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Characterization of anion exchange membranes with natural organic matter (NOM) during electrodialysis
DESALINATION ELSEVIER
Desalination 151 (2002) 43-52 www.elsevier.com/Iocate/desal
Characterization of anion exchange membranes with natural organic matter (NOM) during electrodialysis Hong-Joo Lee, Do Hee Kim, Jaeweon Cho, Seung-Hyeon Moon* Department of Environmental Science and Engineering, Kwangju Institute of Science and Technology (K-JIST), 10ryong-dong, Buk-gu, Gwang]u 500-712, Korea Tel. +82 (62) 970-2435; Fax +82 (62) 970-2434; email." [email protected]
Received 16 January 2002; accepted 3 June 2002
Abstract
Natural organic matter (NOM) is thought to be a major source of fouling during membrane filtration of natural waters. The organic matter present in surface waters was characterized in terms of its molecular weight distribution, acidity and electrokinetic properties. The fouling potentials of anion exchange membranes were predicted by the characterization. Changes in the physicochemical properties of anion exchange membranes were also examined during electrodialysis (ED) process of solutions containing NOM. The ED performances were evaluated for the three anion exchange membranes (AMX, AM- 1 and ACM) in the presence of NOM. Fouling phenomena in terms of current efficiencyand NaC1 flux were in good agreement with the fouling potentials predicted by the characterization results. Observations of the molecular weight distribution and the constituents of NOM revealed that the hydrophobic NOM fraction with high molecular weights deposited mainly on the membrane surface, providing fouling effects on the anion exchange membrane. Keywords: Natural organic matter; Electrodialysis; Fouling; Characterization
1. Introduction
Naturally occurring dissolved organic substances are thought to be a major foulant during membrane filtration of natural waters. Natural organic matter (NOM) has a heterogeneous nature and contains aromatic and aliphatic macromolecules. Each fraction o f the hydrophobic, the transphilic and *Corresponding author.
the hydrophilic NOMs is characterized by molecular weights, charge densities and structures. Accordingly adsorption of the NOM is affected by the solution chemistry of natural waters and the pH value. In addition, the molecular weights and the constituents of the NOM particles are also found to strongly influence adsorption to the membrane surface [ 1]. Fouling phenomena caused by NOM in natural waters has raised interest in micro-
0011-9164/02/$- See front matter © 2002 Elsevier Science B.V. All rights reserved PII: S001 1 - 9 1 6 4 ( 0 2 ) 0 0 9 7 1 - 2
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H.-J. Lee et al. /Desalination 151 (2002) 43-52
filtration, ultrafiltration, nanofiltration and reverse osmosis processes [2]. Fouling is essentially caused by the deposition of foulants on the membrane surface, causing deterioration in the membrane performance through a decline in the flux and an increase in the resistance. Investigation of fouling in pressure-driven membrane processes revealed that adsorption of macromolecules from solution does contribute to membrane fouling as well as pore adsorption/plugging [3]. Recently treatment of surface water using ion exchange membranes has been increasing in water purification such as nitrate removal, demineralization and ultrapure water production [4,5]. Fouling of ion exchange membranes during electrodialysis (ED) occurs due to deposition of foulants on the membrane surface, which is dependent on the electrochemical interaction between the surface and the foulants [6]. Negatively charged organic matter moves toward anion exchange membranes under an electric field, and then they accumulate on the membrane surface. Fouling of ion exchange membranes leads to an unacceptably high resistante or power consumption during ED [7]. Sometimes, fouling causes decrease in the flux of ions and the current efficiency [8]. The extent and the nature of such fouling tendencies in ED are related to the electrochemical and physical properties of the membrane and foulants. Therefore, it is crucial to characterize the foulants and membranes for understanding the fouling phenomena [9,10]. In the previous study [11], it was found that anion exchange membranes had different fouling potentials, which were a determined characterization offoulant (humate) and anion exchange membranes. The fouling potential ofhumate on anion exchange membranes was investigated in ED experiments. Since NOM contains foulants other than humate, it is necessary to study the fouling phenomena with respect to the properties of NOM. In this study the properties of NOM and anion exchange membranes were characterized and the fouling potentials were predicted using the NOM collected in river water. In addition, the fouling phenomena of anion exchange membranes duringED ofsolu-
tions containing NOM were discussed according to the constituents and molecular weights.
