INORGANIC AND ORGANIC SOIL AMENDMENTS USED FOR THE IMMOBILIZATION OF CADMIUM IN CONTAMINATED SOILS - Management of Hazardous Residues Containing Cd - page 103

ability to accumulate Cd varies significantly between species and between
cultivars within a species (DalCorso et al, 2008). Further, this technique may
require extremely long times, e.g. centuries, to achieve the complete
remediation of soil. The requisite time with no agricultural activity during the
remediation process and the necessary biomass treatment after remediation are
the major drawbacks that limits its use (Marques et al, 2009).
Considering the limitations described above, in situ immobilization is
gaining considerable interest over the last decade (van der Lelie et al., 2001).
The aim of immobilization techniques is to use a soil amendment to alter the
soil chemistry and sequester or absorb PTE into the amendment´s matrix. In
this way, the bioavailable fraction is reduced, and the detrimental effects of
PTE on environmental receptors, such as microorganisms, plants, animals,
water bodies, and humans are minimized (Castaldi et al, 2005). A range of
inorganic compounds, such as lime and phosphate fertilizers (Bolan et al.
2003a, b; Hong et al, 2010 a) or organic amendments (Mohamed et al, 2010;
Basta et al., 2001) have been used to immobilize Cd in contaminated soils.
Various mechanisms have been attributed to the effect of these amendments,
including enhanced Cd adsorption (Adriano, 2001); precipitation of metals as
phosphates, hydroxides or carbonates (Basta et al., 2001); and formation of
insoluble Cd-organic complexes in the presence of organic amendments
(Shuman 1999; Farrell, Jones 2010). This Chapter discusses the potential
values of phosphate compounds and biosolids compost, relative to their ability
to immobilize Cd in Cd-contaminated soils.
3.1. Phosphate Compounds
Although phosphate amendments have been initially applied to remediate
Pb-contaminated soil, they also proved to immobilize Cd and Zn in con-
taminated soils (Lambert et al., 1997; Hettiarachichi et al., 1998). Moreover,
addition of phosphorus to soil is the basis of a patented process to reduce
bioavailability of PTE (Pierzynski and Hettiarachchi, 2002). Sources of
phosphate may include either water-insoluble minerals, such as natural or
synthetic apatites and hydroxyapatites, or water-soluble salts, such as
diammonium phosphate and phosphoric acid. Depending on the source, soil
application of phosphate may cause the precipitation of Cd or phosphate-
induced Cd 2+ adsorption.