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| What is Electrodialysis
(ED)?
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. The electrical charges on the ions allow
them to be driven through the membranes fabricated from ion exchange
polymers. Applying a voltage between two end electrodes generates
the potential field required for this. Since the membranes used
in electrodialysis have the ability to selectively transport ions
having positive or negative charge and reject ions of the opposite
charge, useful concentration, removal, or separation of electrolytes
can be achieved by electrodialysis.
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Ion Permeable
Membranes
The ion
permeable membranes used in electrodialysis are essentially sheets of
ion-exchange resins. They usually 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). Ions with a charge opposite to the fixed charge (counter
ions) are freely exchanged at these sites. The concentration of counter
ions (e.g., Na+) is relatively high; therefore, counter ions
carry most of the electric current through the membrane. The fixed charges
attached to the polymer chains repel ions of the same charge (co-ions),
in this case the anions. Since their concentration in the membrane is
relatively low, anions carry only a small fraction of the electric current
through a cation permeable membrane. 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, because the fixed -NR3+
groups repel positive ions. This exclusion, as a result of electrostatic
repulsion, is called Donnan exclusion.
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.
Bipolar Membranes
Bipolar
membranes consist of an anion-permeable membrane and a cation permeable
membrane laminated together. 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. 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.
There
are substantial advantages to water splitting with bipolar membranes.
Since there are no gases evolved at the surface or within the bipolar
membranes, the energy associated with conversion of water to O2
and H2 is saved, and the power consumption is about half
that of electrolytic cells. Compared to the electrodes used in conventional
electrolytic cells, the bipolar membranes are inexpensive. Where
dilute (e.g., < 1 M) acids or bases are needed, bipolar membranes
offer the prospect of low cost and minimum unwanted byproducts.
For
further information on Electrodialysis, see Watts New "Electrochemical
Salt Splitting."
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