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.