An effective approach to sulfate separation from aqueous solutions was developed based on crystallization of sulfate-water clusters with a simple ligand self-assembled in situ from water-soluble subcomponents.
With a free energy of hydration of –1080 kJ/mol, sulfate is one of the most hydrophilic anions found in nature. The extreme water affinity of sulfate originates from its high charge density and its ability to accept multiple hydrogen bonds from water. As a result of its strongly hydrated structure, sulfate is difficult to separate effectively and selectively from aqueous solutions, especially from mixtures containing less hydrophilic anions such as nitrate or perchlorate.
An effective approach to sulfate separation from aqueous solutions was demonstrated in this work based on crystallization of extended sulfate-water clusters with a hydrazone-linked bis(guanidinium) ligand. The ligand was generated in situ by hydrazone condensation in water, thereby bypassing the need for elaborate synthesis, tedious purifications, and utilization of organic solvents. Crystallization of sulfate-water clusters represents an alternative approach to the now established sulfate separation strategy involving encapsulation of the ‘naked’ anion, offering such advantages as reduced thermodynamic penalty associated with anion dehydration and enhanced selectivity due to specific recognition of the clusters in the crystalline state.
This crystallization-based approach to sulfate separation may find applications in nuclear wastes cleanup where sulfate interferes with the vitrification of the waste and increases the volume of the waste forms, or in oil field injection operations where sulfate may create scale problems.