Selective extractions

Päivi M. Kauppila, Geological Survey of Finland, P.O. BOX 1237, FI-70211 Kuopio, Finland, e-mail: paivi.kauppila(at)


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

Appropriate applications

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“).


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