Effects of acid–base cleaning procedure on structure and properties of anion-exchange membranes used in electrodialysis

Abstract

In this paper, the consequences of traditional chemical cleaning on anion-exchange membrane structure and properties are thoroughly assessed. A homogeneous anion-exchange membrane (AEM1) and a heterogeneous one (AEM2) were subjected to ageing protocols in 2M NaOH and 2M HCl up to 700h at room temperature. Moreover, both membranes were exposed to cleaning cycles in which they were soaked alternatively in 0.1M HCl and 0.1M NaOH (30min each) from 100 to 400h to simulate the cleaning-in-place (CIP) procedure commonly used in industrial electrodialysis stacks. Exposure to strong acidic conditions did not result in severe changes of the physicochemical properties of AEM1. In contrast, strong alkaline conditions resulted in the transformation of poly(vinyl chloride) (PVC), i.e. the polymeric binder of AEM1, into a polyene structure as a consequence of dehydrochlorination enhanced by the presence of quaternary ammonium groups. Additionally, a small part of the functional sites were degraded by E1 or E2 elimination reactions. On the other hand, the cleaning cycles caused entanglement modifications in the semi-interpenetrating polymer network structure of AEM1. This was a consequence of the constant changing of the equilibrating solutions nature, which led to membrane inflate–deflate sequences. Therefore, membrane toughness was deteriorated, thus leading to the formation of cracks and fractures, as observed before on the same type of membrane after industrial electrodialysis in which traditional CIP is performed through acid–base sequences. In contrast, the heterogeneous AEM2 suffered degradation in strong acid conditions, whereas it seemed to be more resistant than AEM1 in strong alkaline conditions. Regarding the HCl–NaOH cycles, modifications were more significant for AEM2. This investigation permitted to correlate membrane ageing resulting from such ex-situ acid–base cleaning cycles and that suffered after conventional electrodialysis during traditional CIP.