Wetting, Scaling, and Fouling in Membrane Distillation: State-of-the-Art Insights on Fundamental Mechanisms and Mitigation Strategies

Abstract

Membrane distillation (MD) has been garnering increasing attention in research and development, since it has been proposed as a promising technology for desalinating hypersaline brine from various industries using low-grade thermal energy. However, depending on the application context, MD faces several important technical challenges that would lead to compromised performance or even process failure. These challenges include pore wetting, mineral scaling, and membrane fouling. This review is devoted to providing a state-of-the-art understanding of fundamental mechanisms and mitigation strategies regarding these three challenges. Guided by the fundamental understanding of each membrane failure mechanism, we discuss both operational and material strategies that can potentially address the three technical challenges. In particular, the material strategies involve the development of MD membranes with tailored special wetting properties to impart resistance against different types of membrane failure. Lastly, we also discuss research needs and best practices in future studies to further enhance our ability to overcome technical challenges toward the practical, sustainable, and scalable applications of MD.

Full text

Wetting, Scaling, and Fouling in Membrane Distillation: State-of-
the-Art Insights on Fundamental Mechanisms and Mitigation
Strategies
Thomas Horseman, Yiming Yin, KofiSS Christie, Zhangxin Wang, Tiezheng Tong,*and Shihong Lin*
Cite This: ACS EST Engg. 2021, 1, 117−140
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ABSTRACT: Membrane distillation (MD) has been garnering
increasing attention in research and development, since it has been
proposed as a promising technology for desalinating hypersaline
brine from various industries using low-grade thermal energy.
However, depending on the application context, MD faces several
important technical challenges that would lead to compromised
performance or even process failure. These challenges include pore
wetting, mineral scaling, and membrane fouling. This review is
devoted to providing a state-of-the-art understanding of
fundamental mechanisms and mitigation strategies regarding
these three challenges. Guided by the fundamental understanding
of each membrane failure mechanism, we discuss both operational
and material strategies that can potentially address the three technical challenges. In particular, the material strategies involve the
development of MD membranes with tailored special wetting properties to impart resistance against different types of membrane
failure. Lastly, we also discuss research needs and best practices in future studies to further enhance our ability to overcome technical
challenges toward the practical, sustainable, and scalable applications of MD.
KEYWORDS: membrane distillation, wetting, scaling, fouling, mitigation strategies
■INTRODUCTION
Membrane distillation (MD) is a membrane-based thermal
desalination process that has received extensive and growing
research and development interests in the past few decades.
While MD has multiple configurations, each case involves the
use of a nonwetted (typically hydrophobic), microporous
membrane to serve as an airgap that separates the feed and
distillate solutions from mixing. The transport of water vapor
from the hot, salty feed solution to the cold distillate is driven
by a partial vapor pressure gradient. This partial pressure
gradient is typically induced by the temperature gradient and,
in certain cases, enhanced by a partial vacuum.
1−3
The interest in MD has grown substantially in recent years
due to the increasing demand for modular systems capable of
treating hypersaline water including oil- and gas-produced
water, brine from inland brackish water desalination, and brine
generated in zero liquid discharge (ZLD) processes.
4−7
Because of growing water scarcity and more stringent
regulations, these hypersaline brines are becoming both
unconventional sources of water and hazardous wastes of
increasing environmental concern. MD is the most promising
modular (down-scalable) technology capable of treating high-
salinity feedwater using low-grade thermal energy and, thus,
has several unique advantages for treating hypersaline brine, as
compared to the state-the-of-art desalination process, reverse
osmosis,
1,8,9
or conventional thermal distillation pro-
cesses.
1,10−13
More recently, MD has also been explored as
an advanced technological platform for solar-thermal desalina-
tion due to its ability to implement latent heat recovery.
14−16
Another major category of membrane processes that
envelopes MD is membrane contactor (MC), where a
nonwetted microporous membrane is also used as an airgap.
However, the intended species of mass transfer in MC is the
volatile solute instead of water vapor. For instance, MC has
been actively explored for recovering valuable gases such as
ammonia and methane, sequestrating carbon dioxide, oxygen-
ation/deoxygenation, and removal of volatile contami-
nants.
17,18
The mass transfer in an MC is also driven by a
partial vapor pressure gradient, which is often induced by a
concentration gradient and sometimes assisted by a thermal
Received: June 12, 2020
Revised: August 7, 2020
Accepted: August 11, 2020
Published: October 1, 2020
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gradient and/or partial vacuum (fundamentally, all mass-
transfer processes in MD and MC are driven by a chemical
potential gradient reflected as a partial vapor pressure
gradient).
In both MD and MC, the microporous membranes serve as
not only a medium for vapor transport but also a barrier to
direct liquid permeation and, thus, must be maintained free
from pore blocking and wetting. However, the feedwater in
many promising applications of MD or MC often contains
constituents that promote fouling and wetting of conventional
hydrophobic membranes.
19−24
For example, organic matter,
such as proteins from the food and beverage industry or oil
particles that exist in oil- and gas-produced water, are potent
foulants, especially for hydrophobic membranes.
4,25−27
Low-
surface-tension, water-miscible liquids may exist in the
feedwater of MC, reducing the overall surface tension of the
feedwater and resulting in pore wetting.
28,29
In addition,
synthetic surfactants and natural amphiphiles may also reduce
the surface tension of the feedwater and induce wetting.
30−32
For MD used in treating hypersaline brine, an additional
challenge is mineral scaling, that is, the formation and/or
accumulation of salt precipitates on the membrane surface that
results in significant flux reduction and, in some cases, even
pore wetting.
33−35
All these technical challenges, namely, wetting, scaling, and
fouling, constrain the practical adoption of MD and MC for
treating a wide spectrum of feedwater. In particular, these
limitations pose a paradox for MD as an effective technology
for desalinating and concentrating hypersaline brines: on the
one hand, MD is very promising for such applications due to
its (theoretical) capability of handling hypersaline brine using
low-grade thermal energy; on the other hand, concentrating
brine inevitably increases the concentrations of salts, foulants,
and surfactantswhatever constituents that originally exist in
the feedwaterand thus intensifies the propensity of scaling,
fouling, and wetting and limits the (practical) applicability of
MD in various applications.
1,9,36−38
It is therefore of
paramount importance for the community to gain fundamental
understanding of these challenges facing MD and MC and to
develop effective strategies of mitigating these technical
challenges to enable these membrane-based vapor-transport
processes to achieve their full potential for practical
applications. This review is precisely organized to assess the
recent advances in the fundamental understanding and
technological development we have made as a community
toward addressing these technical challenges.
In this review, we will discuss the fundamental mechanisms
and mitigation strategies for the three technical challenges in
membrane-based vapor-transfer processes: wetting, mineral
scaling, and fouling. We will focus our discussion on MD, as
most of the developments in addressing these challenges have
been made in the context of MD. However, many principles
and strategies to be discussed will also apply to MC. We
highlight the most recent research findings that enrich or
improve our mechanistic knowledge on wetting, scaling, and
fouling in MD desalination. An important part of the
mitigation strategies involves the application of membranes
with special wetting properties, which is a relatively new field
fueled by the recent advances in material science in
understanding and developing surfaces with special wettability.
However, we intend in this review to focus our discussion on
the fundamental mechanisms instead of presenting a
comprehensive survey of different membrane fabrication
methods.
■PORE WETTING
The most essential function of a membrane in MD and MC
systems is to separate an aqueous stream from another liquid
or gas stream in order to facilitate volatile component transport
between the two streams. In MD and MC applications,
Figure 1. (A) Schematic diagram of membrane wetting, featuring three wetting detection techniques based on distillate conductivity, optical
transmittance, and transmembrane impedance. (B) Illustration of how different properties associated with different detection techniques vary in
different stages of the wetting process. We note that distillate conductivity only starts to increase when at least some pores are fully penetrated by
the feed solution. Thus, the distillate conductivity does not capture any information when some pores are only partially infiltrated (but not a single
pore has been fully penetrated). Detection techniques based on transmembrane impedance and optical transmittance can provide information
regarding this stage of partial infiltration.
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membrane failure occurs when the liquid feed solution freely
permeates through the membrane, a phenomena known as
pore wetting.
27,28
In MD, wetting results in reduced salt
rejection and, thus, contamination of the distillate
stream;
2,39−42
whereas, in MC applications, wetting results in
reduced mass-transfer and separation efficiencies.
43−45
In MD,
the simplest and most common method for detecting wetting
is measuring distillate electrical conductivity
46
(Figure 1A).
The major limitation of this technique, however, is that it only
detects wetting once the feed liquid has already fully
penetrated at least a fraction of the membrane pores and
contaminated the distillate.
Recently, in situ techniques based on single-frequency
impedance
47
and light transmittance
48
have been developed
to provide dynamic insights into the wetting process. With the
technique based on single-frequency impedance, the air-filled
pores separating the salty feed solution and distillate stream
can be modeled as an equivalent circuit
49,50
(Figure 1A). The
total impedance of this equivalent circuit decreases as the air-
gap becomes progressively thinner due to the propagation of
feed solution/air interface toward the distillate (Figure 1B).
With the optical technique based on light transmittance
(Figure 1A),theintensityoftransmittedlightisalso
dependent on the thickness of the air-filled pores (Figure
1B). The membrane transitions from being opaque when
unwetted to being translucent when wetted.
48
While the
wetting detection method is not the focus of this review, we
want to emphasize that techniques are being actively developed
to extract more information about the dynamic wetting
phenomenon that has not been accessible by previous
detection techniques that rely on the change of the distillate
properties. The ability to unveil more information about the
dynamic behavior of wetting is critical for enhancing our
fundamental understanding of pore wetting.
Mechanism of Pore Wetting. General Criterion of Pore
Wetting. Historically, pore wetting is explained based on force
balance at the triple phase boundary with the help of an
important concept called liquid entry pressure (LEP). LEP is
the minimum hydrostatic pressure required to push the liquid
into the membrane pores (for ideal cylindrical pores, entry is
equivalent to penetration) and can be estimated using eq 1
51,52
B
r
L
EP 2cos
L0
γθ
=−
(1)
where γLis the liquid surface tension, θ0is the intrinsic contact
angle between the liquid and solid membrane material, ris the
equivalent pore radius, and Bis a geometric factor accounting
for the noncylindrical nature of the membrane pore geometry
(B= 1 for perfectly cylindrical pores). In general, reducing the
surface tension of the feed solution reduces both γLand cos θ0
(as θ0is also a function of γL), thus reducing LEP and
facilitating pore wetting.
