Selective encapsulation of cesium ions using the cyclic peptide moiety of surfactin: Highly efficient removal based on an aqueous giant micellar system

•SF showed high affinity for Cs+ and formed their inclusion complex.•SF selectively encapsulated Cs+ even in the presence of other alkali metal ions.•A highly efficient removal of Cs+ from water using giant SF micelles was achieved through centrifugal ultrafiltration. Cyclic peptide of surfactin (SF) is one of the promising environment-friendly biosurfactants abundantly produced by microorganisms such as Bacillus subtilis. SF is also known to act as an ionophore, wherein alkali metal ions can be trapped in the cyclic peptide. Especially, SF is expected to show high affinity for Cs+ because of the distinctive cavity size and coordination number. In this study, we reported the specific interaction between SF and Cs+ and succeeded in the highly efficient removal of Cs+ from water using giant SF micelles as a natural sorbent. The specific interaction between SF and Cs+ to form their inclusion complex was revealed by nuclear magnetic resonance (NMR) and Fourier transform infrared (FT-IR) spectroscopy. We found that SF micelles selectively encapsulate Cs+, which was suggested by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). A highly effective separation of Cs+ immobilized on the surface of the SF micelles was also achieved through facile centrifugal ultrafiltration in 91% even in coexisting with other alkali metal ions such as Na+ and K+. Thus, the use of the giant micellar system of SF with its high Cs+ affinity and distinctive assembling properties would be a new approach for the treatment of contaminated soil and water.