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Journal of Membrane Science 320 (2008) 292302
Contents lists available at ScienceDirect
Journal of Membrane Science
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m e m s c i
Chemical and physical aspects of organic fouling of forward osmosis membranes
Baoxia Mi , Menachem Elimelech
Department of Chemical Engineering, Environmental Engineering Program, P.O. Box 208286, Yale University, New Haven, CT 06520-8286, USA
a r t i c l e i n f o
Article history:
Received 16 December 2007
Received in revised form 21 March 2008
Accepted 8 April 2008
Available online 22 April 2008
Keywords:
Forward osmosis
Osmosis
Organic fouling
Alginate
Humic acid
Bovine serum albumin
Intermolecular adhesion force
Foulantfoulant interaction
Pressure retarded osmosis
a b s t r a c t
The growing attention to forward osmosis (FO) membrane processes from various disciplines raises the
demand for systematic research on FO membrane fouling. This study investigates the role of various
physical and chemical interactions, such as intermolecular adhesion forces, calcium binding, initial per-meate flux,and membraneorientation,in organic fouling of forward osmosis membranes. Alginate, bovine
serum albumin (BSA), and Aldrich humic acid (AHA) were chosenas model organic foulants.Atomic force
microscopy (AFM) was used to quantify the intermolecular adhesion forces between the foulant and
the clean or fouled membrane in order to better understand the fouling mechanisms. A strong correla-
tion between organic fouling and intermolecular adhesion was observed, indicating that foulantfoulant
interaction plays an importantrole in determining therate and extent of organic fouling. Thefouling data
showedthat FO fouling is governed by thecoupled influence of chemical andhydrodynamic interactions.
Calcium binding, permeation drag, and hydrodynamic shear force are the major factors governing the
development of a fouling layer on the membrane surface. However, the dominating factors controlling
membrane fouling vary from foulant to foulant. With stronger intermolecular adhesion forces, hydrody-
namic conditions for favorable foulant deposition leading to cake formation are more readily attained.
Before a compact cake layer is formed, the fouling rate is affected by both the intermolecular adhe-
sion forces and hydrodynamic conditions. However, once the cake layer forms, all three foulants have
very similar flux decline rates, and further changes in hydrodynamic conditions do not influence fouling
behavior.
2008 Elsevier B.V. All rights reserved.
1. Introduction
Pressure-driven membrane processes microfiltration (MF),
ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO)
have been employed heavily in the field of water purifica-
tion, wastewater reclamation, and desalination. These membrane
processes use hydraulic pressure as the driving force for water
transport through the membrane. Significant hydraulic pressure
is often required for the operation of such processes, especially
in RO desalination, which results in extensive use of prime (elec-
tric) energy. This drawback, coupled with limitations on feed water
recovery, has led to the investigation of alternative approaches towater desalination.
Forward osmosis (FO), a potential alternative to pressure-driven
membrane processes such as RO in certain applications, has been
gaining popularity in recent years. FO uses a concentrated draw
solution to generate high osmotic pressure, which pulls water
across a semi-permeable membrane from the feed solution. The
draw solute is then separated from the diluted draw solution to
Corresponding author. Tel.: +1 203 432 4333; fax: +1 203 432 2881.
E-mail address: [email protected] (B. Mi).
recycle thesolute, as well as to produce clean product water.FO has
been explored for use in seawater desalination [1,2], wastewater
reclamation [35], industrial wastewater treatment [6], and liq-
uid food processing [7]. Pressure-retarded osmosis (PRO), a closely
related process, also utilizes osmotic pressure as the driving force
for water permeation through a semi-permeable membrane. PRO
has been evaluated as a potential process for generating electric-
ity by utilizing the osmotic pressure difference between saline and
fresh waters [8,9] or between a working fluid and a draw solution
in a closed loop [7,10].
The growing interest in FO from various disciplines calls for
more fundamental research that can lead to a better understand-ing of the FO process and further advances in the technology.
Efforts have been made to develop a draw solution that can induce
high osmotic pressure, be easily separated from the clean product
water, and require low energy for regeneration [2,11]. Other stud-
ies have aimed at developing an understanding of water transport
phenomena through FO membranesfor instance, the influence of
internal concentration polarization, membrane structure/material,
and membrane orientation on membrane flux [1215]. These stud-
ies provide importantinformationthat can leadto the development
of new FO membranes with reduced internal concentration polar-
ization and high water permeability.
0376-7388/$ see front matter 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.memsci.2008.04.036
http://www.sciencedirect.com/science/journal/03767388mailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_14/dx.doi.org/10.1016/j.memsci.2008.04.036http://localhost/var/www/apps/conversion/tmp/scratch_14/dx.doi.org/10.1016/j.memsci.2008.04.036mailto:[email protected]://www.sciencedirect.com/science/journal/03767388 -
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One notable research area that hasbeen overlooked so farin this
emerging technology is membrane fouling. It is well known that
membrane fouling is a major obstacle to the efficient application
of membrane technologyin applications involving seawater desali-
nation, wastewater reuse, and water treatment. Numerous studies
have been conducted with pressure-driven membrane processes
to understand the causes of membrane fouling and to develop
strategies for fouling control. However, very few publications have
addressed the problem of FO membrane fouling [6,16].
