, concerns regarding long-term plasticiser migration—leading to eventual coating failure—necessitated additional solutions. Non-Liquid Additives and Acrylic Co-Polymers: To circumvent plasticiser-related durability issues, flexible acrylic co-polymers were blended with PVDF. These acrylics aimed to improve both the coating’s flexibility and its adhesion to the PVC substrate without the long-term leaching problems associated with plasticisers. Systematic evaluation of different acrylic monomers, their molar ratios, and their influence on PVDF crystallinity and surface morphology identified promising formulations that maintained or enhanced PVDF’s intrinsic protective properties. Optimising the manufacturing parameters, including solvent choice, polymer-to-solvent ratios, drying temperatures, and process conditions, was critical to ensuring that the coatings could be produced using the existing dip-coating infrastructure. From the initial tests to the final accelerated weathering assessments, the Thesis provides a robust methodology for producing PVDF-based coatings that are cost-effective, durable, and tailored to local industry constraints. Ultimately, two categories of formulations emerged as viable solutions: PVDF coatings plasticised for improved flexibility, and PVDF/acrylic co-polymer blends that combine durability, UV stability, and reliable adhesion. Accelerated weathering tests confirmed that these coatings can significantly extend the functional lifespan of PVC-coated textiles, offering substantial long-term value. By closing the gap in local beneficiation of fluorspar-derived products, this research not only contributes to the scientific understanding of PVDF-based formulations but also aligns with South Africa’s strategic objectives to bolster its manufacturing capacity and improve economic resilience. The Thesis delivers both theoretical insights and practical frameworks, paving the way for large-scale pilot testing, industrial adoption, and broader economic benefits—representing a significant advancement for local producers and an important step in the sustainable utilisation of South Africa’s abundant mineral resources.
Rapid component-specific characterisation of woody biomass via peak-focused thermogravimetric deconvolution
Tau, Lerato (University of Pretoria, 2026)
The accurate determination of hemicellulose, cellulose, and lignin in wood and pulp materials is a critical step in the paper and pulp industry as it provides insight into the pulping mills efficiencyand informs product development. Traditionally, such analyses rely on laborious, chemical-intensive methods that are time-consuming, costly, and require hazardous reagents. This study investigates a rapid, computationally driven thermal method for lignocellulose characterisation using thermogravimetric analysis (TGA) coupled with the independent parallel reaction (IPR) kinetic model. The IPR model was selected for its computational efficiency and ability to deconvolute overlapping peaks in the derivative thermogravimetric (dTG) curves, which show the rate of mass loss as temperature increases. The initial kinetic parameters, specifically activation energy (Ea) and pre-exponential factor (A), for each component were estimated using a novel top-and-tail approach which linearises selected regions of the dTG curve. The study workflow comprised of three levels: model verification, validation, and sensitivity analysis. Model verification was performed using five published pinewood datasets. The model achieved low root-mean-square error (RMSE) values ranging from 0.015 to 0.100, high adjusted R² values (97.74 % – 99.96 %), and fit qualities above 97.00 %, while the deviations in predicted component mass fractions compared to literature values ranged from 0.2 % to 8.4 %. This verified the model’s ability to accurately deconvolute TGA data and predict lignocellulose composition for pine wood. Subsequently, model validation was conducted using industrially sourced pine and Eucalyptus wood, and their derived pulp samples. The model successfully predicted the hemicellulose, cellulose and lignin content of the wood samples, achieving RMSE values of 0.063 for pine and 0.060 for the Eucalyptus. However, predictions for kraft pulp samples revealed systematic overestimation of lignin content, suggesting that the model encounters limitations in handling samples with low lignin fractions and highlights the need to account for component interactions. This peak-focused deconvolution model demonstrated rapid characterisation by converging within an hour of optimisation, thus, confirming the method’s speed in determining lignocellulose content in woody biomass. Synthetic mixtures were formulated from known quantities of xylan, microcrystalline cellulose, and kraft lignin to represent typical lignocellulosic compositions of wood and pulp. Application of the model to these mixtures showed reasonable predictions for the wood-like compositions (Mixes 1–4), with deviations ranging from 1.7 % to 12.6 %, while mixtures resembling pulp samples (Mixes 5–6) exhibited substantial lignin overestimation. These findings corroborate the validation results, indicating that the model performs well for complex, lignin-rich mixtures but requires refinement for low-lignin materials. Potential refinements for future work include incorporating hybrid kinetic deconvolution approaches to better capture component interactions, supporting model predictions through traditional wet-chemistry methods. Finally, sensitivity analysis was conducted by varying heating rates (5 °C/min, 10 °C/min, 20 °C/min), performing local parameter perturbations (±20%), and altering initial parameter estimates. The model proved sensitive to heating rate variations as the model predictions varied drastically at a slow heating rate (5 °C/min) and a faster heating rate (20 °C/min), the most reliable and accurate model predictions were determined at 10 °C/min. Local sensitivity analysis showed that the kinetic parameters for hemicellulose and cellulose had the largest influence on composition predictions. It was also discovered that parameter initialisation influenced model convergence as some optimisation processes reached local minima instead of the global minima when the initial estimates were varied randomly. These results indicate that the number of model parameters could be efficiently reduced to mitigate parameter compensation effects, and that optimisation across multiple heating rates should be explored in future work to improve model robustness. Overall, this study demonstrates the potential of TGA combined with the IPR model as a rapid, reagent-free, and computationally efficient method for lignocellulosic characterisation in woody biomass. While improvements are needed to refine predictions for pulp samples, the approach provides a promising tool for frequent compositional analysis, reducing reliance on conventional chemical-intensive techniques and supporting industrial process monitoring.
