Synthesis and application of novel hydroxylated thia-crown ethers as composite ionophores for selective recovery of Ag⁺ from aqueous sources

Selective recovery of silver ions Ag⁺ has been a big challenge due to its difficult separation from complex aqueous feed streams. Herein, four novel highly selective 13- to 19-membered thia-crown ethers (TCEs) were successfully developed by intermolecular cyclization of S- and O-containing intermediates. The TCEs have reactive hydroxyl group(s) for coating on solid supports. To evaluate their ability to capture Ag⁺, the TCEs were coated on polypropylene (PP) membrane (TCE@PP) at high loading (∼280 wt%) via wet-incipient technique with glutaraldehyde acetalization. Adsorption results of all TCE@PP reveal high Ag⁺ capacities with q_e ∼124–179 mg g⁻¹, excellent Ag⁺ selectivities with K_d ∼291–778 L g⁻¹ and rapid uptake rate within 1 h. But DH19-TCE4 or 19TCE@PP is the most effective as it achieved 96% Ag⁺ complexation compared with others ∼53–86% at feed C_o = 1.5 mM. Density functional theory calculations reveal that DH19-TCE4 had the closest cavity size (∅_c = 2.35 Å) with Ag⁺ (∅_Ag+ = 2.30 Å), the most negative binding energy (BE = −65.76 kcal mol⁻¹), and the least cavity distortion during Ag⁺ complexation. All adsorbents are reusable and stable with consistent performance even after five cycles of adsorption/desorption runs. Overall results demonstrate the effectiveness of the synthesis strategies for TCEs and their high potential as adsorbents, especially DH19-TCE4, for selective Ag⁺ recovery from aqueous sources.