One potential advantage of using FO is that it operates with
no hydraulic pressure, which may result in lower membrane foul-
ing propensity than pressure-driven membrane processes due to
lessercake layer compaction. Hollowayet al. [6] compared the foul-
ing behavior of FO and RO for wastewater centrate treatment, and
demonstrated a slower flux decline rate in FO than in RO. However,
the lack of systematic, controlled studies on FO membrane fouling
makes it impossible to fully explain the different fouling behaviors
of the two processes. Membrane fouling in RO has been well stud-
ied and the physicochemical mechanisms involved are relatively
understood. For instance, the effects of intermolecular forces, diva-
lent cations, and hydrodynamic conditions (initial flux, cross-flow
velocity) on RO membrane fouling have been studied extensively
[1721]. To date, however, it is not clear how these physical and
chemical interactions impact FO membrane fouling.
The objective of this study is to understand the role of sev-
eral key physical and chemical interactions in organic fouling of
forwardosmosismembranes. Threeorganicmacromolecules algi-
nate, bovine serum albumin (BSA), and Aldrich humic acid (AHA)
were chosen as model foulants. Atomic force microscopy (AFM)
was used to quantify the intermolecular adhesion forces between
the foulant and the clean or fouled membrane. The adhesion forces
were correlated with the membrane fouling behavior to elucidate
thefouling mechanisms of theFO membraneat themolecular level.
The influence of other physical and chemical factors, for instance,
calcium binding, initial permeate flux, and membrane orientation,
was also investigated.
2. Materials and methods
2.1. Forward osmosis membrane
Theforwardosmosis membraneused in this studywas provided
by Hydration Technologies, Inc. (Albany, OR). It has an asym-
metric structure and is made of cellulose acetate supported by
embedded polyester mesh. The total thickness of the membrane
is approximately 50m based on examination of the membrane
cross-section by scanning electron microscopy. Other characteris-
tics of the membrane are given in McCutcheon et al. [2].
2.2. Organic foulants
We used bovine serum albumin (BSA), sodium alginate, and
Aldrich humic acid (AHA) (SigmaAldrich, St. Louis, MO) as model
organic foulants. According to the manufacturer, the molecular
weightof theBSA isapproximately66 kDa. Thesodium alginatewas
extracted from brown seaweed, and its molecular weight ranges
from 12 to 80 kDa. The organic foulants were received in powder
form. Stocksolutions forBSA andalginate(10g/L)were preparedby
dissolving the foulant in deionized (DI) water. Mixing of the stock
solution was performed for over 24h to ensure complete dissolu-
tion of the foulant. The stock solution was stored in sterilized glass
bottles at 4 C without further purification.
AHA stock solution was purified to decrease ash content and
remove bound iron following the procedure described by Hong
and Elimelech [17]. Briefly, AHAsolution (10 g/L) was first prepared
by dissolving AHA powder in DI water. The pH of the AHA solu-
tion was adjusted to approximately 1 by addition of 1 M HCl. Then,
precipitation of AHA took place for 10min. The AHA solution was
then centrifuged at 628.34 rad/s (6000 rpm) for 10min. After cen-
trifugation, the supernatant was discarded and the precipitate was
resuspended in 1 M HClsolution. Theabove cleaning procedurewas
repeated five times. After acid precipitation, the AHA was further
purified by dialysis against DI water. Finally, the solid content of
purified AHA suspension was determined by weighing the mass
after freeze-drying the sample. The concentration of the AHA solu-
tion was adjusted to 10g/L, and the stock was stored in a sterilized
bottle at 4 C.
2.3. Test solutions
The feedsolutionfor fouling experiments contained50 mM NaCl
and200 mg/L foulant; some of the solutions also contained 0.5mM
CaCl2. The ambientpH of the BSA, alginate, and AHA feed solutions
was 6.3, 5.8, and 6.2, respectively. The test solution for AFM force
measurement contained 50 mM NaCl and 20 mg/L foulant, with or
without 0.5 mM CaCl2. However, the AFM tests used lower foulant
concentration than fouling experiments, because a high concen-
tration of foulant interferes with the force measurement. The draw
solution for the fouling experiments was composed of 1.5 or 4 M
NaCl. A commercial software from OLI System Inc. (Morris Plains,
NJ) was used to calculate the osmotic pressures of draw and feed
solutions.
2.4. Bench-scale forward osmosis fouling experiments
The FO fouling experiments were performed with a bench-scale
membrane system as depicted in Fig. 1. The cross-flow membrane
cell was custom built with equally structured channels on both
sides of the membrane. The dimensions of the channels are 77mm
long by 26 mm wide by 3 mm deep. No spacer was used in the
channel to accelerate membranefouling.Co-current cross-flowwasused to minimize strain on the suspended membrane. Variable
speed gear pumps (Micropump, Vancouver, WA) were usedto gen-
erate cross-flows, forming separate closed loops for the feed and
draw solutions. The draw solution tank was placed on a digital
scale (Denver Instruments, Denver, CO) and weight changes were
monitored by a computer to record the permeate flux. A constant
Fig.1. A schematic diagramof thelaboratory-scale forwardosmosis(FO) systemfor
fouling experiments.