Risk and performance assessment of existing treatment plants to remove emerging pollutants from wastewater : a case study of Johannesburg water’s wastewater treatment plants
Muntswu, Arinao D. (University of Pretoria, 2025-10-01)
The occurrence, persistence, and removal efficiency of emerging pollutants were systematically investigated at two wastewater treatment plants (WWTPs) operated by Johannesburg Water, located in the City of Johannesburg, South Africa. A broad-spectrum analysis was conducted using Gas Chromatography-Mass Spectrometry to detect various pollutants. Sampling was carried out at three strategic points within each treatment plant: the influent, reactor effluent, and final effluent, to assess the dynamics and fate of multiple classes of pollutants throughout the treatment process. Across both plants, 1,109 unique compounds were detected; the overall mean match quality was 66.21% (SD = 19.85%), with a 95% confidence interval of 63.91%–68.51%. In Wastewater Treatment Plant A, only 17% of detected compounds fell below the 50% match confidence threshold, with a mean match quality of 73.19% (SD = 17.85%). In contrast, 14.31% of detected compounds fell below the 50% threshold in Wastewater Treatment Plant B. This study aimed to comprehensively assess emerging pollutants’ occurrence, behaviour, and fate within Johannesburg Water’s wastewater treatment system and identify their potential sources and pathways. The results revealed that, although certain pollutants are effectively removed, many compounds persist through multiple stages of treatment. The contaminants identified in both influent and effluent comprised a wide range of chemical classes, including halogenated hydrocarbons, organofluorine compounds, organosilicon compounds, organobromine compounds, pyridine derivatives, antioxidants, heterocyclic compounds, terpenes, insecticides, pharmaceuticals, personal care products, and epoxides. Notably, several compounds, including Bis(chloromethyl) ether, oxalic acid, phenols, hydrazine, santolina triene, benzeneethanamine, isoxazolidine, toluene, indolizine, propargylamine, n-hexane, ethylbenzene, p-xylene, isobutylene epoxide, acetone, 1H-imidazole, fumaronitrile, Bacchotricuneatin C, 4-O-methyl-D-arabinose, pyrrolidine, aziridine, D-limonene, and thiazole, were exclusively detected in the final effluent. Their presence suggests potential formation as transformation products during treatment, raising significant concerns about their possible environmental persistence, bioaccumulation, and associated public health risks within the receiving water bodies in the City of Johannesburg. The variability in biodegradability indicates that while some compounds can undergo natural breakdown during standard wastewater treatment processes, others resist microbial degradation. This resistance necessitates the implementation of more intensive remediation strategies. Compounds with high persistence and low biodegradability pose an even greater risk, as they can infiltrate natural water bodies, adversely affecting aquatic ecosystems and potentially contaminating human water supplies in South Africa.
Bioprospecting and characterisation of heavy metal stress-induced antimicrobials from rhizospheric Bacillus sp. against multidrug-resistant Pseudomonas Aeruginosa (strain ATCC 27853) and Acinetobacter baumannii (strain ATCC-BAA-1605)
Millard, Kylah (University of Pretoria, 2026)
This thesis explores the potential of heavy-metal-contaminated rhizospheres as a source of antimicrobial secondary metabolites capable of inhibiting multidrug-resistant pathogens. X-Ray Fluorescence (XRF) analysis confirmed substantial contamination of soils from Salvage and Recycling in Silverton, creating an extreme environment that selected for stress-resilient rhizospheric microorganisms. Several bacterial isolates were derived from these soils, particularly Bacillus subtilis Steel 7, which, among other isolates, that displayed strong inhibitory activity against Pseudomonas aeruginosa ATCC 27853 and Acinetobacter baumannii ATCC BAA-1605, This demonstrates that heavy-metal stress enhances antimicrobial metabolite production. Chemical profiling of the most active bacterial extract using ultra-high-performance liquid chromatography coupled to photodiode array detection and high-resolution mass spectrometry (UPLC-PDA-HRMS) revealed a complex metabolite mixture, with ions at mass-to-charge ratio (m/z) 132 and 166 recurring across active chromatographic fractions. Through sequential bioassay-guided fractionation and multi-platform structural elucidation, including mass spectroscopy (MS), proton nuclear magnetic resonance (¹H NMR) and carbon-13 nuclear magnetic resonance (¹³C NMR), heteronuclear single quantum correlation (HSQC), and heteronuclear multiple bond correlation (HMBC), the main compound of 132 m/z was identified as leucine in the bacterial extract. The antimicrobial testing of the crude bacterial extract exhibited strong activity whereas the identified leucine compound was inactive against the test pathogens, indicating that the potent antimicrobial activity observed in crude extract is attributable to one or more of the low-abundance metabolites. However, leucine is not individually active against the pathogens tested, it may contribute synergistically (as an adjuvant or otherwise) to the sensitivity or susceptibility of the pathogens to the active antimicrobial compound in the crude extract. Thus, while a major constituent (leucine) in the bacterial secretions was conclusively characterised, the true active compound(s) remain undiscovered in the crude extract. This work demonstrates that extreme environments stimulate antimicrobial secretion and bio-assay guided isolation of the active metabolite(s) clarifies the chemical complexity underlying this activity, and provides a methodological foundation for future high-resolution, bioassay-guided isolation of the active metabolite(s). Based on the findings in this work, stressed rhizospheric microbiomes could serve as a potentially valuable reservoir for novel antibiotic discovery.
Formation thermodynamics of layered double hydroxides
Muire, Hanno (University of Pretoria, 2025)
This research delves into the thermochemistry of layered double hydroxides (LDHs), with hydrotalcite as a key representative of this class of anionic clay compounds. LDHs are characterised by their brucite-like layers, generally composed of divalent and trivalent metal cations intercalated with anions such as carbonate to maintain charge balance. These materials have garnered significant interest due to their versatile applications in environmental remediation, catalysis, and materials science. The synthesis of LDHs is typically carried out through co-precipitation, urea hydrolysis, and hydrothermal dissolution-precipitation. Co-precipitation is the most commonly used due to its simplicity and efficiency. However, it presents challenges, including the high cost of metal salts and the generation of environmentally harmful effluents. In response to these challenges, the study explores more sustainable synthesis methods, focusing on hydrothermal dissolution-precipitation reactions that utilise metal oxides and hydroxides. However, these methods require careful optimisation due to the low solubility of the reactants. A key aspect of this research is the role of water in LDHs, which can appear in various forms, such as adsorbed, intercalated, and excess water. The type and presence of bound water have a significant impact on the stability, formation, and thermodynamic properties of LDHs. This study applies the thermodynamic difference rule (TDR) method to a set of experimental enthalpy data for LDHs, aiming to show that bound water behaves in an ice-like state. To validate this, the TDR method is compared with the simpler additive approach of mechanical mixture models, demonstrating its accuracy in estimating thermodynamic properties, even when the precise amount of bound water is uncertain. By enhancing the understanding of LDH thermochemistry, particularly the role of bound water, this research offers a stepping stone to improve current synthesis methods, develop novel and more sustainable processes, and reduce the need for extensive experimental trials. Ultimately, the study aims to contribute to the broader goal of advancing environmentally friendly practices in synthesising LDHs while also providing reliable thermodynamic data that can be used in modelling and simulation efforts for these important materials.
