How a calcium-doped adsorbent helps deal with nuclear wastewater

Researchers at Korea’s Pusan National University developed a calcium-doped adsorbent that turns unwanted protons in acidic wastewater into agents for facilitating the removal of cesium ions (Cs⁺), a major challenge when it comes to wastewater treatment in nuclear power plants. 

In a paper published in the Journal of Hazardous Materials, the researchers explain that the new material exhibited 68% higher adsorption of Cs⁺ in acidic conditions than in neutral conditions, opening doors to the design of high-performance adsorbents.

According to the scientists, one of the major by-products of the nuclear fission process used for power generation is ¹³⁷Cs (an isotope of cesium), a radioactive element that has a half-life of 30 years and is often removed from nuclear powerplant wastewater via selective adsorption using ion exchangers. However, this process is severely hindered in acidic wastewater where excess protons (H⁺) impair the adsorption ability and damage the lattice structure of the adsorbent.

While looking for solutions to this issue, the Pusan team led by Kuk Cho decided to turn adversity into opportunity and used potassium calcium thiostannate (KCaSnS), a new layered calcium (Ca²⁺)-doped chalcogenide ion exchanger, that utilizes the typically problematic H⁺ ions in acidic wastewater to enhance the cesium ion (Cs⁺) adsorption process. 

Essentially, the Ca²⁺ ions from KCaSnS are leached out by H⁺ and Cs⁺, making way for Cs⁺.

A judo-like approach

“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. 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,” Cho explained. “This provides a large enough space for Cs⁺ to reside after its release from the lattice structure.”

In the study, the team employed a hydrothermal process to synthesize the novel KCaSnS ion-exchange material, which was then used to investigate the adsorption of a non-radioactive isotope of Cs⁺ (to avoid radioactivity exposure) in different solutions with pH values ranging from 1 to 13.

The team found that at pH 5.5 or neutral condition, the Cs⁺ adsorption capacity was 370 mg/g, whereas at pH 2 (strongly acidic), the capacity increased by 68% 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% 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% 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.

These results establish KCaSnS as a promising candidate for the removal of radioactive ions from nuclear wastewater. The insights gained from this study could open up new avenues for the development of high-performance adsorbents for highly acidic environments. 

“The impressive adsorption capacity of KCaSnS can help alleviate the challenges associated with managing radioactive waste by providing a practical solution for reducing the volume of radioactive waste produced during spent fuel reprocessing and decommissioning of nuclear power plants,” Cho noted.