3. Electronic Theses and Dissertations (ETDs) - All submissions

Permanent URI for this community

https://wiredspace.wits.ac.za/handle/10539/45

Browse

Now showing 1 - 20 of 24

An investigation on the presence and characteristics of microplastics in freshwater sediment in the Vaal River
(2022) Ndlovu, Michelle
Microplastics (MPs) have become a bane on the environment for decades, and their impact on freshwater systems is still understudied. With the advancement in research techniques, MPs have been detected in the air, tap water, underground water, food, soil, and human stool. Their potential harmful impacts have drawn significant attention in recent years in the context of environmental protection. In comparison to marine waters, studies of MPs in freshwater sediment are limited. A thorough review of the literature on MPs in freshwater sediment also revealed a lack of standardisation in MP studies, ranging from the true definition of MPs to sampling and analytical methods. In addition to the unavoidable and potentially irreversible ecosystem imbalances caused by MP pollution, the establishment of environmental standard benchmarks and coordinated responses is also hampered by a lack of knowledge on their abundance, distribution, and fate. As such, this study aimed at generating scientific evidence to provide evidence-based recommendations and risk-informed standards to influence and guide policymaking. This project was designed to generate new data on the presence and characteristics of MPs pollution in South African freshwaters, as well as assessing the contribution of land-based activities to MPs pollution in freshwater bodies using the Vaal River as a study case. The main objectives were to determine the presence, abundance, and physical and chemical characteristics (size, shape, colour, and polymer composition) of MPs in sediment samples from the Vaal River. Twenty-five sediment samples were collected from the Vaal River over 3 days using a 500 mL Van Veen grab sampler. Prior to analysis, samples were pretreated for effective MPs extraction and identification. Sample preparation and extraction were performed using modified National Oceanic and Atmospheric Administration (NOAA) laboratory methods for the analysis of the MPs. Samples were first oven-dried to remove moisture, digested using 10% potassium hydroxide (KOH) to remove organic and inorganic matters which may hamper MPs identification, after which, MP particles were separated based on their density. Density separation was conducted using saturated sodium iodide solution (NaI). For MPs analysis, optical microscopy (stereomicroscope) was used to detect and identify MPs in terms of size, shape, and colour. Scanning electron microscopy (SEM) was used to study the surface morphology of the MP particles. The polymer composition was determined by Raman Spectroscopy to identify polymer type. MPs were detected in all samples with varying abundances from 30 to 1096 particles/kg_dry weight (dw) and a mean abundance of 463 ± 284 particles/kg_dw. The examination of the physical properties of MPs revealed a predominance on fragments (63%) and fibers (35%), small-sized particles of less than 0.5 mm (32%), as well as the prevalence of coloured MPs (60%), mostly blue, white, and green. Detected MPs were identified as Polyethylene (PE) “both high and low density”, polypropylene (PP), and polyethylene co-vinyl acetate (PEVA), polyester (PES), polyurethane foam (PU), and polyethylene/hexene-1-copolymer (PEH), according to Raman Spectroscopy analysis. These findings reveal elevated levels of MP contamination within the Vaal, with the prevalence of fragments and fibers (98%) over pellets (2%) indicating that MPs pollution is from secondary sources. Fragments are formed by the breakdown of larger plastic pieces; fibers, on the other hand, are mostly attributed to domestic sewage, and, in this case, it is likely the protracted sewage input around the Vaal triangle. The prevalence of PE, PP, and PEVA is consistent with MPs from secondary sources as they are commonly used in single-use plastic, packing bags, textiles, and containers. These findings provide a benchmark for guiding future monitoring studies and identifying research gaps. In general, anthropogenic activities, including heavy industrial and tourism activities in the area, agriculture, fishing, and wastewater discharge, were identified as potential sources of MPs in this study. The pollution profile, in terms of the physical and chemical properties of the detected MPs, indicates serious threats and health risks. For instance, small-sized MPs that were found to be predominantly abundant in the sediment samples, are considered to be more harmful to aquatic organisms due to their bioavailability (size similarity to natural preys). The large surface area of small MPs also increases the possibility of adsorbing and desorbing other pollutants from the surrounding environment. Coloured MPs may pose additional harm due to the toxicity of most colorant agents. Thus, the high prevalence of small-sized and coloured MPs within the Vaal River is therefore of a particular concern.
Colloidal synthesis of MoSe2, ReSe2, and SnSe2 nanomaterials and their carbon nanocomposites for applications in hydrogen evolution reactions
(2022) Ndala, Zakhele Bafana
The work herein focuses on the synthesis of MoSe2, ReSe2, and SnSe2 nanostructures using the colloidal method. This work is aimed to show that the colloidal method can be used to produce transition metal dichalcogenide (TMD) nanostructures with excellent catalytic activity toward the hydrogen evolution reaction (HER). Moreover, this work demonstrates that the method can be used to incorporate various techniques that are used to improve the catalytic activity of TMD nanostructures. The catalytic activity of the TMD nanostructures was improved by introducing Se vacancies, chemical doping, increasing exposure of active edge sites, and forming hybrid nanostructures with carbon nanomaterials. The colloidal method was successfully used to synthesize MoSe2, ReSe2, and SnSe2 nanostructures. The MoSe2 and ReSe2 were synthesized using selenium powder as the selenium precursor and oleic acid as the solvent/surfactant. SnSe2 was synthesized using selenium powder as the selenium precursor and olelyamine as the solvent/surfactant. The MoSe2 and ReSe2 nanostructures formed spherical structures flowerlike morphologies that were composed of few-layer nanosheets. The SnSe2 nanostructures formed into 2D nanoplates. The onset potential and overpotential of the MoSe2 nanostructures were recorded as 108 mV and 313 mV respectively. The onset potential and overpotential of the ReSe2 were measured to be 168 mV and 331 mV respectively. These materials were used as a baseline and attempts were made to improve their catalytic activity. A change in the selenium precursor from selenium powder to selenourea was shown to result in the introduction of Se vacancies in the MoSe2. This in turn resulted in improved catalytic activity towards the HER, which was attributed to an increase in the number of active sites provided by the Se vacancies. The onset potential and the overpotential of the SnSe2 nanostructures were measured to be 319 mV and 618 mV respectively. However, the SnSe2 nanoplates were electrochemically activated through H+ intercalation, and the catalytic performance of the nanostructures drastically improved. The electrochemically activated SnSe2 nanoplates exhibited exceptional improvements in catalytic performance with an onset potential of 141 mV and an overpotential of 289 mV. The pristine SnSe2 was used as a baseline and attempts were made to improve the catalytic activity of these materials. The effect of surface functionalization of the nanostructures on their catalytic activity toward the HER was studied. Three solvents/surfactants commonly used in colloidal synthesis were studied. These were oleylamine (OLA), oleic acid (OA), and trioctylphosphine oxide (TOPO). The surfactants interacted differently with the TMD nanostructures, but a common observation v was made. The use of TOPO instead of OA which was initially used resulted in an improvement of the catalytic activity. The TOPO synthesized ReSe2 nanostructures had a reduced onset potential and overpotential of 73 mV and 171 mV respectively. The TOPO synthesized MoSe2 nanostructures also had a reduced onset potential and overpotential of 297 mV and 193 mV respectively. The TOPO synthesized SnSe2 had improved catalytic activity with an onset potential and overpotential of 229 mV and 569 mV respectively. This improvement in the catalytic activity was attributed to the degree of passivation on the surface of the nanostructures. The computational studies on the ReSe2 nanostructures showed that OA and OLA result in a high degree of passivation of the nanostructure surface compared to the TOPO, this results in the surfactants blocking more of the active sites which negatively impacts the catalytic activity. TOPO is a much bulkier surfactant, which results in a lower degree of surface passivation. Hybrid nanostructures of the TMDs and carbon nanostructures were produced using colloidal synthesis. Pristine and nitrogen-doped reduced graphene oxide was used for the study. This was done to increase the electrical conductivity of the TMD nanostructures, which would, in turn, result in increased catalytic activity towards the HER. Few-layered ReSe2 nanostructures were grown on rGO (ReSe2-rGO) and N-rGO (ReSe2-N-rGO). The catalytic activity of the ReSe2 improved after incorporating the nanostructures on the carbon nanostructures. The ReSe2-N-rGO had higher catalytic activity than the ReSe2-rGO, this was attributed to the improved electrical conductivity of the N-rGO provided by the nitrogen doping. The onset potential and overpotential of ReSe2-N-rGO were measured to be 115 mV and 218 mV respectively, which were much improved from pristine ReSe2 nanostructures. The SnSe2-NrGO also showed some improvement in the catalytic activity of the nanostructures. However, the MoSe2-N-rGO did not show any improvement and the catalytic activity worsened. This was attributed to the interaction of the MoSe2 with the N-rGO nanosheets, the MoSe2 nanostructures grew independently to the N-rGO nanosheets which were in stark contrast to the ReSe2-N-rGO nanostructures. This limited the interaction of the MoSe2 with the N-rGO and resulted in impaired catalytic activity. Alkali metal doping was used to improve the catalytic activity of SnSe2 nanostructures. Potassium was used to dope the SnSe2 nanostructures. The doping was confirmed using powder X-ray diffraction, X-ray fluorescence spectroscopy, and UV-vis spectroscopy. The potassium doped SnSe2 nanostructure showed an improvement to the catalytic activity compared to the pristine SnSe2 nanostructures. The onset potential and overpotential of the doped nanostructures were measured to be 265 mV and 385 vi mV respectively. This improvement was attributed to the introduction of new active sites on the SnSe2 nanoplates through potassium doping. This was confirmed using the electrochemically active surface area (ECSA) which increased from 8.8 mF/cm2 in the pristine nanostructures to 11.7 mF/cm2 in the doped nanostructures. This work has successfully demonstrated that colloidal synthesis can be used to produce TMDs with excellent catalytic activity. The catalytic activity of some of these materials is comparable to the best catalyst of the same materials reported in the literature that are produced using other methods.
