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274 G. Petruzzelli et al.

However, this conclusion is not completely true since some metals, such as nickel
phytoextraction, have been employed with great efficiency (Ghaderian et al. 2007).
In some cases, technology also provides positive results with non-
hyperaccumulator plants (Pedron et al. 2009; Koopmans et al. 2007). These
successes derive, above all from the characteristics of contaminated soils, for
instance, the pH that determines both the bioavailability of the contaminants and
the conditions necessary for plant growth. Soil properties are the key to
phytoextraction efficiency, but often they are not fully considered in the selection
of the technology. Soils undergo physical, chemical, and biological reactions that
continuously distribute metals among the various soil phases. Retention and release
processes take place depending on each specific metal, soil properties, and time
(Alexander 2000; Ehlers and Luthy 2003). Therefore, ability of the same plants to
uptake metals is quite different among soils with different properties. Moreover, not
all polluted soils and climate characteristics are suitable for all plant growth,
including hyperaccumulators. Therefore a general scheme of phytoextraction that
does not consider the specific properties of the contaminated soil may completely
fail to predict the efficiency of the technology in field applications.

13.2 The Bioavailability of Contaminants: An Undervalued
Aspect

The efficiency of all in situ technologies strictly depends on soil properties, which
regulate the distribution of contaminants among the different soil phases. This is
particularly important for phytoremediation, since plants absorb substances only if
they are present in the soil liquid phase (soil solution). The evaluation of contami-
nant bioavailability is essential for the appropriate application of the technology. In
soil, bioavailability is the result of complex mechanisms of mass transfer and
absorption, which are affected by contaminant properties, the chemical and physi-
cal characteristics of the soil, and the biology of the organisms involved (National
Research Council 2002). The transfer of heavy metals from the solid phase into soil
solutions is fundamental. Only after being released in the aqueous phase, can a
contaminant move freely towards the plant roots and be absorbed. Thus the metal
speciation in soil is critical for phytoextraction under field conditions, while the
concentration in the liquid phase is an essential parameter for the final success of
remediation (Petruzzelli and Pedron 2006). In soils characterized by high contents
of humic acids or the significant presence of clays, metals are strongly retained by
these components, reducing the phytoextraction efficacy. In addition, stronger
bonds between metals and soil surfaces correspond to the increasing time of
residence of metals in soil, a reduction in bioavailability, and therefore a decrease
in the efficiency of phytoextraction (Shelmerdine et al. 2009).