Liam Abrahamsen-Mills

Sb- and Nb-doped Zr and Sn-umbites have been prepared using hydrothermal synthesis with both high purity and yield. All four materials display excellent removal of Cs⁺ cations from acidic, neutral, and basic solutions, abating at least 80% of the Cs present. This performance is retained in the presence of competing Na⁺ cations as well as across the pH range. The most sustained selectivity is observed in acidic media, with evidence of a minor reduction in selectivity under basic conditions. The umbites have successfully been shaped into pellets, introducing macroporosity and retaining the selective uptake of Cs in the presence of excess Na. Through thermal treatment, samples of partially Cs-exchanged umbite can be converted into dense silicate phases where radioactive Cs can be immobilized in a potential wasteform for long term storage. These findings present doped umbites as prospective materials for industrial use with selective abatement properties and capabilities for deployment followed by end of life geological disposal.

Neptunium (²³⁷Np) is an important radionuclide in the nuclear fuel cycle in areas such as effluent treatment and the geodisposal of radioactive waste. Due to neptunium’s redox sensitivity and its tendency to adsorb strongly to mineral phases, such as iron oxides/sulfides, the environmental mobility of Np can be altered significantly by a wide variety of chemical processes. Here, Np interactions with key iron minerals, ferrihydrite (Fe₅O₈H·4H₂O), goethite (α-FeOOH), and mackinawite (FeS), are investigated using X-ray Absorption Spectroscopy (XAS) in order to explore the mobility of neptunyl(V) (Np(V)O₂⁺) moiety in environmental (radioactive waste disposal) and industrial (effluent treatment plant) scenarios. Analysis of the Np L_III-edge X-ray Absorption Near-Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) showed that upon exposure to goethite and ferrihydrite, Np(V) adsorbed to the surface, likely as an inner-sphere complex. Interestingly, analysis showed that only the first two shells (O_ax and O_eq) of the EXAFS could be modelled with a high degree of confidence, and there was no clear indication of Fe or carbonate in the fits. When Np(V)O₂⁺ was added to a mackinawite-containing system, Np(V) was reduced to Np(IV) and formed a nanocrystalline Np(IV)O₂ solid. An analogous experiment was also performed with U(VI)O₂²⁺, and a similar reduction was observed, with U(VI) being reduced to nanocrystalline uraninite (U(IV)O₂). These results highlight that Np(V) may undergo a variety of speciation changes in environmental and engineered systems whilst also highlighting the need for multi-technique approaches to speciation determination for actinyl (for example, Np(V)O₂⁺) species.

⁹⁰Sr sorption to brucite appears enhanced by both humic acid, and organics present in irradiated cyanobacterial supernatant.

Ferrihydrite is the most common iron oxyhydroxide found in soil and is a key sequester of contaminants in the environment. Ferrihydrite formation is also a common component of many treatment processes for cleanup of industrial effluents. Here we characterize ferrihydrite formation during the titration of an acidic ferric nitrate solution with NaOH. In situ SAXS measurements supported by ex situ TEM indicate that initially Fe₁₃ Keggin clusters (radius ∼ 0.45 nm) form in solution at pH 0.12–1.5 and are persistent for at least 18 days. The Fe₁₃ clusters begin to aggregate above ∼ pH 1, initially forming highly linear structures. Above pH ∼ 2 densification of the aggregates occurs in conjunction with precipitation of low molecular weight Fe(III) species (e.g., monomers, dimers) to form mass fractal aggregates of ferrihydrite nanoparticles (∼3 nm) in which the Fe₁₃ Keggin motif is preserved. SAXS analysis indicates the ferrihydrite particles have a core–shell structure consisting of a Keggin center surrounded by a Fe-depleted shell, supporting the surface depleted model of ferrihydrite. Overall, we present the first direct evidence for the role of Fe₁₃ clusters in the pathway of ferrihydrite formation during base hydrolysis, showing clear structural continuity from isolated Fe₁₃ Keggins to the ferrihydrite particle structure. The results have direct relevance to the fundamental understanding of ferrihydrite formation in environmental, engineered, and industrial processes.

Previously, it has been suggested that metal ions complexed to humic acid in the environment might show slower dissociation than those added to humic substances in the laboratory, which has serious implications for the transport of radionuclides in the environment. The dissociation of lanthanide and anthropogenic actinide ions from humic substance complexes has been studied as a function of humic concentration and metal ion:humic concentration ratio. The results suggest that the apparently slower kinetics observed for metal ions complexed in the environment are probably due to the large humic concentrations that are used in those studies. Further, there is no evidence that the dissociation rate constant varies at very low metal ion concentrations. Although humic samples size-fractionated by ultrafiltration showed that more metal may be bound non-exchangeably, there was no evidence for different rate constants. Ultrafiltration of Eu(III)/humic acid mixtures did show a shift in Eu from smaller to larger fractions over a period of two days. Therefore, the results suggest that dissociation rate constants determined in the laboratory at metal ion concentrations higher than those expected in the environment may be used in predicting radionuclide mobility, provided that the humic acid concentration is in the range expected at the site.