Assessment of atmospheric trace metals in the western Bushveld Igneous Complex, South Africa. PG van Zyl et al., 30 September 2013

Transcription

1 Assessment of atmospheric trace metals in the western ushveld Igneous Complex, South Africa PG van Zyl et al., 30 ptember 2013

2 Outline ackground and aims Sampling site and methods Results Trace metal concentrations comparison to ambient air quality standards Temporal variations Size-resolved distribution SEM-EDS analysis Source apportionment Conclusions

3 ackground and aims Atmospheric trace metal species in the atmosphere are mainly attributed to the emission of particulate matter (PM) into the atmosphere tural emissions of trace metals result mainly from different processes acting on crustal minerals, e.g. erosion, surface winds and volcanic eruptions, as well as from natural burning and from the oceans.,,,, Ca, Ti, Cr,, Mn Predominant anthropogenic sources are pyrometallurgy, biomass burning (veld fires), fossil-fuel combustion and incineration metal smelting one of the most important anthropogenic trace metal emission sources. As, Cd, Cu, Ni,, V, Hg, Pb. Trace metals emitted into the atmosphere can cause a variety of health-related and environmental problems, depending on the extent and time of exposure.

4 ackground and aims Only a few studies on the concentrations of atmospheric trace metals in South Africa have been conducted. (Nriagu 1996; Engelbrecht et al., 2002; aker et al., 2002; gabi, 2006; leynhans, 2008) South Africa has the largest industrialised economy in Africa, with significant mining and metallurgical activities. A large fraction of the South African mineral assets is concentrated in the IC covers an area of km 2, which stretches across the North West, Gauteng, Mpumalanga and Limpopo Provinces. This region holds numerous metallurgical operations due to large deposits of V, Ni, Cu, Cr, Co, Sn. 80% of the platinum group metals (PGMs) of the world are also produced from the IC.

5 ackground and aims The IC is geographically subdivided into five limbs, of which the western limb is the most exploited and industrially developed. The western IC forms part of a recently declared air pollution priority area, i.e. the Waterberg-ojanala tional Air Quality Management Priority Area At present limited data exist for atmospheric trace metal concentrations in this region (Piketh, 2005). Atmospheric trace metals were collected in the western IC at Marikana in the North West Province surrounded by more than 30 mines and 11 pyrometallurgical smelters within a 60 km radius, which include Pt, Cr, V, Mn and base metal production..

6 ackground and aims The general aims of this investigation were to compare trace metal concentrations measured at Marikana to existing air quality standards, conduct a temporal (seasonal and diurnal) and size resolved assessment of trace metal species, determine possible sources of trace metal species in the western IC.

7 Sampling site and methods Measurements were conducted at Marikana ( S, E, 1170m AMSL), which is a small village situated approximately 35 km east of Rustenburg. The site was situated in a residential area on the property of the Marikana Municipal clinic, which provided access to electrical supply and also ensured the safe keeping of equipment. (Venter et al., 2012). A mobile monitoring station comprehensively equipped with atmospheric measurement instruments was placed at the site. Measurements included meteorological data, radiation measurements, inorganic gaseous species concentrations (SO 2, NO x, O 3 and CO), as well as physical and chemical properties of aerosol particles (Laakso et al., 2008, Petäjä et al., 2013).

8 Sampling site and methods

9 Sampling site and methods PM 2.5 and PM 10 samples were collected with MiniVol samplers on Teflon filters 5 L/min for 12 hours/day for a period of six days (72 hour samples) for one year (Nov 2008 Oct 2009). The starting time for sampling was altered every six days in order to obtain diurnal samples, i.e. 6 am and 6 pm. Atmospheric trace metals were extracted with hot acid extraction for ICP-MS analysis concentrations of the metals in atmosphere were calculated using the US EPA compendium method IO trace metal species detected (,,,, Ca, Ti, V, Cr, Mn,, Co, Ni, Cu,, As, Cd, Pt, Hg, Pb, e,,, Pd, a, Au, Tl, U). Metal concentrations below the detection limit of the ICP-MS were considered to have concentrations of half the detection limit of the specific metal species considered (a precautionary assumption frequently used in health-related environmental studies).

10 Results Trace metal concentrations Particulate Pd, Hg, Tl and U were below the detection limit of the analytical technique for the entire year. The concentrations of the trace metal species Ca, Co, As, Cd, a and Au were only for 25% or less of the time during the sampling period above the detection limit. Concentrations reported for these metals are therefore likely to be an over-estimate due to the precautionary assumption described previously. These are therefore not discussed further due to the uncertainties associated with their concentrations.

