Theses and Dissertations (Chemical Engineering)

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The impact of ozone demand on the required dosage in dielectric barrier discharge plasma-ozonation systems : investigation into a simulated surface water
(University of Pretoria, 2024-12) Iwarere, Samuel Ayodele; Daramola, Michael Olawale; Tizaoui, Chedly; u17056480@tuks.co.za; Roos, Anche
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.
A comparative study of adsorbents derived from coal fly ash for the adsorption of tetracycline from aqueous solutions
(University of Pretoria, 2024) Tichapondwa, Shepherd Masimba; Yapi, Litha; u18171941@tuks.co.za; Houghton, Eric Emmanuel
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
(University of Pretoria, 2025-01) Focke, Walter Wilhelm; Ramjee, Shatish; chrisdaviddenton@gmail.com; Denton, Christopher David
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
(University of Pretoria, 2024-10) Brink, Hendrik Gideon; Feleni, Usisipho; u21830658@tuks.co.za; Kyomuhimbo, Hilda Dinah
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
(University of Pretoria, 2025-02) Labuschagne, F.J.W.J. (Frederick Johannes Willem Jacobus); Wiessner, Sven; u14006546@tuks.co.za; Jones, Louise
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
(University of Pretoria, 2024-09-25) Merckel, Ryan David; baaschw@gmail.com; Baasch, William Peter
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.
Application of machine learning and knowledge-based systems to support decision-making on fate and behaviour of engineered nanoparticles in aqueous environment
(University of Pretoria, 2025-01) Daramola, Michael Olawale; Musee, Ndeke; ntsikeleloyalezo@gmail.com; Yalezo, Ntsikelelo
In recent decades, modern science has transformed due to the recognition and usage of engineered nanoparticles (ENP), which are substances that have a peripheral dimension in the geometric range of 1 to 100 nm. These materials have evolved into multifunctional materials that are used for engineering and innovations in a wide range of fields, including agriculture, medicine, food, industry, biomedical, and energy. Apart from their unique functionality and numerous benefits, the surge in production of multi-enabled nano-products and their emission in ecology systems, specifically to aquatic systems has raised serious environmental concerns about the potential deleterious effects of ENPs on the aquatic biota. So far, to address the existing environmental safety concerns, a volume of experimental data has been generated using natural freshwater and like systems for characterisation of the ENP colloidal stability. However, this data is knowledge-poor, heterogeneous, highly multifaceted (not easily discernible data variables relationship), and uncertain (due to the multiplicity of data); thus, highly challenging to be utilised to support the decision-making. Therefore, this work describes the application of data modelling techniques for the development of computer-based intelligent systems to support decision-making in dealing with the fate and behaviour of ENPs. At the same time, the study aims to provide a coherent understanding of the mechanisms and interactions that underpin these ENP transformation behaviours in aqueous environments. The field of data modelling has gained prominence across various fields including numerous environmental domains with the advancement of artificial intelligence (AI) research, digital computers, and big data. The modelling techniques of interest in this study included machine learning (ML) (i.e., adaptive neuro-fuzzy inference system (ANFIS), artificial neural network (ANN), support vector regression (SVR), random forest regression (RFR), k-nearest neighbour (KNN), extreme gradient boosting (XGBoost) and multi-linear regression (MLR)) and knowledge-based system (KBS) (i.e., fuzzy logic, semi-quantitative analysis). The results showed that ML was quite useful for modelling heterogeneity and non-linear data. It also revealed that diverse ENP transformation processes are influenced by variant parameters and that significant variables reported experimentally are not fundamentally good predictive variables. The RFR algorithms had the highest performance with the coefficient of determination (R2) and Nash-Sutcliffe efficiency (NSE) greater than 0.80 and 0.70, respectively for predicting the dynamic aggregation of ENPs. Furthermore, to predict the dissolution of nZnO the models that performed the best were the RFR and XGBoost algorithms with R2 values of 0.85 and 0.92, respectively. Overall, ML techniques of RFR, XGBoost, SVR, and ANN models yielded satisfactory to a very good level of accuracy in predicting both the aggregation and dissolution of ENPs. However, MLR showed poor performance, for both processes an indication of no underlying linear relationship between the model inputs and output. In addition, to ML algorithms demonstrating high prediction accuracy, and meta-analysis aiding to quantitatively evaluate highly heterogeneous data from multiple literature sources: to account for the scarcity of quantitative data the domain knowledge was encoded using rules and scores to develop intelligent KBS. These included computer-based semi-quantitative analysis integrated with decision tree classifiers (SQADTC) and fuzzy decision-making systems (FDMS). SQADTC used several weights/scores allocated to different factors (inputs) or linguistic variables and their sub-level (intermediate outputs). The functionality of SQADTC was illustrated using worked case studies of silver (nAg), nZnO, and nTiO2. The results demonstrate that our proposed model can be highly effective and valuable for preliminary screening of the exposure of ENPs. SQA application is relatively cost-effective and easy to use since no software or computational tools are required. In addition, non-experts can easily understand the hierarchical nature, Boolean logic, and visual representations of DTCs; which is highly valuable given that testing each variation of ENPs is tedious and associated with high cost. Furthermore, the FDMS constituted 321 (three hundred and twenty-one) if-then conditional statements in the fuzzy inference system. Modelling results using FDMS in the case studies of nTiO2 and nZnO demonstrated that the representation of qualitative knowledge by fuzzy sets and its application as very successful in handling partial truth information. FDMS provides flexibility to reduce bias and integrate the uncertainty that arises with the modelling of expert intuitions or perceptions. FDMS was able to replicate human-like reasoning using natural language in complex scenarios with no sharp boundaries, which makes the model ideal in various real-world scenarios. Overall, this thesis work offers the application of ML and KBS as a basis to maximise and leverage accessible data (structured input-output data pairs and unstructured expert knowledge) to support ENP monitoring, initial screening, and exposure assessment. The developed decision support system could aid to reduce the costs associated with experimental testing and support the establishment of robust frameworks for nano-safety. This is necessary to balance the advancements in nanotechnology and long-term environmental protection. Additionally, the efficiency of developed models can be extended to other ENPs and readily scaled when new and more information becomes accessible without having to reconstruct the frameworks of these models.
