Sulfate Separation from Aqueous Alkaline Solutions by Selective Crystallization of Alkali Metal Coordination Capsules

Self-assembly of a tris(urea) anion receptor with Na2SO4 or K2SO4 yields crystalline capsules held together by coordinating Na+ or K+ 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+, pH 14), thereby providing for the first time a viable approach to sulfate separation from nuclear wastes.