Strategies Based on Calixcrowns for the Detection and Removal of Cesium Ions from Alkali-Containing Solutions Giuseppe Arena,* Annalinda Contino, Antonio Magrı `, Domenico Sciotto, Giuseppe Spoto, and Alberto Torrisi Dipartimento di Scienze Chimiche, Universita ` di Catania, Viale Andrea Doria 6, 95125 Catania, Italy Calixcrowns have proved to be excellent sequestering agents for cesium ions, even in the concurrent presence in solution of massive amounts of other alkali metal ions. This property has rendered such ligands as good candidates for the selective detection/removal of cesium ions from radioactive liquid wastes coming from nuclear processes. Strategies based on the anchoring of such macrocyclic ligands onto silica-gel supports for ion chromatography analysis and on their inclusion into polymeric membranes are summarized here. Methods to correct for contaminants present on the stationary-phase surfaces are briefly outlined. Advantages offered by the methods presented are also highlighted. 1. Introduction 137 Cs + is a strong γ-emitting radionuclide with a half- life of 30.04 years (Figure 1). Its radioactivity renders this isotope harmful to living tissues, and consequently, the presence of 137 Cs + in the environment is to be avoided. 137 Cs + is neither a primordial nor a cosmogenic radionuclide but originates from human activity. In particular, the accumulation of such a radioactive metal ion in the environment has been caused by nuclear weapon tests carried out in the atmosphere until the 1960s, while currently, further accumulation in the environment comes from nuclear reactor wastes. The 30.04-year half-life of this radionuclide allows its entry into the food chain and water supply, and thus, the detection and the selective removal of 137 Cs + are of major importance. A number of different strategies for dealing with these problems have been developed throughout the last 30 years. 1-4 In particular, scientists have been challenged by the detection and removal of cesium from seawater and river water, where this radionuclide concentrates at the nanomolar level. Such a low concentration, coupled with the large concentra- tion of ions such as Na + ,K + , and Mg 2+ usually present in these waters, has represented a significant difficulty to overcome for the selective detection and/or removal of cesium. Among the other methods, organic ion exchangers have been investigated for achieving selective detection as well as preconcentration of cesium from water solutions 1 , while separation methods based on facili- tated transport through liquid membranessmostly sup- ported liquid membranes (SLMs)sincorporating carri- ers that are specific and selective for the radioactive ions have been investigated with the aim of obtaining a selective and nearly complete removal of such radionu- clides from nuclear reactor medium-activity liquid wastes (MALW). 5 The recent synthesis of single- and double-crowned calix[4]arene derivatives in the 1,3-aternate conforma- tions, 6,7 also called calixcrowns, has represented a step forward in the selective detection of cesium ions in water solutions. In fact, such ligands show a significantly higher affinity for cesium ions than for other alkali metal ions. 6 A variety of papers have confirmed such ionoforic properties of calixcrowns (an overview can be found in ref 8) and have provided a range of single- and double-crowned calix[4]arenes derivatives with en- hanced selectivity for cesium over other alkali metal ions (for example, see refs, 9-12). Here, attention will be paid to the recent synthesis and use of calix[4]crown derivatives as selective station- ary phases for HPLC separations as well as to their inclusion into polymeric inclusion membranes (PIMs). 13,14 Both of these applications are aimed at providing new tools for the selective detection and separation of cesium ions from solutions that also contain other alkali metal ions in relatively large concentrations. 2. Calix[4]crown-Derivatized Stationary Phases The covalent chemical bonding of molecular receptors to solid supports has greatly enhanced the importance * Author to whom correspondence should be addressed. E-mail: garena@dipchi.unict.it. Phone: 39-095-738-5071. Fax: 39-095-580-138. Figure 1. Decay scheme of cesium-137. 3605 Ind. Eng. Chem. Res. 2000, 39, 3605-3610 10.1021/ie000220l CCC: $19.00 © 2000 American Chemical Society Published on Web 10/02/2000