A sequential extraction (fractionation) procedure was adapted for this study and was carried out to compare the distribution of Pb in the soils of three locations, with varying levels of pollution. In all cases, Pb was predominantly accumulated in the residual fraction and in the fraction associated with Fe/Mn oxides. Soil samples were also treated with 0.1 M EDTA (ethylenediaminetetraacetic acid), which was used as an extractive cleaning solution for removing Pb from the soils. Possible relationships between the amounts of Pb extracted with EDTA and the Pb amounts associated with the different sequential fractions were investigated by using multiple regression analysis. The amount of Pb extracted with the cleaning solution was found to be related to the fractions associated with oxides and organic matter (and/or sulphides) which in turn appeared to be interdependent, as a probable consequence of the simultaneous complexation process when the EDTA solution is used. The sand fraction (particle size ranging from 2 to 0.08 mm) showed a favourable influence for recovery purposes. This fact seems to be related to the relatively high ionic radius of Pb which makes diffusion processes within lattice structures difficult.
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Barona et al. (1996) studied this question.