Bioremediation of Heavy Metals from Aquatic Environments 227

The natural process of bioleaching has its limitations therefore, researchers have broadened

this process to treat solid wastes artificially to remove or solubilize metals. These solid wastes are

released from different industries and mining processes causing various health hazards to animal

diversity as well as human health. Different microbes are used in the bioleaching process. Examples

of some microbes are archaea, fungi and acidophilic bacteria (Brandl et al. 2001, Natarajan and

Ting 2014). All three types of acidophilic bacteria such as mesophiles, moderate thermophiles and

thermophiles are used in this process. Fungi can also be used in this process. Some of the fungi that

are used in the bioleaching process include Penicillium chrysogenum, Penicillium simplicissimum,

Aspergillus niger and Aspergillus flavus. The fungal bioleaching process requires a pH from 3.0 to

7.0 and a temperature ranging from 25 to 35°C.

13.2.4 Biotransformation

This is the method by which a chemical compound’s structure is changed, resulting in the production

of a molecule with comparatively higher polarity. In other words, through the process of metal-

microbe interaction, metal and organic molecules are changed from a harmful state to a form that

is substantially less toxic. This technique basically enables microbes to acclimatize to changing

environments.

Microorganisms control trace element transformation (microbial or biotransformation) through

a number of mechanisms such as oxidation, reduction, methylation, demethylation, complex

formation and biosorption. Microbial transformation is important in the behavior and fate of toxic

elements in soils and sediments, particularly Arsenic (As), Chromium (Cr), mercury (Hg), and

Selenium (Se). Biotransformation processes can change the speciation and redox state of these

elements, controlling their solubility and mobility (Kunhikrishnan et al. 2017). These processes are

critical for trace element bioavailability, mobility, ecotoxicity and environmental health. Microbial

cells have a high surface-to-volume ratio, a rapid rate of growth, as well as metabolic activity, and

is simple to maintain sterile conditions for microbes. They are thus ideal organisms for the process

of biotransformation. Condensation, hydrolysis, the creation of new carbon bonds, isomerization,

the addition of functional groups, oxidation, reduction and methylation are all methods that can be

used to carry out this process. These processes might cause metals to volatilize, thus decreasing their

ability (Tayang and Songachan 2021).

13.2.5 Biomineralization

A natural process of mineral production is the biomineralization of heavy metals. Minerals like

phosphates, oxides, sulfates, silicates and carbonates are naturally synthesized in this process, which

involves a variety of mechanisms in living things. Mineral production depends on the presence

of highly variable and reactive surfaces, such as cell walls and extra organic layers with varying

levels of hydration, content and structure. Additionally, there are organic ligands that deprotonate

and impart a net negative charge on the microbial surface as pH rises, including amine, carboxyl,

hydroxyl, phosphoryl and sulfur.

Positively charged potential hazardous metals precipitate unevenly into more stable and

compact mineral products. Phosphate precipitation, carbonate precipitation, oxalate precipitation

and complexation can all result in the immobilization or complexation of metals (Tayang and

Songachan 2021).

13.3 Types of Bioremediations

The process in which incomplete and occasionally complete detoxification of pollutants takes place

by small microorganisms is called biodegradation (Gouma et al. 2014). The more specified term is

bio-mineralization in which bacteria, fungi and plants release different acids and convert them into