Fertiliser-catalysed biochar production: an analytical and techno-economic feasibility study
(University of Pretoria, 2024-06-27) Merckel, Ryan David; brendon.trollip01@gmail.com; Trollip, Brendon
The current agricultural method of burning leftover plant material in farmlands before planting a new crop is outdated. While this technique may be fast and cost-effective, it is very unsustainable. This practice produces large amounts of carbon black, resulting in health issues, and reduces soil fertility [Pang, 2019]. It is also well known that the process used to produce nitrogen-containing fertilisers, the Haber-Bosch process, is thermodynamically unfavourable at high temperatures. To overcome this limitation, the process is operated at extremely high pressures, making the process very energy intensive [Razon, 2014]. Like fossil fuels, the world’s phosphorus reserves are limited and can’t sustain current agricultural practices indefinitely, driving the search for sustainable alternatives [Desmidt et al., 2015]. With 71 billion metric tons of phosphate ore available and a production rate of 0.22 billion metric tons in 2021 [Jasinski, 2022], the use of specialty fertilisers is expected to rise due to the irreplaceable role of phosphorus in farming. The objective of this research was to identify fertilising agents that reduced the pyrolytic onset temperature of biomass, producing a fertilising-biochar. By lowering the pyrolytic ignition temperature, energy reductions were observed, resulting in a more economically viable product. The enriched biochar, containing nitrogen (N), phosphorus (P), and potassium (K), aims to provide essential nutrients to the soil for crop cultivation. 79 prospective fertilising agents, each representing one of the three macronutrients (NPK) required by biomass, were analysed. K2CO3, K3PO4, and Ca(NO3)2·4H2O were chosen to represent potassium, phosphorus, and nitrogen, respectively. As for the biomass of choice, Eucalyptus grandis was selected, due to its widespread abundance, not only in South Africa, but in the world. The fertilising agents were each added in concentrations of 1 wt.%, 2 wt.%, 5 wt.%, 10 wt.%, 20 wt.%, and 50 wt.% to the biomass with deionised water and then dried at 105 °C. Thermogravimetric analysis was conducted on the dried samples with a Hitachi STA7300 horizontal-beam TGA-DTA system and compared to a neat sample of E. grandis. It was established that both K2CO3 and K3PO4 suppressed the pyrolytic onset temperature of the biomass. K2CO3 exhibited an ignition temperature suppression of -11 °C, -25 °C, -48 °C, -71 °C, -83 °C, and -97 °C for 1 wt.%, 2 wt.%, 5 wt.%, 10 wt.%, 20 wt.%, and 50 wt.%, respectively, with a maximum possible suppression of -98 °C. This resulted in an energy saving of 18.7% for the 50 wt.% loading. K3PO4 also showed significant suppression of the pyrolytic onset temperature, with 1 wt.%, 2 wt.%, 5 wt.%, 10 wt.%, 20 wt.%, and 50 wt.% loadings resulting in suppressions of -2 °C, -20 °C, -33 °C, -49 °C, -55 °C, and -75 °C, respectively. The highest fertiliser loading (50 wt.%) indicated an energy saving of 16.7%. Conversely, Ca(NO3)2·4H2O demonstrated no catalytic effect on E. grandis. After establishing that the fertilising agents had a catalytic effect on E. grandis and resulted in an energy reduction, a techno-economic feasibility analysis was conducted to investigate the economic viability of an enriched biochar. To quantify the feasibility, four financial indicators were utilised: the net present value (NPV), internal rate of return (IRR), return on investment (ROI), and payback period (PP). In addition to the sale of the nutrient-enriched biochar, the by-products (synthesis gas and pyrolysis oil) were also offset to increase economic feasibility. Four scenarios for the sale of by-products were considered, with Scenario 2 and Scenario 4 proving to be the most viable. In Scenario 2, electricity was sold to the grid by combusting the synthesis gas and wood vinegar fraction, while the phenolic fraction was distilled and sold separately. In Scenario 4, the phenolic fraction was extracted through distillation, and both the wood vinegar and phenolic fractions were sold. Process parameters including the daily tonnage, fertiliser loading, biochar yield, fertiliser agents ratio, and the final selling price were varied to optimise the final selling price. The first price comparison was between neat biochar to that of coking coal, used in the steel manufacturing industry. It was discovered that at a selling price of 20 R·kg⁻¹, neat biochar attained an IRR of 3.4% and a payback period of 10.4 years (with the plant lifespan set at 20 years). The selling price of neat biochar was calculated as almost four times the current price of coking coal (5.04 R·kg⁻¹). The other price comparison that was investigated was between NPK fertilising products, currently used in agriculture, to the enriched biochar. Due to the difficulty of obtaining bulk prices for K2CO3, K3PO4, and Ca(NO3)2·4H2O, wholesale prices for muriate of potash, diammonium phosphate, and urea were used, respectively. Although the selling price was kept as low as possible, an IRR level of 16% was set to attract investors, given that the plant would require a major fixed capital investment. The analysis indicated that employing a biochar with 51.7% loaded fertiliser resulted in a 32-fold price increase per hectare if the required amount of fertiliser is applied. Using a 5.3% loading, the cost increase would soar to 127 times higher per hectare. The price increase is primarily attributed to the large amount of biochar needed—if an enriched biochar of 5.3% loading is used, 19 times more product is required per hectare compared to using neat fertilisers.
Application of layered double hydroxides in advanced wastewater remediation processes for the removal of phenol, efavirenz, and nevirapine
(University of Pretoria, 2024-12-12) Tichapondwa, Shepherd Masimba; tabana.ls@tuks.co.za; Tabana, Lehlogonolo Shane
The global challenge of water contamination necessitates innovative approaches for wastewater treatment. This study explored modified layered double hydroxide (LDH)-based photocatalysts and adsorbents for removing persistent organic pollutants, focusing on antiretroviral drugs (ARVDs) such as efavirenz (EFV) and nevirapine (NVP). These drugs, commonly found in wastewater due to extensive use in HIV/AIDS treatment, resist conventional treatments and pose environmental and health risks. The synthesis and characterization of an Ag-AgBr-LDH photocatalyst revealed a promising combination of LDH’s durability and Ag-AgBr’s superior visible-light absorption. Techniques like X-ray diffraction, scanning electron microscopy, and BET surface area analysis confirmed the material's suitability. Photocatalytic evaluations showed significant degradation of phenol and ARVDs, with EFV and NVP achieving maximum degradation efficiencies of 84% and 100%, respectively. Response surface modeling identified critical interactions among factors such as initial pH, photocatalyst loading, and pollutant concentration. Continuous flow reactor tests highlighted optimal degradation at a photocatalyst loading of 3 g/L, a flow rate of 10 mL/min, and high light intensity. Parallel investigations assessed the adsorption potential of calcined LDH (CLDH). Characterization revealed rapid adsorption kinetics and physisorption as the dominant mechanism, with hydrogen bonding playing a significant role.
