Scientists at Pacific Northwest National Laboratory (PNNL) and Notre Dame Radiation Laboratory (NDRL) have developed a new computational model of the interactions between solvent molecules and negatively charged ions (anions), particularly those composed of a central atom surrounded by multiple oxygen atoms (oxyanions). Several oxyanions are significant components of the contents of the high-level radioactive waste tanks at the Hanford and Savannah River sites; however, existing models were unable to predict the thermodynamic properties of these species with the accuracy needed for cleanup applications. The PNNL and NDRL scientists found that the errors in these models could be reduced significantly by using a better description of size and geometry of the cluster of solvent molecules surrounding a dissolved oxyanion. For example, in an aqueous solution, the central nitrogen atom in a nitrate ion has a much larger radius and the oxygen atoms much smaller radii than previously assumed. Examination of the electrostatic potential around a dissolved nitrate ion, and of the interactions between the nitrate ion and the surrounding water molecules, showed that the new description is more consistent with the fundamental chemical interactions that govern oxyanion solvation than previous models. The new model can reliably predict the free energy of solvation of several oxyanions of interest in high-level radioactive waste, including perchlorate, formate, nitrate, and nitrite. This information is needed for predicting the evolution the chemistry of the wastes, both during storage in tanks and during treatment and processing. These results were published in the July 31, 2003 issue of the Journal of Physical Chemistry A, which also features a diagram of the new model of the nitrate ion on the cover. This work was supported by the Environmental Management Science Program and made use of the Molecular Science Computing Facility at the William R. Wiley Environmental Molecular Sciences Laboratory at PNNL.

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8/6/03