Ion-exchange properties of brown algae. II. Rate mechanism for calcium-hydrogen ion exchange for particles from Ascophyllum nodosum

The ion-exchange properties of algal particles from A. nodosum have been investigated. The Ca/H ion-exchange rate at hydrogen ion concentration 0·1 n was shown to be strikingly lower, while the selectivity coefficent K was about ten times higher, for A. nodosum than for L. digitata and L. hyperborea stipe particles. Log K showed a marked decrease with increasing calcium equivalent fraction in the algal particles. The higher Ca/H selectivity of algal particles from A. nodosum was shown to be the cause of the observed difference in exchange rate between this alga and the two other species. The ion-exchange rate Ca/H for 36–52 mesh particles from A. nodosum at hydrogen ion concentration 0·1 n was governed by a film-diffusion mechanism and the diffusion of calcium through a thin liquid film enveloping the particles was suggested as the rate-determining step. The rate was particle-diffusion controlled for 10–20 mesh particles. The exchange rate for 36–52 mesh particles increased strongly as the hydrochloric acid concentration was increased from 0·05 to 1 n; this exchange appeared to proceed like a second order reaction at 0·05 n and showed a gradual change to a first order reaction at 1 n-acid.