4. Future developments

• The bioavailability factor (BF), defined as the ratio between the metal content in mobile phase and the total metal concentration in the soil. This value indicates the fraction of the

• The retention factor (RF), the ratio between the amount of metal in the residual fraction (after mineralization and then sequential extraction) and the total amount of metal in the soil. Its value reflects the amount of metal retained in the solid phase. Normally, the

• The transfer factor (TF) or bioaccumulation factor, the ratio between the metal content of certain plant tissues and the total amount of metal in soil. It expresses the degree to which a plant absorbs the metal in the roots and other tissues, usually having much higher values for roots than for stem or seeds. Currently, the accumulation factor in the edible

Phytoremediation can be used in combination with other remediation techniques: chelateassisted remediation, microbial-assisted remediation, and the use of transgenic plants [90, 91]. The 1990 EPA Manual on In situ Treatment of Contaminated Soils mentions the remediation term or ecological restauration, limiting the definition to the physicochemical methods of

The purpose of the biological remediation process is to degrade contaminants and transform them into harmless intermediates and byproducts. The last step is to complete the mineralization of contaminants to carbon dioxide, water, and simple, inorganic compounds. Microorganisms in the rhizosphere can symbiotically interact with roots to increase the absorption of metals from soil or to biodegrade or immobilize certain toxic compounds for plants [93, 94]. The low solubility of heavy metals in the soil solution is an important impediment to their extraction by plants. In order to make the phytoextraction process more efficient, it is necessary to find methods to solubilize the heavy metals, increasing their bioavailability and therefore the ability to be extracted from plants, preferably with accumulation in the aerial parts, easy to remove by harvesting. Until now, besides soil pH reduction, the only viable solution for increasing the mobility of heavy metals in soil is the addition of substances that form soluble compounds with heavy metals existing in the soil in different forms, thus increasing their bioavailability. The use of chelators for soil remediation has started from the finding that these heavy metal complexes are more soluble in aqueous solutions than other combinations. Applying some ligands to the soil, such as EDTA, citrate, or tartrate, results an increased heavy metal mobility, an immediate increase of the mobile fraction amount in the soil and then in the

The use of amendments and fertilizers is also useful to increase the phytoextraction capacity of plants. Adding organic amendments such as compost, green fertilizer, and biosolids is playing

total metal concentration in the soil that is considered available for plants.

retention factor is lower in soils with low pH and low clay content.

parts of the plants is of maximum interest.

immobilizing or extracting heavy metals from the soil [92].

3.4. Other bioremediation techniques

204 Ecosystem Services and Global Ecology

roots and aerial parts of the plants [95, 96].

an important role in metal mobility and plant growth [97, 98].

Phytoremediation requires a greater effort than simply plant cultivation with minimal maintenance, assuming that the concentration of heavy metals in the soil will decrease. In addition, phytoextraction also refers to phytomining. A limited definition of the term "phytomining" is the possibility to use the crop plants to achieve economical production of metals, both from contaminated soils and also from soils that naturally have a high concentration of metals [66]. This extraction for commercial purposes of heavy metals from crop plants is not widely used. Several plant species are used by geologists for mineral prospecting, as indicator plants for the presence of different metals in soils: Equisetum arvense (horsetail) for gold, Alyssum bertolonii and Thlaspi L. for nickel, Viola calaminaria for zinc, and Pteridium aquilinum for arsenic [99, 100].

Another method of improving the cost-benefit of phytoremediation is to extract active principles from plants and used before plant processing. Obviously, if any useful substances (metals or oils) are recovered from the plants or by using the harvested plants for biofuel production, this practice can reduce the related costs of phytoremediation [101, 102].

Recent research in the phytoremediation application includes the use of transgenic plants and removal of metallic nanoparticles from soils [37, 103]. The challenge is to identify genes coding the specific heavy metal hyperaccumulation in plants.
