Pretreatment optimization for microalgae oil yield enhancement and residual biomass characterization for sustainable biofuel feedstock production

Assessment of lipid extraction from cultivated Chlorella pyrenoidosa (cultivated) and commercial Chlorella vulgaris microalgae cells involved comparing novel (microwave and ultrasound) and conventional (Soxhlet) extraction techniques. Designed experiments were conducted to optimize the processing parameters responsible for effecting algal oil yield and quality. Studies revealed that optimized pretreatments (microwave and ultrasound) disrupted the cell wall facilitating the release of lipid from microbial cells. Ultrasound pretreatment was much more effective in terms of extraction efficiency in comparison to microwave and Soxhlet. Ultrasound-assisted technology yielded three times more oil from Chlorella pyrenoidosa (34.30 ± 0.38%) than Chlorella vulgaris (13.07 ± 0.44%) under optimal processing conditions and two times more than Soxhlet extraction. Optimal conditions for Chlorella pyrenoidosa were 26% amplitude, 30-min extraction time, and a solvent-to-biomass ratio of 50 mL/g. Similarly, for Chlorella vulgaris, the most efficient conditions were 50% amplitude, 20-min extraction time, and a solvent-to-biomass ratio of 50 mL/g. The FTIR analysis revealed the presence of esters’ C=O stretching bonds, while TGA results demonstrated significant weight loss between 150.47 and 483.29 °C for Chlorella pyrenoidosa (CP) and 163.85 and 428.50 °C for Chlorella vulgaris (CV). These findings suggest that both microalgae could serve as a promising feedstock for pyrolysis and combustion processes, leading to the production of valuable by-products such as hydrocarbon gas and bio-oil, thereby indicating potential for biodiesel production and as a nutritional supplement from the extracted oil and residual biomass.