The Impact of Gold Mining The Impact of Gold Mining on Mercury Pollution on Mercury Pollution

in the Witwatersrand Basin, South Africain the Witwatersrand Basin, South Africa

E.M. Cukrowska1, J. Lusilao-Makiese1, E. Tessier2 , B. Yalala1,Hlanganani Tutu1 and Luke K Chimuka1

11School of Chemistry, University of the Witwatersrand, P. Bag X3,WITS 2050, Johannesburg,South Africa2 Laboratory of Bioinorganic and Environmental Analytical Chemistry, CNRS, University of Pau, France

E-mail: ewa.cukrowska@wits.ac.za

In South Africa (SA), the largest In South Africa (SA), the largest anthropogenic point sources of anthropogenic point sources of mercury emissions are mercury emissions are combustion combustion sourcessources followed by followed by gold mining gold mining activitiesactivities (reprocessing of old tailing (reprocessing of old tailing dams and artisanal mining).dams and artisanal mining).

In the year 2000, South Africa was In the year 2000, South Africa was ranked ranked 2nd in the world for total Hg 2nd in the world for total Hg emissions and 4th for Hg emissions emissions and 4th for Hg emissions arising from stationary fossil fuel arising from stationary fossil fuel combustion combustion (Pacyna et al., 2006), (Pacyna et al., 2006), though recent studies critically revised though recent studies critically revised previous emission inventories.previous emission inventories.

Trend of global anthropogenic emissions by Trend of global anthropogenic emissions by region 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 speciationThe mercury speciation

ABSORPTIONBIOAVAILAVILITYMETHYLATIONTOXICITY

of Hg depends on its…. chemical formchemical form

Total elemental analysis is not enough to provide such information

Research motivation and objectivesResearch motivation and objectivesMotivation:

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 Assessment of the extent of Hg contamination from historic gold mining sites i.e. to characterization of specific “hot spots” and to determination of Hgof specific “hot spots” and to determination of HgTOTTOT, IHg and MeHg in water, soil, and biota., IHg and MeHg in water, soil, and biota.

To use obtained data for understanding the To use obtained data for understanding the biogeochemical speciation of Hg, its distribution, distribution, transport and fate transport and fate in environmental compartments impacted by mineral processing and energy production in South Africa.

Sampling locationsSampling locations

Sediment, water and plant sampleswere collected near TSFs and inwatersheds 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.

Hg contamination from active gold mining sitesHg contamination from active gold mining sites

Speciation of Hg in sediment cores: sample preparation procedureSpeciation of Hg in sediment cores: sample preparation procedure

Sampling

Freezing (~18C) and Dissection

Freeze-drying and Grinding

Spiking

MicrowaveAssisted

Extraction

Derivatization1μl of organic phase

GC-ICPMS

Data acquisition

Case 1: Vaal River West Complex: a. sedimentsCase 1: Vaal River West Complex: a. sediments

US EPA US EPA Threshold Effect Level (TEL) Threshold Effect Level (TEL) value for Hg in sediment: value for Hg in sediment: 174 ng g174 ng g-1-1

950 – 220 ppbfrom dry to rainy season

120 – 1000 ppbfrom dry to rainy season

8345-1500 ppbFrom dry to rainy season

0.005-1.000 ppb in H2OFrom dry to rainy season

4200ppb

Remobilization of Hg from the water damto surrounding sediments.

Methylmercury formationMethylmercury 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 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

KrugersdorpGame Reserve

Mining Area

High Hg concentration in sediment and water!High Hg concentration in sediment and water!

West Rand HgTOT

0

500

1000

1500

2000

2500

3000

80A 86A 87 92 93 95 97 98 100 105 106

[Hg]

(ng

g-1

)Hg concentration much higher within the mining site than in the Game reserveHg concentration much higher within the mining site than in the Game reserve

0

500

1000

1500

2000

[Hg]

(ng

g-1

)

86A 87 92 93 97 98 100

West Rand sediments

IHg

10 x MHg

Hg speciation in Randfontein sedimentHg speciation in Randfontein sediment

93

0

4

8

12

16

20

0 1 2 3

%MeHg

Dep

th (

cm)

%MHg93

0

4

8

12

16

20

1200 2200 3200

[IHg] (ug/kg)

Dep

th (

cm)

IHg (μg kg-1)

93

0

4

8

12

16

20

0,3 0,45 0,6 0,75

%S

Dep

th (

cm)

%S

93

0

4

8

12

16

20

0,15 0,3 0,45 0,6

%C

Dep

th (

cm)

%C

Correlation with Correlation with Suphur and CarbonSuphur and Carbon

Hg in water samples: Typical case of AMDHg in water samples: Typical case of AMD

Hg speciation in borehole waterHg 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 Case 3: Characterisation and modelling of mercury speciation in

urban air affected by gold mining - assessment of bioavailability urban air affected by gold mining - assessment of bioavailability

(pilot study) (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 AnalysisSample Analysis

HgHgTOTTOT in dust was determined, in dust was determined, after MW digestion, by Anodic after MW digestion, by Anodic Stripping Voltammetry with a Stripping Voltammetry with a gold rotating disc electrode gold rotating disc electrode (ASV-RDE)(ASV-RDE)

Chemical

Gray's solution (g L-1)pH 4.50

artificial lung fluid (ALF) (g L-1)pH 4.50

Phosphoric acid 1.200  -Sodium chloride 2.300 6.0193Ammonium chloride 5.300  -Sodium dihydrogen phosphate

1.700  -

K-acid-phthalate 0.200  Calcium chloride 0.290 0.2771Sodium acetate trihydrate 0.580 0.9526Sodium bicarbonate 2.300 2.6042Sodium citrate 0.590 0.0970Sodium carbonate 0.630  -Sulphuric acid 0.510  -Glycine 0.450  -Citric acid 0.420  -Magnesium chloride -  0.2033Potassium chloride  - 0.2982di-sodium hydrogen phosphate

 - 0.1417

Sodium sulphate anhydrous  - 0.0710

Composition of lung leaching Composition of lung leaching solutionssolutions

Total mercury in dustTotal mercury in dust

Tailings footprint2081 ± 165 µg kg-1

Soweto1575 ± 213 µg kg-1

Reprocessed tailings3196 ± 133 µg kg-1

PPC cement2265 ± 71 µg kg-1

Sandton22 ± 1 µg kg-1

Cemetery2011 ± 327 µg kg-1

CBD111 ± 9 µg kg-1

Industrial area1102 ± 98 µg kg-1

Mineral processing plant1037 ± 36 µg kg-1

Soccer City954 ± 124 µg kg-1

> 2000 µg kg-1

2000-1000 µg kg-1

100 – 999 µg kg-1

<100 µg kg-1

Northern Residential Area

Mining & Industrial Area

Southern residential 466 ± 26 µg kg-1

Industrial area1177 ± 91 µg kg-1

CBD North59 ± 6 µg kg-1

+20 km

Dust samples leaching resultsDust samples leaching results

Sample ID 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

(Mean ± % Standard deviation)Detection limit: 7.11 ng L-1

< 25 µm particle size fraction

Sample ID HgTOT (µg kg-1)

Gray’s Solution(µg kg-1) %

ALF (µg kg-1) %

H2O (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

(Mean ± % Standard deviation)Detection Limit: 6.24 ng L-1

Bulk – Coarse dust

SummarySummary The extremely high Hg concentrations found in water and sediment suggest that post-gold-mining

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.

AcknowledgementAcknowledgementss

I. Weiersbye (APES, Wits University)

NRF (South Africa)

THRIP (South Africa)

Anglo Gold Ashanti (South Africa)

Thank You!