3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Modelling of low temperature oxidation of coal dumps.(1998) Kaitano, Rufaro.Storage and waste dumps from coal mining tend to spontaneously combust. This is mainly as a result of the oxidation process which is accelerated by the availability of oxygen and the exothermic nature of the oxidation process. In cases of poor ventilation the heat accumulation within the bed is thought to lead to the spontaneous combustion of coal. The work in this dissertation aims to investigate the change in oxygen concentration in a bed of coal and also measure the rate of oxidation (oxygen absorption) in a closed reactor under isothermal conditions. Drying rate of coal under nitrogen was also looked into. An analysis of the oxygen concentration profile in a three metre 20 cm ID plastic column filled up with coal has been carried out. As the coal ages (becomes oxidised) its reactivity towards oxygen decreases and changes in the oxygen concentration profile are noticed. Experiments have been carried out up to 8 months and from the results obtained, a simple pseudo-steady-state model has been developed to describe the diffusion of oxygen into a reacting coal bed. The findings could prove useful in trying to find a solution to coal and waste dump fire control. The second experiment is a simple isothermal oxygen absorption experiment in which the rate of absorption of oxygen on a given coal sample is measured at different initial concentrations of oxygen. The initial concentration of oxygen is varied over a fairly wide range in order to determine the dependence of the rate of oxidation on the oxygen concentration. The rate- limiting step in low temperature oxidation of coal is found to be the absorption of oxygen. Moisture also plays a role in coal oxidation. Drying experiments were also carried out so as to quantify and investigate the rate of loss of moisture. Models have been developed which try to explain tile mechanisms involved in the drying process. The modelling suggest that the bound water model is more appropriate to the type of behaviour exhibited during the drying process.Item Modelling of low temperature oxidation of coal dumps.(1998) Kaitano, Rufarostorage and waste dumps from coal mining tend to spontaneously combust. This is mainly as a result of the oxidation process which is accelerated by the availability of oxygen and the exothermic nature of the oxidation process. In cases of poor ventilation the heat accumulation within the bed is thought to lead to the spontaneous combustion of coal. The work in this dissertation aims to investigate the change in oxygen concentration in a bed of coal and also measure the rate of oxidation (oxygen absorption) in a closed reactor under isothermal conditions. Drying rate of coal under nitrogen was also looked into. An analysis of the oxygen concentration profile in a three metre 20 cm ID plastic column filled up with coal has been carried out. As the coal ages (becomes oxidised) its reactivity towards oxygen decreases and changes in the oxygen concentration profile are noticed. Experiments have been carried out up to 8 months and from the results obtained, a simple pseudo-steady-state model has been developed to describe the diffusion of oxygen into a reacting coal bed. The findings could prove useful in trying to find a solution to coal and waste dump fire control. The second experiment is a simple isothermal oxygen absorption experiment in which the rate of absorption of oxygen on a given coal sample is measured at different initial concentrations of oxygen. The initial concentration of oxygen is varied over a fairly wide range in order to determine the dependence of the rate of oxidation on the oxygen concentration. The rate- limiting step in low temperature oxidation of coal is found to be the absorption of oxygen. Moisture also plays a role in coal oxidation. Drying experiments were also carried out so as to quantify and investigate the rate of loss of moisture. Models have been developed which try to explain the mechanisms involved in the drying process. The modelling suggest that the bound water model is more appropriate to the type of behaviour exhibited during the drying processItem Characterization, bioavailability and health risk assessment of mercury in dust impacted by gold mining(2015-05-25) Yalala, Bongani NdhlovuGold mining in South Africa has been the backbone of the economy for many years. With it came economic well-being, the growth and development of satellite towns, cities and metropolitan cities, e.g. Johannesburg-a place of gold. Unfortunately, it also came with adverse effects, most of which are now evident, after a century of mining, with little or no regard for pollution prevention or any form of remediation. Of interest, in this study, is the presence of tailings storage facilities (TSFs) found within the residential areas, in close proximity to commercial district and industry, having been built around them. Currently, some 270 TSFs lie dormant, pregnant with vast number of toxic heavy metals from the initially low efficient but selective gold processing techniques. This led to the deposition of the sand dumps, with high sulphur, iron, chromium, cadmium, arsenic, and mercury amongst other toxic metals. Exposure to oxygen, and water, the pyrites were oxidized and formed acid mine drainage (AMD), which resulted in the leaching out all toxic heavy metals into ground water and surface water causing serious water pollution and environmental degradation. Due to the low efficient gold processing technique, some gold amount was discarded together with the tailings materials. The reprocessing of these dumps led to the generation of dust, which is easily distributed over large areas of land. The unrehabilitated, semi-rehabilitated, and the abandoned TSFs contributed to all forms of pollution, majorly, windblown dust from unprotected tops and sides, AMD leaching toxic heavy metals. In this study, mercury, one of the most toxic elements found within the vast TSFs was determined. This was carried out as part of a larger environmental impact assessment on the effects and scale of pollution from the gold mining in the Witwatersrand. The study area consisted of the greater Johannesburg area, covering commercial business district (CBD), the industrial areas (Aeroton, City Deep, Germiston, Selby, Springs), and the residential areas (Alberton, Boksburg, Centurion, Germiston, Greenside, Sandton, Springs). Dust samples were collected from paved surfaces in the streets, and accessible buildings, were sieved into three sieved into three fractions (PM100, PM50, PM25), and most of the work focused on the smallest size fraction (PM25) in order to study impact of inhalable and respirable dust. Three sequential extraction procedures (modified BCR-the European Community Bureau of Reference, selective sequential procedure (SSE), and novel sequential extraction procedure (n-SEP)) were applied for partitioning and evaluating the mobility, availability and persistence of mercury in urban dusts. Bioavailability of mercury was assessed by leaching dust with artificial gastric and lung fluids which mimicked body conditions. Contamination levels were assessed based on the enrichment factor (EF), contamination factor (Cf) and geoaccumulation index (Igeo) were calculated to further assess the environmental risk and provide a preliminary estimate of the main sources of mercury in street dust. Non-carcinogenic effects and carcinogenic effects due to exposure to urban street dusts were assessed for both children and adults. The total mercury (HgTOT) ranged from 269 to 1350 μg kg-1. In the PM25 size fraction, mercury exhibited the following decreasing order of HgTOT: industrial area > CBD > residential area. This order shows that the HgTOT concentration in the street dust decreased with increased distance from the TSFs. The highlight was that the highest HgTOT was reported in industrial areas next to the TSFs, tailings reprocessing areas, and tailings footprints. Furthermore, in residential areas grossly affected by TSFs and tailings reprocessing, reported high HgTOT values similar to those reported for industrial samples. These results indicated that the presence of TSFs were largely responsible for the mercury found in the dust. The results from the characterization of the dust showed a large concentration of fine particulate matter, with the characteristically high quartz (74 – 98 wt. %), and minor minerals phases such as chloritoid, chlorite, K-feldspar, jarosite, mica, muscovite, pyrite, and pyrophyllite, all below 10 wt. %. These have been known to enrich trace metals, hence a high concentration of mercury. The close proximity of the tailings to the communities led to the determination of bioavailability of mercury from dust. The bioaccessible Hg extracted by lung fluid (up to 3% of HgTOT) was higher than that of gastric fluid (up to 1% of HgTOT) and was related to the mobile pool of Hg in dust. This suggests that human exposure to Hg in dust via inhalation is greater than that via the gastric tract. These values were very similar to the values obtained from water soluble phase in the sequential extraction procedure (average 1.4% of HgTOT). This indicated that these fluids were able to extract the most bioavailable fraction of Hg, which is responsible for most of the transformation reactions involving mercury. Contamination assessment factor was carried out to classify the pollution levels and indicate whether they are from natural or anthropogenic sources. Based on the EF, Cf, and Igeo, 70, 82, and 84% of the street dust samples were classified as heavily enriched, very highly contaminated, and strongly polluted by mercury, respectively, indicating that they are of anthropogenic origin. The human health risk model was useful in identifying the areas of health risks from exposure to mercury pollution. It showed that children were more vulnerable than adults when exposed to mercury in dust via ingestion. The cancer risk for exposure to As, Cd, and Cr by both children and adults was significantly high for oral ingestion of dust. Cr (VI) was the highest contributor followed by As and lastly Cd. For inhalation pathway, the possibility of developing cancer after a lifetime exposure was low and below the acceptable limits (10-6).Item Status and molecular identification of arbuscular mycorrhizal (AM) fungi associated with Acacia spp. on rehabilitated gold and uranium mine tailings(2015-02-04) Buck, Michelle ToniPhytoremeditation of mine tailings provides the most cost-effective means of alleviating their pollutant effects. Research has shown that successful revegetation of mine tailings can be optimised by providing appropriate microbial symbionts for the plants. The aim of this study was to assess the arbuscular mycorrhizal (AM) status of trees currently being used for phytoremediation trials of mine tailings in the Welkom gold fields, and to determine the AM fungal diversity of these sites. The Acacia spp. analysed were growing on rehabilitated gold and uranium mine tailings which had undergone different rehabilitation regimes. Planted acacia trees which had been inoculated with crude AM fungal inocula were present on one mine tailing site as compared to the second mine tailing site on which the acacias were naturally colonisers and the site had been ameliorated with garden refuse. Root and slime samples were collected in early spring and half if each initial sample was used immediately for colonisation analysis and to identify AM fungi through molecular analysis of the small subunit rRNS gene sequences; the other half of each sample was used to produce trap cultures which were used later for colonisation and molecular analysis. Total AM fungal colonisation of initial samples for planted acacies was 19 % and for naturally colonising acacias was 66 %. The total AM fungal colonisation of trap culture samples for planted acacias increased to 32 % and for naturallhy colonising acacias it increased to 78 %. Spore counts of initial samples averaged 402 spores per 100 g-1 soil for planted acacias and 455 spores per 100 g-1 soil for naturally colonising acacias. For trap culture samples, spore counts decreased by approximately 50 %. The AM fungi identified fell within 8 genera, namely, Diversispora, Rhizophagus, Scutellospora, Claroideoglomus, Cetraspora, Sclerocystis, Glomus and Redecker. The study represents a first report utilising molecular biosystematics with AM fungal DNA from colonised roots as the template. The results will assist in making decisions about future AM fungal surveys and applying AM fungal inoculum in phytoremediation trials of mine waste sites. Key words: Phytoremediation, mine tailings, arbuscular mycorrhizal (AM) fungus, Acacia, molecular identification, SSU rRNA gene sequence