of varying surface roughness on the dense film, mitigation of
the membrane fouling via membrane surface modification on
its hydrophilicity, electrostatic charge and roughness are
proposed, which may strongly modify the interaction between
the membrane and the foulants.
4.1 Surface Hydrophilicity and Charge
It has been widely acknowledged that increasing the
hydrophilicity of the membrane surface and imparting a
negative surface charge are very effective means to prevent
fouling, considering the common foulants are often of
hydrophobicity and negative charge (Goh et al. 2018).
The general consensus on the anti-fouling mechanism of
a hydrophilic surface emphasizes the critical role of the
formation of a hydration layer formed via hydrogen bonding
between water and the hydrophilic functional groups on the
surface (Pashley 1981a, b). By largely reducing the possible
hydrophobic–hydrophobic interactions, this layer can prevent or reduce undesirable adsorption of foulants which is
energetically highly unfavorable. Also, this hydration layer
contributes a new steric hindrance to the hydrophobic foulant, effectively reducing the possible direct contact. Thus,
surface hydrophilization is often recommended to reduce
membrane fouling, especially toward the organics, colloids
and microorganisms (Wang and Lin 2017).
The surface charge of membranes is also an important
consideration in reducing the membrane fouling where foulants are charged. Usually, it is appropriate to use a membrane
carrying the same charge as the foulants, giving an electrostatic repulsion contributing to anti-fouling effect (Kochkodan
et al. 2014). Thus, there have been a number of attempts to
incorporate new ionizable functional groups on the surface of
IEM as anti-fouling strategy (Ulbricht 2006; Kato et al. 2003).
High hydrophilicity can decrease the surface charge on
the membrane surface, thus reducing the adhesion of pollutants (Vaselbehagh et al. 2014). To sum up, a high
hydrophilicity prevents the adsorption of foulants through
reducing hydrophobic–hydrophobic interactions, while a
negative surface charge prevents the adsorption of negatively charged foulants through enhancing the electrostatic
repulsion. Such an anti-fouling strategy has seen many
applications, as listed in Table 4 (Ruan et al. 2018; Sri
Mulyati et al. 2012; Vaselbehagh et al. 2014; Mulyati et al.
2013; Zhao et al. 2018b; Grebenyuk et al. 1998; Li et al.
2018b; Hao et al. 2018).
4.2 Surface Roughness
Membrane surface roughness as an important characteristic
of membrane morphology was correlated to the membrane
fouling phenomenon (Kochkodan et al. 2014; Hao et al.
2018; Fernandez-Gonzalez et al. 2017). Membranes with
rougher surfaces are observed to be more favorable for foulants’ attachment: A greater roughness increased the total
surface area to which foulants can approach and attach; i.e.,
the ridge–valley structure favors accumulation of foulants at
the surface (Hao et al. 2018). Similarly, a membrane with a
smooth surface is thus not easily fouled, due to a reduced
contact area for foulant, less propensity for foulant–membrane interaction and a presumably higher shear rate
(Kochkodan et al. 2014; Fernandez-Gonzalez et al. 2017).
Membrane fouling is clearly mitigated via constructing a
smoother and denser sulfonated dopamine (SDA) layer by
introducing the sulfonated group, compared to the counterpart of dopamine (DA)/AEM (Ruan et al. 2018). AEM
images clearly report the membrane morphology change due
to mussel adhesive mimetic random copolymer immobilization on the membrane as shown in Fig. 4. DA/AEM (Ra =
36.5 nm) had a rougher surface than pristine AEM (Ra =
11.7 nm) which may be caused by the nonuniform polymerization and aggregation of DA, while the SDA-coated
one exhibits very smooth surface (Ra = 5.08 nm).
Decreasing surface roughness by surface modification
with polydopamine (PDA) enhances the anti-fouling potential of AEM (Li et al. 2018b). The introduction of PDA
coating helps to construct a smoother and denser
composite-modified layer which consists of graphene oxide
(GO) and PDA (shown in Fig. 5). The electrodeposition of
GO on AEM (GO-M) (Ra = 33.9 nm) led to more ridges and
valleys than the pristine membrane (PM) (Ra = 23.5 nm),
while the surface roughness of GO@PDA-M gave a Ra value
to 26.5 nm. The good anti-fouling ability could be maintained in the long-term operation, with nearly negligible
increase of AEM potential (*0.6 V) for the prepared
GO@PDA-modified one after 20 h fouling, much lower than
that for GO-modified AEM (*2.0 V).
5 Mitigation of Fouling by New Membrane
Synthesis
In addition to the anti-fouling modification of the IEM surface, another promising strategy is to synthesize new functional membranes. The key factor is to incorporate functional
groups of less sensitivity to fouling (e.g., aliphatic polymer
or nanocomposite ones) and optimize the packing in the
newly synthesized membrane phase (Chen et al. 2010).
5.1 Aliphatic-Hydrocarbon-Based Membrane
Almost all AEMs have been prepared from derivatives of
styrene–divinylbenzene copolymers. As a consequence,
Current Strategies for the Design of Anti-fouling …
19
the membrane fouling via membrane surface modification on
its hydrophilicity, electrostatic charge and roughness are
proposed, which may strongly modify the interaction between
the membrane and the foulants.
