Equilibrium and kinetic studies on the adsorption of dark colored compounds from apple juice using adsorbent resin

The kinetics of adsorption of dark colored compounds on adsorbent resin were studied at different adsorbent resin concentrations (1, 2, 4, 8 g adsorbent resin per liter of apple juice) for the temperature range of 20-80degreesC. For equilibrium adsorption curves; the Langmuir and Freundlich models were attempted and model parameters (K-ad, Q(0), K-f and n) were obtained for different temperatures. The adsorption data of dark colored compounds present in apple juice onto macro porous adsorbent resin fitted well both to Langmuir and Freundlich models. Heat of adsorption (DeltaH) value of 10.567 kJ/mol indicated the endothermic nature of the adsorption process. A decrease of Gibbs free energy (DeltaG) with increasing temperature also indicated the spontaneous nature of the process. The adsorption and desorption rate constants were determined separately by applying a geometric approach to the Langmuir model. The adsorption rate constants (k(a)) varied from 0.0093 to 0.1103 min(-1), and the desorption rate constants (k(d)) from 0.0050 to 0.0285 min(-1) at different temperatures and adsorbent resin concentrations. The activation energy of adsorption (E-a) averaged as 21.2 +/- 4.2 kJ/mol, and that of desorption (E-d) as 10.1 +/- 3.3 kJ/mol. (C) 2002 Elsevier Science Ltd. All rights reserved.