2 Category of IEMs
IEMs are typically thin polymeric films containing fixed
charged groups which are ionized in water (Strathmann
2010). Figure 1 summarizes the common categories of IEM
regarding the charge nature and morphology, where based
on the charge nature (or type of ionic functional groups),
IEMs are broadly classified into CEMs and AEMs (Zeng
et al. 2019; Ran et al. 2017), and based on their morphology,
IEMs can be classified into the homogeneous and heterogeneous ones. The forming category according to the functional charged group is further illustrated since the ion
permselectivity is highly related.
IEMs can selectively allow the passage of oppositely
charged ions (counterions) while obstruct similarly charged
ions (co-ions), with the counterions permselectivity following the theory elucidated by Donnan. CEMs generally contain negatively charged groups (e.g., –SO 3
−
, –COO
− , –PO 3
2
− , –PO 3 H
− ). Diverse polymer materials including poly(ether
sulfone) (PES), poly(ether ketone) (PEK), polybenzimidazole (PBI), polyimide (PI), poly(phenylene), polyphosphazene and polyvinylidene fluoride (PVDF) were
investigated as the backbones for CEMs (Ran et al. 2017).
AEMs are generally prepared from positively charged
polyelectrolytes, and quaternized ammonium is the most
conventional conducting groups for AEMs (Nie et al. 2015),
where imidazole, phosphonium, tertiary sulfonium cations
and metal-cation-based polyelectrolytes also serve as
promising candidates for AEMs (Yang et al. 2015; Stokes
et al. 2011; Zhang et al. 2012; Disabb-Miller et al. 2013).
The topological architectures of polymeric ionomers generally contain main chain, side chain, block copolymers and
densely functionalized types (Ran et al. 2017) (Table 2).
3 Category of Ion-Exchange Membrane
Fouling
Previous section introduces the membrane material background, and this section will focus on the foulant part, in
order to link to the possible membrane–foulant interaction.
Indeed, understanding the commonly reported fouling
types and their mechanisms is crucial to the development of
anti-fouling strategies for IEM-based processes. The foulants
intend to adhere to the membrane materials by complicated
foulant–membrane and foulant–foulant attractive interactions from the physicochemical point of view. While thermodynamically, this is a process toward the minimization of
Gibbs free energy of the system (Zhao et al. 2018a; Zhang
et al. 2016).
The fouling behavior of any foulant can be simplified to
approaching–adsorption–accumulation (Wang et al. 2017;
Zhao et al. 2018a). First, the foulants (or their precursors)
approach or contact a membrane surface. Then, these foulants adsorb or attach onto the membrane surface via electrostatic, hydrophobic, van der Waals, hydrogen bonding or
other interactions; finally, foulants accumulate or aggregate
together with each other and then form cake, gel, biofilm or
scaling layers on the membrane surface.
Various membrane–foulant classifications exist. For
example, according to intrinsic characteristics of fouling
behaviors, membrane–foulants can be divided into
non-migratory (e.g., organic colloids, natural organic matters,
biomacromolecules), proliferative (e.g., microorganisms,
bacterial, living cells) and inorganic foulants (e.g., precipitated salts and sparingly soluble salts) (Zhao et al. 2018a;
Zhang et al. 2016; Dydo and Turek 2013; Husson et al. 2013;
Luo et al. 2012). For simplicity, organic fouling, biofouling,
inorganic scaling and colloidal fouling are used to describe
the category of IEMs fouling depicted in Fig. 2 and Table 3.
3.1 Organic Fouling
Common organic foulants to IEMs involve organic substances such as oil, carbohydrates, proteins, humic acid and
aromatics (Husson et al. 2013; Banasiak and Schäfer 2009;
Guo et al. 2014; Shi et al. 2011; Bukhovets et al. 2010; Sri
Mulyati et al. 2012). These organic substances easily stick to
the surface of the membrane and/or lodge themselves inside
the membrane-free volume, and the induced fouling
dynamics is generally relevant to the foulant physical–
chemical properties such as the molecular structure,
hydrophobicity, charging nature, solubility, mobility and
bulk solution concentration (Mikhaylin and Bazinet 2016).
For example, a permeable organic anion which is small
enough to penetrate into membrane-free volume possibly
fouls the membrane inner phase due to a low mobility in the
confined zone. Given a high bulk concentration and its low
solubility, organic molecule adsorption at the membrane
surface also occurs, causing a drastic increase in the membrane stack resistance (Bukhovets et al. 2010).
Generally, anion-exchange membranes are more susceptible to fouling by organic compounds under electrostatic
attraction effect. Researchers have focused on fouling of
AEMs by organic foulants of negative charge, such as
bovine serum albumin (BSA), humate, carboxylic acids and
anionic surfactants (Sri Mulyati et al. 2012). The mixture of
proteins formed an intense layer on membrane surfaces,
which was ascribed to solute charge rather than particle size
and variation in molecular packing (Bukhovets et al. 2010).
Other foulant properties can also be influential.