Design and synthesis of antifolates as potential antimalarial agents
(2024) Butsi, Kamogelo
Malaria still remains a life threatening disease in many parts of the world. According to the World Health Organisation (WHO) approximately 241 million cases of malaria were reported worldwide in 2020, with Africa being the most affected continent. Prevalence of the disease is attributed to the significant increase in resistance towards currently used chemotherapeutic agents. Therefore, the search for new antimalarial candidates is imperative. In this report, we give progress on the synthesis of novel 2,4- diaminopyrimidine derivatives and their respective activity against Plasmodium falciparum. We have successfully developed a synthetic route towards flexible 2,4- diaminopyrimidine derivatives, which entails the synthesis of these target compounds in five steps starting from commercially available esters. The Sonogashira reaction was the key step employed to functionalise the iodinated pyrimidine ring at the 5- position with a suitable acetylene to form an alkyne intermediate. In this way, a four atom linker was introduced between the heterocyclic ring system and the terminal phenoxy ring. Reduction of the triple bond in the alkyne intermediate by a hydrogenation reaction afforded a library of the desired flexible 2,4-diaminopyrimidine analogues. Of the synthesised compounds, 15 were sent for activity assessment against both wild type and mutant PfDHFR enzymes. They inhibited both PfDHFR enzymes at low nanomolar concentrations. Additionally, these compounds were also tested against the human DHFR to evaluate their selectivity. All tested compounds were selective for PfDHFR over human DHFR. Further assessment on the antiplasmodial activity of these compounds were conducted in vitro in a whole cell P. falciparum assay against the drug sensitive TM4/8.2 strain and the multidrug resistant strain V1S. The tested compounds displayed moderate antiplasmodial activity in the micromolar range with IC50 values of 0.28 – 23.4 µM and 17.`6 – 64.0 µM against the drug sensitive strain and multidrug resistant strain, respectively. Unfortunately, they were mild to highly cytotoxic towards VERO cells. Further interrogation of the selectivity index (SI) indicated that the tested compounds did not display any selectivity for the plasmodial parasite over VERO cells. Abstract iii Therefore, inhibition of the Plasmodium strains is not only due to compounds interacting with the parasite but also cytotoxicity of the compounds on the viability of erythrocyte.
Development of a novel passive sampler based on a combination of membrane assisted solvent extraction and molecularly imprinted polymer for monitoring of selected pharmaceuticals in surface waters
(2022) Khulu, Sinegugu
Pharmaceuticals are an important group of persistent emerging pollutants due to being continuously detected in the aquatic environment. Pharmaceuticals are compounds whose therapeutic effects enhance the health of individuals. However, these compounds have the potential to enter the environment as part of effluents from wastewater treatment plants while other sources include improper disposal of expired and unused medication and human excreta. Although these pollutants exist in trace levels (µg L-1 to ng L-1 ) in environmental samples, they have gained a lot of attention in the science community owing to the perceived detrimental effects on human health and ecotoxicological effects in aquatic life. As such, determination of pharmaceuticals in the environment has become an essential component in environmental monitoring studies. Various analytical techniques have been reported mainly grab sampling followed by solid phase extraction being the most reported. However, the challenge has been that these pharmaceuticals exist in trace levels which requires extremely sensitive approaches. At the same time, they exist as mixtures which requires the need for analytical methods which allows for multi-residue analysis at a time. One of the obvious choices of sampling is grab sampling that involves instantaneous collection of samples. The main challenge of such is that this requires large amounts of samples as well as potential to miss the episodic events of pollution. In this regard, recent studies now advocate for passive sampler-based approaches where a sorbent is deployed in the environment over a period of time. This allows for estimation of time-weighted average (TWA) concentrations which caters for episodic events of pollution. In this regard, the purpose of the current study was to develop an alternative passive sampling technique based on a combination of a molecularly imprinted polymer (MIP) sorbent and membrane assisted solvent extraction (MASE) for the determination of pharmaceuticals belonging to five different classes in surface water. The approach was to synthesize a smart polymer using molecular imprinting technology, place it inside a semi-permeable polypropylene membrane and finally place it in a protective chamber. The chamber and its content, now referred to as the passive sampler was then deployed under optimized conditions for monitoring of the five model pharmaceuticals belonging to different groups. The first part of the work involved synthesizing a smart polymer, the MIP for efficient and selective extraction of pharmaceuticals belonging to different groups. This was done by selecting an appropriate template for molecular imprinting process. Cavity tuning experiments which involved the utilization of all the target pharmaceuticals whether as single or multi-template were conducted. The venlafaxine imprinted polymer was successfully selected based on its cross-selectivity for the selected pharmaceuticals. The synthesized polymer attained maximum matrix-matched adsorption capacities ranging from 206 to 418 ng mg-1 for individual pharmaceuticals within 80 min. Batch adsorption and kinetic studies indicated that the binding of the selected pharmaceuticals on the MIP particles resulted in multiple interactions through chemisorption. An analytical method for determination of the target pharmaceuticals was successfully developed using liquid chromatography-mass spectrometry (LC-MS), giving detection limits ranging from 0.03 to 0.31 ng mL-1 and quantification limits ranging from 0.12 - 3.81 ng mL-1 for individual pharmaceuticals. The venlafaxine imprinted polymer was further applied as a selective sorbent for solid phase extraction of an antiretroviral (nevirapine), an antidepressant (venlafaxine), a muscle relaxant (methocarbamol), an anticonvulsant (carbamazepine) and a cardiac stimulant (etilefrine) in dam water samples, yielding recoveries ranging from 43 - 69%. This preliminary data indicated that MIP cross selectivity can be an essential and attractive approach in the monitoring of organic pollutants belonging to different classes in environmental water bodies. This work, presented as Paper 1 in the thesis was published in Polymer Bulletin Journal. The synthesized venlafaxine imprinted smart polymer was then used in combination with a membrane assisted solvent extraction technique, referred to as MASE-MIP for the extraction of these compounds in complex environmental water samples. The MASE-MIP combination utilized the cross-selectivity of the synthesized venlafaxine viii MIP whilst preventing co-extraction of larger molecules to yield cleaner extracts and increased selectivity. After efficient extraction, the sample extracts were analyzed using liquid chromatography-quadrupole time-of-flight mass spectrometry (LCqTOF/MS). The MASE-MIP was optimized for various significant experimental parameters such as the influence of the sample salt content, the stirring rate, the stirring time and the amount of MIP using a central composite design. Optimum extraction conditions for a sample volume of 18 mL were found to be 5 g of salt content, a stirring rate of 400 rpm, an extraction time of 60 min and 50 mg of MIP, yielding good extraction efficiencies ranging from 38 – 91% for individual pharmaceuticals. The optimized MASE-MIP-LC-qTOF/MS method yielded detection limits in the range of 0.09 to 0.20 ng mL-1 and quantification limits ranging from 0.31 to 0.69 ng mL-1 for individual pharmaceuticals. Furthermore, the optimized extraction method was applied in environmental monitoring of selected pharmaceuticals in two important rivers in South Africa. All selected model compounds were detected in the water samples at concentrations ranging from 0.19 to 2.48 ng mL-1 . This illustrated emphasis of a need to continuously monitor the presence of these compounds in environmental waters. The monitoring could be done through the proposed analytical method which has proven to be precise and accurate. This work, presented as Paper 2 in this thesis has been published in Chemosphere journal. The developed MASE-MIP technique was further used in combination with the passive sampling technique to form a MASE-MIP based passive sampler for extraction and monitoring of selected pharmaceuticals in environmental water bodies. This technique utilized the cross-selectivity of the synthesized MIP, the size exclusion and protective membrane and allowed for preconcentration of the target pharmaceuticals into the green receiver solvent (ionic liquid). The passive sampler approach was based on allowing the targeted compounds to diffuse selectively in an integrative manner through the polypropylene membrane which housed an ionic liquid as a green receiving solvent and a MIP. Upon successful diffusion, the analytes were selectively adsorbed by a MIP. The technique was optimized for parameters such as effects of biofouling, ix deployment time and solvent type for the receiver phase. Furthermore, the passive sampler was calibrated in laboratory-based experiments to obtain sampling rates (Rs) for each target pharmaceutical with the view to attain estimated time weighted average (TWA) concentrations of the targeted pharmaceuticals in environmental waters. The optimum matrix-matched sampling rates obtained ranged from 0.0007 - 0.0018 L d-1 for individual pharmaceuticals, whilst the method detection and quantification limits ranged from 2.45 - 3.26 ng L-1 and 8.06 - 10.81 ng L-1 , respectively. Upon deployment in a dam situated in a highly populated township in South Africa, only etilefrine and methocarbamol were detected and quantified at maximum TWA concentrations of 12.88 and 72.29 ng L-1 , respectively. This work is well presented as Paper 3 in this thesis. Paper 3 is a manuscript under review in Water Research journal. The MASE-MIP based passive sampler showed proven ability to selectively extract targeted pharmaceuticals prior to their determination using LC-qTOF/MS. In this case, the presented experimental procedures allow for detection of trace level environmental concentrations of the targeted pharmaceuticals, making them suitable alternative analytical methods that can be utilized for monitoring of these compounds in environmental water bodies.