29,41,51−53
The general criterion for
membrane pore wetting is that the transmembrane pressure,
ΔP, exceeds LEP.
51,54
PLE
PΔ
≥
(2)
This criterion, as specified by the above inequality, applies well
in most cases, provided the LEP can be accurately determined.
If the membrane is composed of perfect cylindrical pores, the
system will transition from an unwetted state (Figure 2A) to a
wetted state (Figure 2C), as long as ΔPexceeds LEP, without
going through any stable transition state (Figure 2B)
observable by measuring the transmembrane impedance or
optical transmittance. This abrupt transition from unwetted
state to wetted state has been observed when (1) the hydraulic
pressure of the feed solution undergoes a significant stepwise
increase and (2) the surface tension of the feed solution is
suddenly reduced by the addition of a large amount of water-
miscible liquids such as alcohol. The absence of observable
transition state (i.e., the nearly instantaneous wetting as shown
in Figure 2D) is consistent with the kinetics of an expanding
Poiseuille flow.
29,55,56
Assuming an ideal cylindrical pore, the
depth of pore infiltration l(t) can be estimated using eq 3
29
Figure 2. Various states in a wetting process. (A) Unwetted state: all pores are neither partially infiltrated nor fully penetrated. (B) Transition state:
some pores are partially infiltrated, but no pore is fully penetrated. (C) Wetted state: some or all pores are fully penetrated. (D) Behavior (i.e., salt
rejection and transmembrane impedance) of wetting induced by having a hydrostatic pressure exceeding the LEP. (E) Behavior of wetting induced
by addition of surfactants.
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lt rPt() 4(LEP)
2
μ
=Δ−
(3)
where ris the pore radius, μis the dynamic viscosity of the
feed liquid, tis time with t=0defined as the point that feed
solution begins intruding into the membrane pores (see Wang
et al.
29
for a more detailed derivation). Calculations based on
eq 3 suggest that liquid penetration through a membrane pore
typically takes only seconds, which explains the absence of an
observable transition state.
Surfactant-Induced Wetting. What happens when the feed
solution contains surfactants? Equation 2 suggests that wetting
would occur just as in the case when low-surface-tension
(LST) water-miscible liquid is added, except that we need a
much lower concentration of surfactants, as they are very
effective in reducing the liquid surface tension. However, both
the impedance and optical transmittance-based techniques
revealed that wetting induced by the addition of surfactants is
transient, not instantaneous; that is, the transition state as
illustrated in Figure 2B is observable for an extended period of
time.
47,48
The observable transition state suggests that
surfactant-induced wetting may have a very different
mechanism as compared to that of wetting induced by
increasing the ΔPor reducing the LEP via adding LST water-
miscible liquids. The key to explaining the dynamic behavior of
surfactant-induced wetting in MD is to understand that (1)
surfactants readily adsorb onto a hydrophobic surface
immersed in water and that (2) surfactants are very effective
in reducing liquid surface tension, and thus it only requires a
very low concentration of surfactants to reduce the LEP to be
below the ΔPvalue.
The dynamic wetting behavior of surfactant-induced wetting
has been recently elucidated and modeled,
29,31,32
and it is
illustrated in Figure 3A. On the one hand, the fast adsorption
of surfactants onto the hydrophobic membrane pore wall
constantly removes surfactant from the solution near the
water/air interface (i.e., the “wetting frontier”). On the other
hand, advective transport due to water vapor flux and diffusive
transport due to concentration gradient replenish surfactants
from the feed solution (outside the pore) to the wetting
frontier. The combination of the adsorption-driven depletion
and transport-driven replenishment results in a surfactant
concentration at the wetting frontier that is lower than the bulk
concentration. The relevant LEP of the system should be
calculated, not based on the surfactant concentration of the
feed solution (in the bulk), but based on the surfactant
concentration and the corresponding surface tension of the
feed solution at the wetting frontier. In this context, eq 2 again
becomes applicable, as the criterion for pore wetting, except
that in this case the LEP of the wetting frontier, not of the bulk
feed solution, should be compared with ΔP.
The theory presented above explains why instantaneous
wetting does not occur even if LEP calculated from the bulk
surfactant concentration is lower than ΔP. However, pore
wetting still occurs gradually, because the pore wall surface has
a limited capability for surfactant adsorption. When the wetted
Figure 3. (A) Mechanism of surfactant-induced pore wetting featuring surfactants adsorption at the wetting frontier and the transport of surfactants
from the bulk feed solution to the wetting frontier via advection and diffusion. (B) “Bridge building”analogy of the dynamic wetting model: passing
the river (equivalent to “wicking through the pore”) requires extending the bridge (equivalent to “saturating the pore surface surrounding the
wetting frontier”). The impacts of (C) bulk surfactant concentration, (D) vapor flux, and (E) surfactant type, on the kinetics of wetting as
quantified by breakthrough time or its inverse. The corresponding variables in the “bridge building”analogy are also given on the top of each panel.
Adapted with permission from (C, D) ref 31 and (E) ref 32; Copyright 2018 and 2019 Elsevier, respectively.
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surface of the pore is saturated, adsorptive depletion of
surfactants no longer occurs, which leads to the increase in the
surfactant concentration at the wetting frontier and, corre-
spondingly, to the reduction of the surface tension and LEP.
Once the LEP becomes lower than ΔP, wetting occurs, and the
wetting frontier propagates toward the distillate. This forward
propagation exposes a new region of the pore to the feed
solution, which again enables rapid adsorption of surfactants to
increase the LEP beyond ΔPuntil that new region is again
saturated with surfactants. Such a process of adsorption,
saturation, and forward propagation of the wetting frontier
repeats itself and results in the experimentally observed
transient wetting. While this phenomenon is described here
as a stepwise process to facilitate understanding, it is
continuous in a real process.
It may be helpful to illustrate this new model for surfactant-
induced wetting with an interesting analogy of “bridge
building”(Figure 3B). To build a bridge (i.e., for the feed
solution to fully penetrate a hydrophobic pore), bricks (i.e.,
surfactant molecules) must be transported to the bridge
frontier and laid there to further extend the bridge. The brick-
laying process (i.e., the adsorption of surfactants to the pore
wall at frontier) is very fast. Therefore, the kinetics of the
bridge-building process is limited by how fast the bricks are
transported to the bridge frontier. This bridge building analogy
leads to several important implications. For example, if the
brick-transporting truck carries more bricks per truck, which is
equivalent to having a higher feed concentration of surfactants
(as the advection term is proportional to concentration), the
bridge building (pore wetting) process is faster (Figure 3C;
note that, if the bulk surfactant concentration is too low,
wetting does not occur, because ΔPis lower than the LEP of
the bulk solution). In addition, if the truck moves faster, which
is equivalent to having a higher flux (as the advection term is
proportional to flow velocity), the bridge building (pore
wetting) process is also faster (Figure 3D). These two
dependences of wetting kinetics on experimental conditions
have been shown experimentally and predicted accurately by
the dynamic wetting model.
31,32
The most interesting conclusion from the model regards
how the surfactant type affects the wetting kinetics. The bridge
building analogy suggests that, if the bricks are bigger
(primarily, longer), the bridge will be extended faster provided
(1) all trucks move at the same speed and (2) each truck
carries the same number of bricks. Equivalently, the dynamic
wetting model predicts that “bigger”surfactants will lead to
faster pore wetting (i.e., shorter breakthrough time), which has
also been experimentally confirmed
31,32
(Figure 3E). Here, the
size of the surfactants can be quantified using surface excess
concentration Γ, which can be evaluated using the Gibbs
adsorption isotherm.
57−59
Specifically, a very nice linear
correlation has been experimentally shown between the surface
excess concentration and the breakthrough time for a series of
surfactants of different charges (Figure 3E, except for sodium
dodecyl sulfate, for which diffusion has an important
contribution to transport. See a detailed explanation in the
work of Wang et. al.
32
). This experimentally validated model
emphasizes the “size”of surfactants as a key property that
influences how efficiently different surfactants induce wetting
(assuming the same molar concentration), which is in contrast
to the previous understanding that focused on the interaction
between the surfactant and the pore surface.
30,60
According to
the dynamic wetting model, the surfactant-pore surface
interaction is irrelevant to the kinetics of pore wetting, because
adsorption, which is much faster than transport of surfactants
to the frontier, is not the rate-limiting step.
The dynamic model for surfactant-induced wetting is the
first quantitative model for predicting the kinetics of surfactant-
induced wetting. The model, which provides a satisfactory
fitting to the experimental data from multiple dimensions, is
based on the assumption that wetting occurs only when the ΔP
exceeds the LEP at the wetting frontier. Another plausible
mechanism for propagation of the wetting frontier is via the
“autophilic effect”, that is, the surfactants can adsorb onto the
unwetted part of the surface ahead of the triple phase boundary
and thereby reduce the surface energy of the unwetted part of
the surface near the wetting frontier, making it hydrophilic.
61,62
As a result, the propagation of the liquid−air interface toward
the distillate is spontaneous and can occur without applied
pressure. The “autophilic”explanation, however, has challenges
in explaining why there exists a minimum bulk surfactant
concentration below which wetting does not occur (Figure
3A). Moreover, the subtle impact of ΔPon the wetting
kinetics, which has been experimentally validated, also appears
to confirm the assumed wetting criterion based on comparing
the ΔPand the LEP. Whether an autophilic effect does play an
important role in surfactant-induced wetting requires further
investigation. However, even assuming an autophilic effect is
important, because the adsorption of surfactants onto a
hydrophobic surface immersed in water is highly energetically
favorable, we speculate that the autophilic effect can only exert
its contribution when a wetted pore surface is saturated with
surfactants. In other words, the bridge building analogy and the
kinetic model of pore wetting focusing on the critical role of
surfactant transport should apply regardless of the exact
mechanism of frontier propagation.
