Transcript Application of the Hyphenated GC

The Impact of Gold Mining
on Mercury Pollution
in the Witwatersrand Basin, South Africa
E.M. Cukrowska1, J. Lusilao-Makiese1, E. Tessier2 , B. Yalala1,
Hlanganani Tutu1 and Luke K Chimuka1
11School
2
of Chemistry, University of the Witwatersrand, P. Bag X3,WITS 2050, Johannesburg,South Africa
Laboratory of Bioinorganic and Environmental Analytical Chemistry, CNRS, University of Pau, France
E-mail: [email protected]

In South Africa (SA), the largest
anthropogenic point sources of
mercury emissions are combustion
sources followed by gold mining
activities (reprocessing of old tailing
dams and artisanal mining).

In the year 2000, South Africa was
ranked 2nd in the world for total Hg
emissions and 4th for Hg emissions
arising from stationary fossil fuel
combustion (Pacyna et al., 2006),
though recent studies critically revised
previous emission inventories.
Trend of global anthropogenic emissions by region
based on Pirrone et al. (1996) (a), Pacyna et al. (2003)
(b), Pacyna et al. (2006) (c), and Pirrone et al. (2010)
(d). Data reported in Fig. d) are for most contributing
countries. AF-Africa; AS-Asia; EU-Europe; NA-North
America; OC-Oceania; SA-South America.
The mercury speciation
ABSORPTION
BIOAVAILAVILITY
METHYLATION
TOXICITY
of Hg depends on its….
chemical form
Total elemental analysis
is not enough to provide
such information
Research motivation and objectives
Motivation:
 Most of the data on mercury in SA concerns total Hg determination only and mostly from
coal fired power stations.
 There is a need for measured speciation data near sources of concern, in order to get a
better quantitative understanding of Hg chemistry in the SA semiarid environment
(atmosphere, soils, water bodies and biota).
Objectives:
 Development, optimization and validation of reliable analytical methods for the
determination of mercury species in different environmental matrices (air, water, sediments
and biological materials).

Assessment of the extent of Hg contamination from historic gold mining sites i.e. to
characterization of specific “hot spots” and to determination of HgTOT, IHg and MeHg in
water, soil, and biota.