4.1 Surface Hydrophilicity and Charge
It has been widely acknowledged that increasing the
hydrophilicity of the membrane surface and imparting a
negative surface charge are very effective means to prevent
fouling, considering the common foulants are often of
hydrophobicity and negative charge (Goh et al. 2018).
The general consensus on the anti-fouling mechanism of
a hydrophilic surface emphasizes the critical role of the
formation of a hydration layer formed via hydrogen bonding
between water and the hydrophilic functional groups on the
surface (Pashley 1981a, b). By largely reducing the possible
hydrophobic–hydrophobic interactions, this layer can prevent or reduce undesirable adsorption of foulants which is
energetically highly unfavorable. Also, this hydration layer
contributes a new steric hindrance to the hydrophobic foulant, effectively reducing the possible direct contact. Thus,
surface hydrophilization is often recommended to reduce
membrane fouling, especially toward the organics, colloids
and microorganisms (Wang and Lin 2017).
The surface charge of membranes is also an important
consideration in reducing the membrane fouling where foulants are charged. Usually, it is appropriate to use a membrane
carrying the same charge as the foulants, giving an electrostatic repulsion contributing to anti-fouling effect (Kochkodan
et al. 2014). Thus, there have been a number of attempts to
incorporate new ionizable functional groups on the surface of
IEM as anti-fouling strategy (Ulbricht 2006; Kato et al. 2003).
High hydrophilicity can decrease the surface charge on
the membrane surface, thus reducing the adhesion of pollutants (Vaselbehagh et al. 2014). To sum up, a high
hydrophilicity prevents the adsorption of foulants through
reducing hydrophobic–hydrophobic interactions, while a
negative surface charge prevents the adsorption of negatively charged foulants through enhancing the electrostatic
repulsion. Such an anti-fouling strategy has seen many
applications, as listed in Table 4 (Ruan et al. 2018; Sri
Mulyati et al. 2012; Vaselbehagh et al. 2014; Mulyati et al.
2013; Zhao et al. 2018b; Grebenyuk et al. 1998; Li et al.
2018b; Hao et al. 2018).
4.2 Surface Roughness
Membrane surface roughness as an important characteristic
of membrane morphology was correlated to the membrane
fouling phenomenon (Kochkodan et al. 2014; Hao et al.
2018; Fernandez-Gonzalez et al. 2017). Membranes with
rougher surfaces are observed to be more favorable for foulants’ attachment: A greater roughness increased the total
surface area to which foulants can approach and attach; i.e.,
the ridge–valley structure favors accumulation of foulants at
the surface (Hao et al. 2018). Similarly, a membrane with a
smooth surface is thus not easily fouled, due to a reduced
contact area for foulant, less propensity for foulant–membrane interaction and a presumably higher shear rate
(Kochkodan et al. 2014; Fernandez-Gonzalez et al. 2017).
Membrane fouling is clearly mitigated via constructing a
smoother and denser sulfonated dopamine (SDA) layer by
introducing the sulfonated group, compared to the counterpart of dopamine (DA)/AEM (Ruan et al. 2018). AEM
images clearly report the membrane morphology change due
to mussel adhesive mimetic random copolymer immobilization on the membrane as shown in Fig. 4. DA/AEM (Ra =
36.5 nm) had a rougher surface than pristine AEM (Ra =
11.7 nm) which may be caused by the nonuniform polymerization and aggregation of DA, while the SDA-coated
one exhibits very smooth surface (Ra = 5.08 nm).
Decreasing surface roughness by surface modification
with polydopamine (PDA) enhances the anti-fouling potential of AEM (Li et al. 2018b). The introduction of PDA
coating helps to construct a smoother and denser
composite-modified layer which consists of graphene oxide
(GO) and PDA (shown in Fig. 5). The electrodeposition of
GO on AEM (GO-M) (Ra = 33.9 nm) led to more ridges and
valleys than the pristine membrane (PM) (Ra = 23.5 nm),
while the surface roughness of GO@PDA-M gave a Ra value
to 26.5 nm. The good anti-fouling ability could be maintained in the long-term operation, with nearly negligible
increase of AEM potential (*0.6 V) for the prepared
GO@PDA-modified one after 20 h fouling, much lower than
that for GO-modified AEM (*2.0 V).
5 Mitigation of Fouling by New Membrane
Synthesis
In addition to the anti-fouling modification of the IEM surface, another promising strategy is to synthesize new functional membranes. The key factor is to incorporate functional
groups of less sensitivity to fouling (e.g., aliphatic polymer
or nanocomposite ones) and optimize the packing in the
newly synthesized membrane phase (Chen et al. 2010).
5.1 Aliphatic-Hydrocarbon-Based Membrane
Almost all AEMs have been prepared from derivatives of
styrene–divinylbenzene copolymers. As a consequence,
Current Strategies for the Design of Anti-fouling …
19