Multiscale design and regeneration of recyclable hydrophobic deep eutectic solvent for efficient Pb2 + extraction via Cl⁻ coordination

Water contamination with lead (II) (Pb²⁺) poses risks to human health and ecosystems, necessitating sustainable and efficient remediation strategies. This study developed recyclable hydrophobic deep eutectic solvents (HDESs) for high-efficiency Pb²⁺ extraction, integrating COSMO-SAC predictions, density functional theory (DFT), and molecular dynamics (MD) simulations to unravel extraction mechanisms. Four DESs were prepared using Aliquat 336 (hydrogen bond acceptor, HBA) paired with decanoic acid, 1-decanol, DL-menthol, or thymol (hydrogen bond donors, HBDs) at a 1:1 molar ratio. The theoretical analyses revealed that thymol-based HDES exhibited superior binding stability with Pb²⁺ via Cl^- coordination, while maintaining robust HBA-HBD interactions. The experimental optimization identified thymol-Aliquat 336 HDES as the optimal extractant, achieving the extraction efficiency of 98.66 % for Pb²⁺ with an initial concentration of 100 μg/mL in the aqueous solution under the following conditions: T = 298.15 K, pH = 1, HBA:HBD = 1:1 molar ratio, and mass ratio of 1:1. After the 5th regeneration cycle using back-extraction with 1 mol/L HCl, the HDES thymol-Aliquat 336 achieved an extraction efficiency of 74.92 %. FTIR and interaction energy analyses confirmed the reversibility of Cl^--Pb²⁺ coordination and the stability of HBD-HBA hydrogen bonding during regeneration. This work advances DES design by coupling multiscale simulations with process investigation, offering a green, cost-effective solution for Pb²⁺ removal with significant potential for industrial application.