The accumulation and distribution of organic compounds in plants includes three phases: enzymatic modification and degradation, conjugation, and sequestration in cell walls [124]. To date, it is anticipated that using some biotechnological approaches and genetic engineering utilizing the knowledge of the metabolic and genetic processes regulating a metal tolerance will add to the plant resistance and accumulation of HMs. Solubility of metal and other contaminants and their bioavailability to the plants is chiefly effected by the chemical properties of soil for example, cation exchange capacity, loading rate, soil pH, redox potential, organic matter, soil texture, and clay content [141, 142]. Normally, higher levels of organic matter or clay and soil pH enhance chelation effect and the metals will be strongly bound to soil for longer periods and will be less bioavailable to plants. Soil temperature is also a crucial parameter accounting for variations in metal accumulation by crops [143, 144]. However, the introduction of various genetic changes in plants can enhance their survival at high concentration of contaminants and it advances their ability to bind or remove toxicants and to influence the synthesis of enzymes thus lessening the toxic effect of HMs [145, 146]. The plants with genetic potential for uptake, extraction, degradation, destabilization, and immobilization of contaminants are the elite implements for cleaning up contaminated soils by phytoremediation processes.
2.8 Environmental Implications of Bioremediation
Bioremediation and phytoremediation, like other remediation technologies do possess both positive and negative impacts.
2.8.1 Advantages of Phytoremediation
1 It is a clean‐up technology, cost‐effective, esthetically pleasing, and environmentally friendly.
2 It has a high probability of public acceptance.
3 It may reduce the entry of contaminants into the environment by preventing their leakage into groundwater systems.
4 It may be used on a larger scale to clean‐up a diversity of contaminants, which is possible with other approaches.
5 Environmental disruption is negligible, and it preserves topsoil in in‐situ treatment.
6 Plants act as soil stabilizers, which minimizes the grasshopper effect, and prevents contaminants from spreading in their surrounding environment.
7 It has the potential to treat a wide‐range of hazardous pollutants in the environment.
8 Sites can be monitored easily with the naked eye.
9 Additional advantages of phytoremediation over bioremediation, physio‐chemical and engineering methods include the production of useful byproducts, such as bioenergy or wood pulp.
10 Phytoremediation also supports bioremediation because plants supply nutrients and provide protection for rhizospheric microorganisms, which promotes remediation of pollutants. Additionally, the plants that are grown during phytoremediation provide stabilization of the soil and could potentially be used for green energy purposes.
11 Lessens the amount of landfill waste further (up to 90%), which can be further used as bio‐ore of heavy metals.
2.8.2 Disadvantages of Phytoremediation
1 It is usually slower than other common treatment technologies and depends upon climatic conditions.
2 For better results spots must be large enough to cultivate and utilize agricultural machinery for planting and harvesting.
3 Contaminants collected in leaves and trunks can be released again to the environment during litterfall.
4 The contaminants should be present within reach of the root zone and should not be bound to the organic portion of soil to be accessible for the plants; typically 10–15 ft for trees and 3–6 ft for herbaceous plants.
5 It is a time‐consuming and slow process – it may take several growing seasons to fully clean‐up a site.
6 Very often the introduction of nonnative species may affect the whole biodiversity
2.9 Conclusion
Bioremediation is a remarkable approach to mitigate contaminants from polluted environments like water and soil, but it is not a permanent solution to an overall contamination problem. We must acknowledge that once pollutants are released into the environment, they cannot be completely degraded because of their movement between various environmental elements and food chains. For that reason, as a foremost strategy, we must stop or reduce the production of those pollutants, which could accumulate in the environment and cause environmental degradation. Second, we must implement advanced environmentally friendly approaches of bioremediation to overcome this problem. Bioremediation reduces capital and operational costs making this approach more economic than others, i.e., ex‐situ and in‐situ cleaning methods. In contrast to traditional methods that degrade soil structure and diminish its fertility bioremediation and phytoremediation enhance soil quality and fertility.
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