Decontamination of enoxacin containing aqueous phase through hydrophobic deep eutectic solvents: Solvent regeneration and quantum chemical insights

The current work addresses the extraction of a drug (enoxacin) from an aqueous environment by liquid-liquid extraction using three hydrophobic deep eutectic solvents (DES) namely, (a) dl-menthol: decanoic acid (1:1), (b) dl-menthol: dodecanoic acid (2:1), and (c) dodecanoic acid: decanoic acid (1:2). The experiments were conducted at varying mass ratios of DES/ water phase at ambient condition followed by quantum chemical (QC) analysis. Within the batch process with a single cycle, the extraction efficiency and the distribution ratio of 82 % and 4.75 respectively, were reported with the DES; dl-menthol: decanoic acid (1:1). Different initial drug concentrations (5 to 20 ppm) were introduced to observe the extraction performance at various contamination levels. Followed by the batch-type extraction experiments, three different recycling schemes have been implemented to assess the reuse and recovery of DESs. A 90 % percentage extraction was achieved by recycling the DES phase with purification through activated carbon (AC) which allowed it to maintain its continuity of the aqueous phase. The higher hydrophobicity of the DES phase with relatively high drug solubility proved to be the driving force of the enhanced DES-drug interaction. This was supported by the QC analysis as comparably higher non-bonded interaction energy was obtained for the DES-enoxacin complex suggesting stronger interaction and compactness. The charge transfer (CT) process confirmed the magnitude of charge transfer between DES and the drug through natural bonding orbital (NBO) charge analysis. Non-bonded interactions with dispersion effect were confirmed by the structural arrangement analysis of the complex supported by frontier molecular orbital (FMO) analysis. The quantum theory of atom in molecules and the non covalent interaction (QTAIM & NCI) analysis suggest hydrogen bonding and dispersion interactions (van der Waals) as the basis of interaction between the molecular groups.