Abstract

This thesis presents a comprehensive study of the synthesis, ion-exchange, and storage behaviour of the microporous zirconium silicate umbite, K₂ZrSi₃O₉·H₂O. The investigation focuses on the removal and immobilisation of radionuclides from aqueous nuclear effluents as a long-term storage strategy prior to consolidation into a permanent waste form. Zeotype ion-exchangers are of increasing interest within the nuclear waste management community due to their robustness and selective behaviour towards medium-lived radionuclides, which pose damaging and persistent risks to the environment and living organisms.

The first reported systematic ion-exchange experiments comprising Ce4+, Co2+, and Nd3+, as well as Cs+ and Sr2+, monitored by inductively coupled plasma-mass spectroscopy (ICP-MS), suggest high uptake values for Ce4+ and Cs+. Contrastingly, poor uptake values were found for Co2+ and Nd3+, there was minimal increase in uptake with higher concentrations and longer contact times. The following order of selectivity was discovered: Nd3+ < Sr2+ < Co2+ < Cs+ < Ce4+, which was maintained when protons were added to the ion-exchange experiments. Ce4+ and Nd3+ are used as inactive surrogates for U4/6+ and Pu3/4+ respectively. Competitive ion-exchanges including common water cations (Ca2+, Mg2+, and Na+) found that umbite retains a high selectivity for Cs+ and Sr2+, despite also having a high affinity for Na+. The consequence of the presence of protons in ion-exchanges meant higher uptakes were achieved, due to the protons accelerating exchange mechanisms. Adsorption isotherms found maximum storage capacities of 80-90% for Ce4+ and Cs+, while as high as 60% for Sr2+. Rietveld refinements were performed on X-ray diffraction (XRD) patterns to characterise the new umbite phases found in the samples after ion-exchange; energy dispersive X-ray spectroscopy (EDS) was utilised to supplement ICP-MS data; and scanning electron microscopy (SEM) detailed the umbite particle morphology.

The first storage investigations were set out to understand the degree of leaching over 20-months. Powder and pellet waste forms stored in acidic and basic media at room temperature or 45°C meant many environments were tested. XRD and SEM confirmed the strong structural integrity of umbite after being exposed to acidic and basic media of 0.05 M for 20-months. A high degree of immobilisation was confirmed for Sr2+ and Co2+, especially in pelletised form. Though, ICP-MS measured high Cs+ leaching, most significantly in basic media. As expected, over increasing time, the degree of leaching increased for Cs+ in basic media and Co2+ in acidic media, even from the pelletised samples. Meanwhile, the degree of Sr2+ remained the same throughout suggesting an equilibrium.

Overall, umbite demonstrates promise as an ion-exchange material for Ce4+ and Cs+, while its immobilisation qualities provide a good temporary storage solution for Co2+ and Sr2+ before consolidation.