Refractometry models for compositional analysis of binary and ternary mixtures
(University of Pretoria, 2024-01-26) Focke, Walter Wilhelm; Du Toit, Elizbé; u15076238@tuks.co.za; Pretorius, Franco
A review of existing compositional binary mixture models for refractive indices was undertaken and showed that they can be recast in forms that are linear in mole or volume fraction. Typically, mole fractions proved to be the better composition descriptor when analysing literature data and the molar refraction was shown to be virtually temperature independent. Depending on whether temperature, density or refractive index measurements of the mixture are available, different correlations were developed to predict the composition (as shown below). Furthermore, refractive index measurements on mixtures of n-alkanes with DEET were taken to determine phase equilibrium behaviour. With extrapolation to infinite molar mass, these alkanes approach polyethylene, which is the material from which DEET-containing mosquito-repellent devices were made. The results show that the microporous structure was likely formed by liquid-liquid phase separation. Lastly, it was shown that Padé approximants provide good representations of binary and ternary refractive index data. The temperature dependence of the pure components was elegantly subsumed in a pure component property, namely, the molar volume. Various constraints were applied to reduce the number of adjustable parameters even further and this proved to be successful.
Poly(lactic acid) filament filled with layered double hydroxide for fused deposition modelling; optimisation of additive fraction and material extrusion parameters
(University of Pretoria, 2024-07-31) Labuschagne, F.J.W.J. (Frederick Johannes Willem Jacobus); Coetzer, R.L.J. (Roelof); u18017763@tuks.co.za; De Bruin, Philip
The optimum layered double hydroxide (LDH) loading in poly(lactic acid) (PLA) filament, layer height, nozzle temperature and infill density levels were determined by statistically maximising the ultimate tensile stress of printed parts. Fused deposition modelling (FDM) is a material extrusion additive manufacturing (AM) method. It allows the printing of complex parts with simple and relatively cheap equipment. After stereolithography, it is the most popular AM method. FDM parameters that influence a printed artefact most are layer height, nozzle temperature and infill density. PLA is a compostable polymer which can be synthesised from renewable sources. It is the most used polymer in FDM and is projected to continue dominating the 3D printing landscape. LDH is an anionic clay with a brucite-like structure. It contains carbonate anions in its interlayer, which can be exchanged with other substances, making it extremely versatile for various applications. From a systematic literature review following preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines it was shown that only one paper describes acrylonitrile butadiene styrene filament filled with LDH. This showed that filaments containing LDH for FDM purposes have scope for research. With PLA being the most popular FDM polymer, a second review using PRISMA guidelines were completed on PLA filled with LDH. No other review on LDH in PLA was found in the 87 articles considered. A 24−1 fractional factorial experiment was used to screen the four factors, and was augmented to fit quadratic models for the respective responses. These included ultimate tensile stress and load, elongation at break, Young’s modulus and impact energy. A central composite design was used to verify the optimum conditions predicted by the derived models. The statistical design of experiments (DoE) considered the following ranges for LDH loading, layer height, nozzle temperature and infill density: 0 % to 10 %, 0.18 mm to 0.42 mm, 190 ◦C to 220 ◦C and 10 % to 100 % respectively. Analysis of variance (ANOVA) was used to derive models and analyse the factors that affect the responses. For tensile properties the optimum combination of factors were at lower levels of layer height, nozzle temperature and LDH loadings at 0.18 mm, 190 ◦C and between 0 % and 4 % respectively. Infill densities between 80 % and 100 % also yielded the maximum tensile properties. Impact properties did not vary statistically in this region either. Inconclusive results were observed for Young’s modulus, and it is expected that another material extrusion parameter affects this response. Even though inferior in strength, parts could be printed with filament containing up to 10 % LDH. It was shown that LDH PLA filament can be made and that artefacts can be printed with FDM. Up to 4 % LDH can be printed without negatively affecting mechanical properties compared to pure PLA, and printing is still possible with LDH loadings as high as 10 %. Because substances can be intercalated into LDH for specific purposes, a legion of applications including medical, environmental and flame retardance applications are theoretically possible. The combination of the benefits of FDM and LDH can lead to tremendous advancements in a variety of fields. Since this is the first work reported on LDH PLA filaments, further research and work is recommended. Development should focus on limiting degradation of PLA when making filled filament, achieving the required strengths for specific applications and testing the efficacy of intercalated substances after printing. The proper use of statistics in research is highly recommended. It was shown that resources are wasted because statistics are not fully exploited, especially in the nature science and engineering establishments. Specifically, it is not necessary to have five or more replications for each experimental point in a statistically designed experimental programme, especially in the screening stage of a DoE. More research in the effect of slicing software on printed parts are also required. It was found that the fracture locations on printed tensile parts depended on the Ultimaker Cura slicer settings instead of the material.
Multispecies bacterial attachment to A106 GB industry-finished steel used in heat exchangers
(University of Pretoria, 2024-06-19) Chirwa, Evans M.N.; Tichapondwa, Shepherd Masimba; alicia.prithiraj@gmail.com; Prithiraj, Alicia
Multispecies bacterial attachment to industrial-finished alloys is not understood. It is not well understood as to why certain bacterial species selectively attach to differently finished steel surfaces. It is also a matter of curiosity as to why the attachment of certain bacteria influences corrosion. Bacterial attachment in heat exchangers leads to biofouling, corrosion, and downtime costs. This study evaluated the synergistic effect of bacterial attachment to smooth and rough (industrial standard) surfaces unique to the petrochemical industry. From the results there were no significant time-related differences in colonisation (p(perm)>0.05), and bacterial levels on the surfaces (p>0.05). However, quantification of surfaces using Atomic Force Microscopy (AFM) showed significant differences (p<0.05) in the root mean square surface roughness (RMS) of the differently finished surfaces, elucidating that bacterial colonisation was not proportional to surface roughness. It was observed that Clostridium sp. colonised the rough surfaces abundantly, and Pseudomonas sp. favoured the rough surface during early colonisation which influenced the corrosion rate. In bacterial presence, the corrosion rate on the rough alloy surface on day 3, exhibited corrosion resistance. This was owing to the synergistic behaviour of the bacteria which selectively attached to the rough surface and formed biofilm. Increased corrosion rates were then observed when compared to the smooth alloy. On the rough surface on day 6, the corrosion rate was observed to be the highest with 38.72 ± 0.15 mm/y. Smooth surfaces exhibited unusual corrosion rates on this day. On day 13 both surfaces exhibited a corrosion protection phenomenon. In light of the findings, it was i observed that there were significant differences observed on day 6, in the corrosion rate value between the rough and smooth surfaces (p<0.05). The growth model confirmed that exponential growth phase took place from day 6. Total Organic Carbon (TOC) results revealed that during bacterial growth, the bacteria utilised the carbon sources and produced acetic acid and lactic acid which played an important role in the corrosion process. Unlike sulfate-reducing bacteria (SRB), Clostridium sp. and Pseudomonas sp. described in this study are rarely reported in the petrochemical environment. These microorganisms are ubiquitous; however, their dominance in these systems showed that they play a significant role in steel corrosion. This study used next-generation sequencing with qPCR into microbial species colonising steel with AFM, which are rarely reported jointly in the literature. These bacteria can survive nutrient-depleted conditions for extended periods. The results provided a basis to explicate metabolic pathways. Long-term steel exposure to the bacterial consortia indicated steel protection rather than corrosion. Innovative insights on carbon-metal bonding were also determined, which could be a basis for future work. The synergistic behaviour of the bacteria provided a new dimension of thinking regarding the corrosion of carbon steel. In this study, the smooth-finished alloy performed best in this process system based on the corrosion evaluation.
