The crossed interdiffusion of sodium nitrate and sulfate through an anion exchange membrane, as studied by Raman spectroscopy

Raman spectroscopy is applied to visualize concentration profiles in solution adjacent to the top of DSV anion exchange membranes in horizontal positions. The concentration profiles of sulfates, nitrates and their fluxes through the membrane are recorded as a function of time during 13 - 15 h experiments. A three layer mathematical model based on extended Nernst - Planck equations with a convective term is developed. The membrane's kinetic and static parameters, such as ionic diffusion and ion exchange coefficients, are determined by separate experiments. Only one parameter, the diffusion boundary layer thickness, delta, is fitted. It is found that the model described fluxes well, especially when the variation of delta is taken into account. At the same time, the model gives nearly linear concentration profiles, whereas the measured profiles are much more complicated. This complicated shape is explained by natural convection and the emergence of the Benard vortexes in the system.