(466a) Light-Responsive Azobenzocrown Ethers for Enhanced Cs+ Extraction from Contaminated Aqueous Sources

Nuclear fission provides a cheaper and cleaner energy source. However, it also holds a highly hazardous consequence when handled improperly (e.g. Chernobyl Power Plant accident) or disrupted by natural calamities (e.g. Fukushima Daiichi Power Plant earthquake and tsunami incident). The accidental release of radioactive fission products (e.g. ¹³⁷Cs⁺) pose threats to the general population and to the environment by soil and water contamination. Thus, continuous monitoring and separation of these ions are essential. Liquid-liquid extraction (LLE) still serve as a good separation technology for radioactive metal ions removal from contaminated waters. LLE involves solute transport, Cs⁺ in this case, from aqueous to organic solvent phase with the assistance of ionophores and other additives pre-dissolved in the organic phase. Crown ethers are known to be inert to radioactive materials and thus are good candidates for LLE. A greener approach is introduced wherein light is used as an external stimulus to enhance the removal of Cs⁺ contaminants.

Herein, photo-responsive azobenzocrown ethers (a-BCEs) were synthesized and applied as Cs⁺ ligands. The prepared a-BCEs and their intermediates were thoroughly characterized by FTIR, FT-NMR, and UV-vis spectroscopy. Cis-trans isomerization of a-BCEs was assessed by scanning 200-1100 nm wavelength range after UV or visible light irradiation. UV-vis absorption studies confirmed cis-trans photo-switching abilities of the compounds within 1 h of irradiation. UV-irradiated a-BCEs showed cis-content estimates ranging from 42% to 88% while visible light-irradiated samples showed trans-content estimates of more than 98%.

The LLE performance of a-BCEs for Cs⁺ was evaluated by its extraction percentage (%E) and distribution coefficients (D). The a-BCEs alone showed better %E towards Cs⁺ in their cis-forms (8-11%) compared to the trans-isomers (5-9%). D values for both cis and trans a-BCEs were obtained in the range of 0.05 to 0.15 where cis is slightly higher than trans. Addition of diethylhexyl phosphate (DEHPA) to a-BCEs improved the transfer of Cs⁺ from aqueous to organic phase and showed higher %E in the cis-forms (21-22%) than the trans-isomers (15-16%) and DEHPA control (14-16%). D values also improved in the range of 0.15 to 0.30 where cis is slightly higher than trans. The findings indicate a synergistic extraction of Cs⁺ by DEHPA and a-BCEs under the influence of UV light. The a-BCEs show potential application for Cs⁺ removal in aqueous systems.

This study was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science and ICT (2020R1A2C1003560, 2021R1A2C2093746, and 2021R1H1A2008284), Basic Science Research Program through the Ministry of Education (2020R1A6A1A03038817).