This study examines the mechanisms of adsorption of anthocyanins from model aqueous solutions at pH values of 3, 6, and 9 by ion-exchange resins making the main component of heterogeneous ion-exchange membranes. This is the first report demonstrating that the pH of the internal solution of a KU-2-8 aromatic cation-exchange resin is 2-3 units lower than the pH of the external bathing anthocyanin-containing solution, and the pH of the internal solution of some anion-exchange resins with an aromatic (AV-17-8, AV-17-2P) or aliphatic (EDE-10P) matrix is 2–4 units higher than the pH of the external solution. This pH shift is caused by the Donnan exclusion of hydroxyl ions (in the KU-2-8 resin) or protons (in the AV-17-8, AV-17-2P, and EDE-10P resins). The most significant pH shift is observed for the EDE-10P resin, which has the highest ion-exchange capacity causing the highest Donnan exclusion. Due to the pH shift, the electric charge of anthocyanin inside an ion-exchange resin differs from its charge in the external solution. At pH 6, the external solution contains uncharged anthocyanin molecules. However, in the AV-17-8 and AV-17-2P resins, the anthocyanins are present as singly charged anions, while in the EDE-10P resin, they are in the form of doubly charged anions. Due to the electrostatic interactions of these anions with the positively charged fixed groups of anion-exchange resins, the adsorption capacities of AV-17-8, AV-17-2P, and EDE-10P were higher than expected. It was established that the electrostatic interactions of anthocyanins with the charged fixed groups increase the adsorption capacity of the aromatic resin by a factor of 1.8–2.5 compared to the adsorption caused by the π–π (stacking) interactions. These results provide new insights into the fouling mechanism of ion-exchange materials by polyphenols; they can help develop strategies for membrane cleaning and for extracting anthocyanins from juices and wine using ion-exchange resins and membranes.

The nutritional value of wines as well as fruit and berry juices is to a large extent determined by the presence of anthocyanins in them, which possess antioxidant activity. To extract these substances from food waste as well as to condition juices and wines, ion-exchange resins and membranes based on them are being increasingly used. The kinetics of sorption of anthocyanins by ion-exchange resins from model solutions with pH of 3, 6, and 9 is studied. It is shown that the Donnan exclusion of coions leads to a change in the structure of anthocyanins in the internal solution of ion-exchange materials compared to the external solution. As a result, the electrostatic interactions of anthocyanins with ion-exchange materials are weakened or enhanced compared to the expected interactions based on pH of the external solution. Large anthocyanin molecules more easily penetrate heterogeneous ion-exchange membranes compared to the resins from which they are fabricated. The reason is the presence of extended macropores localized at the boundaries of the contacts of the reinforcing cloth and composite of the ion-exchange material with an inert binding agent in the membranes.