Rhamnolipids production from citrus waste cellulose extract using oil adapted soil microbiota as biocatalysts : a bioprocess engineering approach
Nkosi, Siyabonga (University of Pretoria, 2025)
The increasing demand for sustainable counterparts to synthetic surfactants has sparked interest in microbial biosurfactants, particularly rhamnolipids, due to their biodegradability, low toxicity, and strong surface activity. However, large-scale production is limited by high substrate costs, pathogenicity of common producer strains, and inefficient process optimization. This study addresses these challenges by valorising orange peel waste (OPW) as a renewable carbon source and employing an oil-adapted soil microbiota. Cellulose was extracted from OPW via sequential maceration, alkaline, bleaching, and organic acid pretreatments, yielding 42.49% (w/w) with a saccharification efficiency of 42.22%. Structural integrity and purity were confirmed using FTIR, SEM, XRD, and NMR. Oil-adapted soil microbiota was screened for biosurfactant production, leading to the selection of E. hormaechei. Preliminary fermentation utilizing the orange peel-derived cellulose hydrolysate (OPDC) produced 1.1 g/L rhamnolipids and a microbial density (OD₆₀₀) of 0.66. This yield represents a substantial performance increase, outperforming prior waste oil-based media by nearly 30%. Initial fermentation results revealed modest rhamnolipid production, necessitating statistical optimization to enhance yield and process efficiency. A Plackett–Burman design was employed to screen ten influential variables, followed by OVAT validation and Response Surface Methodology using Central Composite Design (RSM–CCD). Statistical optimization increased the yield to 5.94 ± 0.11 g/L under optimal conditions (3 g/L OPDC, 0.5 g/L MnSO₄·7H₂O, 0.1 g/L CaCl₂·2H₂O, pH 7.0), representing a 5.5-fold improvement. The biosurfactant exhibited strong surface activity, reducing surface tension from 72 to 27.73 mN/m and achieving an emulsification index (E24) of 69.57%. TLC, FTIR, NMR, and LC–MS confirmed the predominance of mono-rhamnolipid congeners, particularly Rha-C10-C10, which enhance emulsification and hydrophilicity. Kinetic modelling further validated the optimized rhamnolipid yield, with the hybrid Logistic–Luedeking-Piret model accurately predicting a production value of 5.77 g/L, closely matching the experimental yield of 5.94 ± 0.11 g/L. High correlation coefficients for biomass growth (R² = 0.996), substrate utilization (R² = 0.994), and biosurfactant synthesis (R² = 0.996) confirmed the robustness of the model, while substrate yield coefficients (Yₓ/ₛ = 0.070 g/g, Yₚ/ₛ = 0.8 g/g) demonstrated efficient conversion of OPDC into Rhamnolipids. This modelling approach offers a reliable framework for scaling up rhamnolipid production using solid Agro-waste substrates. This integrated approach, combining non-pathogenic microbes, citrus waste valorisation, and advanced modelling, establishes a scalable and environmentally sustainable platform for rhamnolipid production, supporting applications in remediation, industrial formulations, and the circular bioeconomy.
Biogas production in African urban informal settlements : temperature and mixing considerations for continuous digestion of food waste
Kleynhans, Caela A. (University of Pretoria, 2026-01)
This study evaluated a low-cost food waste anaerobic digester (FWAD) designed for African urban informal settlements, where electricity and process control are limited. Eight small-scale reactors were operated under varying mixing, pH control, and temperature conditions to assess the feasibility of stable operation with minimal input. Results showed no significant difference in methane yield between reactors with continuous pH dosing and those adjusted every 48 hours, nor between continuously mixed and minimally mixed systems (ANOVA p > 0.05 for all comparisons). The highest mean methane yield, 0.267 L CH₄ g VS⁻¹, was achieved by the minimally mixed reactor with 48-hourly pH control at 30°C, while the controlled reactor at 37°C produced a comparable 0.247 L CH₄ g VS⁻¹. Total methane production was similar at both temperatures, although gas generation was faster during the first 24 hours at 37°C. Compared to extended post-feeding gas recovery, 58% – 73% of total methane was produced within the 48-hour cycle, suggesting conversion could increase by 30% – 40% with extended liquid retention. Microbial analyses showed compositional differences but consistent performance, indicating functional redundancy. These results confirm the capacity of FWAD for stable, efficient biogas production without continuous energy input.
Production of rhamnolipid from foodwaste and its use in flocculation of Potentially Toxic Elements (PTEs) for enhanced Mine Influenced Wastewater (MIW) treatment to irrigation water using an intergrated Micellar Enhanced Ultrafiltration (MEUF) unit
Ndwandwa, Nolundi (University of Pretoria, 2025)
Food waste management remains a critical global challenge, with approximately 1.3 billion tonnes discarded annually. Although landfilling and open dumping remain the most common practices due to their low cost and simplicity, these methods produce leachates and release methane, a potent greenhouse gas. A circular economy approach offers an alternative, whereby food waste is valorised into high-value products such as biosurfactants. This study investigates a three-phase project that couples sustainable rhamnolipid (RL) production from food waste with the remediation of mine influenced wastewater (MIW), which often contains potentially toxic elements (PTEs) at concentrations exceeding World Health Organization (WHO) discharge limits. In water-scarce regions such as South Africa, reclaiming MIW for agricultural irrigation addresses both water security and rural development, thereby contributing to Africa’s Agenda 2063. Phase 1 examined the selection and characterization of commercially-available biosurfactants suitable for Micellar-Enhanced Ultrafiltration (MEUF), focusing on their critical micelle concentration (CMC), micellar size, and micellar morphology, which govern ultrafiltration efficiency. A phosphate buffer solution (pH 7) was prepared to dilute four biosurfactants (rhamnolipid, lipopeptide, lactonic sophorolipid, and surfactin) to 10 mg L⁻¹ for flocculation experiments with MIW at a 1:1 (v/v) ratio. The mixtures were shaken for 72 h at 25 ºC, centrifuged, and analyzed for conductivity and total dissolved solids (TDS). Surfactin exhibited the most consistent performance, likely due to stable micelle formation, whereas rhamnolipid was more effective at reducing conductivity