On the selective transport of mixtures of organic and inorganic anions through anion-exchange membranes: A case study about the separation of nitrates and citric acid by electrodialysis

Electrodialysis is finding an increasing number of applications, recently also in the separation and purification of organic acids. This technology involves reducing the use of chemicals and the generation of wastes, as compared to conventionally used processes. Gaining insights about the competitive transport between organic and inorganic ions in ED systems is key to achieving efficient separation processes in the bioresource industry. In the present study, the competitive transport of organic (citrates) and inorganic anions (nitrates) through anion-exchange membranes is investigated, under varying pH conditions, ion concentrations, and applied currents, by means of chronopotentiometry and ED experiments. In the absence of nitrate ions and under acidic conditions (pH 2), the resistance of the membrane system is very high because citric acid is mainly present in its undissociated and uncharged form. In the case of sodium citrate (pH 8), the membrane resistance decreases, especially at high current densities, which promote dissociation reactions and the subsequent concentration increase in ionic species near the membrane. Such phenomenon is identified both in chronopotentiometric curves and in long-term ED experiments by a gradual drop in membrane voltage with time. Although being less concentrated than citrates, the selective removal of nitrates is the most effective choice for the separation of both types of species. The selectivity factor of nitrates over citrates reaches the highest values at low applied current densities (below the limiting current density), with stable permselectivity values higher than 20. Such conditions also lead to the least specific energy consumption per nitrates removed (<0.5 kW & sdot;hr & sdot;kg(NO3 -) (-1)). Therefore, ED can be used under such favorable operating conditions after a clarification step in order to remove inorganic ions from fermentation broths and increase the purity of organic acids. At higher current densities, the enhanced transport of citrate anions produces a significant drop in selectivity towards nitrates and increases the specific energy consumption.