Sulfate Separation from Aqueous Alkaline Solutions by Selective Crystallization of Alkali Metal Coordination Capsules RajbanshiArbin MoyerBruce A. CustelceanRadu 2011 Self-assembly of a tris(urea) anion receptor with Na<sub>2</sub>SO<sub>4</sub> or K<sub>2</sub>SO<sub>4</sub> yields crystalline capsules held together by coordinating Na<sup>+</sup> or K<sup>+</sup> cations and hydrogen-bonding water bridges, with the sulfate anions encapsulated inside urea-lined cavities. The sodium-based capsules can be selectively crystallized in excellent yield from highly competitive aqueous alkaline solutions (∼6 M Na<sup>+</sup>, pH 14), thereby providing for the first time a viable approach to sulfate separation from nuclear wastes.