Modeling multicomponent ion transport to investigate selective ion removal in electrodialysis

Abstract

A 2-dimensional multicomponent ion transport model based on Nernst-Planck (NP) equation and electroneutrality assumption is developed for an electrodialysis (ED) cell operated in the ohmic regime. The flow in channels are assumed incompressible, isothermal, and laminar. Donnan equilibrium and flux continuity are considered at ion-exchange membrane (IEM)-solution interfaces. To account for tortuosity effects inside membranes, effective ionic diffusion coefficients are calculated using membranes water volume fractions. The developed multicomponent model is used to elucidate the effects of feed solution properties, cell properties, system hydrodynamics, operational conditions, and membrane properties on selective divalent ion removal in the cell.The results indicate that the selective removal of divalent ions improves with decreasing the cell length, imposed potential, and ionic strength of feed water. Enhanced mixing in spacer-filled cell also promotes selective divalent ion removal. Higher concentrations of fixed charges on the membranes results in greater selectivity toward divalent ions at short cell length and low imposed potentials. With equal concentrations of fixed charges, membranes with high water content are less favorable for selective divalent ion removal. The developed framework enables the optimum selection of cell design, IEMs, spacer design, and operational conditions to selectively remove ions from multicomponent solutions.