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Bioremediation methods of heavy metal contaminated soils

It is estimated that nearly 20 million hectares of soils are contaminated with different heavy metals, both originating from natural sources as well as anthropogenic activities (Liu et al. 2018). Many approaches are employed to reduce heavy metal pollution in soils which include physical, chemical, biological techniques and integrated approaches. Physical approaches of remediation rely mainly on removal of contaminated soil, applying electro-kinetic approaches, and verification while chemical methods include soil washing, immobilization and encapsulation; in biological methods, different organisms (plants and microbes) are used to extract, stabilize, volatilize and remove heavy metals from polluted soils (Klialid et al. 2017). In integrated methods, physical, chemical, or biological approaches are implied in combination. Particularly employed teclmiques in the remediation processes have their own merits and demerits. Liu et al. (2018) advocated in situ and landfilling approaches as more effective than the other methods. Araliah et al. (2019) asseited that an integrated approach using chemical and biological techniques is more effective and environmentally sound. A combination of nanotechnology with the use of microorganisms is also considered as a promising method to reclaim contaminated soils (Cao et al. 2019). In many recent reviews, employment of bioremediation techniques has been favored for reclamation of heavy metal contaminated soils because of their cost-effectiveness, feasibility and eco-friendliness (Ashraf et al. 2019, Fu et al. 2019, Yang et al. 2019, Muthusaravanan et al. 2020).

Table 1. Heavy metals and their consequences on germination, growth, physiological, metabolical and yield attributes of

different plants.

Plants under heavy metal stress	Heavy metals	Nature of heavy metals	Effects on plants	References
Phaseohis vulgaris	As	Non-essential, toxic	At concentration 5 mg dm"⁵, As stress significantly reduced growth, physicochemical activities and protein contents	Stoeva et al. (2005)
Hordeum vulgare	Ni	Essential	100 pM concentration of Ni caused necrosis, chlorosis and reduced growth and mineral distribution	Rahman et al. (2005)
Sesbania drummondii	Pb	Non-essential, toxic	Poor seedling growth, photosynthesis and anti-oxidative responses	Israr et al. (2006)
Brassica pekinensis	Cu	Essential	At higher concentration, growth and nitrogen metabolism reduced	Xiong et al. (2006)
Hordeum vulgare	Cu, Al, Cd	Essential and non-essential	Poor growth and metabolic activities at higher concentrations	Guo et al. (2007)
Elsholtzia atg)n	Pb	Non-essential, toxic	Photosynthesis, leaf growth and membranes’ abnormalities at 200 pM	Islam et al. (2008)
Leucaena leucocephala	Cd, Pb	Non-essential	75 ppm concentration of Pb while 50 ppm of Cd reduced seedlmg root and shoot growth and dry weight	Shafiq et al. (200S)
Vetiveria zizanioides	Cd	Non-essential, toxic	Protem, chlorophyll contents, reduced water uptake and poor root activity	Aibibu et al. (2010)
pakchoi and mustard	Cd	Non-essential	Reduced growth and photosynthesis, shoot and root w'eight decreased	Chen et al. (2011)
Liman usitatissimum	Cd, Cr	Non-essential	Decreased photosynthesis and plant growth attributes	All et al, (2015)
Eichhornia crassipes	Pb	Non-essential	Plant growth and chlorophyll content drastically decreased	Malar et al. (2016)
Entca sativa	Cu, Ni, Zn, Hg, Cr, Pb	Essential/non- essential	Decreased germination and seedling growth	Zhiet al. (2015)
Brassica juncea	Cr, Cd, Pb, Hg	Non-essential	Increased level of heavy metals decreased photosynthesis, biomass and chlorophyll content	Sheetal et al. (2016)
Zea mays	Ni	Essential	At higher concentrations, Ni stress induced low mineral contents, photosynthesis, growth and biomass	Rehman et al. (2016)
Pisum sativum	Co, Pb, Cd	Essential/ Non-essential	Abnormal growth and biomass at concentration 750-1250 ppm	Majeed et al. (2019)
Spinacia oleracea	As, Hg	Non-essential	Abnormal and reduced plant growth, and metal distribution	Zubair et al. (2019)
Triticum aestivum	Zn, Cu	Essential	Germination parameters and seedlmg growth attributes responded negatively to higher concentration of metals	Wang et al. (2019)
T. aestivum	Cu, Pb	Essential non- essential	Reduced plant growth and enzymatic activities	Jiang et al. (2019)
Desmodesmus sp.	Cu	Essential	Negative effect on growth and filament structure	Buayam et al. (2019)
Zea mays	Cu	Essential	At higher concentrations, plant height and weight reduced u'hile mineral uptake w'as disturbed	Reckova et al. (2019)

Table 1 Contd. ...

Heay Metal Pollution in Agricultural Soils: Consequences and Bioremediation Approaches 231

...Table 1 Contd.

Plants under heavy metal stress	Heavy metals	Nature of heavy metals	Effects on plants	References
Petunia hybrid	Cd, Cr, Cu, Ni and Pb	Essential/non- essential	Morphological and biochemical abnormalities	Khan et al. (2019)
Uvginea maritime	Al. Cr, Cd, Pb	Essential/non- essential	Decline in photosynthesis and chlorophyll pigments	Houri et al. (2020)
Nicotiana alata	Cd, Cr, Co, Ni, Pb	Essential/non- essential	Stress imposition, negative effects on physiology' and photosynthesis	Khan et al. (2020)

Bioreinediation in wider terms implies the use of living organisms for minimizing the concentration and adverse effects of heavy metals in contaminated soils. Generally, plants (herbs, shrubs, trees and even algae) are used as remediating agents in most of the polluted sites and the technique is referred to as “phytoremediation”. Microorganisms such as bacteria and some fungi may also be used in remediation processes and this approach is termed as “microbial remediation”. Phytoremediation coupled with the microbial application (bacteria, microfungi, and microalgae) is termed as “microbial assisted phytoremediation” (Dotaniya et al. 2018). Plants used wither alone or in combination with microbes, and employ different strategies such as chelation, accumulation, extraction, volatilization and stabilization of heavy metals (Figure 1).

Figure 1. An illustration of the heavy metal input mto soils and bioremediation strategies employed by plants and microbes.

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