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feed and draw solution temperature of 201 C was maintained
by a water bath (Neslab, Newington, NH). Heat transfer took place
through submerged stainless steel heat exchanger coils within the
water bath.
The protocol for all fouling experiments comprised the follow-
ing steps. First, a new membrane coupon was placed in the unit
before each experiment. Since the membrane has an asymmet-
ric structure, the fouling behaviors at both membrane orientations
were tested. When the membrane active layer is placed against
the feed solution, we refer to this orientation as forward osmosis
(FO) mode, whereas when the membrane active layer is against
the draw solution, we refer to this orientation as pressure-retarded
osmosis (PRO) mode. Next, 2 L feed solution without foulant and
2 L draw solution were added to the feed and draw solution tanks,
respectively. Cross-flows of feed and draw solutions were run for
1 h in their respective closed loops, without passing flow through
the membrane cell, to stabilize the temperature of the system. The
temperature was maintained at 201 C for all experiments. After
reaching the desired temperature, the bypass valves of both cross-
flows were closed to allow flow of feed and draw solution through
both sides of the membrane. The cross-flow velocity for both the
feed and draw solution sides was fixed at 8.5 cm/s. After the initial
fluxstabilized, which took about 1 h, 200mg/L of foulant was added
to the feed solution and the fouling experiment was continued for
2024 h. A computer was used to continuously monitor water flux
throughout the fouling experiment.
Baseline experiments were conducted to quantify the flux
decline due to the decrease in the osmotic driving force during the
fouling experiments as the draw solution is continuously diluted by
the permeate water. The baseline experiments followed the same
protocol as that for the fouling experiments except that no foulant
was added to the feed solution.
2.5. Interfacial force measurement
Atomic force microscopy (AFM) was used to measure thefoulantfoulant and foulantmembrane interfacial forces, follow-
ing the procedures described by Li and Elimelech [19]. The force
measurements were performed with a colloid probe on a Multi-
mode AFM(VeecoMetrology Group, Santa Barbara, CA). The colloid
probe was made by attaching a carboxylate modified latex (CML)
particle (Interfacial Dynamics Corp., Portland, OR) to a commercial
SiN AFM cantilever (Veeco Metrology Group, Santa Barbara, CA).
The CML particle (4.0m in diameter) was attached by Norland
Optical adhesive (NorlandProducts, Inc.,Cranbury, NJ) to the tipless
SiN cantilever and cured under UV light for 20min.
The AFM adhesion force measurements were performed in a
fluid cell filled with the test solution. For each force measurement
experiment, a piece of fresh membrane was set up in the fluid
cell and rinsed with DI water. Then the test solution, containing
50 mM NaCl and 20mg/L foulant, with or without 0.5mM CaCl2,
was injected to fully displace the DI water in the fluid cell. The
test solution was left to equilibrate with the FO membrane for
3045 min to allow foulants to adsorb on the membrane surface
as well as on the colloidal probe. The force measurements were
conducted at five different locations on the membrane. 30 force
measurements were taken at each location to minimize inherent
variability in the force data, which is mainly attributed to the het-
erogeneity of the membrane surface. Only the retracting (pull-off)
force curves were processed and converted. For each solution con-
dition, both theaverages of alladhesionforces at differentlocations
as well as the force distributions are presented.
3. Results and discussion
3.1. Forward osmosis membrane characteristics
Membrane surface morphology is known to play a role in mem-
brane fouling. Therefore, we used AFM to characterize the surface
properties of the FO membrane. As shown in Fig. 2a, the mem-brane has some bumpy areas on the surface, which is primarily
caused by the embedded polyester mesh. The more localized sur-
face morphology,shown in Fig.2b, depicts a roughnesson theorder
of several tens of nanometers. The surface roughness of the FO
membrane does not differ much from a typical RO/NF membrane
[22].
Fig. 3 shows the frequency distribution of the adhesion forces
between theAFM particle (CML)probeand thecleanFO membrane.
The CML particle contains a highly charged layer of carboxylate
functional groups, thereby serving as a surrogate for carboxy-
late rich foulants, such as humic acid and alginate [19]. Since the
membrane surface is clean during the initial stage of fouling, the
adhesion forces between the foulant and the clean membrane
surface determine the initial fouling rate. The adhesion force, F,normalized by the radius of the particle, R, is proportional to the
energy per unit area required to separate the particle and the flat
surface by an infinite distance [19]. Therefore, F/R can be viewed as
a measure of theenergy required topreventa foulant from accumu-
lating on the membrane surface, which makes it a good indicator
for the membrane fouling potential.
