Treatment of high fluoride concentration water by MgAl-CO3 layered double hydroxides: Kinetic and equilibrium studies

MgAl-CO3 layered double hydroxides (LDHs) have been employed to treat high fluoride concentration solution. The influences of solution pH, initial fluoride concentration and other anions in the solution were investigated by a series of batch experiments. A marked decrease in the amount of adsorbed fluoride by LDHs is observed with increasing pH. The extent of fluoride removal in the presence of other anions decreases in the order HCO3−>Cl−>H2PO4−>SO42−. The equilibrium isotherm for fluoride uptake corresponds closely to the Langmuir–Freundlich (L–F) model. The maximum capacity of LDHs for fluoride ions and the Gibbs free energy (ΔG0) for the defluoridation process were calculated to be 319.8±5.7 mg/g and −9.0±0.66 kJ/mol, respectively. The negative value of ΔG0 indicates the spontaneous nature of the treatment process. Four kinetic models have been evaluated in order to attempt to fit the experimental data, namely the pseudo-first order, the pseudo-second order, the modified multiplex and the double exponential models. It was found that the modified multiplex model, involving a rapid first order step and a slow second order step most closely described the kinetics. The activation energies for the two steps are 37.2±5.26 and 72.6±4.52 kJ/mol, respectively, suggesting that the rapid step is controlled by diffusion processes, whilst the second step is controlled by the reaction of fluoride with the LDHs.