Kinetic and thermodynamic evaluation of lipid and pigment extraction from Nannochloropsis oculata by ultrasound-assisted and conventional methods.

Conventional extraction of lipids and pigments involves the use of toxic solvents and long extraction times potentially leading to degradation of the target compounds. Ultrasound-assisted extraction (UAE) has emerged as a promising alternative, capable of enabling high yields under milder conditions and shorter extraction times. This study investigated the extraction kinetics of lipids, chlorophylls, and carotenoids from Nannochloropsis oculata using 75% (v/v) ethanol at 20–60 °C, with and without ultrasound, and compared the results with conventional extraction. Optimal extraction conditions were identified as 8 min with ultrasound for lipids, 30 min without ultrasound for chlorophylls, and 4 min without ultrasound for carotenoids, all at 60 °C, reaching 86.2%, 68.1%, and 67.6% of the total content in the biomass, respectively. The pseudo-second order (PSO) model best described the extraction kinetics for all compounds, presenting higher coefficients of determination (R2) and lower root mean square errors (RMSE) compared to the pseudo-first order (PFO) model. This suggests extracellular film diffusion around the solid particles was not the rate-limiting step of mass transfer. Thermodynamic analysis revealed endothermic behavior and increased process spontaneity at higher temperatures . Overall, ultrasound had minimal effect on extraction yields, suggesting that the disruption observed was mainly driven by the solvent effect. Nonetheless, 75% ethanol proved effective, achieving high recoveries without toxic solvents. These results provide insights for sustainable microalgal biocompound extraction and highlight the need for more complex kinetic models considering both intra- and extracellular mechanisms.