Environmental Engineering Reference
In-Depth Information
authors (Száková et al, 2007; Ghafoor et al., 2008; Hong et al, 2010b).
Conversely, no significant differences in soil pH values were observed
between control and soils treated with C1, C2 and C3 in each sampling date.
Interestingly, in most of the studies reported in the literature, biosolids
compost addition increased soil pH across all treatments in comparison to
control samples (Castaldi et al, 2005; Gadepalle et al, 2009; Tapia et al, 2010).
The lack of significant differences in soil pH values between control, C1, C2
and C3 in this study suggests that pH may be regarded as the key factor
controlling the concentration of exchangeable Cd in Cd contaminated soils. A
negative and significant correlation could be established between EXCH-Cd
and soil pH [EXCH-Cd (mg kg
-1
) = -1,4818 pH + 13,66, R
2
= 0.98, p<0.01]
among all sampling dates.
Changes in Cd Organic Fraction (OM-Cd)
At the end of the studied period, the fraction of Cd associated with organic
matter increased in all amended soil samples and control (Figure 2). Only
C3+L significantly increased OM-Cd compared to control at day 14, 28 and
42. No significant differences between the other treatments and control were
observed.
The formation of stable complexes between Cd and organic matter in
composted biosolids has been reported by other authors (Bolan et al., 2003c;
Lin, Zhou 2009; Chen et al, 2010; Mohamed et al, 2010 and Tapia et al, 2010).
However, these studies also reported an increase in soil pH due to the
amendment´s incorporation. Depending on soil pH, soil organic matter
generally carries more surface charge than do inorganic soil components, as
discussed before. However, in this study, increases in soil pH were only
observed in lime treated soils. Therefore, increase in surface charge of
compost´s organic matter was not the cause for the increase in OM-Cd in non-
lime amended soils. Kaschl et al (2002) studied the relative importance of
different types of organic ligands (dissolved organic matter, humic and fulvic
acids) for the binding of Cd at pH=7. The highest complexing capacity for Cd
was found for the most humified and less soluble organic materials, and could
not be directly related to total acid-titratable or carboxylic groups, indicating
the importance of sterical issues and other functional groups. The high content
of humified organic matter in biosolids (Torri et al, 2003) may have hindered
slight increases in humic substances originated in the compost process,
explaining the lack of significant differences in OM-Cd between C1, C2 or C3.
Moreover, there was no difference in the concentration of OM-Cd between
control and C1, C2 and C3, which may be attributed to the high level of
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