Adsorption of Dark Compounds With Bentonites in Apple Juice

Abstract

The adsorption equilibrium of dark-coloured Compounds from apple juice has been investigated as a function of several variables including activation conditions of bentonite (heat and acid treatments), clay concentrations (2 x 10(-3)-8 x 10(-3) kg clay/dm(3) apple juice) and temperature (range of 296-336 K). The adsorption efficiency with acid-activated bentonite was greater than that with heat-activated and native bentonite, depending on surface properties; specific surface areas were 95.31, 71.95 and 71.76 m(2)/g, respectively. Absorbance data at 420 nm were fitted reasonably well with the Langmuir and Freundlich isotherms. The parameters K, Q(0), K-fr and n were determined for different temperatures. Thermodynamic parameters such as Gibbs free energy (Delta G), enthalpy (Delta H) and entropy (Delta S) change of adsorption were determined as about -3.125, 9.43 and 0.039 kJ mol(-1) K-1, respectively, for acid-activated bentonite. These parameters were also determined for native and heat-activated bentonites. It was shown that the adsorption process was endothermic, spontaneous and controlled by physical mechanism. The adsorption and desorption rate constants (k(a), and k(d)) were obtained separately by applying a geometric approach to the first-order Langmuir model. k(a) varied from 5.717 x 10(-4) to 20.667 x 10(-3) s(-1) and k(d) from 1.425 x 10(-4) to 7.473 x 10(-3) s(-1). The results showed that acid-activated bentonite is more suitable for the adsorption of dark compounds from apple juice. (c) 2006 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved.