INORGANIC AND ORGANIC SOIL AMENDMENTS USED FOR THE IMMOBILIZATION OF CADMIUM IN CONTAMINATED SOILS - Management of Hazardous Residues Containing Cd - page 114

organic matter in the control, together with the fact that Cd is one of the more
weakly bonded ions by soil organic matter (Harter, Naidu 2001).
Nonetheless, an increase in OM-Cd was observed in control and
composts, lime or lime + compost amended soils during the incubation period,
and accounted between 30-50% of the total soil Cd.
Changes in Cd Precipitated Fraction (INOR-Cd)
The amounts of INOR-Cd in control and amended soils are shown in
Figure 3. No significant differences in INOR-Cd were observed with the
addition of C1, C2 or C3 compared to control in all sampling dates. On the
contrary, lime application resulted in a significant increase of INOR-Cd
compared to control. These results are in agreement with some other published
papers which indicate that application of lime can significantly increase the
precipitated fraction of Cd in contaminated soil (Knox et al. 2001; Hong et al,
2010 b), possibly as Cd-carbonate (Knox et al. 2001; Basta et al., 2001) at the
expense of the EXCH-Cd fraction. A positive and significant correlation could
be established between INOR-Cd and soil pH [INOR-Cd (mg kg -1 ) = 0,6443
pH - 2,267 (R 2 = 0,96, p<0.01)] among all sampling dates.
The decrease in the phytoavailability of Cd in the presence of biosolids or
biosolids´ compost has often been attributed to increased complexation by the
organic constituents (Adriano, 2001). However, inorganic compounds present
in biosolids are responsible for inducing the ‗plateau effect' in Cd uptake by
crops (Brown et al., 1998). Biosolids contain from 30 to 60% of inorganic
mineral forms, like Fe, Mn, and Al oxides, silicates, phosphates and
carbonates (Sommers et al., 1976; Torri 2009) that are highly reactive with
PTE. Many authors have documented the role of these inorganic components
in the formation of heterogeneous precipitates as well as their ability to adsorb
Cd (Corey et al., 1987; Essington and Mattigod, 1991; Kuo, 1986).
Furthermore, several authors have reported the predominant role of Fe and Mn
oxides in the almost irreversible adsorption of Cd in biosolid amended soils
(Li et al., 2001; Hettiarachchi et al., 2003 and Merrington and Smernik, 2004).
In this study, fixation was more pronounced in C3+LL. These results may be
due to the presence of elevated concentrations of iron oxyhydroxides in the
compost, in agreement with Buekers et al. (2007) and Torri (2009). Cadmium
specifically adsorbed by these inorganic phases would be expected to have a
low phytoavailability, and could become no phytoavailable as surfaces become
occluded.