Wetting Mitigation Strategies. Pretreatments. There
are limited pretreatment techniques that may mitigate
membrane pore wetting. For example, physical pretreatments
such as microfiltration (MF), ultrafiltration (UF), or nano-
filtration (NF) can effectively remove amphiphilic proteins that
may eventually wet the pores of a hydrophobic membrane.
63,64
However, they cannot mitigate pore wetting induced by
surfactants or LST water-miscible liquids. Surfactants can be
removed from solution via ion exchange,
65
coagulation,
66,67
floatation or foam fractionation,
68−70
or biodegradation.
71,72
While some of the aforementioned pretreatment methods have
not yet been tested for MD or MC and a knowledge gap exists
in matching the appropriate pretreatment method given a real
water application, some studies have shown promise in using
these pretreatment strategies for MD and MC. For example,
dispersing bubbles into a surfactant-containing feed solution
from the textile industry formed floating foams that removed
surfactants from the bulk solution. This foam fractionation
pretreatment in turn eliminated the surfactant-induced wetting
that had previously occurred in the downstream MD process.
70
In another study, MC used for ammonia stripping from a
model wastewater stream was plagued by eventual wetting due
to amphiphilic protein molecules. After the integration of the
MC process with a microfiltration or ultrafiltration pretreat-
ment, the tested poly(tetrafluorethylene) (PTFE) and
polypropylene (PP) membranes realized a twofold and
fourfold increase in the ammonia mass-transfer coefficients,
respectively, and membrane wetting was never observed
throughout the duration of the 30 h experiment.
64
For a
feed solution containing LST water-miscible liquids as often
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encountered in MC, no viable pretreatment exists for wetting
mitigation, as the removal of the wetting agent is precisely the
technical goal of MC itself. In general, if the feedwater has a
composition that may promote pore wetting in MD or MC,
options to mitigate wetting via pretreatment are fairly limited.
Therefore, a different approach based on developing a wetting-
resistant membrane is needed for more versatile and robust
wetting mitigation.
Omniphobic Membranes. Because all wetting processes,
whether induced by surfactants or alcohol, follow the same
principle described by eq 2, we can develop membranes that
resist wetting by LST liquids as a strategy for wetting
mitigation. Membranes that are resistant to wetting by LST
liquids, such as oil, are called oleophobic membranes. In most
cases, oleophobic membranes are also hydrophobic and thus
are referred to as amphiphobic or omniphobic membranes.
73,74
Without specifying the medium, the definitions of wetting
property assume air as the medium. Strictly speaking,
omniphobicity literally refers to resistance to wetting by all
liquids, whereas amphiphobicity refers to wetting resistance to
both water and oil. The difference between the two concepts is
more quantitative than qualitative. For instance, a membrane
may be referred to as amphiphobic if it is resistant to wetting
by mineral oil (γ≈30 mN m−1) but not by ethanol (γ≈22
mN m−1) and n-decane (γ≈24 mN m−1). Therefore, we use
omniphobic membranes here to refer broadly to membranes
with resistance to wetting by LST liquids.
The development of an omniphobic membrane for MD
application was first reported in 2014
28
and has since attracted
extensive research interest.
73−82
For example, omniphobic
membranes have shown promising wetting resistance in
treating produced water from the oil and gas
73,74
and coking
industries.
76
However, the fundamental mechanism of
achieving omniphobicity had been elucidated by the material
science community several years earlier.
83−86
Without going
through the theoretical derivation (interested readers are
referred to these publications
83,84
), here we summarize the
criteria for developing omniphobic membranes and the basic
principle behind such criteria.
The two major criteria for making an omniphobic
membrane or, more generally, an omniphobic surface, are
that (1) the material has a low surface energy and (2) the
surface has a reentrant texture. The first criterion is shared by
both hydrophobic and omniphobic membranes and is often
met by using fluoro-polymers and surface modifiers, which are
known to be of low surface energy and chemically relatively
stable. In describing the wetting state of a general textured
substrate surface, if the liquid is in full contact with the
substrate, the system is in a Wenzel state; if the liquid is
supported by a composite surface comprising the protrusions
of the solid substrate and the air pockets between these
protrusions, the system is in a Cassie−Baxter state.
In the context of membrane wetting, the composite interface
at the wetting frontier that comprises the membrane pores and
the solid surface itself envelopes the aforementioned “surface
texture”. The Cassie−Baxter state corresponds to the unwetted
state depicted in Figure 2A, when the LEP at the wetting
frontier is high enough to deter any liquid infiltration into the
membrane pores toward the direction of the distillate stream.
Likewise, the Wenzel state corresponds to the wetted state
depicted in Figure 2C, when the LEP at the wetting frontier
satisfies the condition stipulated by eq 2 and thus results in
pore wetting. To prevent wetting of any degree, the wetting
frontier needs to be maintained in a Cassie−Baxter state at the
membrane surface. The thermodynamics of wetting suggests
that the maintenance of a Cassie−Baxter state for high-surface-
tension liquids, such as water, can be achieved as long as the
porous membrane is made of low-surface-energy materials.
75,87
However, it also suggests that the Cassie−Baxter state is not
stable (i.e., it has a higher free energy than that of the Wenzel
state) if the surface tension of the liquid is sufficiently low.
Here comes the important role of the reentrant texture (the
second criterion) that sustains a metastable Cassie−Baxter
state.
Figure 4. (A) Local intrinsic contact angles at the local triple-phase boundary for a reentrant structure (top) and a nonreentrant structure
(bottom). (B) Illustration of how a liquid intrusion into a pore with reentrant geometry would change the contact area of the liquid/solid and
liquid/air interfaces. (C) Free energy profile as a function of the position of the liquid/air interface for pores with reentrant and nonreentrant
geometries.
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A reentrant texture is a concave topography in which the
cross-sectional area of a pore increases with the depth of the
pore. A classic reentrant texture is an array of inverted cones.
Figure 4A shows the cross-section of space between two
inverted cones (top) versus that between two cylinders
(bottom). The requirement of a reentrant texture to maintain
a Cassie−Baxter state with even LST liquids can be explained
using both force balance and free energy analysis. Without a
reentrant texture, the maintenance of a Cassie−Baxter state
requires that local contact angle be higher than 90°(Figure 4A
bottom), which can be satisfied if the solid material
constituting the pores has a low surface energy and the liquid
has high surface tension (e.g., water). However, with LST
liquids, a local contact angle does not exceed 90°even with
solid material with low surface energy. In this case, the Cassie−
Baxter state can only be maintained with a reentrant texture
with which the local contact angle can be lower than 90°
(Figure 4A top).
The necessity of reentrant texture for achieving omnipho-
bicity can also be elucidated from the perspective of free
energy. When liquid infiltrates into a pore, part of the solid/air
interface is replaced by a solid/liquid interface (Figure 4B). If
the solid surface has a low surface energy and the liquid has a
high surface tension, the replacement of the solid/air interface
by a solid/liquid interface is energetically unfavorable, which
explains the stable Cassie−Baxter state for water contacting a
hydrophobic membrane. When an LST liquid is considered,
this replacement is energetically favorable even if the pore is
made of low-surface-energy materials. However, with and only
with reentrant texture, the liquid infiltration expands the
liquid/air interface, which is energetically unfavorable. If the
unfavorable expansion of the liquid/air interface outweighs the
favorable expansion of the solid/liquid interface, a free energy
barrier would exist in the process of the transition from the
Cassie−Baxter state to the Wenzel state when the pore is fully
infiltrated (Figure 4C). Therefore, a metastable Cassie−Baxter
state can exist with LST liquid in the presence of a reentrant
structure. On the contrary, the transition from the Cassie−
Baxter state to the Wenzel state would monotonically reduce
the free energy if the system does not have a reentrant texture
that promotes expansion of the liquid/air interface upon pore
infiltration (Figure 4C).
While inverted cones are convenient models for illustrating a
reentrant structure, they are not a practical structure to
engineer on a membrane. Instead, fibers from electrospinning
or nanoparticles from spraying or solution-phase adsorption
are used to construct the reentrant texture for omniphobic
membranes.
74−76,78
Recent studies also find that omniphobic
membranes can be obtained by simple chemical modification
to reduce the surface energy of commercial hydrophobic
membranes, which suggests that the structure of some
commercial hydrophobic membranes is already reentrant.
88
It also suggests that whether a Cassie−Baxter state can be
maintained depends on the intricate interplay between liquid
surface tension, surface energy of the solid, and the pore
geometry. If the membrane is made of material of lower surface
energy and/or has a more robust reentrant structure (the
readers can refer to ref 84 for the theory about the robustness
of oleophobicity), the membrane may become “more
omniphobic”and can sustain stable MD performance with a
feed solution of a lower surface tension. In an extreme case
with a doubly reentrant structure, even a surface of high surface
energy (e.g., silica) can be omniphobic.
89
As mentioned above,
this article is not focusing on membrane fabrication methods.
Therefore, readers who are interested in fabricating omni-
phobic membranes can refer to these review papers.
38,82,90
■MINERAL SCALING
Mineral scaling is extremely relevant to MD, as its most
opportunistic applications are for treating hypersaline indus-
Figure 5. (A) CNT considering contributions from both reduction in bulk free energy and increase in interfacial free energy. (B) Reducing free
energy barrier for nucleation via (1) heteronucleation and (2) oversaturation. (C) Local spatial distribution of temperature and salt concentration
in the direction normal to the membrane, featuring temperature and concentration polarizations. Specifically, T, IAP, and SI represent temperature,
ion activity product, and saturation index, respectively; the first subscripts, F and D, denote the properties of the feed solution and distillate,
respectively; the second subscripts, B and M, denote the properties in the bulk and at the membrane surface, respectively. (D) Spatial distribution
of temperature (represented by color−red symbolizing high temperature and blue symbolizing low temperature) and salt concentration
(represented by density of white dots) in the MD module.
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trial wastewater where reverse osmosis (RO) is not feasible.
1
Depending on the pretreatment methods used, hypersaline
wastewaters often contain ions that form sparingly soluble
minerals (i.e., sulfates, carbonates, and silicates).
91
During MD,
these compounds are concentrated as water evaporates, which
eventually leads to the development of a precipitated layer of
mineral crystals on the membrane surface, also known as
mineral scaling.
