Fluorine effects on U(VI) sorption by hydroxyapatite

In this study, the effects of fluorine doping of HAp and fluoride ions concentration on the sorption behavior of U(VI) by hydroxyapatite (HAp) were investigated. The results demonstrated that the sorption equilibrium of U(VI) on fluorine-doped hydroxyapatite and hydroxyapatite was reached by 75 min of contact time, and the maximum sorption of U(VI) was achieved at a low pH(initial) value of 3. The pseudo-second order model described the kinetic sorption processes, whereas the Langmuir model and the sorption isotherm process indicated that the mechanism was chemisorption on a homogeneous material. The calculated thermodynamic parameters suggested the sorption of U(VI) on materials was a spontaneous and endothermic process. In particular, we determined that a high concentration of fluorine ions in solution markedly decreased the U(VI) sorption capacity, whereas fluorine doping also lowered the U(VI) sorption by HAp. However, fluorine doping increased the stability of HAp in acidic conditions. In addition, it is unlikely that UO2F+ and UO2F2(aq) species adsorbed via F atoms to the adsorbents, as the oxygen atoms or uranium atoms of uranyl ions are already bonded to F atoms, and F- is likely to form U-F complexes first rather than exchange for OH- when the solution contain uranium with very high concentration. The findings reported in this study aid in facilitating the removal of uranium from solutions using HAp and other materials, especially from fluorine-uranium radioactive wastewater in the environment. (C) 2015 Elsevier B.V. All rights reserved.