Published: June 01, 2011 r2011 American Chemical Society 2702 dx.doi.org/10.1021/cg200515w | Cryst. Growth Des. 2011, 11, 2702–2706 COMMUNICATION pubs.acs.org/crystal Sulfate Separation from Aqueous Alkaline Solutions by Selective Crystallization of Alkali Metal Coordination Capsules Arbin Rajbanshi, Bruce A. Moyer, and Radu Custelcean* Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States b S Supporting Information R esearch in the area of supramolecular chemistry of anions 1 started mainly as an academic endeavor aimed at under- standing the underlying principles of anion coordination, but more recently it has been increasingly driven by the need to find solutions to utilitarian problems, generally related to health, environment, or energy. While the fundamental aspects of anion recognition are most often investigated in organic solvent media, real-world problems typically involve much more competitive aqueous environments. 2 One such problem of special interest to us is sulfate separation from radioactive wastes. Sulfate is a problematic component of legacy nuclear wastes, particularly those in the U.S. Department of Energy (DOE) complex, as it interferes with the vitrification process selected for waste disposal, increases the volume of waste forms that must be produced and stored, and reduces their geologic performance. 3 Sulfate removal from aqueous solutions is already challenging enough due to the very strong hydration of this anion (ΔG° h = 1080 kJ/mol). 4 The extreme ionic strength (>6 M) and alkalinity (pH 14) of the waste, as well as the high concentrations of competing anions (mainly NO 3  , NO 2  , OH  , and CO 3 2 ), 5 further increase the complex- ity of the problem. Though examples of sulfate selective receptors have been previously reported, 6 none of them has been demon- strated to work in a viable binding-release cycle under the extremely demanding conditions found in nuclear wastes. A promising approach to effective sulfate recognition and separation from competitive aqueous environments is to take inspiration from Nature’s sulfate-binding protein 7 and completely isolate the anion from the surrounding solvent by encapsulation inside structurally constrained cavities functionalized with comple- mentary binding groups. Though such cryptand-like architectures 8 are often difficult to assemble via traditional organic synthesis, a more practical approach to cage receptors for anions via self- assembly from relatively simple building units has been recently demonstrated. 9 Alternatively, self-assembly of crystalline solids that selectively include targeted anions upon crystallization can be effectively employed for anion separation. 10,11 A distinct advantage of such crystalline “hosts” is that the stiffer environment inside crystals may prevent the structural distortion of the anion-binding cavities and accommodation of competing anions, resulting in superior selectivity. We have recently reported that two molecules of the tripodal ligand L1, consisting of a urea-functionalized tren scaffold, 12 self- assemble with Mg(H 2 O) 6 2þ cations and encapsulate sulfate upon crystallization from competitive aqueous solutions. 13 The rigid and highly complementary binding cavities of these crystalline cap- sules, comprising 12 urea hydrogen bonds to the sulfate, ensured exceptional sulfate selectivity based on shape, size, and charge discrimination. However, in spite of their excellent anion recogni- tion abilities, these capsules have limited utility for sulfate separa- tion from alkaline nuclear wastes, as they do not form under basic conditions (pH > 10) due to Mg(OH) 2 precipitation. We subse- quently made the argument that if similar capsules could be selectively crystallized with alkali metal cations instead, 14 which are tolerant to highly basic conditions, it would potentially provide a viable solution to the problem of sulfate separation from nuclear waste. A Na-based system, in particular, could take advantage of the abundance of sodium ions in the waste, which not only would circumvent the need for adding external ionic components to the waste but also would significantly decrease the solubility of the Received: April 22, 2011 ABSTRACT: Self-assembly of a tris(urea) anion receptor with Na 2 SO 4 or K 2 SO 4 yields crystalline capsules held together by coordinating Na þ or K þ 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 þ , pH 14), thereby providing for the first time a viable approach to sulfate separation from nuclear wastes.