Bipolar Membrane Electrodialysis (EDBM)

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.

Fig.1, Black box description of the EDBM process

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.

The main requirements for a bipolar membrane are:

1. Excellent long-term stability
2. A low passive drop in potential
3. A high rate of water splitting
4. High permselectivity
5. as well as good mechanical stability

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.

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.  

Fig.1, Schematic process of the salt ion split by applying bipolar membranes to their corresponding acids and bases.

The potential difference needed for the electrodialytical production of one-molar solutions is 0.83 V which is equivalent to an energy consumption of 22 Wh. In contrast, the electrolytical production of a theoretical drop in potential is equivalent to an energy consumption of 55 Wh. Further advantages of bipolar membrane technology include the comparatively simple apparatus configuration, the option of a stack-like set-up as well as the low investment costs.

Especially the potential to split water without having reactive gases involved made it a promising technology already fifteen to twenty years ago.

2. Characteristics of EDBM
3. Splits water into acid (H^＋) and alkali (OH^－) with relatively low voltage.
4. Due to the absence of electrode reaction, no oxidation-reduction reaction takes place. Therefore, there will be no byproduct.
5. Produces acid and alkali from inorganic salt and organic acid salt in a single process.
6. Controls the concentration rate of acid and alkali.
7. Different from electrolytic processing, no electrodes are required for through every single cell and, therefore, fewer gases are generated.
8. Produces less waste solution.
9. Can withstand continuous operations for an extended time since no regeneration process, like ion exchange resin process, is required.

3. Application examples of EDBM
4. ・Production of organic acid from organic acid salt
5. ・Production of amino acid from amino acid salt
6. ・Production of acid and alkali from waste solution with salt
7. ・Production of acid and alkali from inorganic salt

Fig. 2, Examples of production with EDMB