Current Density Dependence of Transport Selectivity of Metal Ions in the Electrodriven Process across the Cation Exchange Membrane

Understanding the mechanisms leading to the selective transport of cations in an electrodriven process across a cation exchange membrane is important to design and control the potential gradient-based separation process. In this study, a comprehensive description of the current density (I, over a broad current regime) dependence of transport selectivity (S_i) between cations of the same/different valence is presented. The role of conventional transport mechanisms such as diffusion, electromigration, and electroconvection in controlling the S_i was identified theoretically as well as by multiple experimental approaches. These parameters were found to be dependent on the limiting current density (I_lim). In general, irrespective of the cations involved, S_i (over Na⁺) decreased gradually with increasing I and then increased slowly (and saturated) after I_lim. This extent of variation of S_i was heavily dependent on the charge and hydration state of the cations. At I < I_lim, both diffusion and electromigration processes contributed and, notably, the sorption selectivity outweighed the migration selectivity. At I → I_lim, diffusion was the solitary mechanism responsible for cation transport and migration selectivity was the major contributor in S_i. At I > I_lim, as also validated by the Peclet numbers, the overall transport was dictated by electroconvection.