Three-phase partitioning based on CO2-responsive deep eutectic solvents for the green and sustainable extraction of lipid from Nannochloropsis sp

Three-phase partitioning (TPP) had been developed for lipids extraction from microalgae. However, the volatile and flammable organic solvents (typically t-butanol) were widely used in the conventional TPP. In this study, CO2-responsive deep eutectic solvents (DESs) were used as the alternative solvents and applied for lipid extraction from Nannochloropsis sp. by using a TPP system. These DESs can be separated from lipid and easily recycled by bubbling CO2 and N2 after extraction. The response characteristics of the optimal DES-3 (HBA: HBD = 3:1) were studied by FT-IR, 1H NMR, and 13C NMR. Furthermore, factors influencing the extraction process were studied in detail. The maximal extraction yield (172 mg/g) was obtained under the optimized conditions: ammonium sulfate of 20 wt%, liquid–solid ratio of 35:1, extraction temperature of 80 °C, and extraction time of 90 min, which was much higher than the t-butanol-based TPP system (135 mg/g). The lipid was recovered from DES after extraction, and the fatty acid composition was analyzed after TPP. Finally, the recovered DES was reused for lipid extraction, and the results indicated good cyclic stability for this DES after five cycles. This study presents a green, efficient, and sustainable strategy for lipid extraction and further biofuel production.