Development, validation, quantification and applications of refinery primary reference gas mixtures using various analytical techniques
(2024) Mphaphuli, Gumani Ethel
The carbon-intensive energy economy of South Africa has increasingly become an environmental issue contributing to climate change, with little study done on the refinery gases contribution to air pollution in the country. Refinery gas is a mixture of complex hydrocarbons and non-combustible gases recovered from refining and conversion processes in the fuel industry, with some intermediate products used as energy sources. The sale of these fuels often follows the guidelines of carbon trading schemes as stipulated by the environmental protection frameworks. Furthermore, refinery gas measurements are challenging because of plausible cross-interferences among target gases during preparation and value assignment. Thus, development of technical capabilities for reliable measurements and reference materials for refinery reference gas mixtures is crucial for measurement solution in the energy industry. This study focused on the development of complex multicomponent refinery gas mixtures with target gases of carbon dioxide (CO2), carbon monoxide (CO), oxygen (O2), nitrogen (N2), methane (CH4), ethane (C2H6), propane (C3H8) and 1,3-butadiene (C4H6) in helium (He) at two amount fractions ranges of 1 000 to 4 000 µmol.mol-1 (low range) and 1 to 22 %mol.mol-1 (high range) to shorten the traceability chain while developing technical skills and measurement equivalence. This was achieved through purity analysis of starting materials in accordance with International Organization for Standardization (ISO) 19229:2019 and gravimetric preparation in accordance with ISO 6142:2015. Verification of gas mixtures was done using gas chromatography with thermal conductivity detection for the quantification of components CO, CO2, O2 and N2 and flame ionisation detection for the quantification of components CH4, C2H6, C3H8, and C4H6 in accordance with ISO 6143:2001. Quantification of trace-level impurities during purity analysis was performed on a gas chromatography coupled with a flame ionisation detector, a pulse discharge helium ionisation detector, and a thermal conductivity detector. The final purities of the starting materials were successfully quantified at > 99.99 %mol.mol-1 for iv C2H6, C4H6, and C3H8 and > 99.999 %mol.mol-1 for CH4, CO, CO2, O2, N2 and He. Two ranges of refinery reference gas mixtures were prepared using static gravimetric method. The high range gas mixtures were prepared using a singlestep dilution method, while the low range gas mixtures used a multi-step dilution method that posed challenges related to the pressures, purities, and chemistries of the starting materials. Single-point calibration method was deployed for the verification of the matrixmatched refinery reference gas mixtures. The final percentage relative expanded uncertainty ranged from 0.313 to 3.281% for hydrocarbons (CH4, C2H6, C3H8, and C4H6) and 0.150 to 3.045% for CO, CO2, N2, and O2 at 95% confidence level (k = 2). The analytical method using gas chromatography coupled with various detectors developed in this study was successful in corroborating the gravimetric amount fractions and the verification amount fractions for the specified components of interest in the developed refinery reference gas mixtures accurately within a 1% difference.
Electrocatalytic detection of drugs of abuse using onion-like carbon based electrocatalysts
(2024) Ehirim, Tobechukwu Justice
Substance abuse is a serious problem worldwide. Among abused substances, tramadol and alcohol are one of many. There is an urgent need to use electrochemical method for their detection since electrochemistry methods are simple, low-cost, high sensitivity and can easily be miniaturised. This PhD work reports the first investigation on the application of nanodiamond-derived onion-like carbons (OLC) and conductive carbon black (CB) as (i) electrocatalysts for the detection of tramadol, and (ii) as support materials for nano-sized palladium electrocatalysts (Pd/OLC, Pd/CB, Pd-CeO2/OLC) for the detection of ethanol. The catalysts were characterised with X-ray diffraction (XRD), Raman, Brunauer– Emmett–Teller (BET), Thermal gravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM). For the detection of tramadol, OLC gave the best sensing performance compared to CB. Theoretical calculations (DFT simulations) predict that OLC is better because it allows for weaker interaction energy with tramadol (Ead = -26.656 eV) than CB (Ead = -40.174 eV). OLC-modified glassy carbon electrode (GCE-OLC) shows a wide linear calibration curve (55 – 392 M), high sensitivity (0.0315 A /M), low limit of detection (LoD) and quantification (LoQ) of 3.8 and 12.7 M, respectively. OLCmodified screen-printed electrode (SPE-OLC) successfully detected tramadol in real tramadol drug and human serum. The OLC-based electrochemical sensor promises to be useful for sensitive and accurate detection of tramadol in clinics, quality control and routine quantification of tramadol in pharmaceutical formulations. For the oxidation and detection of ethanol, Pd/CB, Pd/OLC and Pd-CeO2/OLC, were studied as catalysts. In comparison, adding ceria (CeO2) to Pd/OLC, the performance was enhanced significantly than in carbon-only support for palladium. GCE/PdCeO2/OLC shows the best electrocatalytic performance (i.e., high current density, fast electron transport, etc). DFT calculation, supported by XPS and HRTEM data, predict that this high activity may be related to CeO2 modulating the electronic properties of the catalyst. GCE/Pd-CeO2/OLC gave wide linear range for ethanol sensing (38.5 – 286 mM), excellent sensitivity (0.00024 mA mM-1 ) and LoD of ~ 8.7 mM. The GCE/Pd-CeO2/OLC shows excellent potential for application in real samples of commercial alcoholic beverages and human serum, with satisfactory recoveries (89 – 108 %).
Enzyme-MOFs for biocatalytic degradation of organic pollutants
(2024) Dlamini, Mbongiseni Lungelo
The widespread use of pharmaceutically active compounds (PhACs) and the incidental discharge of endocrine-disrupting chemicals (EDCs) into surface water has resulted in the detection of such compounds in effluents of wastewater treatment plant effluents (WWTPs). This indicates that such contaminants are highly stable and recalcitrant, defying total elimination by traditional technologies. These compounds have been designated as emerging contaminants, and involuntary exposure to them may have negative consequences for humans and aquatic life. As a result, there is a need to investigate potential strategies for effectively eliminating such contaminants from water. Among the various remediation technologies, enzymatic biotransformation of these organic pollutants by oxidative enzymes is a green, cost-effective, and considerably sustainable approach. In this study, an Aspergillus-based laccase, immobilized on zeolitic imidazolate frameworks (ZIF) was investigated for its potential to biodegrade three model PhACs (carbamazepine (CBZ), diclofenac (DFC), and norfloxacin (NOR)), and dimethyl phthalate (DMP) - a potential endocrine-disrupting chemical. The laccase was immobilized via a facile one-pot biomineralization covalent binding procedure. The success of this attachment was confirmed by Fourier Transform Infrared Spectroscopy (FTIR) and Nuclear Magnetic Resonance (NMR) spectroscopy analysis, demonstrating peaks and proton shifts attributable to functional groups present in the protein structure of the enzyme. Furthermore, Powder XRay Diffraction (PXRD) analysis revealed a preserved crystallographic structure of the ZIF particles after the laccase attachment. Additionally, the characteristic rhombic dodecahedral morphologies of ZIF crystals were retained after enzyme attachment, confirmed by Scanning Electron Microscope (SEM) micrographs, with slightly agglomerated particles. The immobilized laccase (Lac-ZIF) demonstrated significant resistance to adverse environmental conditions such as extreme pH, elevated temperatures of up to 70 ℃ (relatively higher deactivation energies (Ed)), and harsh organic solvents in comparison to the free laccase. Also, the Lac-ZIF proved to be quite recyclable retaining high efficiencies (>70%) for up to 5 reuse cycles, as well as remarkable storage stabilities of up to 15 d. Even though the immobilization of the laccase resulted in diminished affinities for the substrates (analytes) as illustrated by increasing Michaelis constant (Km) values, the Lac-ZIF still outperformed the free laccase in the removal of all the investigated contaminants. This was attributed to the synergistic adsorption and biodegradation effects provided by the ZIF backbone and the attached laccase enzyme. For the optimization of operational parameters for DFC and NOR removal, the quadratic response surface methodology (RSM) regression model proved effective and viable with high accuracy as confirmed by experimental results. In this sense, the experimental values (93.5% for DFC and 94.9% for NOR) were found to be very close to the predicted removal efficiencies, 93.9 and 95.1% for DFC and NOR, respectively. Finally, the synthesized biocatalyst was used to decontaminate the investigated compounds in real river water samples, and it significantly outperformed the free laccase. Due to its remarkable attributes such as thermal and storage stabilities, strong pH resistance, recyclability, and high degradation potential, the Lac-ZIF is a potential candidate for application in greener and more effective wastewater remediation approaches.
