Computational chemists in France are closer to understanding a process that enhances the extraction of nuclear waste.

Cesium-137 is a toxic and radioactive waste product from nuclear power generation. It can be removed from the waste by an extraction method that uses a type of compound called a calixarene. This forms a complex with the cesium ion (Cs⁺) which can then dissolve in an organic solvent and be removed.

In nuclear waste, however, the counter ion for Cs⁺ is often a nitrate ion, which does not dissolve well in organic solvents and so Cs⁺ is poorly extracted. The extraction can be improved by adding a co-solvent, known as a 'solvent modifier'. Georges Wipff and Nicolas Sieffert at the University Louis Pasteur in Strasbourg wanted to find out how these modifiers work. 'This is important for basic science, as well as for technological applications,' Wipff explained.

Wipff and Sieffert used a type of computer simulation called molecular dynamics to model systems with a calixarene complex of Cs⁺ and nitrate in chloroform, which was modified by a fluorinated alcohol. They looked at systems that contained water as well, as it would be present in the extraction process. They concluded that the modifier worked by improving how well the nitrate ion interacted with the organic phase and by acting as a surfactant, making it easier for the ions to cross the interface between organic solvent and water.

Jean-Claude Bünzli, an expert in lanthanide supramolecular chemistry from the Swiss Federal Institute of Technology at Lausanne, believes that the research will help chemists to design extraction processes 'in a rational and predictive way'. 'The synergy between theoretical chemistry and experiments therefore leads to a new era in solvent extraction, which will help solve the difficult problem of nuclear waste reprocessing and disposal,' he said.

Rachel Warfield