Elegant double-key mechanism drives ion-exchange selectivity John B. Parise, Stony Brook University, DMR-0452444

Analysis of time resolved diffraction data coupled with molecular dynamics calculations reveal the origin of selectivity towards cesium by silicotitinate compounds, proposed for the clean-up of toxic Cs-137. Exchange occurs in two steps:

1) approach of cesium (yellow) forces water to rotate (curved arrow)

2) repulsion between hydrogen on the rotated water molecule and a hydroxyl group on the framework (straight arrow) rotates framework blocks (purple) leading to more favorable positions for cesium in the framework. Grey atoms and bonds are the positions before, and blue atoms and bonds are the positions after approach of cesium.

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• Background: The family of crystalline titanium silicates (CTS), with sitinakite topology (Na 2Ti2O3SiO4•2H2O) and in the hydrogen-exchanged form (H2Ti2O3SiO4•2H2O) the so-called H-CST form, show remarkable ion-exchange selectivity towards Cs+ and Sr2+. This selectivity might make H-CTS and related materials useful ion-exchangers to replace the in-tank precipitation procedures for treatment of radioactive waste solutions at the many radioactive storage facilities through-out the U.S. We discovered, from measurements carried out on partially ion exchanged H-CST, that the origin of the extra-ordinary selectivity for Cs+ lies in the subtle distortions of the framework and speculated this resulted from Cs-H repulsive interactions during the initial stages of exchange. The mechanism whereby the structure distorts remained unknown. The step-by-step mechanism can be revealed by taking “snap-shots” of the on-going reaction using very bright synchrotron-X-ray sources. Because hydrogen is involved neutron scattering data must also be obtained. These time resolved data, collected with time resolutions of seconds-minutes can be interpretted with the aid of molecular dynamics which allows us to build “movies” of the exchange process based on analysis of the diffraction data.

• The new and exciting result: Recently completed analysis of time resolved X-ray and neutron powder diffraction data, aided by molecular dynamics calculations, reveal the detailed mechanism of cesium exchange involves an elegant chain of repulsive interactions unique to this inorganic ion exchanger. Two repulsive interactions provide keys to unlock two structural changes sequentially before the framework distortion, which opens up a new, preferred, site for cesium exchange within the framework, occurs. 1) Immediately upon introduction of cesium solution to the H-CST solid, a repulsive interaction between the cesium and a water molecule sitting in the cavity of H-CST takes place. The hydrogen-atoms on this water molecule rotate away from this cesium to new positions, indicated by the curved arrow. This movement activates the second key - 2) a repulsive interaction with a hydrogen atom attached to the framework. Movement of this hydrogen triggers a distortion in the framework, rotating blocks of the structure to open up favorable sites for additional cesium exchange.

• Graduate student: Aaron Celestian, completed his PhD this summer and will take a faculty position at CUNY, Queens College, NY.

• Collaborations: James Kubicki (Penn State)• Abe Clearfield (Texas A&M)

Elegant double-key mechanism drives ion-exchange selectivity John B. Parise, Stony Brook University, DMR-0452444