Plasma-vernietiging van rubberskroot : teoretiese en eksperimentele onderbou
(University of Pretoria, 2023-12) Crouse, Philippus L.; Van der Walt, Izak J.; aaj421216@gmail.com; Jansen, Arnold Alexander
Afrikaans: Die oorhoofse doelwit van hierdie navorsing was die onwikkeling van ʼn reaktorkonsep en vloeidiagam vir ʼn vervoerbare plasma-gebaseerde eenheid vir die prosesering van skrootrubber. Die projek het voor die internasionale Covid-inperkings afgeskop. In daardie stadium het Necsa reeds oor ʼn ontwerp vir ʼn navorsings-plasma-eenheid beskik. Die konstruksie van die toerusting is eers in 2022 voltooi met `n beperkte begroting. Die inbedryfstelling van die toerusting het deel van hierdie studie gevorm. Die algemene teikenvoermateriaal vir die eenheid is egter munisipale afval. Daar is nie gepoog om enige aspek van die sisteem tydens die studie vir die voer van rubber te optimiseer nie. Die gedrag van die navorsingsisteem ‒ soos hy ontwerp, gebou, en in bedryf gestel is ‒ is dus ge-evalueer vir rubber as voermateriaal. Die uiteindelike uitsette is konsep-voorstelle vir ʼn toekomstige rubbervergassingsisteem ‒ ondersteun deur die nuwe kennis geskep tydens die afhandeling van hierdie projek. Kritieke data vir reaktorontwerp behels die kinetika en die termodinamika van die proses. Die kinetika bepaal die minimum vertoeftyd van die material in die reaktor, en die vertoeftyd pen die grootte van die reaktor vas. Kinetika verwys oor die algemeen na die snelheid van die reaksie, of reaksies, en kan chemiese of fisiese meganismes behels. Die termodinamika van die proses bepaal beide die opbrengs en die energie-vereistes. Die nodige termodinamise data kan grootliks teoreties gegenereer word. Eksperimentele werk is egter nodig om die kinetika te bepaal. Hoewel daar ʼn oorvloed van data oor die termiese gedrag en kinetika van rubber in die oop literatuur beskikbaar is, is dit uiters riskant om nie eksperimentele werk op eie materiaal uit te voer nie; en verder was die formulering van die kinetika nie in die vorm wat vir hierdie projek benodig is nie. Twee noukeurige eksperimentele programme is gevolglik aangepak om die kinetika op `n mikro- en makroskaal vas te pen. Die eerste was ʼn termogravimatriese studie op fyn-gemaalde rubberkrummels, ≤ 100 μm. Die tweede was ʼn videografiese studie van die termiese gedrag van rubberblokkies in ʼn buisoond. Onder inerte toestande piroliseer rubber in die temperatuurgebied 200‒500 °C, om koolstofmonoksied, metaan, en verskeie alkane, alkene, en sikliese organiese verbindings te vorm. Die massaverlies is ordelik 70 %. Indien die residu verder na hoër temperature verhit word, vind addisionele massaverlies plaas ‒ maar slegs van ʼn verdere paar persent. Verskeie opsies vir die benutting van die koolstofhoudende residue is beskikbaar. Die konsep wat in die navorsing getoets is, is die gebruik van die tru-Boudouard-reaksie; dit is die reaksie van koostofdioksied met vaste koolstof, om koolstofmonoksied te vorm. Die termogravimetriese studie het isotermiese sowel as dinamiese lopies oor ʼn wye stel temperatuurprogramme behels. Kinetiese triplette, d.w.s. modelle, aktiveringsenergieë, en pre-eksponesiële faktore, is uit die isotermiese termogramme bepaal. Hierdie data is daarna gebruik as beginwaardes vir direkte passing van die dinamiese krommes. Die hoofbevindinge was dat: die eerste pirolisestap deur ʼn 3D-diffusiemodel beskryf kan word; die hoë-temperatuurpirolise onder inerte toestande deur die Mampel-meganisme; en die tru-Boudouard-reaksie deur ʼn chemiesbeheerde krimpende-partikelmodel. Die buisoondeksperimente het belangrike insig oor die termiese gedrag van makroskopiese rubberblokkies verskaf. Die blokkies verkool rofweg binne die tye wat deur die TGA-kinetika voorspel word, by laer temperature, met ʼn sigbare verkolingsfront wat na binne beweeg. By nagenoeg 800 °C en hoër word die prosestempo egter deur die warmte-oordragtempo bepaal. Die totale pirolisetyd kan bereken word as die som van die inherente pirolisetyd en ʼn warmte-oordragkomponent. Verder word die proses so vinnig dat die druk wat binne die rubber opbou a.g.v. die piroliseprodukgasse, hoog genoeg om mikro-ontploffings en verpoeiering van die blokkies te veroorsaak ‒ in granules van 100 μm en kleiner. Die tru-Boudouard se reaksietempo is direk eweredig aan die koolstofdioksiedkonsentrasie; die diffusiesnelheid van die koolstofdioksied na die reaksieoppervlak bepaal eers die reakietempo bo ~1 200 °C. Die plasmavergassingsreeks het die ondersoek uitgebrei na die kg h-1 skaal. Die werk is uitgevoer m.b.v. ʼn nuut-opgerigte 15 kW(e) plasma-vergassingstelsel, ontwerp om organiese materiaal met lug, suurstof en stoom, of kombinasies daarvan, te vergas waar die reaksiekinetika vinnig is. In die geval van die tru-Boudouardreaksie is die kinetika egter relatief stadig en die vereiste vertoeftyd in die reaktor beduidend. Die uitmekaarspat van die monsters, soos tydens die buis-oondeksperimente waargeneem, is bevestig deur die voorkoms van rubberkooks oral op koue oppervlaktes in die toerusting en die versamelde materiaal in die filter. Die endotermiese reaksie by die aanvanklike blootstelling van monsters in die buisoond is bevestig deur die temperatuurskommelings in die plasmareaktor wat tydens pulserende rubbervoer waargeneem is. Onder eksperimentele kondisies en -gasvloeie was die reaktorvolume van 4.7 L onvoldoende vir die verlangde vertoeftyd en dit het duidelik geword dat reaktorontwerp van kardinale belang sal wees vir ’n praktiese vergassingaanleg. Ten slotte word ŉ prosesvloeidiagram vir plasmavergassing van skrootrubber deur die tru-Boudouardreaksie voorgestel en bespreek, wat ook bedryfsveiligheid, omgewingsveiligheid, statutêre vereistes, en verwysings na prosesmodellering en ŉ aantal tegno-ekonomiese studies uit die literatuur insluit.
