Sustainable Lipid Recovery from Microalgae via Hydrothermal Pretreatment with Natural Deep Eutectic Solvents: Molecular Design, Mechanistic Insights, and Life Cycle Assessment

This study presents a sustainable strategy for biodiesel production from wet microalgae using natural deep eutectic solvents (NADESs) as green alternatives to synthetic deep eutectic solvents (Syn-DESs) in hydrothermal pretreatment. Betaine served as the hydrogen bond acceptor (HBA), paired with seven organic acids as hydrogen bond donors (HBDs), to examine the impact of the HBD structure (specifically carbon chain length and functional groups) on the pretreatment process. Results showed that increasing carbon chain length enhanced protein solubilization but could limit biomass depolymerization, whereas unsaturated bonds facilitated both depolymerization and solubilization, improving the lipid yield. Characterization of hydrochar by TGA and FTIR elucidated structural changes in pretreated microalgae, revealing that the HBD structure, through its influence on NADES acidity and molecular polarity index determined by quantum chemical calculations, affected both depolymerization and solubilization of microalgae. Life cycle assessment indicated that while not all NADES surpassed Syn-DES in performance, the betaine-malic acid NADES demonstrated a superior net energy ratio (NER) of 0.21 (compared to 0.24 for Syn-DES) and reduced CO₂ emissions by 15.36%. Additionally, USEtox modeling showed a 2-order-of-magnitude reduction in ecotoxicity with this NADES. These findings underscore the potential of tailored NADESs to enhance the sustainability and efficiency of biodiesel production from microalgae.