What are the main contributions to the total enthalpy of displacement accompanying the adsorption of some multivalent metals at the silica-electrolyte interface?

The integral molar enthalpies of displacement, Delta(dpl)h, accompanying adsorption of Mg(II), Ca(II), Sr(II), Ba(II), Cd(II), Co(II), Zn(II), and Eu(III) cations from aqueous solutions of metal nitrate onto Spherosil XO75LS at 298 K were determined based on the combination of liquid flow calorimetry and classical adsorption for two degrees of surface coverage: 0.035 mu mol m(-2) and 0.08 mu mol m(-2), in the presence of 0.1 mol L-1 sodium nitrate in the aqueous phase at pH 5, 6, and 7. The displacement was shown to be endothermic and quite independent of the chemical specificity of the adsorbing metal. Two enthalpy effects were postulated to contribute mostly to the positive Delta(dpl)h values, depending on the experimental pH value: (i) cation dehydration upon adsorption and (ii) deprotonation of surface silanols to create negatively charged SiO- sites. Changing proportions among the various adsorbed species, including "free" Eu3+ or Cd2+ cations and hydrolyzed Eu(OH)(2+), Eu(OH)(2)(+) or Cd(OH)(+) cations, were accepted to explain the downward trends in Delta(dpl)h with increasing extent of adsorption for Eu(III) and Cd(II). (C) 2013 Elsevier Inc. All rights reserved.