Novel solid phase synthesis of a visible-light-active p-n heterojunction CuS/ZnS photocatalyst for wastewater treatment applications
(University of Pretoria, 2023-12-21) Chirwa, Evans M.N.; Tichapondwa, Shepherd Masimba; rachel.mugumo@tuks.co.za; Mugumo, Rachel
Water contamination by toxic organic chemicals is a major global environmental concern leading to photocatalytic technologies applied in wastewater treatment. The aim of this work was to investigate a new, simple one-pot combustion synthesis technique for creating sulphur- based CuS/ZnS p-n heterojunction nanocomposite photocatalysts. The study examined the photocatalytic activity and reusability of these nanocomposites in removing rhodamine B (RhB) dye from aqueous systems under visible light irradiation. Rhodamine B is an azo dye that is widely applied in processing operations such as the colouring process in textile industries which provides significant socioeconomic benefits; however, its minute traces in water antagonistically affect the environment and all life forms. In this study, a novel heterointerface strategy is proposed for synthesising p-n heterojunction nanoporous agglomerate nanocomposites. This approach involves a simple one-pot one-step combustion method to that attunes the morphology and band gap energy of visible-light-induced nanomaterials. Various characterisation techniques were employed to analyse the physicochemical properties of the synthesised nanocomposite materials. X-Ray Diffraction (XRD) was used to ascertain the crystallinity and purity of the synthesised materials, while X-Ray Fluorescence (XRF) was utilised to determine the composition of the photocatalyst. To confirm the morphology and elemental chemical composition of the synthesised materials, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Energy-Dispersive X-Ray Spectroscopy (EDS) were conducted. Braunauer-Emmett-Teller (BET) analysis was employed to measure the surface area and pore size distribution of the materials. Furthermore, the optical properties, including the photo absorption range and band gap energy of the synthesised nanocomposite materials, were determined using Ultraviolet-Visible spectroscopy (UV-vis). Several intrinsic reaction parameters affecting the photodegradation process were systematically varied to determine the optimal conditions, including the catalyst composition (CuS, ZnS, and CuS/ZnS), catalyst loading (ranging from 0 to 15 gL -1 ), initial solution pH (ranging from 1 to 13), and initial pollutant concentration (varying from 5 to 100 ppm). The experimental findings revealed that a binary CuS/ZnS catalyst, loaded at 10 gL -1 and with a pH of 5, achieved an impressive 97 % degradation of a 5 ppm RhB dye following 270 minutes of visible light exposure. These results highlighted the significant enhancement in photocatalytic degradation efficiency when pristine ZnS is coupled with highly photosensitive CuS. Specifically, the degradation efficiency improved from 67 % to 97 % within 4 hours of solar irradiation. Moreover, it is noteworthy that the Langmuir-Hinshelwood kinetic model demonstrated the best fit to the data when a loading of 10 gL -1 was employed, yielding an impressive R 2 value of 0.99 and a maximum rate constant (k max ) value of 0.0186 min -1 indicative of pseudo-first-order kinetics rates. Additionally, this composite catalyst exhibited remarkable chemical stability and reusability, as it achieved 83 % RhB dye removal after five recycling runs. Further investigations involving scavenger tests identified the photogenerated holes (h + ) and superoxide free radicals (•O 2 ) as the primary reactive species responsible for degradation process. This comprehensive study provides valuable insight into the design of highly efficient nanomaterials for removing organic pollutants from wastewater, and a possible reaction mechanism is proposed.
