How hydrophobic deep eutectic solvents as liquid-liquid extractants remove Pb2+ from wastewater: Material properties, extraction behavior, and molecular-level mechanism simulation?

Abstract

Hydrophobic deep eutectic solvents (HDES) have emerged as green alternatives to traditional organic extractants in liquid-liquid extraction processes. In particular, the natural non-ionic HDES composed of thymol (Thy) and decanoic acid (Dec) has been found to effectively remove Pb²⁺ from water. This work systematically evaluates the physicochemical properties, extraction behavior, and mechanism of this HDES for Pb²⁺ removal from aqueous solutions. The physicochemical properties of HDES, including melting point, viscosity, density, thermal stability, and hydrophobicity, were characterized to ensure its suitability as a recyclable extractant operating at ambient temperature. The effects of key operational parameters (solvent composition, water-to-HDES ratio, pH, and temperature) on Pb²⁺ removal efficiency were extensively investigated, achieving an optimal extraction efficiency of 98.91 %. The reusability of HDES was demonstrated, highlighting its potential for sustainable applications. Innovatively employed molecular dynamics (MD) simulations to elucidate the underlying extraction mechanism, filling the knowledge gap in the mechanism of non-ionic HDES for heavy metal extraction. The results revealed a dual adsorption mechanism: (i) direct adsorption of Pb²⁺ via coordination interactions with carboxyl groups within HDES and (ii) complexation-mediated indirect adsorption, where lead decanoate (PbDec) complexes form in solution and are subsequently captured by HDES through hydrogen bonding and electrostatic interactions. Coordination interactions and hydrogen bonding are the primary forces driving adsorption, electrostatic forces and van der Waals interactions also contribute to adsorption. These findings provide valuable guidance for the rational design of next-generation HDES for heavy metal remediation, contributing to the advancement of sustainable wastewater treatment technologies.