Aqueous Sulfate Separation by Crystallization of Sulfate–Water Clusters

Abstract

An effective approach to sulfate separation from aqueous solutions is based on the crystallization of extended [SO4(H2O)5(2-)]n sulfate-water clusters with a bis(guanidinium) ligand. The ligand was generated in situ by hydrazone condensation in water, thereby bypassing the need for elaborate syntheses, tedious purifications, and organic solvents. Crystallization of sulfate-water clusters represents an alternative approach to the now established sulfate separation strategies that involve encapsulation of the "naked" anion.

Aqueous Sulfate Separation by Sequestration of [(SO4)2(H2O)4]4− Clusters within Highly Insoluble Imine‐Linked Bis‐Guanidinium Crystals

“…This low solubility implies high stability for the BBIG‐SO 4 crystals. We have proposed that one of the structural factors contributing to the stability of these crystals is the favorable stacking of the planar bis‐iminoguanidinium cations . This proposal is consistent with previous observations that guanidinium cations have a propensity to stack to each other or to aromatic rings .…”

“…The aqueous solubility of the sulfate salt was found to be lower than the corresponding solubilities of the nitrate and chloride analogues, by a factor of about 40 and 4000, respectively. Notably, the solubility of BBIG‐SO 4 is also lower by a factor of 45 than the solubility of the glyoxal‐bis(iminoguanidinium) sulfate salt, previously reported by us . The corresponding solubility product ( K sp ) of BBIG‐SO 4 is 2.4(±0.6)×10 −10 , which is only marginally higher than the K sp of BaSO 4 (1.1×10 −10 ).…”

“…The exceptionally low aqueous solubility of BBIG‐SO 4 is quite unusual for a guanidinium sulfate salt . This low solubility implies high stability for the BBIG‐SO 4 crystals.…”

“…The challenge with anion crystallization from water is to identify anion‐binding ligands that can effectively compete against the strong anion hydration, and that are also able to self‐assemble with the anions of interest into crystals with low aqueous solubility. Along this line, we recently discovered that crystallization of sulfate, in the form of extended [SO 4 (H 2 O) 5 2− ] n clusters, with rigid and planar bis‐guanidinium ligands, can strike a favorable energetic balance that allows for the efficient separation of the highly hydrophilic sulfate anion from water . In this demonstrated prototype, the bis‐guanidinium ligand was synthesized in situ by condensation of glyoxal with aminoguanidinium sulfate, resulting in a sulfate salt with low aqueous solubility ( K sp =3.2×10 −7 ), comparable with that of SrSO 4 .…”

Supramolecular anion recognition in water: synthesis of hydrogen-bonded supramolecular frameworks

“…13 This is perhaps surprising, given its similarity to the thoroughly-explored guanidinium functionality. 14,15 Attracted by the amidinium motif's relatively high p K a , 16 strong hydrogen bonding capability and ease of synthesis, we sought to investigate whether the favourable interaction between this motif and carboxylate anions could be used to prepare self-assembled structures.…”