Aqueous Sulfate Separation by Sequestration of [(SO4)2(H2O)4]4− Clusters within Highly Insoluble Imine-Linked Bis-Guanidinium Crystals
by Radu Custelcean, Neil J. Williams, Charles A. Seipp, Alexander S. Ivanov
Selective crystallization of sulfate with a simple bis-guanidinium ligand, self-assembled in situ from terephthalaldehyde and aminoguanidinium chloride, was employed as an effective way to separate the highly hydrophilic sulfate anion from aqueous solutions. The resulting bis-iminoguanidinium sulfate salt has exceptionally low aqueous solubility (Ksp=2.4×10−10), comparable to that of BaSO4. Single-crystal X-ray diffraction analysis showed the sulfate anions are sequestered as [(SO4)2(H2O)4]4− clusters within the crystals. Variable-temperature solubility measurements indicated the sulfate crystallization is slightly endothermic (ΔHcryst=3.7 kJ mol−1), thus entropy driven. The real-world utility of this crystallization-based approach for sulfate separation was demonstrated by removing up to 99 % of sulfate from seawater in a single step.