3. Phytoremediation: a green technology to remove heavy metals from the soil

Many remediation techniques have been used to respond to the growing number of soils contaminated with heavy metals [33–35].

Decontamination methods currently applied in the majority of sites are mainly characterized by the manipulation of enormous quantities of soil or heavy metal extraction by using chemical reagents. These practices are very expensive and also lead to the loss of soil fertility by changing its physicochemical properties (structure, cationic exchange capacity, etc.), destroying at the same time the microorganisms from the soil and, ultimately, the humus layer [36]. In this situation, other less brutal methods for heavy metal extraction were searched and developed. Bioremediation and phytoremediation in particular are such "mild" remediation methods that maintain or even restore the natural soil fertility [21].

Thus, methods by which plants, natural or genetically modified, alone or in the presence of auxiliary substances cause polluted soils to become less dangerous for humans have been developed [37, 38].

Phytoremediation is defined as a phenomenon of polluting substances extraction by using plants. With all these, there are many types of phytoremediation, so we can state that phytoremediation represents a much broader defined term [39, 40]. Phytoremediation of soils, waters, and sediments is not a new concept; for decades it has been found that some plants can degrade or extract heavy metals and other pollutants from these environmental compartments. Plants have been used for the decontamination of wastewater about 300 years ago. Thlaspi caerulescens and Viola calaminaria were the first species of plants used in the nineteenth century and found to accumulate large concentrations of metals [4].

A strong motivation to apply phytoremediation in historically contaminated sites, in addition to other advantages, is the particularly low cost of this method compared to conventional ones. Table 2 highlights the costs of different soil remediation techniques. Nevertheless, the most frequently applied remediation techniques for contaminated soil in Europe include land excavation or disposal [41].

#### 3.1. Phytoremediation process and techniques

attributable exclusively to mining activities, although these are preponderent in many regions [28–30]. Most of these activities are currently closed, remaining behind enormous quantities of heavy metals that have been deposited in the soil. The volume of tailing dumps discharged has exceeded 10 billion tonnes per year [31]. Usually, these mine tailings are not covered by vegetation caused by a poorly structured soil, being potential sources of heavy metal spread-

Source As Cd Cr Cu Pb Hg Ni Zn Mining and processing of ores √√ √ √√ √ Metallurgy √√ √√ √ √ √√ Chemical industry √√ √√ √ √ √

Paint industry √√ √ √

Textile industry √√ √√ Chemical fertilizer industry √√ √ √√ √√ Petroleum industry √√ √√ √ √ √

Alloy industry √

Table 1. Industrial sources of the most important heavy metals in the soil [27].

Glass industry √ √ √

Paper industry √√ √ √ √

Burning of coals √√ √√ √ √ √

3. Phytoremediation: a green technology to remove heavy metals from

Many remediation techniques have been used to respond to the growing number of soils

Decontamination methods currently applied in the majority of sites are mainly characterized by the manipulation of enormous quantities of soil or heavy metal extraction by using chemical reagents. These practices are very expensive and also lead to the loss of soil fertility by changing its physicochemical properties (structure, cationic exchange capacity, etc.), destroying at the same time the microorganisms from the soil and, ultimately, the humus layer [36]. In this situation, other less brutal methods for heavy metal extraction were searched and developed. Bioremediation and phytoremediation in particular are such "mild" remediation

Thus, methods by which plants, natural or genetically modified, alone or in the presence of auxiliary substances cause polluted soils to become less dangerous for humans have been

methods that maintain or even restore the natural soil fertility [21].

ing through water infiltration or wind [32].

198 Ecosystem Services and Global Ecology

contaminated with heavy metals [33–35].

the soil

developed [37, 38].

The metal extraction or accumulation by plants involves a variety of biological mechanisms and requires direct knowledge of plant physiology and soil science.

