Phytoremediation: A way towards sustainable Agriculture

— Phytoremediation means utilizing the potential of a variety of plants to remediate soil, sludge, sediment and water (surface water and underground aquifers) contaminated with heavy metals at the point or non-point sources. Phytoremediation is solar energy-driven technology, eco-friendly and a cost-effective way of making soil and water pollutants free. It is a process of onsite remediation by using different biological processes of plants. Phytoremediation is known widely by different terms viz., green-remediation, botanic-remediation, agro-remediation, and vegetative-remediation, etc. Pollutants occur in different forms, like organic, inorganic, metallic and non-metallic, etc. Plants can be utilized for phytoremediation of heavy metal polluted soil and water resources. This review gives current understanding of the mechanism of heavy metal remediation by different plant species, therefore encouraging research and development in this area. Phytoremediation further needs a profound understanding of the underlying mechanism and requires pilot level as well as field level studies.


INTRODUCTION
Phytoremediation is a dynamic process that eventually degrades or extracts pollutants in different proportions as they are hazardous and toxic to all living beings. The pollutants are degraded either through accumulation, filtration or dissipation. In the current scenario, the need of the hour is realizing the consequences of heavy metal pollutants in soil and water (Kaur, 2018). Heavy metal pollutants, the half-life is much more than that of organic pollutants like pesticides and petroleum by-products. Uranium in groundwater aquifers of Rajasthan is a great concern to environmentalists as in some or other ways it is affecting the natural occurrence of biogeochemical cycles (Daud et al., 2018). As different phases of the industrial revolution have passed, a variety of remediation technologies have also come into the market to deal with a variety of pollutants. Out of these contaminants, heavy metals like Uranium pose a great threat to the surrounding environment (Papazogloua and Fernandob, 2017). Due to mining and milling, radionuclide contaminants is prevalent in subsurface sediments throughout India. Due to the overexploitation of underground water resources, the water level has declined beyond environmentally acceptable and recoverable levels. Subsequently, heavy metals have found its way to the subsurface level thus entered into water and soil, contaminated them as a result of waste-disposal practices (Bora and Sarma, 2020).

III. PHYTOEXTRACTION
The literal meaning of phytoextraction is, "Phyto" meaning plant and "Extraction" meaning removing (Henry and Fabio, 2001) (as shown in Fig.1). The process of phytoextraction involves translocating the pollutants from the rhizosphere to different plant parts, viz., shoot, leaf, stem, flower, etc. Few plant species have the potential to extract both essential (Cu, Mg, Mo, K, Fe, Mn, Ni, P, and Zn) as well as non-essential metals (Se, B, Cd, Co, Cr, Ag, and Hg). Essential metals are those required by plants in optimum amounts for their growth and development, whereas non-essential metals are toxic even in low amounts (Tang et al., 2019;Gupta et al., 2020).

IV. RHIZOFILTRATION
The ability of plants to filtrate contaminated water aquifer, surface water, and wastewater with heavy metals, agri-waste (Pesticides and Insecticides) through a bunch of roots or adventitious root is known as Rhizofiltration (Akob et al., 2007) (as shown in figure 1). Therefore, it is a modified phytoextraction method by using aquatic vegetation and adsorption of toxic elements primarily into the root zone (Beans, 2017 (Soliman and Sugiyama, 2016). The most widely used choice for hemofiltration is terrestrial plants as they possess fibrous roots and rapid growth rate. The process of hemofiltration can be applied to wetlands, ponds and constructed water tanks. The ultimate fate of rhizofiltered pollutants is rhizodegradation (degradation in roots) and then phytodegradation (degradation in aerial parts of the plant) (Gonzalez et al., 2017).

V. PHYTOSTABILIZATION
It is the process in which plants store toxic metals at a particular site in a non-toxic metallic and immobile form, hence the metal is not able to mobilize to other organelles thus do not interfere with cellular metabolism (as shown in figure 1). Subsequently, the rate of migration of metals gets reduced (Oscar et al., 2016). Therefore, the soil need not be free from contaminants and on-site phytoremediation can be done with potential plants whose roots are capable of growing under polluted soils and thus helps in metal immobilization through root adsorption, metal precipitation, complex formation or reduction (Barcel and Poschenrieder, 2003). Additionally, the metals are stabilized within plant cells from mobile and toxic to immobile form, for example, toxic Cr 6+ gets transformed to Cr 3+ , which is less mobile (James, 2001). The process of phytostabilization seems to be more efficient in the case of fine soils having high organic matter content (Berti and Cunningham, 2000).

VI. PHYTOVOLATIZATION
In this process, plants utilize transpiration to converts heavy metals from more toxic form to less toxic volatile form, thus eradicate pollutants from soil and water (as shown in figure  1). The metals that get volatized through transpiration are, Arsenic, Mercury (Hg, more toxic to Hg 2+ , less toxic) and Selenium (Se, more toxic to (CH3)2Se, 600 times less toxic), etc. Plant species that adopt the phytovolatilization process for removing contaminants are Arabidopsis thaliana and Musk grass.

VII. CONCLUSION
Phytoremediation of contaminated soil and water resources has proved to be a sustainable technology and emerged as one of the eco-friendly agriculture practices. Phytoremediation has a high potential when compared with other traditional and conventional approaches for heavy metal removal. A variety of plant species have shown high performance in hyper accumulation of heavy metals viz., Cadmium, Copper, Mercury, Lead, Zinc, and Uranium, etc. Plants belonging to different families have different abilities to accumulate, detoxify and sequester a variety of heavy metals. However, the phytoremediation research studies are very few in number predominantly at field level. Hence, the need of the hour for phytoremediation research is on developing novel experimental design both at pilot as well as field level in polluted soil and water resources. Furthermore, the procedure for removal of heavy metals augmented biomass necessities to be additionally developed. Additionally, the current circumstances stresses on using the amalgamation and collaboration of traditional methods along with recent phytoremediation practices to deliver an advanced way of heavy metal remediation from both contaminated soil and water resources.

DISCLOSURE
The manuscript does not have any conflict of interest with any author, organization, institute, etc.