Page 285 - Plant-Based Remediation Processes

13 Phytoremediation Towards the Future: Focus on Bioavailable Contaminants 279

mobilize metals to soil appears to increase the bioavailable amount without creating
undesirable environmental side effects (Doumett et al. 2011). The design of plant
microbial consortia, on the other hand, that would be able to modify the rhizosphere
environment, could increase bioavailability and the uptake of heavy metals. In
addition, the foliar treatment with phytohormones such as cytokinin increased the
phytoextraction efficiency of crop plants in mercury-contaminated soil through the
increase in plant biomass and evapotranspiration (Barbafieri and Tassi 2010;
Cassina et al. 2012).
Another solution is to use plants to reduce only the fraction that is the most
hazardous to the environment and human health: the mobile fractions of metals in
soils (Fitz et al. 2003; Wenzel 2009). In this way, plants can be used to decrease
bioavailable metals, while the cleanup time can be substantially shortened. With
this approach based on the concept of bioavailable contaminant stripping (BCS)
introduced by Hamon and McLaughlin (1999), an evaluation of the hazards of the
residual fraction not removed by plants left to risk assessment procedures. This
option is also supported by new legislation which no longer defines pollution on the
basis of target concentrations, but according to the results derived from a site-
specific risk analysis.
This remediation strategy originates from the intrinsic properties of the technol-
ogy, whose applicability is strictly linked to the bioavailability of heavy metals. As
previously highlighted, phytoextraction acts only on the amounts of metals that are,
or may be, bioavailable. Nevertheless, most contaminated sites contain a residual
fraction of metals, which are bound irreversibly to soil surfaces that phytoextraction
cannot remove. The main criticism of the BCS method is that it is unknown how
long it will take to reintegrate mobile metals in soil solutions, once the original
soluble amount has been entirely or in part removed by plants. This problem can be
overcome by enhanced bioavailable contaminant stripping (EBCS) (Petruzzelli
et al. 2011, 2012), which evaluates this amount through the combined use of:
• Chemical extraction with a mobilizing agent, specific for each metal, capable of
rapidly solubilizing the maximum possible amount of a metal, in order to rapidly
simulate the slow release of metallic elements from the soil solid phase.
• Pot experiments in which the selected mobilizing agent is used as an additive
(assisted phytoextraction), which in successive growing cycles must confirm the
absence of the bioavailable fractions.
Thus this amount of mobilized metal can be considered to correspond to the
maximum potentially available, which can be removed by plants in one or more
cycles of growth. The rate of resupply to the depletion of metals in a soil solution
depends on metal pool speciation in the solid phase, where most metals are
irreversibly linked (Lehto et al. 2006). The characterization of available pools
and the ability to resupply metals from less available pools is essential to support
any decision to apply this technology in the field. The use of pot experiments in
addition to chemical extraction can help to evaluate whether there may be unpre-
dictable events that hinder plant growth in contaminated soil or whether the plant
roots are not able to access the metals in soil, for instance, due to poor physical