Bipolar Membrane Electrodialysis for Cleaner Production of Gluconic Acid: Valorization of the Regenerated Base for the Upstream Enzyme Catalysis

Bipolar membrane electrodialysis (BMED) is an environmentally friendly, high-effective technique for the cleaner production of gluconic acid. However, the unsatisfactory purity and low concentration of the regenerated base limit the applicable field of the byproduct for further utilization. In this study, BMED was applied for the cleaner production of gluconic acid from the perspective of the regenerated base. First, four types of cation-exchange membrane were screened for the BMED process by evaluating the performances of current efficiency, purity of regenerated base, and energy consumption. The BMED performances follow the order of CMX > TWEDC > CJMC-5 > CJMC-3. Furthermore, the effects of current density, the volume ratio between salt and base compartment on the BMED performances were optimized. A high base purity of 96.6% was obtained at a current density of 40 mA/cm2, while a high base concentration of 4.58 mol/L could be reached by applying a high volume ratio of 5:1 between the salt and base compartment. Moreover, the mechanism on the leakage of organic salts into the regenerated base was elucidated. The purity of the regenerated base was attributed to diffusion and electromigration. Diffusion dialysis experiments demonstrated that the permeability of gluconate through the CMX was 4.7 times of that through the BP-1. It was also found that the leakage of gluconate into the base could be alleviated at a low current density ranging from 20 to 50 mA/cm2. Finally, the regenerated base was subjected to the enzyme catalyst experiment for conversion of glucose into gluconate. This proof-of-concept study demonstrates that the regenerated base from the BMED process could be valorized to the upstream route for a closed-loop cleaner production.