despite increasing TDS. Oil spreading tests established CMC values of 30 mg L⁻¹ for rhamnolipids and 10 mg L⁻¹ for surfactin. Dynamic Light Scattering (DLS) confirmed micelle formation, and Scanning Electron Microscopy (SEM) revealed rhamnolipid-induced morphological changes, forming spherical micelles in coal MIW and vesicular structures in gold MIW. Phase 2 evaluated the influence of biosurfactant concentration and MIW-to-biosurfactant ratio on MEUF performance. Flocculation studies were conducted at CMC using varying MIW:RL ratios (v/v) (1:1, 1:3, 1:5, 1:7, 1:10), as well as multiple CMC concentrations of 1x1 CMC to 1x10 CMC corresponding to biosurfactant concentrations of 30–300 mg L⁻¹. The experiments were conducted in duplicates. Characterization included particle size distribution (Zetasizer), PTEs removal (ICP-OES). Results indicated that a 1:10 rhamnolipid-to-MIW ratio removed Co, Mg, Se, and Sr by 84–92 %, while Fe and Cr removals peaked at 1:5 (99 %) and 1:3 (74 %), respectively. Increasing CMC generally had limited effects, but Fe removal in gold MIW increased to 97 % at 15× CMC. Treatment shifted MIW pH from acidic (2.7) to near-neutral (6.4), improving PTEs complexation. The most balanced outcome was achieved at 1:5 ratio and 4× CMC (120 mg L⁻¹). Phase 3 focused on the sustainable production of rhamnolipids from food waste. Corn cobs, an abundant by-product of South Africa’s maize industry, were selected as a carbon source. Cellulose nanofibers (CNFs) were extracted through sequential pre-treatments including size reduction, Soxhlet dewaxing (toluene/ethanol, 2:1 v/v, 6 h), alkaline pulping (10 % NaOH, 80 °C, 2 h), bleaching with acidifiedsodium chlorite (95 °C, 6 h), and citric acid hydrolysis (70 %, 120 °C, 4 h), followed by freeze-drying. Characterization of CNFs employed Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDS), and X-ray Fluorescence (XRF). Morphological analysis showed cellulose strands with residual lignin and hemicellulose cross-linking. FTIR confirmed characteristic cellulose, hemicellulose, and lignin peaks. XRD analysis revealed cellulose I peaks at 18°, 26°, and 40°, with crystallinity indices of 50 (CNFs) and 49 % (bleached samples). Pre-treatment processes, including dewaxing, pulping, bleaching, and acid hydrolysis, progressively removed lignin, hemicellulose, and waxes, resulting in purified cellulose suitable for nanofibrillation. While LC-MS analysis also revealed a shift from xylose-rich hemicellulosic material to glucose-dominant cellulose. XRF highlighted changes in elemental composition due to chemical treatments, with final CNFs showing high oxygen and silica content. SEM and DLS also confirmed structural disruption and nanofiber formation. Rhamnolipid biosynthesis was conducted using Pseudomonas aeruginosa ATCC 9027 in a mineral-salt medium (pH 6.68) supplemented with CNFs and waste cooking oil as the carbon source. Inoculum preparation involved culturing the strain on nutrient agar at 35 °C for 72 h, transferring cells into nutrient broth (6 g L⁻¹), and incubating at 35 °C, 150 rpm. Batch fermentations were carried out with carbon source concentrations of 1–2.2 g L⁻¹ at 30 °C, 180 rpm for 21 days in triplicates. After the respective incubation in the mineral media, the production media was centrifuged at a high speed 2000 rpm for 20 min at room temperature (25°C). The supernatant was then acidprecipitated using concentrated hydrochloric acid (HCl) to pH (2.0-3.0). Crude biosurfactant (i.e. rhamnolipids) was then obtained by the acid solvent extraction method, using chloroform: ethanol (2:1 v/v) and freeze dried. Rhamnolipid characterization employed oil spreading tests, FTIR, surface tension analysis, emulsification index (E₂₄), liquid chromatography-mass spectrometry (LC-MS), thin-layer chromatography (TLC), SEM and OST. Fermentation using Pseudomonas aeruginosa ATCC 9027 successfully produced rhamnolipids, with optimal emulsification observed at 2.2 g/L corncobs and waste cooking oil after 21 days. TLC and LC–MS analyses confirmed that the synthesized biosurfactant was predominantly a mono-rhamnolipid, consistent with previously reported structural and chromatographic characteristics. FTIR and H-NMR further confirmed the mono-rhamnolipid structure. The rhamnolipid achieved emulsification activities of 57% for RL-2 and 47% for RL-1 while SEM revealed distinct morphological differences between RL-1 and RL-2: RL1 displayed impure RL, whereas RL-2 exhibited more uniform, closely packed spheroidal clusters which have also been previously reported in literature. OST measurements verified its ability to reduce surface tension with a critical micelle concentration of 150 mg/L for RL-2. This multi-phase investigation demonstrates the potential of food waste valorisation in value-added products and the use of the produced bioproduct in mine-influenced wastewater remediation. Furthermore, the successful extraction of CNFs from corncobs and their use as a carbon source for rhamnolipid production supports the feasibility of a circular bioeconomy approach for sustainable disposal of agro based waste.
Synthesis and performance evaluation of Moringa Oleifera seed coat biosorbent for carbamazepine adsorption from model wastewater
Azeh, Peace S. (University of Pretoria, 2025-12-09)
This study investigates biochar produced from discarded Moringa oleifera seed coats as a low-cost, sustainable biosorbent for removing the persistent pharmaceutical pollutant carbamazepine (CBZ) from water. The seed coats were chemically activated with phosphoric acid and carbonised at 600 °C, producing a meso- and microporous biochar with a high surface area (237.3 m²/g), abundant functional groups, increased graphitisation, and enhanced thermal stability, as confirmed by FTIR, SEM/EDX, BET, XRD, TGA, and elemental analyses; alternative pretreatments showed no performance improvement. Batch adsorption experiments demonstrated excellent CBZ removal (93.32% at 50 ppm), with agitation speed identified as the most significant operational parameter via response surface methodology using a Box–Behnken design. The optimised conditions (400 rpm, 30 °C, 2.15 h) yielded an experimental adsorption capacity of 51.09 mg/g, closely matching model predictions. Kinetic analysis showed that adsorption followed a pseudo-second-order model, indicating chemisorption, while the Temkin isotherm best described equilibrium behaviour. Overall, the results confirm Moringa oleifera seed coat biochar as a promising green adsorbent for pharmaceutical removal in wastewater, supporting circular economy principles and warranting further evaluation in real wastewater systems.
