General Aspects/Case Studies on Sources and Bioremediation Mechanisms of Metal(loid)s 151

are present in microbial excreta and their derivative products (Park et al. 2011). Metallothioneins,

the cysteine-rich polypeptides bind metal(loid)s. Microbial exopolymers, metal-thiolate clusters,

cysteine-containing-glutamyl peptides and phytochelatins are associated with the reduction of

metal-(loid) binding and detoxification (Cobbett and Goldsbrough 2002, Park et al. 2011). The

bacterial species Micrococcus luteus and Azotobacter sp. immobilized Pb, i.e., 490 mg g⁻¹ and

on a dry weight basis about 310 mg Pb g⁻¹ on (Tornabene and Edwards 1972). Other groups of

researchers found that Sulfate-Reducing Bacteria (SRB) effectively remove Zn from the medium.

SRBs use phosphogypsum as a sulfate source for terminal electron acceptors for energy production.

Stenotrophomonas maltophilia reduced the toxicity of Se(III) from the legume rhizosphere of

Astragalus bisulcatus (Gregorio et al. 2005). The plant roots have a significant role in the chemistry

of the metalloids and soil environment (Tao et al. 2004). Metal(loid)s removal from contaminated

soils by plant roots is an emerging eco-friendly remediation technology (Ernst 1996). This

bio-transformation technology in soil depends on pH changes, microbial activity, phytochelatins,

metal binding by root exudates and plant uptake via roots. Plant root exudates form complexes with

organic acid and metal(loid)s in soil (Koo et al. 2010).

9.4 Bioremediation Approaches of Metalloids

The hazardous metallic and metalloid contamination has appeared as a critical problem of

unease with the development of industrial, and agricultural areas with a rising human population.

Additionally responsible for the increasing amount of the metal(oids) are natural biogeochemical

processes. Various kinds of bioremediation methods can be employed to remediate metalloids.

Bioremediation, an ideal method at recovery, is a process that employs microorganisms, fungi and

higher plants to decontaminate or degrade the pollutants in the environment through the processes

of biodegradation, biotransformation and biodeterioration. The addition of fungi and bacteria

enhances the bioremediation process to reduce pollutants to environmentally acceptable and legally

permissible levels (Pointing 2001). When the crop is grown in afflicted areas, metallicoids can enter

the body directly by drinking contaminated water or indirectly through the food chain.

9.4.1 Industrial Wastewater

Industrial effluents are the major sources contributing harmful contaminants to the environment.

Many metalloid industries situated near the coastal areas discharge effluents to shore water bodies.

Such harmful compounds generated in the surroundings also cause various harmful effects on living

organisms throughout the food chain. Printing and dyeing industrial wastewater need to be treated

before discharge. Dyeing, rubbing and desizing are the major sources of water contamination of

effluents from textile industries. The residual dye-containing waters are characterized by visibly

intense color, huge COD, suspended solids and alkaline pH (Anjaneya et al. 2011). Thus, effluents

from these industries create a major concern for the environment. Bioremediation of textile

wastewater has been done using potential organisms like Brevibacillus chashiuensis, Bacillus

subtilus, Pseudomonas species, B. cereus, Micrococcus species and B. mycoide (Mahmood et al.

2013, Durve et al. 2012). Dubey et al. (2019) found that Phormidium mucicola has a maximum

potential of biosorption of 86.12 and 94.63% removal abilities for zinc and copper. Hence, to some

extent, with the aid of biodegradation, it is possible to mitigate the metal(oids) contamination in the

environment because of the increasing growth of industries that pressurize society and all natural

resources. Cyanobacteria have demonstrated a remarkable potential in industrial effluent treatment,

aquatic and terrestrial habitat bioremediation, chemical effluent detoxifying, fertilizing ability, as

a fuel alternative and as a protein source in the food industry. Some species of cyanobacteria viz.

Nodularia species, Cyanothece species, Synechococcus species and Oscillatoria species have great

biosorption and biodegradation abilities in industrial wastewater (Kumar et al. 2011).