Standard Enthalpies of Adsorption of Di- and Trivalent Cations on Alumina

Abstract

Standard enthalpies of Ni(II), Gd(III), and Y(III) adsorption on alumina can be estimated from the temperature effect on apparent equilibrium constants of surface reactions. The results are relatively insensitive to the choice of the adsorption model, although the calculated apparent equilibrium constants of surface reactions at a given temperature are model dependent and they often differ by several orders of magnitude from one model to another. The conditions of the experiment—solid-to-liquid ratio, initial concentration of multivalent cations in solution, presence of various 1:1 and 2:1 electrolytes at various concentrations—do not significantly affect the calculated values of standard enthalpies of adsorption. Assuming that the total number of surface sites and the number of protons released per one adsorbed heavy metal cation do not depend on temperature, the standard enthalpies of adsorption on alumina at constant pH are 65 kJ/mol for Ni, 90 kJ/mol for Gd, and 90 kJ/mol for Y. The presence of alkaline earth metal cations (up to 0.05 mol dm−3) does not significantly influence the adsorption of nickel and gadolinium on alumina. Thus, barium and magnesium do not compete with heavy metal cations for the surface sites.