RADIOELEMENT CHEMISTRY AND GEOCHEMISTRY IN THE CENTRE FOR RADIOCHEMISTRY RESEARCH

The Centre for Radiochemistry Research was established in 1999 as a partnership between BNFL and the University of Manchester. Its focus is on the chemistry of the radioactive elements, with particular emphasis on geochemistry and coordination chemistry. Here, I provide examples of three of our current research areas.

1. Environmental chemistry of artificial radioelements. The authorised, low-level discharges from the BNFL Sellafield reprocessing plant into the Irish Sea have resulted in significant inputs of a range of radioelements. The intertidal sediments of this area provide a rare opportunity to carry out field studies of their migration and transport. ^239,240Pu data can be used to derive a mathematical model of sediment transport, which can be used to estimate unknown historical discharges, for example of ²³⁶U. This isotope is of particular interest since it, alone among the U isotopes, can be used as a fingerprint for irradiated U.
2. Molecular environmental science. The use of laboratory models and advanced spectroscopic analyses, mainly X-ray absorption spectroscopy, allows identification of reaction mechanisms. The mineral mackinawite, tetragonal FeS, is of interest because of its potential redox activity. Pertechnetate is readily reduced to form a TcS₂-like phase and the Tc site within the mineral structure can be identified using theoretical analysis. UO₂²⁺ and NpO₂⁺ are also reduced by FeS but the products are different, with U forming a mixed oxidation state oxide and Np forming a Np(IV) surface complex.
3. Actinide coordination chemistry. A range of actinyl-phosphate-perrhenate (as a pertechnetate analogue) complexes has been synthesised. Perrhenate proves to be a good ligand for UO₂²⁺ ions, with mono- and bidentate coordination, and bridging modes all identified. With NpO₂⁺, the phosphate ligands coordinate to the Np centre while perrhenate acts as a counterion. Using a range of polyoxometalate ligands, a number of different structural motifs have been identified, including similar UO₂²⁺ and NpO₂⁺ complexes, in which a Np-Np interaction can be identified. Electronic spectroscopy of the PuO₂²⁺ analogue in solution suggests a similar interaction.