Another key component of the phytoremediation of heavy metals is volatilization carried out by plants and microbial coimnunities. It is an effective mechanism to convert more toxic heavy metals to less toxic volatile compounds and released by plants or microbes to the atmosphere in modified forms (Tangahu et al. 2011). Taken up by roots, heavy metals are transported to shoots and leaves where they can interact with available biomolecules and may be transfonned into less toxic complexes. Dming the process of transpiration, the transfonned chemicals may escape to the atmosphere thus reducing their loads inside plant tissues and in soils. Once transfonned, the toxicity of heavy metals may be greatly reduced and in the air, their presence may not pose any seiious problems. Microorganisms may not directly paiticipate in phytovolatilization processes; they, however, can enhance their transport to roots and the ability of plants to take them up effectively. The nature of heavy metals plays a key role in phytovolatilization. Generally, mercury, selenium and arsenic are effectively volatilized by Pterisvittata and Arabidopsis thaliana (Sakakibara et al. 2010, Kumar et al. 2017). Limmer and Burken (2016) stated that plants can either directly volatilize pollutants through their stems and leaves or indirectly via root activities in the soil. Aromatic plants and those which have high evaporation potential may be regarded as ideal candidates for phytovolatilization. Muthusaravanan et al. (2020), while referring to Pilon-Smits et al. (1999), listed many plants as potential volatilizers among which Саппа glauca, Colocasiaesculenta, Cyperus papyrus, Azollacaroliniana, Arundodonax, Pterisvittata, Brassica juncea, Lupinus sp., Liriodendron tulipifera and Typhaangiistifolia effectively volatilized selenium. Anarado et al. (2019) reported that Murrayakoenigii, Ocimumgratissimum, Amaranthushybridus, Capsicum annuum and Morin gaol eifera showed some degree of volatilization of heavy metals from the contaminated environment; however, since these vegetables are consumed by humans and could offer health issues.

Besides phytoextraction, phyto-stability and phytovolatilization of heavy metals by plants, microbes and plants-microbes, some other strategies such as rhizofiltration, phytodegradation, phyto-accumulation/hyper-accumulation, plants or microbes-mediated detoxification of heavy metals and phytomining are also widely acknowledged strategies in cleaning up of polluted soils (Tangahu et al. 2011, Suman et al. 2018). Ma et al. (2016a) discussed the role of plants and microbes in phytoremediation processes. They outlined that plants either alone or assisted by microbes cany' out detoxification, bioaccumulation, bioleaching, bioexclusion, mobilization and immobilization, and biotransfonnation, which effectively contiibutes to minimizing heavy metal concentration in soil. Mosa et al. (2016) also provided a comprehensive mechanism of heavy metal removal by different microorganisms. They described that microbes could reduce heavy metal concentration in the contaminated environment by biosoiption and bioaccumulation, siderophore formation, and production of biosurfactants.

Studies have revealed promising results for whether the bioremediation agents (plants and microbes) were applied alone or in combination. Strains of Microbacterium (G16) and Pseudomonas flnorescens (G10) were found to promote the growth and lead uptake in Brassicanapus under Pb-contaminated soils (Sheng et al. 2008), while Achromobacterxylosoxidans (AxlO) contributed to growth and biomass increase in B. juncea with an enhanced potency of Cu uptake in the host plant (Ma et al. 2008). In other reports, bacterial species Flavobacterium sp., Rhodococcus sp., Vdriovoraxparadoxus, Chrysiobacteriumhumi, Rahtoniaeutropha, Microbacterium, Pseudomonas flnorescens, and/5 aeruginosa were identified as effective synergists to stimulate extraction of heavy metals (Cd, Cr, Zn, Pb, Co, Ni) and pesticide residues from the polluted soils with augmentative effects on the growth of different plants (Belimov et al. 2005, Rajkumar and Freitas 2008, Ahemad and Khan 2011, Sobariu et al. 2017).

Similarly, different plant species such as Tithoniadiversifolia and Helianthus annum (Adesodun et al. 2010), Pterisvittata, P. cretica, Boehmerianivea, and Miscanthusfloridulu (Sun et al. 2014), Piper marginathum and Stecherusbifidus, Jatropacurcas and Capsicum annuum (Marrago- Negrete et al. 2016), microbial assisted-Ricinuscommunis (Annapurna et al. 2016), Lenina minor (Bokhaii et al. 2016), compost and biochar assisted-MoringaoIeifera (Ogundiran et al. 2018), Lenina minor and Azollafiliculoides (Amare et al. 2018), Brassica campestris, Rorippapalustris, Sinapisarvensis and Thlaspiatrens (Drozdova et al. 2019), Coronopusdidymus (Sidliu et al. 2020), and Corchoruscapsularis (Saleem et al. 2020) have been repoiled for varying degrees of tolerance to heavy metals and phytoremediation potentials of different heavy metals in different environmental conditions.

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