The frequency distribution is obtained from a total of 150 force
measurements at five different locations. The distribution plot is
presented to illustrate the spread of intermolecularadhesion forces
obtained during force measurements. Fig. 3 shows that the adhe-
sion forcesbetweenthe foulant andthe clean membraneare spread
over a wide range, from 0.1 to 0.9 mN/m, which is most likely due
Fig. 2. AFM images of the FO membrane active layer. The images were taken in 50 mM NaCl solution. The scanned areas are: (left) 20m20m and (right) 2m2m.
Note also the difference in the vertical scales of the images.
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Fig. 3. Frequency distribution of the adhesion forces between the AFM particle
probe and the clean FO membrane (representing foulantmembrane interaction).
Test solution: 50 mM NaCl and pH 7. The force measurements were performed at 5
different locations on the membrane, with 30 measurements at each location (i.e.,
total 150 measurements).
to the surface roughness of the forward osmosis membrane. The
results indicate that membrane surface morphology greatly influ-
ences the foulantmembrane interactions.
3.2. Baseline experiments
Since the fouling experiments were performed in batch mode,
the osmotic driving force for water flux kept decreasing due to the
dilution of draw solution and concentration of feed solution. There-
fore, the flux decline in the fouling experiments is caused not only
by membrane fouling but also by the decrease in osmotic driving
force. In order to separate the effects of fouling and decrease of
osmotic driving force, we conducted baseline experiments to quan-tify the flux decline due to the progressive decrease in the osmotic
driving force.
As shown in Fig. 4, the baseline experiments were conducted
in both FO and PRO modes under conditions corresponding to
the fouling experiments. Fig. 4a shows the permeate flux decline
as a function of accumulated permeate volume. The water flux
behavior in these experiments as a function of the corresponding
draw solution osmotic pressure, as calculated based on the dilu-
tion of draw solution by permeate flow, is presented in Fig. 4b.
The osmotic pressures of the 50 mM NaCl feed and 1.5/4M NaCl
draw solutionsare 233047.5 Pa(2.3 atm)and 7498050/26040525Pa
(74/257 atm), respectively. The FO membrane pure water perme-
ability coefficient obtained under hydraulic pressure (RO mode) is
3.61012 m/(s Pa). Note,however, that theactual water fluxof the
FO membraneis much lowerthan thevalue calculated based on the
water permeability coefficient and the osmotic driving force. This
behavior is attributed to internal concentration polarization effects
as described in detail in our recent publications [12,13].
The fluxdecline curves shown in Fig. 4 wereused as baselines to
normalize the flux decline curves obtained from the fouling exper-
iments. The baseline flux was first divided by its corresponding
initial fluxto obtain a normalization factor.Then, theflux data from
the fouling experiment was divided by the normalization factor to
obtain the correctedflux. Toavoidconfusion,the flux decline curves
from the fouling experiments presented in this paper use the cor-
rected, normalized flux instead of the actualobserved flux. In other
words, the flux decline curves shown in Figs. 512 solely represent
the effect of membrane fouling.
3.3. Chemical aspects of fouling: Role of calcium and
intermolecular forces
3.3.1. Alginate
To study the effect of calcium on alginate fouling, fouling exper-
iments were performed with feed solutions of 50mM NaCl with or
without Ca2+ (0.5mM). The flux decline curves obtained for each
fouling condition (Fig. 5a) are compared with the corresponding
adhesion forces (Fig.5b and c). Note, however, thatthe alginate con-
centrations are lower in the force measurements than the fouling
experiments (20 and 200 mg/L, respectively). This is because high
foulant concentration is needed for accelerated fouling, whereas
high alginate concentration interferes with intermolecular interac-
tions in the AFM force measurements.
Calcium ions have been shown to enhance alginate fouling in
reverse osmosis membrane systems [8,9]. Similarly, for the FO
membrane, a much more severe flux decline is observed duringalginate fouling in the presence of calcium ions compared to that
in the absence of calcium ions (Fig. 5a). Consistently, the aver-
age adhesion force in the presence of calcium ions is 0.66 mN/m,
about twice the average force without calcium ions. The results
demonstrate that the presence of calcium ions enhances the inter-
molecular adhesion between alginate molecules, resulting in more
severe membrane fouling.
The greater adhesion between alginate molecules in the pres-
ence of calcium ions is attributed to intermolecular bridging by
Fig. 4. Baseline tests with the clean FO membrane in FO and PRO modes. The permeate flux decline is plotted against (a) the accumulated permeate volume and (b) the
corresponding osmotic pressure of the draw solution. The feed and draw solutions initially contain 50 mM NaCl and 4.0/1.5M NaCl, respectively. Other test conditions:
cross-flow velocity of 8.5 cm/s, pH of 7.0, and temperature of 201
C. Note that a water flux of 10m/s corresponds to 36.0 L m
2 h
1 or 21.2gal ft
2 d
1 (gfd).
