Selective Removal of Sulfate from Water by Precipitation with a Rigid Bis-amidinium Compound

A simple, readily prepared biphenyl bis-amidinium compound (1⋅Cl₂) is able to selectively precipitate sulfate from water. The precipitant is effective at concentrations as low as 1 mM and shows complete selectivity against monovalent anions, and high selectivity even against CO₃²⁻ and HPO₄²⁻. It is highly effective (>90 % sulfate removed) in both seawater and highly acidic conditions relevant to mining waste-streams. X-ray crystallography reveals that 1⋅SO₄ forms a tightly packed, anhydrous, structure where each sulfate anion receives eight hydrogen bonds from amidinium N−H hydrogen bond donors.