Desalination: Organic Fouling and Reverse solute selectivity in forward osmosis: Role of Working temperature and inorganic draw solutions

7/25/2019 Desalination: Organic Fouling and Reverse solute selectivity in forward osmosis: Role of Working temperature and

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Organic fouling and reverse solute selectivity in forward osmosis: Role of working

temperature and inorganic draw solutions

Jiyong Heo a, KyoungHoon Chu b, Namguk Her a, Jongkwon Im b,c, Yong-Gyun Park d, Jaeweon Cho e,Sarper Sarp f, Am Jang g, Min Jang h, Yeomin Yoon b,a Department of Civil and Environmental Engineering, Korea Army Academy at Young-Cheon, 135-1 Changhari, Kokyungmeon, Young-cheon, Gyeongbuk 770-849, South Koreab Department of Civil and Environmental Engineering, University of South Carolina, Columbia, SC 29208, USAc National Institute of Environmental Research, Environmental Research Complex, 42 Hwangyeong-ro, Seo-Gu, Incheon 404-170, South Koread Environmental Process Design Team, GS E&C, 33 Jongro, Jongro-gu, Seoul 110-702, South Koreae School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology (UNIST),UNIST-gil 50, Ulju-gun, Ulsan 689-798, South Koreaf Qatar Environment and Energy Research Institute, Qatar Foundation, Education City, CP4, PO Box: 5825, Doha, Qatarg

School of Civil and Architecture Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-16 Gu, Suwon, Gyeonggi-do 440-746, South Koreah Department of Civil Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia

H I G H L I G H T S

We examined the inuence of high levels of organic foulants on FO membrane performance.

We evaluated the inuence of temperature on the forward water and reverse salt ux behavior.

Lower water viscosities played a dominant role in enabling concentrative internal concentration polarization.

a b s t r a c ta r t i c l e i n f o

Article history:

Received 31 March 2015

Received in revised form 16 June 2015

Accepted 16 June 2015

Available online xxxx

Keywords:

Forward osmosis

Unpressurized pressure-retarded osmosis

Fouling

Temperature

Inorganic draw solutions

Reverse solute ux selectivity

The water ux of several draw solutions (DSs, solutes: KCl, NaCl, CaCl2, Na2SO4) and fouling propensity of two

different organic foulants (humic acid and alginate) were systematically investigated using forward osmosis (FO)and unpressurized pressure-retarded osmosis. In addition, reverse solute selectivity was evaluated to characterize

the water and salt transport mechanisms at different temperatures and in the presence of four different inorganic

DS compounds. The inuence of solutionviscosity has signicantimplications in FO applications, because the water

molecules easily penetrated and diffused throughout the FO membrane active layer (AL) and supporting layer (SL)

with increasing temperatures, which is mainly correlated with the lower water viscosities with increasing temper-

atures. The results indicated that the water ux on average signicantly increased from 9.5 to 13.7 and 24.9 LMH

when the operating temperature was increased from 5 to 20 and 45 C, which corresponded to a 44 and 262% in-

crease in thewaterux,compared to theFO modeat 5 C. However,the water ux andviscosity exhibited generally

constanttrends withrespectto the elevating temperature.In addition, elevating temperature increasedthe reverse

solute ux selectivity(RSFS), not onlyby decreasing the internalconcentration polarization (theAL and SL) and the

wettability within the effective porosityof the SL, but alsovia the improvement of water moleculediffusion kinetics

rather than solute diffusion. In addition, the RSFS was inversely related to the solute permeability of the different

DSs and followed the order Na2SO4 NCaCl2 NNaCl NKCl. These results have signicant implications for the predic-

tion of water ux behavior and the selection of DSs at different temperatures in osmotically driven FO processes.

2015 Elsevier B.V. All rights reserved.

1. Introduction

Osmotically driven membrane processes (ODMPs) involve spontane-

ouswater movement by means of a natural chemical gradient in which a

solution tends to move from a state of lower osmotic pressure to one

higher osmotic pressure through a semi-permeable membrane. ODMPs

have great potential in sustainable water purication, since they reduce

the energy required for seawater desalination and typically exhibit low

propensities for membrane fouling [13]. A number of ODMPs, including

forward osmosis (FO) and pressure-retarded osmosis (PRO), have been

studied [1,416]. Prior studies havefocusedon (i) understanding internal

and external concentration polarization (ICP and ECP, respectively),

which reduce the deleterious effects of the water ux[4,7]; (ii) mem-

brane fabrication techniques for higher waterux environments[11,17,

18]; (iii) module development with feed solutions (FSs) containing

Desalination xxx (2015) xxxxxx

Corresponding author.

E-mail address:[email protected](Y. Yoon).

DES-12620; No of Pages 9

http://dx.doi.org/10.1016/j.desal.2015.06.012

0011-9164/ 2015 Elsevier B.V. All rights reserved.

Contents lists available atScienceDirect

Desalination

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 / d e s a l

Please cite this article as: J. Heo, et al., Organic fouling and reverse solute selectivity in forward osmosis: Role of working temperature andinorganic draw solutions, Desalination (2015),http://dx.doi.org/10.1016/j.desal.2015.06.012
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various foulants and scalants [6,9,10,1416,19]; and (iv) development of

possibleorganicand inorganic drawsolutions (DSs) based on reverse salt

diffusion [8,12,13]. In these studies, theperformance of ODMPs was inu-

enced by the following temperature-dependent factors [13,2023]:

membrane properties, solution properties, module conguration,

and operating conditions. Particularly, the temperature directly

inuenced the thermodynamic characteristics of the FSs and mem-

brane properties[ 2426], which, in turn, directly inuenced the

water permeability, salt permeability, and reverse solute

ux selectivity(RSFS)[13,21].

