New adsorbent shows promise in helping remove Cs+ from nuclear wastewater

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 S^2- 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 S^2- 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.