Studies on the thermodynamics of exchange in clays—III Nickel exchange on Na-, Ca- and Mg-Montmorillonites

Abstract

Exchange equilibria involving Ni ions on Na-, Ca- and Mg-montmorillonites were studied with the help of the thermodynamic models of earlier workers. The exchange isotherms and free energy changes were indicative of a lower preference of Ni for the base saturated montmorillonites. The exchange was in accordance with the Hofmeister series. Enthalpy changes indicated a stronger binding of Ni on Ca- and Mg-montmorillonites than on Na-clay. Entropy findings were indicative of a dehydrated and disordered arrangement of Ni ions during their adsorption on Na-montmorillonite. The effect of dehydration on the entropy was the reverse in the case of Ca- and Mg-montmorillonites. The diversity of the exchange equilibria involving Ni, Na, Ca and Mg ions on the montmorillonite surface was further confirmed from our observations on the activity coefficients and the values of Δ G m x , Δ H m x and Δ S m x .