One of the major byproducts of the nuclear fission process used for power generation is 137Cs, a radioactive isotope of Cs that has a half-life of 30 years and is often removed from nuclear-power-plant wastewater via selective adsorption using ion exchangers. However, this process is severely hindered in acidic wastewater where excess protons impair the adsorption ability and damage the lattice structure of the adsorbent.
Now, researchers from Pusan National University (Busan, South Korea, www.pusan.ac.kr) found a way to turn this adversity into an advantage. In their work, to be published in the August issue of the Journal of Hazardous Materials, they introduce potassium calcium thiostannate (KCaSnS), a new layered Ca2+-doped chalcogenide ion exchanger. It utilizes the typically problematic H + ions in acidic wastewater to enhance the adsorption of Cs+. Essentially, the Ca2+ ions from KCaSnS are leached out by H+ and Cs+ ions, making way for Cs+ ions.
“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 Kuk Cho, professor at the Department of Civil and Environmental Engineering.
In the study, the team used a hydrothermal process to synthesize the 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, the Cs+ ion-adsorption capacity was 370 mg/g, whereas at pH2, the capacity increased by 68% to 620 mg/g. Remarkably, this trend was completely opposite to what previous studies had established.