Microwave-assisted extraction of phenolic compounds from Justicia spicigera: Kinetic and thermodynamic study

Based on an initial approach to the extraction of phenolic compounds from Justicia spicigera using microwaves, this study focused on evaluating the effect of different temperatures (35–80 °C) and solvent-to-feed ratios (70% ethanol, 10–40 mL/g) on the extraction process. A second-order kinetic model was applied to evaluate the mass transfer parameters: effective diffusivity, the mass transfer coefficient, the initial extraction rate, and the equilibrium concentration of total phenolic compounds. This model showed a good fit to the experimental data (R 2 >0.90). Temperature had a positive effect on the extraction of phenolic compounds from J. spicigera . As the extraction temperature increased, both the effective diffusion coefficient values (from 7.04 × 10⁻¹² to 8.29 × 10⁻¹² m²/s) and mass transfer coefficient improved (from 6.22 × 10⁻⁷ to 2.56 × 10⁻⁶ m/s). In addition, from an initial ratio of 20 mL/g, the Biot number was ˃50, indicating that internal mass transfer was the main factor involved in controlling the microwave-assisted extraction rate (MAE). The negative values obtained for Gibbs free energy indicated that the mass transfer process was both feasible and spontaneous, while the positive enthalpy and entropy change values indicated that the extraction process was endothermic and irreversible. The extraction and diffusion kinetics models provide information that will be useful for the efficient design and implementation of future processes implemented to obtain phenolic compounds from J. spicigera using an MAE system.