Evaluation of ion transport properties characterizing concentration polarization in membrane-solution system under different factors

The ion transport properties across the membrane under conditions of different membrane types, solution concentrations, flow rates and temperatures were investigated in a four-compartment reactor. By combining linear sweep voltammetry and chronopotentiometry, the limiting current density (I-lim), the ion transition time (tau) and the difference between ion transport numbers in the membrane and the solution (t(m)-t(s)) were determined. And the diffusion boundary layer thickness (delta) of the membrane-solution system at steady-state conditions was measured by electrochemical impedance spectroscopy. The results show that the use of Selemion membrane and the increase of solution concentration, flow rate and temperature, I-lim and tau increase, t(m)-t(s) and delta decrease. This means the concentration polarization of the system is weaker and complete concentration polarization is more difficult to occur. At the same time, I-lim, tau and t(m)-t(s) are strongly related to solution concentration and temperature, while the diffusion boundary layer thickness is mainly affected by solution concentration and flow rate. Additionally, I-lim of anion exchange membranes is larger than that of cation exchange membranes due to the difference in migration rates of anion and cation.