Hydrophobicity of sodium dodecylbenzene sulfonate
(SDBS) due to the indissociable organic chain may foul the
Current Strategies for the Design of Anti-fouling …
15
IEMs are typically thin polymeric films containing fixed
charged groups which are ionized in water (Strathmann
2010). Figure 1 summarizes the common categories of IEM
regarding the charge nature and morphology, where based
on the charge nature (or type of ionic functional groups),
IEMs are broadly classified into CEMs and AEMs (Zeng
et al. 2019; Ran et al. 2017), and based on their morphology,
IEMs can be classified into the homogeneous and heterogeneous ones. The forming category according to the functional charged group is further illustrated since the ion
permselectivity is highly related.
IEMs can selectively allow the passage of oppositely
charged ions (counterions) while obstruct similarly charged
ions (co-ions), with the counterions permselectivity following the theory elucidated by Donnan. CEMs generally contain negatively charged groups (e.g., –SO 3
−
, –COO
− , –PO 3
2
− , –PO 3 H
− ). Diverse polymer materials including poly(ether
sulfone) (PES), poly(ether ketone) (PEK), polybenzimidazole (PBI), polyimide (PI), poly(phenylene), polyphosphazene and polyvinylidene fluoride (PVDF) were
investigated as the backbones for CEMs (Ran et al. 2017).
AEMs are generally prepared from positively charged
polyelectrolytes, and quaternized ammonium is the most
conventional conducting groups for AEMs (Nie et al. 2015),
where imidazole, phosphonium, tertiary sulfonium cations
and metal-cation-based polyelectrolytes also serve as
promising candidates for AEMs (Yang et al. 2015; Stokes
et al. 2011; Zhang et al. 2012; Disabb-Miller et al. 2013).
The topological architectures of polymeric ionomers generally contain main chain, side chain, block copolymers and
densely functionalized types (Ran et al. 2017) (Table 2).
3 Category of Ion-Exchange Membrane
Fouling
Previous section introduces the membrane material background, and this section will focus on the foulant part, in
order to link to the possible membrane–foulant interaction.
Indeed, understanding the commonly reported fouling
types and their mechanisms is crucial to the development of
anti-fouling strategies for IEM-based processes. The foulants
intend to adhere to the membrane materials by complicated
foulant–membrane and foulant–foulant attractive interactions from the physicochemical point of view. While thermodynamically, this is a process toward the minimization of
Gibbs free energy of the system (Zhao et al. 2018a; Zhang
et al. 2016).
The fouling behavior of any foulant can be simplified to
approaching–adsorption–accumulation (Wang et al. 2017;
Zhao et al. 2018a). First, the foulants (or their precursors)
approach or contact a membrane surface. Then, these foulants adsorb or attach onto the membrane surface via electrostatic, hydrophobic, van der Waals, hydrogen bonding or
other interactions; finally, foulants accumulate or aggregate
together with each other and then form cake, gel, biofilm or
scaling layers on the membrane surface.
Various membrane–foulant classifications exist. For
example, according to intrinsic characteristics of fouling
behaviors, membrane–foulants can be divided into
non-migratory (e.g., organic colloids, natural organic matters,
biomacromolecules), proliferative (e.g., microorganisms,
bacterial, living cells) and inorganic foulants (e.g., precipitated salts and sparingly soluble salts) (Zhao et al. 2018a;
Zhang et al. 2016; Dydo and Turek 2013; Husson et al. 2013;
Luo et al. 2012). For simplicity, organic fouling, biofouling,
inorganic scaling and colloidal fouling are used to describe
the category of IEMs fouling depicted in Fig. 2 and Table 3.
3.1 Organic Fouling
Common organic foulants to IEMs involve organic substances such as oil, carbohydrates, proteins, humic acid and
aromatics (Husson et al. 2013; Banasiak and Schäfer 2009;
Guo et al. 2014; Shi et al. 2011; Bukhovets et al. 2010; Sri
Mulyati et al. 2012). These organic substances easily stick to
the surface of the membrane and/or lodge themselves inside
the membrane-free volume, and the induced fouling
dynamics is generally relevant to the foulant physical–
chemical properties such as the molecular structure,
hydrophobicity, charging nature, solubility, mobility and
bulk solution concentration (Mikhaylin and Bazinet 2016).
For example, a permeable organic anion which is small
enough to penetrate into membrane-free volume possibly
fouls the membrane inner phase due to a low mobility in the
confined zone. Given a high bulk concentration and its low
solubility, organic molecule adsorption at the membrane
surface also occurs, causing a drastic increase in the membrane stack resistance (Bukhovets et al. 2010).
Generally, anion-exchange membranes are more susceptible to fouling by organic compounds under electrostatic
attraction effect. Researchers have focused on fouling of
AEMs by organic foulants of negative charge, such as
bovine serum albumin (BSA), humate, carboxylic acids and
anionic surfactants (Sri Mulyati et al. 2012). The mixture of
proteins formed an intense layer on membrane surfaces,
which was ascribed to solute charge rather than particle size
and variation in molecular packing (Bukhovets et al. 2010).
Other foulant properties can also be influential.
Hydrophobicity of sodium dodecylbenzene sulfonate
(SDBS) due to the indissociable organic chain may foul the
Current Strategies for the Design of Anti-fouling …
15