Signicant temperature variations have been observed in several

practical membrane separation applications, including municipal and

industrial feed water purication [20,22,25,26]. These temperature

variations inuence the key characteristics of membrane separation

processes,such as solute mass transfer, water movement, concentration

polarization(CP), and membrane fouling, which are strongly inuenced

by changes in solution viscosity, foulant characteristics, water diffusivi-

ty, and osmotic pressure. For example, the viscosity of water and solute

rejection decrease with increasing feed water temperature; in contrast,

solute diffusivity and solution osmotic pressure increase with increasing

feed water temperature, thereby leading to a greater water ux at higher

temperatures [22]. Thus, CP can be reduced at higher feed water temper-

atures by enhancing salt transport in the membrane support layer (SL),

which, in turn, affects the solute passage and osmotic pressure during

the membrane separation process. Withrespect to membrane fouling, in-

creasing the temperature typically leadsto a decrease in humic acid (HA)

colloids, which decrease specic cake resistance and result in more

porous deposits on the membrane surface[26]. In addition, biolm for-

mation is enhanced at higher temperatures, particularly when wastewa-

ter is used as the FS.

The inuence of temperature on FO membrane processes has also

been studied by evaluating reverse solute diffusion as a function of

water permeation. For example, Phillip et al. [13]demonstrated that the

reverseux selectivity was not dependent on the membrane structural

characteristics or the bulk DS concentrations for organic and inorganic

solutions. However, the reverseux selectivity wasdependent on the se-

lectivity of the active layer (AL) and the osmotic pressure, both of which

varied withtemperature [8,13]. The inuence of working temperature onFO separation performance was studied by Zhao et al.[20], who demon-

strated that higher temperatures favor higher initial uxes, water recov-

eries, and concentration factors; however, greater membrane scaling was

observed when the FS was introduced at temperatures of 25, 35, and

45 C. You et al.[23]reported that the mass diffusion kinetics, such as

the water trans-membrane ux in the FO process, could be improved

signicantly by increasing the temperature. In the temperature range of

20 to 45 C, the FO ux positively correlated with the water ux and

bulk solution temperature. Xie[21]recently reported the inuence of

temperatures between 20 and 40 C on NaCl RSFS. In practical terms, in-

creasing the temperature contributed to an increase in pure water and

NaCl permeability coefcients, thereby suggesting that higher tempera-

tures signicantly increased solute diffusivity and decreased water

viscosity. Moreover, in the Xie's study, the RSFS did not change withincreasing temperature, which suggested no change in membrane

properties.

Relatively little is still known about the inuence of increasing

temperature on the correlation of waterux and solution viscosity,

RSFS, and FO membrane fouling propensity in the presence of different

inorganic DSs in FO- and PRO-mode operations. Therefore, in the study

reported herein, the specic objectives of the research included the

following: (i) examination of the inuence of high levels of organic

foulants (i.e., 60 mg L1 of HA and alginate) and secondary efuents

on the foulingcharacteristics of FO membranes at various temperatures

(5, 20, and45 C) in both FO and unpressurized PRO (uPRO) modes; (ii)

evaluation of the inuence of increasing temperature (5 up to 45 C) on

the forward waterux and reverse saltux behavior (i.e., the solution

viscosity and solute selectivity coefcients); and (iii) determination of

the inuence of temperature on the RSFS and FO membrane properties,

which was evaluated using previously studied reverse draw solute per-

meation modeling. This systematic assessment of temperature inu-

ence will improve the understanding of fouling propensity, forward

water, and reverse solute ux behavior and enable better selection of

DSs for osmotically driven FO processes.

2. Materials and methods

2.1. FO membranes and characterization

Commercially available at-sheet cellulose triacetate (CTA) FO

membranes were obtained from Hydration Technologies, Inc. (Albany,

OR, USA). These FO membranes were developed specically to exhibit

unique characteristics (e.g., greaterhydrophilicity and less ICP) compared

with other semi-permeable membranes. The ICP was reduced through

the use of a thin, synthetic,asymmetricCTA AL; embeddedultrathin poly-

ester woven mesh SLs were incorporated for mechanical strength [27].

CTA content in the membrane was conrmed by Fourier-transform in-

frared (FTIR) spectroscopy (Fig. S1); the higher frequency peak at

1737 cm1 associated with acetyl functional groups of CTA was detected

on the membrane surface. After it was cut to the desired size, the FO

membrane was preserved by soaking in deionized (DI) water and stored

away from direct sunlight in a refrigerator at 4 C. According to the sup-

plier, the membrane exhibited 99.4% salt rejection and an overall tap

water ux of 7.7 and 12.0 L m2 h1 (LMH) in the FO and uPRO

modes, respectively. The electrophoretic mobility of an FO membrane

was less negative than the surface potential; the zeta potential (ZP)

ranged from4 to 8 mV. The water contact angles of the FO mem-

brane selective layer (or AL) and the back layer (or SL) were 61.3

0.8and 66.4 1.3, respectively.Moredetailswith respectto membrane

properties are provided separately[28].

