152

Bioremediation for Sustainable Environmental Cleanup

9.4.2 Soil Contamination

Soil contamination is on the rise, which is catalyzed by the modern development of industry, chemical

fertilizer uses on large-scale and pesticides, etc. The demand for food is increasing continuously,

which is stressing the already contaminated soil for growing crops. Hence, there is a major need

for the decontamination of soil, which can be brought about physio-chemically and biologically

(Jin et al. 2021). But, increasing the quantity of various substances in the soil and waters because

of the outbreak of industries has generated harrowing conditions for people and water bodies (Dixit

et al. 2015). The plant-based techniques have an excellent capacity to degrade, extract and stabilize

the contaminants in phytoremediation, which has become evident as a great alternative in respect of

cost and is eco-friendly. Metals are introduced in underground water aquifers through soil by water

flow, these metals are introduced into soil by anthropogenic activities (Shabir Hussain 2012).

Microbial remediation techniques make use of microorganisms with unique functions that help

in reducing the pollutants by degrading them, by converting them into non-toxic substances with

their metabolism under environmentally adapted conditions. Some of the limitations in microbial

remediation are that the microbes mutate easily due to poor genetic stability and are not able to

remove the pollutants completely. The inability to compete with the indigenous strain is a significant

need that easily affects their performance (Rudakiya et al. 2019). About 72 species of acidophilic

thermophilic species have been isolated by primary and secondary screening for resistance to heavy

metal concentrations and their ability to biosorption (Umrania 2006). Phosphate-dissolving bacteria

can detoxify metalloids (Saranya et al. 2018, Li et al. 2016). Plants convert pollutants into non-toxic

forms (Jin et al. 2021). Alfalfa plants possess potentially reduced metalloids (Agnello et al. 2016).

9.5 Bioremediation Techniques for Heavy Metal(oid)s Removal

Enhancement

Inorganic metals/metalloid pollutants like Cu, Hg, Zn, As, Cd, Mn and Se appear in the environment

mainly as cations and anions rely only on the plant vascular system for their translocation and uptake

(Dhankher et al. 2012). Inorganic pollutants are therefore changed (reduced/oxidized), transported

within plants and volatilized (Se, Hg) in a few instances, but they cannot be treated as such. Many

bioremediation techniques, such as microorganism-based techniques, have demonstrated their

potential for degrading and detoxifying many organic as well as inorganic pollutants. In comparison

to other traditional approaches, biological systems are barely resistant to environmental extremes;

hence they have an edge over other approaches as they are less expensive (Cunningham and

Ow 1997).

Plant-based remediation solutions, also known as phytoremediation, have sparked increased

interest because they are potentially more cost-effective, have low adverse effects and are

ecologically sound (Cunningham and Ow 1997). Metal ions are taken up from the root and

delivered to the above-ground components through the shoot system during phytoremediation,

where they concentrate. The components of the plant are harvested, and so is the metallic build­

up, leading to the elimination of pollutants (i.e., metals) from the site (Nandakumar et al. 1995).

Plants have demonstrated the capability to survive predominantly at higher levels of metal

contaminates and organics toxicity by quickly converting them into less toxic metabolites in many

circumstances. This can be accomplished through phytoextraction (the intake and recovery of

metallic pollutants in the form of above-ground biomass), rhizofiltration (the filtering of metals

in root systems) or phytostabilization (the stabilization of waste sites through erosion control

and large-scale evapotranspiration), among other methods (Cunningham and Ow 1997). The

phytoremediation procedures are not mutually exclusive, and they can be employed in tandem for

greater effectiveness and efficiency. Phytostabilization, phytoaccumulation in harvest-worthy plant

tissues (phytoextraction or rhizofiltration) in rare situations, and phytovolatilization are among

the phytoremediation strategies available for inorganics. However, bioremediation techniques for