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

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(+) >/= Ca(2+)) > Mg(2+). 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.