4 The Mechanism of Aging and Degradation
of IEM
The aging and degradation of the IEM result into the varying
of the membrane characteristics like increased resistance or
even membrane damage, which greatly reduces the membrane lifetime and process efficiency. The mechanism of
aging and degradation of IEM are summarized as the following two parts, fouling and degradation under extreme
conditions.
4.1 Membrane Fouling
The phenomenon that foulant deposit on the surface and
pores of the membrane often under the concentration
polarization condition is called membrane fouling (Sata et al.
1996). Membrane fouling is also a major cause of aging of
IEM. The interaction between pollutants and membranes
mainly includes electrostatic interaction, van der Waals
force, solvation, and spatial stereoscopic effects. According
to the nature of pollutants, it can be divided into organic
pollution and inorganic pollution. Inorganic pollution is
mainly caused by the formation of Ca
2+ , Mg
2+ or other
multivalent ions on the surface or inside of the ionic membrane, and it is caused by polarization or solution supersaturated. Organic pollution is formed by the deposition or
penetration of proteins, humic acids, surfactants or other
macromolecular organic substances on the surface of the
membrane or inside the membrane free volume or pore,
mainly due to the electrostatic interaction or bonding
between functional groups even of same charge. For
example, Fig. 2 illustrates the structure of a commercially
available hydrocarbon-type AEM and a suggested mechanism for fouling by aromatic compounds where two aspects
of fouling mechanism are indicated: affinity between anions
and the oppositively charged fixed groups of the AEM, and
affinity as p–p interactions between the aromatic membrane
matrix and compounds (Tanaka et al. 2011). In addition, the
molecular size of the organic matter and the network structure of the film also affect the interaction of the organic
matter to the membrane base film (Lu et al. 2015).
It is reported that fouling caused a clear loss of
ion-exchange sites for both AEM and CEM, as well as other
membrane characteristics’ variation (Ghalloussi et al. 2013).
The adsorption of organic colloidal particles in the membrane free volume occurred and a higher swelling degree
was observed due to the hydrophilic nature of foulant.
Higher swelling of the IEM will cause breakage of some
bonds and polymer chains, resulting in macroscopic defects
(defects, voids, cavities) in the membrane that are filled with
external solutions. In long-term operation, these defects may
form through non-selective channels. Colloidal associations
formed by organic fouling (e.g., amino acids) in the
nano-pore may contaminate the membrane (Sata 2004),
Table 5 Scanning electron microscopy (SEM) picture and membranes resistance of used and new IEM
Ion-exchange membranes
SEM picture
Resistance (X)
Membranes resistance (X cm
2
)
AEM
Fresh
12.8
9.46
After 1800-hour use
20.2
14.93
CEM
Fresh
15.4
11.34
After 1800-hour use
22.3
16.45
Reproduced with permission (Ting-qing et al. 2014)
32
L. Han
of IEM
The aging and degradation of the IEM result into the varying
of the membrane characteristics like increased resistance or
even membrane damage, which greatly reduces the membrane lifetime and process efficiency. The mechanism of
aging and degradation of IEM are summarized as the following two parts, fouling and degradation under extreme
conditions.
4.1 Membrane Fouling
The phenomenon that foulant deposit on the surface and
pores of the membrane often under the concentration
polarization condition is called membrane fouling (Sata et al.
1996). Membrane fouling is also a major cause of aging of
IEM. The interaction between pollutants and membranes
mainly includes electrostatic interaction, van der Waals
force, solvation, and spatial stereoscopic effects. According
to the nature of pollutants, it can be divided into organic
pollution and inorganic pollution. Inorganic pollution is
mainly caused by the formation of Ca
2+ , Mg
2+ or other
multivalent ions on the surface or inside of the ionic membrane, and it is caused by polarization or solution supersaturated. Organic pollution is formed by the deposition or
penetration of proteins, humic acids, surfactants or other
macromolecular organic substances on the surface of the
membrane or inside the membrane free volume or pore,
mainly due to the electrostatic interaction or bonding
between functional groups even of same charge. For
example, Fig. 2 illustrates the structure of a commercially
available hydrocarbon-type AEM and a suggested mechanism for fouling by aromatic compounds where two aspects
of fouling mechanism are indicated: affinity between anions
and the oppositively charged fixed groups of the AEM, and
affinity as p–p interactions between the aromatic membrane
matrix and compounds (Tanaka et al. 2011). In addition, the
molecular size of the organic matter and the network structure of the film also affect the interaction of the organic
matter to the membrane base film (Lu et al. 2015).
It is reported that fouling caused a clear loss of
ion-exchange sites for both AEM and CEM, as well as other
membrane characteristics’ variation (Ghalloussi et al. 2013).
The adsorption of organic colloidal particles in the membrane free volume occurred and a higher swelling degree
was observed due to the hydrophilic nature of foulant.
Higher swelling of the IEM will cause breakage of some
bonds and polymer chains, resulting in macroscopic defects
(defects, voids, cavities) in the membrane that are filled with
external solutions. In long-term operation, these defects may
form through non-selective channels. Colloidal associations
formed by organic fouling (e.g., amino acids) in the
nano-pore may contaminate the membrane (Sata 2004),
Table 5 Scanning electron microscopy (SEM) picture and membranes resistance of used and new IEM
Ion-exchange membranes
SEM picture
Resistance (X)
Membranes resistance (X cm
2
)
AEM
Fresh
12.8
9.46
After 1800-hour use
20.2
14.93
CEM
Fresh
15.4
11.34
After 1800-hour use
22.3
16.45
Reproduced with permission (Ting-qing et al. 2014)
32
L. Han