Adsorption of some divalent cations from aqueous solution on precipitated silica

Abstract

The adsorption of the cations Co^II, Ni^II, Cu^II, Zn^II and Cd^II on the K⁺ form of a precipitated silica is reported. The adsorption isotherms of Co^II and of Ni^II at pH 7.0 and different temperatures follow the Langmuir isotherm. The endothermic ‘apparent’ heats of adsorption, 2 and 10 kJ mol^–1, respectively, are compared with those for Cu^II and Cd^II. The limiting amount adsorbed, X_m, of the four ions decreases as the pH is lowered. An ion-exchange mechanism between hydrated K⁺ ions in the outer Helmholtz layer and metal, M^II, ions in solution is proposed. The results also suggest that (MOH)⁺ ions are adsorbed at the higher pH values. A plot of X_m at pH 6.0 and 323 K for the five ions studied against atomic number exhibits the Irving–Williams sequence.

At pH 3.0, i.e. close to the point of zero charge of silica, the adsorption results have been used to calculate the free energy of specific adsorption, ΔG, using the Grahame equation. ΔG was –11.1, –10.7, –13.4, –13.3 and –12.1 kJ mol^–1 for M = Co, Ni, Cu, Zn and Cd, respectively. Specific adsorption is tentatively ascribed to the formation of surface coordination complexes.