微生物诱导铀沉积矿化的微观行为及机理研究

Radioactive wastes will be continually produced due to the development of nuclear weapons, nuclear power and nuclear techniques. As one of the most frequent radionuclide contaminants in ground and surface waters, uranium has a long half-life and significant biological/chemical toxicity. Therefore, great efforts have been devoted in recent years to understand the fate and mobility of this contaminant in the environment by exploring the interaction between uranium and inorganic and organic components in soil and subsoil, such as rock, minerals, humic substance and microorganism. Among these environmental media, microorganism, the most small, simple and widely spread organisms in nature, should been pay much more attention because of the impressive characteristics, such as high surface-to-volume ratio, the ability to quickly breed and to inhabit hostile environments. Microbial activities, especially biomineralization induced by microorganisms, play a considerable role on chemical and migration behavior of radioactive elements in natural environment. However, the available knowledge on the microscopic behavior and mechanism of biomineralization process is still limited until now. On the basis of our previous research results, this project tries to explore the microscopic behavior and involved mechanism for biomineralization of uranium. The main research contents will include: (1) the systematic investigation on uranium biomineralization induced by the representative microorganisms isolated from typical soils in Southwest China; (2) the structure analysis and characterization of the biomineralized products of uranium; (3) the study of the involved biochemistry mechanism and processes in microorganisms induced uranium biomineralization. This project is of great importance for deep understanding the impacts and rules on uranium biomineralization by microorganisms. Besides the structure of the products will be confirmed, the essential relationship between the structure of biosediment and microorganism as well as the involved mechanism will be further revealed on a molecular level by this project. Moreover, these results can contribute to better comprehending of the chemical speciation, dissolution, precipitation, and mobilization of uranium in environment, and be beneficial in the development of a potential bioremediation method for radionuclides contaminated soils. Additionally, it also provides a reference for the safety assessment of disposal of radioactive waste.

微生物是自然界中分布极为广泛、个体微小且构造简单的低等生物，具有比表面积大、繁殖速度快、对环境适应能力强等特点。微生物诱导的沉积矿化行为对于放射性元素/核素在环境中的化学与迁移行为具有显著影响，但其微观行为与机制尚需深入探讨。本项目拟选取西南某铀污染土壤中的优势微生物，探讨铀在土壤微生物中的还原沉积/矿化行为规律及影响因素、铀沉积物的组分与结构，以及微生物诱导铀还原沉积/矿化的生化机制与过程等，以全面了解微生物诱导铀沉积矿化的条件及影响规律，确定铀在微生物细胞中沉积物的化学结构，从生物分子水平上揭示微生物诱导铀沉积矿化的微观行为与机理，厘清微生物成分或种类与铀沉积物结构之间的内在联系。上述研究结果不仅有助于深入了解铀在环境中的化学形态、迁移沉降、转化等行为，还有望为放射性核素污染土壤的生物修复提供一种新的思路与方法。同时，对放射性废物的地质处置与安全性评价也具有一定的参考价值。

微生物诱导的沉积矿化行为对放射性元素/核素在环境中的化学与迁移行为具有显著影响，但关于其微观行为与机制的研究尚存在诸多不足。本项目从我国某铀污染土壤中筛选出对铀具有显著矿化效果的菌株，系统考察其诱导U(VI)矿化的行为，确定沉积物的结构，从分子水平上揭示微生物诱导铀沉积/矿化的微观行为与机制，取得了如下进展：（1）明确该铀污染土壤中微生物主要由14个细菌门类群和11个真菌门类群组成，其中5个优势细菌门类群和6个真菌门类群；（2）筛选出3株能显著诱导铀矿化的细菌，经鉴定为革兰氏阳性菌B. Thuringiensis X-27，革兰氏阴性菌Enterobacter sp. X-57和Lelliottia sp. X-110；（3）对上述三种微生物诱导沉积矿化的微观行为与机制进行了系统研究，发现有氧条件下，微生物诱导铀矿化为先吸附再矿化的渐进过程；U(VI)在不同菌上的矿化部位和产物不同，可在胞内、胞外或胞内外同时发生，产物为NH4UO2PO4•3H2O和NaUO2PO4•3H2O；矿化主要涉及细胞的应激响应、转运活性、代谢活性与氧化磷酸化等，且在不同阶段存在不同的响应特性；矿化可减弱U(VI)对细胞的毒性；（4）构建了环境兼性厌氧菌诱导铀还原体系，建立了微生物还原体系中U(Ⅳ)的定量分析方法；发现厌氧环境中电子供体是还原的必要条件，且还原体系不同，电子供体不同，电子传递途径也有所不同；还原体系中，U(Ⅵ)的生物还原、生物吸附与非还原生物矿化共存，并相互制约，还原生成的U(Ⅳ)产物为无定型沉积物；（5）通过EXAFS和TRLF分析进一步发现，微生物诱导铀的矿化或还原体系中铀优先与含磷基团配位，含磷基团不足时再与羧基配位；有氧条件下微生物中含磷基团与铀进行单齿配位，厌氧条件下单齿和双齿并存。上述研究结果不仅加深了对微生物诱导U(VI)铀沉积/矿化和还原的微观行为与生物学机制的认识，也为放射性污染土壤的生物修复提供了一种新的思路与方法。.到目前为止，基于本项目已在国内外发表和交流学术论文20篇，其中正式发表论文SCI收录10篇，EI收录1篇，多人次参加国内外学术交流。先后有5名博士生、4名硕士生参与本课题的研究。基于本项目研究结果，已采集中核集团某废弃铀矿渣及矿坑水进行理化和微生物种群分析，拟对其进行修复/治理实验研究。