process or mechanism. But later, it is discovered that they are not only lethal but their existence can persuade different mechanism of metal resistance in microorganisms. Microbes and metal ion communication can take place through different mechanisms. These mechanisms are classified depending on the pathway of communication of metal ions with microbes such as active and passive uptake of metal ions [63]. The biological interaction of microbes and metal ions transform the ions from toxic to less toxic or few accessible forms or arrest metal ions to inhibit their opening into bioprocess (Figure 3.1). The various interactive mechanisms are available such as biosorption, bioleaching, biovolatilization, bioimmobilization, and bioaccumulation. The EPS of biofilm bacterial cell also interact with metal ions, due to communication among positive charge metal ions and negative charge EPS of cell surface [64].
Figure 3.1 Interaction of metal with EPs and binding on bacterial cell surface.
3.5 Biodetoxification Mechanisms
The accumulation of heavy metals in food chain and their toxicity affects to biological system creates various problems. These can also enter to water bodies and contaminate soil through agricultural extract, industrial wastes, domestic runoff, and other commercial activities. We can eliminate or reduce heavy metal from contaminated sources. Therefore, there are different types of detoxification technology that have been utilized to eliminate heavy metals from contaminated sources. These detoxification technologies are briefly described as follows:
➢ Biosorption
➢ Bioleaching
➢ Biovolatilization
➢ Bioimmobilization
3.5.1 Biosorption
The capability of biological materials to accumulate or bind heavy metals present in the wastewater of polluted water bodies through metabolically facilitated or physico-chemical pathways is called biosorption. All microorganisms (algae, bacteria, fungi, and yeast) are proved to be potential metal biosorbents. This method of treatment is having some advantages such as cost effectiveness, highly efficient and effective, reduction chemical and biological slurry, no extra nutrient necessity, revival of biosorbent, and probability of metal reclamation [65]. The sorption of metal can take place by microorganisms following two different processes: active process and passive process [66].
Active process: This process is metabolism dependent and also called as bioaccumulation process. In this process, transport of metal through the membrane of cell with a subsequent accumulation of intracellular metal facilitated through metabolism of cell. Only viable cells can perform bioaccumulation, which are also often linked with a mechanism of resistance initiated through microorganism in the existence of a toxic metal [66].
Passive process: This process is a metabolism-independent process, otherwise known as biosorption. This is a physic-chemical process, normally includes four mechanism (adsorption, ion-exchange, complexation, and precipitation) and this mechanism helps to transport metal inside the cell [66].
Adsorption: The adsorption occurs with the help of van der Waals’ force [52, 64]. The selective materials for adsorption of Cr(VI) and Ni(II) are “crushed initiated carbon > bagasse > fly ash” and “crushed initiated carbon >fly ash >bagasse”, respectively. The lower pH of 6.0 is suitable for removal of Cr(VI) and pH 8.0 is appropriate condition for removal of Ni(II) ions. The limitation of adsorption is that the ability is very low and their use for industrial runoffs treatment cannot be defensible [22].
Ion-exchange: In biosorption process, the ion-exchange method was first introduced by Volesky and Holan (1995) and is backing through numerous current studies [66]. In passive absorption, the ion exchange method has essential role. In this mechanism, the biomass is displayed toward metal because the first metal aliquots are continuously discharged in to the solution while the second metal is combined, and a portion of second metal is combined to the bio-sorbent. This assay is suitable for Cu2+ and Pb2+ removal [67].
Complexion: In this method, the complex formation on the cell surface after communication between metals and functional groups of microorganisms occurs for metal removal from the solution. The magnesium, copper, calcium, mercury, zinc, and cadmium accumulation via Pseudomonas syringae takes place and is removed by simply complexion mechanism. The organic acid may produce by microorganisms may chelate toxic metals, resulting in makeup of metallo-organic molecules [65].
Figure 3.2 Mechanism of biosorption.
Precipitation: The precipitation may be dependent or autonomous to metabolism process. The metal elimination from solution is frequently linked by the functional defense system of microorganisms. The microorganisms behave in the occurrence of noxious metal generating compounds such as metallothionein and phytochelatins and induce the precipitation method. This method is not dependent upon the cellular metabolism; rather, it may be a chemical reaction significantly occurs among the metal and the cell surface, with an indigenous aggregation of ions of metal and their subsequent precipitation. The mechanism of biosorption is described overhead may occur concurrently (Figure 3.2) [65, 66].
3.5.2 Bioleaching
In bioleaching, metal cations are mobilized from almost insoluble ores by complexation and biological oxidation method. The application of microorganisms for recovery of heavy metals is currently a universal enhanced biotechnological method [68]. This technique is mainly accepted by mining industries in order to extract metals incorporate in low-grade sulfide ores. In this method, solid remains are discarded as waste material, while metals are transformed to solution phase. Presently, bioleaching method is used in several metal removal applications from ground water, sludge, soil, and sediments [69]. The biomining is a universal word for two techniques such as bioleaching and bio-oxidation techniques. There are two types of leaching method such as contact and non-contact. In non-contact leaching method, metals are mainly removed via planktonic bacteria that oxidize the surface mineral ions in solution. The Fe3+ ions arise from the bacteria and interact with the surface of mineral, where they decreased and oxidized the moiety of sulfide and release the Fe2+. So, again Fe2+ ions pass into the cycle to continue the reaction again. In contact leaching method, the maximum cells adhere to the surface of sulfide minerals. This is an electrochemical method in which the suspension of sulfide minerals occur between the borders of bacterial cell and the sulfide mineral surface and this area is occupied by EPS [68].
The EPS plays an important role in this bioleaching process through an interfacial method, which takes place in natural environment. During the process of up-taking of heavy metal (Fe3+) ions, uronic acids facilitate to produce exopolymers among cell wall and surface of metal sulfide. These complexes involved in electrostatic interaction to form primary attachment, oxidizing attacks to deterioration of metal sulfide and act as nutrient [70] (Figure 3.3). The chemo-lithotrophic bacteria such as Acidothiobacillus sp., Leptosperillium sp., and archaea are involved in