Through the rhizosphere (the interface between plant roots and soil), the water is absorbed by the roots to replace the evaporated water from the leaves. The metals in the soil solution (free ions or organometallic complexes) can move together with water (by convection or mass transfer) as the plant absorbs the water needed for vital processes. Absorption of water from the rhizosphere creates a hydraulic gradient directly from the ground to the surface of the roots. This concentration gradient or hydraulic control ensures the diffusion of ions from the soil particles to the deficient layer surrounding the roots [45, 46].

The elimination by plants of exudates and metabolites play an important role in the phytoremediation process. Thus, enzymes such as dehydrogenase, hydrolase, peroxidase, and phosphatase are released at the plant-soil interface and contribute to the degradation of


Table 2. Costs of different soil remediation methods [42–44].

some soil compounds [47]. Plant enzymes named metallothioneins and phytochelatins bind the heavy metals increasing the extraction of these elements [48, 49].

Phytoimobilization represents a combination of phytoextraction (heavy metals are extracted from the soil by perennial plants, but they are not harvested) and phytostabilization (fallen leaves are collected, the soil being treated to immobilize heavy metals). This technique uses tolerant species to the target pollutant which will form a "vegetal carpet" in areas where natural vegetation is absent due to the high concentration of pollutants. The method is already

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Evapotranspiration – plants also have the ability to influence local hydrogeological conditions. Thus, plants are capable of intercepting a significant amount of rain on the surface of their leaves. This intercepted water is evaporated directly into the atmosphere, not reaching the ground. Simultaneously, infiltrations are reduced, so the method can also be used to limit the

The presence of vegetation above a groundwater body has the effect of a "pump," on the one hand reducing the amount of water in the area of the rhizosphere; on the other hand, extracting the heavy metals the groundwater may have lower heavy metal concentrations

Rhizodegradation, also called photosynthesis or plant-assisted degradation, represents the transformation of existing organic contaminants into the soil due to the bioactivity in the rhizosphere. Plants metabolize organic pollutants (including with the help of associated microorganisms) at the level of the roots, turning them into less or no toxic compounds. A symbiotic relationship is established between plants and microorganisms in the soil. Plants increase the pH of the soil and provide the nutrients needed for the microorganisms. These contribute to soil clean-up, thus providing the rhizosphere more conducive to the development of the roots [61]. Many pollutants can be degraded into harmless products or can be transformed into energy and feed sources for plants or soil organisms. But then, natural substances removed from plant roots (e.g., sugars, alcohols, phenols, carbohydrates, and acids) contain organic

Rhizofiltration is based on the property of plant roots that grow in well-aerated water to

Phytodegradation, also known as phytotransformation, refers to the absorption of organic pollutants from soil, sediments, and water and their subsequent transformation by plants. Depending on the concentration and composition, as well as the plant species and local conditions, an organic pollutant may be able to pass through the protective barrier of the rhizosphere. In this case, it may suffer a transformation process inside the plant. The transforming mechanisms are very diverse, the resulting products being stored in vacuoles or

In order to be absorbed by the plant through the roots, an organic pollutant must be soluble in the soil solution. Once the pollutant has reached the plant, it can be stored and/or biotransformed in the plant biomass through lignification (binding the pollutant or its byproducts within the plant lignin) or can be further metabolized to carbon dioxide and water (mineralization) [35]. Plants capable of causing pollutant degradation are the phreatophytes

successfully used in the case of tailing dumps from the mining industry [59].

[60]. The presence of plants at the surface of the soil also prevents its erosion.

carbon that feeds soil microorganisms, stimulating their biological activities.

precipitate and concentrate toxic metals from the pollutant effluents.

(species of Populus, Salix) or grains (rye, Sorghum) [62, 63].

accumulation of water in the ground.

embedded in plant tissues [50].

In fact, phytoremediation is based on the extension of already existing processes in different ecosystems, with other processes that occur under different conditions, different degrees of contamination, different pollutants, and plant species.