2.2. Feed and draw solutions

Sodium alginate (SA, SigmaAldrich, St. Louis, MO, USA) and HA

(SigmaAldrich, St. Louis, MO, USA) were used as model organic

foulants for polysaccharides and natural organic matter (NOM), respec-tively. These organic macromolecules are the major cause of organic

fouling during membrane ltration of surface water, seawater, and

wastewater efuent. To ensure the consistency of the organic composi-

tion in the FS, stocksolution was prepared by concentratingSA and HA.

SA has been widely used in membrane fouling studies to simulate

wastewater efuent organic matter containing high levels of soluble

microbial products. SA was received from the manufacturer in powder

form with a 1280 kDa molecular weight; a 2 g L1 stock solution of

SA in DI water was prepared. HA,which is a mixture of complexpoly-

electrolytes with carboxylic and hydroxyl functional groups, has

been used extensively as a model organic foulant for high molar mass

NOM, and its characteristics can be found elsewhere[29]. HA stock

solution was prepared by dissolving in DI water and then sequentially

ltering through GF/F (0.7-m) glass microberlters (Whatman Inc.,Piscataway, NJ, USA) and Durapore (0.45 m) membrane lters

(Millipore Inc., Billerica, MA, USA) to remove any impurities and partic-

ulate matter. Stock solutions were stored in sterilized glass bottles at

4 C and used in subsequent experiments. The molecular weights of

the HA were about 15 kDa, as outlined in our previous study[30]. A

total organic carbon (TOC) analyzer (Shimadzu, Corp., Kyoto, Japan)

was used to measure the organic carbon contents of the stock solutions

and FSs.

TheFS for theFO experiments was prepared by further dilution of SA

and HA stock solution with DI water to obtain a desired concentration of

60 mg L1 dissolved organic carbon (DOC). The ionic strength of the FS

was adjusted by adding NaCl to maintain a nal background ionic

strength of 1 mM. The pH was adjusted to 6.5 by addition of 0.1 M

NaOHand 0.1M HCl, as needed.DS was prepared by dissolvingdifferent

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concentrations of various inorganic compounds (NaCl, Na2SO4, CaCl2, or

KCl) (SigmaAldrich, St.Louis, MO, USA) in DI water.These four inorganic

compounds were chosen as DS solutes, since their thermodynamic prop-

erties are well established.

2.3. Bench-scale FO set up

The FO experiments were conducted with an in-house, bench-scalestainless-steel plate and frame as an FO cell coupled with an FS and a DS

tank. The setup was built with symmetric, rectangular double-channels

on bothsides of the membrane to enable simultaneousowing of theFS

and DS; the channel dimensions were 76 mm 27 mm (length

width), providing an effective membrane coupon area of 41.04 cm2.

Fig. S2 shows the schematic diagram of the bench-scale FO system

used in these experiments. The design and operating details for the FO

system have been recently reported elsewhere[28]. Each set of each

experiment were carried out for more than 12 h: FO-mode runs

were initiated with DI water as FS to the membrane active layer

and a 1 M NaCl DS; uPRO-mode runs were initiated with DI water

as FS to the membrane support layer and a 1 M NaCl DS. The ionic

strength of the FS was maintained at a NaCl concentration of 1 mM.

The solution temperature was varied from 5 to 45 C, and was con-

trolled to within 1.5 C using a water heater/chiller (Fisher Scientic

Isotemp Chillers, Pittsburgh, PA, USA) throughout the baseline and

fouling tests. Mesh spacers were employed to support the FO mem-

brane at a constant ow rate of 0.19 L min1 (corresponding to a

crossow velocity of 9.8 cm s1) for the FS and DS. Flow rates were

monitored using ow meters (Dwyer, Michigan City, IN, USA) during

all experiments.

2.4. FO membrane fouling experiments and reverse salt selectivity

The FO fouling experimental protocol included the following steps.

First, a new membrane coupon was placed where the CTA membrane

was sandwiched in the unit cell, as described previously. Next, the sys-

temwas thoroughlyrinsed with DI water,and concurrent cross-ows ofDI water solutions were maintained for more than 1 h to stabilize the

system. At this stage, the FS with 60 mg L1 DOC (foulant) was intro-

duced via the feed reservoir described above. To achieve a water ux,

1 L of a 2 M NaCl stock solution was added to the DS tank. Both the DS

and FS were continuously mixed using a magnetic stirrer. The DS reser-

voir was placed on a digital balance (AV8101, Ohaus, NJ, USA), and the

waterux in the DS was recorded by monitoring the weight change of

the DS; the weight data were automatically logged into a computer

every 10 min. Prior to each main experiment, baseline experiments

were conducted to quantify the ux decline associated with declining

osmotic driving as the DS was gradually diluted by the permeate

water. Baseline experiments also enabled correction of the ux

curves obtained during fouling runs, and they enabled direct com-

parison of the fouling and temperature effects and quantication ofNaCl reverse diffusion from DS to FS, even when no foulant was added

to the FS.

To quantify the reverse solute ux during the FO experiments, the

solute concentration in the FS was monitored at 3-min intervals using

a calibrated conductivity meter with a cell constant of 1 cm1 (Thermo

Scientic, Vernon Hills, IL, USA). These measurements enabled the

determination of the pure water permeability of the membrane AL, A,

the solute permeability of the membrane AL, B, and the structural

parameter of the membrane SL,S, by treatingA,B, andSas adjustable

parameters to enable simultaneous tting of experimental water and

reverse salt ux data to corresponding governing equations, as de-

scribed in our recent publication[28]. Some runs were duplicated to

conrm the reproducibility of the FO fouling and temperature effect

experiments.

