New adsorbent can remove radioactive cesium ions from nuclear wastewater

New adsorbent can remove radioactive cesium ions from nuclear wastewater
by Staff Writers
Busan, South Korea (SPX) Jun 21, 2023

Nuclear power is typically considered a cleaner way of generating power compared to fossil fuels. It does not release air pollutants and greenhouse gases like carbon dioxide as by-products. However, it creates radiotoxic waste that needs proper treatment to prevent adverse environmental and health conditions.

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 (NPP) 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.

Recently, a team of researchers led by Prof. Kuk Cho from Pusan National University, Korea, found a way to turn this adversity into an advantage. In their breakthrough work, made available online on 16th May 2023 and to be published in Volume 455 of the Journal of Hazardous Materials on 5th August 2023, they have presented potassium calcium thiostannate (KCaSnS), a new layered calcium (Ca2+)-doped chalcogenide ion exchanger. It 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+.

"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 S2- under acidic conditions. This provides a large enough space for Cs+ to reside after its release from the lattice structure," explains Prof. Cho, speaking of the mechanism underlying the action of KCaSnS.

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 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 (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. Remarkably, 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 S2- 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.

Research Report:Leaching of structural Ca2+ ions from a chalcogenide adsorbent by H+ lifts Cs(I) uptake