Orange peel waste valorisation through limonene extraction using bio-based solvents

Orange peel waste (OPW) can be an effective feedstock for extraction of natural bioactive components such as limonene, a high value-added chemical broadly exploited for food, pharmaceutical, and cosmetic industrial applications. Extraction of limonene from OPW has been conventionally performed via solvent extraction using hexane, a hazardous petrochemical solvent currently restricted under international regulations. In this work, we have conducted a comparative assessment of the performance of a variety of green solvents for sustainable valorisation of OPW through limonene extraction. In particular, cyclopentyl methyl ether (CPME), ethyl lactate (EL), isopropyl alcohol (IPA), polyethylene glycol 300 (PEG 300), isopropyl acetate (IAc), dimethyl carbonate (DMC), methyl ethyl ketone (MEK), 2-methyl-tetrahydrofuran (2-MeTHF) and ethyl acetate (EAc) have been evaluated as hexane replacement for the recovery of limonene from OPW. Initially, a preliminary solvent screening was carried out using the COnductor-like Screening MOdel for Real Solvents (COSMO-RS) to estimate the solubility of limonene in the proposed solvents and rank their theoretical extraction performance. Afterwards, experimental studies were performed to determine the limonene extraction yields and optimize the operating conditions (temperature, time and solvent load) for limonene recovery from OPW using the various solvents, as confirmed by gas chromatography mass spectrometry (GC-MS) analysis. Overall, results support that CPME and 2-MeTHF bio-based solvents significantly outperform the benchmark petrochemical solvent hexane by increasing limonene extraction yields up to 80% and 40% respectively at optimum operating conditions. Moreover, recovery and reuse of these solvents in consecutive extraction cycles was successfully accomplished, while scanning electron microscopy analysis (SEM) suggests that solvent effects on biomass structure disruption could be beneficial for further bioprocessing.