Activity Coefficients and Selectivity Values of Cu⁺⁺, Zn⁺⁺ and Ca⁺⁺ Ions Adsorbed in the Nitella flexilis L. Cell wall during Triangular Ion Exchanges

The triangular exchanges Cu++−Zn++−Ca++ have been performed in the Nitella flexilis cell wall. The selectivity sequence for the exchange of the cations is Cu++ » Zn++ > Ca++. The presence of two types of exchange site has been confirmed. Possibly due to the Jahn-Teller effect, the cupric ion forms inner-sphere coordination with functional groups of the sites. This results in lower rotional mobilities and lower activity coefficients of the curpric ions immobilized close to the negative charges in the Stern layer. At higher equivalent fractions, the cupric ions in the diffuse layer are more mobile and have activity coefficients close to one.