Environmental Engineering Reference
In-Depth Information
diammonium phosphate, and phosphate rock as stabilizing agents in Cd-highly
contaminated soils. Addition of all tested phosphate fertilizers induced a shift
of Cd mobile forms towards more stable fractions; however, the shifting
degree occurred at different rates depending on the phosphate fertilizer used,
and followed the sequence: triple superphosphate > diammonium phosphate >
phosphate rock. Consequently, stabilization efficiency appeared to be
correlated with dissolution of phosphate sources.
Other studies have shown that many PTE-phosphates are thermo-
dynamically stable and relatively insoluble in natural environments (Santillan-
Medrano, Jurinak, 1975; Nriagu, 1984; Vieillard and Tardy, 1984; Ruby et al.,
1994). The possibility of forming cadmium phosphate [Cd
3
(PO
4
)
2
] upon the
addition of soluble phosphate compounds in Cd contaminated soils has been
proposed, without direct evidence (Ma et al., 1993; Cotter-Howells, Caporn,
1996). In effect, Bolan et al. (2003 a) found no evidence of Cd
3
(PO
4
)
2
formation in soil samples even at the highest level of KH
2
PO
4
and Cd addition.
Moreover, the solubility of Cd
3
(PO
4
)
2
has been shown to be too high to control
the concentration of Cd in soil suspensions involving iron and aluminium
hydrous oxides (Kuo, 1986; Soon, 1981). Thus, although the precipitation as
Pb or Zn phosphates has been proved to be one of the main mechanisms for
the immobilization of Pb and Zn in contaminated soils (Hettiarachchi et al.
2000; McGowen et al., 2001) no conclusive evidence was provided up to this
time on the existence of new solid phases of Cd-phosphate compounds upon
addition of phosphates on Cd-contaminated soils.
Problems Associated with the Use of Phosphate as an Immobilizing
Agent
Numerous studies have demonstrated that phosphates should be applied at
very high rates to decrease Cd availability in field contaminated soils
(McGowen et al., 2001; Hong et al, 2009; Hong et al. 2008), resulting in new
environmental problems. Surface runoff from croplands with high levels of
soil phosphorus may originate phosphorus enrichment of streams, lakes and
estuaries (Sharpley et al., 1996). Accumulation of phosphorus in water bodies
can result in rapid algae growth, oxygen depletion when algae decompose, and
accelerated eutrophication in saline and fresh waters (Sims et al., 1998; Lin et
al., 2009).
Although phosphate fertilizers provide effective immobilization of Cd in
highly contaminated soil (Wang et al, 2008), their application may also result
in soil acidification (Chen et al., 2007c; Thawornchaisit, Polprasert 2009). The
effect of phosphate addition on soil pH depends on the buffering capacity of
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