Sulfate Separation from Hanford Waste Simulants by Selective Crystallization of Urea-Functionalized Capsules

Crystallization of urea-functionalized capsules self-assembled from a tripodal anion receptor (L1) was evaluated as a means to selectively separate sulfate from aqueous alkaline solutions simulating Hanford waste compositions. The crystallizing solids consist of anionic SO ₄(L1) ^2- ₂ capsules, and Mg(H ₂O) ²⁺ ₆ or Na ₂(H ₂O) ²⁺ ₄ hydrated cations, alternating in three-dimensional frameworks with NaCl-type topology. While both frameworks encapsulate sulfate selectively upon crystallization through the formation of complementary hydrogen bonds from the urea groups, the separation efficacy depends strongly on the nature of the cation, the pH, and the nature and concentration of competing anions in the solution. Crystallization of the Mg-based capsules provides an efficient sulfate separation from mildly alkaline solutions (pH < 9.5), with more basic conditions leading instead to Mg(OH) ₂ and L1 precipitation. On the other hand, crystallization of the Na-based capsules proved efficient from highly alkaline solutions (pH = 14) with compositions similar to those found in the Hanford wastes.