Water quality, health risk assessment, and treatment options for groundwater in Matsa, Mamvuka and Manyii villages, Limpopo Province
Tshikombeni, Ondwela Venetia (University of Pretoria, 2025-04)
Water sources in rural areas are mostly contaminated by different pollutants such as pathogens, heavy metals, and chemicals. The communities of Matsa, Mamvuka, and Manyii have been without a municipal treated water source. Therefore, they depend on spring water to meet their basic water demand. This study addresses the water quality issues associated with the consumption of contaminated water in these villages due to the lack of existing research assessing the water quality of the spring water utilised. The study evaluated the physicochemical and microbiological quality of groundwater in Matsa, Mamvuka, and Manyii villages, Limpopo Province, South Africa. A water quality assessment was carried out on 48 samples, and the method of interviews using a questionnaire was used to determine the communities' practices of water collection and storage. The questionnaire was distributed to 15 households in the Matsa, Mamvuka, and Manyii villages, with 5 people from each village being interviewed, and adults over 18 years were randomly selected for an interview questionnaire. The physicochemical parameters were measured using the 7-in-1 water quality meter (Hanna Instruments), and turbidity was measured using the turbidimeter (Thermo Scientific Eutech TN 100). Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) and Discrete Analyser were used to analyse heavy metals and anions, whereas the membrane filtration method was used for microbial analysis. Various water treatment methods, including chlorine tablets, boiling, plasma technology, ceramic filters, and bleach, were applied to determine the most effective water treatment for the three villages. Ecological Structure Activity Relationships (ECOSAR) was used to test the ecological risks posed by the various water treatment methods on the environment. Statistical analysis was performed using Microsoft Excel to test the significance of the influence of seasons on physicochemical, heavy metal, and anion parameters, as well as treatment methods. PCA and Box and Whisker were used to describe the distributions of metals and ions across the sampling sites. Most of the physicochemical parameters’ concentrations in drinking water samples were within the regulatory standards limits of South African National Standards 241 and the World Health Organisation, except for S10 (Manyii spring), which exceeded the ≤ 5 as it recorded 10.75 and 51.5 NTU for the dry and wet seasons. Some heavy metals, such as Fe and Al, exceeded the SANS regulations in both seasons. For the dry season, Fe was above the limit at S3 (2.03 mg/L), while Al exceeded at S2 (0.34mg/L). In the wet season, these metals exceeded the limits at S3 (4.3mg/L) and S10 (2.095 mg/L) for Fe. Meanwhile, Al exceeded at points S6 to S12 with concentrations of 0.315, 0.785, 0.415, 0.855, 0.345, and 0.45 mg/L, respectively. According to SANS 241, these metals should not exceed the values of 2 and 0.3 for Fe and Al, respectively. Moreover, As (0.046 mg/L at S2, and 0.027 mg/L at S12 during the dry season,) Mn (0.15 mg/L and 0.31 mg/L at S7 during both seasons), and Ni (0.102 mg/L at S6 during the dry season) exceeded the SANS 241 values of ≤0.01,0.1, and 0.07 mg/L, respectively. Average highest E. coli detected from Matsa (1312.5 CFU/ 100 mL), Mamvuka S1(330 CFU/ 100 mL), Mamvuka S2 (1327.5 CFU/ 100 mL) and Manyii (2340 CFU/ 100 mL) during the wet season exceeded levels detected in the dry season (892.5, 340, 350 and 542.5 CFU/ 100m L) respectively; whereas, for Total coliform, the levels detected in the wet season did not have a specific trend as compared to the levels of E. coli. However, the average highest recorded level among all sampling points was found to be in S5 (9547.5 CFU/ 100 mL) for the dry season, and in S10 (13125 CFU/ 100 mL) for the wet season. Furthermore, the microbial results showed that 100% of the sampling points were contaminated with E. coli and Total coliforms. The water treatment methods employed showed no statistical difference from one another. It was therefore concluded that the water purification methods tested are all effective and can all treat water, as there is no statistical evidence that one method outperforms the other. However, toxicological results showed that bleach and Plasma may pose high risks to the environment and aquatic animals; therefore, the use of other methods (chloribe tables, ceramic filters, and boiling) is recommended to avoid contaminating the environment. The calculated sum Water Quality Index of the sampled water showed that the heavy metals play a role in contributing to low water quality in these communities. The dry and wet seasons WQI sum range within the poor water quality class (160.17 and 121.84). The carcinogenic effects using the Hazard Index (As) through ingestion and dermal pathways (4.80e+00 and 1.26e+00) and Carcinogenic Risk (As_2.16e-03 and 5.66e-04) and (Ni_ 2.08e-02 and 5.45e-03) for children and adults during the dry season showed that the usage of springwater could present significant health risks, as the indices were above the recommended value by the United States Environmental Protection Agency (USEPA). The questionnaire results showed that the residents from the three study areas lack access to potable water, as 46.68% of the respondents responded that they use spring water, whereas 26.66% buy from those with boreholes, and the other 26.66% depend on their private borehole. These communities have been without potable drinking water for over 10 years. Furthermore, during the summer season, most households practice harvesting rainwater in the study areas. Water in these communities is used for all domestic needs, and stored in covered drums or tanks, mostly for less than a week before another collection. Meanwhile, storage practices are good, but most of the residents are not aware of the associated health risks of consuming contaminated water, with only a few associating untreated water with illnesses such as cholera and diarrhea. Based on the results from the water quality analysis, as well as the questionnaire, this study recommends the need for municipalities to invest in providing potable drinking water for the study areas and suitable water treatment methods to treat water before consumption. This can be done through the provision of tankers, maintaining the available municipal pipes, and drilling boreholes. Residents from these communities should practice treating water before consumption to minimise health risks, and this can be done by using chlorine tablets, bleach, and boiling methods, as they can easily be employed in rural areas.