To use obtained data for understanding the biogeochemical speciation of Hg, its
distribution, transport and fate in environmental compartments impacted by mineral
processing and energy production in South Africa.
Hg contamination from active gold mining sites
Sampling locations
Sediment, water and plant samples
were collected near TSFs and in
watersheds from goldfields within
the Wits Basin.
A pilot study was carried out on
a few collected air samples from
a closed ventilation shaft
(Vaal River site) in order to assess
the level of atmospheric Hg emission.
Dust samples were collected from
Greater Johannesburg area.
Speciation of Hg in sediment cores: sample preparation procedure
Derivatization
1μl of organic phase
Sampling
GC-ICPMS
Spiking
Freezing (~18C)
and Dissection
Data acquisition
Freeze-drying and Grinding
Microwave
Assisted
Extraction
Case 1: Vaal River West Complex: a. sediments
US EPA Threshold Effect Level (TEL) value for Hg in sediment: 174 ng g-1
950 – 220 ppb
from dry to rainy season
120 – 1000 ppb
from dry to rainy season
8345-1500 ppb
From dry to rainy season
0.005-1.000 ppb in H2O
From dry to rainy season
4200ppb
Remobilization of Hg from the water dam
to surrounding sediments.
Methylmercury formation
Methylation occurs at deeper layers in
sediments.
MeHg enrichment in the region of
high IHg and low redox.
There is a risk for MeHg to enter the
water system!
Gaseous Hg (TGM) measurements in air samples from the ventilation shaft
High TGM concentrations at the outlet
of the ventilation shaft
Typical background TGM in pristine and open areas:
1 - 4 ng m-3 (Wangberg et al., 2008)
TGM within urban areas: up to 15 - 25 ng m-3
(Wang et al., 2007)
The obtained TGM concentrations are likely to trace an underground gaseous Hg
source from the old mine shafts connected to the ventilation shaft.
Case 2: Hg
contamination from
historic gold mining
sites
Krugersdorp
Game Reserve
Mining Area
High Hg concentration in sediment and water!
Hg concentration much higher within the mining site than in the Game reserve
West Rand HgTOT
3000
[Hg] (ng g-1)
2500
2000
1500
1000
500
0
80A
86A
87
92
93
95
97
98
100
105
106
Hg speciation in Randfontein sediment
West Rand sediments
[Hg] (ng g-1)
2000
1500
IHg
1000
10 x MHg
500
0
86A
87
92
93
97
98
100
93
93
%MHg
%MeHg
0
1
IHg (μg
kg-1)
[IHg] (ug/kg)
2
3
1200
0
4
8
12
8
12
16
16
20
20
93
93
%C %C
%S %S
0,3
0,45
0,6
0,15
0,75
0
0
4
4
8
12
Depth (cm)
Depth (cm)
3200
4
Depth (cm)
Depth (cm)
2200
0
0,3
0,45
0,6
8
12
16
16
20
20
Correlation with
Suphur and Carbon
Hg in water samples: Typical case of AMD
Hg speciation in borehole water
Methylation occurs in deep water
(reducive conditions) but,
due to its mobility, MHg migrates to
shallow levels.
Case 3: Characterisation and modelling of mercury speciation in
urban air affected by gold mining - assessment of bioavailability
(pilot study)
Investigations of atmospheric mercury have been mostly done on gaseous
species. Although, to assess human expose to mercury, especially in urban areas,
the inhalable dust should be included in a study.
The aim of this study was to determine the magnitude of mercury pollution in this
urban area and assess its bioavailability.
Dust samples were collected on inhalation levels (1-2 m above a ground). They were later
separated into different fractions by micro sieving.
Bioavailability of mercury in inhalable dust (25 µm) was tested by leaching collected samples
with artificial lung fluid (ALF, pH 4.5), Gray’s solution (pH 7.4) and water. The leaching
conditions were selected to mimic lungs environment (incubator at 300C, time 24 hrs,
rotation of samples 150 rpm). Total concentrations of mercury in dust fractions were also
determined after microwave digestion.
Sample Analysis
HgTOT in dust was determined, after
MW digestion, by Anodic Stripping
Voltammetry with a gold rotating
disc electrode (ASV-RDE)
Chemical
Phosphoric acid
Sodium chloride
Ammonium chloride
Sodium dihydrogen
phosphate
K-acid-phthalate
Calcium chloride
Sodium acetate trihydrate
Sodium bicarbonate
Sodium citrate
Sodium carbonate
Sulphuric acid
Glycine
Citric acid
Magnesium chloride
Potassium chloride
di-sodium hydrogen
phosphate
Sodium sulphate anhydrous
Gray's
solution
(g L-1)
pH 4.50
1.200
2.300
5.300
1.700
artificial
lung fluid
(ALF)
(g L-1)
pH 4.50
6.0193
-
0.200
0.290
0.580
2.300
0.590
0.630
0.510
0.450
0.420
-
0.2771
0.9526
2.6042
0.0970
0.2033
0.2982
0.1417
-
0.0710
Composition of lung leaching solutions
Total mercury in dust
+20 km
Sandton
22 ± 1 µg kg-1
Northern Residential Area
Cemetery
2011 ± 327 µg kg-1
CBD North
59 ± 6 µg kg-1
CBD
111 ± 9 µg kg-1
Tailings footprint
2081 ± 165 µg kg-1
Reprocessed tailings
3196 ± 133 µg kg-1
Mineral processing plant
1037 ± 36 µg kg-1
Industrial area
1102 ± 98 µg kg-1
Industrial area
1177 ± 91 µg kg-1
PPC cement
2265 ± 71 µg kg-1
Mining & Industrial Area
Soccer City
954 ± 124 µg kg-1
Soweto
1575 ± 213 µg kg-1
Southern residential
466 ± 26 µg kg-1
> 2000 µg kg-1
2000-1000 µg kg-1
100 – 999 µg kg-1
<100 µg kg-1
Dust samples leaching results
Bulk – Coarse dust
HgTOT
(µg kg-1)
Gray’s Solution
(µg kg-1)
%
ALF
(µg kg-1)
N3 fly over
37.86 ± 4.32
26.77 ± 1.14
71
23.43 ± 0.60
61.9
15.10 ± 1.35
RCS entrance
28.71 ± 1.05
20.70 ± 1.05
72
17.31 ± 0.67
60.3
11.07 ± 3.01
RCS exit
15.26 ± 3.57
11.48 ± 1.6
75
9.86 ± 3.15
64.6
7.79 ± 0.52
De Korte Str.
6.07 ± 1.15
4.87 ± 0.3
80
4.09 ± 1.73
67.3
3.23 ± 0.55
Smit Street
6.11 ± 1.80
4.92 ± 0.6
81
4.22 ± 1.16
69.1
3.25 ± 0.60
Sample ID
%
H2O
(µg kg-1)
(Mean ± % Standard deviation)
Detection Limit: 6.24 ng L-1
< 25 µm particle size fraction
HgTOT
(µg kg-1)
Gray’s Solution
(µg kg-1)
%
ALF
(µg kg-1)
%
H2O
(µg kg-1)
N3 fly over
22.77 ± 3.10
14.31 ± 7.03
63
10.86 ± 2.15
48
9.32 ± 1.31
RCS entrance
20.20 ± 3.29
16.84 ± 7.12
83
13.76 ± 5.86
68
7.89 ± 1.19
RCS exit
11.12 ± 4.57
8.24 ± 1.85
74
7.18 ± 2.12
65
5.76 ± 0.83
De Korte Str.
4.40 ± 1.56
3.46 ± 1.34
79
3.04 ± 4.15
69
2.18 ± 1.10
Smit Street
4.34 ± 2.14
3.02 ± 1.14
70
2.77 ± 1.52
64
2.18 ± 0.77
Sample ID
(Mean ± % Standard deviation)
Detection limit: 7.11 ng L-1
Summary
 The extremely high Hg concentrations found in water and sediment suggest that post-goldmining activities may be important contributors of Hg to watersheds through probable
remobilization that might occur at specific hot spots on a seasonal basis.
 Localized point sources of Hg likely exist throughout the entire gold mining region.
 Methylation of Hg is occurring close to the sources, which can allow MeHg to enter the food
web.
 These point sources offer the most treatable target areas for investigation of possible
remediation projects.
 TGM determination has shown a probable underground pollution source. Although, further
air sampling in both shafts and open sites is needed to confirm these preliminary results.
 Extremely high concentration levels of mercury was found in air and dust in industrial areas.
Especially high levels were detected around presently reprocessed old gold tailings dumps.
The levels dropped significantly in CBD area but still showing elevated concentrations. They
depend strongly on prevailing wind directions, dry and wet seasons, and day time.
 Leaching experiments showed that the Gray’s solution extracts more mercury from the dust
followed by ALF and much lower by water. This suggests that most of the mercury inhaled
with dust will be extracted in the deep interstitial parts of the lungs. In the upper region of
lungs extraction will occur to a lesser extent.
Acknowledgements

I. Weiersbye (APES, Wits University)

NRF (South Africa)

THRIP (South Africa)

Anglo Gold Ashanti (South Africa)
Thank You!