3. Results and discussion

3.1. FO membrane characterization

The intrinsic properties of FO membranes were investigated by

calculating the selectivity of the dilutive ICP of the AL of FO membranes

based on the solution-diffusion (SD) model approach. The membrane

selectivity is related to the ratio ofAto B, whereAis the intrinsic pure

water permeability coef

cient andB is the intrinsic salt permeabilitycoefcient of the FO membrane AL. The parameters for determining

the selectivity coefcient are similar to those presented in a previous

study based on the reverse osmosis (RO) mode[7,31]. Although accu-

rate prediction of the characteristic transport parameters of hydrophilic

FO membranes is difcult using the SD model, the inherent discrimina-

tion of RO- and FO-mode parameters enabled determination of the FO

membrane characteristics with different DS solutes. The selectivity

values were calibrated using the data determined experimentally from

RO-mode and previously described methods [28]. Briey, the selectivity

values (A/B) were determined using a pressurized, dead-end congura-

tion mode with a stirring speed of 300 rpm. To determineA and B, the re-

jection of inorganic salts and the pure waterux in the RO mode were

plotted vs. applied pressure based on the SD model approach.

The FO membrane water permeability coefcient (A) was

0.517 L m2 h1 bar1, which is slightly higher than that reported in

our previous research. The difference was attributed to the variation

between experimental protocols and membrane coupons. Fig. S3

shows the membrane selectivity (A/B) for thefourtypes of draw solutes

investigated in this study. The A/B values followed the order

Na2SO4 NCaCl2 NNaCl NKCl. The determination ofA and B for the FO

membrane coupon and the four types of draw solutes (i.e., the solute

mass transfer properties of the FO membrane and the DS solute ions)

elucidated the trends in solute permeation through the FO membrane

[32]. KCl exhibitedthe highest membrane selectivity, despite the smaller

size of the cation Na+ compared with the cation K+. However, K+ cat-

ions are less hydrated than Na+ cations, leading to greater permeability

of K+ through the FO membrane compared with Na+. This result con-

rms thehigher solutetransferof KClthan that of NaCl based on solution

electro-neutrality. In addition, the membrane selectivity was greater indivalent salts than in monovalent salts. TheA/Bvalues of divalent salts

(7.93 0.13 and 9.24 0.57 bar1 for CaCl2and Na2SO4, respectively)

were approximately 3.54.0 times higher than the membrane selectivity

of themonovalent salts (A/B values of 2.65 0.28and 2.27 0.13 bar1

for NaCl and KCl, respectively). The higher solute permeability of NaCl

and KCl was attributed to the smaller hydration radii of Na+

(0.358 nm), K+ (0.331 nm), and Cl (0.332 nm) compared to those of

Ca2+ (0.421 nm) and SO42 (0.379 nm), indicating that the divalent

salts have better separation properties than the monovalent salts in the

FO membrane.

3.2. Effects of solution temperature and FO membrane orientation on water

ux and organic fouling

As indicated by the Van't Hoff equation, temperature directly inu-

ences the osmotic pressure of the solution and other thermodynamic

properties, such as diffusion coefcient, viscosity, etc[13,20,22]. Since

the FO process involves two independent streams between feed and

draw solutions on each side of the membrane, the inuence of temper-

ature on each solution may be inuenced by the FO membrane orienta-

tion (FO vs. uPRO). The inuence of temperature and membrane

orientation on the organic fouling and waterux were evaluated in

the FO experiments using isothermal FS and DS temperature stages

and solution temperatures of 5, 20, and 45 C. Fig. 1 shows the inuence

of temperature on the pure waterux (baseline, using DI water as feed)

and waterux(using 60mg L1 HA or SA as foulants) in the FO process

at different membrane orientations (FO and uPRO modes).

3J. Heo et al. / Desalination xxx (2015) xxxxxx



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As shown inFig. 1, the water ux improved signicantly when the

temperature of both of the solutions was increased from 5 to 45 C.

When the operating temperature of both of the solutions was increased

from 5 to 20 and 45 C, the water ux (baseline) increased from 9.5 to13.7 and 24.9 LMH, respectively. These increases corresponded to a 44

and 262% increase in the water ux, respectively. The temperature

signicantly inuenced the magnitude of the water ux in the FO

mode, since it greatly reduced the ICP across the membrane. The in-

creasein thewaterux at highertemperatures wasattributed to chang-

es in thermodynamic properties, such as the osmotic pressure, diffusion

coefcient, and/or the viscosity of the FS and DS. As the temperature

was increased from 5 to 20 and 45 C, the osmotic pressure increased

(data not shown: however, the data is available on request), which

was expected to increase the net driving force and, hence, enhance

the water ux during the FO process. In addition, the water viscosity

decreased, thereby resulting in an increase in the water self-diffusivity

at higher temperatures, which, in turn, resulted in an increase in the

mass transfer via reducing the inuence of ICP.

Similar increasing trends in the water ux were observed in the

uPRO mode. With an increase in temperature from 5 to 20 and 45 C,

the pure water ux (baseline) increased from 15.1 to 25.9 and 41.4

LMH, respectively. The increase in the water ux in the uPRO modewas related to both thermodynamic properties and the ICP phenome-

non in the membrane support layer. The temperature variation inu-

enced the water ux, which inuenced the thermodynamic properties

(i.e., the water viscosity and diffusivity). A lower driving potential

(membrane ux) was observed with increasing DS concentration as a

necessary consequence of dilutive ICP in the membrane SL in the FO

mode [27,33], which was also inuenced by the temperature variations.

