Päivi M. Kauppila, Geological Survey of Finland, P.O. BOX 1237, FI-70211 Kuopio, Finland, e-mail: paivi.kauppila(at)gtk.fi
Selective extractions are used to study the leachability and dissolution of contaminants from mine waste materials. There is a wide range of extraction methods available targeting to leach a certain chemical fraction of potential contaminants in the waste (e.g. Hall et al. 1996). The objective of the extractions is to simulate the mobilization of the contaminants in specific environmental conditions. The selective extractions provide a better measure of the impacts of the metals on the environment than the measurements of the total concentrations.
Description of the method
Prior to analysis, waste material is freeze-dried (tailings) or dried in a room temperature (waste rocks). Tailings are typically sieved under 2 mm grain size and waste rocks are crushed under 2 mm grain size using a jaw-crusher and ground in a carbon steel bowl. After pre-handling, waste material is leached with a specific extractive solution in a selected temperature for a defined period. The chemical composition of the leachate solution is then measured using ICP-AES and/or ICP-MS techniques. Table 1 lists examples of selective, single extractions typically used for mine wastes and other geological materials.
Two of the most common selective extractions used for mine waste characterisation are the hot aqua regia and strong nitric acid leach (Niskavaara 1995) to determine the concentrations of the acid-soluble elements. The element concentrations measured from these acid leach solutions reflect the element concentrations that are bound to mica and clay minerals, salt minerals and sulphide minerals (Doležal et al. 1968, Räisänen et al. 1992b). Elements adsorbed to the surface of minerals and mineral precipitates may also be examined using weak leaching methods with either a sequential or single extractions, such as distilled water, ammonium acetate or ammonium oxalate leach (Dold & Fontboté 2001, Heikkinen & Räisänen 2008).
Table 1. Examples of typical selective extraction solutions used in mine waste characterization.
|Extract solutions||Geochemical fraction||Liquid-solid ratio||Equilibration time, Temperature||References|
|Distilled water||Water-soluble fraction||1:50||1 h, RT||Dold 2003|
|0.01 M NH4Cl,
|Physically adsorbed, easily leachable, bioavailable fraction||1:50
|2 h, RT
|Heikkinen & Räisänen 2008|
|1 M NH4-acetate, pH 4.5||Exchangeable, carbonates,||1:10
|2h, RT||Gatehouse et al. 1977, Sondag 1981, Dold 2003, *Heikkinen & Räisänen 2008|
|0.2 M NH4-oxalate, pH 3.0 in darkness||Fe(III)oxyhydroxides||1 h RT||Dold 2001, 2003|
|0.2 M NH4-oxalate, pH 3.0 in darkness||Fe(III)oxyhydroxides||1:100||3 h, RT||Räisänen et al. 1992a, Heikkinen & Räisänen 2008|
|0.2 M NH4-oxalate, pH 3.0 in water bath||Fe(III)oxides||2 h, 80°C||Dold 2003|
|H2O2 35% in water bath||Organics and secondary Cu-sulfides||1h, heated in water bath||Sondag 1981, Dold 2001|
|Aqua regia (1:3 HNO3 HCl)||Sulphides||1:100||Overnight RT, 2 h 90°C||Doležal et al 1968, ISO11466, Heikkinen & Räisänen 2009|
|KClO3-12 M HCl + 4 M HNO3||Sulphides||1:50||30 min RT + 20 min 90°C||Chao & Sanzalone 1977, Hall et al. 1992, Heikkinen & Räisänen 2008|
|HF-HClO4-HNO3||Silicates and residual oxides||1:20||200°C + 20 min 90°C + 1 h 90°C + overnight 70°C + 120°C||Hall et al. 1996|
RT = room temperature
Selective extractions can be used to assess the forms and mobility of potential harmful elements in different types of mine wastes (e.g. tailings, waste rock). To simulate the release of elements under a various environmental conditions, selective extractions can be made sequentially or non-sequentially in a series (see “Sequential extractions“).
Chao, T.T. & Sanzolone, R.F. 1977. Chemical dissolution of sulphide minerals. Journal of Research of the U.S. Geological Survey, Vol 5(4): 409–412.
Dold, B. 2001. A 7-step sequential extraction for geochemical studies of copper sulfide mine waste. In: Securing the Future, International Conference on Mining and the Environment, June 25 – July 1, 2001, Skellefteå, Sweden: proceedings, Vol. 1. Stockholm: The Swedish Mining Association, 158–181.
Dold, B. 2003. Speciation of the most soluble phases in a sequential extraction procedure adapted for geochemical studies of copper sulfide mine waste. Journal of Geochemical Exploration 80:55-68.
Dold, B. & Fontboté, L. 2001. Element cycling and secondary mineralogy in porphyry copper tailings as a function of climate, primary mineralogy, and mineral processing. Journal of Geochemical Exploration 74, 3–55.
Doležal, J., Provondra, P. & Šulcek, Z. 1968. Decomposition techniques in inorganic analysis. Iliffe Books Ltd, London. 224 p.
Gatehouse, S., Roussel, D.W. & Van Moort, J.C. 1977. Sequential soil analysis in exploration analysis. Journal of Geochemical Exploration 8, 483–494.
Hall, G.E.M., Vaive, J.E., Beer R. & Hoashi, M. 1996. Selective leaches revisited, with emphasis on the amorphous Fe oxyhydroxide phase extraction. Journal of Geochemical Exploration 56:59–78.
Heikkinen, P.M. & Räisänen, M.L. 2008. Mineralogical and geochemical alteration of Hitura sulphide mine tailings with emphasis on nickel mobility and retention. Journal of Geochemical Exploration 97, 1–20.
Heikkinen, P.M. & Räisänen, M.L. 2009. Trace metal and As solid-phase speciation in sulphide mine tailings – Indicators of spatial distribution of sulphide oxidation in active tailings impoundments. Applied Geochemistry 24, 1224–1237.
ISO11466:1995. Soil quality – Extraction of trace elements soluble in aqua regia.
Niskavaara, H. 1995. A comprehensive scheme of analysis for soils, sediments, humus and plant samples using inductively coupled plasma atomic emission spectrometry (ICP-AES). In: S. Autio (Ed.) Geological Survey of Finland, Special Paper 20, 167–175.
Räisänen, M.L., Hämäläinen, L. & Westerberg, L. 1992a. Selective extractions and analysis of metals in organic stream sediments. Analyst 117, 623–627.
Räisänen, M. L., Tenhola, M. & Mäkinen, J. 1992b. Relationship between mineralogy and the physicochemical properties of till in central Finland. Bulletin of the Geological Society Finland Volume 64 (1): 35–58.
Sondag, F. 1971. Selective extraction procedures applied to geochemical prospecting in an area contaminated by old mine workings. Journal of Geochemical Exploration 15, 645–652.