INORGANIC AND ORGANIC SOIL AMENDMENTS USED FOR THE IMMOBILIZATION OF CADMIUM IN CONTAMINATED SOILS - Management of Hazardous Residues Containing Cd - page 108

carbon can facilitate the transport of Cd through the formation of soluble
complexes (Antoniadis and Alloway, 2002; Kaschl et al, 2002; Pan, Zhou
2007). Although these compounds may reduce Cd adsorption onto soil
surfaces (Guisquiani et al., 1998), they have been shown to be less readily
available for plant uptake (Hamon et al., 1995; Han et al., 2001).
Throughout the world, there have been increasing interests in the
utilization of waste materials such as biosolids or biosolids compost as low-
cost adsorbents rather than disposal in a landfill. Traditionally, biosolids have
been viewed as one of the major sources of PTE accumulation in soils, and a
large volume of work has been carried out to examine the mobilization and
bioavailability of biosolid-borne metals in soil (Merrington, Madden 2000; Li
et al, 2001; Li et al, 2006; Torri, Lavado 2008 a, Torri, Lavado 2008 b).
Biosolids contain a high proportion of humified organic matter. Torri et al
(2003) reported that between 29-45% of biosolids-borne carbon was
recalcitrant in three representative soils of the Pampas Region, Argentina.
During composting, organic compounds are transformed through successive
activities of different microbes into stabilized humic substances (García et al.,
1993; Paré et al., 1998).
Although biosolids and biosolids compost may contain Cd themselves,
further studies have shown that biosolid addition to soil enhanced the ability of
soil in adsorbing heavy metals, thereby limiting its bioavailability (Merrington
and Smernik, 2004, Tapia et al, 2010). Actually, soil application of this
organic waste has improved Cd immobilization in extremely contaminated
soils (Li et al., 2001; Bolan et al., 2003 c; Brown et al., 2003; Hettiarachchi et
al, 2003; Tapia et al, 2010, among others), aiding the re-establishment of
vegetation (Castaldi et al., 2005; Gadepalle et al, 2009). The presence of
phosphates, aluminum compounds and other inorganic minerals in biosolids is
also responsible for the retention of Cd, preventing the increased metal
availability suggested in the ‗time bomb' hypothesis (Li et al, 2001;
Hettiarachchi et al., 2003; Torri, Lavado 2009). Such observations imply that
the sequestration capacity of biosolids of sludge origin might be indefinite due
to the residual effects of the mentioned constituents. This observation
complements those of Mahler et al (1987) and Brown et al. (1998) which
indicated that either inorganic phases in biosolids or recalcitrant organic
carbon are responsible for maintaining low plant availability in biosolids-
amended soils. Currently, there is intense debate whether the organic or the
inorganic fraction is responsible for Cd and other PTE immobilization (Li et
al., 2001; Stacey et al., 2001, Bolan et al., 2003 c, Hettiarachchi et al., 2003,
Merrington et al., 2003; Kumpiene et al., 2008).