34
Mineral scaling can occur via the deposition
of precipitates formed in the bulk solution or direct nucleation
and growth of precipitates on the membrane surface. In either
case, scaling reduces process efficiency or leads to process
failure, and it must be properly addressed to maximize the
potential for applying MD toward addressing broader water
treatment challenges.
92
Mineral scaling is mostly detected by monitoring the water
vapor flux over time.
93,94
Typically, an appreciable decrease in
water vapor flux is indicative of scaling, as the scaling layer
blocks the membrane pores. Although this method involves a
straightforward indicator that is directly relevant to membrane
performance, it only allows scaling to be detected after much of
the membrane surface has been covered and provides little
information regarding the growth kinetics, spatial distribution,
morphology, and composition of the scale layer. Ex situ
membrane autopsy has helped elucidate scaling mechanisms
and how they relate to operation parameters and membrane
characteristics. A suite of standard material characterization
techniques has been used for membrane autopsy, including
scanning electron microscopy (SEM) with energy-dispersive
X-ray spectroscopy (EDS), Fourier-transformed infrared
(FTIR) spectroscopy, atomic force microscopy (AFM), X-ray
diffraction (XRD), and X-ray photoelectron spectroscopy
(XPS).
95−97
There has also been some progress on the development of in
situ scaling observation techniques. For example, optical
coherence tomography (OCT) has revealed that hardly any
scaling particles were visible before the vapor flux decline.
98,99
Also, a laser-based optical technique has been used to measure
the concentration profiles of metal chloride salts in the MD
feed solution near the membrane surface.
100
However, more
powerful and easy-to-implement in situ scaling detection
methods are still in need to better elucidate the dynamics of
scaling, especially during the early stages of crystal growth.
Mechanism of Scaling. General Scaling Criterion.
Mineral precipitates form when the species’concentration
product exceeds its solubility product. Like other mineraliza-
tion processes occurring in nature, the formation of mineral
scales in membrane desalination follows the theories
established under the framework of classic nucleation theory
(CNT).
101
The CNT describes the change of free energy
(ΔG) associated with the formation of a nascent cluster of a
new solid phase (from ions), which is expressed as the sum of
two terms (eq 4,Figure 5A)
102
Gn A
l
n
μγ
Δ
=− Δ +
(4)
where nis the number of ions in a cluster, Δμrefers to the
chemical potential difference of the ions between their solution
phase and solid phase, Ais the surface area of the cluster, and
γln refers to the interfacial energy between the liquid and the
nucleus. The first term on the right side of eq 4 represents the
energy change associated with the formation of bulk mineral
from ngrowth units (i.e., bulk free energy), which is always
negative under supersaturated conditions; the second term
regards the energy gain (penalty) associated with increasing
the interfacial area (i.e., interfacial free energy). Assuming a
spherical nucleus of radius r, the change in the free energy
during homogeneous nucleation is given by eqs 5 and (6)
102
Gr N
vrr() 4
34
hom A
m
3
ln
2
πμ πγ
Δ
=− Δ+
(5)
where NAand vmare the Avogadro number and molar volume,
respectively. The chemical potential difference Δμis depend-
ent on the degree of oversaturation
kT K
ln IAP
B
sp
i
k
j
j
j
j
j
j
y
{
z
z
z
z
z
z
μ
Δ
=
(6)
where kBis the Boltzmann constant, Tis the absolute
temperature, IAP is the ion activity product, and Ksp is the
equilibrium constant (or solubility product). As shown in
Figure 5A, ΔGhom(r) reaches a maximum value (ΔG*) when
the mineral nucleus grows to a critical size r*. Only the nuclei
with sizes larger than r*can evolve into thermodynamically
stable minerals. Therefore, ΔG*represents the height of the
energy barrier of mineral nucleation.
Equations 5 and (6) bridge the degree of saturation of the
feed solutions to the thermodynamic energy barrier of scale
formation. Saturation index (SI) is often used in the literature
to quantify the scaling potential of the feedwater in membrane
desalination.
95,103−106
While there are multiple definitions of
SI in the literature (it has been defined as ln(IAP/Ksp),
log10(IAP/Ksp), or (IAP/Ksp)), the SI based on all these
definitions consistently increases as the solution becomes more
concentrated. With an increase of SI, Δμincreases (according
to eq 6) and ΔGdecreases (according eq 4 or (5)), resulting in
a lower nucleation energy barrier (ΔG*in Figure 5B) and
easier mineral formation.
For heterogeneous nucleation, which could take place on
membrane surfaces or foreign particles in the bulk solution, the
interfacial energy at the liquid-substrate (γls), substrate-nucleus
(γsm), and liquid-nucleus (γls) interfaces needs to be
considered.
102
The presence of a substrate surface decreases
the interfacial free energy (Figure 5C), which also leads to a
reduction of the nucleation energy barrier. The energy barrier
of heterogeneous nucleation (ΔGhet
*) relates to that of
homogeneous nucleation (ΔGhom
*) by a correction factor
called the wetting function f(θn/w)
107
Gf G
G
() 1
4(1 cos )
(2 cos )
het n/w hom n/w 2
n/w hom
θθ
θ
Δ*=Δ
*=−
×+ Δ
*(7)
where θn/w is the contact angle of a nucleus (assuming a
spherical cap geometry) on the substrate in water as the
medium (we note that θnshould not be misinterpreted as the
in-air water contact angle of the membrane substrate, which
will be denoted as θw). Because the morphology of the nucleus
is determined by the crystal structure and not necessarily a
spherical cap, θn/w is more of a hypothetic metric to quantify
the interaction between the mineral nucleus and the substrate
based on the Young equation.
cos n/w ls sn
ln
θγγ
γ
=
−
(8)
Most membrane substrates in MD are nonpolar polymers that
do not have polar interaction (also called acid−base
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interaction according to Van Oss
108
). Therefore, the γls and γsm
terms in eq 8 can be written as
109
2
ls l s l
D
s
D
γ
γγ γγ=+− (9)
and
2
sn s n s
D
n
D
γ
γγ γγ=+ − (10)
where γl,γs, and γnare the surface energy of the liquid,
substrate, and nucleus, respectively, and γl
D,γs
D, and γn
Dare the
contributions of dispersion force (also called Lifshitz-van der
Waals interaction by Van Oss) to the surface energy of the
liquid, substrate, and nucleus, respectively. Considering eqs 9,
(10), and the equality of γs≈γs
Dfor a nonpolar substrate, eq 8
can be rewritten as
cos 2( )
n/w
ln sl
D
n
D
ln
θ
γγ γγ γ
γ
=
−− −
(11)
Depending on the temperature, γl
Dis slightly higher than 20
mN m−1for water. Without known exception, γn
Dis always
larger than γl
D(e.g., γn
D≈37 mN m−1for silica
110
and γn
D≈47
mN m−1for gypsum
111
). Therefore, l
D
γ
−
n
D
γis negative,
and thus cos θn/w positively correlates with γs. In other words,
as the substrate becomes more hydrophobic (as a result of
reduced γs), the contact angle θn/w in eq 7 also increases (as
cos θn/w decreases).
The value of f(θn/w)ineq 7 ranges between 0 and 1 when
θn/w changes from 0 to π, consistent with the above discussion
that ΔGhet
*is lower than ΔGhom
*. The occurrence of
heterogeneous nucleation is more favorable than homogeneous
nucleation, and thus the presence of a surface (e.g., membrane)
promotes scale formation. While ΔGhom
*is a constant at a
specific temperature and SI, a difference in membrane
materials may result in different energy barriers for
heterogeneous nucleation. This partially lays the foundation
of fabricating scaling-resistant membranes by tuning mem-
brane surface properties to inhibit heterogeneous nuclea-
tion.
95,96,103,112,113
The CNT theory also provides insights into the change of
scaling potential due to the concentration and temperature
polarization in an MD process. Concentration polarization is a
phenomenon in which solutes accumulate within a boundary
layer near the membrane surface as a result of the advective
transport of solute toward a nonpermeable boundary and the
back-diffusion due to concentration gradient (Figure 5C).
Concentration polarization leads to a higher IAP at the
membrane surface than in the bulk, which consequently affects
the SI at the membrane surface. Besides, temperature
polarization refers to a decrease of feedwater temperature
from the bulk to the membrane surface due to the limited heat
transfer rate across the boundary layer (Figure 5C).
114
Temperature polarization reduces the temperature at the
membrane surface as compared to that of the bulk and, also,
thereby alters SI via changing the temperature-dependent Ksp.
For example, the solubility of NaCl and silica is enhanced with
increased temperature,
115,116
whereas an inverse correlation
between temperature and solubility was reported for calcite
(CaCO3) and Na2SO4.
116−118
There are also minerals for
which the correlation between solubility and temperature is
not monotonic. For example, the solubility of gypsum (CaSO4·
2H2O) peaks at ∼40 °C.
119
The combination of the effects of
concentration and temperature polarization may result in an SI
at the membrane surface that differs substantially from that in
the bulk, which adds to the impact of heterogeneous
nucleation.
In practical MD membrane modules, the mass and heat
transfer across the membrane also causes spatiotemporal
variations in temperature and solute concentration along the
channels. As more water is recovered, the solute concentration
in the feed stream gradually increases. However, such an
increase in solute concentration occurs more temporally than
spatially, as the maximum water recovery per pass is limited to
a few percent.
10,11
Therefore, the scaling potential always
increases over time, as water recovery increases. In contrast,
the spatial distribution of temperature along the module is
substantial due to the large amount of latent heat transfer
associated with vapor transfer (Figure 5D). Consequently, the
dependence of scaling potential on the module position (at a
given moment) is strongly dependent on the type of
correlation between solubility and temperature.
Understanding Mineral Scaling Beyond CNT. Scaling in
MD is a rather complex process that involves multiple
fundamental phenomena in parallel and/or in sequence.
These phenomena include, but are not limited to, ion
transport, nucleation, precipitate growth, particle deposition,
and adhesion. While understanding CNT and concentration/
temperature polarization are important to understand scaling,
knowledge gaps still exist to precisely predict long-term
membrane performance in MD subject to mineral scaling.
