Utilising X-ray Tomography to Study Metal Oxide Dissolution in Aqueous Environments

Abstract

Metal oxide dissolution in aqueous environments is a crucial phenomenon in various fields, from the operation of pressurised water reactors to the long-term storage of nuclear waste. Despite its importance, the complex dissolution mechanisms of metal oxides remain poorly understood due to the limitations of conventional experimental methods. This thesis pioneers the use of X-ray tomography to study the real-time dissolution behaviour of metal oxides, focusing on copper(II) oxide (CuO) as a model system. The research begins by developing a reliable method for synthesising CuO specimens and investigating the effects of post-oxidation cooling rate on their composition and morphology. X-ray diffraction, X-ray photoelectron spectroscopy, and electron microscopy techniques reveal the presence of distinct CuO phases (monoclinic and cubic) with markedly different aqueous stabilities. This discovery highlights the critical role of specimen preparation in determining the crystalline structure and dissolution properties of metal oxides. Next, ex situ lab-based X-ray tomography experiments are conducted to study CuO dissolution in EDTA solutions at various pH levels. Novel flow cells are designed and optimised to enable controlled exposure of specimens to the solutions. The resulting tomograms provide valuable insights into the morphological changes during dissolution, including the formation of a low-density colloidal layer and a dissolution-resistant shell. Building upon these findings, in situ synchrotron X-ray tomography experiments are performed to capture the real-time dissolution dynamics of CuO. A custom experimental setup is developed, and advanced data processing techniques, such as convolutional neural networks, are employed for accurate segmentation of the high-resolution tomograms. The results reveal the complex interplay between the dissolution-resistant shell, preferential dissolution along specific crystallographic planes, and the formation of a colloidal gel layer. This thesis makes significant contributions to the field of metal oxide dissolution by establishing X-ray tomography as a powerful tool for studying these processes in unprecedented detail. The insights gained from the ex situ and in situ experiments shed light on the fundamental mechanisms governing CuO dissolution and pave the way for future investigations of more complex oxide systems under reactor-relevant conditions. The developed methodologies and data analysis techniques have broad applicability across various fields, from materials science to drug dissolution studies.

Date of Award	17 Mar 2025
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Fabio Scenini (Supervisor), Philip Withers (Supervisor) & Brian Connolly (Supervisor)