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296 B. Mi, M. Elimelech / Journal of Membrane Science 320 (2008) 292302
Fig. 5. Effectof calcium on alginatefoulingof theFO membrane(FO mode).Experi-
mental conditions forfoulingexperiments: draw solutioncontaining 4 M NaCl; feed
solution containing 200 mg/L alginate and 50 mM NaCl, with/without 0.5mM Ca2+;
cross-flow velocity of 8.5 cm/s; pH of 5.8; temperature of 201 C. (a) Flux decline
curves with alginatein thepresence andabsenceof Ca2+; water fluxes arecorrected
to account for the dilution of draw solution and reduction of the osmotic driving
force during the fouling run. (b and c) Frequency distribution of the foulantfoulant
adhesion forces for alginate in the absence of Ca2+ and in the presence of 0.5 mM
Ca2+, respectively. The test solutions for the force measurements had a pH of 5.8,
contained20 mg/Lalginate and 50mM NaCl, with/without calcium. The membrane
surface area for the fouling tests is 0.002m2. Note that a water flux of 10m/s
corresponds to 36.0 L m2 h1 or 21.2gal ft2 d1 (gfd).
Fig. 6. TEM cross-sections of alginate fouling layer formed in a fouling experiment
with 200 mg/L alginate, 50 mM NaCl, and 0.5mM Ca2+. The alginate gel layer was
peeled off the membrane surface after the fouling experiment.
calcium ions, resulting in the formation of a cross-linked alginate
gellayer on themembrane surface. Alginate is a linearcopolymer of
mannuronic and guluronic acids that contain abundant carboxylicfunctional groups. Calcium ions bind preferentially to the carboxy-
late groups of alginate in a highly cooperative manner and form
bridges between neighboring alginate molecules, leading to the
formation of a gel network [18]. The alginate gel layer formed in
the fouling experiment with 200 mg/L alginate, 50 mM NaCl, and
0.5mM Ca2+ was peeled off the membrane surface and examined
under TEM. As demonstrated in Fig. 6, the alginate gel layer is com-
posed of cross-linked long chain molecules forming a relatively
thick network structure.
3.3.2. BSA
The effect of calcium on BSA fouling is also studied by per-
forming fouling experiments and force measurements with and
without Ca2+. The flux decline curves obtained from each foulingrun (Fig. 7a) are compared with the adhesion forces measured by
AFM (Figs. 7b and c). The BSA concentrations in the force mea-
surements and the fouling experiments were 20 and 200 mg/L,
respectively.
Fig. 7a demonstrates that, unlike alginate, BSA fouling is not
affected by the presence of calcium ions. Consistently, the adhe-
sion forces measured in the absence and presence of calcium ions
are comparable, which is also roughly the same as the adhesion
force with alginate in theabsence of calcium ions. Our results again
demonstratethat theintermolecular force is a good indicator of the
fouling behavior of organic foulants. The reason that calcium does
not affect BSA fouling could be due tothe low content of carboxylic
groups in BSA molecules, which greatly reduces the possibility of
forming complexes and a cross-linked foulant layer with calcium
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B. Mi, M. Elimelech / Journal of Membrane Science 320 (2008) 292302 297
Fig.7. Effect ofcalciumon BSAfouling ofthe FOmembrane(FO mode).Experimental
conditionsforfoulingexperiments:drawsolutioncontaining4 MNaCl;feedsolution
containing 200mg/L BSA and 50 mM NaCl, with/without 0.5mM Ca2+; cross-flow
velocity of 8.5 cm/s; pH of 6.3; and temperature of 201 C. (a) Flux decline curves
ofBSA inthe presenceandabsenceof Ca2+; water fluxes arecorrected to account for
the dilution of draw solution and reduction of the osmotic driving force during the
fouling run. (b and c) Frequency distribution of the foulantfoulant adhesion forces
for BSA in the absence of Ca2+ and in the presence of 0.5mM Ca2+, respectively. The
test solutionsforthe force measurementshad a pHof 6.3,contained20 mg/LBSAand
50 mM NaCl, with/without calcium. Note that a water flux of 10m/s c orresponds
to 36.0L m2 h1 or 21.2gal ft2 d1 (gfd).
Fig. 8. Effect of calcium on AHA fouling of the FO membrane (FO mode). Exper-
imental conditions for fouling experiments: draw solution containing 4 M NaCl;
feedsolution containing 200 mg/LAHA and50 mMNaCl,with/without0.5 mMCa2+;
cross-flow velocity of 8.5 cm/s; pH of 6.2; temperature of 201 C. (a) Flux decline
curves of AHA in the presence and absence of Ca2+; water fluxes are corrected
to account for the dilution of draw solution and reduction of the osmotic driving
force during the fouling run.(b and c) Frequency distribution of the foulantfoulant
adhesion forces for AHA in the absence of Ca 2+ and the presence of 0.5 mM Ca2+,
respectively. The test solutions for the force measurements had a pH of 6.2, con-
tained 20 mg/L AHA and 50 mM NaCl, with/without calcium. Note that a water flux
of 10m/s corresponds to 36.0 L m2 h1 or 21.2gal ft2 d1 (gfd).
