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

英国有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敦雷，这些地点的土壤和地下水受到放射性污染。实际上，地下水被放射性核素污染是一个全球性问题，核设施的退役将需要控制和消除污染。然而，目前对地下微生物如何影响受污染土地中放射性核素的流动性和行为知之甚少。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：锝、铀、镎和钚。所有这些放射性核素都是长寿命的，U和Tc目前是世界上许多地点的优先污染物，并在塞拉菲尔德报告为污染物，而Np和Pu将是放射性废物和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，并且当与它们的还原形式相比时，在它们的氧化状态下通常更移动的。该项目的重点是了解这些放射性核素与微生物和沉积物的相互作用，从污染的核场地在英国在“氧化还原循环”作为还原条件的发展和还原系统被再氧化。在地下环境中，微生物控制氧化还原化学，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，这些相互作用可能会影响环境行为的Tc，U，Np和Pu通过改变它们的氧化还原状态（或形态）。发生的放射性核素-微生物相互作用可分为两组：1。直接相互作用，其中微生物是酶介导的放射性核素形态的变化; 2。间接相互作用，其中微生物代谢的还原产物，如Fe（II）或硫化物，可导致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促和非生物反应之间的平衡如何控制污染环境中放射性核素的氧化还原循环的了解相对较少。理解这些放射性核素-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比Np和Pu的放射性危害小，我们将把大部分实验集中在U和Tc上，将Np和Pu的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Tc，U，Np和Pu的酶促转化，其中关键微生物存在于地下环境中以及还原和再氧化系统中。此外，我们将采取沉积物从DY和SF，并使用地球化学技术来检查的行为的放射性核素在复杂的沉积环境中的还原条件的发展，当我们再氧化的沉积物。我们还将使用分子生态学技术来确定控制沉积物地球化学的关键微生物。反过来，我们将分离纯培养的关键微生物参与沉积物中的氧化还原循环，并使用这些微生物研究模型系统中还原和再氧化过程中的放射性核素行为。这些实验将在旨在了解酶转化的纯培养系统和旨在检查真实的沉积环境中的转化的复杂沉积物实验之间架起桥梁。这些多学科方法将使我们在了解放射性核素锝、铀、镎和钚的氧化还原循环行为方面取得重大进展。

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.