Separation of organics and salts with ion-exchange membranes: effect of matrix and organics

Separation of organics and salts with ion-exchange membranes: effect of matrix and organics Ion-exchange membranes (IEM) are expected to be a good candidate to achieve a selective separation between organics and salts, since they are less prone to fouling because of the lack of pressure during operation. This is useful for the application of IEM in the treatment of complex streams by electrodialysis and reverse electrodiaysis for example, especially when wastewater and seawater are used. However, although fouling of IEM has received attention in literature, the transport mechanics of organics is not well understood yet. In a previous study, we showed that the presence of NaCl greatly influences the transport of organics and that this transport seems to be mainly diffusion driven in the presence of salts [1], indicating that operational parameters such as the applied potential have little influence on the transport, but rather matrix and membrane properties will have an effect. This study focusses on the effect of different salt types (NaCl, MgCl2, Na2SO4) and the direction of the transport of the organics relative to the transport of the salts. Both trace organic contaminants (TOrC) and organic acids (OA) are used as a model for organics. Results Three different hypotheses were tested in this research; (1) transport of organics is different in the presence of multivalent salt ions versus monovalent ones, (2) the transport direction of the organics with respect to the salt is important and (3) organics transport is mainly diffusion driven. The tests with multivalent salt ions clearly show that the transport of negatively charged organics is higher in the presence of Na2SO4 and the transport of positively charged organics is higher in the presence of MgCl2 when compared to NaCl. This can be explained by the lower diffusion coefficient of both SO42- and Mg2+ compared to Cl- and Na+ respectively. When salts and organics are dosed in different compartments, Donnan dialysis plays a significant role in diffusion, causing an increase in the organics transport. This was observed both in experiments with and without an external driving force, further confirming the diffusive nature of the organics transport. This was further endorsed by the experiments with OA, where no difference in transport rate can be observed between experiments with and without external driving force. Furthermore, experiments with OA in the absence of salts show a significantly higher transport rate, both with and without a constant current density. This study confirms the diffusive nature of organics transport in the presence of salts. Ongoing research focusses on the influence of membrane characteristics on the transport behaviour of organics, with the ultimate goal of developing tailor-made membranes for the selective separation between organics and salts. This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 685579 References [1] Vanoppen, M., Bakelants, A.F. a M., Gaublomme, D., Schoutteten, K.V.K.M., Bussche, J. Vanden, Vanhaecke, L. et al. (2015) Properties Governing the Transport of Trace Organic Contaminants through Ion-Exchange Membranes. Environmental Science & Technology, 49, 489–97.

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