Articles
Comparative adsorption of rooibos polyphenols from NADES and ethanol extracts: isotherm, kinetic, and thermodynamic insights
Article number
1466_63
Pages
519 – 526
Language
English
Abstract
In this study, the adsorption characteristics of polyphenols in rooibos leaf spent (RLS) were investigated using HPD-100 resin, with the aim of separating the polyphenols from RLS for potential applications in food preservation.
The extracts were obtained using a natural deep eutectic solvent (NADES) composed of choline chloride and lactic acid (ChCl-La) as the extraction solvent, while 60% (v/v) ethanol was used as a conventional solvent for comparison.
The experimental data were fitted to pseudo-first-order, pseudo-second-order, and intraparticle diffusion models, and kinetic parameters, rate constants, equilibrium sorption capacities, and coefficient of determination (R2) were determined.
Subsequently, the equilibrium sorption capacity was modelled using the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) adsorption isotherm models at two temperatures (25 and 35°C). Adsorption of rooibos polyphenols on HPD-100 resin was exothermic and physisorptive for both NADES (ChCl-La) and ethanol systems.
For ChCl-La, the pseudo-second-order model and the Freundlich isotherm (R2=0.991 at 25°C and 0.968 at 35°C) showed the best fit, indicating surface-controlled, heterogeneous adsorption that weakened with temperature.
In contrast, ethanol adsorption followed intraparticle diffusion control, best described by the D-R model (R2=0.869; E=0.0037 kJ mol‑1 at 25°C and R2=0.899;0.0039 kJ mol‑1 at 35°C). Overall, ChCl-La exhibited higher adsorption affinity but greater thermal sensitivity, while ethanol showed lower affinity with better thermal stability.
The extracts were obtained using a natural deep eutectic solvent (NADES) composed of choline chloride and lactic acid (ChCl-La) as the extraction solvent, while 60% (v/v) ethanol was used as a conventional solvent for comparison.
The experimental data were fitted to pseudo-first-order, pseudo-second-order, and intraparticle diffusion models, and kinetic parameters, rate constants, equilibrium sorption capacities, and coefficient of determination (R2) were determined.
Subsequently, the equilibrium sorption capacity was modelled using the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) adsorption isotherm models at two temperatures (25 and 35°C). Adsorption of rooibos polyphenols on HPD-100 resin was exothermic and physisorptive for both NADES (ChCl-La) and ethanol systems.
For ChCl-La, the pseudo-second-order model and the Freundlich isotherm (R2=0.991 at 25°C and 0.968 at 35°C) showed the best fit, indicating surface-controlled, heterogeneous adsorption that weakened with temperature.
In contrast, ethanol adsorption followed intraparticle diffusion control, best described by the D-R model (R2=0.869; E=0.0037 kJ mol‑1 at 25°C and R2=0.899;0.0039 kJ mol‑1 at 35°C). Overall, ChCl-La exhibited higher adsorption affinity but greater thermal sensitivity, while ethanol showed lower affinity with better thermal stability.
Publication
Authors
Z.P. Mina, K.A. Jimoh, T.M. Pfukwa, O.A. Fawole
Keywords
rooibos leaf spent, natural deep eutectic solvent (NADES), adsorption isotherms, intraparticle diffusion, physisorption
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