Typically, membrane performance refers to vapor flux and salt
rejection. These two metrics can be easily measured and are
most relevant in practical operation. A widely investigated
parameter that is particularly relevant to understanding the
kinetics of mineral scaling is the induction time. The relevant
induction time is the period from process initiation to the
onset of irreversible flux decline due to mineral scaling.
Notably, the induction time of flux decline caused by mineral
scaling in membrane desalination is fundamentally distinct
from the induction time of scale formation, which refers to the
time between the generation of a supersaturated solution and
the formation of the first nuclei.
120
The induction of scale
formation, a physicochemical phenomenon, is believed to
occur much earlier than the onset of flux decline, an
operational phenomenon. The relationship between those
two “inductions”is still unknown and requires further
investigation.
The roles of heterogeneous and homogeneous nucleation in
regulating scaling behavior in MD have not been fully
understood. Although heterogeneous nucleation occurs more
favorably than homogeneous nucleation due to the reduction
of the energy barrier, both may take place in an MD process,
especially considering the high salinity of the feedwater. The
contributions of the heterogeneous and homogeneous
nucleations to mineral scaling are difficult to quantify
separately, but a better understanding of their respective
contributions is essential for predicting scaling kinetics and
developing scaling mitigation strategies. For example, homoge-
neous silica nucleation has been shown to play a dominant role
in initiating the decline of water vapor flux, while
heterogeneous silica nucleation facilitates the membrane pore
blockage.
121
In another study, filtering a supersaturated
calcium sulfate feed solution to remove homogeneously
precipitated crystals proved to limit the rate of flux decline
due to scaling.
122
If that is proven generally true for all
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common scaling species, the inhibition of precipitation in the
bulk solution and the abatement of precipitate deposition onto
the membrane surface will be more effective in mitigating
membrane scaling than hindering heterogeneous nucleation by
altering membrane surface properties.
In addition, to develop an improved general framework for
understanding mineral scaling in MD, we also need to acquire
a better understanding of the species-dependent behavior of
MD scaling. Different types of scaling could have distinct
mechanisms of precipitate formation at the molecular level and
different consequences in their macroscopic scaling behavior
(Figure 6).
35
Some scales, including gypsum, calcite, and NaCl,
are formed via a crystallization process (Figure 6A). In the case
of gypsum scaling, it has been shown that crystal growth causes
scaling intrusion (into the pores) and pore deformation
(Figure 6B). The pore deformation eventually leads to
membrane wetting, as LEP is reduced with larger deformed
pores.
35,123,124
In contrast, silica scaling results from the
polymerization of silicic acid followed by gelation (Figure
6C).
121
Consequently, it does not result in the same intrusion
and pore deformation phenomena observed in gypsum scaling
(Figure 6D). In consequence, MD with silica scaling is only
plagued with fluxdeclinebutnotmembranewetting.
Therefore, mineral scaling formed by crystallization should
receive special attention in MD desalination of wastewater with
mixed salts, due to its potential for causing membrane wetting.
It would be valuable to evaluate whether wetting-resistant
membranes, such as those described above, are also effective to
mitigate membrane wetting caused by inorganic scaling.
Scaling Mitigation Strategies. Pretreatments. The use
of antiscalants is the most straightforward approach of
controlling mineral scaling in membrane desalination including
MD.
125−134
For example, organic phosphonate derivatives
were found to mitigate the precipitation of calcium scales in
the MD treatment of seawater RO brine,
133
while the
application of a proprietary polymeric compound was reported
to retard calcite scaling in MD with coal seam gas RO brine as
the feedwater.
135
Additionally, carboxylic-based polymeric
molecules also inhibit the nucleation of calcite and gypsum
in MD.
128
Antiscalants typically serve as nucleation inhibitors
that hinder homogeneous nucleation in feed solutions. The
two major mechanisms that are likely responsible for scaling
inhibition are (1) the formation of soluble complexes in the
bulk solution, which decreases the SI by reducing availability of
the “free”ions for precipitation, and (2) the direct adsorption
onto the nuclei surface, which retards the mineral growth.
9,37
Antiscalants that are widely used in industry are weak acids
such as phosphonate derivatives and polymeric molecules
anchored with carboxylic groups. Under near-neutral con-
ditions of desalination, such antiscalants are partially or fully
deprotonated, exposing negative active sites that are able to
form complexes with multivalent cations in the solution.
136−139
For example, poly(acrylate acid) (PAA), a carboxylic derivative
polymer with pKaof 4.4,
140
is highly deprotonated at
approximately the pH of 7 and may strongly chelate with
Ca2+ to reduce its activity for precipitating with CO32−or
SO42−to form calcite or gypsum.
141,142
However, this complex
formation mechanism is challenged by the fact that the
antiscalants can be highly effective at a very low concen-
trationmuch lower than the concentration of Ca2+ stoichio-
metrically.
143,144
Another possible mechanism for the role of antiscalants in
scaling inhibition is the adsorption of antiscalant molecules
onto the surface of crystal nuclei via either electrostatic
interaction or ligand exchange.
142−147
Adsorption of anti-
scalants may contribute to scaling mitigation in two different
ways. First, the adsorption of antiscalants on the nuclei surface
increases the interfacial free energy at the liquid-nucleus
interface,
148
thereby increasing the energy barrier of nucleation
according to the CNT as discussed in the previous section.
This consequently reduces the nucleation rate and extends the
induction time of scaling.
128,145,148
Second, the attachment of
antiscalants reduces the active nuclei surface area for crystal
growth.
149,150
Figure 6. (A) Mechanism of gypsum scaling-induced wetting via pore deformation under crystallization pressure. (B) SEM-EDS images showing
gypsum intrusion into the membrane pores. (C) Silica scaling via gelation of silica nanoparticles. (D) SEM-EDS images showing the absence of
silica intrusion into membrane pores. Adapted with permission from ref 35.
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Besides the use of antiscalants, other strategies have been
applied for the mitigation of mineral scaling in MD.
Coagulation and softening are common approaches to remove
scalants from the feedwater prior to the MD process.
151
For
example, precipitative softening has been shown to reduce the
concentrations of several scale-forming species (e.g., Ca2+,Sr
2+,
and silica) in produced water from shale oil and gas
production, decreasing the corresponding SI of sparingly
soluble salts such as calcite, silica, and strontianite (SrCO3).
152
This resulted in the effective mitigation of mineral scaling and
improved water recovery in the subsequent MD process. In
addition to these chemical-based pretreatments, nanofiltration
has been explored for softening the feedwater before MD to
mitigate scaling.
33
Superhydrophobic Membranes. The use of a super-
hydrophobic membrane can complement the pretreatment
strategies and contribute to the overall reduction in membrane
scaling potential.
95,96,112,113,153,154
Superhydrophobic mem-
branes are characterized by their extreme nonwetting property.
In general, a surface is typically considered superhydrophobic if
(1) the static water contact angle is higher than 150°and (2)
the sliding angle is below 10°(alternatively, contact angle
hysteresis, which is the difference between the advancing and
receding contact angles, can be used instead of a sliding angle).
In the MD literature, superhydrophobic membranes are
sometimes also referred to as slippery membranes.
1,112,113
Membrane superhydrophobicity is achieved by minimizing the
liquid−solid contact area via introducing rough, and preferably
hierarchical, surface texture. If we denote ϕas the areal fraction
of a solid−liquid interface over the entire liquid interface (with
both solid and air), the apparent water contact angle θw,A (i.e.,
the water contact angle actually measured with the membrane)
relates to the intrinsic contact angle θw,0 (i.e., the water contact
angle measured with a molecularly smooth, nonporous surface
made of the same material as the membrane) via the following
equation.
155
cos (cos 1) 1
w,A w,0
θϕθ=+− (12)
The simplest and most common fabrication technique to
create a superhydrophobic membrane is to coat the membrane
surface with nanoparticles grafted with perfluorinated or other
ultralow surface energy,
95,96
although many other techniques
can also be used.
96,112
In all cases, the major requirement is
that the membrane surface has a high degree of roughness and
low surface energy.
Several studies have shown that scaling can be mitigated
with superhydrophobic membranes to different extents
depending on the scaling type and other factors.
95,96,112,113,156
With the use of superhydrophobic membranes, gypsum scaling
was substantially delayed, and NaCl scaling was not even
observed.
96,112,113
Also, the superhydrophobic membrane also
showed promising effectiveness in reducing mineral scaling in
MD treatment of industrial wastewater such as the blowdown
water from the cooling tower of a power plant.
95
Although the
exact mechanisms of scaling mitigation by superhydrophobic
membranes are unclear, there are several possible explanations.
First, the small liquid−solid contact area with a super-
hydrophobic membrane reduces the area available for crystal
deposition or nucleation on the membrane surface (Figure
7A). For example, consider a hydrophobic and super-
hydrophobic membrane made from the same material with
θw,0 = 105°. For the rough, superhydrophobic membrane with
θw,A = 165°, the areal fraction of liquid−solid contact ϕis
estimated to be only ∼5% based on eq 7. In comparison, for
the hydrophobic membrane with θw,A = 120°,ϕis estimated to
be ∼67%. The larger liquid−solid contact offers more area for
heterogeneous nucleation and crystal deposition and/or
adhesion (Figure 7B).
Figure 7. Possible mechanisms for the scaling mitigation with superhydrophobic membranes (top) relative to conventional hydrophobic
membranes (bottom). (A, B) Reducing solid/liquid contact area. (C, D) Reducing nucleation propensity by reducing surface energy. (E, F)
Enhancing boundary cross-flow and eliminating stagnant zones in a partially wetted pore. Adapted with permission from ref 96.
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The second possible explanation of the scaling mitigation by
a superhydrophobic membrane is the reduced propensity for
heterogeneous nucleation, which can be explained by the CNT
and eq 7 introduced in the last section. Equation 7 suggests
that the nucleation on an interface with higher θn/w is more
difficult, because f(θn/w) is a monotonically increasing function
for θn/w ∈[0,π]. As discussed earlier, θn/w positively correlates
with the intrinsic (in-air) water contact angle θw,A. Therefore,
we should expect that nucleation on a surface with lower
surface energy γsalso has a higher energy barrier than
nucleation on a surface with higher γs. Consequently,
nucleation propensity is substantially lower with a super-
hydrophobic membrane for two reasons. First and most
importantly, with a superhydrophobic membrane, there is a
larger fraction of the liquid−air interface (Figure 7C), which
has the lowest nucleation propensity (as γs≈0 for air).
