Environmental and Agricultural Microbiology. Группа авторов

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 metal recovery from sulfide minerals. These bacteria are gram negative, aerobic, and able to survive below pH 3.0 and temperature 25°C–35°C. Leaching bacteria are mostly belongs to proteobacteria (Acidithiobacillus sp., Acidiphilium sp., Acidiferrobacter sp., and Ferrovum sp.), Nitrospirae (Leptospirillum sp.), Firmicutes (Alicyclobacillus sp. and Sulfobacillus sp.), and Actinobacteria (Ferrimicrobium sp., Acidimicrobium sp., and Ferrithrix sp.) [68, 69]. But among all bioleaching bacteria, Thiobacillus sp. involved in solubilization of metal sulfide because it takes carbon dioxide from atmosphere for cellular synthesis, pulls their energy from oxidation of elemental sulfur, reduces sulfur compounds, and results in production of ferric ions and sulfuric acids, which are entangled in heavy metal extraction. The frequently used bacteria for bioleaching process are Acidothiobacillus thiooxidans, Acidothiobacillus ferrooxidans, and Leptospirillum ferrooxidans, and these are able to grow in high acidic condition (pH 1.5–3.0) [69].

Figure 3.3 Mechanism of bioleaching.

      The oxidation of metal sulfide by Fe/S oxidizing bacteria is defined through two distinct pathways such as polysulfide and thiosulfate pathway [68, 69]. These mechanisms depend on metal sulfide reactivity with protons (acid solubility) [69]. In case of thiosulfate pathway, metals are acid-insoluble such as pyrite (FeS₂), molybdenite (MoS₂), and tungstenite (WS₂), and Fe³⁺ ions occur through metal sulfide extraction. This reaction results the production of metal cations (M⁺) and thiosulfate that oxidizes to sulfuric acid. The production of sulfuric acid creates acidic condition so T. ferrooxidans and L. ferroxidans catalyze Fe³⁺ ions for recycling. In case of polysulfide pathway, metals are acid soluble such as sphalerite (ZnS), galena (PbS), arsenopyrite (FeAsS), chalcopyrite (CuFeS₂), and hauerite (MnS₂) through electron extraction by iron(III) ions and proton attack. In this mechanism, polysulfide is the main intermediate form and can be oxidized to sulfuric acid by using bacteria A. ferroxidans and A. thiooxidans [71]. In bioleaching process, maintenance of acidic condition is essential because the optimum action of Fe/S oxidizing bacteria and to retain metals constant in solution phase.

      3.5.3 Biovolatilization

The transformation of metals by microbes into their volatile forms is known as bio-volatilization and contributes in the alteration of metal from soluble state to gaseous state. This biovolatilization process can remove metal from solid phase by utilizing microbes. Therefore, this process can be applied for both wastewater treatment and solid waste treatment. If the gas form of volatilized metals can trick from wastewater treatment method, they can be consequently recovered [72]. The metals commonly connected with their methylation and alkylation of biovolatilization method by microbes, whereas volatilization of mercury and arsenic may also be facilitated by their removal [72, 73]. Biovolatilization is a common method for mercury and arsenic in environment through which detoxification approaches applied on soil and water based on transformation of highly toxic compound to nontoxic or less toxic compound and highly volatile for removal of metals (Figure 3.4) [73].

Figure 3.4 Mechanism of biovolatilization.

      In contaminated environment, bacteria developed resistance resulting due to the aforesaid mechanism which further leads to mercury detoxification. The reductase enzyme (Mercury(II)reductase) of the bacteria causes a reduction of Hg²⁺ to nontoxic Hg⁰, and hence, a diffusional loss of Hg⁰ from bacterial cell takes place. The mercuric reductase coded by merA gene is important for reduction of inorganic Hg while cytosolic mercuric lyase enzymes coded by merB gene breaks the C-Hg bond of most organomercury [69]. Earlier studies reported that bacteria involved in this mechanism and resistance to Hg such as Bacillus sp., Pseudomonas sp., Psychrobacter sp., Halmonas sp., Luteimonas sp., and Micrococcus sp. are isolated from highly polluted area [74]. The elemental mercury is highly volatile and the gas phase needs some special treatment to immobilize it. The Hg⁰ produced by volatilization and it is removed into gas phase by fast oxidative absorption process and recovered. This technique can be applied on soil, wastewater, and sediment [69].

The biovolatilization process also involve in arsenic removal from contaminated soil and water. In soil, arsenic could be converted into volatile byproducts and removed. Both aerobic and anaerobic microorganisms are involved in the evolution of volatile arsenicals. The volatilization of arsenic by microorganisms depends upon several factors like arsenic compound, concentration, and moisture of soil, organic materials, temperature, other similar components, growth of microbes, and ability of volatilization of arsenic. Biovolatilization of arsenic is by lessening of As(V) to As(III) with end product of TMAs. Currently, Escherichia coli have expressed arsenite S-adenosylmethionine methyltransferase gene (arsM), which is cloned from Rhodopseudomonas palustris and is capable to form methylate inorganic arsenic to TMA volatile form. In indigenous bacteria, arsM gene has capability to remove As through volatilization from soil. The strains express arsM gene in aquatic system such as Sphingomonas desiccabilis and Bacillus idriensis. The arsenic resistant bacteria can express arsM gene for biovolatilization of arsenic and these bacteria can engineered under laboratory condition to apply in aquatic and soil environment [73].

      3.5.4 Bioimmobilization

Currently, bioimmobilization process is used in bioremediation, biodegradation, bio-control, pesticide use, and the manufacture of numerous compound products like antibiotics, enzyme or steroids, and amino acids. In this technique, metal can immobilized using microbial biomass by biosorption to cell walls or by extracellular substances and some common procedures are using for immobilization such as adsorption on exteriors, flocculation, cross connecting of cells, nanocoating, entrapment, covalent bonding to carriers, and encapsulation. The bacteria persuade immobilization mechanism to reduce