Thermochemical investigation on the adsorption of some divalent cations on modified silicas obtained from sol–gel process

Silicas with one or two immobilised ethylenediamine molecules, presenting electron donor Lewis bases, have been prepared through sol–gel method. Characterisation by various techniques indicated equivalent quantities of nitrogenated groups on both kinds of silicas. The adsorption enthalpy changes obtained from the interaction between basic sites on pendant groups and M(II) {Hg(II), Cu(II) and Co(II)} and were determined from calorimetry, carried out at 298.15 K in aqueous solution. From these determinations, the maximum capacity of adsorption (Ns), the adsorption enthalpy at different coverages of the surface (ΔadsHm) and the integral enthalpy for the maximum adsorption (ΔsatHm) were obtained. The determined energy of distribution of the adsorption sites indicates the heterogeneous adsorbentes. It is shown that Ns values are very similar for both silicas, despite the differences of the total number of immobilised groups. All reactions have favourable enthalpic contributions. Other thermodynamic quantities are discussed in terms of stability of the complexes at the solid/solution interfaces.