Microbial fuel cells : the effect of granular activated carbon and graphite as growth substrates on bioelectrogenesis by Geobacter sulfurreducens
(University of Pretoria, 2019-12) Badenhorst, Heinrich; u29079498@tuks.co.za; Van Heerden, Xandra
There is an increasing demand for environmentally sustainable alternative energy sources as the supply of fossil fuels dwindles and the concern about our carbon footprint continues to increase. Bioelectrogenesis is a process that directly converts biomass into electricity. Bioelectrochemical systems hold great potential to breach the gap into sustainable and green energy. Bioelectrochemical systems can be thought of as fuel cells with regenerative, living microbial catalysts. The aim of this investigation was to study the effect of the addition of carbon particles to the anodic chamber of a microbial fuel cell (MFC) containing Geobacter sulfurreducens in order to change the conditions of the MFC. The effect of the crystallinity of the carbon on the efficiency of the cell was evaluated. A crystalline carbon in the form of graphite particles was added to the initial growth media to increase the electrical conductivity of the anolyte. An amorphous carbon, granular activated carbon (GAC) particles, was added to the growth media to increase microbial growth. It was then decided which parameter needed to be prioritised if only one carbon substrate were to be added. A mixture of the two carbon substrates was added to the growth media to investigate what, if both the microbial growth and the conductivity in the MFC were increased, the effect would be on bioelectrogenesis. The effect of various growth periods of G. sulfurreducens prior to inoculation into the MFCs was also investigated. The experiments was thus limited to screening the effects of the bacteria growth media and growth time for the different carbon substrates and mixtures thereof. An increased growth period, 4 months, appeared to be more advantageous for bioelectrogenesis. The addition of GAC particles proved to be advantageous to the microbial growth as the growth was improved nearly 6 fold compared to the control MFC containing G. sulfurreducens only. FE-SEM imaging and BET analyses confirmed that a large and rough surface area is ideal due to the ample attachment sites available for microbes. The biofilm thickens after only 3 weeks of growth. It was evident that the addition of GAC particles to the system was beneficial for bioelectrogenesis. The maximum power density of the MFC containing GAC particles with a 4 months growth period was increased by 6 times compared to the MFC containing pure G. sulfurreducens. The average total energy generated by the MFC containing GAC particles was also 41 % higher than that of the pure G. sulfurreducens control MFC. The overall outputs were improved by the mere addition of GAC particles to the growth media. The addition of the graphite particles to the system had to the opposite effect. The microbial growth was inhibited, which directly caused the bioelectrogenesis to be extremely low. The average energy density of the graphite containing MFC is almost equal to the blank MFC, i.e., the MFC containing no microbial community. This suggests that growth must be prioritised over conductivity. Without microbial growth, increased conductivity does not help the system. The energy density of the MFC comprising the 1:1 mixture of graphite and GAC particles as growth substrate increased the average energy density of the control MFC by 134 %. The addition of the mixture of the two carbon substrates showed a synergistic effect since adding only pure GAC to the MFC increased the average energy density by 41 % compared to the control MFC, and by the addition of pure graphite particles to the MFC had the opposite effect of producing lower energy density than the control MFC. Therefore, mixing the two neat carbon sources and adding the mixture to the MFC, caused the synergistic effect of increasing the energy density of the MFC by more than triple than that of the MFC containing only pure GAC. With the mixture of GAC and graphite particles, both parameters were improved, i.e., microbial growth was improved compared with the control MFC and the conductivity in the system was increased, increasing the electron transfer efficiency and consequently the bioelectrogenesis. One of the industrial applications of MFC systems where the impact can be maximised, is in waste water treatment, since the outcomes include both power generation and the removal of organic compounds in waste streams. It is well documented that numerous microbial fuel cells generate power by oxidation of compounds in wastewater. One study predicts, assuming 100 % efficiency, that the wastewater from a town of 150 000 people could be used to generate approximately 2.3 MW of power; but realistically, a power of 0.5 MW can be expected. From this review it is mentioned that up to 80 % of the chemical oxygen demand (COD) of the wastewater can be removed by using an MFC and that the power generated could be used on site to power additional wastewater treatment. The second most promising application is the use of MFC systems in the biomedical industry as implantable devices. Currently most implanted biomedical devices are powered by batteries, which need to be recharged or replaced, necessitating additional surgeries for the patients. A method for continual electricity generation within the body would revolutionise biomedical devices. The use of MFCs as power sources for implantable devices in humans is a promising focus point. MFCs offer advantages over existing technologies such as lithium-ion batteries in implantable devices such as the heart pacemakers. The MFC would ideally use a biological metabolite fuel source (i.e. glucose or lactate) which is available in physiological fluids such as blood. It is unlikely that MFCs can replace the enzymatic glucose sensors that are currently used, but it was found that a well-designed MFC system, operating in a continuous flow regime, is implanted into the large intestines and utilises the natural flora of microbes within the intestines, could provide adequate power for cardiac pacing. This is one of the most promising future research areas. There are several variables that impact MFC power outputs, therefore extensive future research is still required. The one major problem that needs to be addressed is the longevity of many types of MFCs, most of which would currently be capable of meeting demands for biomedical devices implanted for short-term applications only.
Simultaneous pH and EC control in hydroponics through real-time manipulation of the ammonium-to-nitrate ratio in the nutrient solution
(University of Pretoria, 2024-03) Nicol, Willem; Brink, Gideon Hendrik; u17306290@tuks.co.za; Bosman, Roger Clive
A control algorithm was developed and tested on a hydroponic ebb-and-flow system to assess the efficacy of the proposed control scheme. Three experimental runs were conducted with the purpose of testing the proposed control system on Brassica oleracea var. acephala. The first was an ideal case under sterile conditions, the second was under non-sterile conditions where bacteria were allowed to colonise the plant roots, and the last was a baseline run where nitrate was applied as the only nitrogen source. The system was able to control pH to within 0.5 of the set point (in this case 6.1) while EC control was sufficient to ensure that a steady stream of nutrients were available to the plants at all times. Relative growth rates were fast at maximum average values of between 0.20 per day and 0.21 per day for all of the runs and the yield of organic leaf matter was essentially the same across all the runs at 83 % to 86 % of total plant mass. Finally, the plants grown under the proposed control system were observed to exhibit some improvement in protein and chlorophyll content while the other nutritional characteristics considered were essentially unchanged between treatments. This was all accomplished without having to add any additional toxic ions like Cl- and Na+ as is the case in conventionally controlled systems.
Inactivation of critically ranked carbapenem resistant bacteria and genes in a batch atmospheric plasma reactor
(University of Pretoria, 2023-12-31) Iwarere, Samuel Ayodele; Daramola, Michael Olawale; Chedly, Tizaoui; Unuofin, John O.; u28094001@tuks.co.za; Mosaka, Thabang B.M.
Wastewater treatment plants (WWTPs) have been observed to be direct key reservoir of both antibiotic resistant bacteria (ARBs) and antibiotic resistant genes (ARGs) associated with human infection as high concentrations of ARBs and ARGs have been detected in recycled hospital water. Among the ARBs, the carbapenem-resistant Acinetobacter baumannii and carbapenem-resistant Pseudomonas aeruginosa are ranked as priority 1 (critical) pathogens by the World Health Organisation (WHO) as they constitute a major threat to public health. Moreover, from the heuristic search of literature, it was observed that not only do conventional WWTPs fail to efficiently prevent the discharge of ARBs and ARGs into freshwater environments, but majority of extant advanced treatment technologies are also riddled with bottlenecks that oftentimes outweigh their proficiency. This has warranted the need for treatment technologies that have the capacity to completely obliterate pathogens (ARBs) as well as inactivate their resistance genes (ARGs). In this regard, this study investigated non-thermal plasma (NTP) technology as an alternative disinfection step to inactivate these bacteria and their ARGs. Culture based method and polymerase chain reaction (PCR) were employed in confirming the carbapenem resistance gene blaNDM-1 in Acinetobacter baumannii (BAA 1605) and Pseudomonas aeruginosa (27853). Suspensions of carbapenem-resistant Acinetobacter baumannii (24 h culture) and ATCC Pseudomonas aeruginosa (16 hr culture) were prepared from the confirmed isolates and were subjected to plasma treatment at varying time intervals (3 min, 6 min, 9 min, 12 min and 15 min) in triplicates. The plasma treated samples were evaluated for re-growth and the presence of the resistance gene. The treatment resulted in a 1.13 log reduction after 3 min and the highest ≥8 log reduction (i.e. 99.999999 %) after 15 min for Acinetobacter baumannii. For Pseudomonas aeruginosa, the treatment resulted in a 0.68 log reduction after 3 min and the highest ≥8 log reduction after 12 min. The concentration of the blaNDM-1 gene decreased with time, proving that NTP can inactivate ARGs. The log reduction and gel images suggest that plasma disinfection has a great potential to be an efficient tertiary treatment step for WWTPs. However, there are many factors that still need to be optimised, such as reaction time to completely inactivate the ARGs and removal of biofilms in the way of the treatment of ARBs such as Pseudomonas aeruginosa; before implementation is possible as this technology is yet gradually gaining commercial and industrial espousal.
