Modeling the electric transport of sulfuric and phosphoric acids through anion-exchange membranes

Abstract

The electric transport of the H 2SO 4 and H 3PO 4 mixture through two types of anion-exchange membranes (strong base: ACM; weak base: AAV) was investigated. The H 2SO 4 removal efficiency from the mixture and the H 3PO 4 retention efficiency were calculated. It was found that the effectiveness of the separation process is diminished by the association of phosphoric anions with the fixed charges which reduces the effective charge concentration. The transport of acids was quantitatively described by the model based on the extended Nernst–Planck equation and the Donnan equilibria. It satisfactorily describes the electro-membrane process except its last stage when H 2SO 4 concentration at the cathode side of a membrane is of a low value. When the association equilibrium between H 2PO 4 − and the fixed charges of the ACM membrane is introduced into the model, the concentration changes of H 3PO 4 could be much better described. Assuming the ideal Donnan equilibrium to occur, the transport of the both acids cannot be described by the same parameter related to the membrane porosity, tortuosity and the ion diffusivities. Regarding H 3PO 4, different ionic equilibria were taken into account in order to show their effects on the values of optimal model parameters.