Silicate mystery cleared up

Australian researchers have shed light on the role of silicon in nanocrystalline materials that can selectively extract ions from radioactive waste.

Radionuclides such as ¹³⁷Cs and ⁹⁰Sr account for only a small proportion of the volume of older waste, but are responsible for most of the radioactivity. Therefore, selective extraction by a small volume of solid matrix would reduce waste volumes and simplify waste disposal.

Vittorio Luca and co-workers at the Australian Nuclear Science and Technology Organisation have now uncovered the role of silicon in removing radioactive ions.

It has been known for some time that antimony pyrochlores, M₂Sb₂O₆. nH₂O, when modified by adding silicate (SiO₄^4-), can selectively remove Cs⁺ (and to a lesser extent, Sr²⁺) from mildly acidic solutions. However, the poorly crystalline nature of these materials has so far prevented their structures being determined.

Using a series of diffraction, NMR and thermogravimetric experiments, Luca’s team found that antimony pyrochlores doped with silicate contain silicon, not in lattice sites or on the surface of the nanoparticles, but in the hexagonal tunnels in the pyrochlore structure. They identified the silicon species as Si(OH)₄, which is usually unstable, but can exist in the confined environment of the tunnels. The presence of Si(OH)₄ results in a slight decrease in the tunnel dimensions, which is thought to be the reason behind the selectivity for Cs⁺ and Sr²⁺.

These results should help to develop new materials with enhanced extraction properties and will be important in the nuclear industry.

David Barden