Enhancement of Mass Transfer Through a Homogeneous Anion-Exchange Membrane in Limiting and Overlimiting Current Regimes by Screening Part of Its Surface with Nonconductive Strips

Abstract

A series of anion-exchange membranes based on a Neosepta AMX-Sb homogeneous membrane (Japan) have been studied by applying parallel nonconducting strips of a 100 to 600 μm width with the interstrip distance ranging from 400 to 1900 μm. The current–voltage characteristics of the membranes and the pH of a NaCl solution (of 0.02 mol/L concentration) have been measured in the course of passing the solution through the desalination compartment of a flow-through electrodialysis cell. Two sets of membranes with a nonconducting surface fraction s_nc of 5 to 60%, in which the pattern steps on the surface are 1000 and 2000 μm, have been considered. It has been shown that the limiting current density, i_lim, depends on the nonconducting surface fraction: i_lim exceeds the corresponding value for the initial membrane in the case when s_nc is in the range from 5 to 20%, reaching a maximum approximately at s_nc = 10% followed by a decrease with the further increase in s_nc. At s_nc = 10%, the value of i_lim is greater when the inhomogeneity step is 2000 μm. It has been assumed that the growth in both the limiting current density and the rate of mass transfer through the modified membranes is due to electroconvection. The obtained experimental results correlate well with known mathematical models describing ion transport in membrane systems with allowance for electroconvection in the case of homogeneous and heterogeneous membranes.