Molecular Modeling of Cation and Water Adsorption in Crystalline Titanosilicate and Polyoxoniobate Materials

Permanent disposal of high-level nuclear waste requires efficient separation of trace amounts of various radioactive elements from alkaline waste solutions in order to minimize the final disposal volume required for vitrification. Development of highly selective ion exchangers with increased capacity and kinetics is desired for a more cost-effective separation process. In collaboration with researchers at Savannah River National Laboratories, Sandia National Laboratories, and Texas A&M University, various classes of high-capacity ion exchangers, such as crystalline titanosilicates (CST), pharmacosiderates, and polyoxoniobates are under investigation to enhance their ion exchange performance. Molecular simulations have the potential to be a valuable design tool in guiding future synthesis efforts. The accuracy of molecular simulations is critically dependent upon the quality of the intermolecular force field used to describe the interactions. Due to the large number of experimental x-ray diffraction studies that have characterized both materials, a semi-empirical intermolecular force field potential model is developed for crystalline titanosilicates and polyoxoniobates materials. In addition, ab initio methods, such as density functional theory (DFT) simulations, are investigated to provide additional insight into the electronic structure of these materials and improve upon the force field. Once a suitable potential model is obtained, molecular dynamics and Monte Carlo simulations are applied to predict preferred sorbent sites for cations and water molecules in various cation exchanged structures to supplement experimental x-ray diffraction studies. In particular, this study focuses on understanding the interplay between cations and water molecules in these ion exchangers. It has been amply demonstrated that water can significantly alter ion exchange performance due to its interaction with the exchangeable cations. To this end, replica exchange Monte Carlo simulations as well as grand canonical Monte Carlo simulations are conducted to better understand sorptive properties of the system, such as adsorption isotherms and heats. The computed properties compare well with existing experimental measurements, giving indication that the physics of each system is adequately described through the intermolecular potential model. Given that an accurate potential model has been developed, the ultimate goal is to utilize molecular modeling techniques to understand what controls the selectivity for certain ions and to determine if this selectivity can be enhanced through structural modification of the materials.