Batch electrodialysis of ammonium nitrate and sulfate solutions

Abstract

Batch electrodialysis of ammonium nitrate and sulfate solutions at potentiostatic conditions was investigated using the PC SA and SK membranes. The influence of the initial concentration (0.3–1.5M) and applied voltage (15, 20V) on the current efficiency, CE, and minimal diluate concentration was investigated. It was found that for both salts the average CE was comparable and was in the range 67–80%, depending on the conditions. For the concentrate concentration not exceeding 0.8M, the lowest diluate concentration was below 0.002, 0.005M for sulfate, nitrate, respectively. In the final stage of electrodialytic process, the correlation between the time dependence of the instantaneous CE and the changes of electric current and of potential drop on the membrane stack was analyzed. It was found that by estimating inflection point of these dependencies it is possible to find the moment of process when CE decreases and the energy consumption increases approximately twice. The concentration changes were fitted using the model based on the extended Nernst-Planck equation and ideal Donnan equilibrium. The results of each experiment were fitted with a very good accuracy, however, it was not possible to fit all the experimental results with one set of the model parameter values. The volume changes were satisfactorily approximated by a two parametric equation relating volume flux with electric current density and concentration difference. The effective electroosmotic coefficient referring to a membrane pair corresponds to ca. 6–8mol of water per 1F of electric charge passed through the membrane stack.