An ion-exchange membrane is a
semi-permeable membrane
Semipermeable membrane is a type of synthetic or biologic, polymeric membrane that allows certain molecules or ions to pass through it by osmosis. The rate of passage depends on the pressure, concentration, and temperature of the molecules o ...
that transports certain dissolved ions, while blocking other ions or neutral molecules.
Ion-exchange membranes are therefore electrically conductive. They are often used in desalination and chemical recovery applications, moving ions from one solution to another with little passage of water.
Important examples of ion-exchange membranes include the
proton-exchange membranes, that transport
cations
An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
, and the
anion exchange membranes used in certain
alkaline fuel cells to transport
anions
An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
.
Structure and composition
An ion-exchange membrane is generally made of
organic or
inorganic
An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bondsthat is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemistry''.
Inor ...
polymer
A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...
with charged (ionic) side groups, such as
ion-exchange resin
An ion-exchange resin or ion-exchange polymer is a resin or polymer that acts as a medium for ion exchange, that is also known as an ionex. It is an insoluble matrix (or support structure) normally in the form of small (0.25–1.43 mm radiu ...
s.
Anion-exchange membranes contain fixed cationic groups with predominantly mobile anions; because anions are the majority species, most of the conductivity is due to anion transport. The reverse holds for
cation-exchange membranes.
The so-called heterogeneous ion-exchange membranes have low cost and a thicker composition with higher resistance and a rough surface that can be subject to fouling. Homogeneous membranes are more expensive, but have a thinner composition with lower resistance and a smooth surface, less susceptible to fouling. Homogeneous membrane surfaces can be modified to alter the membrane permselectivity to protons, monovalent ions, and divalent ions.
Selectivity
The selectivity of an ion-exchange membrane is due to
Gibbs-Donnan equilibrium and not due to physically blocking or electrostatically excluding specific charged species.
The selectivity to the transport of ions of opposite charges is called its permselectivity.
[
]
Applications
Ion-exchange membranes are traditionally used in electrodialysis
Electrodialysis (ED) is used to transport salt ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential difference. This is done in a configuration called an electrodialysis ...
or diffusion dialysis by means of an electrical potential or concentration gradient, respectively, to selectively transport cationic and anionic species. When applied in an electrodialysis desalination process, anion- and cation-exchange membranes are typically arranged in an alternating pattern between two electrodes (an anode and a cathode) within the electrodialysis stack. A galvanic potential is supplied as a voltage generated at the electrodes.[
A typical industrial electrodialysis stack consists of two chambers: a product-water chamber and a concentrate-reject chamber. During stack operation, salts are transferred from the product to the concentrate. As a result, the reject stream is concentrated up while the product stream is desalted.][
Exemplary applications of ion-exchange membranes utilized in electrodialysis and EDR include seawater desalination, ]industrial wastewater treatment
Industrial wastewater treatment describes the processes used for Wastewater treatment, treating wastewater that is produced by industries as an undesirable by-product. After treatment, the treated industrial wastewater (or effluent) may be reus ...
of highly scaling waters, food and beverage production, and other industrial wastewaters.[
]
References
{{reflist
See also
* Proton-exchange membrane
Laboratory techniques
Analytical chemistry