**6. The use of plants for biological remediation of heavy metal polluted soils**

Phytoremediation is one of the best techniques for treatment of heavy metal-polluted sites. It is an *in situ* strategy that uses vegetation and associated microbiota together with agronomic practices to aid in metal remediation [79]. It is based on the use of special type of plants to decontaminate soil by inactivating metals in the rhizosphere or translocating them in the aerial parts [56]. Some plants developed mechanisms to remove ions selectively from the soil to regulate the uptake and distribution of metals. Potentially useful phytoremediation technol‐ ogies for heavy metal-polluted sites include phytoextraction, phytostabilization and rhizofil‐ tration [75].

Phytoextraction uses hyperaccumulating plants to remove metals from soil by absorption into the roots and shoots of the plant. The aboveground shoots can be then harvested to remove metals from the site and subsequently stored as hazardous waste or employed for the recovery of metals. The ideal plant for phytoextraction should grow rapidly, produce high amount of biomass and be able to tolerate and accumulate high metal concentrations in shoots [80]. Hyperaccumulating plants belong to the families of *Brassicaceae*, *Fabaceae*, *Euphorbiaceae*, *Asterraceae*, *Lamiaceae*, and *Scrophulariaceae* [77]. Studies indicate that many *Brassica* species, such as *B. juncea*, *B. napus* or *B. rapa* exhibit enhanced accumulation of Zn and Cd [81] In comparison to conventional methods like e.g. soil excavation (*ex situ* remediation), phytoex‐ traction is time consuming, but on the other hand it is cost-effective and less labor-intensive [9].

Phytostabilization is based on the use of plants to limit the mobility and bioavailability of metals in soil. Plants used in this method are characterized by high tolerance of metals in surrounding soils together with their low accumulation. Phytostabilization can be carried out through the process of sorption, precipitation, complexation, or metal valence reduction. This technique is useful for the removal of Pb, As, Cd, Cr, Cu, and Zn [82]. This process is advan‐ tageous because in this case disposal of hazardous material/biomass is not required, and it is very effective when rapid immobilization is needed to preserve soils or ground and surface waters [82].

Rhizofiltration (or phytofiltration) removes metals from contaminated soil via absorption, concentration and precipitation by plant roots. This technique is used to remove pollutants from groundwater and aqueous-waste streams rather than for the remediation of polluted soils [76]. Apart from the above described phytoremediation methods, some authors [83] include also phytovolatization and phytodegradation.

Phytovolatization involves the use of plants to volatilize pollutants from their foliage such as Se and Hg, while phytodegradation uses plants and associated microorganisms to degrade organic pollutants. Even though phytoremediation strategies are inexpensive, effective, environmentally friendly and can be implemented *in situ*, a substantial proportion of metal pollutants are unavailable for root uptake by field grown plants [84]. Therefore, methods of increasing phytoavailability of heavy metal pollutants in soil and their transport to plant roots are vital to the success of *in situ* phytoremediation. In this case it is useful to apply microbial populations that are able to affect trace metal mobility and availability to plants, through the release of chelators, acidification and redox changes [85]. It was proved that the presence of rhizosphere bacteria increases the available concentrations of various heavy metals to hyper‐ accumulative plants [80]. Microbial populations may be used not only for increasing metal bioavailability to plants, but also for the promotion of hyperaccumulative plant growth through N2 fixation, production of phytohormones and siderophores, and transformation of nutrients [26]. Figure 2 summarizes the mechanisms of plant-mediated remediation of contaminated soils.

followed by sedimentation, ion exchange, reverse osmosis and microfiltration [78]. Neverthe‐ less, physicochemical techniques for heavy metal remediation are generally costly and have side effects [37]. Therefore, continuous efforts have been made to develop techniques that are

**6. The use of plants for biological remediation of heavy metal polluted soils**

Phytoremediation is one of the best techniques for treatment of heavy metal-polluted sites. It is an *in situ* strategy that uses vegetation and associated microbiota together with agronomic practices to aid in metal remediation [79]. It is based on the use of special type of plants to decontaminate soil by inactivating metals in the rhizosphere or translocating them in the aerial parts [56]. Some plants developed mechanisms to remove ions selectively from the soil to regulate the uptake and distribution of metals. Potentially useful phytoremediation technol‐ ogies for heavy metal-polluted sites include phytoextraction, phytostabilization and rhizofil‐

Phytoextraction uses hyperaccumulating plants to remove metals from soil by absorption into the roots and shoots of the plant. The aboveground shoots can be then harvested to remove metals from the site and subsequently stored as hazardous waste or employed for the recovery of metals. The ideal plant for phytoextraction should grow rapidly, produce high amount of biomass and be able to tolerate and accumulate high metal concentrations in shoots [80]. Hyperaccumulating plants belong to the families of *Brassicaceae*, *Fabaceae*, *Euphorbiaceae*, *Asterraceae*, *Lamiaceae*, and *Scrophulariaceae* [77]. Studies indicate that many *Brassica* species, such as *B. juncea*, *B. napus* or *B. rapa* exhibit enhanced accumulation of Zn and Cd [81] In comparison to conventional methods like e.g. soil excavation (*ex situ* remediation), phytoex‐ traction is time consuming, but on the other hand it is cost-effective and less labor-intensive [9].

Phytostabilization is based on the use of plants to limit the mobility and bioavailability of metals in soil. Plants used in this method are characterized by high tolerance of metals in surrounding soils together with their low accumulation. Phytostabilization can be carried out through the process of sorption, precipitation, complexation, or metal valence reduction. This technique is useful for the removal of Pb, As, Cd, Cr, Cu, and Zn [82]. This process is advan‐ tageous because in this case disposal of hazardous material/biomass is not required, and it is very effective when rapid immobilization is needed to preserve soils or ground and surface

Rhizofiltration (or phytofiltration) removes metals from contaminated soil via absorption, concentration and precipitation by plant roots. This technique is used to remove pollutants from groundwater and aqueous-waste streams rather than for the remediation of polluted soils [76]. Apart from the above described phytoremediation methods, some authors [83] include

Phytovolatization involves the use of plants to volatilize pollutants from their foliage such as Se and Hg, while phytodegradation uses plants and associated microorganisms to degrade

easy to use, sustainable and economically feasible.

772 Environmental Risk Assessment of Soil Contamination

tration [75].

waters [82].

also phytovolatization and phytodegradation.

**Figure 2.** Mechanisms of phytoremediation involved in purifying contaminated soils and physiological processes that occur in plants during phytoremediation.
