INORGANIC AND ORGANIC SOIL AMENDMENTS USED FOR THE IMMOBILIZATION OF CADMIUM IN CONTAMINATED SOILS - Management of Hazardous Residues Containing Cd - page 102

toxicity and mobility of Cd through soil profiles (Alloway, 1990; Helmke and
Naidu, 1996). Single extraction and/or sequential extraction procedures are
used to estimate the distribution of PTE associated with particular phases in
soils and sediments. A large number of sequential extraction schemes have
been proposed for soils, generally attempting to identify PTE held in any of
the following fractions: soluble, exchangeable, sulfide/carbonate-bound,
organically bound, oxides-bound, and residual or lattice mineral bound
(Tessier et al, 1979; Sposito et al, 1982; Shuman 1985; Rauret et al, 1999).
Assessment of metal mobility and bioavailability in soils and sediments using
sequential extraction procedures's assumes that mobility and bioavailability
decrease in the order of extraction. Thus, metals in the exchangeable fractions
are most mobile and bioavailable, whereas metals in residual fraction are
tightly bound and least mobile under natural environmental conditions.
Technologies to minimize the chances of Cd reaching the food chain aim to
reduce the concentration of bioavailable forms by increasing adsorption,
precipitation or entrapment of Cd in crystal lattices (Hamon et al, 2002).
3. R EMEDIATION T ECHNIQUES
Different remediation techniques have been developed in order to reduce
PTE and, in particular, Cd bioavailability in soils (Chen, Lee 1997; Gray et al.,
2006). Several physical-chemical treatments based on excavation, landfilling,
thermal treatment, acid leaching and electro-reclamation have been proposed.
Unfortunately, these methods are not suitable for practical application, because
of their high cost and low efficiency, limiting its use on vast contaminated
areas (Khan et al, 2005). Moreover, they are often environmentally invasive
and do not permit a natural reshaping of the environment (Lombi et al., 2002).
In contrast, bioremediation is cost-effective and environmentally friendly. Two
bioremediation technologies, phytoremediation and stabilization, have been
widely investigated for the remediation of Cd polluted sites (Dushenkov et al,
1997; Carrillo-González et al, 2006).
Phytoremediation of Cd-contaminated soils is a fast-expanding
technology, which uses specific plants species which have an innate ability to
tolerate and accumulate Cd (Blaylock and Huang, 2000; Ghosh, Singh 2005;
2004). This technology is increasingly being considered as an alternative to the
conventional engineering-based remediation methods and has the advantage of
being cost effective, environmental friendly, and less disruptive to the soil and
therefore the associated site ecosystem (Marques et al, 2009). However, the