degradation is seen as one main driver for membrane aging.
The degradation of membrane refers to the loss of availability of the membrane due to the destruction of the stability
and/or functionality of the polymer in the IEM under certain
physical or chemical factors, such as high temperature or
high pH. For example the oxidizing substances in the water
like O 2 , Cl 2 , etc., generated by the polar chamber reaction,
can cause oxidative damage to the membrane, rendering
functional group degradation.
Thus, it is necessary to summarize the mechanism of
aging and degradation of ion-exchange membrane. At the
same time, the review paper also proposed prevention and
control measures for the aging and degradation of
ion-exchange membranes according to the current research.
2 The Membrane Characteristics and Their
Relationship of the Ion-Exchange
Membrane
The main membrane characteristics related to the aging and
degradation of the IEM include exchange capacity, water
content and swelling degree, thickness and contact angle of
membrane, mechanical strength, conductivity, transport
number, permeability. Because these performance parameters vary upon the degree of membrane aging and degradation, we list some details of them in Table 1.
The ion-exchange capacity and water content are two
critical parameters of IEM directly or indirectly affecting
IME performance. A simple plot between these two is seen
in Fig. 1, with the water content of the membrane plotted as
the Y-axis and the exchange capacity as the X-axis, then the
reciprocal of its slope referring to the concentration of the
fixed group (Xu and Huang 2008). In general, for given
mechanical strength of the membrane, the higher the concentration of the fixed group, the better the performance of
the membrane, and the direction pointer is marked on Fig. 1.
3 Aging and Degradation in IEM
Applications
In the applications of IEM, such as electrodialysis (Ghalloussi et al. 2013), electrodialysis reverse (Zhang 2017),
biopolar membrane electrodialysis (Mani 1991; Hwang and
Choi 2006), electrodeionization (Ting-qing et al. 2014), fuel
cell (Tanaka 2015b), etc., the aging and degradation of the
IEM result in changes of the membrane characteristics, e.g.,
decreased ion-exchange capacity, increased membrane
resistance, inactivation of functional groups, changes of the
hydrophilicity, etc., which in turn reduces the use efficiency
of the membrane.
3.1 Electrodialysis
The electrodialysis (ED) process is a combination of an
electrochemical process and a dialysis diffusion process.
Driven by an external direct current (DC) electric field, ions
migrate under fixed directions and are selectively allowed to
pass through the respective IEMs. With assistance of
repeatable units consisting of alternatively arranged AEM
and CEM, two solutions of different salinity are produced,
the diluate and the concentrate. Next to the diluate and the
concentrate chambers, there are anode chamber where the
oxidation reaction occurs (the anode solution is acidic) and
cathode chamber where reduction reaction occurs (the
cathode solution is alkaline). Ghalloussi et al. (2013) used
the IEM supplied by Eurodia Industry SA to find that the
ion-exchange capacity of almost all membranes decreased
significantly after two years of dealing with a solution
containing a weak organic acid in the ED process, and the
change of other membrane characteristics is also shown in
Table 2.
All
the
investigated
membranes
experienced
ion-exchange capacity decrease after use with only exception as CEM2, which is the relatively stable sample in any
aspects of membrane characteristics (not further detailed).
This decrease varies from ca. 70% for CEM1 to ca. 85% for
AEM1. Varying water contact angle before and after use for
the IEM is also observed, indicating the membrane
hydrophilicity changed. Used CEM1 becomes more
hydrophobic, in line with decreased water content, volume
fraction of the inter-gel solution. Clearly opposite change
was found for AEM1 which is more hydrophilic after use.
The above data indicates clear aging phenomenon.
Loss of hydrophilic sulfonic acid functional sites of
CEM1 results in a decrease in water content and membrane
thickness as well as an increase in contact angle. Thus, the
membrane becomes denser and its pores are narrower, which
was manifested by a decrease in the volume fraction of the
inter-gel solution. Lower ion-exchange capacity and narrower pores result in a decrease in the concentration and
mobility of the counter-ions, increased adsorption of common ions, resulting in a loss of specific conductivity and
permselectivity, but contributes to higher conductivity.
One can conclude that for CEM1, the great loss of
ion-exchange capacity (activated sites of the function
groups) and the hydrophilicity as well as associated
decreased conductivity and permeability are all the consequence of membrane aging. The aging on AEM1 probably
differs from that for CEM1, considering the great probability
of organic anion adsorption (likely due to the hydrophilic
groups such as carboxyl and hydroxyl groups of the organic
acids). That explained the hydrophilization of AEM1,
together with increased thickness and water content.
