Adsorption of dark compounds with bentonites in apple juice

Absorbance data at 420 nm were fitted reasonably well with the Langmuir and Freundlich isotherms. The parameters K, Q0, Kfr and n were determined for different temperatures. Thermodynamic parameters such as Gibbs free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) 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 (ka and kd) were obtained separately by applying a geometric approach to the first-order Langmuir model. ka varied from 5.717×10−4 to 20.667×10−3 s−1 and kd from 1.425×10−4 to 7.473×10−3 s−1. The results showed that acid-activated bentonite is more suitable for the adsorption of dark compounds from apple juice.