Bipolar Membrane Electrodialysis Technology

What Is Bipolar Membrane Electrodialysis?

Bipolar membrane electrodialysis (BMED) is a system that combines electrodialysis (ED) and bipolar membrane (BPM), developed for its ability to convert anions and cations into acids and bases, respectively. BMED is composed of BPM, cation exchange membrane (CEM) and anion exchange membrane (AEM) in a certain arrangement, involving three basic membrane structures:

Three-chamber BMED
It contains CEM, AEM and BPM in each membrane repeating unit to achieve acid and alkali separation and recovery from salts.
Two- chamber base-BMED
It contains CEM and BPM in each membrane repeating unit for processing weak acid salts (organic acid salts) to obtain relatively pure bases.
Two-chamber acid-BMED
It contains AEM and BPM in each membrane repeating unit for converting weak base salts into relatively pure acids.

Schematic diagram of BMED. [1]

How Bipolar Membrane Electrodialysis Works

Under the action of a DC electric field, the bipolar membrane can dissociate water and obtain hydrogen ions and hydroxyl ions on both sides of the membrane. Taking advantage of this feature, a bipolar membrane electrodialysis system composed of a bipolar membrane and other anion and cation exchange membranes can convert salts in aqueous solutions into corresponding acids and bases without introducing new components.

Cathodic reaction:

2H₂O+2e→H₂+2OH⁻

Anodic reaction:

H₂O→1/2O₂+2H++2e

Overall electrolysis reaction at the electrode:

3H₂O→1/2O₂+H₂+2H⁺+2OH⁻

Bipolar membrane water dissociation reaction:

Bipolar Membrane Electrodialysis Technology

Bipolar Membrane Electrodialysis Compared with Other Membrane Processes

Taking the membrane treatment process for obtaining organic acids from fermentation broth as an example, the characteristics of ED, ion exchange and BMED were compared in terms of additives, products, production scale and recovery effects. Overall, BMED technology has clear advantages, including:

Low energy consumption and small device size can save investment;
There is no oxidation or reduction reaction in the whole process, no side reaction products, and no pollution.

Bipolar Membrane Electrodialysis Technology

New Process of Bipolar Membrane Selective Electrodialysis

The new process of bipolar membrane selective electrodialysis (BMSED) combines selective electrodialysis (SED) and BMED processes. In BMSED, the common AEM and CEM in the BMED stack are replaced with corresponding monovalent selective membranes (MVA and MVC). The new BMSED process can achieve multiple goals, including:

Desalination of concentrated brine: This process works well when treating brine with a concentration of 70-105 g/L.
Separation of monovalent and polyvalent ions: Na⁺/Ca²⁺ and Cl^-/SO₄^2- have permselectivities of 5 to 10 and 50-60, respectively.
Production of high-purity acid/base: The concentrations of NaOH and HCl in the base/acid solution are increased to 2.2 and 1.9 mol/L respectively, and the purity of NaOH and HCl is close to 99.99%.

BMSED process diagram. [2]