Depending on the mode of action on the pollutants and the place where the action takes place, the following phytoremediation mechanisms and biological processes are distinguished: phytoextraction, phytostabilization, phytoimobilization, evapotranspiration, rhizodegradation, rhizofiltration, phytodegradation, and phytovolatilization [35, 50, 51].

Heavy metals in the soil are only suitable for phytoextraction, evapotranspiration, phytostabilization, and phytoimobilization [35, 52]. Phytoextraction is by far the most studied and applied method. Phytodegradation and rhizodegradation processes, as well as phytovolatilization, are specific to organic pollutants; the major difference between these and other processes applicable to metals is the complete mineralization of the pollutant after degradation.

From the point of view of the place where the remediation takes place, this procedure is exclusively in situ, without excavation of contaminated site, in all types of phytoremediation [52, 53]. Also, from the point of view of the processes that occur, they can be either in plant, by the absorption of the metals in the plant (phytoextraction, rhizofiltration, phytovolatilization), or ex plant due to the action of the excreted enzymes by the plants or the microorganisms associated with the plants (phytoimobilization), either combined (in the case of phytostabilization).

Phytoextraction is based on the cultivation of large biomass plants and the ability to extract large amounts of heavy metals from the soil, accumulating them in the plant tissues. These plants are harvested using conventional farming methods and then dried and incinerated, the resulting ash being stored [54].

Starting from the necessity of finding solutions for the decontamination of areas polluted with heavy metals of anthropogenic origin, the concept of "heavy metal phytoextraction" was introduced for the first time by Baker and Brooks in 1983 [55].

Phytostabilization refers to plant ability to stabilize pollutants, thereby reducing their mobility and bioavailability. In the case of nonagricultural land, especially those with a high degree of pollution, a method of mitigating the risk of pollution can be the reduction of the possibility of moving heavy metals into the soil [56].

From the point of view of the area where the pollutant fixation takes place, phytostabilization can take place in the rhizosphere, on the root membranes, or in the root cells. This method applies especially to tailings dumps, but the main disadvantage of this technique is that the metals remain in the soil.

Phytostabilization research is still in the laboratory phase, with very few applications in the field. These include the use of plants as Brassica juncea for the stabilization of lead and cadmium from both mine and tailing dumps; Rubus ulmifolius to stabilize arsenic, lead, and nickel; or lemon grass to stabilize copper in mine tailings [49, 57, 58].

Phytoimobilization represents a combination of phytoextraction (heavy metals are extracted from the soil by perennial plants, but they are not harvested) and phytostabilization (fallen leaves are collected, the soil being treated to immobilize heavy metals). This technique uses tolerant species to the target pollutant which will form a "vegetal carpet" in areas where natural vegetation is absent due to the high concentration of pollutants. The method is already successfully used in the case of tailing dumps from the mining industry [59].

some soil compounds [47]. Plant enzymes named metallothioneins and phytochelatins bind

In fact, phytoremediation is based on the extension of already existing processes in different ecosystems, with other processes that occur under different conditions, different degrees of

Depending on the mode of action on the pollutants and the place where the action takes place, the following phytoremediation mechanisms and biological processes are distinguished: phytoextraction, phytostabilization, phytoimobilization, evapotranspiration, rhizodegradation,

Heavy metals in the soil are only suitable for phytoextraction, evapotranspiration, phytostabilization, and phytoimobilization [35, 52]. Phytoextraction is by far the most studied and applied method. Phytodegradation and rhizodegradation processes, as well as phytovolatilization, are specific to organic pollutants; the major difference between these and other processes applicable to metals is the complete mineralization of the pollutant after degradation.

