Thermodynamics applied to the study of the limits of sequential extraction procedures used for the speciation of trace elements in sediments and soils

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Abstract
Thermodynamics are used to study the limitations of the sequential extraction procedures for the speciation of heavy metals in soils or sediments. According to the thermodynamics, the non‐selectivity of the chemical reagents used in the sequential extraction procedures can be demonstrated. For example, the changes in acidity and organic complexing reagents occurring during these procedures may contribute to the extraction of metallic cations from different compartments. Re‐adsorption phenomena may also be explained. Validation of sequential extraction procedures remains a serious and controversial problem; thermodynamics may also establish the chemical conditions which would have been involved in clearly allowing validation or invalidation of these procedures. Thermodynamics confirm the inadequacy of sequential extraction procedures for trace element speciation in soils and sediments and outlines the problems of using the results of these procedures for the estimation of the mobility and bio‐availability of trace elements.