The environmental behaviour of redox active radionuclides - a combined biogeochemical and geomicrobiological approach.

英国留下了大量受污染的核工业场地。其中两个主要地点位于塞拉菲尔德和Dounreay，这些地点的土壤和地下水已受到放射性污染。事实上，放射性核素对地下水的污染是一个全球问题，核设施的退役将需要控制和消除污染。然而，目前对地下微生物影响放射性核素在受污染土地上的流动性和行为的方式知之甚少。我们已经确定了与放射性核素污染特别相关的四种放射性核素：氚、铀、钚和钚。所有这些放射性核素都是长期存在的，U和Tc目前是世界各地许多地点的优先污染物，在塞拉菲尔德被报告为污染物，而Np和Pu将是放射性废物和受污染土地中的重要中期环境污染物。此外，它们都是氧化还原活性的，与它们的还原形式相比，它们在氧化状态下通常更具流动性。该项目的重点是了解这些放射性核素与微生物和来自英国受污染核设施的沉淀物的相互作用，在氧化还原循环期间，随着还原条件的发展和还原系统的氧化。在地下环境中，微生物控制着氧化还原化学，最近的许多研究强调了微生物可以与氧化还原活性放射性核素相互作用的事实。反过来，这些相互作用可能通过改变它们的氧化还原状态(或形态)来影响TC、U、NP和Pu的环境行为。发生的放射性核素-微生物相互作用可分为两类：1.直接相互作用，即微生物在酶作用下调节放射性核素物种形成的变化；2.间接相互作用，即微生物代谢的减少产物，如Fe(II)或硫化物，可引起物种形成的非生物变化。目前，对微生物与放射性核素相互作用的直接和间接机制以及酶反应和非生物反应之间的平衡如何控制受污染环境中的放射性核素氧化还原循环的了解相对较少。了解这些放射性核素-微生物相互作用的基本机制是本提案的重点，我们将使用三种不同的方法来解决这个问题。由于U和Tc的放射性危害小于Np和Pu，我们将把大部分实验集中在U和Tc上，将我们的Np和Pu工作限制在几个关键系统上。我们将使用地质微生物学技术来研究在地下环境和还原和再氧化系统中发现的关键微生物对Tc、U、Np和Pu的酶转化。此外，我们将从DY和SF采集沉积物，并使用生物地球化学技术来研究放射性核素在复杂沉积环境中随着还原条件的发展以及当我们将沉积物重新氧化时的行为。我们还将使用分子生态学技术来识别控制沉积物生物地球化学的关键微生物。反过来，我们将分离沉积物中参与氧化还原循环的关键微生物的纯培养，并使用这些微生物来研究模型系统中还原和再氧化过程中的放射性核素行为。这些实验将在为了解酶转化而设计的纯培养系统和为检查真实沉积环境中的转化而设计的复杂沉积物实验之间架起桥梁。这些多学科方法将使人们能够在理解放射性核素的氧化还原循环行为方面取得实质性进展，这些放射性核素包括氚、铀、铍和钚。

The UK has a substantial legacy of contaminated nuclear industry sites. Two of the principle sites are at Sellafield and and Dounreay, and the soils and groundwaters at these sites have been contaminated with radioactivity. Indeed, groundwater contamination with radionuclides is a global problem, and decommissioning of nuclear facilities will require control and removal of the contamination. However, little is currently understood of the way that sub-surface microorganisms affect the mobility and behaviour of radionuclides in contaminated land. We have identified four radionuclides that are of particular relevance to radionuclide contamination: technetium; uranium; neptunium; and plutonium. All of these radionuclides are long-lived, and U and Tc are currently priority pollutants at a number of sites throughout the world and are reported as contaminants at Sellafield, whilst Np and Pu will be significant medium term environmental contaminants in radioactive wastes and in contaminated land. Additionally, they are all redox active and are commonly more mobile in their oxidised states when compared to their reduced forms. This project focuses on understanding the interactions of these radionuclides with microorganisms and sediments from contaminated nuclear sites in the UK during 'redox cycling' as reducing conditions develop and as reduced systems are reoxidised. In sub-surface environments, microorganisms control redox chemistry, and many recent studies have highlighted the fact that microorganisms can interact with redox active radionuclides. In turn these interactions may affect the environmental behaviour of Tc, U, Np and Pu by altering their redox state (or speciation). The radionuclide-microbe interactions that occur can be split into two groups: 1. Direct interactions, where the microbe is enzymatically mediating changes in the radionuclide speciation and; 2. indirect interactions, where reduced products of microbial metabolism such as Fe(II), or sulfide can cause abiotic changes in speciation. Currently, there is a relatively poor understanding of both the direct and indirect mechanisms of microbial interactions with radionuclides, and how the balance between enzymatic and abiotic reactions controls radionuclide redox cycling in contaminated environments. Understanding the fundamental mechanisms of these radionuclide-microbe interactions is the focus of this proposal, and we will tackle this problem using three different approaches. As U and Tc are less radiologically hazardous than Np and Pu, we will focus the majority of our experiments on U and Tc, limiting our Np and Pu work to several key systems. We will use geomicrobiology techniques to examine the enzymatic transformations of Tc, U, Np and Pu with key microorganisms found in sub-surface environments and in reducing and reoxidising systems. In addition, we will take sediments from DY and SF, and use biogeochemistry techniques to examine the behaviour of the radionuclides in complex sedimentary environments as reducing conditions develop, and when we reoxidise the sediments. We will also use molecular ecology techniques to identify key microorganisms controlling the biogeochemistry of the sediments. In turn we will isolate pure cultures of key microbes involved in redox cycling in the sediments, and use these microorganisms to investigate radionuclide behaviour during reduction and reoxidation in model systems. These experiments will bridge between the pure culture systems designed to understand enzymatic transformations, and the complex sediment experiments designed to examine transformations in real sedimentary environments. These multidisciplinary approaches will allow substantial advances in understanding the redox cycling behaviour of the radionuclides technetium, uranium, neptunium and plutonium.