As shown inFig. 2, water molecules easily passed and diffused through

the AL and SL of the FO membrane upon increasing the temperature in

the uPRO mode. As a result, increasing the temperature from 5 to 20

and45 C generated an increasein thewateruxof 72and 274%, respec-

tively, in the uPRO mode.

Organic fouling in the FO and uPRO modes was evaluated using single

foulants (60 mg L1

of HA and SA). According to the literature, a

Time (hr)

0 2 4 6 8 10 12 14

Wa

terflux(Lm

-2h

-1)

0

2

4

6

8

10

12

Baseline - DIFeed - HA (C

0= 60 mg L

-1as DOC)

Feed - SA (C0= 60 mg L

-1as DOC)

a

Time (hr)

0 2 4 6 8 10 12

Waterflux(Lm

-2h

-1)

0

3

6

9

12

15

18

Baseline - DI

Feed - HA (C0= 60 mg L -1as DOC)


-1as DOC)

b

Time (hr)

0 2 4 6 8 10 12

Waterflux(Lm

-2h

-1)

0

5

10

15

20

25

30

Baseline - DI

Feed - HA (C0= 60 mg L

-1as DOC)


-1as DOC)

c

Time (hr)

0 2 4 6 8 10 12 14

Wa

terflux(Lm-2h

-1)

0

3

6

9

12

15

18

Baseline - DIFeed - HA (C

0= 60 mg L-1as DOC)

Feed - SA (C0= 60 mg L-1as DOC)

d

Time (hr)

0 2 4 6 8 10 12

Waterflux(Lm-2h

-1)

0

5

10

15

20

25

30

Baseline - DI


-1as DOC)


-1as DOC)

e

Time (hr)

0 2 4 6 8 10

Waterflux(Lm-2h

-1)

0

10

20

30

40

50

Baseline - DI


-1as DOC)


-1as DOC)

f

Fig. 1.Pure waterux (baseline, using DI water as feed) and water ux (using 60 mg L1 HA or SA as foulant) with various temperatures at different membrane orientations: (a) 5 C,

(b) 20 C and (c) 45 C i n the FO mode and (d) 5 C, (e) 20 C and (f) 45 C in the uPRO mode. Operating conditions: pH = 6.5; ionic strength = 1 mM; and NaCl = 2 M.




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maximum foulant concentration of 60 mg L1 simulates the efuent

organic matter of wastewater [15,34,35]. The two different foulants

were used to investigate two different characteristics of fouling; HA is

more hydrophobic than SA. Experiments were carried out over morethan 12 h to thoroughly evaluate thefoulant effects. The inuence of tem-

perature variation on organic fouling was also investigated; hydrody-

namic shear was enhanced by decreasing viscosity with respect to

increasing temperature. The results of fouling runs in the FO and uPRO

modes, with temperature variations from 5 to 45 C, are shown in

Fig. 2; decreases in the ux percentage after fouling are presented as

normalized uxes in this gure. In the normalizedux decline (compar-

ing between foulant and temperature effects), SA fouling at lower

temperature conditions was the most pronounced, followed by middle

and higher temperature conditions.

In the uPRO mode, low temperatures caused more severe fouling,

based on the observed changes in the solution characteristics (water

viscosity) and fouling patterns. The fouling propensity was more severe

at low temperature because the viscosity of an alginate solution was

variable with temperature (decreasing temperature resulting in in-creasing viscosity), and also the longer chins can be formed at higher

viscosities even similar concentrations of alginate solution. In addition,

the more severe SA fouling and ux decline with decreasing tempera-

ture was also attributed to cake layer formation[10,15], which is the

formation of a cake-enhanced, cross-linked, thick gel layer on the mem-

brane surface. A schematic description of this change for the tempera-

ture is depicted inFig. 3. The fouling rate increased with the volume

concentration factor (VCF), as shown in Fig. S4. The VCF provides a

more reasonable comparison of the organic fouling and ux decline

than the permeate volume or time, since the concentration of retained

organic foulants on the membrane surface in the FS was highly variable

with respect to the condensing volume. This variability signicantly in-

uenced thephysicaland chemical properties of themembraneand sol-

uteat theinterface of the membrane. Expressed as VCF (VCF = 1.03.0,

corresponding to a recovery of 058.7%), the fouling resistance was sig-

nicant with respect to SA at low temperatures, with water ux losses

of about 30.2% compared with the baseline experiments.

Fouling experiments at different temperatures were alsoconducted to

evaluate the inuence of membrane orientations, namely, the AL facing

the FS in the FO mode and the SL facing the FS in the uPRO mode.

Fig. S5 compares the average water ux proles in the FO and uPRO

modes for the HA and SA organic foulants (at a loading of 60 mg L1).

Generally, greater ux reduction was observed with SA. In the FO mode,

the water uxesat the end of 20h testruns in the absence of SA at differ-

ent temperatures (5, 20, and 45 C) were 9.5, 13.7, and 24.9 LMH, respec-

tively,while those in the presence of SA exhibited lower water ux values

of 8.6, 12.7, and 21.9 LMH, respectively. In the uPRO mode, theux reduc-

tion was 24% greater, with a water ux of 25.9 and 19.8 LMH at 20 C in

the absence and presence of 60 mg L1 of SA, respectively. In general,the water ux dropped by more than 14.9 and 30.2% compared with

baseline tests based on temperature variations in the uPRO mode. In

uPRO mode, more severe HA fouling were observed since the porous SL

is exposed to the foulant containing feed solutions, as shown in Fig. S6.

