Sequential extraction procedure
Päivi M. Kauppila, Geological Survey of Finland, P.O. BOX 1237, FI-70211 Kuopio, Finland, e-mail: paivi.kauppila(at)gtk.fi
Sequential extractions (or sequential selective extraction procedures) are widely used to study element speciation in soils, sediments, and mine wastes (e.g. Tessier 1979, Sondag 1981, Hall et al. 1996, Dold and Fontboté 2001, Dold 2003, Heikkinen and Räisänen 2008). In the method, solid material is partitioned into specific fractions by a sequential series of selective extractions using appropriate reagents, and it aims to simulate the release of elements under a various environmental conditions. It provides information on the origin, mode of occurrence, bioavailability, mobilization and transport of trace elements and other elements of interest (Tessier et al. 1977). For instance, based on sequential extractions it is possible to evaluate into which minerals trace metals are adsorbed or bound to.
In the method, the dried, sieved or ground sample is leached with a series of chemical, selective solutions sequentially, i.e. the sample is subjected to extractive solutions one after another. Between each successive extraction, supernatant is separated from the residue by centrifugation and analyzed for trace metals and other elements of interest using ICP-methods, and the residue is washed with deionized water. Sequential extraction series typically includes five to seven different extractions with an increasing strength of reactivity, the last leach being the digestion of residual minerals with strong acids to measure the total concentrations of elements. Accuracy of the procedure is checked by comparing the sum of the element concentrations in the individual fractions with the total concentrations.
Extractions can also be made non-sequentially i.e. parallel, i.e. separate subsamples are leached with extractive solutions of increasing aggressivity, assuming that the stronger extractants also dissolve the phases leached with the weaker solutions. After extractions, each supernatant is centrifuged and filtered and elements are determined from the solutions with ICP-AES/MS. The amount of each fraction is then calculated by subtracting the concentration of the previous (weaker) extraction from the next step. The sum of the fractions equals the total concentrations (100%). By using parallel selective extractions some undesired reactions (e.g. precipitation) caused by remnants of previous reagents or sample losses during filtrations may be avoided during the procedure (Hall and Pelchat 2005, Alakangas and Öhlander 2006).
Sequential extraction and non-sequential (parallel) methods are often criticized because of the uncertainty in the selectivity of specific extractions applied. To improve the certainty, the design of the procedure and the interpretation of the results require detailed data of the mineralogy and the geochemical processes evaluated to take place in the studied material (Dold 2003, Heikkinen 2009). One way to decrease the uncertainty is to measure the minerals that have leached in each extraction using e.g. XRD or DXRD methods (Dold 2003).
There are number of sequential methods available to study element speciation in soils and sediments (e.g. Tessier et al. 1979, Sondag 1981, Hall et al. 1996), but only a few of the methods have been designed specifically for mine wastes. For example, Dold (2003; Table 1) has developed a sequential extraction procedure applicable for copper sulphide mine wastes (Dold and Fontboté 2001, 2003), Heikkinen and Räisänen (2008, 2009) have applied parallel selective extraction schemes to study Ni mine and Cu mine tailings, and Gunsinger et al. (2006) have used sequential extraction procedure to study metal retention on Fe precipitates in tailings. In mine wastes, the complex processes of sulphide oxidation and subsequent retention of mobilized elements by secondary phases via precipitation and sorption requires careful planning and design of the extraction procedure.
Table 1. Example of the sequential extraction procedure for mine wastes (Dold 2003).
|Extraction steps||Extraction solution||Preferentially dissolved minerals|
||Deionized H2O||Secondary sulphates|
||1 M NH4-acetate, pH 4.5||Calcite, vermiculite-type mixed layer, adsorbed and exchangeable ions|
||0.2 M NH4-oxalate in darkness, pH 3.0||Schwertmannite, two-line ferrihydrite, secondary jarosite, MnO2|
||0.2 M NH4-oxalate, pH 3.0 in water bath 80°C||Goethite, jarosite, Na-jarosite, hematite, magnetite, higher ordered ferrihydrites|
||35% H2O2 in water bath||Organic material, covellite, chalcocite-digenite|
||KClO3 + HCl, 4 M HNO3 boiling||Rest of the sulphides|
||HNO3, HF, HClO4, HCl digestion||Silicates, residual|
The method can be used to specify chemically different fractions of elements, particularly contaminants, in mine wastes to evaluate their mobilisation potential and retention in the waste materials. It will give an estimate of the conditions needed that contaminants are leached from the waste material. Sequential extractions are applied to understand the long-term behaviour of mine wastes and to optimize their remediation.
- Combined with mineralogy provides a powerful tool to study element mobilisation and retention processes in mine wastes.
- Sequential extraction method is usually not a routine method for analytical laboratories, and as such it requires detailed instructions by the customer.
- Reprecipitation of elements may occur between extractions causing inconsistent results in sequential extraction
- Sum of the various extractions may differ from the total concentrations of elements
- Fractions are not really specific for the extractions – they only give an approximation of the element partitioning in the studied material
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