Chemistry Reference
In-Depth Information
HX
solution
MOH
solution
B
A
C
B
Acid
Salt
Base
H
+
OH
-
OH
-
H
+
X
-
M
+
H
2
O
H
2
O
MX
solution
B - Bipolar membrane
C - Cation membrane
A - Anion membrane
Fig. 19.21
Bipolar membrane
separation.
good water permeability from the external solution
to the interface and have a thin interface between
the cation and anion layers to give efficient, low-
resistant transport of hydrogen and hydroxide ions.
The attractions of bipolar membranes are that they
avoid the production of excess quantities of by-
product gases and also reduce the energy costs asso-
ciated with electrode polarisation in the more
conventional electrohydrolysis approach. Bipolar
membranes are used with either two- or three-
compartment cells. The basic three-compartment
cell, shown in Fig. 19.21, for a typical sodium sulfate
treatment consists of a cation-exchange, an anion-
exchange and a bipolar membrane. In operation,
positive and negative ions migrate through the
respective monopolar membranes and concentrate
in compartments on opposite sides of the bipolar
membranes. The hydrogen and hydroxide ions gen-
erated by the bipolar membrane diffuse back into the
adjacent compartments, in opposite directions, to
produce the alkali and acid solutions.
Bipolar membranes also can be used in a two-cell
configuration, regenerating only one base or acid.
For example, in an anion/bipolar membrane con-
figuration the anions move through the anion-
exchange membrane and combine with the H
+
ions
arising from the bipolar membrane to form the acid
product. This type of cell is useful for converting salts
of weak bases (e.g. ammonium nitrate) to a salt/base
mixture and a relatively pure acid. Conversely, the
cation/bipolar membrane two-compartment cell is
useful for the processing of the salts of weak acids
(of organic acids) to give a relatively pure base
stream and a mixed acid/salt stream. Potential appli-
cations of bipolar membranes in recycling or efflu-
ent control [55] are numerous.