Thus, minimalux decline was observed in experimental runs of more

than 12 h in the FO mode in the presence of both types of organic foulant

NormalizedfluxJw

/Jw,0

(%)

60

70

80

90

100

FO mode

uPRO modeHumic acid

a

5 20 45

NormalizedfluxJw

/Jw,0

(%)

60

70

80

90

100

FO mode

uPRO modeSodium alginate

b

5 20 45

Fig. 2. Comparison of normalized wateruxfor (a) humicacid and (b) sodiumalginatein

theFO anduPROmodes with respectto thetemperature.Operating conditions: pH = 6.5;

ionic strength = 1 mM; and NaCl = 2 M.

Active layer

effm

Low Temperature

Support layer

Feed

Draw

effm

High Temperature

Support layer

Active layer

Draw

Feed

Fig. 3. A conceptualillustration forthe inuence oftemperature onthe uPRO membrane foulingand concentrationpolarizationprole (m: osmotic pressuredifferenceacross themembrane,

eff: effective osmotic pressure).




6/9

(HA and SA). The FO mode exhibited less fouling propensity to both

organic foulants than the uPRO mode. However, greater ux decline

was observed in the PRO mode in the presence of SA in the FS. The con-

centrative ICP (in the uPRO mode) exhibited higher ux and greater

ux decline in the porous membrane than the dilutive ICP (in the FO

mode), while the uPRO mode was not resistant to fouling on the mem-

brane surface. This behavior was attributed to two processes: (i) The ICP

inuence (SA fouling can lead to the enhanced ICP) [10]and (ii) the

foulant layer roughness induced by the membrane orientations (uPROorientation was more prone to fouling). In the uPRO mode, the ICP was

less severe, but other factors, including greater membrane surface

porosity and rougher structure, resulted in rapid fouling and, conse-

quently, greater ux decline. For example, more porous and rougher

membranes enabled greater poreblocking by foulants in the FS, leading

to foulantfoulant interactions and theinitiationof cake layer formation

on the SL. These processes then promoted enhanced ICP and a reduction

in the differential osmoticpressure across the membrane, thereby leading

to a ux decline in the uPRO mode. As shown in Fig. 2b, SA did not cause

an obvious water ux loss in the FO mode throughout the ltration pro-

cess. In contrast, greater ux reduction was observed after SA was intro-

duced during the uPRO mode at low temperatures. This behavior was

attributed that the kinetics of SA sol/gelling process was favored in the

uPRO mode at low temperatures (i.e., the SA gels set more with low tem-

perature) and the synergistic interactions between the higher solution

viscosity and denser cake layer formation (uPRO orientation was more

prone to SA fouling compared to the FO orientation) on the membrane

surface.

3.3. Effects of solution temperature on waterux behavior during the FO

process: The role of different types of draw solution

The FO water ux behavior in the presence of different DSs was sys-

tematically studied at different solution temperatures. FO-mode tests

were performed to evaluate the inuence of mono- (NaCl, KCl) and

divalent (CaCl2, Na2SO4) inorganic DS compounds in a simple matrix

with an AL-facing FS conguration for the optimal operating parameters

in FO desalination applications at different temperatures. In this cong-

uration, the permeate water ux behavior with NaCl, KCl, and CaCl2in

the FO membrane (average ux values of 8.43, 9.10, and 9.89 LMH,

respectively, at 20 C) was greater than that observed for Na2SO4DS

(average ux of 7.11 LMH at 20 C), as shown in Fig. 4. In the FO mode,

the osmotic driving force resulting from the concentration difference

across the membrane and the osmotic pressure (based on theoreticalcalculations) can be extremely high when using divalent inorganic com-

pounds in the DS instead of monovalent inorganic compounds; the

osmotic pressures of 1 M NaCl, KCl, CaCl2, and Na2SO4were 45.3, 44.8,

62.9, and 54.1 bar, respectively. Theoretically, divalent inorganic com-

pounds have great potential for providing high water recovery and

waterux. However, the water ux was signicantly reduced. In this

study, all of the DS solutes studied exhibited a similar inuence on the

water ux behavior based on their respective osmotic pressure differ-

ences; however, the DS containing Na2SO4exhibited the lowest water

ux behavior, even though this solute has the highest net bulk osmotic

pressure basedon osmoticpressure calculations. This apparent discrepan-

cy between observed and expected behavior was attributed to the differ-

ent diffusivities of the divalent DSs, which were inversely proportional to

their osmotic pressures. Low water ux was thus observed, since the ICP

effect is inversely proportional to the solute diffusivity, which was greater

in the presence of larger divalent ions. The mass transfer of these DSs in

FO membrane follows an inverse order of the membrane selectivity

(A/B) except for CaCl2 because the ICPinuence is inversely proportion-

al to thesolutemass transfer.Thus, greater dilutive ICP in the SL induced

a loss of osmotic pressure in DSs with CaCl2 and Na2SO4 throughout the

entire ltration process. TheICP increase observed in the presence of di-

valent DSs resulted in a net osmoticpressure decrease in the osmotical-

ly driven FO process. However, the absolute viscosity of both FS and DS

decreased with increasing temperature, which enhanced the diffusivity

of the water ux through the membrane AL and porous layer, thereby

Permeate volume (mL)

