Ferric chloride based downstream process for microalgae based biodiesel production

•FeCl3 can integrate downstream processes of microalgae-based biodiesel production.•FeCl3 can coagulate microalgae cells at pH 3.•Microalgae cells can be effectively destroyed by the Fenton reaction at 90 °C.•Free fatty acids can be converted to biodiesel by FeCl3 under acidic condition.•Two-step esterification process can solve the problem of alkaline catalyst.

In this study, ferric chloride (FeCl3) was used to integrate downstream processes (harvesting, lipid extraction, and esterification). At concentration of 200 mg/L and at pH 3, FeCl3 exhibited an expected degree of coagulation and an increase in cell density of ten times (170 mg/10 mL). An iron-mediated oxidation reaction, Fenton-like reaction, was used to extract lipid from the harvested biomass, and efficiency of 80% was obtained with 0.5% H2O2 at 90 °C. The iron compound was also employed in the esterification step, and converted free fatty acids to fatty acid methyl esters under acidic conditions; thus, the fatal problem of saponification during esterification with alkaline catalysts was avoided, and esterification efficiency over 90% was obtained. This study clearly showed that FeCl3 in the harvesting process is beneficial in all downstream steps and have a potential to greatly reduce the production cost of microalgae-originated biodiesel.