Selective separation of aqueous sulphate anions via crystallization of sulphate–water clusters

Abstract

In this study, the potential of oxoanion separation with the ligands N-4-methylbenzyl-4-amino-1,2,4-triazole (L₁), N-4-fluorobenzyl-4-amino-1,2,4-triazole (L₂), N-4-hydroxy benzyl-4-amino-1,2,4-triazole (L₃), and N-3-methoxy-4-hydroxybenzyl-4-amino-1,2,4-triazole (L₄) was explored. The salts of sulphate, nitrate, and perchlorate were crystallized in the form of [HL₁]⁺[HSO₄]⁻·H₂O, [HL₂]⁺[HSO₄]⁻·H₂O, 2[HL₃]⁺[SO₄]²⁻·2(H₂O), and 2[HL₄]⁺[SO₄]²⁻·H₂O, [HL₃]⁺[NO₃]⁻·H₂O and [HL₄]⁺[NO₃]⁻, and [HL₃]⁺[ClO₄]⁻ and [HL₄]⁺[ClO₄]⁻, respectively. Competitive crystallization and competitive sulphate-binding experiments demonstrated special recognition of the sulphate–water clusters (in the forms of [HSO₄⁻·H₂O]_n or [SO₄²⁻·H₂O]_n) via the parallel packed 3-D architectures of HL₁⁺, HL₂⁺, and HL₄⁺ cations, which were far more effective and selective than the non-3-D structure of HL₃⁺ cation. The observed selectivity for sulphate, nitrate, and perchlorate was found to be anti-Hofmeister. In addition, the selective separation of sulphate anions in seawater was investigated, which demonstrated the real-world application of the present strategy.