Sustainable Extraction of Per- and Polyfluoroalkyl Substances (PFAS) from Water Using Hydrophobic Deep Eutectic Solvents: Experimental and Modeling Studies

Per- and polyfluoroalkyl substances (PFAS) are a class of synthetic chemicals widely used in industrial and consumer products due to their water- and grease-resistant properties, but their persistence in the environment and potential to cause adverse health effects, including cancer, liver damage, and endocrine disruption, have raised significant environmental and public health concerns. In this study, molecular modeling was employed to identify effective hydrophobic deep eutectic solvents (HDESs) for the removal of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), two highly persistent emerging contaminants, from synthetic wastewater. Using the Conductor-like Screening Model for Real Solvents (COSMO-RS), a systematic screening of 129 potential HDES constituents was conducted, focusing on key performance indicators such as distribution ratio, selectivity, and performance index at infinite dilution, with toluene serving as the benchmark for these performance indicators. This modeling effort identified the top-performing HDES constituents, predominantly derived from terpene and terpenoid families, with promising extraction capabilities and favorable properties for liquid-liquid extraction (LLE). Further analysis, incorporating toxicity and multicriteria evaluation metrics, identified three HDESs, trioctylphosphine oxide: 2-methylbenzothiazole (1:9), trioctylphosphine oxide (TOPO): menthyl acetate (1:4), and 1,8-cineole: menthyl acetate (2.5:1), as promising candidates for PFOA and PFOS extraction, providing a basis for experimental validation and optimization.

Building on this foundation, experimental investigations were conducted to evaluate the effectiveness of DESs for PFOA extraction. A multicriteria assessment of ten DES formulations was conducted, focusing on parameters such as extraction performance, density, viscosity, environmental impact, and hydrophobicity. Among the DESs tested, a mixture of trioctylphosphine oxide and lauric acid (TOPO:LauA, 1:1) demonstrated an exceptional single-stage extraction efficiency of 99.74%, significantly outperforming the toluene benchmark (82.26%). Remarkably, the exceptional extraction efficiency (>98 %) of TOPO:LauA was sustained across diverse operational conditions. The latter included a broad pH and temperature spectrum (3–9 pH; 15–100 °C), solvent-to-feed ratios down to 1:7 (w/w), contaminant concentrations as low as 0.1 ppm, rapid equilibration within 1 min, and reusability throughout seven cycles without any observable degradation, as verified by spectroscopic analysis. TOPO:LauA also demonstrated exceptional selectivity for PFOA in the presence of other PFAS or a synthetic wastewater matrix.

Moreover, computational quantum chemistry modeling using density functional theory (DFT) and COSMO-RS, implemented in Materials Studio and COSMOtherm software, was employed to elucidate the molecular interactions and solvation dynamics underlying the extraction process. This provided critical insights into the molecular mechanism of extraction, strongly aligning with our experimental findings. The development of TOPO:LauA for the extraction of aqueous PFOA represents a significant breakthrough in water treatment, surpassing the efficacy of traditional extraction solvents and other PFAS conventional removal methods, such as adsorption and membrane separation.