Second, because the fabrication of a superhydrophobic
membrane typically uses materials (e.g., perfluorinated
compounds) with a lower γsthan that used for fabricating
hydrophobic membranes (e.g., poly(vinyl difluoride) or
polypropylene), the nucleation propensity on the liquid−
solid interface is also lower with a superhydrophobic
membrane (Figure 7C) than with a hydrophobic membrane
(Figure 7D).
The third explanation of scaling mitigation by a super-
hydrophobic membrane is the slip boundary condition for
liquid flow along the membrane surface (Figure 7E). The slip
boundary condition may have two major impacts on scale
formation.
113
The first impact is the reduced residence time for
crystal growth and interaction with a superhydrophobic
membrane surface as compared to that for a flow along a
hydrophobic membrane with a no-slip condition (Figure 7F).
The second impact is the higher flow velocity and turbulence
intensification near the surface of a superhydrophobic
membrane. This second effect may reduce the oversaturation
level near the membrane surface by promoting better mixing to
minimize the concentration polarization. It may also aid in
dislodging precipitated crystals or even preventing their
deposition.
117
The first and second explanations of scaling mitigation
concern the free energy of a system, which is essentially a
thermodynamic argument. In comparison, the third contribu-
tion is a hydrodynamic argument. While all these explanations
are mechanistically reasonable, to what extent each explanation
contributes to the overall effect of scaling mitigation by
superhydrophobic membrane is practically difficult to quantify.
Operation Strategies. Besides pretreatment and the use of
superhydrophobic membranes, various operation strategies can
also reduce mineral scaling in MD. These strategies include
water flushing,
133
microbubble aeration,
157
electrophoretic
mixing,
158
flow/temperature reversal,
159
and MD integrated
with a crystallizer,
116,160−167
which hinder scaling by either
removing scalants from the feedwater or disrupting the
nucleation process. For example, periodic flushing using
deionized water has been shown to maintain a stable water
vapor flux of MD in gypsum-saturated feed solutions, probably
due to the removal of gypsum nuclei from the membrane
surface.
133
However, the use of deionized water (product of
MD) for cleaning is not desirable, and the effectiveness of this
approach diminishes as more water is recovered. In another
study, flow and temperature reversal between feed and
distillate streams was proved effective in mitigating mineral
scaling when using MD to treat hypersaline water from the
Great Salt Lake.
159
Furthermore, microbubbles have been also
used to effectively reduce vapor flux decline caused by salt
precipitation. The authors proposed that the negatively
charged microbubbles not only reduced the effect of
concentration polarization but also attracted divalent ions
(e.g., Ca2+ and Mg2+) at the water−air interface to reduce their
availability in the bulk solution for scale formation.
157
Scaling
mitigation can also be achieved by applying an alternating
current to the surface of electrically conducting membranes
fabricated via depositing a network of carbon nanotubes on a
hydrophobic membrane.
158
Such a strategy has been shown to
be effective for mitigating both gypsum and silica scaling, likely
due to the electrophoretic mixing that disrupts the
concentration polarization layer. Also, the generation of an
electrical double layer creates a concentration imbalance
Figure 8. (A) Top-down SEM image and (B) cross-section elemental mapping based on SEM with EDS for a hydrophobic membrane scaled with
gypsum. (C) Top-down SEM image and (D) cross-section elemental mapping based on SEM-EDS for a superhydrophobic membrane. In both
cases, the membranes were subject to intermittent back-purging of the same frequency, duration, and intensity. (E, F) Illustration of how (E) partial
pore intrusion into hydrophobic membrane or (F) the lack of it with superhydrophobic membrane influences the effectiveness of scaling mitigation
by purging or pulse flow. Adapted with permission from ref 174.
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between anions and cations, which also contributes to slowing
the scale formation.
158
Another integrated process that has the benefit of pure water
recovery while simultaneously recovering valuable precipitates
is membrane distillation-crystallization (MDC). In MDC, MD
is used to concentrate the aqueous solution to the desired
supersaturation, while a crystallizer is used to precipitate and
remove mineral crystals from the solution. While the
integration of MD with a brine crystallizer may allow for an
enhanced water recovery ratio and delay scaling induction
time,
168,169
the MD membrane is still subjected to high
saturation solutions and, thus, eventually suffers from mineral
scaling.
170,171
Backwashing is a common strategy for membrane cleaning
in microfiltration.
172,173
Because an MD membrane needs to
be maintained nonwetted, back-purging (with gas) can be used
instead of backwashing (with water). It has been found that
intermittently back-purging a hydrophobic membrane cannot
mitigate gypsum scaling due to the robust adhesion of the scale
layer to the hydrophobic membrane surface (Figure 8A).
However, when intermittent back-purging is combined with
superhydrophobic membranes, gypsum scaling can be virtually
eliminated (Figure 8 B), even though the use of a
superhydrophobic membrane alone without back-purging can
only delay gypsum scaling.
174
Back-purging a hydrophobic
membrane was ineffective, because the partial intrusion of the
feedwater into the pores promotes very robust adhesion of the
formed crystal to the membrane (Figure 8C,E). In comparison,
the minimum solid−liquid contact area and lack of pore
intrusion in the case of a superhydrophobic membrane makes
back-purging effective (Figure 8D,F).
In a more recent study, a similar approach integrating
superhydrophobic membrane and pulse flow (i.e., flow with a
variable hydraulic pressure) was shown to be also effective in
eliminating gypsum scaling.
175
Again, neither pulse flow with
hydrophobic membrane nor superhydrophobic membrane
alone without pulse flow was effective in mitigating gypsum
scaling. While pulse flow is likely more practical than back-
purging in a real operation, both studies reveal that the same
importance of synergy between the material and operation
strategies in mitigating gypsum scaling. Regardless of the
specific operation approach, recharging the air layer on the
surface of a superhydrophobic membrane seems to be critical
to sustain long-term MD performance against gypsum scaling.
Other approaches toward this goal of air-layer recharging,
including the direct injection of nanobubbles or the in situ
generation of bubbles via electrolysis, may be explored. Finally,
it is not entirely clear why a superhydrophobic membrane
alone, without back-purging or pulse flow, can effectively
eliminate scaling by NaCl but not gypsum. We can only
postulate at this point that the crystal morphology plays an
important role. Elucidating the impact of scaling type on the
effectiveness of different scaling mitigation approaches is
important for developing strategies that are universally effective
for real feedwater with complex compositions.
■FOULING
Fouling is a common problem to all membrane processes and
is an umbrella term that can include organic fouling, inorganic
fouling, and biological fouling. In MD processes, mineral
scaling has been referred to as inorganic fouling in some cases.
However, inorganic fouling in other membrane processes often
involves only inorganic particles that originally exist in the
feedwater, excluding mineral scaling. This type of inorganic
fouling (sometimes referred to as colloidal fouling) is not a
major concern in MD or MC, as inorganic particles can be
readily removed using simple pretreatments. Therefore, we will
focus on organic and biological fouling in this discussion.
Generally speaking, fouling is a membrane failure common
in both MD and MD applications, where species in the feed
solution accumulate on the membrane surface and block its
pores, reducing the flux of water vapor (in MD) and volatile
component (in MC), and it may eventually lead to pore
wetting.
124,176
From an operational perspective, the low
operating pressure in MC and MD reduces the propensity to
form a dense, compact, irreversible foulant layer as compared
to pressure-driven membrane processes such as RO or
NF.
177,178
From a material perspective, however, MD and
MC membranes are inherently prone to organic fouling due to
the long-range hydrophobic interaction between the hydro-
phobic membrane and many common hydrophobic organic
foulants.
179,180
The most common method for detecting membrane fouling
is by simply monitoring the flux of water vapor (in MD) or
volatile species (in MC), which declines as the membrane
becomes fouled. Several studies have also used electro-
impedance spectroscopy to detect organic fouling in
situ.
181−184
In brief, the working mechanism is that the foulant
layer adds electrical resistance to the membrane surface. This
new detection technique allows for the elucidation of the
mechanisms and time scales of organic fouling, which in turn
enables early fouling detection, more efficient operation, and
more effective fouling mitigation in practice.
Organic foulants relevant to MD and MC include proteins,
humic acids, and, in some cases, emulsified oil droplets. Oil
fouling is relevant to MD, because MD has been actively
explored for treating oil- and gas-produced water.
4,73,152
Biological fouling (or just biofouling) is caused by the growth
of bacteria, fungi, and algae, especially by the formation of
biofilm. Technically, organic foulants are also found in
biofouling, as substances from microbes, including proteins
and extracellular polymeric substances, are organics. In this
section we will review the mechanisms in which these foulants
interact with the membrane surfaces, making the distinction
between the saline conditions typical to MD and lack thereof
in MC. In particular, we will discuss the effects of solution
composition on fouling mechanisms and how these mecha-
nisms apply to each of the individual foulant categories. Finally,
we will review the common, and most effective, mitigation
strategies that have shown promise for practical MC and MD
applications.
Mechanism of Organic Fouling. In general, organic
fouling is a phenomenon where organic colloids interact with a
membrane surface in an attractive manner and accumulate to
cover the membrane surface. Colloidal interaction is often
modeled by the Derjaguin−Landau−Verwey−Overbeek
(DLVO) theory, which considers the relative contributions
of the electric double layer (EDL) and van der Waals (vdW)
interactions. The EDL interaction, which is repulsive for two
similarly charged interacting surfaces, is strongly dependent on
the ionic strength. In the context of MD, which is most
promising for desalinating hypersaline brine, the ionic strength
is so high that no interaction energy barrier should exist,
because the vdW contribution always outcompetes the EDL
contribution. Therefore, it may be argued that the DLVO
theory is less relevant to MD due to the lack of interaction
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energy barrier in any practical condition. The DLVO theory
may be relevant to understanding fouling propensity in the
context of MC, where salinity (and thus ionic strength) can be
quite low. Even so, the DLVO theory should only be used to
provide qualitative reasoning for the observed phenomena but
should not be used as a predictive tool to quantify the fouling
propensity. Specifically, a microporous hydrophobic membrane
is different in many aspects from the idealized impermeable
surface in a particle deposition to which the DLVO theory has
been applied.
