An experimental study on the ion-exchange behavior of the smectite of Cabode Gata (Almeria, Spain): FEBEX bentonite

Binary cation-exchange processes (Na-K, Na-Mg, and Na-Ca) have been investi gated at 25 degreesC in samples of bentonite from Cabo de Gata (Almeria, SE Spain). Exchange isotherms and Vanselow selectivity coefficients indicate that the preference of this bentonite for K+, Ca2+, and Mg2+ ions is greate r than that for the Na+ ion. The change in (i)(j)Kv as a function of the eq uivalent fraction of cation M, E-M, is interpreted on the basis of the pres ence of different types of adsorption/exchange sites (tetrahedral, octahedr al, and edge sites). For the Na/K exchange, (K)(Na)Kv exhibits a steep decr ease for E-K < 0.25-0.3. This is interpreted as due to preferential exchang e at the tetrahedral sites. However, the opposite behavior is observed in t he case of Ca and Mg, with a noticeable increase of (i)(Na)Kv for E-Ca(EMg) > 0.6-0.8. The compared selectivities of Ca and Mg for this smectite are n early equal. As far as the exchanger is concerned, the Na-Ca and Na-Mg exch ange is nearly ideal, in the raoultian sense. Thermodynamic exchange consta nts and the corresponding Gibbs free energies of reaction have been compute d from (i)(j)Kv coefficients. This allow us to quantify the exchanger's aff inity sequence, which appears to be Nat(+) < K+ < Mg2+ < Ca2+. (C) 2001 Aca demic Press.