New remedy for radioactive wastewater
Scientists have discovered a new material which is able to remove caesium and strontium from the wastewater generated by spent fuel rod reprocessing. Peter Minting reports
A number of inorganic molecules including transition metal ferrocyanides, and hydrated polyvalent metal trisilicates are known to be selective caesium binders.
Some crystalline antimonic acids and silico-antimonates are also known to be selective for strontium. Both groups of compounds are thermally and chemically stable in the presence of radioactivity but neither group has a selective affinity for both elements.
New research has resulted in the synthesis of two titanium silicate materials which are capable of binding to caesium and strontium. Like the metal ferrocyanides, trisilicates and antimonates, titanium silicates are crystalline inorganic compounds with an open framework structure. This structure provides sites for radioactive elements to bind to. In the case of the titanium silicates, binding sites are available for both types of cations. The compounds synthesized, hydrous crystalline sodium silicotitanate and sodium titanium silicate (STS) have a high affinity for caesium and strontium ions and according to the researchers: "Could be regarded as promising materials for alkaline nuclear waste remediation."
Titanium silicates could be used to help minimize waste streams which have to be disposed of as hazardous waste, or even remediate contaminated groundwater.
The research teams have now looked at the effectiveness of STS, otherwise known as M2Ti2O3Si4 nH20 (M=H, Na). This was tested on a variety of solutions in the laboratory, including simulated seawater and groundwater.
The process is basically ion exchange, where the caesium and strontium ions are able to displace sodium ions or protons. The STS molecule has tunnels which are the right size for caesium and strontium but too large for smaller ions. This is important because in nuclear waste and the environment sodium, potassium, calcium and magnesium levels exceed those of caesium and strontium by several orders of magnitude.
Affinity of STS for the elements can be expressed as a coefficient, Kd, based on the volume to mass ratio of the components involved and the concentrations of the elements before and after the reaction.
Once bound to the STS, the caesium and strontium could be removed from the solution by, for example, membrane filtration. Not all of the caesium and strontium will bind to the STS and affinity is reduced in both cases with increasing acidity and with increasing concentrations of salts such as calcium chloride. Affinity is hardly affected in alkaline conditions but Kd values for strontium are reduced in solutions contaminated with organic molecules.
According to Dr Sergey Mikhalovsky, a member of the Brighton research team: "Caesium is usually the main problem because it is so mobile and usually more difficult to remove from solution than strontium." Fortunately caesium affinity is not seriously affected by low levels of acidity or solute content, so it may be robust enough for use in a treatment process.
Mikhalovsky said: "We would welcome an opportunity to work with the UK's nuclear industry." But according to Dr Jean Watson of BNFL: "The process already used at Sellafield has been designed to last until the plant is decommissioned." Contaminated water at Sellafield is passed over a sodium aluminosilicate resin before being discharged to the sea. But an STS-based product might be more effective at radioactive element removal