11 Results Trace metal concentrations Concentration Annual Annual Maximum 3- range of 72-hour standard limit averages month averages samples Ti V 1 (b) # Cr 2.5x10 4 (a) * Mn 0.15 (a) Ni 0.02 (b) Cu Pt Pb 0.5 (a),(b),(d), (c) e a WHO air quality guidelines for Europe b European Commission Air Quality Standards c tional Ambient Air Quality Standards of the US EPA d tional Air Quality Act of the South African Department of Environmental Affairs # 24-Hour limit value * WHO guideline for Cr(VI) concentrations associated with an excess lifetime risk of 1: Month limit value l concentrations in µg m -3

12 Ave concentration PM10 / (µg.m -3 ) Ave concentration PM2.5 / (µg.m -3 ) Results asonal variation Cr Cr Cr e Ti V Cr Mn Ni Cu Pt Pb Others e Ti V Cr Mn Ni Cu Pt Pb Other

13 Results asonal variation

14 Results Diurnal variation PM Night 4% PM 2.5 Night 96% PM 2.5 Day 62% PM Day 38% PM % PM %

15 Results asonal variation

16 Results Size distribution 100% 90% 80% 70% 60% 50% 40% PM PM % 20% 10% 0% Pt Cu e Pb Ti Cr Mn V Ni

17 Results SEM-EDS analysis

18 Results Source apportionment PCFA operates on sample-to-sample fluctuations of the normalised concentrations. PCFA was applied as an explorative tool in this investigation the dataset was not large enough for definitive source apportionment. Factor analysis of smaller datasets is more susceptible to statistical artefacts, such as autocorrelation or the influence of outliers. Therefore, only the most apparent groupings of metal species relating to expected sources in the region were identified. The dataset used for PCFA was the combined ambient concentrations determined for all 27 metal species in both the PM 2.5 and PM 10 fractions. Six factors were retained from the dataset.

19 Results Source apportionment Factor 1- Species 25% or less above the detection limit e Ca Ti V Cr Mn Co Ni Cu As Pd Cd a Pt Au Hg Tl Pb U Factor 5 - Unidentified e Ca Ti V Cr Mn Co Ni Cu As Pd Cd a Pt Au Hg Tl Pb U Factor 4 - Crustal e Ca Ti V Cr Mn Co Ni Cu As Pd Cd a Pt Au Hg Tl Pb U

20 Results Source apportionment Factor 2 - Vanadium related e Ca Ti V Cr Mn Co Ni Cu As Pd Cd a Pt Au Hg Tl Pb U Factor 3 - ase metals related e Ca Ti V Cr Mn Co Ni Cu As Pd Cd a Pt Au Hg Tl Pb U Factor 6 - rrochromium related e Ca Ti V Cr Mn Co Ni Cu As Pd Cd a Pt Au Hg Tl Pb U

21 Conclusions Atmospheric had the highest annual average concentration, while was the second most abundant species. With the exception of Ni, NONE of the trace metals measured at Marikana during the sampling period exceeded local and international standard limit values. The wet removal processes of atmospheric PM 10 trace metals were more significant than wind generation thereof indicated by the temporal variation, i.e. higher concentrations during the dry season. A less significant seasonal trend is observed for the trace metal concentrations in the PM 2.5 fraction, which was attributed to a faster replenishment of smaller particles into the atmosphere after rain events, as well as the close proximity of the sampling site to the major PM 2.5 sources (anthropogenic).

22 Conclusions During night-time 4% of the total trace metal concentration was in the PM fraction, while the PM fraction contributed to 38% of the total daytime trace metal concentrations. 79% of the total trace metal levels that were measured were in the PM 2.5 fraction indicating a strong influence of anthropogenic sources. 40% and more of a specific metal was in the PM 2.5 fraction, with Cr, V, Ni, and Mn occurring almost completely in the PM 2.5 fraction. SEM also indicated that a large fraction of the particles was likely to originate from anthropogenic activities and from wind-blown dust. SEM-EDS also detected non-metallic S that is usually associated with the PGM pyrometallurgical industry.

23 Conclusions Explorative factor analysis of the dataset for all 27 metals detected resolved four meaningful emission sources, i.e. vanadium related, base metal related, crustal and ferrochromium related correlating well with known sources occurring in the western IC.

24 Thank you

25 Results asonal variation 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Pt Pt Pt Pt Ni Pt Pt Pt Ni Cr 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Pt Ni Ti Pt Pt Ni Ti Ti Pt Ni Ni Ni Cr Ni Cr Ni Cr Ni Cr e Ti V Cr Ni Cu e Ti V Cr Mn Ni