Mass-Action Expressions of Ion Exchange Applied to Ca2+, H+, K+, and Mg2+ Sorption on Isolated Cells Walls of Leaves from Brassica oleracea1

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The cation exchange properties of cell walls isolated from collard (Bassica oleracea var acephala D.C.) leaves were investigated. Cation sorption on cell walls was described by mass-action expressions of ion exchange, rather than by the traditional Donnan equilibrium. The mass-action expressions enable the selectivity of the wall for one cation over another to be determined unambiguously from ion exchange isotherms. We found that: (a) the cation composition of the wall varied as a function of the solution cation concentration, solution cation composition, and pH in a way predicted by mass action; (b) the affinity of the wall for divalent cations increased as the equivalent fraction of divalent cation on the wall increased, and as the concentration of divalent cations in solution increased; (c) the selectivity of the wall for any metal cation pair was not altered by the concentration of H+ in solution or on the wall; (d) H+ sorption on the wall may be treated as a cation exchange reaction making it possible to calculate the relative affinity of the wall for metal cation pairs from H+-metal (Me) titration curves; and (e) the relative affinity of the wall for the cations we studied was: H+ ≫ (K+ ≥ Ca2+) > Mg2+. A cation-exchange model including surface complexes is consistent with observed cation selectivity. We conclude that metal cations interact with the wall to minimize or eliminate long-range electrostatic interactions and suggest that this may be due to the formation of site-specific cation-wall surface complexes.

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