INTERACTION OF METALS AND PROTONS WITH ALGAE .4. ION-EXCHANGE VS ADSORPTION MODELS AND A REASSESSMENT OF SCATCHARD PLOTS - ION-EXCHANGE RATES AND EQUILIBRIA COMPARED WITH CALCIUM ALGINATE

Sorption is frequently described in terms of adsorption isotherms (Langmuir, Freundlich), but metal sorption on algae has been recognized to be an exchange process where sorption is accompanied by the release of ions. An experimental ion-exchange constant for Zn displacing Ca from Rhizoclonium was used to calculate concentrations over a wide enough range to assess interpretations given to Langmuir and Scatchard plots. While such plots may be convenient to describe maximum sorption and systematize data, misinterpretations can occur at low metal concentrations in an ion-exchange system. Values of K-ex for seven metals displacing Ca from Vaucheria correlated with formation constants of the metal acetates and with K-ex of the metals on calcium alginate, a model of cell wall components, indicating the bonding of metals to carboxylate groups of algal cell walls. Rates of metal desorption from Vaucheria by EDTA-Li are inversely related to binding strengths, and rates of Cd desorption from calcium alginate justify assumptions in the K-ex expression. Removal of Cd with a calcium alginate column was investigated for possible application to water treatment.