Measurement of Membrane Characteristics Using the Phenomenological Equation and the Overall Mass Transport Equation in Ion-Exchange Membrane Electrodialysis of Saline Water

Abstract

The overall membrane pair characteristics included in the overall mass transport equation are understandable using the phenomenological equations expressed in the irreversible thermodynamics. In this investigation, the overall membrane pair characteristics (overall transport number 𝜆, overall solute permeability 𝜇, overall electro-osmotic permeability 𝜙 and overall hydraulic permeability 𝜌) were measured by seawater electrodialysis changing current density, temperature and salt concentration, and it was found that 𝜇 occasionally takes minus value. For understanding the above phenomenon, new concept of the overall concentration reflection coefficient 𝜎∗ is introduced from the phenomenological equation. This is the aim of this investigation. 𝜎∗ is defined for describing the permselectivity between solutes and water molecules in the electrodialysis system just after an electric current interruption. 𝜎∗ is expressed by the function of 𝜇 and 𝜌. 𝜎∗ is generally larger than 1 and 𝜇 is positive, but occasionally 𝜎∗ becomes less than 1 and 𝜇 becomes negative. Negative 𝜇 means that ions are transferred with water molecules (solvent) from desalting cells toward concentrating cells just after an electric current interruption, indicating up-hill transport or coupled transport between water molecules and solutes.