Experimental study and modeling of the influence of pH on the selective extraction of organic acid by electrodialysis

This study investigates the influence of pH on the selective transport of organic acids in electrodialysis, focusing on a reaction–transport mechanism enhancing weak acid dissociation at the anion-exchange membrane. Electrodialysis experiments using sodium acetate at pH 4.8 show that acetic acid transfer to the concentrate is sustained even after sodium depletion, confirming the acid dissociation reaction-mechanism near the membrane. The study is extended to equimolar mixtures of acetate with formate, lactate, and butyrate at various initial pH values. Results showed that the selectivity varies with the pH and acids dissociation constants. Formate/acetate selectivity increased at lower pH due to a higher dissociation degree of formate (lower pKa) compared to acetate, while acetate/lactate selectivity decreased. In contrast, acetate/butyrate selectivity remains nearly constant due to their similar pKa values. A previously developed model is extended to account for pH-dependent ion transport and solvent fluxes. Experimental data provide parameters, including hydration numbers and anion and cation permselectivities. A corrective coefficient σ is introduced to reflect membrane-driven dissociation mechanism. The resulting model accurately predicts solute concentration and pH variations, supports the hypothesis of a constant permselectivity over the investigated pH range, and offers insight into the individual transfer of each ion. It can thus be used to determine operating conditions, including pH, that maximize selectivity in electrodialysis. Finally, these findings show that, for mixtures of monocarboxylic acids, a reduced description with a single corrective parameter σ is sufficient to quantitatively predict composition and pH variations while treating permselectivity as constant. • Selectivity varies with pH, acids pKa and permselectivities • Counter-ion transfer drives ED selectivity in OA systems • Permselectivity is not influenced by the pH • Permselectivity-based previous model is extended to any pH conditions • Corrective coefficient introduced to account for acid dissociation mechanism