Characterisation of fischer-tropsch wax/LLDPE blends
(University of Pretoria, 2023-08-17) Focke, Walter Wilhelm; Ramjee, Shatish; u19409088@tuks.co.za; Mhlabeni, Thobile Lillian
Wax is often used as a processing additive in polymer compounding, particularly in thermoplastic processing, due to its ability to improve processability. Wax acts as an external lubricant in the polymer melt, reducing the melt viscosity and increasing the melt flow rate. It’s addition, in masterbatching operations, facilitates improved dispersion of additives and fillers, as well as easier mixing and extrusion. Furthermore, the addition of wax to the polymer can reduce the processing temperature, leading to energy savings and reduced wear on processing equipment. However, the effectiveness of wax as a processing additive is strongly dependent on the type and amount of wax used, as well as the specific polymer being processed and its processing conditions. This study investigated the flow behaviour and compatibility characteristics of Fischer-Tropsch (F-T) wax blended with linear low-density polyethylene (LLDPE), for its possible application as a processing aid package for highly filled pigment masterbatches. The samples were prepared by melt blending using extrusion. The blends were prepared in predetermined quantities of the F-T wax and LLDPE in increments of 10 wt-%. This study provides a survey of characterisation methods and principles using rheology, differential scanning calorimetry (DSC), and hot stage polarised optical microscopy (POM). Both sample preparation and characterisation work were conducted in a temperature range of 120 – 180 °C. Rheological behaviour of the F-T wax/LLDPE blends were measured using the cone-and-plate configuration. The results showed that small additions of LLDPE to F-T wax increased the viscosity of the blend significantly. The composition dependence of the zero-shear melt-viscosity of the blends was adequately represented by the Friedman and Porter mixing rule with alpa = 3.4. This is equivalent to the expression in which the viscosity is calculated via the weight-average molecular mass of the mixture, i.e., ηo = KM^3.4. This implies that the zero-shear melt viscosity was dominated by polymer chain entanglement. The activation energy for viscous flow was found to be insensitive to blend composition. A linear relationship in all the Han plots, i.e., the plots of the logarithm of the storage modulus (G') against the logarithm of the loss modulus (G") was observed. Within the experimental uncertainty, they were essentially unaffected by variations in blend composition, temperature and the applied angular frequency. Additionally, the Cole-Cole plots supported the notion that the wax/LLDPE blends were miscible in the molten state. These results suggest full miscibility of the F-T wax/LLDPE blend system down to temperatures as low as 120 °C. The melting and crystallisation behaviour were studied using hot-stage optical microscopy and differential scanning calorimetry (DSC) in isothermal and dynamic modes. DSC results revealed significant LLDPE melting point depression increasing with increasing wax content. Optical microscopic monitoring of isothermal crystallisation, of the LLDPE phase, showed that adding wax decreased the size of the polymer spherulites. Beyond 50 wt-% wax, it was not possible to distinguish the spherulites at the magnification applied (25). Overall, it was found that increasing the wax content delayed the onset of crystallisation, decreased the overall crystallinity, and reduced the size of the crystallites of the LLDPE-rich phase. The results from both techniques were consistent with partial co-crystallisation of the two components. In summary, all the results indicate full miscibility of the wax and the LLDPE in the melt and partial co-crystallisation in the solid state. Furthermore, in the dynamic DSC scans, the near complete absence of a wax-like melting peak for the blend containing 10 wt-% wax suggests complete miscibility at that concentration.
Assessment of a continuous-flow atmospheric air dielectric barrier discharge in the degradation of tramadol, cefixime, and carbamazepine in aqueous solutions
(University of Pretoria, 2024-02-26) Samuel, Iwarere; Michael, Daramola; samuelo.babalola@gmail.com; Babalola, Samuel Olatunde
Active pharmaceutical contaminants which are constantly released into both surface and ground water through wastewater treatment plants (WWTP), run-off from agricultural fields, excretion, and disposal of unused or expired medicines into sewage, have become a global concern because of their effects on the aquatic ecosystem and human health. Several studies have examined the use of non-thermal plasma reactors like the dielectric barrier discharge (DBD) in the degradation of various pharmaceutical compounds with significant degradation and mineralization efficiencies. However, most studies are either conducted in batch mode with small solution volumes or in pure synthetic solutions. Also, the working gases have mostly been pure synthetic oxygen gases, which can increase the associated cost of treatment. In this study, the performance of a continuous-flow atmospheric air dielectric barrier discharge was assessed specifically for the degradation of tramadol, cefixime, and carbamazepine, which are among the commonly discovered pharmaceuticals in the water cycle. By selecting pharmaceutical contaminants from different drug classifications, this study aimed to show the efficacy of a DBD reactor in degrading a wide range of pharmaceutical residues, irrespective of their physicochemical properties. At alternating current (AC) voltage range of 6 – 8 kV and frequency of 20 kHz, 93% degradation of tramadol was observed in 60 min, >99% degradation of cefixime in 8 min, and 92% degradation of carbamazepine in 40 min. Also, the degradation efficiency of each pollutant was susceptible to the operation conditions of the DBD, including applied voltage, initial concentration of pollutant, pH, conductivity, water matrix, and water flow rate. The chemical species generated were investigated with a spectrometer while radical scavenging experiments were used to establish their respective roles in the degradation of the pollutants. Experiments conducted in real wastewater effluent confirmed that the presence of 𝐻��𝐶��𝑂��3− used as pH buffers played a scavenging role in the degradation of analgesic tramadol in the matrix as the ion reacted with hydroxyl radicals (•OH) thereby reducing its oxidizing power. Also, a toxicity test revealed that the plasma-treated tramadol solution was less toxic to Escherichia coli as opposed to the untreated solution. A new idea was investigated, which was to understand what happens when a metal ion catalyst (Fe2+) is mixed with •OH radical scavengers. In this case, the reactor was able to still achieve significant degradation of cefixime due to the increased production of H2O2 in the aqueous solution. The reactor’s performance was also compared with UV-systems for the degradation of carbamazepine at similar experimental conditions. The degradation results obtained in 40 mins were 6.5%, 17.8%, 89%, 91%, and 98% for UV-only, UV/Fe, UV/H2O2, UV/Fe/H2O2, and plasma systems, respectively. The plasma system also had the highest energy efficiency (75.24 kWh/m3) and the least required energy cost of treatment (13 USD/m3) compared to the UV-systems considered. Thus, an assessment of the laboratory-scale studies has demonstrated the feasibility of the novel continuous-flow atmospheric air dielectric barrier reactor in the degradation of tramadol, cefixime, and carbamazepine pollutants in mono-component solutions. This technology has shown potential for field-scale studies as it can be incorporated into existing wastewater treatment plants to degrade active pharmaceutical residues that escape the various treatment stages. However, considering that pharmaceutical pollutants always exist as mixtures and not as a single component in solution, future studies should consider the efficacy of the reactor in degrading a mixture of the pollutants in different water matrices, including real pharmaceutical waste samples.