The impact of ozone demand on the required dosage in dielectric barrier discharge plasma-ozonation systems : investigation into a simulated surface water
Roos, Anche (University of Pretoria, 2024-12)
The provision of clean and safe water and sanitation (Sustainable Development Goal 6) and the promotion of good health and well-being (Sustainable Development Goal 3) are critical challenges faced globally, exacerbated by rapid population growth and industrialisation leading to increased contamination of freshwater sources. Waterborne diseases, responsible for approximately 1.4 million annual deaths as reported by the World Health Organization (WHO), highlight the urgency of addressing these issues. Traditional water treatment methods such as boiling, chlorination (addition of chorine tablets) or liquid chlorine bleach, ceramic filters, and UV disinfection have limitations, including inadequate inactivation of all microorganisms and the formation of undesirable by-products, necessitating the exploration of advanced oxidation processes (AOPs), specifically ozone treatment. Extensive studies have been done on the generation of ozone using plasma technology. However, depending on the combination of electrode material, feed gas, current (alternating or direct), and type of discharge use (corona, direct barrier discharge), a wide range of ozone concentrations generated from nonthermal plasma has been reported. Consequently, optimizing the ozone dose is essential for effective water disinfection, as residual ozone can pose toxicity risks for human consumption. This research emphasizes the development and optimisation of advanced plasma-ozonation systems, specifically tailored to the properties of reactor materials and the targeted disinfection outcomes. There remains ongoing research that produces conflicting results regarding whether Chemical Oxygen Demand (COD) and Total Suspended Solids (TSS) or COD and Total Organic Carbon (TOC) are the most effective indicators of ozone demand in water bodies for predicting the appropriate ozone dosage required for efficient ozonation. This study investigates the relationship between ozone demand and key water quality parameters, specifically TSS, COD, and TOC. By analysing these relationships, the research aims to predict optimal conditions for treating contaminated water through a Dielectric Barrier Discharge (DBD) plasma ozonation system. Understanding how these parameters influence ozone demand will provide valuable insights into enhancing the efficiency of the ozonation process, enabling better treatment strategies for contaminated water sources. This work ultimately seeks to contribute to the development of effective water treatment protocols that leverage DBD plasma ozonation technology. Utilising Box-Behnken Design (BBD) in Response Surface Methodology (RSM), statistical analyses were conducted to optimise ozone generation in a high-voltage direct current (DC) dielectric barrier discharge (DBD) reactor fed with oxygen. Key operational variables—gas flow rate, applied voltage, and frequency—were assessed. Analysis of variance (ANOVA) revealed that frequency was the only factor with a significant impact on ozone concentration, with the quadratic model yielding a maximum ozone concentration of 1.09 mg/L.3 The influence of optimised ozone doses on simulated water containing glucose, phenol, and humic acid was evaluated by monitoring COD, TOC, TSS. Results indicated no significant reduction in COD and TOC with glucose, underscoring the importance of pH and functional groups for effective ozonation. Ozonation of phenol and HumeGro (a fertilizer consisting of humic acid) samples yielded maximum reductions of 49% and 54% for COD, and 11.6% and 38% for TOC, respectively. Predictive modelling showed that COD and TOC could be effectively analysed using linear or logarithmic transformations, while TSS (93% reduction in HumeGro samples) demonstrated variability that did not conform to linear models. TSS consists of particulate matter with varying characteristics such as size, composition, and density, which can lead to inconsistent responses during ozone treatment. Consistent trends were observed in repeat experiments with surface water. In conclusion, the integration of COD and TOC as indicators, combined with the strategic modelling of ozone doses, underscores the importance of a data-driven approach in optimising the DBD plasma ozonation system. This not only enhances the treatment efficiency but also aligns with broader environmental goals of reducing organic pollutants in water systems. However, several factors require further optimisation, including the filtration of TSS and the reaction time. Additionally, the current setup must undergo scaling up before it can be deemed suitable for point-of-use (POU) implementation.
Environmental and thermal aging studies on plastics and biopolymers : changes in mechanical properties and emissions of volatile organic compounds (VOCs)
Nzimande, Monwabisi Cyril (University of Pretoria, 2024-07)
Plastics, especially biopolymers, degrade in response to environmental conditions such as UV radiation, heat, and moisture, resulting in surface changes and decreased in mechanical properties. As biopolymers replace traditional plastics, evaluating their long-term environmental impact is critical. This study aims were to investigate the effects of extended environmental aging on the surface and mechanical properties and the emission of volatile organic compound of biopolymers, including polybutylene adipate terephthalate (PBAT), poly (lactic acid) (PLA), polybutylene succinate (PBS) and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH).Biopolymers samples were prepared by melting and compounding in a twin-screw extruder, followed by injection moulding into dumbbell-shaped specimens. The changes in the structural, chemical, thermal, and mechanical characteristics of the biopolymer before and after were investigated by FT-IR, TS, TG-FTIR and Py-GC/MS. The visual appearance of biopolymers after exposure to accelerated aging for 1000 hours was studies. Photo-oxidation led to the formation of chromophores and double bonds, leading to PBAT, PHBH and PBS change color from white to yellow. PLA, which was before transparent due to its amorphous structure, became opaque white because of UV-induced cold crystallization. After 1500 hours of aging, tensile strength in PBAT increased by 12%, while for PBS and PHBH tensile strength decreased by 72% and 12%, respectively. PLA showed the highest decrease after 1000 hours due to UV-induced photodegradation and hydrolysis. Strain at break decreased significantly in PBS (98%), PBAT (64%), and PHBH (60%) after 1500 hours. FTIR spectroscopy confirmed the presence of carbonyl groups in PBS, PLA, and PBAT by revealing C=O and C-O bond peaks at 1710 cm−1 and 1046-1100 cm−1, respectively. To evaluate the effect of photooxidation, the carbonyl index was measured using FTIR analysis, which effectively measures oxidative degradation. The carbonyl index of the biopolymers decreased with aging indicating a gradual decrease in oxidative breakdown. TG-FTIR of biopolymers showed the present of degradation products majorly oligomers, unsaturated carboxylic acids, CO2, CO, and H2O. VOCs analysis revealed that no toxic emissions such as benzene or toluene were emitted from the recycled plastics before and after aging. The study showed that the highest concentration of VOCs was 3-formyl-benzoic acid in PBAT, 2-butenoic acid in PHBH, 2,5-furandione in PBS, and 1,4-dioxane-2,5-dione in PLA. The absence of harmful substances indicates that emissions are within acceptable levels, showing Py-GC/MS's efficiency in monitoring environmental safety.
