Sorption of U(VI) species on hydroxyapatite

The sorption of uranyl (UO22+) cations to hydroxyapatite was studied as a function of the amount of sorbent, ionic strength, U(VI) concentration, pH and temperature. The rate of uranyl sorption on hydroxyapatite decreased with increased uranyl concentrations. The amount sorbed decreased with increased ionic strength and increased with pH to a maximum at 7-8. The sorption data for UO(2)2+ were fitted well by the Freundlich and Dubinin-Radushkevich (D-R) isotherms. The anions Cl-, NO3, SO42- and CH3COO- decreased the sorption of uranium on hydroxyapatite while S2O32- slightly increased it. The sorbed uranium was desorbed by 0.10M and 1.00 M solutions of HCl and HNO3. The thermodynamic parameters for the sorption of UO22+ were measured at temperatures of 298, 313, 323 and 333 K. The temperature dependence confirmed an endothermic heat of sorption. The activation energy for the sorption process was calculated to be +2.75 +/- 0.02 kJ/mol.