From the point of view of the place where the remediation takes place, this procedure is exclusively in situ, without excavation of contaminated site, in all types of phytoremediation [52, 53]. Also, from the point of view of the processes that occur, they can be either in plant, by the absorption of the metals in the plant (phytoextraction, rhizofiltration, phytovolatilization), or ex plant due to the action of the excreted enzymes by the plants or the microorganisms associated

Phytoextraction is based on the cultivation of large biomass plants and the ability to extract large amounts of heavy metals from the soil, accumulating them in the plant tissues. These plants are harvested using conventional farming methods and then dried and incinerated, the

Starting from the necessity of finding solutions for the decontamination of areas polluted with heavy metals of anthropogenic origin, the concept of "heavy metal phytoextraction" was

Phytostabilization refers to plant ability to stabilize pollutants, thereby reducing their mobility and bioavailability. In the case of nonagricultural land, especially those with a high degree of pollution, a method of mitigating the risk of pollution can be the reduction of the possibility

From the point of view of the area where the pollutant fixation takes place, phytostabilization can take place in the rhizosphere, on the root membranes, or in the root cells. This method applies especially to tailings dumps, but the main disadvantage of this technique is that the

Phytostabilization research is still in the laboratory phase, with very few applications in the field. These include the use of plants as Brassica juncea for the stabilization of lead and cadmium from both mine and tailing dumps; Rubus ulmifolius to stabilize arsenic, lead, and

with the plants (phytoimobilization), either combined (in the case of phytostabilization).

the heavy metals increasing the extraction of these elements [48, 49].

rhizofiltration, phytodegradation, and phytovolatilization [35, 50, 51].

introduced for the first time by Baker and Brooks in 1983 [55].

nickel; or lemon grass to stabilize copper in mine tailings [49, 57, 58].

contamination, different pollutants, and plant species.

200 Ecosystem Services and Global Ecology

resulting ash being stored [54].

metals remain in the soil.

of moving heavy metals into the soil [56].

Evapotranspiration – plants also have the ability to influence local hydrogeological conditions. Thus, plants are capable of intercepting a significant amount of rain on the surface of their leaves. This intercepted water is evaporated directly into the atmosphere, not reaching the ground. Simultaneously, infiltrations are reduced, so the method can also be used to limit the accumulation of water in the ground.

The presence of vegetation above a groundwater body has the effect of a "pump," on the one hand reducing the amount of water in the area of the rhizosphere; on the other hand, extracting the heavy metals the groundwater may have lower heavy metal concentrations [60]. The presence of plants at the surface of the soil also prevents its erosion.

Rhizodegradation, also called photosynthesis or plant-assisted degradation, represents the transformation of existing organic contaminants into the soil due to the bioactivity in the rhizosphere. Plants metabolize organic pollutants (including with the help of associated microorganisms) at the level of the roots, turning them into less or no toxic compounds. A symbiotic relationship is established between plants and microorganisms in the soil. Plants increase the pH of the soil and provide the nutrients needed for the microorganisms. These contribute to soil clean-up, thus providing the rhizosphere more conducive to the development of the roots [61]. Many pollutants can be degraded into harmless products or can be transformed into energy and feed sources for plants or soil organisms. But then, natural substances removed from plant roots (e.g., sugars, alcohols, phenols, carbohydrates, and acids) contain organic carbon that feeds soil microorganisms, stimulating their biological activities.

Rhizofiltration is based on the property of plant roots that grow in well-aerated water to precipitate and concentrate toxic metals from the pollutant effluents.

Phytodegradation, also known as phytotransformation, refers to the absorption of organic pollutants from soil, sediments, and water and their subsequent transformation by plants. Depending on the concentration and composition, as well as the plant species and local conditions, an organic pollutant may be able to pass through the protective barrier of the rhizosphere. In this case, it may suffer a transformation process inside the plant. The transforming mechanisms are very diverse, the resulting products being stored in vacuoles or embedded in plant tissues [50].

In order to be absorbed by the plant through the roots, an organic pollutant must be soluble in the soil solution. Once the pollutant has reached the plant, it can be stored and/or biotransformed in the plant biomass through lignification (binding the pollutant or its byproducts within the plant lignin) or can be further metabolized to carbon dioxide and water (mineralization) [35]. Plants capable of causing pollutant degradation are the phreatophytes (species of Populus, Salix) or grains (rye, Sorghum) [62, 63].