A comparative study of adsorbents derived from coal fly ash for the adsorption of tetracycline from aqueous solutions
Houghton, Eric Emmanuel (University of Pretoria, 2024)
Emerging contaminants (ECs) like tetracycline (TC) are generated from various human activities, such as pharmaceutical manufacturing, agricultural runoff, and wastewater discharge. The presence of TC poses severe risks to the environment and public health. Various methods have been proposed for removing TC from water, such as chemical precipitation, membrane filtration, and advanced oxidation processes. However, adsorption has gained prominence as an effective method for removing tetracycline due to its simplicity, cost-effectiveness, and ability to achieve high removal efficiencies. Fly ash is a hazardous byproduct of coal combustion, often discarded in landfills, posing environmental risks. However, it can be repurposed into low-cost adsorbents through various modifications and treatments. Utilizing fly ash-derived adsorbents for tetracycline removal not only helps reduce waste but also provides an eco-friendly and cost-effective alternative to conventional, more expensive adsorbents. This study aimed to develop and evaluate various fly ash (FA)-derived adsorbents for TC removal, leveraging waste materials for environmental sustainability. Firstly, FA was acid-treated with hydrochloric acid to produce acid-modified FA (AM-FA). Secondly, FA was base-treated with sodium hydroxide to yield base-modified FA (BM-FA). Additionally, Zeolite Na-P1 (ZNa-P1) was synthesized from FA using hydrothermal treatment. These three adsorbents were subjected to adsorption tests to compare their adsorption performance. Furthermore, silica nanoparticles (SiNPs) were derived from FA (FA-SiNPs) and subsequently FA-SiNPs was doped with iron to create Fe-SiNPs. For comparison, silica nanoparticles were also synthesized directly from a pure sodium silicate solution (SSSNPs). All silica nanoparticle-based adsorbents (i.e. FA-SiNPs, Fe- SiNPs, SSSNPs) underwent adsorption tests to compare their adsorption efficiency. The comparative adsorption test among FA, AM-FA, BM-FA and ZNa-P1 revealed that BM-FA and ZNa-P1 removed 76 % and 90 % of TC, respectively, compared to 35 % with unmodified FA. AM-FA had the lowest performance, removing just 11 % of TC. ZNa-P1's superior performance was linked to its high zeolite purity, with a high cation exchange capacity (CEC) of 6.37 meq/g and a surface area of 35.7 m2/g. BM-FA, had a larger surface area of 110.8 m2/g, but exhibited a lower CEC of 3.42 meq/g. The adsorption efficiency of these adsorbents was more closely related to CEC than surface area. Optimal TC removal with ZNa-P1 was achieved at 7.5 g/L dosage and pH 5. The adsorption of TC on ZNa-P1 followed pseudo-second-order kinetics and the Langmuir isotherm model, with a maximum capacity of 46.34 mg/g at 30 °C. Thermodynamic studies with ZNa-P1 indicated that the process was spontaneous and endothermic. The adsorption mechanism of TC on ZNa-P1 involved ion-exchange, hydrogen bonding, and electrostatic attraction. The comparative adsorption tests among FA-SiNPs, SSSNPs, and Fe-SiNPs revealed that Fe-SiNPs demonstrated superior performance, removing 59 % of tetracycline, compared to 30 % and 20 % removal by FA-SiNPs and SSSNPs, respectively. The enhanced removal efficiency of Fe-SiNPs was attributed to the iron content, which facilitated TC adsorption through chelation. Optimal TC removal using Fe-SiNPs was achieved at a dosage of 5 g/L and within a pH range of 4-5. The adsorption of TC on Fe-SiNPs followed Elovich kinetics and the Langmuir isotherm model, with a maximum capacity of 32.31 mg/g at 30 °C. Thermodynamic studies with Fe-SiNPs indicated that the adsorption process was spontaneous and exothermic. The adsorption mechanism of TC on Fe-SiNPs was chemisorption involving electrostatic attraction and hydrogen bonding. This study highlights the potential of FA-derived adsorbents, particularly ZNa-P1 and Fe-SiNPs, as sustainable solutions for removing TC from contaminated water. The findings contribute to advancing waste utilization strategies particularly, adsorption, for environmental remediation.
Acetylated nanocellulose as a reinforcement for cellulose acetate
Denton, Christopher David (University of Pretoria, 2025-01)
Current research has shown that the use of cellulose nanofibres (CNFs) can be used in the reinforcement of biodegradable plastics. Problems arise with certain polymers, such with cellulose acetate (CA), due to agglomeration of the nanofibres. Hydrogen bonds will form between cellulose fibres in a process called hornification, which results in a poor-quality material and a waste of CNFs. It is thus necessary to modify the fibres such that they disperse into the CA matrix. The partial acetylation of CNFs can achieve this without destroying the fibrous network. However, there is a problem with this solution. The fibres when made can contain above 75 % water and have to remain in suspension or else irreversible agglomeration will occur. A recent discovery shows that xanthan gum can prevent this from happening by acting as a capping agent for the fibres preserving the fibrillated network during drying. Since the acetylation process can be affected by the presence of water this allows for a simple solvent swap to a solvent that does not affect the reaction. It is first necessary to demonstrate that acetylated CNFs will perform as expected without complicating the methodology. CNF acetylation was carried out through the use of acetic anhydride after the nanocellulose solvent was swapped for acetic acid. Azeotropic distillation was used to ensure that as much water as possible is removed from the CNF. Acetylation was confirmed through FTIR analysis quantitatively compared the peaks from the double bonded oxygen in the acetyl group. In testing, various CNF contents and degrees of acetylation were used and in total sixteen different combinations were incorporated into CA films which were made through the solvent casting. All films were plasticized to 25 % with triacetin and solvent cast in watch glasses. Microscopy images of the films revealed that acetylation of the CNF can reduce the agglomeration of the fibres by 400 % in the CA matrix, a clear indication that acetylation limits the hornification of the CNF. TEM imaging also shows the improved dispersion of the fibres iii in deliberately collapsed CNF samples. Other experimental procedures showed that the modified films will have improved optical transparency and mechanical properties. UV-vis showed an acetylated CNF had 35 % less absorbance compared to its unmodified counterpart. The effect on the Youngs modulus from the CNF can also be increased by over 250 % through modification. Improvements in the tensile strength were limited due to CA already having a high tensile strength. Effects on the rheology and viscosity of the modified CNF still need to be researched to gain a better understanding of the process on the materials shear thinning ability. This research has demonstrated that a biodegradable fibre reinforcement material can be used in certain polymers which previously posed a problem due to poor dispersion. This opens the door for more biodegradable polymers to be used in industry without the worry of the materials not being tough enough. There is potential for further development in this area, especially around polyhydroxyalkanoates, but the ability to minimise the fibre diameter is a big step in the right direction.
Laccase immobilized on metal-polymer composites for wastewater treatment and sensing applications
Kyomuhimbo, Hilda Dinah (University of Pretoria, 2024-10)
This study explores the multifunctional applications of laccase, a versatile multi-copper enzyme found in fungi, plants, and bacteria. Laccase facilitates the direct reduction of molecular oxygen to water while oxidizing various electron donors, making it appealing for biotechnological applications across sectors such as food, paper and pulp, wastewater treatment, pharmaceuticals, and biosensing. Despite its advantages, challenges related to high costs, instability under harsh conditions of pressure and temperature, and non-reusability in continuous processes necessitate the exploration of enzyme immobilization techniques. In this thesis, laccase enzyme was immobilized on metal-polymer composites comprising either zinc oxide nanoparticles (ZnONPs) or silver-doped ZnONPs (Ag@ZnONPs) embedded in chitosan, polyvinylpolypyrrolidone (PVPP) and polyaniline (PANI) polymers. Characterization studies, including UV-Vis spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD), demonstrate that Ag doping increased the surface area and reduced the bandgap energy without significantly altering nanoparticle size. The resulting polymer composites exhibit high stability and surface area, enabling effective dye adsorption and immobilization matrices for laccase enzyme. The composite beads were studied to remove four dyes: Bismarck brown, orange G, brilliant blue G, and indigo carmine from sewage wastewater and papermill industrial effluent. Applying these composite beads showcases their efficacy with an efficiency of over 90% and 70% chemical oxygen demand (COD) and over 60% and 80% dye removal from wastewater and papermill effluent respectively. Insights from liquid chromatography-mass spectrometry (LC-MS) indicate that laccase facilitated the cleavage of azo bonds and the formation of smaller compounds that were further mineralized by Aspergillus sp. that only thrive on the degradation by-products. Additionally, laccase-activated composites demonstrated significant antibiotic degradation capabilities, removing over 20% more tetracycline and ciprofloxacin when laccase was included, and exhibiting altered degradation pathways with reduced antibiotic activity over time. Finally, the development of a laccase-immobilized ZnO-polyaniline (PANI) nanocomposite biosensor for detecting CTAB highlights the potential for innovative analytical techniques in environmental monitoring. The biosensor exhibited a high sensitivity and wider dynamic linear detection ranges of 0.5 – 100 µM, 200 – 500 µM and 700 – 1900 µM. Overall, the findings underscore the potential of laccase and its immobilization strategies for sustainable biotechnological applications across various domains.
