Identification of number and type of cations in water-soluble Cs⁺ and Na⁺ calix[4]arene-bis-crown-6 complexes by using ESI-TOF-MS

The treatment of cesium-contaminated wastewater has become one of the biggest issues. The selective Cs⁺ removal from wastewater containing competitive alkali metal ions such as Na⁺ is desired to reduce the volume of sludge. Therefore, the present work focused on water-soluble calix[4]arene-bis-crown-6 (W-BisC6) to selectively capture Cs⁺. For characterization of the complex, UV–vis spectroscopy is commonly used, however, due to the limited availability of information it can be hard to quickly identify the specific structures of some complexes. In this work, the electrospray ionization time of flight spectrometry (ESI-TOF-MS) is successfully utilized to identify the number and type of cations in W-BisC6-cation complexes. ESI-TOF-MS accurately recognized 4 types of complex (W-BisC6–Na⁺, W-BisC6–Cs⁺, W-BisC6–2Na⁺, W-BisC6–Na⁺–Cs⁺), and the experimental and simulated results were almost perfectly matched. It also revealed the difficulty of W-BisC6–2Cs⁺ complex formation under the present conditions. Thus, this technique is significantly helpful for rapid identification of the specific structures of complexes during Cs⁺-contaminated wastewater treatment.