Environmental and Agricultural Microbiology. Группа авторов

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Environmental and Agricultural Microbiology - Группа авторов


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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].

Schematic illustration of the 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

      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].

      3.5.4 Bioimmobilization


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