The effect of layered double hydroxides on the vulcanisation and properties of elastomer compounds
Jones, Louise (University of Pretoria, 2025-02)
Elastomers such as natural rubber (NR) and solution styrene butadiene rubber (SSBR) are widely used, especially in the tyre industry. These elastomers are crosslinked, most often via sulphur vulcanisation, in order to achieve the desired mechanical properties. In recent years the use of ZnO as a crosslinking agent in sulphur vulcanisation has been cause for concern due to the fact than Zn can be harmful to aquatic life. As such, alternatives to ZnO and methods to reduce the amount of Zn present in the rubber are desirable. In this work, layered double hydroxides (LDHs) were investigated as alternative crosslinking agents to ZnO in order to either reduce or eliminate Zn in the vulcanisation formulation. Three different stearate modified LDHs, namely CaAl-St, CaZnAl-St and CaFeAl-St were synthesised via one-step coprecipitation and added to NR, synthetic isoprene rubber (IR) and four different SSBR grades (one unfunctionalised and three functionalised grades) at different concentrations in the place of ZnO and stearic acid. The trimetal LDHs were synthesised by making a 10 mol % substitution of Ca with either Zn or Fe. The LDH synthesis was successful. XRD showed the characteristic LDH peaks and FTIR supported these results. In the case of the stearate modified LDHs, XRD showed that there was a clear shift in the primary peak to a lower 2θ value compared to the unmodified LDH, indicating that the basal spacing was increased and that the stearate anions were successfully intercalated into the LDH. SEM showed the presence of hexagonal platelets characteristic of LDH. Amorphous material was also present. In the NR and IR study, it was found that these LDHs are not satisfactory crosslinking agents. Rheology data showed that the samples cured with LDH did not reach nearly the same level of crosslinking as the samples cured with ZnO and stearic acid, even when Zn was present in the LDH. The results indicated that a higher concentration of Zn is required in these elastomers for sufficient crosslinking to take place. The mechanical properties also reflected the low crosslink density. The LDH-cured samples had tensile properties similar to that of NR and IR cured with only sulphur and an accelerator. In SSBR the LDH behaved differently. It was found that even 3 phr CaZnAl-St was sufficient for crosslinking, since rheology data showed that those samples reached similar levels of cure compared to the samples cured with ZnO and stearic acid. This equates to approximately 54 times less Zn in the vulcanisation formulation compared to the standard. Increasing the LDH concentration slightly increased crosslink density. In two of the functionalised grades, it is also possible to completely eliminate Zn, as the samples cured with CaFeAl-St reached similar levels of cure as the reference sample. In one of the functionalised grades CaAl-St also reached a similar level of cure. This indicates that there is potential for further investigation on how Zn-free LDH could be used to crosslink some SSBR grades. The mechanical property data showed that the LDH cured samples had tensile behaviour similar to the reference samples, indicating that the presence of LDH does not negatively affect the mechanical properties, nor does it affect properties such as glass transition temperature significantly. This work therefore highlights the potential of using CaAl-based LDHs as alternatives to ZnO in SSBR, thereby reducing the amount of Zn required for sulphur vulcanisation. Further work is required to study the effect of the LDHs in SSBR filled with reinforcing fillers such as silica, which could enable the use of these LDHs in a commercial tyre formulation in future.
Design, construction and commissioning of a counter-flow rotary pyrolyser
Baasch, William Peter (University of Pretoria, 2024-09-25)
Biochar is a sustainable carbon sequestrate and a strong contender for renewable carbon based materials. Biomass is a key commodity which should be exploited in the pursuit of the energy transition due to the wide variety of fossil fuel-like derivatives it can produce. The conversion of biomass to value-added products usually focuses on pyrolysis vapour and noncondensable gases because of their properties and possible use cases. In general, biochar is typically consumed in the process, via combustion, to improve the energy efficiency of these processes. Biochar has a variety of uses that are often is regarded/sacrificed in favour of the aforementioned pyrolysis products, and therefore further analysis and research into biochar would not only benefit the biochar value chain but also aid in understanding the technical and economic uses of the solid pyrolysis product. Extensive research has been done on biochar and its potential usage, but further practical research into biochar reactor design, construction, and commissioning allows the exploration of biochar generation and the quality of the final product. This will also allow the evaluation of the commissioned experimental apparatus and what further may be modified to produce increased product generation rates and higher product quality. Ignoring biochar as a value-adding product could result in research that develops a suboptimal solution to the energy transition. While biochar may not pose immediate benefits to the pyrolysis process besides additional energy-generating capacity via combustion the resulting product has potential in various areas of great value and therefore additional research into the manufacturing of this value-adding product would be of great benefit. For the investigation of biochar as a value-adding product, the requirement to produce a potentially large amount of repeatable samples was identified and therefore the requirement for a biochar-focused pyrolysis reactor was identified. The Rotary Kiln Pyrolyser (RKP) In-situ Activation (ISA) was designed, constructed, commissioned and operated. Throughout the exploration of this research topic the design, construction, commissioning and operation of a biochar-focused reactor was a major objective. This objective was much more difficult to achieve due to the complexities of the processing material and the products produced. Therefore, three prototype versions of the reactor (RKP-ISA) were constructed. The first rendition was prototype 1 where areas of improvement were identified and the lessons learnt from the execution of the first prototype were implemented on the second prototype. Thereafter the second prototype was analysed and further modifications were made to improve the operation and quality of the products produced. An investigation of the reactor performance was completed via the analysis of the produced biochar samples. Experimental runs were completed utilising the prototype 2 reactor and the quality of the resulting biochar was determined through thermogravimetric and elemental analyses. The biochar analysis indicated that further improvement to the prototype 2 reactor could be implemented to produce improved samples. The modifications made to the reactor resulted in improved biochar stability and quality. The body of work completed demonstrates the advancement of the reactor design and operation, but further investigation can still be completed regarding the modification of biochar for specific use cases and the scaling up of the established process for industrial application.

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