28
L. Han
The degradation of membrane refers to the loss of availability of the membrane due to the destruction of the stability
and/or functionality of the polymer in the IEM under certain
physical or chemical factors, such as high temperature or
high pH. For example the oxidizing substances in the water
like O 2 , Cl 2 , etc., generated by the polar chamber reaction,
can cause oxidative damage to the membrane, rendering
functional group degradation.
Thus, it is necessary to summarize the mechanism of
aging and degradation of ion-exchange membrane. At the
same time, the review paper also proposed prevention and
control measures for the aging and degradation of
ion-exchange membranes according to the current research.
2 The Membrane Characteristics and Their
Relationship of the Ion-Exchange
Membrane
The main membrane characteristics related to the aging and
degradation of the IEM include exchange capacity, water
content and swelling degree, thickness and contact angle of
membrane, mechanical strength, conductivity, transport
number, permeability. Because these performance parameters vary upon the degree of membrane aging and degradation, we list some details of them in Table 1.
The ion-exchange capacity and water content are two
critical parameters of IEM directly or indirectly affecting
IME performance. A simple plot between these two is seen
in Fig. 1, with the water content of the membrane plotted as
the Y-axis and the exchange capacity as the X-axis, then the
reciprocal of its slope referring to the concentration of the
fixed group (Xu and Huang 2008). In general, for given
mechanical strength of the membrane, the higher the concentration of the fixed group, the better the performance of
the membrane, and the direction pointer is marked on Fig. 1.
3 Aging and Degradation in IEM
Applications
In the applications of IEM, such as electrodialysis (Ghalloussi et al. 2013), electrodialysis reverse (Zhang 2017),
biopolar membrane electrodialysis (Mani 1991; Hwang and
Choi 2006), electrodeionization (Ting-qing et al. 2014), fuel
cell (Tanaka 2015b), etc., the aging and degradation of the
IEM result in changes of the membrane characteristics, e.g.,
decreased ion-exchange capacity, increased membrane
resistance, inactivation of functional groups, changes of the
hydrophilicity, etc., which in turn reduces the use efficiency
of the membrane.
3.1 Electrodialysis
The electrodialysis (ED) process is a combination of an
electrochemical process and a dialysis diffusion process.
Driven by an external direct current (DC) electric field, ions
migrate under fixed directions and are selectively allowed to
pass through the respective IEMs. With assistance of
repeatable units consisting of alternatively arranged AEM
and CEM, two solutions of different salinity are produced,
the diluate and the concentrate. Next to the diluate and the
concentrate chambers, there are anode chamber where the
oxidation reaction occurs (the anode solution is acidic) and
cathode chamber where reduction reaction occurs (the
cathode solution is alkaline). Ghalloussi et al. (2013) used
the IEM supplied by Eurodia Industry SA to find that the
ion-exchange capacity of almost all membranes decreased
significantly after two years of dealing with a solution
containing a weak organic acid in the ED process, and the
change of other membrane characteristics is also shown in
Table 2.
All
the
investigated
membranes
experienced
ion-exchange capacity decrease after use with only exception as CEM2, which is the relatively stable sample in any
aspects of membrane characteristics (not further detailed).
This decrease varies from ca. 70% for CEM1 to ca. 85% for
AEM1. Varying water contact angle before and after use for
the IEM is also observed, indicating the membrane
hydrophilicity changed. Used CEM1 becomes more
hydrophobic, in line with decreased water content, volume
fraction of the inter-gel solution. Clearly opposite change
was found for AEM1 which is more hydrophilic after use.
The above data indicates clear aging phenomenon.
Loss of hydrophilic sulfonic acid functional sites of
CEM1 results in a decrease in water content and membrane
thickness as well as an increase in contact angle. Thus, the
membrane becomes denser and its pores are narrower, which
was manifested by a decrease in the volume fraction of the
inter-gel solution. Lower ion-exchange capacity and narrower pores result in a decrease in the concentration and
mobility of the counter-ions, increased adsorption of common ions, resulting in a loss of specific conductivity and
permselectivity, but contributes to higher conductivity.
One can conclude that for CEM1, the great loss of
ion-exchange capacity (activated sites of the function
groups) and the hydrophilicity as well as associated
decreased conductivity and permeability are all the consequence of membrane aging. The aging on AEM1 probably
differs from that for CEM1, considering the great probability
of organic anion adsorption (likely due to the hydrophilic
groups such as carboxyl and hydroxyl groups of the organic
acids). That explained the hydrophilization of AEM1,
together with increased thickness and water content.
28
L. Han