’This is one of the first attempts to use a MOF for iodine capture,’ said chemist Tina Nenoff of Sandia’s Surface and Interface Sciences Department.
The process reduces the volume of high-level wastes, which is said to be a key concern of the Sandia researchers. ’The goal is to find a methodology for highly selective separations that result in less waste being interred,’ said Nenoff.
Part of the challenge of reprocessing is to separate and isolate radioactive components that can’t be utilised as fuel. The Sandia team focused on removing iodine from spent fuel.
It studied known materials, including silver-loaded zeolite — a crystalline, porous mineral with regular pore openings, high surface area and high mechanical, thermal and chemical stability. Various zeolite frameworks can trap and remove iodine from a stream of spent nuclear fuel, but need added silver to work well.
’Silver attracts iodine to form silver iodide,’ Nenoff said. ’The zeolite holds the silver in its pores and then reacts with iodine to trap silver iodide.’
But silver is expensive and poses environmental problems, so the team set out to engineer materials without silver that would work like zeolites but have a higher capacity for the gas molecules. It explored why and how zeolite absorbs iodine, and used the critical components discovered to find the best MOF, named ZIF-8.
’We investigated the structural properties on how they work and translated that into new and improved materials,’ said Nenoff.
MOFs are crystalline, porous materials in which a metal centre is bound to organic molecules by mild self-assembly chemical synthesis. The choice of metal and organic results in a very specific final framework.
The trick was to find a MOF that was highly selective for iodine. The Sandia researchers took the best elements of the zeolite Mordenite — its pores, high surface area, stability and chemical absorption — and identified a MOF that can separate one molecule, in this case iodine, from a stream of molecules. The MOF and pore-trapped iodine gas can then be incorporated into glass waste for long-term storage.
The Sandia team is also said to have fabricated MOFs, made of commercially available products, into durable pellets. The as-made MOF is a white powder with a tendency to blow around. The pellets provide a stable form to use without loss of surface area, Nenoff said.
Sandia has applied for a patent on the pellet technology, which is expected to have commercial applications.