Phytovolatilization is applied exclusively for the treatment of soils contaminated with As, Hg, or Se, metals that may exist in the gaseous phase. This method uses plants capable of extracting these metals from the soil and volatilizing them in the atmosphere [64]. Plants extract volatile compounds from soil, including metals, and evaporate them through the leaves. Due to the particular toxicity of these metals, which once released can no longer be controlled, the method is still subject of controversy.

#### 3.2. Metal accumulative plant species

The ability of plants to accumulate extraordinarily high levels of some metals and other pollutants has reached an increase interest over the past few years.

In general, heavy metals are phytotoxic to plants, but there are plants capable of absorbing and storing metals in their various tissues (roots, leaves), used successfully in soils rich in heavy metals and known as hyperaccumulative plants. Brooks and his colleagues used this term for the first time in 1977 to describe plants that are able to accumulate more than 0.1% Ni (>1000 μg/g) in their leaves. Hyperaccumulative plants (hyperaccumulators or metallophytes) are those plants capable of accumulating 100 times larger quantities of metal than common plants considered non-accumulating [65, 66].

3.3. Factors that are influencing the phytoremediation process

Table 3. Examples of hyperaccumulative plants and the targeted heavy metal/heavy metals.

transformations, and diffusion [82, 83].

tion in the soil and its concentration in the soil solution.

The success of extensive application of phytoextraction depends on several key factors: the soil physicochemical properties, the degree of soil contamination, the possibility that the metal is absorbed in the roots, and the ability of plants to accumulate metals and then translocate them into the air [77, 78]. The soil properties affecting the bioavailability of heavy metals to plants include soil pH, redox potential, organic matter, clay content, and cation exchange capacity [79]. The low bioavailability of metals in the soil represents a major factor that is limiting the potential for the use of phytoextraction in the case of many heavy metals [77, 80, 81]. A major objective of the phytoremediation studies in historical areas contaminated by heavy metals is to increase the availability of metals to be absorbed from the soil by plants. On the other hand, in the case of phytostabilization, it is preferable to reduce the heavy metal availability in soils. Particularly, the mobility of metals in soil is directly influenced by their chemical species. The chemical characterization of metals determines their behavior and toxicity in the environment [82]. The metal species represent the specific forms of an element including isotopic composition, electron or oxidative state, and complex or molecular structure [3]. Several chemical forms of metals include free metal ions, metal complexes dissolved in solutions and adsorbed on solid surfaces, and metal species coprecipitating in their own solids or in other metals with much higher concentrations. The metal species modify both toxicity and certain processes such as volatilization, photolysis, adsorption, atmospheric deposition, acid-base balance, polymerization, electron transfer reactions, solubility and precipitation equilibrium, microorganism

Hyperaccumulators Heavy metal References Thlaspi caerulescens Zn, Cd, Cu [66, 69] Brassica juncea Pb, Cd, Ni [49, 70] Arabidopsis halleri Cd, Zn [71, 72] Phytolacca acinosa Mn [73] Alyssum bracteatum Ni [55, 74] Brassica napus Zn [75] Sedum alfredii Zn [76]

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There are also plant-related factors contributing to the efficiency of phytoremediation: rapid growth and high biomass producers, the presence of an extensive root system capable of exploring large soil volumes, a good tolerance for high metal concentrations, a high transfer factor (TF > 1), and adaptability to polluted areas under different climatic conditions [82–87]. The availability or retention of a metal in soil and plants can be expressed by several indices [88, 89]: • The modified distribution coefficient (Kmd), defined as the ratio between the metal concentra-

Hyperaccumulative plants are spread all over the world, although they are very rare plants and are found only in certain areas. The approximately known number for these plants is about 500 species belonging to a number of plant families. The majority are "obligate metallophytes," species that occur only on metalliferous soils; a smaller, but increasing number of plant species are "facultative hyperaccumulators" that hyperaccumulate heavy metals when occurring on metalliferous soils, although they commonly grow on normal, non-metalliferous soils [67].