Evaluation of membrane crystallization for the recovery of freshwater and mineral salts from high-saline wastewater
(2024) Chimanlal, Indira Camryn Theresa
Exploring drug delivery systems tailored for [Pt(phen)(acylthiourea)]+ complexes
(2022) Manama, Nthabiseng
Three [Pt(phen)(Ln -O,S)]Cl complexes (PtLn), where phen represents 1,10-phenanthroline and L n -O,S various N,N-di(alkyl)-N`-acylthiourea ligands were successfully synthesized and characterized with yields ranging from 66% to 75%. Generation 4 (G4), acetamide functionalized generation 4 (G4-Ac) and generation 5 (G5-Ac) PAMAM and folic acid functionalized generation 4 (G4-FA) PAMAM dendrimers were successfully synthesized with yields ranging from 80% to 94% and characterized using 1H NMR, 13C NMR, FT-IR and mass spectrometry (ESI and MALDI-TOF) where applicable. Host-guest adduct formation between G4 and PtLn complexes were challenging since G4 was found to exist as a polycation even near-neutral pH. The positively charged amine surface resulted in a small amount of complex degradation and inhibition of encapsulation due to electrostatic repulsion between the dendrimer and the cationic complex. To mitigate this, we acetylated the amine surface of the dendrimer and studied the host-guest interaction of this variant. The solvent was found to play an important role in the encapsulation process with DMSO-d6 showing the most promising results. Host-guest interactions of G4-Ac and PtL1 in DMSO-d6 were investigated in detail. Upon addition to G4-Ac, shifting of the aliphatic and aromatic peaks that belong to PtL1 were observed in the 1H NMR spectrum with a significant shift of 0.25 ppm and 0.26 ppm for Hd and Hd` of PtL1. At 300K, DOSY data showed the association between the complex and dendrimer with significant slower diffusion in the mixture (PtL1: 0.3 x10-10 m2 /s, G4-Ac: 0.2 x10-10 m2 /s) as compared to the free species (PtL1: 1.0 x10-10 m2 /s, G4-Ac: 0.4 x10-10 m2 /s). The observed 1H NMR chemical shifts and diffusion coefficient was a molar fraction-weighted average between the “bound” and “free” state of PtL1 which indicated that the dendrimer-complex interaction was in fast-exchange relative to the NMR time-scale. In efforts to further ascertain the extent of the interaction, viscosity measurements were conducted in order to calculate each respective hydrodynamic radii (RH) and volumes (VH) which will give a clear indication of host-guest association. The increase in viscosity for the mixture was attributed to an increase in concentration and increase in ionic strength since the complex is cationic. An 84% and 49% increase in VH was observed in PtL1 and G4-Ac respectively which indicates the formation of the host-guest complex. Dendrimer cavity size played an important role in encapsulation as NOE crosspeaks were observed between the aromatic protons of PtL3 and internal G5-Ac protons whereas none were observed between the complexes and G4-Ac. The mixture of [TBA]Cl and G4-Ac was briefly studied to investigate the effect of ionic strength on the dendrimers diffusion behaviour. An increase in viscosity was observed as a result of increased concentration and ionic strength. A 31% increase in G4-Ac VH was observed while a 40% decrease in [TBA]Cl VH was observed which provided evidence of dendrimer aggregation in the presence of a salt. We iv also investigated the host-guest interaction of well-known 5-Fluorouracil (5FU) as a positive control. Host-guest interactions of G5-5 Fluorouracil (5FU) indicated an electrostatic interaction as seen by the shifting of surface G5 protons (g` and f`) upon addition of 5FU as a result of proton abstraction from the weakly acidic 5FU by the basic surface amine groups of G5. The free complexes, 5FU (positive control), free dendrimers and dendrimer-drug mixtures were screened against A549 lunger cell line to evaluate their cytotoxic activity at 50 μM and 5 μM in DMSO-d6/D2O (6:1). The solvent displayed minimal cytotoxic activity which was attributed to the presence of deuterium as deuterated solvents were found to be more cytotoxic compared to their non-deuterated counterparts in literature. The free complexes and dendrimer-complex mixtures were highly active with less than 1% cell viability at 50 μM. Free 5FU displayed less activity with a cell viability of 62.3% while G5+5FU displayed improved activity with a cell viability of 19.2% at 50 μM. G4-Ac +PtL1 and G4-Ac +PtL2 displayed higher cytotoxicity compared to their free counterparts at 5 μM which could be indicative of effective drug delivery although the results are preliminary. PAMAM dendrimers were found to be biocompatible with a cell viability of 53.30%, 58.78% and 64.53% for G4-Ac, G5 and G5-Ac respectively at 50 μM and acetamide functionalized dendrimers displayed less cytotoxicity compared to the amine terminated dendrimers.
Fischer-Tropsch catalysis: osmium and platinum promoted cobalt supported in/on hollow carbon spheres
(2024) Molefe, Tshepo
The Fischer-Tropsch synthesis (FTS) is a process is used to convert traditional energy carriers such as coal, biomass, and natural gas into hydrocarbon products. The process involves a catalyst assistance in the conversion of syngas to hydrocarbons, which is usually composed of cobalt or iron supported on high surface area materials such as carbon-based materials, metal oxides, silica or alumina. In this thesis, the studies are focused on the fundamental processes involved in FT catalysts by using a hollow carbon sphere support. The study uses hollow carbon sphere (HCS) nanoreactors as the catalyst support and evaluates the effect of the nanoreactor's on the FT process, using Os and Pt nanoparticles as the reduction promoter for a Co catalyst encapsulated inside the HCSs. The synthesis and utilization of Os and Pt nanoparticles loaded on/in Co@HCS FT catalysts to give promoter-Co@HCS (e.g., OsCo@HCS & PtCo@HCS) and promoter/(Co@HCS) (e.g., Os/(Co@HCS) & Pt/(Co@HCS)) catalysts are reported. The use of Os as a promoter for FTS has received very little attention. In previous studies, Os has been shown to be a poor FT catalyst, but its use as a catalyst promoter has not been studied. The hollow morphology of the HCSs nanoreactor was used to investigate the effect of Os and Pt promoter nanoparticle location relative to the Co3O4 (Co 10 wt %) nanoparticles (dCo = 3.5 – 12.5) and on the effect of Os and Pt on the reduction behaviour and activity of the Co FT catalyst. Electron microscopy, in situ PXRD, TGA, PDF, TPR and BET studies revealed that the prepared catalysts were successfully synthesized with Co nanoparticles well dispersed. The Co nanoparticles had a high degree of stability as catalysts because they were encased in a carbon nanoreactor shell with a large surface area which showed good stability against sintering. The use of PXRD, PDF and TPR studies provided information on the Co phases and reduction pathways of the Co3O4 metal catalyst and the spillover effect from the Os and Pt promoters. Co3O4, CoO and Co (fcc) phases were observed for the promoter-Co@HCS and promoter/(Co@HCS) catalysts. More interesting was the observation of the Co hcp phase in the promoter-Co@HCS catalysts, indicating the importance of the promoter-Co interaction. The Co (hcp/fcc) phase ratio increased with the increase in promoter percentage loading and intimacy The synthesis of Co nanoparticles with varying thickness of a carbon shell was conducted with Os promotion on the outside of the shell to investigate the secondary hydrogen spillover effect. 4 The information obtained from the PXRD and TPR data revealed that the intimacy of the Os promoter with the Co catalyst in a thin and medium carbon shell (Os/(Co@HCS16) and (Os/(Co@HCS28)) promoted the reduction of CoOx to give metallic Co hcp and fcc phase due to secondary spillover effect. Whereas, when the Os promoter and Co were separated by a thicker carbon shell (Os/(Co@HCS51)) no metallic Co phase was observed. The study highlights the importance of Os-Co interaction and the ease of H2 diffusion through the carbon shell in determining the reduction of Co oxides. More interesting was the observation of the Co hcp phase in the Os/(Co@HCS16)) catalyst, indicating the importance of the promoter-Co intimate interaction. Nitrogen doping of HCS support was investigated in this thesis. The XPS, BET, and TPR studies showed that N-HCSs can offer a special unique material that is suitable for use as a catalyst support. Nitrogen doping increased the number of catalyst anchoring sites on the support, improved the thermal stability of the material, and assisted in immobilizing the catalyst nanoparticles during the reaction. These properties can improve the efficiency and stability of the catalyst, making nitrogen-doped HCS an attractive material for use as a support in catalytic reactions. In order to explain the synergism during the FT reaction, hydrogen spillover was invoked once more. The primary spillover process (Co and promoter inside a HCS) produced a catalyst that gave a higher Fischer-Tropsch activity (e.g. OsCo@HCS, 38.5 – 46.4 x10-6 molCO/gCo.s) and outperformed unpromoted catalyst (e.g. Co@HCS, 27.8 x10-6 molCO/gCo.s) and catalysts where the promoter and Co were separated (e.g. Os/(Co@HCS), 25.1 – 36.4 x10-6 molCO/gCo.s) by a mesoporous carbon shell (secondary spillover effect) in regards to the FTS activity and C5+ production. In short, secondary hydrogen spillover effect on FTS performance was studied as a function of shell thickness. A shorter distance between the Os promoter and Co catalyst improved CO conversion and enhanced the reduction of CoO to Co0 . Thicker shell (Co@HCS_51nm) reduced reactant flow and led to lower CO conversion and greater selectivity to C5+ hydrocarbon production. The Os promoter and nitrogen doping increased the FTS catalytic performance. Nitrogen doping improved the hydrocarbon selectivity of the Co FTS reaction towards the production of long-chain hydrocarbons. In the spent catalysts, nitrogen doped catalysts revealed less particle sintering.
