Novel methods in harvest of microalgae and extraction of lipids from dilute microalgal suspension for biofuel
Date of Award
Doctor of Philosophy (PhD)
Mechanical and Aerospace Engineering
James E. Smith
Algal biofuels., Biomass energy., Microalgae., Renewable energy sources.
A method of using organic solvent (hexane, 1-butanol, ethyl acetate) to harvest freshwater microalgae Nannochloropsis sp. by tuning microalgal cell surface hydrophobicity with low pH was reported. Harvesting experiments were performed with culture suspension of 60 mL with biomass concentration of 0.35 g/L and 0.85 g/L. Due to cell surface hydrophobicity, microalgal cells attach to the organic solvent and water interphase after separation. Suspension pH of 3 resulted in the highest harvesting efficiency (92-99%) depending on the organic solvent with processing time of 3 min. This method is efficient, cost-effective and eco-friendly as the supernatant can be reused as growth medium. Hexane has found to be only temporarily impedes the growth rate, while ethyl acetate could be used as an alternative carbon source for growth when used in dilution. A procedure, called Acid and Surfactant Mediated Liquid-Liquid Lipid Extraction (ASMLLLE), for lipid extraction from dilute microalgae suspension with biomass density of 1.16 g/L was reported. This method does not require harvesting or dehydration of the microalgal biomass with extraction performed at room temperature. Conventional biphasic organic solvent extraction methods suffer from limited contact between hydrophobic extraction solvents and algal biomass dispersed in the aqueous phase, therefore require energy intensive algae harvesting and long extraction time. In the present study, we show the merits of liquid-liquid extraction performed with a partially water-miscible organic solvent, ethyl acetate, to extract both polar and non-polar lipids from microalgal biomass in dilute aqueous suspension. This method employs a high hydrophilic-lipophilic balance surfactant, sodium dodecyl sulfate (SDS), to disrupt the microalgal cell membrane, transport the intracellular lipids, and retain biomass debris in the aqueous phase after extraction. Citric acid was used to enhance the disruption of the microalgal cell wall and membrane. Optimum conditions were 1.5% (w/v) SDS, microalgal suspension pH of 3.5, 6:6 (v/v) ethyl acetate to microalgal suspension ratio, with 30 sec extraction time. The theoretical fatty acid methyl ester (FAME) and total lipid content was quantified by a modified Bligh and Dyer (B&D) method. The results showed that ASMLLLE method achieved 83-84.4% of the theoretical FAME yield and 79.6-81.1% of the theoretical total lipid yield depending on the phase separation method used. No differences in terms of FAME profile were observed when compared to the B&D method.
Zhang, Chen, "Novel methods in harvest of microalgae and extraction of lipids from dilute microalgal suspension for biofuel" (2017). Dissertations. 136.