Bipolar Membrane Electrodialysis: A Cleaner Production Technique

Introduction:

The basic aspect of electrodialysis with bipolar membranes (EDBM) is the combination of electrodialysis for salt separation with electrodialysis water splitting for the conversion of a salt into its corresponding acid and base. The bipolar membranes enhance the splitting of water into protons and hydroxide ions.

Bipolar membranes are a special type of layered ion exchange (IX) membrane where the two polymer layers one is only permeable for the anions and the other only for cations. Unlike membrane processes EBM isn’t applied for separation purposes but to get a reaction in the bipolar junction of the membrane where the anion and the cation permeable layers are in direct contact.

Process:

Water splits into hydroxide ions and protons. The produced hydroxide ion and proton are separated by migration in the respective membrane layer out of the membrane.

Reaction:

2H2O ↔ H3O+ + OH-

Unlike a water splitting at electrodes during electrolysis, no gases are formed as a side product to this reaction, nor are gases used. Electrodialysis with bipolar membranes (EDBM) can replace electrolysis with water splitting at the electrodes and can also have a wider variety of applications.

Schematic process of the salt ion split by applying bipolar membranes to their corresponding acids and bases

Bipolar Membrane Electrodialysis (BMED)

This phenomenon well known for being highly systematic ,eco-oriented and sustainable in the field of gluconic acid cleaner production. The byproduct generated during the process has limited applications as it was found to be more dilute than concentrated and was also found to be contaminated beyond usable limits. With the regenerated base as the main viewpoint, cleaner production of gluconic acid is the main goal here.

Initially the four cation exchange membranes are screened on the basis of parameters being; Current efficiency, Purity of regenerated base or byproduct and Energy consumption. The BMED process conducted ranked in the following order; CMX>TWEDC>CJMC-5>CJMC-3.Optimization of current density post effects, volume ratio between salt and base compartment regions followed. Base purity amounting to 96.6% was gained at a current density of 40 mA/cm^2,while a high base concentration of 4.58 mol/L was reached on application of 5:1 high volume ratio between salt and base compartments. Leakage of organic salts into regenerated base was elucidated. Electromigration and Diffusion were held positively responsible for the purity of regenerated base. From Diffusion dialysis it was known that gluconate permeability through CMX was. 4.7 times of that through BP-1. It was also observed that gluconate leakage into base could be avoided at low current density amounting to 20 - 50 mA/cm^2.

Towards the end, the regenerated base was made to undergo enzyme catalysis process with the aim being conversion of glucose to gluconate. This proof equipped process shows that base regenerated from BMED is capable of being valorized to upstream route for closed loop cleaner production.

References:

https://pubs.acs.org/doi/10.1021/acs.iecr.1c04657#:~:text=Bipolar%20membrane%20electrodialysis%20(BMED)%20is,the%20byproduct%20for%20further%20utilization.

https://eurodia.com/en/electrodialysis/