Physico-chemical local equilibrium influencing cation transport in electrodialysis of multi-ionic solutions

• Electrodialytic removal of solutions containing ternary mixture of electrolytes
• Physico-chemical local equilibriums during cation transport
• Theoretically estimated LCD could closely (± 5–7%) predict experimental data.
• Cations with higher charge had low transport number, high adsorption and resistance.

Electrodialysis (ED) of solutions containing ternary combination of electrolytes (NaCl, KCl, CaCl2, MgCl2, FeCl3) was carried out. Besides linear velocity of the fluid flowing through the ED cell, physical properties of cations and their interaction with the gel phase of membrane significantly influenced ion transport. The role of concentration polarization was evaluated from limiting current density (LCD) measurement. Theoretically estimated LCD values based on effective diffusivity of each ions and mass transfer coefficients (Sherwood number, Sh correlation) satisfactorily (error ± 5–7%) predicted experimental data. Measurement of transport number, adsorption equilibrium, water uptake and membrane resistance were carried out for each composition to understand the role of cations — membrane equilibrium in ion transport. Cations with higher charges showed reduced transport number, higher adsorption and higher resistance over cation exchange membrane (CEM). Water uptake measurement of equilibrated CEM showed negligible difference when cations of similar charges were interchanged while a meager 1.5% variation was noted when cations of different charges were interchanged.