A classical force field for tetrahedral oxyanions developed using hydration properties: The examples of pertechnetate (TcO4-) and sulfate (SO42-)

Abstract

Radioactive pertechnetate, 99TcO4-, is one of the most problematic ionic species in the context of the clean up and storage of nuclear waste. Molecular simulations can be used to understand the behavior of TcO4- in dilute aqueous solutions, providing reliable potentials are available. This work outlines the development of a new potential model for TcO 4 - and competing SO42-, optimized using their hydration properties, such as the Gibbs hydration free energy (calculated using Bennett's acceptance ratio method). The findings show that the TcO4- oxyanion has a very low hydration free energy (-202 kJ mol-1) compared to other anions (Cl-, I-, SO42-) leading to fast water exchange dynamics and explaining its observed high mobility in the aqueous environment. Its hydrated structure, investigated using ion-water radial distribution functions, shows that it is unique amongst the other anions in that it does not possess well-defined hydration shells. Since contaminants and ubiquitous species in the aqueous environment are often present as tetrahedral oxyanions, it is proposed that the approach could easily be extended to a whole host of other species.