MXene-based nanostructures for electrochemical CO oxidation
(2024) Hussien, Belal Salah Mohammed
2-D MXene based nanostructure owns several unique physicochemical and catalytic properties. Herein, this research used MXene (Ti3C2Tx) for the electrochemical COoxid reaction experimentally for the first time. This work divided to two sections, the first is using mono metal decorated Ti3C2Tx and the second using binary metals decorated Ti3C2Tx to investigate and compered the electrochemical COoxid reaction activity of mono and binary metals with Ti3C2Tx. At first, Ti3C2Tx (TX=OH, O, and F) well-ordered and highly exfoliated 2-D nanosheets used as substrate for the NPs growth and prepared via the selective chemical etching and delamination of MAX phase (Ti3AlC2) with sonication assistance to form Ti3C2Tx nanosheets. After that, Mono and binary metals were prepared via using a facile method by in situ impregnation of Pd or Pt or both salts with Ti3C2Tx in aqueous medium under sonication without using reducing agent or surfactant. The as prepared Pt/Ti3AlC2, Pd/Ti3AlC2, and PtPd/Ti3AlC2 composition and structure were characterized by the scanning electron microscope (SEM), Energy Dispersive X-Ray Analyzer (EDX), the transmission electron microscope (TEM) equipped with high-angle annular dark-field scanning transmission electron microscopy (HAADF-SEM), energy dispersive spectrometer (EDS), The X-ray photoelectron spectroscopy (XPS) spectra and The X-ray diffraction patterns (XRD). The electrochemical COoxid activity were explored using The cyclic voltammogram (CVs), linear sweep voltammogram (LSV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) for all prepared of different samples using Gamry electrochemical analyzer using a three-electrode system contains a Pt wire (counter electrode), Ag/AgCl (reference electrode), and glassy carbon ((3mm) WE) in an aqueous solution saturated with CO of three electrolytes acidic (0.1 M HClO4), neutral (0.5 M NaHCO3) and basic (0.1M KOH) at different sweep rate. The first study showed the electrochemical activity and effect of mono metals NPs of Pd/Ti3C2Tx compared with metal-free Ti3C2Tx in acidic electrolyte, Interestingly, Ti3C2Tx displayed poorer COoxid activity and the integrating of Pd NPs enhance the activity, ascribed to the combination between outstanding physical and chemical properties of Ti3C2Tx and the catalytic advantages of Pd. In contrast, the second study the electrochemical COoxid activity for binary compared with mono NPs decorated Ti3C2Tx. Results showed PdPt/ Ti3C2Tx was substantially superior to Pd/ Ti3C2Tx, Pt/ Ti3C2Tx, and metal-free Ti3C2Tx in three electrolytes experimentally, owing to the electronic and synergetic effect of PdPt and physiochemical properties of Ti3C2Tx. This study may pave the way for the employment of Ti3C2Tx in electrochemical C
Property correlations in materials for energy applications utilising advanced X-ray and photophysical techniques
(2024) Shnier, Adam
Hybrid-perovskites are an emerging material class, with optoelectronic properties highly suited for photovoltaics (PV), light emitting diode, and phosphor applications. Among PVs they have seen a meteoric rise in photon conversion efficiency (PCE) over the last decade, exceeding that of thin-film and multi-crystallite Si PV. Despite the rapid evolution of PCE within these materials, environmental and thermal stability remain the major impediment for their commercial viability. The use of A site cation substitution is an approach to enhance performance and stability in the versatile family of hybrid perovskites. In this thesis, the effect of A site cation substitution on the phase transitions and stability is explored, for both cations which appropriate for the APbX3 structure, and a larger organo-ammonium cation (A′ ) associated with the formation of lower dimensionality hybrid perovskite derivative structures. Both approaches are reported in literature to provide exceptional stability and PV PCE improvements. The hybrid perovskite (F AP bI3)0.85(MAP bBr3)0.15 was characterised using in-situ variable-temperature X-ray diffraction (VT-XRD) data. Sequential Rietveld refinements were employed to explore phase stability, and structural characteristics of this data (FA = formamidinium, MA = methylammonium). A more gradual α → β phase transition is identified, compared to the single cation systems in literature. The effect of Caesium (Cs) substitution on (F AP bI3)0.85(MAP bBr3)0.15 has been characterised across a series of VT-XRD measurements through parametric Rietveld refinement. The introduction of Cs is correlated to divergence between the observed onset temperatures for the cubic to tetragonal and a 0a 0a 0 to a 0a 0 c + (Glazer notation) transitions in the systems, which occur for the α/β phase transition. Beyond the performance improvements, the inclusion of larger A′ cations, have been associated with structural defect passivation, and electronic property tuning due to quantum confinement effects in perovskite derived structures. Hybrid perovskite based systems containing the A′ cation, propylammonium are studied to gain insights on phase stability and phase formation, and optical properties in these systems. The phase composition, structural III properties, and optical properties of (PABr)2x(MABr)1−xPbBr2, 0 ≤ x ≤ 1 and P ABr/MAP bBr3 systems were studied using ex-situ XRD, optical absorbance spectroscopy and photoluminescence measurements (PA = propylammonium). These systems were fabricated by spin coating, with the former from a single well mixed precursor solution and latter a 2-stage process, where PABr was added in various concentrations to a fully formed MAPbBr3 film. Mixed phase compositions were formed of (P A)2MAn−1P bBr3n+1 2- dimensional (2D) and quasi-2D hybrid-perovskite derivative phases, where n determines the layer thickness and n = ∞ is a MAPbBr3 phase. The difference in the kinetic and thermodynamic aspects of film formation in these systems was leveraged to propose a set of controlling factors responsible for the phase formation behaviour of the varied formation conditions.
Qualitative and quantitative characterization of arsenic-binding peptides by LC-ESI-MS for the development of an arsenic point of care device for the South African mines.
