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 beenperformed in the Nitella flexilis cell wall. The selectivitysequence for the exchange of the cations is Cu⁺⁺ » Zn⁺⁺> Ca⁺⁺. The presence of two types of exchange site has beenconfirmed. Possibly due to the Jahn-Teller effect, the cupricion forms inner-sphere coordination with functional groups ofthe sites. This results in lower rotional mobilities and loweractivity coefficients of the curpric ions immobilized closeto the negative charges in the Stern layer. At higher equivalentfractions, the cupric ions in the diffuse layer are more mobileand have activity coefficients close to one.