This renewal proposal outlines our current progress and future research plans for ion exchangers: novel metal niobate and silicotitanate ion exchangers and their ultimate deployment in the U.S. Department of Energy (DOE) complex.  In our original study, several forms (including Cs exchanged) of the heat-treated Crystalline Silicotitanates (CSTs) were fully characterized by a combination of high-temperature synthesis and phase identification, low-temperature synthesis and phase identification, and thermodynamics.  This renewal proposal is predicated on work completed in our current EMSP program: we have shown preliminary data of a novel class of niobate-based molecular sieves (Na/Nb/M/O, M = transition metals) that show exceptionally high selectivity for divalent cations under extreme conditions (acid solutions, competing cations) and novel silicotitanate phases that are also selective for divalent cations.  Furthermore, these materials are easily converted by a high-temperature in situ heat treatment into a refractory ceramic waste form with low cation leachability.  The new niobate-based waste form is a perovskite phase, which is also a major component of Synroc, a titanate ceramic waste form used for sequestration of high-level wastes (HLW) from reprocessed, spent nuclear fuel. These new niobate ion exchangers also showed orders of magnitude better selectivity for Sr²⁺ under acid conditions than any other material. 

The goal of the program is to provide DOE with alternative materials that can exceed the performance of monosodium titanate (MST) for strontium and actinide removal at the Savannah River Site (SRS), remove strontium from acidic waste at Idaho National Engineering and Environmental Laboratory (INEEL), and sequester divalent cations from contaminated groundwater and soil plume.   This program will provide DOE with materials for near-term, lab-bench simulant testing and eventual deployment.  The proposed work will provide information on the structure/property relationship between ion exchanger frameworks and selectivity for specific ions, allowing for the eventual "tuning" of framework for specific ion exchange needs.  To date, DOE sites have become interested in onsite testing of these materials; ongoing discussions and initial experiments are occurring with Dr. Dean Peterman, INEEL (location of the DOE/EM Waste Treatment Focus Area) and Dr. John Harbour, SRS.  Yet the materials have not been optimized, and further research and development of the novel ion exchangers and testing conditions with simulants are greatly needed. In addition, studies of the ion exchanger composition versus ion selectivity, ion exchange capacity, and durability of final waste form are needed.

Team: Tina Nenoff, Mark Phillips, May Nyman, Emily Michaels, SNL; Yali Su, M. Lou Balmer, PNNL; Alexandra Navrotsky, Hongxu Wu, UC Davis

Funding: DOE/EMSP, FY99-03