Читать книгу Environmental and Agricultural Microbiology - Группа авторов - Страница 65

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 the heavy metal concentration [69, 75]. The metabolism and intrinsic property of some bacteria associated with cell wall structure and the presence of extracellular polymeric substances are able to tolerate metal ions. Some other bacteria resist to metal by using resistance mechanisms such as active transport, efflux pump, intra- and extracellular sequestration, methylation, toxic chemical transfer to less toxic chemical through enzymatic transformation of redox reaction, and sensitivity reduction of cellular targets to metal (Figure 3.5) [76].

The heavy metals are reduced by using immobilization process. The Cr(VI) is reduced to Cr(III) by using both anaerobic and aerobic microorganisms. The presence of oxygen in aerobic condition and the reduction of Cr(VI) by microbes are generally catalyzed though soluble enzyme and lessening of Cr(VI) to Cr(III) by microbes as an eco-friendly method [77]. The bacterial strain such as E. coli, Pseudomonas putida, Desulfovibrio sp., Bacillus sp., Shewanella sp., Arthobacter sp., Microbacterium sp., and Cellulomonas sp., which reduce Cr(VI) isolated from contaminated area [78]. Arsenic compound used as an electron donor or accepter by microorganisms and possess the detoxification of arsenic, with pushes up to the membrane level of cells to eradicate As(III) from cells and metabolites of cell, finally As(V) removal arise [69]. Anaerobic bacteria are capable to reduce contaminated As(V) to As(III) and sulfate to elemental sulfur and precipitates in the form of arsenite sulfide [79]. Therefore sulfide precipitation is a useful mechanism for reduction of arsenic. The EPS of Chryseomonas luteola immobilized the metal ions such as cadmium, cobalt, nickel, and copper through adsorption [64].

Figure 3.5 Schematic diagram of bioimmobilization.