Mi Elimelech JMS 2008

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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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B. Mi, M. Elimelech / Journal of Membrane Science 320 (2008) 292302 293

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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294 B. Mi, M. Elimelech / Journal of Membrane Science 320 (2008) 292302

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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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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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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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-


10/11


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.


11/11


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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