Effect of membrane type on cation migration during green tea electromigration and equivalent mass transported calculation

摘要

In a previous exploratory study, the feasibility of selectively extracting EGCG from a green tea solution using an electrodialysis cell was evaluated. Among tested membranes, only the UF-1000Da membrane had allowed an EGCG significant migration. Furthermore, pH and conductivity of green tea solution were affected by membrane type used. With UF membrane, the pH and conductivity increased in a linear fashion while for the three other anionic membranes tested, both parameters decreased. The purposes of this present study were to evaluate the equivalent mass transported and to identify the current carriers during an electromigration treatment to explain the variations in pH, conductivity and catechins migration observed in the previous work. For the cationic/anionic configurations, a drastic demineralization (96% for K~+, 93% for Mg~(2+), 86% for Na~+ and 75% for Ca~(2+)) occurred in the tea compartment after 60 min of treatment. For the cationic/UF configuration, the increases in potassium and sodium cations (18% for K~+ and 357% for Na~+) and the demineralization in magnesium and calcium (56% for Mg~(2+) and 29% for Ca~(2+)) which occurred in the green tea compartment, were also in accordance with the electromigration configuration. Furthermore, it appeared from these results that the main cation contributing to the current transport, during the first 20-30 min of treatment was potassium. The calculation of the theoretical equivalent mass for the cationic/anionic configurations, showed a lack of migrating cations after 30 min of treatment, and the use of electrogenerated H_3O~+ at the interface of the anionic membrane as current carrier. For the cationic/UF configuration, comparison between theoretical and experimental equivalent mass migrated was not applicable, due to the more complex ion migration in relation with this specific configuration.