Scaling of cation exchange membranes by gypsum during Donnan exchange and electrodialysis

Gypsum (CaSO₄*2H₂O) scaling in homogeneous CMV and heterogeneous MK-40 cation exchange membranes (CEM) was investigated under Donnan exchange and electrodialysis (ED) regimes. Scale formation effects were examined in terms of counter-ion flux decline and changes in the potential difference across the scaled-membrane. Under ED regime, changes in the overall electrical stack resistance and the extent of water splitting were also studied as a function of the current density. Similar to findings with corresponding anion exchange membranes (AEM's) studied in our previous work (Asraf-Snir et al., 2014, 2016) [1,2], scaling in the homogeneous cation exchange membrane (CMV) is mainly characterized by surface precipitation, while in the heterogeneous cation exchange membrane (MK-40), scale grows mainly in the membrane interior. The effect of internal scale on the membrane performance was more severe than the effect of surface scale, indicated by earlier and sharper flux decline (both with diffusion and electromigration) as well as larger increase of the potential across the membrane, and thus of the overall stack electrical resistance. It was found, however, that CEM's are less susceptible to scaling than AEM's, indicated by belated and less significant flux decline, smaller membrane potential difference increase and more modest increase in stack electrical resistance. This implies that the chemical nature and structure of the functional groups inside the membrane matrix may play an important role in the scaling process on and within ion exchange membranes.