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solution, while capacity is related to the ability of the soil to resupply metals in the
soil solution following depletion due to plant uptake (Hough et al. 2005). The
processes that determine bioavailability are the release of elements from the solid
phase of soil and their uptake in soluble form by the root system of the plant.
Bioavailable metal pools in soil decrease with time, due to both plant uptake and
aging processes, which poses severe limitations to the amount of metals that can be
removed by the technology. Both the theoretical modeling and the considerations
deriving from the cases of application on a real scale show that phytoremediation is
naturally limited by the considerably long time required, since it is a technique
related to the growth cycles of plants. Decades of remediation would be necessary
in many cases, which reduce the appeal of phytoremediation, especially if rapid
results and a total removal of pollutants are required.
In order to increase the efficiency of phytoextraction, fertilizers can be used to
enhance the productivity of selected plants, positive results have reported recently
in the case of the boron-contaminated soils (Giansoldati et al. 2012). Amendments
such as organic acids or synthetic chelators can be added to soil in order to facilitate
desorption of metals from the solid phase and to increase, consequently, their
solubility (assisted phytoextraction). However, the use of chelators able to form
stable and water-soluble complexes with toxic metals can increase their
concentrations in the soil solution for a long time and in excess of the translocation
capacity of plants (Luo et al. 2005; Santos et al. 2006; Cao et al. 2007), being of
potential concern for their leaching into the subsoil or into ground or surface waters.
The use of natural low molecular weight organic acids such as citric, malic, oxalic,
and tartaric acids and the natural amino acid, glutamic acid, which are characterized
by a much lower toxicity and higher biodegradability, has been proposed as an
alternative (Wu et al. 2004; Evangelou et al. 2006; Doumett et al. 2008). According
to their rapid biodegradability, these ligands show a short persistence in soil
(Evangelou et al. 2008). Repeated applications may therefore be suitable for
maintaining metal bioavailability in soils high enough to support plant metal
uptake.
Other promising possibilities consist in enriching the rhizosphere of plants with
rhizobacteria that promote growth. The biogeochemistry of inorganic contaminants
may be substantially influenced by the processes that happen in the rhizosphere. In
the rhizosphere, while uptaking metals, roots induce changes in soil water transport
and, by the exudation of proton, hydroxyl ions, and organic acids, can modify pH,
redox conditions, and the chemical speciation of metals (Fitz and Wenzel 2002;
Vetterlein et al. 2007; Wenzel 2009; Lin et al. 2010). Finally genetic engineering
has made it possible to increase the tolerance and the accumulation of metals in
species already characterized by a high production of biomass (Bizily et al. 2000;
Meagher and Heaton 2005; Hussein et al. 2007). To sum up, in the case of heavy
metal pollution, the application of phytoremediation on a large scale presents some
problems and, in most cases, excellent results have not yet been achieved. In order
to optimize the technique, research is moving in different directions. The use of
genetically modified plants (Meagher et al. 2000) seems to offer important
prospects, including economic benefits, and the addition of new agents that