0 100 200 300 400 500

Waterflux(Lm-2h

-1)

0

5

10

15

20

25

1M Na2SO4DS, 20o

C1M Na

2SO

4DS, 30 oC

1M Na2SO

4DS, 45 oC


0 100 200 300 400 500

Waterflux(Lm

-2h

-1)

0

5

10

15

20

251M NaCl DS, 5 oC

1M NaCl DS, 20 oC

1M NaCl DS, 45 oC


0 100 200 300 400 500

Waterflux(Lm

-2h

-1)

0

5

10

15

20

251M KCl DS, 5 oC

1M KCl DS, 20 oC

1M KCl DS, 45 oC


0 100 200 300 400 500

Waterflux(Lm

-2h

-1)

0

5

10

15

20

25

1M CaCl2DS, 5 oC

1M CaCl2DS, 20 oC

1M CaCl2DS, 45 oC

a

c

b

d

Fig. 4. Permeate waterux behavior with various DSs of (a) NaCl, (b) KCl, (c) CaCl2, and (d) Na2SO4at different temperatures in the FO membrane. Operating conditions: pH = 6.5 and

ionic strength = 1 mM; and NaCl = 2 M.




7/9

inducing a positiveinuence on the waterux, as shown in Fig. 4. (AllDSs

exhibited increased water ux with increasing FS and DS temperatures.)

3.4. Relating water viscosity to FO membrane waterux

The water ux is an important factor in FO membrane desalination

applications. The average waterux and multiple water viscosity data

were compared. The permeate ux is principally dominated by temper-

ature; in this study, the experimental water

uxes were signi

cantly in-creased by increasing the solution temperature, as shown in Fig. S7.This

result was attributed to several factors: uid viscosity, concentration

polarization, water self-diffusivity, and the mass transfer coefcient. In

this study, we isolated the inuence of temperature variations on FS

viscosity and predicted the waterux trends for the most appropriate

correlations between FO membrane and FS properties. Thecontribution

of water viscosity was investigated at different temperatures to the

water ux behavior; for comparison, the baseline experiment was

based on the entire FO and uPRO processes (Fig. 5a). The water ux

(Jw) behavior across the FO membrane can be expressed by Darcy's

law (Eq.(1))[10]:

Jw P= Rtotal 1

where Pis the transmembrane pressure (Pa),is the absolute viscosity

of the permeate (kg m1 s1), andRtotalis the total resistance of the

membrane (m1). TheJwvalues as a function of the solution temper-

ature for both the FO and the uPRO process were positive, as shown in

Fig. 5a. Good agreement was observed between the experimental water

ux data and the solution viscosity inuence. TheJwvalues exhibited

generally constant trends with respect to the temperature variations; the

values ofJwwere 2.42 0.34, 3.99 0.16, 2.65 0.24, 3.03 0.33,

2.56 0.47, and 6.81 0.43 109 mPa for 1 M NaClFO, 2 M NaClFO,

1 M KClFO, 1 M CaCl2FO, 1 M Na2SO4FO, and 2 M NaCluPRO DSs,

respectively.

Solution viscosities were calculated at different temperatures, which

were proportional to the membrane inherent resistances that are

shown inFig. 5b. This phenomenon suggested that the waterux was

closely related to the solution viscosity, and that the solution viscosity is

a critical factor in the FO process at different temperatures. In addition,

the average water ux and water viscosity results suggested that the

membrane inherent resistance was not only greater in the FO modethan in the uPRO mode, but also increased more in the presence of diva-

lent solutes than in the presence of monovalent solutes, which is consis-

tent with the results discussed above. Therefore, in real FO applications,

the operating temperature must be optimized to maximize the water

ux andltration performance.

3.5. Inuence of solution temperature on reverse ux selectivity of the FO

process

We previously reported that both the type and concentration of the

salt inuence the ICP in narrow membrane pores and thus affect the

waterux and reverse soluteux performance. The reverse solute ux

typically increased linearly with increasing water ux, depending on

the membrane selectivity and the nature of the thermodynamic proper-

ties of the solutions. The RSFS is dened as the ratio of the water ux,

Jw, to the reversesolute ux,Js, in the FO process. We previously reported

that the RSFS is independent of the membrane SL properties and can

quantitatively describe the FO membrane performance according to

Eq.(2)[8,21]:

RSFS n A=B RT 2

wheren is number of dissolved species created by the draw solute, Tis

the absolute temperature of the solution (K), and Ris the ideal gas con-

stant (L bar K1 mol1).

As shown in Fig. 6a, the reverse soluteux increased as a function of

water ux regardless of different DSs. In the FO mode, increasing DS

temperature resulted in increasing water ux as a result of concentra-

tive ICP, which was induced by the increase in the osmotic drivingforce and reduction in the solution viscosity (Fig. 6b). Increasing the

DS temperature also resulted in an increase in the reverse solute ux,

which was caused by the elevated concentrative ICP of the draw solute

across the FO membrane SL.Fig. 6b also shows the dependence of the

membrane selectivity (the ratio of the water and solute permeability

coefcients deduced in the RO mode, as described in Fig. 1). Based on

the RSFS trends associated with theA/Bcoefcients in Eq.(2), the in-

crease in the RSFS was higher in the presence of divalent salts (CaCl2and Na2SO4) than in the presence of monovalent salts (NaCl and KCl).

