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

[Display omitted] ► Molar integral enthalpy of displacement is measured at constant pH and ionic strength. ► Reliable data are provided to test the thermodynamic models of heavy metal adsorption. ► Displacement is endothermic, irrespective of the nature of the adsorbing metal. ► Cation dehydration during sorption is shown to be responsible for this endothermicity. ► The adsorbed species likely include “free” and hydrolyzed cations. The integral molar enthalpies of displacement, Δdplh, 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 298K were determined based on the combination of liquid flow calorimetry and classical adsorption for two degrees of surface coverage: 0.035μmolm−2 and 0.08μmolm−2, in the presence of 0.1molL−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 Δdplh 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 Δdplh with increasing extent of adsorption for Eu(III) and Cd(II).