(2022) Moepya, Refilwe
Long-term occupational exposure to inorganic arsenic (iAs), through inhalation, skin contact, or ingestion, can induce a wide range of negative health effects, including cancer, neurological disorders, and cardiovascular diseases. Colorimetry, inductively coupled plasma mass spectrometry (ICP-MS), and atomic absorption spectrometry (AAS) are some of the currently used techniques for the detection of arsenic. However, these techniques are expensive, technically challenging and it is difficult to make real-time decisions due to the time that lapses between sample collection, transportation, analysis, and data processing. As a result, affordable and fast arsenic-monitoring systems are highly required. The research presented in this dissertation explores the development of peptides that are derived from arsenic resistance repressor protein (ArsR) as the molecular recognition element of an arsenic monitoring point of care device. This study aims to qualitatively and quantitatively investigate the binding affinity of arsenic to the ArsR derived peptides using the LC-ESI-MS. The ArsR-derived peptides (Pep1-RJM, Pep2-RJM, Pep3-RJM) were synthesized using the solid-phase peptide synthesis strategy based on the Fmoc approach. The synthesized peptides were purified using semi-preparative high-performance liquid chromatography (prep-HPLC) and characterized using liquid chromatography-tandem mass spectrometry (LCMS). The qualitative and quantitative binding activity of the ArsR-derived peptides to the arsenate (iAs(V), arsenite (iAs(III)), dimethylarsenic acid (DMA(V)), and Phenylarsine oxide (PAO(III)) standards was monitored by measuring the difference between the concentration of original unbound forms of the peptide and the concentration of peptide-arsenic complex using a combination of reversed-phase liquid chromatography and electrospray ionization mass spectrometry. The pentavalent arsenic metabolites were successfully reduced to the better binding trivalent state using L-cysteine while the peptides disulphides bond were reduced using a fivefold molar excess of dithiothreitol (DTT). The effect of various parameters such as pH, temperature, initial concentration of As(III) metabolites, reaction time, and peptide concentration on the binding efficiency was investigated. Optimum As(III) binding was observed at pH8, at a temperature of 40 °C, an initial As(III) concentration of 60 µM, a binding reaction time of 15 minutes, and a peptide dosage of 25 µM. The condensation reaction of Pep1-RJM with arsenic showed a fixed 1:1 coordination without the formation of clusters even at higher molar concentrations of the As(III). The equilibrium constant for the interaction of Pep1-RJM with iAs(III), DMA(III), and PAO(III) that leads to the formation of covalent arsenic-sulphur bonds (As-S) was successfully measured using the peak areas of the extracted ion chromatograms of the LC-ESI-MS. The following order of binding affinities was obtained from the LC-ESI-MS: pep1-PAO (4.11 x 108 M-1) > pep1- iAs (3.47 x 108 M-1 )> pep1-DMA ( 1.68 x 108 M-1 ). Findings suggest that thiol-containing peptides derived from the ArsR protein are capable of effectively binding arsenic in urine and the LC-ESI-MS is an efficient tool in the quantification of arsenic binding.
Synthesis and biological evaluation of pyrimidine and isoquinoline inhibitors as potential antimalarial antifolates and transmission-blocking agents
(2024) Somandi, Khonzisizwe
Malaria continues to be a serious threat, in particular to the African region. According to the World Health Organisation, in 2021, 247 million malaria cases were reported globally of which 95 % and 96 % of malaria related deaths were in the African region. The persistence of the disease, amongst it being difficult to treat and kill, is also attributed to its resistance to currently used antimalarial agents, including class II antifolate drugs such as pyrimethamine, which are used to target the P. falciparum dihydrofolate reductase (PfDHFR) enzyme. However many other drugs have lost activity because of mutations in the active site of the enzyme. The first component of the research described herein has been to synthesise 2,4- diaminopyrimidine analogues that work by disrupting folate metabolism by inhibiting PfDHFR. In a four step synthetic approach we have successfully prepared a series of pyrimidine-2,4-diamines possessing a flexible four atom linker at the 5-position of the pyrimidine ring (in yields of 33-96 %), which could prove advantageous in avoiding clashes with mutant amino acids in the enzyme active site. Enzyme inhibition assays of the compounds have shown successful inhibition of the wild-type (WT) and quadruple mutant (QM) PfDHFR in nM ranges (Ki-WT; 1.27 – 242.72 nM and Ki-QM; 13.01 – 208.23 nM). A moderate antiplasmodial activity in vitro was observed for all compounds assessed against the drug sensitive strain IC50 (TM4/8.2) 0.42 – 28.0 µM and the drug resistant strain IC50(V1S) 3.72 – 53.7 µM. The second component of the research focuses on the synthesis of transmission-blocking analogues that target the sexual stage of the malaria parasite life cycle and work by inhibiting stage IV/V gametocytes which prevents the transmission of the parasite from the human host back to the feeding mosquito. A series of 3-substituted-isoquinolin-1-yl benzamides, derivatives of the hit compound MMV1581558, have been successfully prepared in a synthetic protocol that involves only two steps from relatively simple precursors, in yields ranging between 14 – 68 %. All analogues are undergoing biological assessment against stage IV/V gametocytes and currently we have only received the results of the asexual blood stage activity assay, with most analogues displaying only moderate activity (IC50 1.18 – 7 µM). Additional biological assays are still underway
Synthesis and characterization of agricultural waste carbon-based structures for application in sensing
(2022) Magagula, Lindokuhle Precious
Rapid population and economic growths, excessive use of fossil fuels, and climate change have contributed to a serious turn towards environmental management and sustainability. The agricultural sector is a big contributor to (lignocellulosic) waste, which accumulates in landfills and ultimately gets burnt, polluting the environment. In response to the current climate change crisis, policy makers and researchers are respectively encouraging and seeking ways of creating value-added products from generated waste. Recently, agricultural waste is making a regular appearance in articles communicating about the production of a range of carbon and polymeric materials worldwide, this has led to a promising concept of waste to wealth in the modern world. The use of biomass waste such as corncob (CC) for the extraction of cellulose nanocrystals (CNCs), synthesis of carbon quantum dots (CQDs), and preparation of activated carbon (AC), has recently gained interest in the area of waste recycling and management. Further, the new materials generated from this waste promise to be effective and competitive in emerging markets. In this study, CC waste was used as a feedstock for preparation of CNCs, CQDs, and AC (shown in figure 1), for sensing applications. CNCs extracted from CC using acid hydrolysis were compared to the CNCs prepared from commercial microcrystalline cellulose (MCC). The CNCs from CC and MCC revealed comparable thermal, surface/structural, and crystallinity. These were confirmed by various characterization techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric Analysis (TGA), and Fourier transform infrared (FT-IR). For further comparison on the effect of the hydrolysis, nitro -oxidation was used to prepare nitro-oxidized cellulose nanocrystals (NOCNCs) from CC. The crystallinity indexes of the NOCNCs was obtained to be 74.37 %, which was significantly higher than that of MCC-CNCs (70.24 %), and CC-CNCs (69.12 %). TEM analysis confirmed that the CNCs had different morphologies, while SEM was used to determine the morphological properties of the samples prior to acid hydrolysis. The as-prepared CC-CNCs and MCC-CNCs were then utilized to prepare highly luminescent nitrogen doped carbon materials, with a high degree of functional groups, sensitivity, and selectivity towards Fe3+ . CQDs showed great potential for fluorescent sensor applications. Incorporation of surface functional groups such as nitrogen and oxygen containing groups were confirmed by FT-IR and X-ray photoelectron spectroscopy (XPS) analysis which showed that the prepared N-CQDs were highly functionalized with these heteroatoms, resulting in an excitation-dependent fluorescence emission. The iii detection limit of Fe3+ was obtained to be 70 nM and 75 nM, for the CC-CNCs and MCC-CNCs derived fluorescent carbon materials, respectively. Due to its natural porous nature, the corncob was also utilized to prepare activated carbons by chemical activation with potassium carbonate (activating agent) at 800 °C using varied ratios of impregnation. Highly porous corncob derived activated carbon (ACC) material with a surface area of 1523.2 m2 /g and a pore volume = 0.81 cm3 /g was. The as-prepared ACC was then decorated with various percentage loadings copper oxide nanoparticles (CuO NPs) was achieved, which produced composites with surface areas and porosity. Simple and room temperature operable sensors based on ACC and the composites were designed on gold-plated interdigitated electrodes (IDEs) embedded on a printed-circuit board (PCB) substrate. The results showed that CuO NPs play an important role in enhancing sensor performance of the ACC since its incorporation improved on the conductivity and response when compared to the ACC-based sensor. The ACC/PVA/CuO 15% sensor demonstrated good reproducibility of the sensing signal when exposed to 100 ppm ethanol vapors for up to four cycles. The sensor exhibited a response and recovery of 125 and 130 seconds, respectively, when exposed to 100 ppm of ethanol. Hence, the ACC/CuO composites could be a future candidate for ethanol gas sensing application at room temperature.