This trend is inversely proportional to the solute permeability coef-

cients.The slopes of the RSFS curves as a functionof solution temperature

in the FO mode were positive. The RSFS was expected to vary slightly

with the temperature (as suggested by Eq.(2)), since it is representative

of the ratio of the reverse solute ux to the water ux. The reverse soluteux linearly increased with increasing water ux and induced lower

solution viscosities with increasing temperature. However, as shown in

Fig. 6b, increasing solution temperatures exerted greater inuence on in-

creasing water ux than salt ux. This relative increase in the water ux

was attributed to thefollowing phenomena: (i)The reductions in viscos-

ity with increasing temperature were mainly attributable to hydrophilic-

ity, which increased the wettability of the SL of the FO membrane; (ii)

during the FO process, less severe ICP decreased the resistivity to solute

diffusion within the SL, thereby enhancing the back diffusivity of the

draw solutes from the membrane AL and causing decreased permeation

of the draw solute towards the feed; and (iii) the temperature range

from 5 to 45 C in this study may have been sufciently wide to induce

changes in the membrane structural parameters; at higher temperatures,

a breakdown in thermally stable conditions may have led to the

Temperature (oC)

0 10 20 30 40 50

Jave

10

*

-9m

Pa)

0

2

4

6

8

10

1 M NaCl-FO

2 M NaCl-FO

1 M KCl-FO1 M CaCl

2-FO

1 M Na2SO

4-FO

2 M NaCl-PRO

a

Resistance(*1015

m-1)

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.61 M NaCl-FO2 M NaCl-FO

2 M NaCl-PRO

1 M KCl-FO

1 M CaCl2-FO

1 M Na2SO4-FO

b

Fig. 5. (a)Javevalue with respectto thetemperature and(b) membrane resistanceat dif-

ferent DSs in the FO and uPRO modes. Operating conditions: pH = 6.5 and ionic

strength = 1 mM.




8/9

improvement of water molecule diffusion kinetics rather than solutediffusion, thereby leading to a greater increase in the water ux com-

pared with the salt ux[25]. In summary, higher temperatures led to

an increase in the RSFS, not only via greater reduction of CP resistances

of FO membrane (ALas well as SL), but also via increasing thewettability

within the effective porosity of the SL.

4. Conclusions

The waterux and fouling propensity of several DSs (based on KCl,

NaCl, CaCl2, and Na2SO4) were evaluated to systematically investigate

the temperature inuence on available FO membranes in FO and

uPRO processes. The RSFS at different temperatures was also evaluated

to characterize the transport mechanisms. The membrane selectivity

(A/B) based on the SD model approach followed the order KCl NNaCl NCaCl2 NNa2SO4. The higher solute permeabilities of NaCl and

KCl were attributed to the smaller hydration radii of Na+ and Cl com-

pared to those of Ca2+ and SO42. The temperature signicantly inu-

enced the magnitude of the water ux in the FO mode, since increasing

temperature greatly reduced the ICP across the membrane, thereby

resulting in changes in the thermodynamic properties, such as osmotic

pressure, diffusion coefcient, andviscosity of the FSs and DSs. Signicant

ux reduction was observed after SA was introduced into the FS at low

temperatures. Theux reduction was attributed to the synergistic inter-

actions between the solution viscosity and the formation of a more

dense cake layer on the membrane surface. The osmotic pressures of

the divalent DSs were inversely proportional to their solute diffusivities,

since the inuence of ICP was greater for the larger, divalent ions. The

water ux was closely related to the solution viscosity and was a

dominant factor during FO processes at different temperatures. Possible

changes in the membrane structural parameters occurred as the temper-

ature ranged from 5 to 45 C; a breakdownin thermally stable conditions

and an improvement of water molecule diffusion kinetics rather than

solute diffusion resulted in an increase in the RSFS as a function of tem-

perature. These results have signicant implications for the performance

of FO applications at elevated temperatures. Operating temperatures

must be optimized to maximize the water ux.

Acknowledgments

This research was supported by a grant (code 15IFIP-B088091-02)

from Industrial Facilities & Infrastructure Research Program funded by

Ministry of Land, Infrastructure and Transport of Korean government.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.

doi.org/10.1016/j.desal.2015.06.012.

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64.

Ave. water flux (L m-2

h-1

)

0 5 10 15 20 25

Reversesolu

teflux(molem-2h

-1)

0.00

0.05

0.10

0.15

0.201 M NaCl

1 M KCl

1 M CaCl2

1 M Na2SO

4

2 M NaCl

a

Temperature (oC)

0 10 20 30 40 50

Reversefluxselectivity(Lmol-1)

50

100

150

200

600

700

800

900 1 M NaCl1 M KCl

1 M CaCl2

1 M Na2SO

4

2 M NaCl

b

Fig. 6.(a) Reverse soluteux as a function of average waterux and (b) reverseux se-

lectivity at varioustemperaturesin theFO mode with different DSs. Operatingconditions:

pH = 6.5 and ionic strength = 1 mM.


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Please cite this article as: J. Heo, et al., Organic fouling and reverse solute selectivity in forward osmosis: Role of working temperature andinorganic draw solutions Desalination (2015) http://dx doi org/10 1016/j desal 2015 06 012
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Desalination: Organic Fouling and Reverse solute selectivity in forward osmosis: Role of Working temperature and inorganic draw solutions