ARSENIC SPECIATION IN MINE TAILINGS AND THE ROLE OF IRON OXY-HYDROXIDES

Barbara Palumbo, Mark Cave, Ben Klinck, and Joanna Wragg British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK

INTRODUCTIONThe south-west of England is extensively contaminated with heavy metals and metalloids arising from centuries of mining activity in the region. The distribution of arsenic has been studied around the Devon Great Consols mine set that lies on the east bank of the River Tamar in the Tavistock district of Devon. In the early years of operation the mine raised copper ores from lodes consisting of chalcopyrite, pyrite, arsenopyrite and minor cassitierite. In the mid 1800's at about the same time as copper ore production declined the use of arsenic as a pesticide developed and the mine had a new lease of life as an arsenic producer. Mining activity at the site ended in 1930 due to depressed world markets. Today the arsenic works and associated buildings are surronded by extensive derelict land with piles of waste rock, calciner wastes and ash, and tailings.

METHODOLOGY Approximately one hundred soil samples (0-15 cm depth) and tailings were randomly collected over the site and in the surrounding area. Soils were also sampled from a mineralised area not affected by past mining activities and from outside the mineralised area. Chemical sequential extractions coupled with direct observations using electron microscopy techniques have been carried out to characterise arsenic partitioning in the solid phases. Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to complement the chemical sequential extraction procedure in order to evaluate the speciation of arsenic. The physiologically based extraction test (PBET) developed by Ruby et al., 1996 has been used to measure the bioaccessibility of arsenic in soil.

References- CAVE, M R, MILOWDOSWKI, A E, and FRIEL, H. 2003. Evaluation of a method for Identification of Host Physico-chemical Phases for Trace Metals and Measurement of their Solid-Phase Partitioning in Soil Samples by Nitric Acid Extraction and Chemometric Mixture Resolution. Geochemistry: Exploration, Environment, Analysis. (Accepted)- RUBY, M V, DAVIS, A, SCHOOF, R, EBERLE, S, AND SELLSTONE, C M. 1996. Estimation of Lead and Arsenic Bioavailability Using a Physiologically Based Extraction Test. Environmental Science & Technology, VOL. 30, 422-430

PBET TEST- Physiologically Based Extraction Test

Centrifugation

Nitric acid

Soil

0.45 m filter membrane

leachate

Separate aliquots of nitric acid of increasing concentration. Passed through the sample under centrifugal force. Determination by ICP-AES.Chemometric data processing.Identification of physico-chemical hosts and the metal distributions within the sample under test.

Ca-S-Al

0200400

600800

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1 3 5 7 9 11 13

Ca-(Si-Al-Cu)

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Fe-Ca-(Si)

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Ca-Al-(K-Si)

S-K

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TailingsDGC Soils

Outside DGC soilsBackground soils

H Todsworthy Farm soils

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N=73

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Outside DGC soilsBackground soils

Higher Todsworthy Farm soils

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60

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Shaf t

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Tamar

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Sinks

Sinks

Drain

Drain

Issues

Little

162.5m

161.2m

161.8m

Shaf ts

Spring

Spring

Spring

Issues

Cottage

Brimbles

Woodland

Honeytor

Hawkmoor

Rock View

(disused)

(disused)

(disused)Path (um)

(disused)

(disused)

(disused)

(disused)(disused) (disused)

Path (um)Path (um)

(disused)

(disused)

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(disused) (disused)

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(disused)

(disused)

(disused)

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(disused)

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Wheal Emma

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Wheal JosiahWheal Josiah

Leat (disused)

Blanchdown Wood

Blanchdown Wood

Wheal Anna Maria

Wheal Anna Maria

Devon Great Console Mine

Devon Great Consols Mine

%[ Shafts pointsMineral veinMineral vein uncertain

# Cluster1 # Cluster2 # Cluster3 # Cluster4

CISMeD TEST-Chemometric Identification of Substrates and Metal

Distributions

RESULTS AND INTERPRETATION

Arsenic concentrations show an asymmetric distribution in the soils at the mine with the lower quartile value and the upper quartile value of 867 mg/kg and 8449 mg/kg respectively and the median value of 2105 mg/kg. The soils in the near vicinity of the mine site have a similar median value to those on the site but have a much smaller spread of values. Soils from Higher Todsworthy Farm, representing soils over a mineralised area that has not been affected by mining activities have arsenic concentrations ranging from 123 mg/kg to 205 mg/kg (median 163 mg/kg). The soils collected near Bere Alston, which represent an unmineralised background area away from mining, show values for arsenic of 59-172 mg/kg (median 71 mg/kg).

