Eutectic solvents as a novel extraction system for microalgae biorefinery

The common lipid extraction from microalgae involves sequential of energy-intensive processes and the use of harmful organic solvents. Eutectic solvents (ES), a novel class of designer solvents, hold a great potential as alternative solvents. Not only they are easy to prepare and have tailorable properties, but they are also able to permeabilize the cell wall of microalgae. However, the lack of vapor pressure makes the solvent regeneration difficult. Therefore, in this thesis, a semi-hydrophobic eutectic solvent was developed to extract lipids from microalgae, with a viable solvent regeneration step. Moreover, a preliminary biorefinery process was also explored.In Chapter 2, we screened several semi-hydrophobic ES. The combination of polar imidazole and nonpolar hexanoic acid showed tuneable hydrophobicity depending on the composition. At low imidazole presence, the mixture dissolved lipids and the lipid solubility decreased at higher imidazole content. This principle was then applied to separate the ES from the dissolved lipids. With this approach, about 75% of lipids could be recovered with a high purity (> 85%).Since retrieving imidazole from the altered ES was difficult, we explored other auxiliaries which can also shift the ES hydrophobicity (Chapter 3). Polar antisolvents, such as water, methanol, and ethanol, were observed to reduce the lipid solubility in the ES. The reduction of lipid solubility increases with the antisolvent polarity and amount. Furthermore, since the antisolvents were (moderately) volatile, a large amount of antisolvents could be loaded to obtain higher purity (up to 100%) and recovery (> 90%) when compared to the previous approach. With this approach, more than 90% of ES could be regenerated through evaporating the antisolvents. Methanol was selected as the best antisolvent as it offered the ease of regeneration without sacrificing the lipid recovery.The novel ES was then applied to intact Nannochloropsis oceanica for lipid extraction (Chapter 4). The yield of lipids extracted with the ES was comparable to the standard Bligh & Dyer method using chloroform/methanol. The extraction yield was found to benefit from the low imidazole content (0 – 15 mol%; hydrophobic state), a high temperature, longer incubation time, and high solvent-to-biomass ratio. Interestingly, the moisture from the wet biomass enhanced the lipid extraction, which was found to decrease with freeze-dried biomass. In addition, supplementation of water could reverse the adverse effect of drying. This result implied that the disruption and drying of biomass was not necessary to ensure the high lipid yield.We also developed further the ES-based process towards microalgae biorefinery (Chapter 5). First, the separation of algae lipid from the ES was performed by addition of methanol at low temperature (-20 °C). With this approach, 60% of the lipids could be recovered, which was significantly lower than the finding in Chapter 3. Furthermore, the ES were reusable after three iterative cycles without significant drop of extraction efficiency. Besides that, the process was found to be scalable with slightly lower efficiency and recovery. Furthermore, denatured proteins and carbohydrates could be obtained from the defatted biomass through an aqueous extraction.Several key findings and challenges from this thesis are summarized in Chapter 6. Besides that, some ideas for tackling the current drawbacks of the proposed process are also discussed. Additionally, we include recommendations for further research to gain understanding of molecular interactions between the ES components and target molecules. Ultimately, task-specific ES for biorefining could be tailored based on the obtained knowledge.