Development of a Deep Eutectic Solvents-Based Microfluidic System for Highly Efficient Separation of Cesium Ions from Aqueous/Acidic Solutions

The development of a rapid and highly efficient method of the separation of trace amounts of cesium (Cs) ions is of critical importance to the advancement of sustainable analysis of Cs isotopes in the fields of nuclear waste management and environmental monitoring. In this study, we developed a novel microfluidic extraction system using a hydrophobic deep eutectic solvent (H-DES) synthesized by menthol, undecanoic acid, and a BOBCalixC6 (Calix(4)arene-bis(octylbenzo-18-crown-6)) extractant and evaluated its applicability as a separation method for Cs ions for the first time. The micro-extraction experiments demonstrated that extraction equilibrium was reached within 60 s, which was 10 times faster than bulk extraction, achieving the complete separation of Cs ions from the nitric acid phase into the H-DES phase. From the extraction stoichiometry and thermodynamics studies, the microextraction mechanism of Cs ions into the H-DES phase was found to be more entropy-driven and spontaneous, differing from the conventional 1-octanol phase. BOBCalixC6 in the H-DES phase enabled the preferential extraction of Cs ions, even from multielement solutions. Furthermore, nearly 100% micro-back-extraction of Cs ions from the loaded H-DESs to an aqueous phase was achieved within 120 s at 60 °C. This is significantly better than bulk extraction taking 60 min or a 1-octanol system achieving less than 40% micro-back-extraction. The results demonstrate that the H-DES-based microfluidic approach has significant potential as an advanced separation method for the target metal elements.