Hydrotalcite Colloidal Stability and Interactions with Uranium(VI) at Neutral to Alkaline pH

Chris Foster; Samuel Shaw; Thomas S Neill; Nick Bryan; Nick Sherriff; Louise S Natrajan; Hannah Wilson; Laura Lopez-Odriozola; Bruce Rigby; Sarah J Haigh; Yi-Chao Zou; Robert Harrison; Katherine Morris

doi:10.1021/acs.langmuir.1c03179

Hydrotalcite Colloidal Stability and Interactions with Uranium(VI) at Neutral to Alkaline pH

Langmuir. 2022 Mar 1;38(8):2576-2589. doi: 10.1021/acs.langmuir.1c03179. Epub 2022 Feb 15.

Authors

Affiliations

¹ Research Centre for Radwaste Disposal and Williamson Research Centre, Department of Earth & Environmental Sciences, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K.
² National Nuclear Laboratory, Chadwick House, Warrington Road, Birchwood Park, Warrington WA3 6AE, U.K.
³ Centre for Radiochemistry Research, Department of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K.
⁴ Sellafield Ltd., Hinton House, Birchwood Park Avenue, Risley, Warrington, Cheshire WA3 6GR, U.K.
⁵ Department of Materials, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K.
⁶ Nuclear Fuel Centre of Excellence, Department of Mechanical, Aerospace and Civil Engineering, The University of Manchester, Sackville Street, Manchester M13 9PL, U.K.

Abstract

In the United Kingdom, decommissioning of legacy spent fuel storage facilities involves the retrieval of radioactive sludges that have formed as a result of corrosion of Magnox nuclear fuel. Retrieval of sludges may re-suspend a colloidal fraction of the sludge, thereby potentially enhancing the mobility of radionuclides including uranium. The colloidal properties of the layered double hydroxide (LDH) phase hydrotalcite, a key product of Magnox fuel corrosion, and its interactions with U(VI) are of interest. This is because colloidal hydrotalcite is a potential transport vector for U(VI) under the neutral-to-alkaline conditions characteristic of the legacy storage facilities and other nuclear decommissioning scenarios. Here, a multi-technique approach was used to investigate the colloidal stability of hydrotalcite and the U(VI) sorption mechanism(s) across pH 7-11.5 and with variable U(VI) surface loadings (0.01-1 wt %). Overall, hydrotalcite was found to form stable colloidal suspensions between pH 7 and 11.5, with some evidence for Mg²⁺ leaching from hydrotalcite colloids at pH ≤ 9. For systems with U present, >98% of U(VI) was removed from the solution in the presence of hydrotalcite, regardless of pH and U loading, although the sorption mode was affected by both pH and U concentrations. Under alkaline conditions, U(VI) surface precipitates formed on the colloidal hydrotalcite nanoparticle surface. Under more circumneutral conditions, Mg²⁺ leaching from hydrotalcite and more facile exchange of interlayer carbonate with the surrounding solution led to the formation of uranyl carbonate species (e.g., Mg(UO₂(CO₃)₃)^2-_(aq)). Both X-ray absorption spectroscopy (XAS) and luminescence analysis confirmed that these negatively charged species sorbed as both outer- and inner-sphere tertiary complexes on the hydrotalcite surface. These results demonstrate that hydrotalcite can form pseudo-colloids with U(VI) under a wide range of pH conditions and have clear implications for understanding the uranium behavior in environments where hydrotalcite and other LDHs may be present.