Effect of Feed Solution pH on Efficiency of Electrodialysis Extraction of Tartrates

Abstract

Resource-efficient and environmentally sustainable electrodialysis (ED) is increasingly being used for the separation and purification of organic acids, including the extraction of their anions from wines, juices, and biochemically processed waste products. In this study, tartaric acid transport through the CJMA-3 anion-exchange membrane was investigated using voltammetry, chronopotentiometry, and ED experiments. It was shown that when using a Na_xH_(2–x)T solution at pH 9.0, which contains only divalent tartrate anions T²⁻, the transport patterns are similar to those well-known for strong electrolytes. However, at pH 2.5 or 3.0, the solution contains a mixture of undissociated tartaric acid molecules H₂T and monovalent anions HT⁻. Upon entering the membrane, some of the HT⁻ anions dissociate. Protons are expelled into the depleted solution due to the Donnan effect, while the newly formed divalent anions T²⁻ migrate through the CJMA-3 membrane. The reduction in HT⁻ concentration near the membrane stimulates the irreversible dissociation of H₂T. Under the influence of the electric field, protons are removed from the reaction zone and migrate into the solution, while anions move into the membrane. Thus, tartrate transport through the anion-exchange membrane occurs even when the feed solution primarily contains undissociated acid molecules. These mechanisms lead to empirical limiting currents significantly exceeding theoretical limiting current values. The energy consumption for extracting 20% of tartrates from a 0.022 M Na_xH_(2–x)T solution is 0.22 (pH 9.0), 0.32 (pH 3.0), and 0.57 kWh/kg (pH 2.5). The duration of ED increases in the following order: pH 3.0 \( \ll \) pH 9.0 < pH 2.5.