Arsenic relative bioaccessibility median values are15% for the mine soils and 13% for the soils surrounding the mine. The median values for the background soils and the soils over the mineralisation are of 9.95% and 9.13%, respectively. The tailings samples, although containing the highest total arsenic content, show a lower median (5.05%) relative bioaccessibility compared to the soils.

Total arsenic

Arsenic relative bioaccessibility

Extraction profiles of a black furnace ash sample showing the 6 identified physico-chemical components (a) and the distribution of the extracted arsenic within those components (b).

( a ) ( b )

BSEM image showing detail of arsenic-rich iron oxyhydroxide cement coating the surfaces of altered waste fragments. It shows banded co l l o fo rm oxyhydroxide gel material d i s p l a y i n g s h r i n k a g e (desiccation) cracks. This is encrusted by more crystalline acicular oxyhydroxide.

Solid phase speciation of arsenic

Sequential extraction data have been used to help elucidate the nature of the physico-chemical forms of As in the different soil types and mine waste material and also understand which of these forms are responsible for the mobile and bioaccessible fraction. The data processing using the CISMeD method of analysis (Cave et al, 2003) allows to characterise the soil by resolving number and composition of the physico-chemical components present in various proportions in the soil.In all samples the physico-chemical component that makes up the most significant percentage of the total extract contains mainly Fe, arsenic and S in variable proportion, but Fe is always predominant. This component is extracted over the range 0.5M-5M (step 9 to 14) with more defined windows of extraction at 0.5M and 5M

Typical results are shown in the partitioning of arsenic in a black furnace ash sample, Figures (a) and (b), which comprises poorly sorted gravel in a sandy matrix. Most of the arsenic is extracted in a component dominated by Fe and a secondary Fe-Ca-Si rich component. Petrographic analysis was also undertaken, which allows the results of the chemometric analysis to be validated. The arsenic-iron association shows up clearly as an arsenic rich iron oxyhydroxide cement coating vesicular, glassy, slag fragments, Figure (c).X-Ray absorption near edge structure (XANES) analysis indicates that As(V) is the dominant oxidation state in the mine waste materials and soils, Figure (d). Quantitative fits of EXAFS spectra using theoretical standards indicate As(V) in tetrahedral coordination with oxygen and second and third-neighbour Fe atoms, ruling out the presence of arsenopyrite. Second and third neighbour As-Fe EXAFS distances imply either adsorption of the arsenic onto an iron oxide/hydroxide substrate or incorporation of the arsenic into a mixed metal oxide phase. The two different As---Fe distances may reflect the presence of doubly oxo-bridged and singly oxo-bridged species.

HNO3

HNO .3

( c )

( d )Relationship of arsenic bioaccessibility to geochemical control

© Crown Copyright. All rights reserved. License number GD272191/2003

Spatial distribution of the geochemical soil clusters at the Devon Great Consols Site

By using k-means clustering of the soil physico-chemical variables the soils have been grouped into 4 clusters. Clusters 1 and 2 group the soils characterised by low arsenic concentration. Cluster 4 contains soils with very high concentrations of arsenic, Fe and S. Cluster 3 has intermediate arsenic values. Spatial plotting of the clusters locates cluster 4 around the arsenic works area, identifying clearly the area of major pollution.

Multiple Linear Regression (MLR) is used to further analyse the relationship between the bioaccessible arsenic content of the soil and other soil parameters for Cluster 1 and Cluster 2. The total element content, pH and organic matter were used as predictor variables. For Cluster 1, 91.8% of the variance in the bioaccessible arsenic is modelled using the total arsenic and the total iron content. Total arsenic has a positive coefficient and iron has a negative coefficient. This suggests that there is a portion of arsenic bound to an iron-rich source that holds arsenic in a non bioaccessible form. For Cluster 2, 93.8% of the variance is modelled entirely by the total arsenic content. These results suggest the presence of at least two different sources of arsenic in the clusters, an iron sulphide contribution in cluster 1 and an iron oxide source in cluster 2 affecting the different relative bioaccessibility of arsenic.

X-ray absorption near edge structure (XANES) spectra of mine waste and soil samples. A model compound PbFe (SO )(AsO )(OH) is included. 3 4 4 6

The dashed vertical lines show the theoretical absorption edge position for As(0) (11865eV) and As(V) (11870eV).

R a d i a l s t r u c t u r e functions (RSF) for mine waste and soil samples.

Bere Alston

Devon Great Consols

Tavistock

H. Todsworthy Farm

SAMPLE SITES REFERRED TO IN THE TEXT

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