A classical force field for tetrahedral oxyanions developed using hydration properties: The examples of pertechnetate (TcO4(-)) and sulfate (SO4(2-))

• Published in: The Journal of chemical physics 2015-11, Vol.143 (17), p.174502-174502
• Main Authors: Williams, Christopher D, Carbone, Paola
• Format: Article
• Language: English
• Subjects: Anion exchanging; Aqueous environments; Computer simulation; Contaminants; Distribution functions; Free energy; Hydration; Physics; Radial distribution; Radioactive wastes
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.4934964

Radioactive pertechnetate, (99)TcO4(-), 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 TcO4(-) and competing SO4(2-), 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(-), SO4(2-)) 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.