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ions. The concentrations of carboxylic functional groups in various
foulants, represented by carboxylic acidity, have been determined
by potentiometric titration in previous studies [23]. The carboxylic
acidity of BSA is around 1 meq/g, much lower than that of alginate,
3.5meq/g.
3.3.3. Humic acid
The flux decline curves obtained with AHA are shown in Fig. 8a.The correspondingadhesion forces measuredwith andwithout cal-
cium ions are shown in Figs. 8b and c. The AHA concentrations
in the force measurements and fouling experiments were 20 and
200 mg/L, respectively. The flux decline curves demonstrate that
AHA fouling is enhanced by the presence of calcium ions, although
not to the large extent shown for alginate. This behavior is con-
sistent with AHA fouling behavior observed in RO systems [20].
However, the measured adhesion forces are not consistent with
the fouling data. Fig. 8b shows that the adhesion forces measured
in the absence of calcium ions are unexpectedly large and are dis-
tributed over a very wide range. This observation is attributable to
the presence of colloidal aggregates of AHA of varying morphology
and size (from a few nanometers to hundreds of nanometers) [24].
The colloidal aggregates interfere with the intermolecular force
measurements, thus producing false results of adhesion forces.
3.3.4. Correlation between fouling and intermolecular adhesion
force
Fig. 9 compares the fouling behaviors of the three foulants: BSA,
AHA, and alginate. In the absence of calcium ions (Fig. 9a), the flux
decline rates for the three foulants are relatively slow and are only
slightly different. Upon addition of calcium ions, however, the dif-
ferences in the fouling rates with the three foulants become more
significant, where alginate fouls the membrane much faster than
AHA and BSA (Fig. 9b).
The average adhesion forces (F/R) corresponding to each fouling
experiment are also listed in Fig. 9. We observe a strong correlation
between organic fouling rate and intermolecular adhesion forces.
A stronger adhesion force is generally associated with a faster fluxdecline. The correlation between organic fouling and intermolec-
ular adhesion confirms our previous finding in RO systems that
foulantfoulant interaction plays an important role in determining
the rate and extent of organic fouling [19]. Strong foulantfoulant
adhesion causes faster accumulation of foulant on the membrane
surface, thereby resulting in more severe membrane fouling. Our
results suggest that the foulant adhesion force can serve as a good
indicator of the fouling potential in FO membrane systems.
Fig. 9. Comparison of the fouling behaviors of BSA, AHA, and alginate in the FO mode. (a and b) Absence and presence of Ca 2+, respectively. Water fluxes are corrected to
account for the dilution of draw solution and reduction of the osmotic driving force during the fouling run. Experimental conditions for fouling experiments: draw solution
containing 4M NaCl; feed solution containing 200 mg/L foulant and 50mM NaCl, with/without 0.5mM Ca2+; cross-flow velocity of 8.5cm/s; temperature of 201 C. Note
that a water flux of 10m/s corresponds to 36.0 L m2
h1
or 21.2gal ft2
d1
(gfd).
The strong correlation between organic fouling and intermolec-
ular adhesion also provides a mechanistic understanding at the
molecular level for the different effects of calcium on the foul-
ing behaviors of the three foulants. Calcium ions are known to
form complexes with the carboxylic functional groups in organic
macromolecules, therefore increasing foulantfoulant intermolec-
ular adhesion. The carboxylic acidities for BSA, AHA, and alginate
are 1.0, 3.4, and 3.5 meq/g, respectively, as determined by poten-
tiometric titration in our previous studies [17,23]. The carboxylic
acidity of BSA is the lowest among the three foulants, indicating
that BSA molecules have the least opportunity to form complexes
with calcium. Therefore, the impact of calcium on the adhesion
force between BSA molecules is the smallest among the three
foulants, resulting in the slowest fouling rate. For AHA and algi-
nate, although they have similar carboxylic acidity, the adhesion
force of alginate is much higher than AHA in the presence of cal-
cium ions. The difference is attributed to the unique alginate gel
forming mechanism. The alginate gel formation not only requires
carboxylate functional groups to form complexes with calcium,
but also needs certain structural characteristics to allow effec-
tive intermolecular bridging. For example, in the linear copolymer
structure of alginate, the polyguluronic acid blocks bind much
more effectively with calcium ions than the polymannuronic acid
blocks. Also, it was found that only polyguluronic acid blocks
over a certain size can be involved in calcium cross-linking, with
the larger being more effective in cross-linking [25]. Therefore,
alginate structural characteristics areimportant forforming a cross-
linked network by intermolecular bridging, which explains why
alginate has a much stronger force andcorrespondingly faster foul-
ing rate than AHA, even though the two have similar carboxylic
acidity.
3.4. Effect of membrane orientation
The absence of hydraulic pressure in FO processes allows the
membrane to be operated in two orientations. When the mem-
brane active layer is placed against the feed solution, we refer tothis orientation as FO mode, whereas when the membrane active
layer is against the draw solution, we refer to this orientation as
pressure-retarded osmosis (PRO) mode. Since internal concentra-
tion polarizationis more pronounced in FO mode than in PRO mode
[17], which significantly diminishes the effective osmotic driving
force for water flux,the draw solution usedin FOmode has a higher
concentration (4 M NaCl) than in PRO mode (1.5 M NaCl) in order
to obtain the same initial flux.
