ABSTRACT
The extent of the hazard posed by trace elements in the soil to organisms depends on their concentrations and chemical speciation in the solid, liquid, and gaseous phases. Limiting the exposure pathways will help to decrease acute and chronic risks. Potentially the most cost-effective strategies to achieve this involve the socalled “mild” remediation techniques (Vangronsveld and Cunningham 1998). Mild remediation options for reducing trace element exposure include deep ploughing, phytoremediation, and chemical immobilization (Osté 2001). Deep ploughing can work if only the upper soil layer is contaminated and the underlying soil has a sufficient fertility and buffer capacity to function as a topsoil. Phytoextraction uses plants to remove soil contaminants by translocating them into plant tissue. The treatment duration and lack of commercially available plant strains with high biomass currently limit its implementation. Furthermore, in soils contaminated with a number of metals, 1 or more elements may limit the phytoextraction potential, e.g., high soil Cu content can reduce the growth of
Thlaspi caerulescens
, and thus Zn and Cd phytoextraction. Chemical immobilization, also called “in situ immobilization,” “inactivation,” or “attenuation,” comprises several methods that aim at reducing potential exposure via the soil or wastes. Mench et al. (1994), Vangronsveld and Cunningham (1998), and Singh and Osté (2001) provide definitions. In this chapter,
chemical immobilization
is defined as an amendment added to the contaminated soil or waste that renders trace elements less bioavailable by altering chemical forms so that toxicity is reduced via a range of exposure pathways, for example, the soil solution, the gaseous phase, or the ingested particles. These technologies need to result in suitable conditions for living organisms. At least 2 options can be adopted. The first option is to promote naturally occurring processes that can alter both speciation and
concentration in solid phases and the soil solution. This may be time consuming or even impossible. An alternative option is to introduce 1 or several compounds via an amendment, leading, for example, to new solid reactive phases or the presence of an essential element for the transformation process. Different mechanisms can be involved, including sorption, acid-base reaction, precipitation, oxidation-reduction, and demethylation.
