Ageing of ion-exchange membranes in electrodialysis: A structural and physicochemical investigation

Understanding the ageing mechanisms of ion-exchange membranes (IEMs) used in electrodialysis (ED) for food industry applications constitutes a major challenge. In this regard, four used membranes (two cationic ones and two anionic ones) were analyzed at the end of their useful life (2 years of ED operation) and compared with their respective fresh new samples to assess the evolution of their structural and physicochemical characteristics, and explore their deterioration. The conductivity, ion-exchange capacity, water content, contact angle and counterion transport number were determined for each new and used membrane. Scanning electron microscopy (SEM), energy dispersive X-ray (EDX), and FTIR analyses were also performed. The used cation-exchange membranes (CEMs) suffered significant degradation; nonetheless, they were generally more robust and resistant than the anion-exchange counterparts, which were more unstable. A significant degradation in the polymer matrix of both membrane types was found. Both used CEMs and anion-exchange membranes (AEMs) lost a part of ion-exchange sites, and their specific electrical conductivity decreased. However, the CEMs became denser with lower water content and higher surface hydrophobicity, while the water content and the thickness of the AEMs increased in about two times. The permselectivity of used CEMs decreased, while that of AEM analogues was not modified. A physicochemical model schematically describing the time evolution of membrane structure and properties was built on the basis of the microheterogeneous model and the experimental data. The main idea was to present colloid aggregates adsorbed by the membrane as nanoparticles occupying a part of membrane pores. This model was used to explain the variation of the totality of equilibrium and transport properties, and in particular, the apparent contradiction between the loss of ion-exchange capacity and the increase in permselectivity of the used AEMs.