Determination of heavy metal composition of particulate matter in a typical chrome and platinum mine area in the Limpopo Province, South Africa

Abstract

The Greater Tubatse Municipality (GTM) is found in the northern part of the Sekhukhune District Municipality in the Limpopo Province of South Africa. It is an area plagued by poor air quality resulting from the activities of chrome and platinum mining, ferrochrome smelters, agricultural and forestry activities, domestic and waste burning practices, as well as vehicular traffic and road dust. This study aimed to contribute to current knowledge on the characteristics and sources of ambient particulate matter (PM) in the area. It also sought to identify the relevant sources, the chemical composition of the pollution stemming from these sources, the amount that each source contributes to PM and the influence of meteorological processes on the spatial distribution of PM in the GTM. PM, PM10 and PM2.5 were measured using inexpensive University of North Carolina gravimetric passive samplers. Computer Controlled Scanning Electron Microscopy was used to characterise the mass concentrations of the PM. The mean annual concentration values for PM10 for the various measurement sites in the area were: Site 1 (32.02 μg.cm-3), Site 2 (31.28 μg.cm-3), Site 3 (38.11 μg.cm-3), Site 4 (24.1 μg.cm-3), Site 5(24.65 μg.cm-3), and Site 5 (20.98 μg.cm-3). These values were below the South African annual NAAQS of 40 μg.cm-3. The PM2.5 mean annual concentrations were as follows: Site 1 (11.8 μg.cm-3), Site 2 (4.7 μg.cm-3), Site 3 (4.8 μg.cm-3), Site 4 (3.0 μg.cm-3), Site 5(3.2 μg.cm-3), and Site 5 (2.5 μg.cm-3). The annual PM2.5 concentration was below the South African annual NAAQS of 20 μg.cm-3. The spatial interpolation of PM chemical components showed that metal concentrations were highest closer to the industrial facilities with annual chromium concentrations exceeding the New Zealand annual limits for both Cr(III) (0.11 μg.cm-3) and Cr(IV) (0.0011 μg.cm-3) in the study area which is dominated by chromium mining and ferrochrome smelters. The HYSPLIT model was run to determine the regional and trans-boundary air transport to the receptor locations. The HYSPLIT cluster transport pathways indicated both regional and trans-boundary transport of air masses from the North-West Province in South Africa, Zambia (passing over the Limpopo Province in South Africa), Zimbabwe (passing over Limpopo Province), Mozambique and the Indian Ocean (passing over Northern KwaZulu- Natal Province and Swaziland). The TAPM model and GIS software were used to determine the distribution of PM10 from industrial point sources and to consider their possible impacts on human health using modelled hourly data. Meteorological parameters such as Monin-Obukhov length, mixing height, ventilation coefficient and air pollution potential calculated using the TAPM model output data were unable to accurately predict the spatial variability of PM and its chemical components in a complex terrain. Source contributions were analysed using the positive matrix factorization (PMF) model. The average source contribution in the study area were vehicle (15.1%), chrome smelter (17.3%), industrial coal combustion (19.7%), wood combustion (24.5%) and crustal material (23.6%). The PM10 spatial distribution from industrial sources showed the highest concentration of 27.3 μg.cm-3. The seasonal distribution indicated that PM10 was high during winter (27.3 μg.cm-3) and low in autumn (15.2 μg.cm- 3). The overall PM10 distribution was to the west and south-west of the industrial point sources. The findings from this study on chemical composition, source profiling and source apportionment, and the analysis of the possible impacts from industrial point sources described here can inform the development and implementation of cost-effective strategies to mitigate the scourge of air pollution encountered in South Africa and many other low- and middle-income countries.