Kinetics and Thermodynamics for Anionic Dye Adsorption on Calcined Layered Double Hydroxides

The adsorption thermodynamics and mechanism of acid red 88 (AR88), acid orange 3 (AO3) and acid violet 90(AV90) on MgAl Layered double hydroxides with 3:1 Mg/Al molar ratio were investigated. The adsorption isotherms were measured at different temperatures, and the functions of thermodynamics were also calculated. The results indicated that the adsorption process on MgAl-LDO were well consistent with the Langmuir equation and were also spontaneous and exothermic. Three kinetics models were used to describe the kinetics experimental data, and it was found that the pseudo-second order kinetics model was best fitted. The Gibbs free energy was calculated to be 7 to 15 kJ . mol(-1) because of Hydrogen bonds between the layers and dye molecular. According to the calculation of Materials Studio5.5, the mechanism of adsorption process was that most dye anions were adsorbed on the surface of MgAl-LDO accompanied with intercalated into the layer.