Promoting circular economy and sustainable construction practices : investigating the integration of waste upcycling for a greener future, with a focus on eggshell and eggshell membrane utilization
(University of Pretoria, 2023-09-20) Brink, Deon; ainasam000@gmail.com; Aina, Samuel Tomi
As climate change continues to take the centre stage among issues of global discuss, carbon heavy industries including construction and agriculture continue to discover possible ways to foster the implementation of cleaner production technology as well as non-hazardous and sustainable waste disposal solutions. While previous studies have made commendable progress in proposing solutions, much more is still desired. A hybrid approach involving the principles of circular economy and material fusion by adsorption was employed on eggshells. Waste eggshells were valorised and up cycled with its major constituents finding effective use along the chain. This study examined the efficient separation of eggshells from its membrane, the recovery of valuable biochemical compounds and the optimized calcination of the bare shell to produce calcium oxide which is a viable cement replacement material. In addition, the adsorption kinetics of the shell membrane with silver nanoparticles and the effectiveness of the nanocomposite as a cytotoxic additive in cement mortar was investigated. The results demonstrated that in seventeen minutes, acetic acid efficiently weakened the bond between the shell and its semipermeable membrane while minimizing the dissolution of calcium and maximizing the leaching of valuable compounds and proteins like collagen. The calcined, leached, and separated shell produced calcium oxide of comparable quality to that derived from limestone during cement production. Further up the value chain, the separated membrane was employed as an adsorbent for silver nanoparticles and the antimicrobial property of the nanocomposite was exemplified on Pseudomonas aeruginosa and Bacillus subtilis. Pseudo-first order, Pseudo-second order, Two-phase adsorption, Crank internal mass transfer model, and Weber and Morris (W&M) kinetic models were employed to further understand the adsorption process. All the models were adequately fitted with R2 values ranging from 0.922 to 0.990 for both AgNPs and AgNO3. Both Langmuir and Freundlich Isotherm models were also fitted. The adsorption process was optimum at a pH of 6, 25 ◦C, and after 48 h of agitation. Compared to previous studies, a remarkable antimicrobial activity against Pseudomonas aeruginosa and Bacillus subtilis was exhibited resulting in 27.77% and 15.34% cell death, respectively. Analysis of variance and Tukey multiple comparison statistical tests were carried out to highlight the significance of the cell inhibition results at P<0.05 and 95% confidence level. B. subtilis exhibited significant tolerance to metabolic inhibition while P. aeruginosa was significantly inhibited. Following this impressive outcome, the cytotoxic property of the nanocomposite was tested in concrete mortar. This is to combat the ever-increasing nfrastructure budget relating to the maintenance of concrete structures damaged by microbial attacks in moist environments. Waste activated sludge obtained from a local domestic wastewater treatment plant, Pseudomonas aeruginosa, and Bacillus subtilis were studied in both aerobic and anaerobic conditions. Waste activated sludge was most susceptible to the nanocomposite with up to 50% cell death recorded for a 2% cement replacement. In contrast, Pseudomonas aeruginosa and Bacillus subtilis experienced a maximum of 9% and 5% cell death, respectively. For the same cement replacement approximately 5% reduction in compressive strength and no significant change in Tensile strength were measured. This minimal loss in compressive strength is negligible compared to be antimicrobial benefits. Furthermore, considering that about 8.7 million metric tons of eggshell was produced globally in 2021, this alternative use of the shell and its membrane is of great importance to the environment. This include reduction in needed landfill, cost saving and reduction in the generation of GHGs. This study presents a multifaceted approach to the valorisation of waste eggshells, demonstrating their potential as a valuable resource in addressing environmental and industrial challenges. This research underscores the importance of exploring innovative solutions that not only reduce waste but also contribute to the development of cleaner and more sustainable practices in various industries. As we continue to grapple with the pressing issues of climate change and resource conservation, such initiatives pave the way for a more environmentally responsible and economically viable future. This specific upcycling of eggshell and its membrane is of particular economic importance. To start with, the cost associated with managing over 8 million tons of eggshell waste could be invested in more profiting ventures. Through the valorisation process, the shell membrane and other biochemical compounds recovered are valuable sources of income due to the high demand of some of these substances, e.g., collagen. Also, against the backdrop of escalating infrastructure budgets worldwide, employing eggshell membrane as an antimicrobial additive in concrete could result in up to 40% savings in maintenance cost. To achieve these benefits however, it is paramount for the construction and waste industry to start looking into the possible challenges. These include collection, logistics, large scale implementation and more. Likewise, new policies surrounding waste disposal and reusage need to be implemented. Finally, new standards and safety regulations need to be developed while critically exploring and understanding the limitations of waste composites in concrete. In conclusion, this research demonstrates novelty by proposing an optimised eggshell calcination process by ensuring the recovery of the shell membrane and valuable biochemical compounds such as collagen prior to calcination. This will not only ensure the production of high-grade calcium oxide but will also in turn upcycle the net worth of eggshells. Secondly, the study proposes a novel ESM/Nanosilver antimicrobial composite through adsorption. This research also aims to enhance the adsorption efficiency of chemically produced AgNPs and AgNO3 by utilizing the eggshell membrane optimally separated from the shells. Thirdly, this research incorporated the antimicrobial ESM/Ag nanocomposite into cement mortar with the aim of producing an antimicrobial mortar. Lastly, this study demonstrated innovative contribution to academia through 5 publications and 6 conference presentations.

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