Modelling the hydrophilic extraction of the bark of Eucalyptus nitens and Eucalyptus globulus: Adsorption isotherm and thermodynamic studies

Eucalyptus species have shown to have polyphenolic compounds with proven antioxidant activity. To obtain these active compounds, the basic aspects of the extractive process need to be studied. In this work, the adsorption process of polyphenols obtained from Eucalyptus nitens and Eucalyptus globulus bark was evaluated, expressed as adsorption isotherms to describe solute/solid interactions. Also, the thermodynamic parameters of the extraction process were calculated. Firstly, the extractive process was optimized by a Box-Benhken experimental design for each Eucalyptus species, maximizing the extraction yield, the total phenols content and its antioxidant capacity. Subsequently, the solute/solid interactions of the phenolic/bark system were determined at various concentrations, and temperatures; allowing even later, to model multistage extractions of phenolic compounds from bark powder. The extracted and non-extracted bark powders were morphologically (SEM) and physically (BET adsorption) characterized. The extractive process led surface changes for both species, being the BET areas similar (55 m(2)/g) and with porous in the mesoporous range. The equilibrium data were best adjusted to Freundlich isotherm, indicating a simple modelling type, which can be used for global studies of the large-scale extractive process for the modelling of phenolic multi-extractions. Thermodynamic parameters AH, AG and AS demonstrated an exothermic and spontaneous process with restricted movement of adsorbate molecules during adsorption, and low values of AG and free adsorption energy, E, calculated from D-R model indicated a physical adsorption type for both eucalyptus species. The validation of the isotherm models presented results that are consistent with the experimental data obtained, through a multi-extraction of five stages for both species of eucalyptus.