**3. Phytoremediation**

Phytoremediation is considered an effective, aesthetically pleasing, cost effective and environmental friendly technology for the remediation of potentially toxic metals from the environment. Plants in phytoremediation accumulate contaminants through their roots and then translocate these contaminant in the aboveground part of their body [38,39]. The notion of using metal accumulator plants for the removal of heavy metals and several other contaminants in phytoremediation was first introduced in 1983, but this idea has already been implanted for the last 300 years [40]. Phytoremediation is known by different names such as agro-remediation, green remediation, vegetative remediation, green technology and botano remediation [4,41,42].

Use of vegetation, soil and micro biota along with other agrochemical practices makes the vegetative remediation an appealing green technology for the accumulation of different heavy metals [43,44]. The application of in situ and ex-situ remediation is applicable in a phytoremediation process. In situ application is used more commonly because it reduces the multiplication of contaminant in water and airborne waste, which ultimately minimize the risk to the adjacent environment [45]. More than one type of pollutant can be treated on site by the phytoremediation without the need for a disposal site. It also reduces the spread of contamination by preventing soil erosion and leaching [46]. The clean up cost of phytoremediation is far less than other conventional techniques of remediation, which is the utmost advantage of this technique [47]. Phytoremediation is a relatively straightforward technique as it does not require any highly specific personnel and exclusive equipment. This is applicable for the remediation of large scale area where other conventional techniques prove to be extremely inefficient and costly as well [48].

An enormous number of contaminants can be remediated by phytoremediation technology such as insecticides, chlorinated solvents, Polycyclic aromatic hydrocarbons (PAHs), Polychlorinated biphenyl (PCBs), petroleum hydrocarbons, radio nucleosides, surfactants, explosive elements and heavy metals [48,49]. There are a number of plant species that have the ability to accumulate significantly higher concentrations of heavy metals in different parts of the body, such as a leaf, stems and root, without showing any sign of toxicity [50,51].

#### *3.1. Characteristics of Phytoremediation Plants*

Plants should have the following characteristics in order to make the phytoremediation an eco-sustainable technology: native and quick growth rate, high biomass yield, the uptake of a large amount of heavy metals, the ability to transport metals in aboveground parts of plant, and a mechanism to tolerate metal toxicity [52–55]. Other factors like pH, solar radiation, nutrient availability and salinity greatly influence the phytoremediation potential and growth of the plant [51,56].

#### 3.1.1. Mechanism of Phytoremediation

Phytoremediation follows different mechanisms such as phytoextraction, phytostabilization, phytovolatilization and rhizofiltration during the uptake or accumulation of heavy metals in the plant [4,41]. The different mechanisms involved in the phytoremediation process are briefly describe below.
