INORGANIC AND ORGANIC SOIL AMENDMENTS USED FOR THE IMMOBILIZATION OF CADMIUM IN CONTAMINATED SOILS - Management of Hazardous Residues Containing Cd - page 107

the soil, the nature of phosphate compounds and the extent of phosphate
adsorption (Havlin et al., 1999). Dissolution of phosphate fertilizers,
particularly triple superphosphate and diammonium phosphate, yield the
release of phosphoric acid (Chrysochoou et al, 2007; Spuller et al, 2007),
decreasing soil pH. Thus, phosphate-induced variation in soil pH may
influence the solubility of Cd in soils, as reported by Levi-Minzi and
Petruzzelli (1984). It has often been observed that the phytoavailability of Cd
increased with decreasing pH (Naidu et al., 1994). Hence, if immobilization of
Cd is followed by soil acidification, the contaminated soil will continue to
pose a potential ecological risk and the remediation treatment may not be
sustainable without continued management inputs. Consequently, co-
application of liming materials with phosphate fertilizer would be necessary to
offset soil acidification, especially in non-alkaline soils.
3.2 Organic (Biosolids) Amendments
Organic matter has been of particular interest in studies of PTE retention
by soils, because of its significant impact on cation exchange capacity (CEC),
and more importantly, the tendency of transition metal cations to form stable
complexes with organic ligands (Elliott et al., 1986; Chen, 1996). Organic
materials provide a large number of non-specific and specific sorption sites for
metals from which they may be difficult to displace (Shuman, 1999). Thus, in
situ application of organic amendments is becoming increasingly widespread
for its low cost and beneficial effect on soil fertility (Tapia et al, 2010).
Organic matter has quite a different effect on Cd availability depending
upon whether it is in soluble or insoluble forms. Soil addition of insoluble
(humified) organic amendments encourage the formation of stable organo-
cadmium complexes, which diminishes Cd solubility (Udom et al. 2004;
Mohamed et al, 2010). The ability of humic substances to bind Cd was
attributed to their high content of oxygen-containing functional groups,
including carboxyl, phenol, hydroxyl, enol, and carbonyl structures of various
types (Rocha et al. 2009). Some studies reported different interactions between
humic acids and Cd, depending on the nature and properties of functional
groups, especially regarding their content of phenolic groups (Datta et al.,
2001).
Dissolved or soluble organic carbon consists of several types of low
molecular weight organic compounds, such as polyphenols, simple aliphatic
acids, amino acids and sugar acids (Fox, Comerfield 1990). Soluble organic