To be considered a hyperaccumulator, the concentration of heavy metal should be 2–3 times greater than in leaves of most species growing on normal soils and at least one order higher than the usual range found in plants from metalliferous soils. The proposed threshold criteria (in g metal per g of dry leaf tissue) are 100 for Cd, Se, and Tl; 300 for Co, Cr, and Cu; 1000 for As, Ni, and Pb; 3000 for Zn; and 10,000 for Mn [68].

The growth of certain plants on soils contaminated by heavy metals leads to their adaptation to the pollution conditions and the assimilation of toxic elements into the vegetal organism. Of course, not all plants are resistant to the action of pollutants, as not all are able to accumulate significant amounts of toxic elements. The vast majority of plants are able to overaccumulate only one heavy metal from the soil, even if the soil is polluted with several such elements. Special abilities for the simultaneous bioaccumulation of several heavy metals have been proven by Thlaspi caerulescens for zinc, cadmium, and copper and Brassica juncea (Indian mustard) for lead and cadmium [49, 66]. Other hyperaccumulative plant species are shown in Table 3.

Thlaspi caerulescens has been extensively studied and is used in most studies as a model plant for assessment the mechanisms of metal translocation, accumulation, and tolerance and for investigating the physiological and biochemical mechanisms of metal accumulation in plants [72].


Table 3. Examples of hyperaccumulative plants and the targeted heavy metal/heavy metals.

#### 3.3. Factors that are influencing the phytoremediation process

Phytovolatilization is applied exclusively for the treatment of soils contaminated with As, Hg, or Se, metals that may exist in the gaseous phase. This method uses plants capable of extracting these metals from the soil and volatilizing them in the atmosphere [64]. Plants extract volatile compounds from soil, including metals, and evaporate them through the leaves. Due to the particular toxicity of these metals, which once released can no longer be controlled, the method

The ability of plants to accumulate extraordinarily high levels of some metals and other

In general, heavy metals are phytotoxic to plants, but there are plants capable of absorbing and storing metals in their various tissues (roots, leaves), used successfully in soils rich in heavy metals and known as hyperaccumulative plants. Brooks and his colleagues used this term for the first time in 1977 to describe plants that are able to accumulate more than 0.1% Ni (>1000 μg/g) in their leaves. Hyperaccumulative plants (hyperaccumulators or metallophytes) are those plants capable of accumulating 100 times larger quantities of metal than common

Hyperaccumulative plants are spread all over the world, although they are very rare plants and are found only in certain areas. The approximately known number for these plants is about 500 species belonging to a number of plant families. The majority are "obligate metallophytes," species that occur only on metalliferous soils; a smaller, but increasing number of plant species are "facultative hyperaccumulators" that hyperaccumulate heavy metals when occurring on metalliferous soils, although they commonly grow on normal, non-metalliferous

To be considered a hyperaccumulator, the concentration of heavy metal should be 2–3 times greater than in leaves of most species growing on normal soils and at least one order higher than the usual range found in plants from metalliferous soils. The proposed threshold criteria (in g metal per g of dry leaf tissue) are 100 for Cd, Se, and Tl; 300 for Co, Cr, and Cu; 1000 for

The growth of certain plants on soils contaminated by heavy metals leads to their adaptation to the pollution conditions and the assimilation of toxic elements into the vegetal organism. Of course, not all plants are resistant to the action of pollutants, as not all are able to accumulate significant amounts of toxic elements. The vast majority of plants are able to overaccumulate only one heavy metal from the soil, even if the soil is polluted with several such elements. Special abilities for the simultaneous bioaccumulation of several heavy metals have been proven by Thlaspi caerulescens for zinc, cadmium, and copper and Brassica juncea (Indian mustard) for lead

Thlaspi caerulescens has been extensively studied and is used in most studies as a model plant for assessment the mechanisms of metal translocation, accumulation, and tolerance and for investigating the physiological and biochemical mechanisms of metal accumulation in plants [72].

and cadmium [49, 66]. Other hyperaccumulative plant species are shown in Table 3.

pollutants has reached an increase interest over the past few years.

is still subject of controversy.

