Adsorption kinetics of fluoride on bone char and its regeneration

The adsorbent of bone char (BC), produced from the pyrolysis of crushed animal bones, was dominated by the mesopores of the Brunauer–Emmett–Teller (BET) surface area. The optimal condition for defluoridation with BC was a pH level near 5.0. Chloride and nitrate ions could increase fluoride adsorption capacity in contrast with the effect of sulfate and carbonate ions. The interchangeability between fluoride and hydroxyl groups on BC sorbent was proved by the Fourier transform infrared spectroscopy. Langmuir equation had a better correlation coefficient than the Freundlich equation at various temperatures. Thermodynamic parameters such as ΔG°, ΔH°, ΔS°, Ea and S*, have been calculated to describe the nature of fluoride adsorption onto BC. Negative ΔG° and ΔH° values at various temperatures indicate a spontaneous process, and its exothermic effect, respectively. However, a positive ΔS° value represents an increasing process for entropy. The Ea and S* values ranging from 5 to 40 kJ·mol–1 and 0 to 1, respectively, demonstrated that the adsorption is dominated by physical process, although the adsorption kinetic process was involved external diffusion, intraparticle diffusion and chemical reaction equilibrium stage. A high concentration of NaOH solution increases efficiency of removing adsorbed F– ions from the BC surface.