A Thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy
2015. / Gold 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).
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/20168 |
| Date | 25 May 2015 |
| Creators | Yalala, Bongani Ndhlovu |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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