X-ray Studies of Ions at Liquid-liquid Interfaces in Model Systems for Solvent Extraction

Solvent extraction is a primary technology for separating actinide from lanthanide elements in the recycling of used nuclear fuel. The transfer of metal ions from aqueous to organic phases underlies the process of solvent extraction. Ongoing developments of this process are aimed at optimizing the efficiency and kinetics of the separation and recovery of base, rare earth, and precious metals, as well as the reprocessing of spent nuclear fuel and nuclear waste. Although the interaction of metal ions with solutes at the organic-aqueous interface is likely to determine the efficiency and kinetics of extraction, little is known about the mechanism of ion transport across this interface. Numerous challenges are encountered, including those posed by demands of using synchrotron X-rays to probe a deeply buried liquid-liquid interface, the necessity to stabilize the ion distribution at the liquid-liquid interface, and the extraneous signals produced by ions in the neighboring bulk phases. Here I will report how these challenges are addressed as well as the X-ray findings, including an intermediate state at the liquid-liquid interface during forward extraction process of Er(III), Y(III) and Sr(II), and ion distribution at the liquid-liquid interface for the backward extraction process of Eu(III). I will also report the advances in the X-ray technique used in this study.