Another important interaction that is strongly relevant to the
organic fouling of hydrophobic membranes is the long-ranged,
hydrophobic interaction.
179,180,185
The hydrophobic interac-
tion is associated with the configurational rearrangement of
water molecules and tends to be an order of magnitude
stronger and longer-ranged than the vDW interactions,
decaying exponentially with distance.
179
The exact mechanism
of the widely observed hydrophobic interaction has long been
debated. One probable explanation is the capillary bridging
between two surfaces by an air bubble that tend to develop on
a hydrophobic surface.
186,187
With this mechanism, a hydro-
phobic interaction is all the more relevant to MD and MC, not
only because hydrophobic membranes are used but also
because an air layer is intrinsically present in a nonwetted
membrane in a Cassie−Baxter state. Considering the
substantially high strength of the hydrophobic interaction
and the heavily shielded EDL interaction in high ionic
strength, we argue that the long-ranged hydrophobic
interaction dominates the membrane fouling process, making
hydrophobic organic foulants very troublesome for sustaining
long-term MD or MC performance.
188,189
This is particularly
true for oil foulants that are not heavily emulsified by excessive
surfactants.
Mechanism of Biofouling. Biofouling pertains to both
MC and MD when feedwater containing a high content of
biological microorganisms, such as those in the wastewaters of
the pharmaceutical, food and beverage industries, and
municipal wastewater treatment facilities, is in direct contact
with the membrane surface.
27,190
The high-salinity and high-
temperature conditions of the feed solution in MD may lead
one to expect a low potential for biofouling.
191
However, there
are bacteria and other microorganisms that are resistant to high
salinity and temperature,
192
and moderate temperature can
even promote the growth of thermophilic bacteria.
176,193,194
While the mechanism of biofouling is complicated due to the
presence of multiple foulant species with different physico-
chemical properties, it is important to discuss the general
process and time scales of biofouling.
Biofouling is a particularly unwelcome fouling phenomenon,
because it would lead to the formation of a large gel-like
structure on the membrane surface. Such a structure, namely
“biofilm,”is difficult to remove physically or via chemical
cleaning.
195
The growth of biofilm is typically characterized by
several unique stages that develop consecutively
195
(Figure 9).
In the early stages of biofouling, a conditioning film develops,
where organic foulants such as proteins, humic substances, and
other organic matter deposit on the membrane surface in a film
with thickness on the order of nanometers.
195,196
The
conditioning film from organic fouling serves as a precursor
for further foulants attachment, as it alters the surface
properties exposed to the feed solution.
197−200
Next, several
attractive interactions, including vdW, hydrophobic, and
hydrogen-bonding interactions, result in reversible bacteria
cell attachment, while larger aggregate particles and protein
aggregates (namely, “protobiofilms”) irreversibly attach to the
conditioning film.
197
These two processes occur on the time
scale of seconds to minutes. After a few hours, the bacteria cells
bind irreversibly to the surface. This irreversible attachment is
due to the reinforcement of the extracellular polymeric
substances (EPS), which are produced within the microbe
colonies of the biofilm and mostly consist of protein,
polysaccharides, some humic substances, and
DNA.
176,193,195,201
The kinetic growth and mechanical properties of the EPS are
heavily influenced by the salinity and specific ionic species.
202
For example, the presence of divalent cations such as calcium
and magnesium is known to reinforce the biofilm by forming
complexes with the polysaccharides in the EPS matrix, forming
a more dense, interconnected network.
195,201,203
Furthermore,
divalent cations in the feedwater aid the initial adhesion of
organic foulants, such as humic substances, by forming
complexes in the bulk that settle more easily and deposit on
the membrane surface.
134,204−206
After several hours up to
days, the biofilm is fully reinforced by the constant secretion of
Figure 9. Different stages of biofilm formation and the respective time scales. Dissolved and colloidal organics deposit onto the surface and form a
conditioning film to which bacteria adhere. Protobiofilm and bacteria then directly deposit onto the conditioning film. These two processes occur
on the time scale of minutes to seconds, but eventually the EPS excreted from the bacteria enhances the cell attachment and facilitates the
formation of a robust and continuous biofilm. The biofilm eventually becomes a source from which bacteria and bacterial clusters are dispersed.
Adapted with permission from ref 196.
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EPS and can even begin to spread and grow with the
colonization of new areas on the membrane by living bacteria
from within the biofilm.
195
Fouling Mitigation Strategies. Pretreatments and
Membrane Cleaning. Well-established physical and chemical
pretreatments can be used to reduce the concentration of
foulants in the feed solution. Physical pretreatments are often
very effective in mitigating membrane fouling. For example,
pretreatment with nanofiltration or ultrafiltration very
effectively limits flux decline due to fouling.
33,36,42
Coagu-
lation/flocculation is a highly cost-effective approach to
remove foulants from the feed solution by forming flocs that
enmesh the foulants. These flocs can be removed by
sedimentation, filtration through porous media, or micro-
filtration.
5,151,207
For example, coagulation pretreatment of
recirculating cooling water with poly(aluminum chloride) has
shown to improve elimination of total organic carbon and
reduce the water vapor flux decline due to organic fouling in
MD.
151
Other methods that may also be effective include
flotation (especially for oil foulants) and activated carbon
adsorption. A recent study using MD to treat real produced
water from the shale oil and gas industry showed that
precipitative softening/coagulation using aluminum sulfate
followed by walnut shell filtration can very effectively reduce
fouling in MD.
152
We expect that pretreatment methods or
treatment trains that have been proven effective for fouling
mitigation in other membrane processes are also effective for
MD and that the choice of pretreatments for fouling mitigation
depends largely on the feedwater characteristics rather than the
membrane processes.
In addition to pretreatment, there are additional chemical
and physical strategies that are effective in removing foulants
that have already attached to membrane surfaces. For example,
the use of micro/nanobubbles via direct injection into the feed
Figure 10. Comparison between in-air (top) and underwater (bottom) wetting properties for (A) hydrophobic membrane, (B) omniphobic
membrane, and (C) composite membrane with a hydrophobic substrate and a hydrophilic coating.
Figure 11. Illustration of the effectiveness of different types of membranes with special wetting properties in mitigating wetting, fouling, and scaling.
A Janus (h) membrane is a membrane with a hydrophobic (thus “h”) substrate and an in-air hydrophilic and underwater oleophobic coating
(denoted by the red line in the figure). A Janus (o) membrane is a membrane with an omniphobic (thus “o”) substrate and an in-air hydrophilic
and underwater oleophobic coating. An omniphobic-slippery membrane is both omniphobic and superhydrophobic (i.e., slippery). The superscript
“*”for “F”(representing fouling) indicates the uncertainty or limited information for the effectiveness evaluation, primarily because of the multiple
types of fouling that have not been comprehensively assessed for the respective membranes. These effectiveness ratings are based on the following
studies: superhydrophobic membranes,
95,96,112,113,153,154,174
omniphobic membranes,
28,73−82
Janus (h) membranes,
38,188,189,219,220,235−237
Janus (o)
membranes,
219,238
and omniphobic-slippery membranes.
239
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solution
157,208−211
or using ultrasound or electrolysis to induce
bubble nucleation near the membrane surface
212−215
has been
shown to effectively limit and remove organic foulants.
Generally speaking, the bubbles both physically dislodge
foulants from the membrane surface and remove foulants
from the feedwater by capturing them at the air−liquid
interface of the bubbles (similar to flotation).
215−217
In a study
where membrane biofouling occurred in the MD desalination
of seawater from the South China Sea, ultrasonic cleaning
effectively removed the organic foulants, and transmembrane
water vapor flux was restored to its initial rate.
213
Other
common membrane cleaning methods, including those using
cleaning agents based on alkaline, acidic, surfactant, and
chelating agents, have also been shown to be effective to
different extents for removing the foulant layer.
23,218
While pretreatment methods that are effective for organic
fouling mitigation can also reduce biofouling to a certain
degree via reducing the microbe concentration of the feed
solution, methods for cleaning organic fouling may not be
effective for alleviating biofouling, especially after biofilms have
formed on the membrane surface. The most effective approach
for preventing biofouling or cleaning a biofilm once it forms is
to apply disinfectants (e.g., chlorination).
178
Because most
disinfectants are also strong oxidants, disinfection is not
applicable for RO or NF membranes that are primarily based
on a polyamide that would suffer a performance loss after
prolonged exposure to oxidants.
178
For MD, however, applying
disinfectants can be a viable approach, because hydrophobic
membranes, especially those made from fluoropolymers, are
typically chemically stable.
Fouling-Resistant Membranes. Many recent studies on
MD fouling are focused on oil fouling, because it presents a
critical challenge when MD is used to desalinate oil- and gas-
produced water. Hydrophobic MD membranes are inherently
prone to oil fouling due to the strong hydrophobic interaction.
Oil fouling is fatal to a hydrophobic membrane, because it is
difficult to be reversed due to pore wicking by oil (Figure
10A). It may be expected that an omniphobic membrane is
resistant to oil fouling, because an omniphobic surface is, by
definition, both hydrophobic and oleophobic. However, it is
important to emphasize that all wetting properties, without a
prefix descriptor, are defined in air. An omniphobic surface
(and membrane) is in-air oleophobic but not underwater
oleophobic. In fact, an omniphobic membrane is underwater
oleophilic,
38
because the low surface energy of the material
required for fabricating an omniphobic membrane leads to a
strong hydrophobic interaction just as that between oil
droplets and a hydrophobic membrane. However, an
omniphobic membrane differs from a hydrophobic membrane
by having a reentrant structure that prevents the oil droplets
from wicking into the pores (Figure 11B). In other words,
while oil droplets can still foul an omniphobic membrane by
covering the membrane surface and blocking pores for vapor
transfer, the fouling can be reversed by cleaning due to the
absence of pore wicking. Interestingly, if the oil-in-water
emulsion is stabilized by a high concentration of surfactants, an
omniphobic membrane would not even be fouled, because
now both the oil droplets and the membrane surface are
rendered hydrophilic due to the adsorbed surfactants, thereby
eliminating the attractive hydrophobic interaction.
38
While the
same principle should apply to hydrophobic membranes,
hydrophobic membranes are susceptible to wetting by
surfactants.
