Phytoremediation is a technique whi

Phytoremediation is a technique which utilizes plants to remove, eliminate, or decrease environmental contaminants (heavy metals, organics, radionuclides, explosives), attenuating the related risk (Barbafieri, 2001; Li et al., 2002). It is based on several natural processes which involve plants:
- direct sorption of metals or moderately hydrophobic organic compounds;
- accumulation or transformation of chemicals by lignification, metabolization, volatilization;
- catalysis and degradation of organic compounds by enzymes released by plants;
- exudates release in the rhizosphere, with pH modification, carbon and microbial activity increase, and contaminant degradation.
Below are some techniques utilized in phytoremediation for removing inorganic contaminants from soils based on the above processes, that the clean-up industry is already utilizing or seriously considering to adopt.

Phytostabilization
Phytostabilization is a process in which plants tolerant to contaminant metals are used to reduce the mobility of contaminant metals, thereby reducing the risk of further environmental degradation by leaching into the groundwater or by airborne spread (Smith and Bradshaw, 1979; Losi et al., 1994; Vangronsveld et al., 1995a). In in situ metal stabilization, soil amendments can be combined with the use of metal-tolerant plants to enhance plant growth and stabilizing effect of the treatment (Vangronsveld et al., 1995a; 1995b; 1996). Metal-tolerant plants immobilize contaminants at the interface root-soil by absorption, precipitation or complexation, thus reducing mobility and migration to groundwater, or to the food chain.
Stabilization may occur:
- in the root zone: proteins are released (by roots) in the rhizosphere, and determine precipitation of contaminants;
- on cell walls: proteins associated with root cell walls may stabilize contaminant outside the root cell, thus impeding contaminant to be transported inside the plant (barrier effect);
- in root cells: proteins present on root membranes may enhance contaminant transport inside the cell, where it is sequestered in vacuoles (Fig. 1).
Arboreal and shrubby plants plants seem to be more prone than herbaceous ones to apply phytostabilization, owing to different sorption ways and metal mobility in the plant organs. Some ornamentals, like bay (Laurus nobilis), pitosphor (Pitosphorum tobira), oleander (Nerium oleander) proved effective in metal stabilization (Carratù et al., 2001). Among the herbaceous plants, monocotyledons, both native (e.g. Lolium perenne) and cultivated (e.g. Zea mays, Avena sativa) are more suitable than dicotyledons (Argese et al., 2001).
Phytostabilization is particularly suitable at sites where it is important to keep metals in non-mobile form, in order to impede dispersion, like it happens for chromium (Bini et al., 2000a; 2007). Moreover, when metal concentration is very high, phytoextraction would require long time to achieve the objectives of land restoration.

Rhizofiltration is a process in which plant roots absorb, precipitate and concentrate heavy metals from polluted wastewater streams (Dushenkov et al., 1995). It proved particularly effective in taking up As from wetland areas (Sette et al., 2001).