Single extraction operational speciation

More widely applied to determine the potential, plant and human bioavailability are the methods of PTMs speciation which involve selective chemical extraction techniques. Estimation of the plant- or human-available element content of soil using single chemical extractants is an example of functionally defined speciation, in which the function is plant or human availability. In operationally defined speciation, single extractants are classified according to their ability to release elements from specific soil phases. Selective sequential extraction procedures are examples of operational speciation (Ure and Davidson, 2002). 

Speciation is defined as the process of identifying and quantifying the different defined species, forms or phases present in a material or the description of the amounts and types of these species, forms or phases present . In some cases, it is possible to identify, by using single or sequential extractions, operationally defined determinations which identify groups of metals without clear identification. In this situation, it is possible to refer to, for example, ethylenediaminete-traacetic acid (EDTA)-extractable trace metals. The reasons why speciation is important is that metals and metalloids can be present in many forms, some of which are toxic. 

Delhi soils by studying its speciation in the soil profile and to assess if there was any spatial variability. Soils representing the Aravali Ridge and the alluvial floodplains of river Yamuna were collected as a single, undisturbed core up to a depth of lm and the profile differentiated into four layers- 0-17 cm, 17-37 cm, 37-57 cm, and 57-86 cm. Pseudo total Aluminum and Iron in the soils were speciated into the operationally defined species (weakly exchangeable, organic matter complexes, amorphous oxides and hydroxides, and crystalline or free oxides) by widely recommended selective extraction procedures. Both A1 and Fe in these soils are bound predominantly as Fe oxides and silicates and have only very low percentages in the easily mobilizable pools. 

A number of fractionation procedures have been developed over the last 15 years in order to distinguish between the various aqueous forms of aluminium. These methods include dialysis, ion exchange (both batch and column), HPLC, F ion-selective electrode, NMR, species specific extractions, filtration and computational techniques. All of these procedures usually measure operationally defined aluminium fractions i.e. groups of aluminium species are measured, rather than a single species, and the values obtained depend upon the precise procedure used for the analysis). Therefore, present speciation procedures may measure slightly different forms of aluminium depending on the conditions, and thus they should accordingly provide different results. 

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