Metal(loid)s Toxicity and Bacteria Mediated Bioremediation 175

cost of biosurfactant production and the poor productivity of biosurfactants serve as a barrier to their

commercial applications in bioremediation. Further, endeavors are needed in this area to recycle and

reuse biosurfactants in order to decrease total costs via the development of cost-effective recovery

techniques.

10.4.1.3 Bioaccumulation

Bioaccumulation is a complicated and dynamic procedure that requires the absorption and

deposition of heavy metal ions in microbial intracellular components. The metabolism-dependent

heavy metal transporting mechanisms determine its effectiveness. Heavy metals move across cell

membranes via ion channels, protein pathways and carrier-mediated movement (Mishra and Malik

2013). The lipid bilayer allows for passive diffusion and could even enable toxic metals to enter the

cell through endocytosis. Heavy metal active transportation is observed in a number of microbial

species, including pseudomonas, Micrococcus, Bacillus and Aspergillus. However, this technique

is not very useful for bioremediation since heavy metal deposited in the cell may mediate harmful

effects on microbial metabolism. The efflux pathway releases heavy metals into the environment

after a certain level of build up. Despite these restrictions, bioaccumulation has been utilized as

a complete bioremediation method when the requirements of a minimum growing medium with

low harmful impact on cells during treatment are required. In one study, the removal of Hg via

bioaccumulation was shown to be more efficient than biosorption when using a live seaweed

Ulva lactuca (Henriques et al. 2015).

10.4.1.4 Biosorption

Microorganisms, including bacteria, are used to extract toxic metals from sites like contaminated

sewage, soil and sediments (Bano et al. 2018). Biosorption is a metabolism-independent, passive

absorption mechanism that binds heavy metals to the cell membrane. It comprises both the

physical and chemical bonding such as electrostatic, covalent, exopolysaccharides, ion-exchange,

Van der Waal’s force and microprecipitation with different functional groups (Montazer-Rahmati

et al. 2011). Several factors, including acidity, temperature, ionic strength of the environment,

permeability, origin, pre-treatment of biosorbents, amount and speciation of heavy metals, affect the

adsorption process (Zhu et al. 2013, Fomina and Gadd 2014). The bacteria Ochrobactrum MT180101

showed high biosorption efficiency for copper chelated with other compounds (Sun et al. 2021).

The affinity for metal binding is attributed to the presence of functional groups on the microbial

cell that includes alkanes, amides, amines, as well as negatively charged exopolysaccharides

(EPS). Furthermore, EPS modifications like acetylation, carboxymethylation, and methylation may

increase the affinity for metal ions (Gupta and Sar 2020).

10.4.1.5 Bioprecipitation

The process of altering soluble toxic metals ions into insoluble species like hydroxides, carbonates,

phosphates and sulfide groups by microorganisms is known as bioprecipitation. In this approach,

microbe-assisted precipitation is not contingent on the activity of microbes, as it could be present

in live and dead cells. Moreover, it may result in the precipitation of toxic metals that are directly

linked to the cells. Ambient factors such as pH levels and redox potentials influence the efficiency of

bioprecipitation. Sulfate-reducing bacteria produce hydrogen sulfide in an anaerobic environment

using organic substances as a nucleophile, and they can precipitate metal ions. Biological

oxidation of soluble ferrous iron under oxidative circumstances produces Fe (III) hydroxides that

co-precipitate additional ions such as sulfide, Cd, and U indirectly (Kaplan et al. 2016, Rinklebe and

Shaheen 2017). The leaching capability and the availability of toxic metals in sediments are greatly

reduced by these insoluble complexes. By oxidation, the sulfide group is resolubilized into the

aqueous phase. Hence, it is a critical process to track fluctuations in environmental redox potential

and microorganisms’ activity.