Reliability of comparisons based on sequential extraction procedures applied to soil samples: The thermodynamic point of view

Sequential extraction procedures are often used to establish the distribution of trace metals in soil or sediment samples. The aim of this work was to examine, from a thermodynamic point of view, the ability of these procedures to answer the two following questions. Does the comparison of the results obtained by the application of one extraction procedure to polluted and non polluted soil samples yield information about the distribution of the added amount of trace metal? Does the application of a sequential extraction procedure to a soil sample before and after the reaction of a given chemical reagent allow us to predict the reactivity of this reagent with respect to the different compartments identified by the procedure ? Simulation of a given sequential extraction procedure is easy and reflects the many currently discrepancies of these procedures. Sequential procedures do not appear to be useful to compare polluted and non polluted soil samples from the point of view of the repartition of the metal between compartments. From the same point of view, sequential extraction procedures do not seem reliable in estimating the reactivity of a chemical reagent with respect to the different compartments able to adsorb trace metals.