Ion migration characteristics during the bipolar membrane electrodialysis treatment of concentrated reverse osmosis brine

Bipolar membrane electrodialysis (BMED) plays a unique role in liquid phase mass transfer and acid-alkali preparation, so BMED wastewater treatment and reuse applications are attracting increasing attention. Herein, BMED was used to treat the concentrated brine (ROC) produced during reverse osmosis (RO), recover valuable resources and produc acid and alkali. The effects of voltage, feed concentration ratio (FCR), and flow velocity on ion flux during BMED were systematically surveyed, and unusual phenomena were thoroughly analyzed during ion transport. The results showed that the main driving force for ion transport was operating voltage, a critical parameter determining operating time. Concentration polarization and concentration gradient caused by a high feed concentration significantly affected ion electromigration stability, and ion hydration characteristics were nonnegligible factors determining ion transport speed. Flow velocity had no marked impact on the experimental outcomes. Furthermore, water transport occurred with salt ions during liquid-phase mass transfer. Under the optimal operating conditions (voltage 24 V, FCR 2:1, flow velocity 55 L/h), the BMED system showed excellent, continuous and stable operation, and efficient production of mixed acid (0.88 mol/L) and mixed alkali (0.72 mol/) was achieved. The BMED process cost was 10.537 yuan/kg NaOH; therefore, process applications of BMED are economically feasible.