Electronic Theses and Dissertations (Masters)
Permanent URI for this collectionhttps://hdl.handle.net/10539/38003
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Item Silver nanoparticle-modified cellulose nanocrystals for fouling control in membrane distillation(University of the Witwatersrand, Johannesburg, 2023-06) Mpala, Josephine Tshepiso; Nthunya, Lebea; Richards, Heidi; Etale, AnitaA global reduction in water resources and the growing demand for fresh water has motivated the quest for the development of sustainable water-augmenting technologies. Membrane distillation (MD) is envisaged as an attractive desalination technology, surpassing cost challenges faced by conventional desalination technologies. Yet, its industrial commercialization faces multiple limitations, including the production of low water fluxes, membrane wetting and membrane fouling. This study sought to investigate the performance of silver nanoparticles (AgNPs) embedded on cellulose nanocrystals (CNCs) (CNC-capped AgNPs) to lessen the impact of biofouling in MD. This was conducted through coating the polyvinylidene fluoride (PVDF) membrane with CNCcapped AgNPs. Prior to coating with CNC-capped AgNPs, PVDF membrane properties were improved (for MD suitability) through blending with polyvinylpyrrolidone (PVP) and functionalized carbon nanotubes (f-CNTs). The resulting membrane had an improved overall porosity, and a respective increase in surface roughness (75%) and mechanical strength (45%). Pristine CNC-capped AgNPs’ characterization presented stable AgNPs with minimal leaching. Transmission electron microscopy (TEM) micrographs revealed a uniform dispersion of spherically shaped AgNPs exhibiting 13.3 ± 3.4 nm average diameter. The presence of AgNPs on the surface of CNCs afforded excellent thermal stability and good anti-microbial activity, mainly against E. coli, P. aeruginosa, S. aureus, S. epidermis, and S. saprophyticus. Following membrane modification, preliminary anti-bacterial tests conducted on the CNC/AgNP-modified PVDF membrane revealed a 98.7%, 52.3%, 78.0%, 53.9% and 93.3% reduction of E. coli, P. aeruginosa, S. aureus, S. epidermis, and S. saprophyticus cells, respectively, demonstrating its ability to control biofouling. Although the CNC/AgNP-modified PVDF membrane exhibited improved membrane properties, such as high surface roughness, high liquid entry pressure (LEP), and good hydrophobicity, its performance in MD (with artificial seawater as the feed stream) was poor, producing the lowest average water flux (0.179 ± 0.0303 kg/m2 /hr) compared to the unmodified PVDF membrane (0.528 ± 0.0838kg/m2 /hr), mainly due to pore blockage. However, upon spiking the artificial seawater with a monoculture of G. Stearothermophilus, the CNC/AgNP-modified PVDF membrane displayed the most stable water flux while the unmodified PVDF membrane’s water flux decreased by 79.3% over the 24-hour (h) period. This was attributed to the formation of a biofouling layer on the PVDF membrane which was absent on the CNC/AgNP-modified PVDF membrane. The AgNPs on the surface of the membrane afforded minimal bacterial deposition during operation. These results ascertain the possibility of biofouling minimization in MD using CNC-capped AgNPs, contributing to MD’s body of work for its ultimate realization for up-scaling.Item Tailored Fabrication of MXene/Chitosan Nanocomposites as Efficient Adsorbents for Heavy Metals Removal from Wastewater(University of the Witwatersrand, Johannesburg, 2023-08) Ibrahim, Yassmin Ahmed Ismail; Eid, Kamel; Ozoemena, Kenneth IkechukwuMXene (Ti3C2Tx) has been extensively utilized in water purification systems, including toxic metal ions removal, owing to the unique layered structure and abundant oxygen surface groups. However, challenges such as aggregation and solubility of Ti3C2Tx nanosheets in water have prompted the need for innovative strategies. In this study, we introduce a i3C2Tx-incorporated chitosan matrix (MX/CS) adsorbent designed to address solubility concerns during water treatment. MX/CS adsorbents are tested towards the capture of “cadmium” (Cd 2+) and “Zinc” (Zn2+) ions in aqueous solutions at varied pH values (i.e., acid, neutral and alkaline), initial ions concentrations (25, 50 and 100 ppm), and varied Ti3C2Tx loading (i.e., 1, 5 and 10), to study the optimization adsorption parameters. In addition, the Ti3C2Tx nanosheets were activated/alkalinized at ratio (2:1, i.e., 2MX:OH/CS), where more negative-ions sites are provided, thus, enhancing the preferential sorption for heavy metal ions in terms of high adsorption capacities, and kinetics than the non-activated samples (MX-10/CS). Freundlich isotherms are predominated for the Cd2+ and Zn2+ ions adsorption on both adsorbents. A selectivity study reveals that Zn2+ ions got adsorbed faster on the adsorbents than Cd2+ ions because of its low atomic radii and electronegativity. Finally, the