Kinetics, isotherm and thermodynamic modeling of liquid phase saponin sorption in soils

Abstract

The sorption behavior of saponin onto clay, paddy, silty loam and sandy soil was thoroughly evaluated in this study. The adsorption kinetic, isotherm and thermodynamic were conducted in batch analysis. Pseudo-first order, pseudo-second order and Elovich kinetic models were applied to evaluate the kinetics of the adsorption. Based on the models, the adsorption of saponin onto soils were mostly governed by physisorption while chemisorption also plays a role in the adsorption process in clay and paddy soil. Mechanism of adsorption was determined by adopting the intraparticle diffusion and Boyd models. The two models concluded that intraparticle diffusion is not the rate-limiting step in saponin uptake in all four soil types. Equilibrium isotherm was evaluated by using Langmuir, Freundlich and Dubinin-Radushkevich isotherm models. The data obeys both Langmuir and Freundlich model, however, Langmuir model tends to overestimate the qe value of the given soil. Clay soil showed highest maximum adsorption, followed by paddy, silty loam and sandy soil. Effects of temperature variation is minimal while variation in pH value was significant where optimum adsorption was achieved at near-neutral pH range. The thermodynamic study showed that the adsorption process was exothermic and spontaneous.