Investigation of metal pollution in soils by single and sequential extraction procedures

It is generally recognized that the knowledge of the total concentrations is not sufficient to understand the behaviour of metals in soils and to predict their availability and mobility, which influence their release and their interaction with other environmental compartments. These characteristics are usually investigated with single or sequential extraction procedures. Single extractions are commonly performed with complexing agents, diluted acids or electrolytes, and in many applications are aimed at the estimation of the most potentially mobile metal fraction and/or of the proportion amenable for plant uptake. In sequential extraction procedures, the soil under investigation is treated with reagents of different chemical properties, which are usually applied in order of increasing strength and give rise to the leaching of metals and to the partitioning of their total content into different fractions; sequential extraction results provide information on metal association to soil components and help to predict the risks associated to metal contamination. This chapter deals with the state-of-the-art of single and sequential extraction techniques, with reference to the following topics: the types and properties of extracting reagents; batch and dynamic flow-through procedures; the possible drawbacks and limitations; other approaches to metal extraction, such as kinetic procedures and leaching with non-specific reagents. Examples of applications of single and sequential extractions to the investigation and management of metal-polluted soils are reported throughout the chapter and a case study on metal availability in polluted soils in Piedmont (Italy) is described in detail.