Defluorination of Wet-Phosphoric Acid： Equilibrium and Kinetic Studies

This study describes the advantages of using iron-oxide-pillared MF (montmorillonite) and EDTA (ethylenediaminetetraacetic acid)-pillared MD (montmorillonite) for the removal of fluoride in the phosphoric acid. The MF and MD were characterized by using BET (Brunauer-Emmett-Teller) and SEM (scanning electron microscope) for there to appreciate their textural and structural properties and in order to appraise their capacity in the fixation of the fluorine experimental results of fluorine adsorption show a defluoration of the phosphoric acid passing beyond 90% with both clays with a light performance for the MD. The kinetics of fluorine adsorption on MF and on MD has been measured at different temperatures. Indeed, the adsorbed quantity increases more favorably to temperatures more high. This kinetics could be described by two models: pseudo-first-order or pseudo-second-order, which depends on temperature and the nature of adsorbent. At different temperatures used and with various adsorbents (MD and MF, the pseudo second order model was the most reliable to determine the order of kinetics of adsorption of fluorine, which is also reflected a good correlation coefficient (above 0.99). The fluoride sorption was reasonably explained with Langmuir or Freundlich isotherms. Thermodynamic parameters such as ΔG°, ΔH°, ΔS° and kinetic one Ea (apparent activation energy) were calculated in order to understand the nature of sorption process. Results indicate that the mechanism of adsorption of the ion of fluoride on the pillared montmorillonite is an exothermic reaction. During equilibrium conditions, both the systems showed negative Gibbs energy, which indicates stability of system. The values of apparent activation energy for the both clays show that the adsorption of F - ions is easier on the MF.