~ 1987 ~ International Journal of Chemical Studies 2019; 7(5): 1987-1994 P-ISSN: 2349–8528 E-ISSN: 2321–4902 IJCS 2019; 7(5): 1987-1994 © 2019 IJCS Received: 10-07-2019 Accepted: 12-08-2019 Ajay Kumar Department of biosciences, Himachal Pradesh University, Shimla, Himachal Pradesh, India Naresh Thakur Department of Crop Improvement CSKHPKV, Palampur, Himachal Pradesh, India Correspondence Ajay Kumar Department of biosciences, Himachal Pradesh University, Shimla, Himachal Pradesh, India Phytoremediation: Green technology for heavy metal clean up from contaminated soils Ajay Kumar and Naresh Thakur Abstract Heavy metal concentration is being continuously increasing in the soil due to modern industrialization and anthropological activities. Heavy metals being nonbiodegradable and with long biological half-lives when entered the food chain, their concentrations get increase increasing with each trophic level due to biomagnification. This increased concentration of heavy metals also poses a threat to human life. A recent new solution to cope up with this problem is the use of green plants for the removal of toxic heavy metals from soil and convert it into harmless and reusable form by a process called phytoremediation. This technology has many advantages over conventional methods for the heavy metal clean up from the soil. Still, there are so many challenges to make this technology practically feasible and useful on a large scale. The present review focuses on the mechanisms of HM uptake, transport, and plant tolerance mechanisms to cope with heavy metals. Keywords: Heavy metals, metal uptake, phytoremediation, translocation, tolerance mechanism Introduction Phytoremediation can be defined as a process in which green plants remove, sequester, or stabilize many organic and inorganic contaminants including heavy metals, radionuclides as well as for organic pollutants like polynuclear aromatic hydrocarbons, polychlorinated biphenyls, and pesticides to render them harmless (Greipsson, 2011) [25] . Heavy metals are difficult to remove from contaminated soils mainly because, being inorganic contaminants, they are bound to the soil matrix and cannot be easily mineralized. Thus heavy metal can be removed from contaminated soils by physical, chemical, and biological methods of remediation (Cunningham and Ow, 1996) [12] . As per now different physico-chemical and engineering, techniques have been developed and are being employed to remove toxic heavy metal ions from polluted soils and waters. They, however, are associated with several disadvantages which include a negative effect on soil properties and biodiversity, and also these techniques are quite expensive (Padmavathiamma and Li, 2007) [43] . Phytoremediation technology offers an eco-friendly alternative to this problem. It is a novel, cost-effective, efficient, environment and eco-friendly, in situ applicable, remediation strategy (Vithanage et al., 2012; Feng et al., 2017) [75, 18] . Plants detoxify the soil contaminants without affecting the topsoil, hence conserving its utility and fertility. Also plants aid in increasing the fertility of the soil by the input of organic matters (Mench et al., 2009) [38] . Hyperaccumulators plant species accumulate toxic heavy metals in above ground parts. These plants can take up large amounts of metals in their shoots without showing significant signs of toxicity. This makes hyperaccumulators ideal candidates for metal phytoremediation and phytomining. A hyperaccumulator plant can be distinguished from a non-hyperaccumulator by its capability to absorb and accumulate 50–100 times than nonaccumulators plants. Hyperaccumulators achieve a shoot-to-root metal concentration ratio called translocation factor (TF) of greater than one (Badr et al., 2012) [4] . The other factor which is very important for phytoremediation and identification of hyperaccumulator species is bioconcentration factor. It is the ratio of metal ion concentration in plant tissue to the soil. About 0.2 % of all known plant species are classified as HM accumulators (Rascio and Navari-Izzo, 2011; Sarma, 2011) [48, 55] . The phytoremediation concept is gaining good public acceptance and can be applied over large scale field sites where other remedial measures are not cost-effective. This green technology for heavy metal detoxification is classified into five subgroups (Alkorta et al., 2004; Thakur et al., 2016) [2, 69] which areas: (a) phytoextraction: Metal translocation to shoots is a crucial biochemical process and is desirable in an effective phytoextraction because