Characterisation of dust fallout around the City of Tshwane (CoT), Gauteng, South Africa

Abstract

The aim of the project was to study the mineral, chemical and morphological characteristics of the fallout dust samples. The dust fallout samples were collected at five different sites following the internationally accepted standard procedure for collection and analysis, South African National Standard and American Standard for Testing Methods (ASTM) D1739-98 (2010). Passive single buckets containing ¾ of distilled water and 10.0 ml of sodium hypochlorite (NaOCl) solution, hoisted at a height of about 2.5 m were exposed from March – June 2013 and were collected every month, taken to the laboratory for characterization. First, the samples were filtered and mass concentrations per month were gravimetrically measured. The samples were then prepared for further characterization by the MicroScan Particle Analyser (MSPA)(Microscope UOP UB 100j) which measured the mineral content and the Scanning Electron Microscope - Energy Dispersive X- ray (SEM - EDX) measured the images of different particles (size, shape) and their elemental content. The results from the MicroScan analysis showed the prevalence of sandy rounded-shaped quartz (SiO2) particles across all sites. Minerals detected within the crustal material at all sites were quartz (SiO2), haematite (Fe; Fe2O3; Al2O3), feldspar (Na, K, Mg, Ca: AI2Si2O8), garnet (SiO4), aluminium silicates (AI2Si2O8) and calcium hydroxide [Ca (OH)2]. Evidently, sites A, B and C dominated the abundance of these minerals. On the other hand, sites D and E showed fine and rounded quartz and quartzite with various trace metal oxides. Measured elements in decreasing concentrations were Si, Ca, P, Al, K, S, Fe, Cu, Ti and Mg. Both coarse particle (2.5 - 10.0 μm) and fine particle sizes ranges (< 1.0 to 2.5 μm) were detected. Large irregular shaped particles sizes were common and were linked to crustal material. The results provide good baseline information which requires further characterization of samples by different analytical techniques and validation of possible sources. The ICP-MS was used to determine the elemental composition of dust fallout and the mean concentration of the elements of concern ranged from 1.1 - 2.6 for As, 0.2 - 0.6 for Cd, 17.0 - 21.9 for Ni, 449.8 - 751.1 for Pb, 9.9 - 22.3 for V and 101.7 - 293.0 μg/m3 for Zn. The elements such as Cd exceeded the set limit 0.005 μg/m3 by OSHA (2015) and NIOSH (2007); Pb exceeded the set standard of 0.5 μg/m3 set by OSHA (2015), NIOSH (2007), WHO (2000), USEPA (2012) and DEA (2013). The rest of the elements fell within the set guidelines by various regulatory national and international bodies. The study concludes that, although, all the sampling sites recorded the dust fallout rates within the SANS 1929 (2011) and DEA (2013) average RESIDENTIAL limit of 600 mg/m2/day, the future accumulation and exceedances cannot be ruled out owing to the increasing developments in urban areas