202 Ecosystem Services and Global Ecology

soils [67].

3.2. Metal accumulative plant species

plants considered non-accumulating [65, 66].

As, Ni, and Pb; 3000 for Zn; and 10,000 for Mn [68].

The success of extensive application of phytoextraction depends on several key factors: the soil physicochemical properties, the degree of soil contamination, the possibility that the metal is absorbed in the roots, and the ability of plants to accumulate metals and then translocate them into the air [77, 78]. The soil properties affecting the bioavailability of heavy metals to plants include soil pH, redox potential, organic matter, clay content, and cation exchange capacity [79].

The low bioavailability of metals in the soil represents a major factor that is limiting the potential for the use of phytoextraction in the case of many heavy metals [77, 80, 81]. A major objective of the phytoremediation studies in historical areas contaminated by heavy metals is to increase the availability of metals to be absorbed from the soil by plants. On the other hand, in the case of phytostabilization, it is preferable to reduce the heavy metal availability in soils.

Particularly, the mobility of metals in soil is directly influenced by their chemical species. The chemical characterization of metals determines their behavior and toxicity in the environment [82]. The metal species represent the specific forms of an element including isotopic composition, electron or oxidative state, and complex or molecular structure [3]. Several chemical forms of metals include free metal ions, metal complexes dissolved in solutions and adsorbed on solid surfaces, and metal species coprecipitating in their own solids or in other metals with much higher concentrations. The metal species modify both toxicity and certain processes such as volatilization, photolysis, adsorption, atmospheric deposition, acid-base balance, polymerization, electron transfer reactions, solubility and precipitation equilibrium, microorganism transformations, and diffusion [82, 83].

There are also plant-related factors contributing to the efficiency of phytoremediation: rapid growth and high biomass producers, the presence of an extensive root system capable of exploring large soil volumes, a good tolerance for high metal concentrations, a high transfer factor (TF > 1), and adaptability to polluted areas under different climatic conditions [82–87].

The availability or retention of a metal in soil and plants can be expressed by several indices [88, 89]:

• The modified distribution coefficient (Kmd), defined as the ratio between the metal concentration in the soil and its concentration in the soil solution.

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

4. Future developments

5. Conclusions

Conflict of interest

The authors declare no conflict of interest.

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,

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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 goal of phytoremediation is to improve the functioning of ecosystems. Plants are considered veritable "ecosystem engineers," and bioremediation by using plants is appreciated as a special applied form of ecosystem services. Assessment of the bioremediation applicability and effectiveness may be required for specific ecosystems, at least until the technology becomes firmly demonstrated and established. Extensive studies of field conditions are required in

Thus, further research is still needed before implementing this technique in a large scale. Before becoming a commercially widely applicable process, phytoremediation requires a commitment to resident population and to local authorities in polluted regions, as well as financial and time resources. At the same time, it has the potential to offer low costs for its application and is considered a green alternative to conventional technologies for soil remediation.

Decontamination of polluted soils by using bioaccumulative plantsis proposed as an environmentalfriendly alternative to the traditional physicochemical methods, being a sustainable method with

order to implement this technique in historically heavy metal-contaminated areas.

a great potential in the terms of environmental protection and cost management.

this practice can reduce the related costs of phytoremediation [101, 102].

the specific heavy metal hyperaccumulation in plants.


#### 3.4. Other bioremediation techniques

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

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

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