Equilibrium isotherms and kinetics studies for the sorption of fluoride on light weight concrete materials

The sorption kinetics of F− on light-weight concrete materials have been investigated at different pHs, agitation rates and temperatures of solution. The pseudo second-order kinetic model was successfully applied in the removal of fluoride ions from synthetic solution. The optimum values of kads for pH (6.9), temperatures (333 K) and agitation rate (235 rpm) were obtained as 0.300, 0.255 and 0.147 g mg−1 min−1, respectively. The equilibrium time of sorption studies using jar tests was defined as 60 min. The surface area of light weight concrete was 22 m2 g−1. The experimental results have been analyzed using Freundlich, Langmuir, Brunauer, Emet and Teller (BET) and Dubinin–Radushkevich (D–R) sorption isotherm models. From D–R sorption isotherm, the mean energy of sorption was defined as 8.68 kJ mol−1. The sorption activation energy (Ea) was calculated using Arhenius equation. Thermodynamic parameters such as ΔH°, ΔS° and ΔG° were calculated from the slope and intercept of linear plot of ln KD against (1/T). The ΔH° and ΔG° values of fluoride sorption on light weight concrete materials show endothermic heat of sorption. But negative free energy value, indicating that the process of fluoride sorption is favored at high temperatures.