Willem Verboom, associate professor of organic chemistry at the University of Twente, The Netherlands, considers some of the latest advances in separating highly radioactive components of nuclear waste

 

Nuclear energy, a sustainable energy source with a low emission rate of greenhouse gasses, is receiving a renewed interest. Clearly, a key problem that faces the nuclear energy industry is the radioactivity of its waste and the environmental risks associated with it. 

Decades of research have been devoted to finding a suitable treatment of nuclear waste to minimise its hazard risks. One of the results of this research has been the development and industrialisation of the PUREX process (plutonium and uranium recovery by extraction). This is used to recycle uranium and plutonium, two of the main contributors to the radiotoxicity of the waste from spent nuclear fuel. Treating nuclear waste in this way reduces its radiotoxicity, but it still remains radioactive for thousands of years. Major contributors to the long radioactivity time span of the remaining waste are the minor actinide elements - mainly neptunium, americium, and curium - and fission products. The main goal of the treatment of this waste is to significantly decrease its volume and radiotoxicity by isolating these elements and then transforming them into less toxic elements. This is known as the partitioning and transmutation concept. 

Solvent extraction is one of the techniques under development for the separation of the minor actinides from nuclear waste. In this technique, the metal is selectively extracted from an aqueous solution of the waste into an organic phase using an extracting molecule or ligand. 

 

Multicoordinate ligands can be used to extract harmful components from nuclear waste

 

Over the years many ligands have been prepared providing greater insight into the theoretical aspects of the ligand-actinide binding. One important factor is the softness of the coordinating atoms; softer (less electronically dense) atoms than oxygen tend to bind more strongly with minor actinides than with lanthanides, which are a series of less harmful elements present in large excess in nuclear waste. The preorganisation of ligands, so that they more closely resemble the structure of the metal-ligand complex, greatly improves the extraction properties. 

Preorganisation can be achieved by connecting the ligands to a molecular platform or scaffold; the large structure formed in this way is known as a multicoordinate ligand. Multicoordinate ligands offer several benefits over normal ligands: they can encapsulate the metal (providing a lipophilic skin); they can easily attain a coordination number equal to that of the metal ion, which results in better binding; and they allow relatively simple attachment to solid supports or lipophilic anions. 

Finding the ideal multicoordinate ligand relies on fine tuning the balance between the conformation of the platform and the type of coordinating atoms. This approach has already resulted in several multicoordinate ligands that efficiently and selectively extract minor actinides at very high acidities (nitric acid is a component of nuclear waste). However, there is still a strong need for simpler and even more efficient multicoordinate ligands to tackle the worldwide problem of nuclear waste. 

Read Willem Verboom's review on 'Multicoordinate ligands for actinide/lanthanide separations' in issue 2, 2007 of Chem. Soc. Rev.