Isothermal kinetics of ethanolic extraction of total hypericin from pre-extracted Hypericum perforatum flowers

Hypericum perforatum L., Hypericaceae (St John's wort) is a highly familiar plant in the medicinal community which recently showed good antiviral activities including against some types of coronavirus.

Establishing the kinetics of isothermal extraction of total hypericin (TH) from the pre-extracted flowers of H. perforatum flowers.

The solvent extraction of TH from the remaining solid residue after the pre-extraction procedure was performed providing isothermal conditions at the temperature of 313 K, 323 K, and 328 K (±1 K) using ethanol as a solvent. The feasibility of mathematical modelling of the isothermal kinetics of TH extraction was explored applying some of the frequently used kinetic models of solvent extraction: first-order reaction model, film theory model, unsteady-state diffusion model, parabolic diffusion model, Elovich's equation. The kinetic complexity was examined using the differential isoconversional method.

The kinetics of isothermal solvent extraction of TH is a kinetic elementary process with the unique rate-determining step. It was found that the kinetics of isothermal extraction of TH can be best described employing the theoretical Jander three-dimensional (3D) diffusional model and its suitability for modelling the investigated extraction was confirmed with statistical parameters [adjusted linear correlation coefficient (R² _adj ) = 0.998-0.999 and the standard error (SE) = 0.005-0.006]. The values of the model kinetic parameters (rate constant (k_M /min^-1 ), activation energy (E_a  = 21.0 ± 4.9 kJ/mol) and pre-exponential factor (lnA = 3.1 ± 2.2 min^-1 ) were calculated.

Based on the model mechanism of the kinetics of the investigated extraction a new mathematical model is suggested and the controlling step of the overall process was found.