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Fig. 10. Effect of membrane orientation on FO membrane fouling by alginate, BSA,
and AHA. Thefluxdeclinecurvesof alginate, BSA, andAHAare shownin (a), (b), and
(c), respectively. Water fluxes arecorrected to account for the dilution of draw solu-
tion and reduction of the osmotic driving force during the fouling run. Membrane
orientations are represented by two modes: FO (membrane active layer facing feed
solution) and PRO (membrane active layer facing draw solution). The feed solution
contained 200mg/L foulant, 50mM NaCl, and 0.5mM Ca2+. The draw solution con-
tained 4 M NaCl for FO mode and 1.5M NaCl for PRO mode. The cross-flow velocity
and temperature are similar to those in Fig. 5. Note that a water flux of 10m/s
corresponds to 36.0L m2 h1 or 21.2gal ft2 d1 (gfd).
The effects of membrane orientation on the fouling of alginate,
BSA, and AHA are shown in Fig. 10. We observe that membrane
orientation has different effects on the three foulants. Fouling with
alginate is not affected by membrane orientation, with a similar
flux decline obtained in FO and PRO modes. In contrast, for BSA and
AHA, the flux decline is more severe in the PRO mode than in the
FO mode. This observation is most notable with AHA (Fig. 10c). The
different membrane orientation effects suggest that the different
organic foulants have different fouling mechanisms.
Membrane fouling is generally governed by the coupled influ-
ence of chemical and hydrodynamic interactions [20]. Chemical
interactions, such as calcium binding, have been found to affect
intermolecular adhesion forces and membrane fouling behavior.
Hydrodynamic interactions, such as permeation drag resulting
from convective flow toward the membrane and shear force
resulting from cross-flow parallel to the membrane, influence
the deposition and accumulation of foulant molecules on the
membrane surface. The relative importance of these factors in con-
trolling organic fouling of FO membranes depends on the foulant
type and membrane orientation.
Since the FO membrane has an asymmetric structure, charac-
terized by a dense active layer on top of a porous support layer,
membrane fouling occurs on different surfaces in FO and PRO
modes. In FO mode, with the membrane active layer against the
feed solution, foulant deposition/accumulation occurs on top of
the active layer. Foulant deposition is affected by both permeation
drag and shear force, resulting from the permeate flux and bulk
cross-flow, respectively. In PROmode, however, withthe membrane
porous support layer against the feed solution, foulant deposition
takes place within the porous structure of the membrane. Since
cross-flow velocity vanishes within the porous support layer, the
influence of hydrodynamic shear forces is absent at the initial stage
of fouling in PRO mode.
As the two membrane orientations provide different hydro-
dynamic conditions during membrane fouling, the effect of
membrane orientation can be used as an indicator for the role
of hydrodynamic conditions in membrane fouling. Since mem-brane orientation shows no influence on flux decline with alginate
(Fig. 10a), we conclude that hydrodynamic interactions do not play
a dominant role in alginate fouling. Instead, chemical interactions
(calcium binding) play a more important role. Calcium binding
results in a highly structured gel layer,which is relatively unaffected
bychanges in hydrodynamicconditions. ForBSA andAHA, however,
thecalcium-binding effects are less significant, andthe influence of
hydrodynamic interactions becomes moreimportant. In PROmode,
the absence of cross-flow within the membrane porous support
layer precludes shear force as a mechanism to drive foulant away
from the membrane. Therefore, fouling with BSA and AHA is more
severe in the PRO mode than in the FO mode. The marked flux
decline with AHA in the PRO mode is attributed to cake layer for-
mation due to lack of shear force as well as hindered back diffusionof AHA aggregates in the porous structure.
3.5. Effect of initial permeate flux
The effects of initial flux on fouling of alginate, BSA, and AHA
are shown in Fig. 11 for fouling in the PRO mode. For each fouling
experiment, the flux decline is plotted in two different forms: the
permeate flux versus permeate volume (Figs. 11ac) and the corre-
sponding normalized flux versus permeate volume (Figs. 11df). A
faster flux decline is generally noticed for all three foulants at the
early stages of filtration when the initial permeate flux is high, but
the degree of influence varies from foulant to foulant.
For alginate and AHA, the influence of initial flux is relatively
small, but for BSA, the higher initial flux causes significantly
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300 B. Mi, M. Elimelech / Journal of Membrane Science 320 (2008) 292302
Fig. 11. Effect of initial flux on membrane fouling in the PRO mode (membrane active layer facing draw solution). (a, b, c) The flux decline curves for alginate, BSA, and AHA,
respectively. (d, e, f) The corresponding normalized flux for the three foulants. Water fluxes are corrected to account for the dilution of draw solution and reduction of the
osmotic driving force during the fouling run. The draw solution contained 4 M or 1.5M NaCl. The feed solution contained 200 mg/L foulant, 50mM NaCl, and 0.5mM Ca2+.
