title: Aqueous Sulfate Separation by Sequestration of [(SO4)2(H2O)4]4-Clusters within Highly Insoluble Imine-Linked Bis-Guanidinium Crystals.
abstract: 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.
authors.affiliation: Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831-6119, USA. custelceanr@ornl.gov.
authors.forename: Radu
authors.initials: R
authors.lastname: Custelcean
authors.affiliation: Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831-6119, USA.|Department of Chemistry, The University of Tennessee, Buehler Hall 1420 Circle Dr., Knoxville, TN, 37996-1600, USA.
authors.forename: Neil J
authors.initials: NJ
authors.lastname: Williams
authors.affiliation: Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831-6119, USA.|Department of Chemistry, The University of Texas at Austin, 1 University Station-A5300, Austin, TX, 78712-0165, USA.
authors.forename: Charles A
authors.initials: CA
authors.lastname: Seipp
authors.affiliation: Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831-6119, USA.
authors.forename: Alexander S
authors.initials: AS
authors.lastname: Ivanov
authors.affiliation: Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831-6119, USA.
authors.forename: Vyacheslav S
authors.initials: VS
authors.lastname: Bryantsev
keywords: anions; cluster compounds; crystallization; guanidines; self-assembly