Separation of carboxylates using electrodialysis: a key process for the electro-refinery concept

Abstract

Electrodialysis, using anion exchange membranes, is an effective technique for separating carboxylates in aqueous solutions. By leveraging the different pKa values of acids, selective separation can be achieved, enhancing the value of the aqueous streams. Thus, electrodialysis can serve as both a separation and purification method. In this work, the transport of carboxylates (oxalate and acetate) through four types of commercial anion exchange membranes (AEMs) has been evaluated. The main characteristics of this transport, particularly those relevant to the purification of streams necessary for the development of electro-refining processes of organic products, have been determined. Significant differences were observed among the membranes, with Fumasep FAB-PK-130 showing the highest efficiency in separating oxalate through electrochemically assisted crystallization, achieving 50% crystallization of oxalic acid in under 10 h. Additionally, pH control enabled the separation of oxalates and acetates, resulting in an aqueous stream containing only oxalic acid from an equimolar mixture of oxalic and acetic acids. However, the results are quite different from those expected according to the acid-base equilibrium of bulk solution that relates the dissociated ionic species and protonated molecular species in both compounds. This highlights the existence of local pH profiles that influence this separation and that need to be understood to achieve effective purification.