Hydrophobic deep eutectic solvents as green solvents for phenolic compounds extraction from olive mill wastewater

This study investigates the use of deep eutectic solvents for extracting phenolic compounds from olive mill wastewater. Four hydrophobic deep eutectic solvents were selected using the COSMO-RS model: [menthol: lactic acid] [1:2], [menthol: octanoic acid] [1:1], [menthol: camphor] [6:4], and [menthol: borneol] [3:1]. The highest phenolic yield (79.2 %) was achieved with [menthol: lactic acid] [1:2] at a solvent/biomass mass ratio of 3 and temperature of 333.15 K within 5 minutes. The performance of the hydrophobic deep eutectic solvents correlated with their acidity values (alpha) in the order: borneol < octanoic acid < camphor < lactic acid. Notably, [menthol: lactic acid] [1:2] achieved 100 % extraction of caffeic acid and 46.7 % of vanillic acid, while [menthol: borneol] [3:1] extracted 100 % of kaempferol. After three extraction cycles, the final total phenolic compouns extraction efficiency (79.2 %) resulted in a good reusability of the MENT:LA [1:2] based HDES. These findings highlight the potential of hydrophobic deep eutectic solvents for sustainable recovery and valorization of phenolic compounds from olive mill wastewater.