**3. Phytoremediation process**

Phytoremediation is defined as the process which uses green plants for the relief, transfer, stabilization or degradation of pollutants from soil, sediments, surface waters and ground‐ water.

In order to be used in phytoremediation, the selected plant species must be tolerant for the pollutant to be extracted, to quickly develop high biomass, to accumulate metals in harvestable parts, to have a well-developed root system and have a high bioaccumulation factor. This factor must be 20 or more for the phytoremediation to reduce the contamination of soil by 50% over a period of 10 crops [7]. The level of metal bioaccumulation and recovery is directly propor‐ tional to the quantity of biomass.

Plants that accumulate high levels of metals are known as hyperaccumulators and can accumulate 50-100 times more metal than a normal plant. There are about 400 hyperaccumu‐ lator species and the level of concentration is 10000 mg/kg for Zn and Mn, 1000 mg/kg for Cu, Co, Ni and As, and 100 mg/kg for Cd [8].

*Thlaspi caerulescens* species is the most studied and known as a hyperaccumulator plant with tolerance capacity for high concentrations of heavy metals in soil (e.g. Cu, Zn, Ni, Cd). Other tolerant species for heavy metals is *Berkheya coddii*, which was studied in South Africa, on ultramafic soils enriched with Ni [9]. The authors found, in leaves, values of Ni concentration about 18000 mg/kg, exceeding several times the metal content in soil (1300 mg/kg) without presenting symptoms of toxicity.

Uptake by the root is the most important way to get trace elements in plants, but have been observed absorption and adsorption processes of metals at level of other tissues. Metal uptake in plants is influenced by the species-specific ability, pedological factors, of which the most important are pH, Eh, fluid regime, clay content, cation exchange capacity, nutrient balance and concentration of other heavy metals. Also, the weather demonstrated some indirect effects on metals absorption rate in plants, mainly due to the influence on the amount of water in the environment [10]. In general, a high-temperature environment positively influences the absorption of micro-nutrients by plants [11].

A disadvantage of phytoremediation is that many hyperaccumulator plants produce a small quantity of biomass. For example, *Thlaspi caerulescens* produces only 2-5 tons/ha, but are plants that produce a larger quantity of biomass 9 t/ha for *Alyssum bertolonii*, or even 22 t/ha for *Berkhya coddii* [12].

Hyperaccumulation involves the absorption, transport and translocation of metals in tissues, where can be stored large amounts of these elements. One of the most studied mechanisms for the metal isolation use metallothionein-derived peptides and phytochelatins. Metal binds to organic sulfite in cysteine, which form the majority of metallothionein-derived peptides. It has been shown that metallothioneins and phytochelatins are stimulated by exposure to metals [13].

and pH of 6 and at a level of only 40 mg/kg at a pH of 5. These levels of equilibrium make the

Phytoremediation is defined as the process which uses green plants for the relief, transfer, stabilization or degradation of pollutants from soil, sediments, surface waters and ground‐

In order to be used in phytoremediation, the selected plant species must be tolerant for the pollutant to be extracted, to quickly develop high biomass, to accumulate metals in harvestable parts, to have a well-developed root system and have a high bioaccumulation factor. This factor must be 20 or more for the phytoremediation to reduce the contamination of soil by 50% over a period of 10 crops [7]. The level of metal bioaccumulation and recovery is directly propor‐

Plants that accumulate high levels of metals are known as hyperaccumulators and can accumulate 50-100 times more metal than a normal plant. There are about 400 hyperaccumu‐ lator species and the level of concentration is 10000 mg/kg for Zn and Mn, 1000 mg/kg for Cu,

*Thlaspi caerulescens* species is the most studied and known as a hyperaccumulator plant with tolerance capacity for high concentrations of heavy metals in soil (e.g. Cu, Zn, Ni, Cd). Other tolerant species for heavy metals is *Berkheya coddii*, which was studied in South Africa, on ultramafic soils enriched with Ni [9]. The authors found, in leaves, values of Ni concentration about 18000 mg/kg, exceeding several times the metal content in soil (1300 mg/kg) without

Uptake by the root is the most important way to get trace elements in plants, but have been observed absorption and adsorption processes of metals at level of other tissues. Metal uptake in plants is influenced by the species-specific ability, pedological factors, of which the most important are pH, Eh, fluid regime, clay content, cation exchange capacity, nutrient balance and concentration of other heavy metals. Also, the weather demonstrated some indirect effects on metals absorption rate in plants, mainly due to the influence on the amount of water in the environment [10]. In general, a high-temperature environment positively influences the

A disadvantage of phytoremediation is that many hyperaccumulator plants produce a small quantity of biomass. For example, *Thlaspi caerulescens* produces only 2-5 tons/ha, but are plants that produce a larger quantity of biomass 9 t/ha for *Alyssum bertolonii*, or even 22 t/ha for *Berkhya*

Hyperaccumulation involves the absorption, transport and translocation of metals in tissues, where can be stored large amounts of these elements. One of the most studied mechanisms for the metal isolation use metallothionein-derived peptides and phytochelatins. Metal binds to

low-zinc soil to release in the soil solution dangerous amounts of this element.

**3. Phytoremediation process**

312 Environmental Risk Assessment of Soil Contamination

tional to the quantity of biomass.

presenting symptoms of toxicity.

*coddii* [12].

Co, Ni and As, and 100 mg/kg for Cd [8].

absorption of micro-nutrients by plants [11].

water.

Phytoremediation of heavy metal polluted soils involves the following processes (Figure 2):


Heavy metals behave differently and have a different mobility depending on plant species. Therefore, Pb, Cr and Cu tend to be stabilized and retained in the root, Cd, Ni and Zn are more easily translocated to aerial tissues, and Cd is transported even to the harvestable tissues of plants [15].

For Pb there are some plant species, such as *Brassica juncea*, *Vetiveria zizinioides*, *Cardaminopsis halleri*, *Cynodon dactylon* and *Sorghum halepense* presenting hyperaccumulative capacity. To improve the ability of plants to accumulate heavy metals, the polluted soil can be amended with chelates that increase the metal bioavailability. This method has disadvantages of the high risk of metals to be more easily leached to the groundwater, in addition to the higher cost of remediation.

**Figure 2.** Phytoremediation of heavy metals polluted soil [2]

Among the plants that can be used in phytoremediation of mine tailings, studies have focused on *Eriophorum angustifolium*, a plant resistant to substrates with a wide range of pH from 10.9 to 2.7. Other species of plants that can be grown in a low pH environment are *Carex rostrata*, *Eriophorum scheuchzeri*, *Phragmites australis*, *Typha angustifolia*, *Typha latifolia*, which grows to a pH value of 2.1, 4.4, 2.1, 3.0 and 2.5 respectively [14].

The studies made since 1977 by the American biologist Dr. Robert Brooks [16], have shown that metals can be extracted from plants (e.g. Ni, Zn, Pb and Au), but the facility of this process depends on the density and solubility of the elements. From the first experiments was obtained 0.01 g of Ni from few kilograms of plant biomass and, more recently, 10 g of Au were obtained from a two hectares of rape culture, established in the vicinity of abandoned mines in Cali‐ fornia.
