4.9
Research on the Possibilities of Using Plants
and Microorganisms for the Biological Extraction of Heavy
Metals From Contaminated Soils
Recent researches report on the possibilities of using MOs for the biological
extraction of HMs from soils contaminated by anthropogenic activities, with sources
of pollution, such as Pb (Pb, As), wood (As, Cr), pesticides (Pb, As, Hg), mining
sites, etc. Comparison of technologies in the study of decontamination of soils
polluted with HMs (As, Cd, Cr, Hg, Pb, Ni, Zn, or Cu) included the following:
insulation (covering) by covering and vertical or horizontal barriers; solidification/
stabilization by cementation, microencapsulation, and vitrification; and separation–
concentration by soil cleaning, soil washing, pyrometallurgical, electrochemical, or
phytoremediation methods.
Phytoremediation for the biological extraction of HMs from contaminated soils
can be achieved by phytoextraction, phytostabilization, or rhizofiltration.
Phytoextraction (followed by harvesting and plant treatment) utilizes plants that
have a hyperaccumulation capacity in tissues (Sarma et al. 2021).
Hyperaccumulate plants accumulate in the leaves over 0.1% Ni, Co, Cu, and Cr
or 1% Zn and Mn (dry matter) irrespective of the concentration of metals in the soil.
Ni and Zn were readily absorbed in plant tissues, as well as Cu and
Cd. Phytostabilization (fixing in roots) uses plants to limit the mobility and
bioaccessibility of metals. It is achieved by producing compounds by plants capable
of immobilizing contaminants at the soil-root interface. By increasing the pH in the
soil, additional stabilization is achieved.
The method is applicable to many metals, especially Pb, Cr, and
Hg. Phytostabilizing plants can tolerate metals and immobilize them in the soil by
adsorption, precipitation, complexation, and reduction. Three varieties of herbaceous plants have been marketed for phytostabilization: Agrostis tenuis, Parys for
Cu waste; A. tenuis, cv Coginan for waste with Zn and Pb acid; and F. rubra, cv
Merlin for waste with Zn and Pb limestone.
Rhizofiltration uses the roots of terrestrial plants to absorb, concentrate, and
precipitate metals from wastewater, including leachates of contaminated soils,
translating them into tissues, more easily than by phytoextraction. Terrestrial plants
develop fibrous root systems with a larger surface of action than aquatic plants.
Another type of rhizofiltration that is fully developed uses water plants in wetland
or wetland plants to treat wastewater or leachate from contaminated soils. These
biotechnologies use the decontamination activity of a complex system of plants and
microorganisms. It is to be noted that one of the goals of phytoremediation is to
achieve major reductions in the cost of decontamination (Ignatius et al. 2014).
106
M. Butu et al.
Research on the Possibilities of Using Plants
and Microorganisms for the Biological Extraction of Heavy
Metals From Contaminated Soils
Recent researches report on the possibilities of using MOs for the biological
extraction of HMs from soils contaminated by anthropogenic activities, with sources
of pollution, such as Pb (Pb, As), wood (As, Cr), pesticides (Pb, As, Hg), mining
sites, etc. Comparison of technologies in the study of decontamination of soils
polluted with HMs (As, Cd, Cr, Hg, Pb, Ni, Zn, or Cu) included the following:
insulation (covering) by covering and vertical or horizontal barriers; solidification/
stabilization by cementation, microencapsulation, and vitrification; and separation–
concentration by soil cleaning, soil washing, pyrometallurgical, electrochemical, or
phytoremediation methods.
Phytoremediation for the biological extraction of HMs from contaminated soils
can be achieved by phytoextraction, phytostabilization, or rhizofiltration.
Phytoextraction (followed by harvesting and plant treatment) utilizes plants that
have a hyperaccumulation capacity in tissues (Sarma et al. 2021).
Hyperaccumulate plants accumulate in the leaves over 0.1% Ni, Co, Cu, and Cr
or 1% Zn and Mn (dry matter) irrespective of the concentration of metals in the soil.
Ni and Zn were readily absorbed in plant tissues, as well as Cu and
Cd. Phytostabilization (fixing in roots) uses plants to limit the mobility and
bioaccessibility of metals. It is achieved by producing compounds by plants capable
of immobilizing contaminants at the soil-root interface. By increasing the pH in the
soil, additional stabilization is achieved.
The method is applicable to many metals, especially Pb, Cr, and
Hg. Phytostabilizing plants can tolerate metals and immobilize them in the soil by
adsorption, precipitation, complexation, and reduction. Three varieties of herbaceous plants have been marketed for phytostabilization: Agrostis tenuis, Parys for
Cu waste; A. tenuis, cv Coginan for waste with Zn and Pb acid; and F. rubra, cv
Merlin for waste with Zn and Pb limestone.
Rhizofiltration uses the roots of terrestrial plants to absorb, concentrate, and
precipitate metals from wastewater, including leachates of contaminated soils,
translating them into tissues, more easily than by phytoextraction. Terrestrial plants
develop fibrous root systems with a larger surface of action than aquatic plants.
Another type of rhizofiltration that is fully developed uses water plants in wetland
or wetland plants to treat wastewater or leachate from contaminated soils. These
biotechnologies use the decontamination activity of a complex system of plants and
microorganisms. It is to be noted that one of the goals of phytoremediation is to
achieve major reductions in the cost of decontamination (Ignatius et al. 2014).
106
M. Butu et al.