Synergistic Integration of C12-C16 Cationic Surfactants for Flocculation and Lipid Extraction from Chlorella Biomass

Microalgae lipids could be a good alternative feedstock for liquid fuel, but complex processing steps with significant energy demands present roadblocks. Combining biomass harvesting and lipid recovery into one simple step can lower complexity and energy costs. This study evaluated the use of C12-C16 cationic surfactants in this synergistic manner. First, we determined that a dose as low as 0.45 mM of C16-alkyl-chain cationic surfactant (hexadecyltrimethylammonium, CTAB) led to >85% biomass-harvesting rate for Chlorella biomass, and good harvest was correlated to a slightly positive zeta potential. Second, the cationic surfactants disrupted cell structures (detected by transmission electron microscopy) and led to lipid recovery (measured as fatty acid methyl esters, FAME) as high as 90% using nontoxic ethyl acetate (EA) as the solvent and without altering the FAME distribution; for context, EA was able to extract less than 1% of FAME from control (not surfactant-treated) Chlorella. Disruption and high FAME yield were associated with surfactants' critical micelle concentration (CMC): a lower CMC required a lower concentration of surfactant to disrupt the cells. The synergistic benefits of cationic surfactants can be attained by maintaining a slightly positive zeta potential for effective flocculation and adding the minimum concentration of surfactant needed for cell disruption.