Adsorption kinetics and thermodynamics characters of organic acids from apple juice by magnetic chitosan microspheres

Abstract: The aim of this research was to study the kinetics and thermodynamics of organic acids adsorption from apple juice by magnetic chitosan microspheres. Magnetic chitosan microspheres of 0.500 g were accurately added to 30 mL clarified apple juice in which organic acid concentration was 2.32 mg/mL. Oscillatory adsorption of organic acids by magnetic chitosan microspheres was carried out at 3 temperature gradients (298, 318 and 338 K) respectively and the adsorption amount at different adsorption time was calculated. The adsorption kinetic curves at 3 temperatures were obtained by plotting the organic acid adsorption amounts with the adsorption time. At the same time, 0.500 g magnetic chitosan microspheres were accurately added to 30 mL clarified apple juice with different initial concentrations of organic acid (2.782 4, 3.744 5, 4.649 6, 5.772 6, 6.892 2, and 7.814 1 mg/mL). Oscillatory adsorption was respectively carried out at 3 temperature gradients until adsorption equilibrium was reached and the adsorption capacity at equilibrium was determined after magnetic separation under a magnetic field. The adsorption capacity at equilibrium was plotted with the organic acid concentration at equilibrium after adsorption to obtain adsorption isotherms under 3 temperature gradients. The Lagergren pseudo-first-order kinetic model, Lagergren pseudo-second-order kinetic model, Elovich model and internal diffusion model were used to fit the adsorption kinetic data. Three isothermal adsorption models (Langmuir, Freundlich and Temkin) were used to analyze adsorption thermodynamics properties. According to the adsorption kinetic curves, it was seen that initial adsorption rate tended to increase with the increasing of the temperature. Meanwhile adsorption amount at equilibrium (112.374 9, 105.558 4, and 100.424 9 mg/g) decreased with the increasing of the temperature, which demonstrated that the adsorption process was exothermic and higher temperature impeded the adsorption reactivity. By comparing the correlation coefficient, it was found that adsorption kinetic process conformed to the Lagergren pseudo-second-order kinetic model better (R2＞0.998). The adsorption constant declined gradually with the temperature increasing, which also offered a proof of heat release about the adsorption process. Also, the equilibrium adsorption amount calculated by the Lagergren pseudo-second-order kinetic model was close to the data obtained from the experiment. Adsorption thermodynamics conformed to Langmuir isothermal adsorption model (R2＞0.998), which indicated that the adsorption process was a monolayer chemical adsorption. Thermodynamic parameters Gibbs free energy change and standard enthalpy change were below zero, and standard entropy change was above zero, which indicated that the adsorption of organic acid to magnetic chitosan microspheres was a spontaneous and exothermic process along with the increasing of entropy. The kinetics and thermodynamics process can provide technical basis for the adsorption process of organic acid from apple juice using the magnetic chitosan microspheres.