To evaluate the permeability of heavy-metal ions that exist as cation and anion species in seawater, we proposed a model equation that expresses the permeation rates of monovatent and divalent anions across an anion-exchange membrane using the permeation rates of Br- and SO42-. The permeability of zinc across a cation- and an anion-exchange membrane was evaluated using the cation- and anion-model equations. We analyzed the permeation rates of zinc species (EZn2+, EZnCl+, EZnCl3-, EZnCl42-). We found that EZnCl+ and EZn2+ decreased with an increase in the concentration of chloride ions in the feed solution, indicating that E-Zn(C) decreases with an increase in the concentration of chloride ions. Moreover, EZnCl3- and EZnCl42- increase with the concentration of chloride ions in the feed solution, indicating that E-Zn(A) increases with the concentration of chloride ions. (c) 2005 Elsevier B.V. All rights reserved.