By squeezing a porous solid, scientists surprisingly made its cavities open wider, letting in – and trapping – europium ions. Given the similarities between europium and uranium ions, the team, based at the University of South Carolina, Yonsei University (Korea), and Stanford University, thinks the innovation could represent a promising new avenue for nuclear waste processing.
The focus of their work is natrolite, one of the many examples of aluminosilicate minerals called zeolites, which contain tiny, regularly spaced pores. Zeolites come in more than a hundred different forms, and the composition of each variant determines the size of the cavity and thus the kinds of molecules and ions that can be retained, or excluded, within.
As a result, zeolites can separate and sort chemical species: Added to a solution containing a mixture of ions, they can selectively retain only those ions that can fit within the pores.
The authors are building on a series of studies that demonstrate how to assert control over the kinds of guests that zeolites will hold within their cavities. The team uses a stimulus that is seldom used to control cavity size: pressure.
Working with natrolite, a natural zeolite with a 2:3:10 ratio of Al:Si:O in the framework, the team reported in Angewandte Chemie that they managed to coax trivalent Eu3+ ions to exchange with K+ ions within the material's nanoscale cavities. The immobilized ions were then trapped within after the pressure was removed.
"With natrolite, people have always said you can't get Eu3+ in there. But under pressure, you can," said Thomas Vogt, one of the co-authors and a professor in the department of chemistry and biochemistry in the College of Arts and Sciences at the University of South Carolina.
The result is counter-intuitive in that the pressure – applied hydrostatically in a diamond-anvil cell and typically exceeding 1 GPa (more than 10,000 atm) – can cause the cavities within natrolite to expand in volume. This auxetic behavior essentially opens a window for larger ions to migrate within the pores, and then they remained trapped there after the pressure is released, said Vogt.
The exchange of europium ions shows promise for nuclear waste processing. "The Eu3+ radius is 108.7 picometers, which is close to the 103 picometers of U4+," Vogt said. "And we've demonstrated we can exchange Eu3+ for K+ – the aliovalent exchange replaces 90% of the potassium."
Beyond that, Vogt thinks studying the behavior of natrolite under pressure could afford insight for researchers working to better understand the workings within the earth's crust. "We've developed the picture of how applying pressure leads to this non-intuitive volume increase and even uptake of water," he said. "And these are common materials in the earth's crust. Mineralogy, tectonics, even fracking – there are a lot of areas where the results could be of interest."
