The nuclear disasters of Chernobyl and Fukushima presented an urgent need for finding solutions to treatment of radioactive wastes. Among the by-products of nuclear fission is radioactive Cs-137, which evokes an environmental hazard due to its long half-life (> 30 years) and high solubility in water. In this work, a water-soluble organic ligand, readily obtained from alloxan and 1,3,5-benzenetriol, has been found to selectively bind and precipitate Cs+ ions from aqueous solutions. The special rigid structure of the ligand, which consists of a ``tripodal'' carbonyl base above and below an aromatic plane, contributes to the size-driven selectivity towards the large Cs+ ions and the formation of a giant, insoluble supramolecular complex. In addition to the low costs of the ligand, high yields and effectiveness in precipitating Cs+ ions, the Cs- complex revealed a high endurance to continuous doses of gamma-radiation, increasing its potential to act as a precipitating agent for Cs-137.