Synthesis and characterization of heteroatomdoped carbon nano-onions supported bimetallic nanoparticles for application in both direct alcohol and ammonia fuel cells
(2024) Sikeyi, Ludwe Luther
The pressing demand for inexpensive, highly active and operational stability electrocatalyst materials for direct alcohol fuel cells (DAFC) and direct ammonia fuel cells (AFC) has inspired remarkable research and growing interest in this field. The main pressing concerns related with the commercial use of these fuel cells (FCs) are their significant expense, low activity and the insufficient long-term stability of the commonly used Pt/C electrocatalysts due to facilitate the intrinsic slow kinetics for the alcohol and ammonia oxidation reaction. As such, the substitution of Pt/C with novel and more successful nanocatalyst materials is critical. The high activity and long-term operational stability of the nanocatalysts rely to a large extent upon the attributes and properties of the carbon support material. In this work, unique support materials were designed from onion-like carbon nanoparticles (OLCNs) for Pt and Pd catalysts to enhance their activity in FCs. OLCNs and the sulphur doped OLCNs were synthesized by a flame pyrolysis (FP) method to give p-OLCNs and S-OLCNs (S = 1.6 %) respectively. The synthesis of nitrogen doped (N-OLCNs) and oxygen functionalized nano-onions was achieved using chemical vapour deposition and reflux methods, respectively. Various Pd and Pt based electrocatalysts were then prepared for pristine, doped and functionalized OLNCs using a microwave-assisted synthesis method and their electrocatalytic activity was evaluated for the ethanol oxidation reaction (EOR). Based on a half-cell electrochemical investigation of the ethanol oxidation reaction in alkaline electrolyte, nitrogen and sulphur doped electrocatalysts demonstrated enhanced EOR catalytic activity and better stability, as well as fast electron transfer when compared to the equivalent undoped and commercial Pd/C electrocatalysts. To further explore the use of OLCNs as support material, bimetallic (Pd/Ag and Pd/Zn) electrocatalysts supported on N-OLCNs were prepared by a one pot synthesis method using sodium borohydride as a reducing agent. The Pd/NOLCN, Pd/Ag/N-OLCN, Pd/Zn/N-OLCN and Pd/C electrocatalysts were studied in iii the methanol oxidation reaction (MOR). The bimetallic (Pd/Ag/N-OLCN and Pd/Zn/N-OLCN) electrocatalysts exhibited superior anti-poisoning tolerance, better electrocatalytic stability and fast charge transfer resistance when compared to monometallic (Pd/N-OLCN and Pd/C) electrocatalysts in the methanol electrooxidation in alkaline media. The improved catalytic performance could be attributed to the strong metal–support interaction Overall, the FP method offered a facile synthesis route for the production of nanocarbons. The Pt or Pd nanoparticles supported on the surface of the modified OLCNs resulted in enhanced electrocatalytic activity in both alcohol and ammonia FCs..
Synthesis and characterization of supported copper-based catalysts for thermo-catalytic hydrogenation of carbon dioxide into methanol
(2022) Duma, Zama Gift
The abatement of anthropogenic sources of carbon dioxide (CO2) requires carbon capture and utilisation technologies which are innovative. The storage of green hydrogen (H2) in methanol is a viable route to utilise captured CO2. The thermocatalytic hydrogenation of CO2 to green methanol and/or other value-added chemicals using heterogenous catalysts presents an alternative, sustainable opportunity towards carbon neutrality. The valorisation of CO2 aims to offset the deleterious effects of greenhouse gas emissions which have already sparked global warming and climate change. This study reports the use of heterogeneous copper-based catalysts supported on metalorganic frameworks (MOFs) for the thermocatalytic hydrogenation of CO2 to methanol. Zirconium-based UiO-66 and aluminium fumarate MOF (AlFum MOF) were selected as supports due to their desirable physicochemical properties such as high surface area, pore-volume, and relatively high thermal stability. Copper-based catalysts were prepared via a co-precipitation process, to form Cu/ZnO/Al2O3/MgO (CZA) catalysts that had a similar elemental composition to a commercial-grade catalyst, and slurry phase impregnation was utilised to yield bimetallic Cu-ZnO MOF-supported catalysts. The Mg-promoted CZA catalyst prepared in this work had relatively larger CuO crystallites of 7.2 nm compared to 5.2 nm in the commercial catalyst. The latter also exhibited a larger specific surface area of 96.7 m2 /g compared to 37.9 m2 /g which is attributed to the difference in average crystallite size. In addition, despite the discrepancy in physicochemical properties, the CO2 conversions and methanol selectivity of the Mg-promoted CZA catalyst were found to be 30.8% and 24.2% compared to 28.9% and 15% in the commercial catalyst, respectively. Cu and Zn were supported on zirconium-based UiO-66 and AlFum MOFs via slurry phase impregnation. The crystallinity, specific surface area, and pore volume of MOFsupports were observed to decrease after metal loading and thermal activation of the catalyst precursors. The MOF-supported catalysts exhibited CO2 conversions ranging from 0-45.6% and methanol selectivity of up to 15.7%. The Cu/ZnO/UiO-66 exhibited the greatest methanol productivity of 128 gMeOH/Kgcat/h compared to 51.8 gMeOH/Kgcat/h in the commercial catalyst. All the MOF-supported catalysts exhibited stability within v the 24-hour evaluation period where equilibrium CO2 conversions were reached within 1-2 hours.
Synthesis of onion-like nanocarbons from cooking oil for the adsorptive and photocatalytic removal of Cr(VI) and methyl orange from aqueous solutions
(2024) Ntuli, Themba Dominic
Rapid industrialization and urbanization have coincided with increased concentrations of organic and inorganic pollutants in wastewater. These include trace metal ions and dyes such as Cr(VI) ions and methyl orange, respectively. Such pollutants tend to bioaccumulate in living organisms, resulting in toxic effects for humans and biota alike and they should thus, be removed from water streams. Two of the common and widely applied technologies for the removal of pollutants from wastewater are adsorption and photocatalysis, typically using carbon materials. This is because carbonaceous materials can serve as adsorbents and/or support materials for semiconductor nanoparticles. This study explores the use of onion-like nanocarbons (OLNCs) as the material of interest for the removal of pollutants in water. The synthesis of OLNCs was achieved by the flame pyrolysis of vegetable cooking oil (olive oil) and waste cooking oil. First, the OLNCs synthesized from olive oil and waste cooking oil were used as adsorbents for the removal of Cr (VI). It was found that the optimum condition for the removal of Cr(VI) ions took place at pH = 2 and CO = 10 mg/L. The values found were t = 720 min and Q0max = 26.53 mg/g for the olive oil OLNCs and t = 360 min and Q0max = 47.62 mg/g for the waste cooking oil OLNCs. The improved removal of Cr(VI) ions by the OLNCs from waste cooking oil was attributed to increased surface oxygen moieties as a result of the oxygenation that took place during the cooking process. The removal mechanism for both adsorbents was consistent with the adsorption coupled reduction removal pathway as indicated by the presence of Cr(III) on the surface of the adsorbent and the presence of Cr(VI) ions in the solution. The experimental data followed the pseudo-second-order (PSO) model and the Langmuir adsorption isotherm. The OLNCs from waste cooking oil demonstrated higher adsorption efficiency than the OLNCs from olive oil, an indication of how waste material could be converted into a useful material. Oxygen functional groups play a role in the surface adherence of chromium ions. Therefore, H2O2 and KOH were used as oxidizing agents for the OLNCs from olive oil. The OLNCs were chemically modified with 10% and 25% of H2O2 and 0.1 M and 0.2M of KOH respectively to improve their adsorption efficiency. Generally, the H2O2-treated adsorbents showed higher adsorption efficiency than the KOH-treated adsorbents. The Q0max values of the chemically modified adsorbents were significantly higher than iv that of the unmodified OLNCs from olive oil (26.53 mg/g) but relatively lower than those of waste cooking oil (47.62 mg/g). The Freundlich adsorption isotherm fitted both adsorbents indicating heterogeneous surface coverage of the surface. The PSO kinetic model was followed by both adsorbents predicting the possible chemisorption mechanism. TiO2 is a well know and widely used photocatalyst, and to further explore the effect of OLNCs in photocatalysis, nanocomposites of OLCNs and TiO2 nanoparticles (TiO2/OLNCs) were prepared via hydrothermal synthesis. This was achieved by varying the mass of the OLNCs (10, 20, 30, and 50 mg) added to 100 mg of TiO2. The materials were labelled as TC-10, TC-20, TC-30, and TC-50 corresponding to the OLNCs mass. The nanocomposites were then used as photocatalysts in the photocatalytic degradation of methyl orange (MO). It was found that the photocatalytic degradation rate of MO using the TiO2/OLNCs was more rapid than TiO2 alone in this order TC-10 > TC-20 > TC-30 >TC-50. The waste cooking oil OLNCs were also used as a support material for iron oxide nanoparticles to form iron oxide/OLNCs nanocomposite. The mass of the OLNCs was varied as follows (25, 50, 100, and 170 mg) the materials were labelled FeC-25, FeC50, FeC-100, and FeC-170. The nanocomposites were then used as photocatalysts for the photocatalytic reduction of Cr(VI) ions to Cr(III) ions. The reduction of Cr(VI) ions to Cr(III) ions was rapid, taking place in the first 15 min. This was followed by subsequent adsorption of the Cr(III) ions through the carbonyl groups of the nanocomposite. Overall the results showed that the flame pyrolysis method was successfully used to convert vegetable cooking oil to carbon nanomaterials (OLNCs). The OLNCs showed promise in the removal of Cr(VI) and (MO). The results are comparable with other carbon nanomaterials, an indication that the as-synthesized OLNCs have potential applications in adsorption, reduction and photocatalysis.
