Characterisation of atmospheric particulate matter from opencast coal mining activities and adjacent communities

Abstract

Fugitive coal dust emissions are a cause of concern from a government and public perspective, particularly so in communities living in proximity of the coal mines. Atmospheric dust contains various constituents and coal dust is only one of the constituents. However, the general perceptions or views have always been that a significant proportion of dust consists of fugitive coal dust which is respirable. Compounding these perceptions is that currently, there is a relative level of difficulty in presenting dust analysis information in a manner which identifies and characterises the coal component of dust.

The aim of this study is the characterisation of atmospheric PM from opencast coal mines and adjacent communities through the application of various microscopic, spectroscopic, thermogravimetric and thermal optical techniques to distinguish the carbon component originating from coal mining and from other sources such as biological and combustion sources in coal mining regions.

Atmospheric particulate matter (PM) of 10 micrometers (µm) or smaller collected from three opencast coal mines and adjacent communities in the Mpumalanga and Limpopo Provinces in South Africa using both active and passive sampling were used as a case study. The analytical techniques include Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDS), reflectance microscopy (reflectance), optical microscopy, X-Ray Photoelectron Spectroscopy (XPS), Raman Spectroscopy (Raman), Thermogravimetric Analysis (TGA), Isotope Ratio Mass Spectrometry (IRMS) and thermal optical analysis.

Atmospheric PM sampled through passive samplers was analysed through SEM-EDS and optical microscopy, while the atmospheric PM samples from active monitoring (filtration) were analysed using Raman, reflectance, TGA, XPS, IRMS and thermal optical techniques Optical microscopy on the passive samples did enable the quantification of PM10-2.5 concentrations for PM from the opencast coal mines and adjacent communities. At the magnifications used, accurate quantification of PM2.5 is not possible. Useful information on the morphology and chemical composition of individual particles was obtained, but this was not adequate for source apportionment, as the mineral composition in mines and in adjacent residential areas is often similar.

For the active monitoring campaign, PM10 daily average concentrations ranged from 59 µg/m³ to 90 ug/m³. The PM10 daily NAAQS was only exceeded at one of the residential stations viz. Clewer.

The differentiation between elemental carbon (EC) and brown/ organic carbon (OC) of the individual particles could not be achieved through SEM-EDS, nor through XPS, Raman, TGA, reflectance microscopy of bulk samples.

Thermal optical analysis allowed the quantification of the EC/OC fraction on actively collected bulk samples and indicated a clear distinction between the EC and OC contents as well as the OC/EC ratio from the mine and residential samples. The analysis of the stable carbon (C) and nitrogen (N) isotope ratios (δ13C/δ12C) and (δ15N/δ14N) of bulk samples showed that the values obtained for both ratios and the relationship between them could also distinguish between samples from the mines and the adjacent residential areas. Further development to refine the source apportionment based on a combination of these techniques is proposed, which will require further sampling and detailed analysis of potential carbonaceous sources.