adsorbents will be generated and prepared for additional adsorption cycles to test their stability. The second part of this work is to present the novel fabrication of multifunctional hydrophobic polymeric foam MX nanocomposites for large-scale ultrafast wastewater treatment. Likewise, the foam nanocomposite will be tested for the adoption of multi-ions solution over wide pH rage to demonstrate the applicability of the novel adsorbent for large-scale applications. Overall, this research contributes to the advancement of water treatment technologies by enhancing the stability of MXene-based adsorbents and introducing an innovative fabrication method for hydrophobic polymeric foam MX nanocomposites. The outcomes demonstrate the applicability of these novel adsorbents for efficient and scalable water purification solutions.Item A comprehensive analysis of urban river pollution – the case of the Hennops river in Gauteng Province, South Africa(University of the Witwatersrand, Johannesburg, 2023) Letseka, Thabiso Esaiah; Chimuka, L.; Richards, L.H.The water quality of rivers is declining at an alarming rate due to pollution from anthropogenic activities associated with urbanization. To ensure ecological restoration and management of rivers, engaging in pollutant source apportionment, evaluation, and monitoring of water quality is of great significance. The study delivers a comprehensive assessment of the state of pollution in the Hennops river catchment facing pollution threats from rapid urbanization. The water quality assessment of the Hennops river was performed through chemical, microbiological, microplastics analysis and ecotoxicological approaches, spanning from upstream region in Tembisa to the downstream Hartbeespoort Dam. Standard methods were employed to assess physiochemical properties of the river’s water. Electrical conductivity and pH fell within the accepted criteria based on the standard water quality guidelines. However dissolved oxygen (DO) levels were below acceptable limits, ranging from 1.53 mg L-1 to 6.47 mg L-1. This signifies a substantial demand for oxygen in the river, likely due to the discharge of sewage from leaking pipes and wastewater treatment plants. This sewage introduces a high volume of organic matter, leading to an increased oxygen demand in the water. Microbiological pollution indicators were employed to assess the microbial water quality of the river. The study's findings revealed elevated bacterial counts, with Escherichia Coli (E. coli) reaching up to 2 250 cfu mL-1 upstream and decreasing to 30 cfu mL-1 downstream. These high counts suggest faecal contamination in the river water. Similar trends were observed with total coliform counts, high coliform counts 170 000 cfu mL-1 in the upstream which remained detectable even downstream and beyond the Hartbeespoort Dam, despite the dilution effects within the dam. The dam was identified as the primary repository for pollution originating upstream. Grab sampling followed by solid phase extraction (SPE) and the passive sampling using a Polar Organic Integrative Sampler (POCIS), were employed as sample preparation methods for preconcentration of methocarbamol, etilefrine, nevirapine, carbamazepine and venlafaxine from river water with subsequent analysis on Liquid Chromatography coupled to quadrupole time of flight mass spectrometry. Both methods yielded good figures of merit with limits of quantification in the range of 0.57 to 2.12 ng mL-1 for POCIS and 0.19 to 1.82 ng mL-1 for SPE. The compounds were detected in the water but at low levels (µgL-1 ), with detected concentrations of carbamazepine in the range 0.62 ng mL-1 – 0.32 ng mL-1 , methocarbamol detected in the range 0.11 ng mL-1 - 0.14 ng mL-1 and venlafaxine 0.50 ng mL-1 – 0.44 ng mL-1 using POCIS. The detected concentrations using SPE were in the range 0.13 ng mL-1 – 0.19 ng mL-1 for carbamazepine, while nevirapine and venlafaxine were detected although below limit of quantification. This underscores the advantage of using passive samplers, which enable the detection of fluctuating contaminant concentrations over time, in contrast to the one-time measurements obtained through grab sampling. In the case of microplastics in the water and sediment samples, five polymer types were identified: polyethylene (PE), polypropylene (PP), high density polyethylene, (HDPE), polyester and polystyrene. The predominant polymer type in surface water was PE (48.6 %), and that in sediment was PP (52.7 %). PE and PP were the most abundant polymer types in both phases, and as these also the leading polymers in plastics production. 80% of the identified microplastics were found to be fibre with most dominant sizes of 1-2 mm for sediments and 0.5-1 mm in water samples. The conducted tests deemed the river water not suitable for irrigation, drinking or recreational purposes and not capable to support aquatic life.