Bromide ion removal from contaminated water by calcined and uncalcined MgAl-CO3 layered double hydroxides

A fundamental investigation on the uptake of bromide ion from contaminated water by calcined and uncalcined MgAl-CO3 layered double hydroxides (LDHs) were conducted in batch mode. The uptake capacity of calcined LDHs (CLDH) is higher than that of uncalcined LDHs, due to their different mechanisms which are confirmed by powder X-ray diffraction, FT-IR spectroscopy and TG-MS measurements. The former mechanism is based on the reconstruction of CLDH to Br-LDHs, whilst the latter is related to the surface adsorption. It has been found that the LDHs calcined at 500 degrees C with Mg/Al molar ratio of 4 represents the highest capacity to remove bromide ion from aqueous solution. The equilibrium isotherms of uptake of bromide by CLDH were well fitted by the Langmuir equation, and thermodynamic parameters such as Delta G0, Delta H0 and Delta S0 were calculated from Langmuir constants. The negative value of Delta H0 confirms the exothermic nature of adsorption. Three kinetics models were used to fit the kinetics experimental data, and it was found that the pseudo-second order kinetics model could be used to describe the uptake process appropriately. The value of Ea was calculated to be 79.9 kJ/mol, which suggests that the process of uptake bromide is controlled by the reaction rate of bromide with the CLDH rather than diffusion.