Researchers from the Pusan National University in South Korea have developed a new calcium-doped ion exchanger for the removal of radioactive cesium from acidic nuclear power plant wastewater. The findings have the potential for developing more efficient and effective methods of remediating radioactive contamination.
Led by Kuk Cho, the researchers designed a novel layered potassium calcium thiostannate (KCaSnS), a calcium (Ca2+)-doped chalcogenide ion exchanger. The process overcomes the difficulty in removing cesium-137 from acidic wastewater, where protons (H+) impair the adsorption ability and damage the lattice structure of the adsorbent.
The study, titled “Leaching of structural Ca2+ ions from a chalcogenide adsorbent by H+ lifts Cs(I) uptake,” was recently published in the Journal of Hazardous Materials.
Transformative approach: With the KCaSnS, the typically problematic H+ ions in acidic wastewater enhance the Cs+ adsorption process. Essentially, the Ca2+ ions from KCaSnS are leached out by H+ and Cs+, making way for Cs+.
“Through a transformative approach, the troublesome proton was converted into a functional agent by incorporating Ca2+ into the Sn–S matrix, resulting in a metastable structure,” said Cho. “Moreover, Ca2+ is a harder Lewis acid than Cs+ and can thus leave the lattice easily because of its weaker affinity to the Lewis soft base S2- under acidic conditions. This provides a large enough space for Cs+ to reside after its release from the lattice structure.”
Increased capacity: In the study, the team used the hydrothermal process to synthesize the novel KCaSnS ion-exchange material, which was then used to investigate the adsorption of a nonradioactive isotope of Cs+ in different solutions with pH values ranging from 1 to 13.
The team found that at neutral pH 5.5, the Cs+ adsorption capacity was 370 mg/g, whereas at acidic pH 2, the capacity increased by 68 percent to 620 mg/g. This trend was completely opposite to what previous studies had established.
The researchers attributed this observation to the fact that under neutral conditions, the Ca2+ was leached out only from the interlayers, which accounted for around 20 percent of the total spots available for Cs+ to be adsorbed by the S2- ions in the Sn–S matrix.
In contrast, under highly acidic conditions, nearly 100 percent of Ca2+ ions were leached out from both the interlayer and the backbone structure, allowing more Cs+ ions inside the lattice. Additionally, in all cases, interlayer K+ was involved in the ion exchange.