Module 6. Membrane processing

Lesson 32


Electrodialysis is an electromembrane process in which ions are transported through ion permeable membranes from one solution to another under the influence of a potential gradient.

Different Types of Membranes

1. Ion permeable membranes

Membranes for electrodialysis are typically hydrocarbon films with ion exchange functional groups attached to the polymer chains. Hydrocarbon membranes are usually categorized as homogeneous or heterogeneous.

For heterogeneous membranes, the film-forming polymer is usually polyethylene or polyvinylidene fluoride, but other polymers could also be used. These membranes are thick, opaque and mechanically strong, but they tend to have higher resistance than homogeneous membranes.

Typical homogeneous membranes have a polymer matrix of styrene cross linked with divinylbenzene (DVB) and ion exchange functional groups of sulfonic acid or quaternary amines.

They are essentially sheets of ion-exchange resins.

They also contain other polymers to improve mechanical strength and flexibility.

The resin component of a cation-exchange membrane would have negatively charged groups (e.g., -SO3-) chemically attached to the polymer chains (e.g., styrene/divinylbenzene copolymers).

Attachment of positive fixed charges (e.g., -NR3+ or C5H5N+R where commonly R = CH3) to the polymer chains forms anion permeable membranes, which are selective to transport of negative ions.

Ion-exchange polymers such as poly(styrene sulfonic acid) are water soluble, so crosslinking is needed to prevent dissolution of ion permeable membranes. Divinylbenzene is used to cross link polystyrene chains.

The degree of cross-linking and the fixed-charge density affect the membrane's properties in opposite ways. Higher crosslinking improves selectivity and membrane stability by reducing swelling, but it increases electrical resistance.

High charge density reduces resistance and increases selectivity, but it promotes swelling and thus necessitates higher crosslinking.

A compromise between selectivity, electrical resistance, and dimensional stability is achieved by proper adjustment of crosslinking and fixed-charge densities.




Method of preparation of ion permeable membranes

Cation exchange membranes are made by adding a sulfonic acid functional group to the benzene ring of the styrene group, usually by treatment with concentrated sulfuric acid, sulfur trioxide or chlorosulfonic acid.

Anion exchange groups can also be added to the benzene ring, but a key reagent for that procedure, chloromethyl methyl ether, is a dangerous carcinogen. That danger is avoided by replacement of styrene with chloromethylstyrene and treatment of the polymer with trimethylamine to form a quaternary amine functional group. Alternative monomers for anion membranes include vinylpyridine or methylvinylpyridine, both of which are quaternized with methyl iodide after polymerization.

Ion permeable membranes are also made by swelling existing films with styrene and DVB, which can then be post-treated to add functional groups, or by grafting of ion exchange functional groups directly onto the polymer matrix of existing films.

2. Bipolar membranes

Bipolar membranes consist of an anion-permeable membrane and a cation permeable membrane laminated together.

Multiple bipolar membranes along with other ion permeable membranes can be placed between a single pair of electrodes in an electrodialysis stack for the production of acid and base from a neutral salt.

When this composite structure is oriented such that the cation-exchange layer faces the anode it is possible, by imposing a potential field across the membrane, to spit water into proton and hydroxyl ions.This results in the production of acidic and basic solutions at the surfaces of the bipolar membranes.

Potential drop of only 0.8 V is necessary for modern bipolar membrane performance.

Last modified: Thursday, 27 September 2012, 5:24 AM