Selective Separation of Organics and Inorganics with Ion-Exchange Membranes: Influence of Solution Matrix and Organics Properties

Abstract

Introduction Many industrial processes results in the production of complex waste streams, often containing a combination of both organics and inorganics. Examples can be found in the food industry where for example salt is used to treat potatoes, olives, cheese, … RO brines can also contain a mixture of both organics and salts. The treatment of these streams is not straightforward, as the organics limit the applications of common physico-chemical treatment methods (e.g. by fouling of membranes) and the salts hamper conventional biological treatment. Furthermore, these conventional techniques are aimed at recovering the water or simply treating the streams before discharge. By selectively separating the organics from the salts, valorisation of both the inorganics (e.g. recycling back to the system) and the organics (e.g. as biogas) can be facilitated. Ion-exchange membranes (IEM) are expected to be a good candidate to achieve this selective separation, since they are less prone to fouling because of the lack of pressure during operation. However, although fouling of IEM has received attention in literature [1–4], the transport mechanics of organics is not well understood yet. In a first 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 [5]. This is of great importance not only for the cases described above, but also for other systems that encounter a mixture of organics and inorganics, such as membrane electrolysis and microbial electrosynthesis processes. 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. Materials and methods A 4-cell pair PC Cell 64004 ED set-up was used for the experiments, equipped with Fujifilm Type I membranes (8x8 cm²). Both experiments with a constant current density and experiments without an external driving force were executed, to distinguish between diffusive and electromigrative transport of the 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 SO4 2- 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.