Bio-based solvents for polyphenol recovery: Transforming olive mill wastewater into high-value resources

Abstract

Spain generates approximately 1.8 million m³ of olive mill wastewater (OMW) annually, posing significant environmental and economic challenges while presenting opportunities for resource recovery. This study investigates the technical feasibility of extracting high-value polyphenols from OMW using bio-based solvents, supporting circular bioeconomy initiatives. Three OMW types—washing (W), centrifuge (C), and storage pools (B)—were sampled at the start and end of the olive harvesting campaign. Polyphenols were extracted using ethyl acetate (EA), 2-methyltetrahydrofuran (2-MeTHF), and cyclopentyl methyl ether (CPME) at varying solvent-to-wastewater ratios and extraction times. The study quantified total polyphenols, flavonoids, antioxidant capacity, and key polyphenols in both wastewater and extracts, alongside evaluating solvent recyclability. Results showed that the highest polyphenol content was found in storage pool wastewater (884 mg GAE L⁻¹ on average). EA and CPME achieved the highest total polyphenol extraction efficiency (41.5 and 38.5 % respectively), while 2-MeTHF was the most efficient for flavonoids (42.8 %). CPME demonstrated the highest solvent recovery (55 %), highlighting its sustainability. Hydroxytyrosol and tyrosol were the predominant polyphenols across all wastewater types, followed closely by vanillin. EA and 2-MeTHF extracts exhibited strong antioxidant capacity (up to 73.7 %). These findings underscore the potential of bio-based solvents for efficient and environmentally friendly polyphenol recovery, promoting sustainable waste valorization and circular economy practices in the olive oil industry.