| dc.description.abstract | The presence of trace transition metal species in the atmosphere can be attributed to
the emission of particulate matter into the atmosphere by anthropogenic activities, as
well as from natural sources. Trace metals emitted into the atmosphere can cause
adverse health–related and environmental problems. At present, limited data exists
for trace metal concentrations in South Africa. In this investigation, the general aim
was to determine the concentrations of trace metals in atmospheric aerosols in the
industrialised western Bushveld Igneous Complex, as well as to link the presence of
these species in the atmosphere to possible sources in the region.
The measurement site was placed in Marikana, a small rural town situated 35 km
east from Rustenburg in the North West Province of South Africa. It is surrounded
by numerous industrial and metallurgical operations. MiniVolumeTM samplers and
Teflon® filters (2 ;m pores) were utilised to collect PM2.5 and PM10 particulate
samples. The MiniVolumeTM samplers were programmed to filter 5 litres of air per
minute for 12 hours per day, over a six–day period. The starting time for sampling
was altered every six days, in order to obtain both day and night samples. Sampling
was performed for a period of one year.
The collected samples were chemically analysed with inductively coupled plasma
mass spectroscopy (ICP–MS). Surface analysis of the sampled filters was performed
with a scanning electron microscope (SEM) in conjunction with energy–dispersive
spectroscopy (EDS). The dataset was also subjected to factor analysis in an attempt
to identify possible sources of trace metal species in the atmosphere.
The concentrations of 27 trace metals (Be, B, Na, Mg, Al, K, Ca, Ti, V, Cr, Mn, Fe,
Co, Ni, Cu, Zn, As, Se, Pd, Cd, Ba, Pt, Au, Hg, Tl, Pb, U) were determined. Pd, Hg,
Tl, U, Ca, Co, As, Cd, Ba and Au were above the detection limit 25% or less of the
time during the sampling period. With the exception of Ni, none of the trace metals
measured at Marikana during the sampling period exceeded local and international
standards. Higher Ni levels were possibly due to base metal refining in the region.
Pb, which is the only metal species that has a standard prescribed by the South
African Department of Environmental Affairs (DEA), did not exceed any of the standards. It is also significant to refer to Hg that was below the detection limit of the
analytical instrument for the entire sampling period.
The impact of meteorological conditions revealed that wet removal of atmospheric
PM10 trace metals was more significant than the wind generation thereof. During the
dry months, the total trace metal concentrations in the PM10 fraction peaked, while
PM10 particles were mostly washed out during the wet season. Wind speed showed
an unexpected inverse pattern compared to wet deposition. A less significant
seasonal trend was observed for the trace metal concentrations in the PM2.5 fraction,
which was attributed to a faster replenishment of smaller particles into the
atmosphere after rain events.
Separation of trace metal concentrations into PM10–2.5 and PM2.5 fractions indicated
that 79% of the total trace metal levels that were measured were in the PM2.5
fraction, which indicated a strong influence of industrial and/or combustion sources.
Fractionalisation of each of the trace metal species detected showed that for each
metal species, 40% and more of a specific metal was in the PM2.5 fraction, with Cr,
V, Ni, Zn and Mn occurring almost completely in the PM2.5 fraction.
Surface analysis with SEM supported results from the chemical analysis, which
indicated that a large fraction of the particles was likely to originate from
anthropogenic activities and from wind–blown dust. SEM–EDS also detected nonmetallic
S that is usually associated with the Pt pyrometallurgical industry that is
present in the western Bushveld Igneous Complex.
Correlations between Cr, V, Ni, Zn and Mn revealed that the main sources of these
species were pyrometallurgical industries. Explorative factor analysis of the
unprocessed and Box–Cox transformed data for all 27 metals detected, resolved four
meaningful emission sources, i.e. crustal, vanadium related, base metal related and
chromium related. Comparison of trace metal species to other parameters measured
(e.g. CO, BC) also indicated pyrometallurgical activities and wind–blown dust to be
the main sources of trace metals in this region. | en_US |
