Kinetics, isotherm, and thermodynamics of adsorption of sterol on strong acid ion exchange resin

Kinetics, isotherm and thermodynamics of sterol adsorption on styrene-divinylbenzene based ion-exchange resin with strong acid was investigated at temperature in the range of 298 to 313 K using a model solution of stigmasterol in n-heptane with initial concentration in the range of 0.3×10^-3 to 1.8×10^-3 kg/kg-solution. Adsorbent (5 wt%) was added to the model solution and isothermal adsorption was performed at 3.33 rps for 7.2×10³ s. Kinetics of sterol adsorption was analysed based on pseudo-first-order and pseudo-second-order models. The results revealed that pseudo-second-order model agreed with the experimental data, much better than pseudofirst- order model. At the equilibrium of adsorption, adsorption capacity (qe) decreased when the temperature was increased. This result indicated that sterol adsorption was exothermic. Analysis of adsorption isotherm data based on Langmuir, Freundlich and linear models showed that Freundlich was the best model that could predict the adsorption isotherm data. Adsorption equilibrium constants calculated based on Freundlich model at various temperatures were used to calculate Gibb's free energy change (ΔG), enthalpy change (ΔH), and entropy change (ΔS). The increase of ΔG with respect to temperature indicated that the adsorption was more favourable at lower temperatures. The negative value of ΔH indicated that the adsorption was exothermic and agreed well with the effect of temperature on adsorption capacity. The negative value of ΔS indicated associative adsorption and decrease of the randomness between the solid/liquid interfaces due to the adsorption.