Researchers at the US Department of Energy’s Sandia National Laboratories have created a new class of molecular cage that removes certain radioactive chemicals from hazardous waste whilst keeping other chemicals out.
The new microporous materials, named Sandia Octahedral Molecular Sieves (SOMS) could be useful in microelectronics fabrication and other industries where purification of, or extraction from, liquid process or waste streams is a significant or costly problem.
Chemically, SOMS can be likened to a sponge that draws in divalent cations into its microscopic pores and snares them at negatively charged bonding sites that have been vacated by ions with weaker charges.
The SOMS are selective about which ions they capture because the sizes of openings on their crystalline surfaces can be adjusted precisely by altering the recipes followed to make them. By varying these openings from 4 to 15 angstroms, the researchers are able to select the sizes of ions or molecules that can and cannot get into the pores.
‘Not only are SOMS fascinating as a new material,’ said Sandia principal investigator Tina M Nenoff, ‘they possess many unique properties that are useful in waste cleanup and industrial processing.’
The new SOMS are said to be extremely sensitive to strontium-90, one of the two most prevalent radioactive constituents of liquid hazardous waste inside the 177 underground storage tanks at the US Department of Energy’s Hanford environmental remediation site.
In lab tests the SOMS are said to have trapped 99.8 percent of strontium-90 ions in parts-per-million concentrations from solutions containing chemically similar and highly abundant sodium ions.
‘We can tune the pore size and the chemistry of the framework on the nano scale so the SOMS materials capture cations on the bulk scale very selectively and efficiently, and in all types of environments,’ said Nenoff.
When heated to about 500C°, the SOMS collapse into a dense glass-like material called perovskite, and its shrunken pores lock the cations tightly into its crystalline structure.
Bricks made from the dense SOMS are said to be impervious to leaching and stable against high pH, radiation, and heat, which might make them ideal for disposal.
‘This unique, unexpected property gives SOMS the added bonus of being ready for a waste repository or landfill after only minimal processing,’ said Nenoff
Liquid waste or processing solutions could be pumped through columns containing the SOMS, she said. When the SOMS became fully saturated with the desired cations, they could be made more dense and disposed of safely.
Technically SOMS are sodium niobium oxide with transition metals such as titanium or zirconium added to give the SOMS their microporosity and ion exchange properties.
Sandia researchers are already studying ways to use SOMS to extract and reuse cobalt from copper-mine electro-refinement waste streams.