Equilibrium and kinetics study of Gd(III) and U(VI) adsorption from aqueous solutions by modified Sorrel's cement

Modified Sorrel's cement was prepared by the addition of ferric chloride. The modified cement (MF5) was analyzed and characterized by different methods. Adsorption of Gd(III) and U(VI) ions in carbonate solution has been studied separately as a function of pH, contact time, adsorbent weight, carbonate concentration, concentration of Gd(III) and U(VI) and temperature. From equilibrium data obtained, the values of Delta H, Delta S and Delta G were found to equal -30.9 kJ.mol(-1), -85.4 J.mol(-1)., K-1, and -5.4 KJ . mol(-1), respectively, for Gd(III) and 18.9 kJ . mol(-1), 67.8 J . mol(-1) K-1 and -1.3 KJ . mol(-1), respectively, for U(VI). The equilibrium data obtained have been found to fit both Langmuir and Freundlich adsorption isotherms. The batch kinetic of Gd(III) and U(VI) on modified Sorrel's cement (MF5) with the thermodynamic parameters from carbonate solution were studied to explain the mechanistic aspects of the adsorption process. Several kinetic models were used to test the experimental rate data and to examine the controlling mechanism of the adsorption process. Various parameters such as effective diffusion coefficient and activation energy of activation were evaluated. The adsorption of Gd(III) and U(VI) on the MF5 adsorbent follows first-order reversible kinetics. The forward and backward constants for adsorption, k (1)and k (2) have been calculated at different temperatures between 10 and 60 degrees C. Form kinetic study, the values o Delta H* and Delta S* were calculated for Gd(III) and U(VI) at 25 degrees C. It is found that Delta H* equals -14.8 kJmol(-1) and 7.2 kJmol(-1) for Gd(III) and U(VI), respectively, while Delta S* were found equal -95.7 Jmol(-1)K(-1) and -70.5 Jmol(-1)K(-1) for Gd(III) and U(VI), respectively. The study showed that the pore diffusion is the rate limiting for Gd(III) and (VI).