Limitations and prospects in bioremediation approaches

The process of bioremediation offers excellent opportunities for utilizing plants, microorganisms or plants assisted by microorganisms to clean up the polluted environment with heavy metals and other organic and inorganic pollutants. However, bioremediation strategies have some limitations in application. The major problem concerned with the processes is the identification of suitable plants and microbial candidates for phytoremediation. Since the majority of the plant species and microorganisms exhibit limited tolerance to heavy metal stress, using of non-tolerant bioremedial agents would not work efficiently in cleaning up the polluted environment. Microbial and plant species which are tolerant to heavy metal stress can work better to remediate the contaminants from the given environment. They also show variable efficacy because some species may have long life period, low biomass, low uptake potential for heavy metals, and not well-developed rooting system. Ideally, plants with hyperaccumulation potential, desired biomass, broad leaves, and extensive roots can promote the process of phytoremediation. Further, the most widely known strategies in phytoremediation such as phytoextraction, phytostability, phytoaccumulation, and phytovolatilization may not be exhibited by a single plant species. Similarly, microbes have low biomass and limited mechanisms of remediation due to lack of roots, and their variable responses to heavy metal stress. Consumable plants are not ideal for phytoremediation because they may cause food chain contamination.

Besides the limitations, bioremediation has several advantages over traditional clean-up methods. Physical, chemical, thermal and other methods have labor and cost burdens in addition to their lack of feasibility in different sites, possible secondary pollution they may cause, and problems concerned with the disposition (Shanna et al. 2018). Bioremediation, in contrast, seems more effective, eco-friendly, cost-effective, relatively feasible, and less hazardous to the environment (Awa and Hadibarata 2020, Haq et al. 2020). After identification of plants and microbes which have ideal characters of heavy metal tolerance, high potential of metal uptake and translocation, rhizofilteration, accumulation, volatilization, biodegradation and transformation of heavy metals could be used as alternative agents to other methods. To prevent food chain contamination, weeds, and wild or cultivated but non-edible plants can be applied as phytoremediation agents. To further augment their efficacy, suitable bacterial, algal and fungal strains may be utilized to assist them in bioremediation of heavy metals. Selvi et al. (2019) favored an integrative approach to maximize the efficiency of bioremediation by employing amendments and upgraded traditional strategies. Utilization of transgenic plants that are modified for phytoremediation purposes may also be considered as potential bioremedial agents in the reclamation of heavy metal polluted soils (Gunarathne et al. 2019).

Conclusions

Heavy metal pollution (Hg, Cu, Ni, Pb, Cd, Cr, Sn, Zn, Fe, Mn, Mo, As, Al, Co, Ag) in land and aquatic enviromnent is one of the major abiotic constraints affecting microorganisms, plants, animals and humans. In agricultural land, the presence of heavy metals is of particular concern because they drastically affect cultivated plants and may lead to food chain contamination. Natural sources of heavy metal pollution are weathering phenomena, the release of volcanoes and flooding of land with water containing heavy metals while antluopogenically, industrial operations, wastewater irrigation, use of agrochemicals and mining activities are adding substantial quantities of heavy metals to soils. Except for essential heavy metals in smaller quantities, most of the heavy metals are toxic to flora and fauna and trigger a number of abnormalities in them. Plants challenged with heavy metal stress may expeiience altered germination, growth, physiology, development and yield. To prevent domestic plants and subsequent food chain contamination, cleanup remediation of the heavy metal polluted environment is necessary. Bioremediation which employs various plants and microorganisms is an attractive cleanup approach for reclamation of polluted soils due to cost-effectivity, feasibility and eco-friendliness as compared to traditional methods. Plants may be used either alone or they may be assisted by different microorganisms to extract, accumulate, volatilize, biodegrade, bio-transform and bio-stabilize heavy metals present in the soil. Besides its potential benefits and evident outcomes, bioremediation approaches need more rigorous research and attention for a wide-scale utilization in the reclamation of heavy metal pollution.

 
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