STANDARD ENTHALPIES OF ADSORPTION OF DIVALENT AND TRIVALENT CATIONS ON ALUMINA

Standard enthalpies of Ni(II), Gd(III), and Y(III) adsorption on alumi na can be estimated from the temperature effect on apparent equilibriu m constants of surface reactions. The results are relatively insensiti ve to the choice of the adsorption model, although the calculated appa rent equilibrium constants of surface reactions at a given temperature are model dependent and they often differ by several orders of magnit ude from one model to another, The conditions of the experiment - soli d-to-liquid ratio, initial concentration of multivalent cations in sol ution, presence of various 1:1 and 2:1 electrolytes at various concent rations - do not significantly affect the calculated values of standar d 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 adsorp tion 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 o f nickel and gadolinium on alumina, Thus, barium and magnesium do not compete with heavy metal cations for the surface sites. (C) 1997 Acade mic Press.