Synthesis of zirconium disulphide nanomaterials and their nanocomposites with radially aligned nanorutile and polyaniline for room temperature sensing of volatile organic compounds
(2022) Fadojutimi, Paul Olawale
Integration of 2D nanomaterials with a polymer or semiconductor metal oxide could help in the development of low-cost sensors for rapid detection of volatile organic compounds (VOCs) at room temperature. This study focuses on the fabrication of robust room temperature sensors of pristine radially aligned nanorutile, and zirconium disulphide/polyaniline (PANI) nanocomposites for chemical sensing of VOCs. ZrS2 was fabricated using both bottom-up and top-down methods of synthesis. Heat up and hot injection methods were employed to fabricate arrays of morphologies of ZrS2 nanomaterials using the colloidal method. However, the nanomaterials showed high oxophilicity which was confirmed by both XRD and XPS. The XPS peak of S2p was conspicuously absent while the peak Zr3d was very noticeable in the XPS spectra. XPS and EDS measurements indicated replacement of sulphur atom by the O atom on the surface of the nanomaterials. The stability study showed the nanomaterials were not stable in ambient environment. Nanoparticles of 11 nm and few layered nanosheets were obtained when bulk crystal samples of ZrS2 were exfoliated in cyclohexyl-2-pyrrolidone and N-methyl (-2-) pyrrolidone. Isopropanol served as a green solvent for the exfoliation of few-layered ZrS2 from the bulk crystal sample compared to amide solvents which are not environmentally friendly. However, the pristine ZrS2 nanomaterials could not sense VOCs at room temperature, this could be as a result of low conductivity and number of layers of the nanomaterials obtained using nanomaterials exfoliated in isopropanol. The sensitivity of raw PANI was greatly enhanced with loading of ZrS2 nanomaterials. The sensor displayed responses of 0.43, 0.58, 1.04 and 0.34% which correspond to methanol, ethanol, isopropanol, and acetone vapours respectively. The relatively better responses of the sensor were credited to the synergistic effect of ZrS2/PANI composite structure. The sensor showed good response to low concentrations (7.7 ppm, 11 ppm, 5.8 ppm and 6.1 ppm) which correspond to methanol, ethanol, isopropanol, and acetone respectively. The sensor was more sensitive to isopropanol compared to other alcohols tested for in this work. The behaviour of the sensor changed from p-type to n-type on exposure to ethanol vapour at elevated relative humidity. The sensor displayed good sensitivity, reproducibility, rapid response and recovery times towards alcohols and stability over 60 days. The hierarchical morphology, high surface area, high porosity and humidity contributed immensely to the titania sensor in the sensing of VOCs at room temperature. The TiO2 sensor showed high sensitivity with responses of -38.27, -86.75, -9.83 and 1.24% which correspond to methanol, ethanol, isopropanol, and acetone respectively. The sensor is more sensitive to ethanol gas compared to other chemical vapours tested. The sensor displayed good sensitivity, reproducibility, rapid response and recovery times towards alcohols and stability over 45 days. The surface area of the nanorutile decreased by 35% on loading of ZrS2, this could the main reason there was no response observed when the nanocomposite of TiO2-ZrS2 was tested for chemical sensing at room temperature. The active sites for adsorption of the vapour were not available probably due to the covering of pores of the nanorutile as well as low conductivity of ZrS2 at room temperature. Both sensors of nanocomposite of zirconium disulphide and polyaniline as well as titania could find application in breath analysers since the least detection for a breath analyser is reported to be 200 ppm.
The crystal engineering of two antibacterial pharmaceutical ingredients: cocrystals and molecular salts of a series of sulfa-drugs and the polymorphism and cocrystals of an isoniazid derivative
(2024) Scheepers, Matthew Clarke
Crystal engineering is the design and synthesis of new solid forms by using the knowledge of intermolecular interactions and crystal packing. The goal of crystal engineering is to improve the properties of materials without altering the chemical identity of the materials. This work can be divided into three major parts. The first part deals with a series of sulfa drugs and its cocrystals. The second part deals with exploring the cocrystals of 3,5-dinitrobenzoic acid, in particular using Hirshfeld surfaces. The last part deals with the polymorphism of an isoniazid derivative and the cocrystals formed with it. In all cases these new solids were characterized by single crystal X-ray diffraction (SC-XRD), powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), and Fourier-transform infrared spectroscopy (FTIR). When designing new solid forms (cocrystals and molecular salts) it would be ideal to use a knowledge based system on the use of synthons. Sometimes this knowledge is limited and can hinder future work. Sulfa drugs such as sulfamethazine (sz), sulfapyridine (sp), sulfathiazole (st) and sulfamethoxazole (sm) have been shown to exhibit unique solid-state behaviour such as cocrystal-salt polymorphism and tautomerism. The number of known cocrystals and molecular salts consisting of the mentioned drugs are limited, which hinders research into exploring this interesting solid-state behaviour. Therefore, the structural landscape of these sulfa drugs was expanded in hopes of observing new cases where this solid-state behaviour is observed. Cocrystals and molecular salts of these sulfa drugs were synthesized using various techniques, such as solvent evaporation and mechanochemical grinding (both dry and liquidassisted grinding). Twenty-five different coformers were chosen to use for these cocrystallization experiments, most of which are either benzoic acid/benzoic acid derivatives or a pyridine derivative. sz formed eight successful cocrystals and one molecular salt with benzoic acid and its derivatives. SC-XRD showed that eight of the coformers that interacted iii with sz formed the sulfonamide-carboxyl synthon; the only exception to this was sz + 4- hydroxybenzoic acid, which the sulfonimide-carboxyl synthon formed instead. Five cocrystals and one molecular salt containing st, three cocrystals and one molecular salt of sm, and four cocrystals and one molecular salt containing sp were also synthesized. Most of these included using 2-aminopyridine or one of its derivatives. The second part deals with the cocrystals and molecular salts of 3,5-dinitrobenzoic acid (dnba). dnba is often used as a popular coformer for crystal engineering purpose. However, very little work centered on dnba with coformers has been reported. In this work we report new multicomponent crystals containing dnba, which include one hydrate, one solvate, one molecular salt, and four cocrystals. The coformers include: 2-acetylpyridine, 3-cyanopyridine, flufenamic acid, 4-dimethylaminobenzophenone, pyridoxine, theophylline, and thiourea. In addition to the strong hydrogen bonding expected, several weaker intermolecular interactions were identified using Hirshfeld surfaces, which included C−H···π bonding, π-hole, and π···π interactions. The Hirshfeld surfaces indicated that these weaker interactions had a significant effect on the packing of these multi-component crystals. These multi-component crystals were compared with the crystal structures reported in the Cambridge structural database (CSD), which has given some significant insights in the structural landscape of dnba. Isoniazid (inh) is a simple, useful active pharmaceutical ingredient (API) used to treat Myobacterium tuberculosis. It is composed of a hydrazine and pyridine ring. The hydrazine group can be modified in a multitude of ways, including using a Schiff-base condensation reaction. This is achieved by using an appropriate ketone or aldehyde. In this work isoniazid was derivitised using diacetone alcohol (4-hydroxy-4-methyl-2-pentanone), generating N’- [(2E)-4-hydroxy-4-methylpentan-2-ylidene]pyridine-4-carbohydrazide (iz4h4m2p). This derivative was found to be polymorphic, existing in two forms, form I and form II. The iv difference between these two forms is distinguished by the major hydrogen bond pattern, which is either a chain hydrogen bond motif formed between the hydroxyl group and amide groups (the pyridine ring is not involved) or a ring based hydrogen bond motif that forms dimers with the hydroxyl group forming a hydrogen bond to the pyridine ring. Form I was found to be metastable with respect to form II, with form I converting to form II upon heating before melting. Form II does not convert to form I. In addition to exploring the polymorphism of iz4h4m2p, several cocrystals of iz4h4m2p were synthesized. The coformers chosen were benzoic acid derivatives. Most of these cocrystals formed a hydrogen bond between the carboxylic acid and pyridine ring. This left the hydroxyl group of iz4h4m2p to form a hydrogen bond to the amide group, forming a chain hydrogen bond motif similar to the one observed in form I. The only deviation from this was observed in cocrystals where the benzoic acid included a hydroxyl group of its own, such as 2,5-dihydroxybenzoic acid.

Browse

Browsing 3. Electronic Theses and Dissertations (ETDs) - All submissions by School "Chemistry"

Results Per Page

Sort Options