On the basis of underwater wettability, a robust material
strategy to develop an MD or MC membrane resistant to oil
fouling is to apply a (in-air) hydrophilic coating layer on a
hydrophobic membrane surface
38,188,189,219,220
(Figure 10C).
An in-air hydrophilic surface is typically underwater
oleophobic due to the hydration layer that results from the
strong attractive interaction between water and the high
surface energy moieties of the hydrophilic coating. This
hydration layer prevents oil fouling, as the spreading of oil on
the membrane surface requires dehydration of the coating
layer, which is energetically unfavorable.
189
The mitigation of
fouling using such a hydration layer is more robust, as the
“nonfouling”condition is a thermodynamically stable state.
Finally, we note that in-air hydrophilicity is only a necessary,
but not a sufficient, condition for underwater oleophobicity.
To develop a robust underwater oleophobic surface, the effects
of electrostatic interaction between oil and surface must also be
considered. For example, while a negatively charged (in-air)
hydrophilic surface is robustly resistant to fouling by negatively
charged oil droplets,
221,222
a positively charged (in-air)
hydrophilic surface has shown to be underwater oleophobic
only upon its initial contact with a solution containing
negatively charged oil droplets. Over the time scale of tens
of minutes, the positively charged (in-air) hydrophilic becomes
increasingly oleophilic and wetted by the oil droplets of
opposite charge.
221
Expectedly, the MD membrane with a
negatively charged hydrophilic coating is substantially more
robust in sustaining a stable MD performance than that with a
positively charged hydrophilic coating, because the former is
robustly underwater oleophobic, while the latter is not.
Ultimately, it is the underwater wetting property that matters
for oil-fouling resistance.
In general, this rationale also applies to the mitigation of
organic foulants other than oil droplets, such as NOM and
proteins. Hydrophilic surfaces with identical charge to the
organic foulants in question are more robust in mitigating
fouling due to the presence of a hydration layer and
electrostatic repulsion.
223−231
This is especially true for organic
foulants with hydrophobic moieties such as proteins. Some-
what paradoxical to the observation that an in-air hydrophilic,
underwater oleophobic surface imparts robust fouling resist-
ance, some studies have also reported that superhydrophobic
membranes are also effective for mitigating fouling by organics
such as humic substances.
232−234
Similar to the mechanisms
for scaling mitigation, mitigating organic fouling with a
superhydrophobic membrane is more attributable to decreased
liquid−solid contact between the feed solution and membrane
surface. Because there is still some small fraction of liquid−
solid contact, long-term biofouling experiments show a small
flux decline, as the humic substances eventually attach to the
surface where there is that liquid−solid contact.
232,233
■OUTLOOK
Despite the several unique advantages of MD in desalinating
hypersaline brine, MD also faces multiple unique challenges as
compared to pressure-driven membrane processes that have
been scaled up for practical applications. For example, virtually
all “sweet spot”applications for MD involve concentrating
high-salinity streams, which inherently has a higher propensity
for mineral scaling than other membrane processes treating
feedwater with low to moderate salinity.
1,6
Furthermore, pore
wetting is a technical challenge that is relevant to MD (or MC)
but not to any other membrane processes. Lastly, while fouling
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is common in all membrane processes, the use of a
hydrophobic membrane and the presence of an air layer in
MD processes enhances the hydrophobic interaction and,
thereby, increases the propensity of fouling by hydrophobic
organics.
189
Nonetheless, the absence of an applied hydraulic
pressure in MD prevents the formation of a very dense foulant
layer, which may alleviate irreversible fouling.
177,178
Any of
these three problems, scaling, wetting, and fouling, if not
properly addressed, can substantially compromise the perform-
ance or even lead to a complete process failure of MD (or
MC).
Fortunately, the rich collective experience in other
membrane processes can help us address the problems of
fouling and scaling using the proper measures of pretreatment,
feedwater conditioning, (e.g., dosing antiscalants or disinfec-
tants), and membrane cleaning. These approaches, though not
scientifically novel, are often economical and effective over a
broad range of foulants or scalants. Other process innovations,
such as the use of micro/nanobubbles, intermittent back-
purging, and pulse flow, have also been proven effective in
enhancing the robustness of the MD performance.
157,174,175
However, these approaches have their limitations. For example,
the challenge of pore wetting is difficult to overcome by
removing wetting agents from the feedwater due to the very
limited capability of achieving such a separation cost-
effectively. In this case, developing novel MD membranes
with special wettability provides an alternative path for
addressing the challenge of pore wetting. For fouling and
scaling, developing better membranes is also an important
supplement to, or even a substitute of, pretreatments and
operational measures. In certain cases, the material and
operational approaches are synergistic and do not function
effectively without each other.
Thanks to the recent development of novel membranes with
special wetting properties, we now have effective solutions to
each of the three major challenges in MD: wetting, scaling, and
fouling. The general rules of thumb are that (1) omniphobic
membranes can prevent wetting, (2) superhydrophobic
membranes are effective in mitigating mineral scaling, and
(3) Janus membranes (i.e., composite membranes) with an in-
air hydrophilic and underwater oleophobic coating are capable
of reducing fouling, in particular, oil fouling. However, none of
these membranes have been proven to be a robust and
universal solution to all three challenges. To engineer
resistance against multiple failure mechanisms, it is possible
to integrate two types of wetting properties into one composite
membrane. For example, a Janus (o) membrane, which
comprises an omniphobic substrate and a fouling-resistant,
hydrophilic surface coating, has been shown to be effective in
mitigating both wetting and fouling.
219
In another example, a
membrane that is both omniphobic and superhydrophobic
(also known as slippery) has been shown to be simultaneously
wetting and scaling resistant.
239
These general principles are
summarized in Figure 11.
The effectiveness of different membranes in mitigating
organic and biological fouling is less clear-cut, as different types
of foulants behave very differently, and not all types of fouling
(e.g., oil, natural organic matter, and biological foulants) have
been tested with membranes with different types of wetting
properties. Membranes that are resistant to fouling by natural
organic matter may not be resistant to oil fouling, and vice
versa. Moreover, even the oil-fouling resistance of the same
membrane also depends on whether and to what extent the oil
droplets are stabilized by surfactants. When oil droplets are
stabilized by excessive surfactants, the membrane may fail via
the mechanism of wetting instead of fouling.
38
In some cases, all types of failure mechanisms in MD have
been referred to as “fouling”. For example, a membrane might
be referred to as “anti-fouling”, while it was actually tested
against a variety of agents that would induce wetting, scaling,
and fouling (of the narrower definition).
9,36,240
While putting
all these mechanisms under the umbrella of fouling may sound
pragmatically convenient from an operation point of view (it
essentially becomes a proxy term for “membrane failure”),
doing so obscures the fundamental difference behind these
mechanisms and is unconstructive for the systematic under-
standing of membrane failure and the development of
mitigation strategies. In other cases, however, the membrane
fails via multiple mechanisms concurrently, and distinction
between them is inherently difficult, because one type of failure
can induce the other. For example, it has been reported that
both organic fouling and mineral scaling can result in pore
wetting.
37
In the case of gypsum scaling, wetting is caused by
pore deformation instead of the reduced surface tension of the
feed solution due to the presence of surfactants or LST
liquids.
35
In addition to more clearly distinguishing between different
failure mechanisms, we also need to be more mindful and
precise in categorizing a membrane with special wettability.
For instance, a superhydrophobic membrane can be both
oleophobic and oleophilic. When a study shows that a
superhydrophobic membrane is wetting resistant without
actually measuring the oil-wetting property of the membrane,
the conclusion can be confusingthe wetting resistance may
likely be attributable to the oleophobicity imparted by the re-
entrant structure instead of the superhydrophobicity imparted
by the high degree of roughness.
Finally, although a limited number of MD studies was
performed using real feedwater such as RO brines or industrial
wastewater, a majority of reported studies on addressing the
challenges of wetting, scaling, and fouling in MD, especially
those related to novel membrane development, used simple
feed solutions. Most studies either focused on a single type of
membrane failure or investigated multiple mechanisms
separately. While these studies are important to enhancing
our fundamental understanding, practical MD processes often
involve more complex feed solutions and the synergy of
different failure mechanisms. Examples include, but are not
limited to, scaling by different types of minerals, simultaneous
mineral scaling and organic fouling, and simultaneous fouling
and wetting. Therefore, future research efforts on pretreatment,
operation, or membrane development should be more directed
toward understanding the combined effects of multiple failure
mechanisms using feed solutions with more complex
compositions. Such efforts will potentially bridge the gap
between fundamental understanding and engineering practice,
thereby enabling MD to become a reliable process for treating
various types of hypersaline brine.
■AUTHOR INFORMATION
Corresponding Authors
Tiezheng Tong −Department of Civil and Environmental
Engineering, Colorado State University, Fort Collins, Colorado
80523, United States; orcid.org/0000-0002-9289-3330;
Email: tiezheng.tong@colostate.edu
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Shihong Lin −Department of Chemical and Biomolecular
Engineering and Department of Civil and Environmental
Engineering, Vanderbilt University, Nashville, Tennessee 37235-
1831, United States; orcid.org/0000-0001-9832-9127;
Email: shihong.lin@vanderbilt.edu
Authors
Thomas Horseman −Department of Chemical and
Biomolecular Engineering, Vanderbilt University, Nashville,
Tennessee 37235-1831, United States; orcid.org/0000-
0002-4660-1448
Yiming Yin −Department of Civil and Environmental
Engineering, Colorado State University, Fort Collins, Colorado
80523, United States
KofiSS Christie −Department of Civil and Environmental
Engineering, Vanderbilt University, Nashville, Tennessee 37235-
1831, United States; orcid.org/0000-0002-7039-7889
Zhangxin Wang −Department of Chemical and Environmental
Engineering, Yale University, New Haven, Connecticut 06511,
United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acsestengg.0c00025
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
The authors are grateful for the generous support by the
National Science Foundation via Grant Nos. 1739884 (S.L.),
1705048 (T.H. and Z.W.), and DGE-1145194 (K.S.S.C.) and
by the Bureau of Reclamation (USBR) under the Department
of Interior via DWPR Agreement R18AC00108 (T.T.).
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