Adsorption of Anthocyanins by Cation and Anion Exchange Resins with Aromatic and Aliphatic Polymer Matrices

Abstract

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.

Understanding of Adsorption and Desorption Mechanisms of Anthocyanins and Proanthocyanidins on Heterogeneous and Homogeneous Cation-Exchange Membranes

“…This experimental fact was due to Donnan’s exclusion of protons from the membrane, which were the products of the photolysis reactions of anthocyanins and other components of cranberry juice. A similar phenomenon in the case of ion-exchange resins, which were soaked in a model solution of anthocyanins, was observed in Sarapulova et al [ 15 ] and [ 21 ]. The CJMC-5 membrane had rather large macropores in the ion-exchange material in addition to the macropores localized in the threads of the reinforcing fabric.…”

“…Hence, when the pH value of the mixture of solvents was 6, anthocyanin cations should have lost their electrical charge and turned into neutral molecules. However, the pH value inside the membrane was 1–3 units lower than pH of the external solution ( Table 5 ) due to Donnan’s exclusion of protons from the cation-exchange membranes—products of protolysis of anthocyanins [ 21 ]. At such pH values, some of the anthocyanins remained cations and cannot be desorbed from the membranes.…”

“…Increasing the pH value of the desorption solution to 10 solved this problem. Anthocyanins acquired a negative charge, and consequently, the electrostatic repulsion of these anions from the negatively charged fixed groups facilitated the extraction of these substances from the membranes [ 21 ].…”

“…Optical images of the surface and cross-section of pristine and fouled coupons were then obtained by optical microscopy. In parallel, optical images of the cranberry juice at different pH values were also obtained to produce a color scale as previously reported by Pismenskaya et al [ 21 ]. The juice was diluted 10 times before recording color.…”

Ion and Molecule Transport in Membrane Systems 2.0

“…It was found that preheating of the source solution to 60 °C allowed the purity of isolated proteins (such as β-lactoglobulin) and their recovery yield to be increased. N. Pismenskaya et al [ 14 ] examined the mechanisms of adsorption of anthocyanins (which is a strong natural antioxidant) by cation- and anion-exchange resins from aqueous solutions at different pH values. It was shown that the kinetic and equilibrium of adsorption can be understood when taking into account that the pH of the internal solution of a cation-exchange resin is two to three units lower, while that of an anion-exchange resin is two to four units higher than the pH of the external solution.…”