Process optimization and highly efficient oil extraction from Spirogyra as environmentally friendly and the fastest growing algae for biodiesel production via SC CO2 and UAE green methods

• Comparative optimization of green oil extractions from Spirogyra algae. • Application of ethanol modified SC CO₂ and ultrasound-assisted (UAE) methods. • Using Spirogyra algae as fast-growing, eco-friendly source for biodiesel production. • RSM optimization of the key operational parameters of oil extraction. • Maximum oil yield of 37.99 % achieved via modified SC CO₂ and 32.43 % by UAE. The present work is a comprehensive comparative study on process optimization of oil extraction from Spirogyra as the fastest growing species of algae and also environmentally friendly using two green extraction techniques of ethanol modified supercritical carbon dioxide (ethanol-SC CO 2 ) and ultrasound-assisted extraction. The physicochemical properties of the extracted lipid were characterized. Also, functional groups present in the lipid were identified through Fourier-transform infrared spectroscopy (FT-IR), and the fatty acid profile was determined with GC analysis. Process optimization was conducted using Response Surface Methodology (RSM) to enhance lipid recovery activity and studied the impact of each operational parameter. In the ethanol-SC CO 2 method, operational parameters including temperature, pressure, and extraction time were assessed, whereas in the ultrasound-assisted method, extraction time, ultrasonic power, and solvent-to-oil ratio were examined. The results showed that ethanol-SC CO 2 method was more efficient than ultrasound-assisted method for Spirogyra algae oil extraction. Under optimized conditions, a maximum lipid yield of 37.99 % was achieved from the proposed model using ethanol-SC CO 2 at 19.24 MPa pressure, 74.93 °C temperature, and 119.05 min extraction time which was a very good agreement with corresponded experimental lipid yield (37.70 %). Also, the optimum condition of the ultrasound-assisted extraction was at extraction time of 42.5 min, ultrasonic power = 351.3 W, and a solvent-to-oil ratio of 13.75 with a maximum lipid yield of 32.43 %. Also, it was observed that among the saturated fatty acids, palmitic acid was dominant (41.22 %), while oleic acid represented the major unsaturated fatty acid (14.08 %).