Computational studies of chemistry underpinning the Enhanced Actinide Removal Plant at Sellafield

位于塞拉菲尔德的强化锕系元素去除厂（EARP）是英国最重要的放射性污水处理厂之一。EARP通过与添加NaOH从酸性废水中沉淀的氧化铁絮团结合，从后处理废水中去除锕系元素和选定的裂变产物。从历史上看，EARP处理的放射性废水来自，例如，Magnox后处理工厂(其中添加硫酸亚铁作为还原剂将Pu从U中分离出来(将Pu（VI）还原为Pu(IV)，而将U保留为U(VI))，但是，随着Sellafield场址从当前的后处理操作过渡到操作后清理和加速退役活动，EARP接收的废水成分将发生性质变化。因此，详细了解在EARP中发生的铁（氧）氧化物形成过程，以及这些物种如何与放射性元素相互作用，不仅将支持当前工厂效率的优化，而且将允许更好地预测效率随流出物组成变化的变化。虽然最近取得了重大进展，但关于铁（氧）氧化物形成和锕系元素被除去的过程的许多细节仍不清楚。三种最常见的铁（III）（氧）氧化物相是水合铁、赤铁矿和针铁矿。水合铁在热力学上是亚稳态的，通常是从酸性铁溶液中析出的第一相；它结构不良，呈纳米晶状，被认为是随着EARP pH升高而形成的主要相。2016年，共同监事发现Fe13 Keggin团簇在非常低的pH下形成，并在pH 1,2以上开始聚集，因此与非常酸性的Fe（III）溶液的碱水解形成水合铁有关。最近，共同负责人报告了一项关于水立方铁纳米颗粒形成过程中钚吸附的EXAFS研究，1得出结论，在水立方铁形成过程中，随着pH值的增加，Pu（IV）通过四核内球配合物进行了强烈的吸附，同时注意到“在没有额外信息的情况下，不清楚水立方铁表面Pu（IV）四核配合物的确切性质。”然而，一种可能性是与Fe13凯金单元的“方形”窗口结合。他们还发现，虽然沉淀形成PuO2不是主要的Pu（IV）固存途径，但有证据表明，PuO2是一个次要产物。该博士项目将使用基于密度泛函理论的分子量子化学技术，通过计算研究铁基簇形成的机制，以及这些簇如何在与EARP相关的条件下与锕系元素结合。学生将首先研究Fe13 Keggin形成的过程，从单体Feaq3+，通过亲和和亲和反应，产生多核中间物质。有了这个，工作将继续研究这些多核铁团簇，包括Fe13 Keggin，与锕系元素的相互作用。最初的重点将放在Pu（IV）上，与之前获得的实验数据联系起来。我们将建立实验观察到的最可能的候选内球，四齿物种，以及它形成的机制。我们还将探索生成PuO2小产物的途径。一旦Pu（IV）的工作完成，将研究单核和多核铁簇与其他与EARP相关的锕系元素的相互作用；关键目标是U（VI）、Np（IV）、Np(V)和Am（III）。学生与所有监督团队的定期会议将确保计算与以前和正在进行的实验工作的持续密切联系，并为计算研究的方向提供有价值的观点和背景。参考文献1。[3]王晓峰，张建军，地球化学学报，2019,33(2):444 - 444。JS Weatherill等人，Environ。科学。抛光工艺。浙江农业学报，2016,50,9333-9342

The Enhanced Actinide Removal Plant (EARP), located on the Sellafield site, is one of the UK's most crucial radioactive effluent treatment plants. The EARP removes actinides and selected fission products from reprocessing effluents by association with a ferric iron (oxyhydr)oxide floc, which is precipitated from acidic effluent streams by the addition of NaOH. Historically, the EARP has treated radioactive effluents from, for example, the Magnox reprocessing plant (in which ferrous sulfamate is added as a reductant to separate Pu from U (reducing Pu(VI) to Pu(IV) while leaving U as U(VI)) but, as the Sellafield site transitions from its current reprocessing operations to post-operational clean-out and accelerated decommissioning activities, the effluent compositions that the EARP receives will change in character. Hence, detailed understanding of the iron (oxyhydr)oxide formation processes occurring in the EARP, and how these species interact with radioactive elements, will underpin not only optimisation of current plant efficiency, but will allow better prediction of changes in efficiency as effluent composition varies. While important progress has recently been made,1, 2 many details of the processes by which iron (oxyhydr)oxides form, and actinides are removed, remain unclear.The three most common Fe(III) (oxyhydr)oxide phases are ferrihydrite, hematite and goethite. Ferrihydrite is thermodynamically metastable, and is typically the first phase to precipitate from acidic ferric solutions; it is poorly structured and nanocrystalline, and is believed to be the dominant phase which forms as the EARP pH rises. In 2016, the co supervisors showed that Fe13 Keggin clusters form at very low pH and begin to aggregate above pH 1,2 and hence are implicated in the formation of ferrihydrite from base hydrolysis of very acidic Fe(III) solutions.Very recently, the co supervisors reported an EXAFS study of plutonium sorption during ferrihydrite nanoparticle formation,1 concluding that Pu(IV) strongly adsorbs via a tetradentate inner-sphere complex during the formation of ferrihydrite as the pH increases, while noting that "the exact nature of the Pu(IV) tetranuclear complex on the ferrihydrite surface is unclear without additional information. However, one possibility is bonding to the "square" window of the Fe13 Keggin unit". They also find that, while precipitation to form PuO2 is not the dominant Pu(IV) sequestration pathway, there is evidence that PuO2 is a minor product.This PhD project will study computationally, using molecular quantum chemical techniques based on density functional theory, the mechanisms by which Fe based clusters form, and how those clusters bind actinides, in conditions relevant to the EARP. The student will begin by examining the process of Fe13 Keggin formation from monomeric Feaq3+, via olation and oxolation reactions which create multinuclear intermediate species. With this in hand, work will proceed to study the interactions of these multinuclear Fe clusters, including the Fe13 Keggin, with actinides. Initial focus will be on Pu(IV), to link with the previously obtained experimental data. We will establish the most likely candidate for the experimentally observed inner-sphere, tetradentate species, and the mechanism(s) by which it forms. Pathways to the PuO2 minor product will also be explored. Once the Pu(IV) work is complete, the interactions of mono and multinuclear Fe clusters with other actinides of relevance to the EARP will be studied; key targets are U(VI), Np(IV), Np(V) and Am(III). Regular meetings of the student with all of the supervisory team will ensure continual close linking of the calculations with previous and ongoing experimental work, and provide valuable perspective and context to the direction of the computational research.References1. KF Smith et al., ACS Earth and Space Chemistry, 2019, 3, 2437-24422. JS Weatherill et al., Environ. Sci. Technol., 2016, 50, 9333-9342