Читать книгу X-Ray Fluorescence in Biological Sciences - Группа авторов - Страница 71

In the environment, heavy metal pollution is a serious problem. However, it is well known that microbial remediation is highly efficient and indispensable in repairing heavy metal pollution. Thus, understanding the mechanism of using micro‐organisms in bioremediation may help to improve remediation efficiency.

The biomineralization of heavy metals inside and outside the micro‐organism cells can precipitate heavy metals, which then reduces their bioavailability. Biomineralization is an important bioremediation method that includes phosphate mineralization, carbonate mineralization, and sulfide mineralization. XAS results showed that Shewanella oneidensis MR‐1 could reduce dissolved U(VI) to UO₂ minerals with poor solubility to achieve the purpose of bioremediation. The addition of S, Si, and P elements in the environment could promote the formation of EPS (Extracellular Polymeric Substances), and the proportion of UO₂ increases by 55–95%, which improved the efficiency of bioremediation significantly [24]. Uranium oxides were mineralized to secondary minerals by fungus, and the main processes were as follows: (1) When the fungus cells were exposed to UO₃ and U₃O₈, the cells secreted large amounts of oxalic acid, which caused UO₃ and U₃O₈ to dissolve and form (UO₂)²⁺ complexes; (2) U was adsorbed by cells, and the adsorption capacity could reach 80 mg/g; (3) The dissolved (UO₂)²⁺ further complexed with P to form UO₂HPO₄ and [UO₂(HPO₄)₂]²⁻, and finally mineralized to form the uranyl phosphate mineral NH₄(UO₂)(PO₄).3H₂O and H₂(UO₂)₂(PO₄)₂.8H₂O [25].

The chemical species of heavy metals in the rhizosphere microenvironment are one of the important factors that affect the process of plant absorption of heavy metals. Study on the mechanism of interaction between rhizosphere micro‐organisms and heavy metals have important significance for the development of phytoremediation. It has been shown that arbuscular mycorrhizal fungi could enhance the capacity of plants to absorb Cr through mineralized and immobilized Cr in the rhizospheric environment, and the main mechanism listed as follows: (i) When exposed to Cr⁶⁺ solution, the EPS secreted by fungal cells increased significantly, and Cr mainly combined with the EPS on the cell surface to form a granular substance; (ii) Cr⁶⁺ was reduced to Cr³⁺, and mainly accumulated in fungal cells in the form of phosphate, histidine, acetate and K₂CrO₄; (iii) EXAFS results showed that Cr was mainly combined with O, P, and C atoms in fungal cells; (iv) Most of Cr was absorbed by arbuscular mycorrhizal fungi, and only a small amount of Cr entered the plants, which alleviates the toxic effect of Cr on plants; (v) Cr in plant root cells mainly existed in the form of acetate, histidine analogs, phosphate, and K₂CrO₄.