The cross-flow velocity and temperature are similar to those in Fig. 5. Note that a water flux of 10m/s corresponds to 36.0 L m2 h1 or 21.2gal ft2 d1 (gfd).
higher flux decline. Since the effect of initial flux on membrane
fouling is attributed mainly to the permeation drag resulting from
convective flow toward the membrane, our results indicate that
the stronger permeation drag exerted on BSA molecules results in
the formation of a cake layer. The large difference in the fouling
behavior with BSA for low and high permeation drags reflects a
transition in the fouling layer from a loose fouling layer structureto a much more compact cake layer. Changes in permeation drag
appear to have less effect on alginate and AHA, because the fouling
layers under both conditions are already in the form of a cake layer.
Once a cake layer forms, fouling becomes much less sensitive to
changes in hydrodynamic conditions.
3.6. Coupled influence of intermolecular adhesion and
hydrodynamic forces
Our previous discussion has shown that intermolecular adhe-
sion and hydrodynamic forces, mainly permeation drag and shear
force, are the major factors governing the development of a fouling
layer on the membrane surface. However, the dominating fac-
tors that control membrane fouling vary significantly with organic
foulant type. Membrane fouling with alginate is rapid due to inter-
molecular binding of foulants by calcium, but is relatively insen-
sitive to hydrodynamic conditions (cross-flow and initial flux). In
contrast, BSA fouling is more subject to hydrodynamic interactions
than to calcium effects. The relative influence of intermolecular
adhesion and hydrodynamic forces on fouling in the FO and PRO
modes is conceptually illustrated in Fig. 12. The corresponding nor-malized flux decline curves are also shown in this figure.
Fig. 12 shows a clear trend in the coupled influence of inter-
molecular adhesion and hydrodynamic interactions on membrane
fouling. With stronger intermolecular adhesion forces, hydrody-
namic conditions for favorable foulant deposition leading to cake
formation aremore readily attained. This is demonstratedin Fig. 12
for alginate, which forms a cake layer under all three tested hydro-
dynamic conditions: FO, PRO, and PRO at high initial flux. For AHA,
the foulant with moderate intermolecular adhesion force, the for-
mation of a cake layer is absent in the FO mode (the least favorable
depositionconditions), but is formed when the shearforce vanishes
in the PRO mode (more favorable deposition conditions). For the
foulant with very weak intermolecular adhesion force (BSA), cake
layer forms only in thePRO mode at high initial flux(the most favor-
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B. Mi, M. Elimelech / Journal of Membrane Science 320 (2008) 292302 301
Fig.12. Schematic illustration of the coupled influenceof intermolecularadhesion and hydrodynamic forces on membrane fouling by alginate,AHA, and BSA.The horizontal
axis compares the different foulants with respect to intermolecular adhesion forces (from strong, left, to weak, right). The vertical axis represents the different hydrody-
namic/deposition conditions, from more favorable (bottom) to less favorable (top) deposition conditions. The corresponding normalized flux decline curves are shown on
top. For these experiments, the feed solution contained 200 mg/L foulant, 50mM NaCl, and 0.5 mM Ca2+. Other test conditions are similar to those in Fig. 5.
able deposition conditions). The favorable deposition conditions
leading to cakeformation areindicatedin Fig.12 by theshaded area.
Once a cake layer forms, a rapid flux decline is observed, but fur-ther changes in hydrodynamic conditions do not affect the fouling
behavior. In addition, when a cake layer forms, the intermolecu-
lar adhesion force has no effect on fouling. This is demonstrated
by the similar flux decline curves with the different foulants in the
PRO mode at high initial flux.
4. Conclusion
A strong correlation between organic fouling and intermolecu-
lar adhesion force was observed, indicating that foulantfoulant
interaction plays an important role in determining the rate and
extent of organic fouling. Thefouling data showedthat FO fouling is
governed by the coupled influence of chemical and hydrodynamic
interactions. Calcium binding, permeation drag, and hydrodynamic
shear force are the major factors governing the development of
a fouling layer on the membrane surface. However, the dominat-
ing factors controlling membrane fouling can vary from foulant tofoulant. With stronger intermolecular adhesion forces, hydrody-
namic conditions for favorable deposition and cake formation are
more readily attained. Alginate has the strongest intermolecular
interactions due to calcium binding, and it forms cake layer under
all tested hydrodynamic conditions. On the contrary, having weak
intermolecular interactions, BSA forms cake layer only at the most
favorable hydrodynamic conditions. AHA behavior lies in between
that of alginate and BSA. Before forming a compact cake layer,
fouling is sensitive to intermolecular interactions and changes in
hydrodynamic conditions. However, once cake layer forms, a rapid
flux decline is observed, and changes in hydrodynamic conditions
or intermolecular adhesion have little effect on the fouling behav-
ior. All three foulants exhibit very similar flux decline curves when
cake layers form.
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Acknowledgement
The authors are grateful for the financial support received from
the California Department of Water Resources under Award Num-
ber 4600 007446.
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