Hydrophobic deep eutectic solvents based on fatty acids for enhanced phenol extraction in supported liquid membrane systems

The extraction of phenol from industrial wastewater remains a significant challenge due to its toxicity and durability. This work examines the feasibility of fatty acid-based hydrophobic deep eutectic solvents (HDESs) as an environmentally friendly substitute for phenol removal in supported liquid membrane (SLM) systems. The HDESs synthesized from dodecanoic acid with octanoic, nonanoic and decanoic acids were analyzed in terms of hydrophobicity, viscosity and stability. Further characterization employing FTIR to validate the stability of the aforementioned HDESs in aqueous conditions was performed. Within the series of synthesized HDESs, the SLM based on [Dod-Non] showed the highest degree of phenol removal, 81 % within 36 h, with greater permeability and flux. This high efficiency could be attributed to the [Dod-Non]-based SLM having the required balance of hydrophobicity and viscosity for enhanced phenol transfer from the aqueous phase. The main operational parameters, including feed pH, initial phenol concentration, stripping solution concentration, and flow rate, were studied systematically. The study revealed that a feed pH as low as 2.0, and moderate NaOH stripping concentrations (0.2 M) were optimum for phenol extraction. The stability test demonstrated no substantial variation in the performance of the HDES-based SLMs of solvent loss after multiple extraction and stripping cycles. This work shows that fatty acid-based HDESs are a promising solution for the development of sustainable and effective SLM treatment of aqueous phenol.