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

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.

References
1. KF Smith et al., ACS Earth and Space Chemistry, 2019, 3, 2437-2442
2. JS Weatherill et al., Environ. Sci. Technol., 2016, 50, 9333-9342