2. Experimentals 2.1. Characterization o f natural organic matter
(NOM) NOM was concentrated from the Nakdong River, located in the southeast of Korea, using a reverse osmosis (Saehan, Korea) and was stored at 4°C. All of the samples were filtered with a 0.45 ~tm cellulose acetate membrane filter. The dissolved organic carbon (DOC) content of NOM was measured with a total organic carbon analyzer (DC-180, Dohrmann, USA). Prefiltered NOM solution was processed through XAD-8 and then XAD-4 resin columns to isolate the hydrophobic, transphilic, and hydrophilic fractions in order to further investigate the constituents ofNOM [ 12]. The organic matters adsorbed to XAD-8 and XAD-4 resins were eluted and collected using a 0.01 N NaOH solution, which corresponds to the hydrophobic and transphilic fractions, respectively. The hydrophilic NOM was obtained from an effluent from the XAD-8/4 resins. The molecular weight distribution of NOM was measured using a size-exclusion chromatography method with a liquid chromatograph (Waters 510, Waters, USA) with a Protein-Pak 125 column (Waters, USA) [13]. Standard solutions with various polystyrene sulfonates (250, 1,800, 4,600 and 8,000 Dalton) were used for the estimation of a molecular weight calibration. The zeta potential values of NOM as a function of pH were determined by electrophoretic mobilities using a commercial electrophoresis measurement apparatus (ELS-8000, Otsuka Electronics, Japan) in 0.01 M KCI [14]. Using an automatic titrator (702 SM Titrion, Metrom, Switzerland), the carboxylic and the phenolic acidities were measured for non-isolated NOM substances. After a sample of 50 mL was acidified to pH 3, a titrant (0.05 N NaOH) was added. The pH value was measured to obtain a titration curve and then carboxylic and phenolic
H.-J. Lee et al. / Desalination 151 (2002) 43-52
acidities were calculated and expressed as meq/g DOC of NOM [11]. 2.2. Characterization o f the anion exchange membranes
Commercial anion exchange membranes, NEOSEPTA ®AMX, AM- 1 and ACM (Tokuyama Corp., Japan), were used in this study. The characterized properties were ion exchange capacity, electrical resistance and the zeta potential. The virgin and fouled anion exchange membranes were presoaked in 0.1 N NaOH solution to exchange the counterion from CI- to OH- [15]. After the surface water drops were removed, the concentration of exchanged OH- was estimated by the amount of 0.01 N HCI consumed by the automatic titrator with an end-point of pH 7. Then, the exchange capacity was expressed as meq/g-dried membrane. Furthermore, the electrical resistance in 0.5 M NaC1 solution at 25°C was measured by an LCZ Meter, NF 2321 (NF Electronic Instruments, Japan) set at 100 KHz and expressed as f~cm2[8]. The streaming potential of the membrane surface was measured using a streaming potential cell, BI-EKA (Brookhaven Instruments Corp., USA), and then the zeta potentials were estimated using the Smoluchowski-Helmholtz equation [ 16,17]. The dependence of the zeta potential on the pH was observed for the anion exchange membranes. Consequently their isoelectric points were determined at the pH where the zeta potential approaches zero, giving a net surface charge of zero. For the streaming potential measurements, a KCI solution was used as a reference electrolyte. 2. 3. ED in the presence o f N O M with different cell set-ups
ED experiments were performed for three anion exchange membranes (AMX, AM-1 and ACM) to observe the fouling phenomena. The two-cell pair unit, consisting of an anion exchange membrane and a common cation exchange membrane (CMX) with an effective area of 100 cm 2 each, was assembled in a TS- 1 ED stack with flow-sheet
45
spacers (Tokuyama Corp., Japan). The initial diluate solution in each experiment was 5.0 L of 0.1 M NaCI containing 110 mg/L of non-isolated NOM as DOC at a solution pH of 5.5-7.0. As an initial concentrate solution, 5 L of 0.05 M NaC1 was used in the ED experiments. The flow rates of the diluate and concentrate were maintained at 0.2-0.3 L/min. For the electrode rinse solution, 800 mL of 3 wt.% Na2SO 4 was used. For the chemical cleaning of fouled membranes, base (1.0 wt.% NaOH) and deionized water were circulated sequentially for 20 min in the concentrate and diluate compazlrnents between the experiments. The ED performances for different cell configurations were mainly compared in terms of the NaCI removal rate in the diluate solutions and the cell resistance changes.
3. Results and discussion 3.1. Characterization o f N O M
The dissolved organic carbon (DOC) of the concentrated NOM was 550 mg/L. The molecular weights of the organic substances in NOM were in the range of 100-3,500 Dalton, having an average molecular weight of 1,570. It was found that the NOM fractions isolated with XAD-8/4 resins were 58% hydrophobic, 22% transphilic and 20% hydrophilic constituents. The average molecular weights of the hydrophobic, the transphilic and the hydrophilic NOM constituents were 1,840, 1,650 and 670, respectively. The molecular weight distributions of the hydrophobic NOM, transphilic, and hydrophilic NOMs are presented in Fig. 1. The hydrophobic and transphilic NOMs contain mostly hydrophobic and hydrophilic acids, respectively, as the representative charge property of NOM, while the hydrophilic NOM is mostly comprised of hydrophilic neutrals such as polysaccharides [ 18,19]. The fouling potentials during membrane processes are related to the interaction between the properties of the membrane surfaces and those of the foulants. Therefore, the fouling potential
46
H.-J. Lee et al. / Desalination 151 (2002) 43-52 0.4
10 ¸ pH
0,2-//
0.3
_~ if:
/
/ -10
/
-20
/~/
0.1
• N
-30 -40
0.0
II'
1000
2000 3000 4000 Molecularweight
5000
6000
-50
Fig. 1. Molecular weight distributions of isolated NOM in the feed solution.
Fig. 3. Zetapotentialsofthe hydrophobicandthe transphilic NOM as a functionofpH; electrolyte 10 mM NaCI.
of the membrane may be determined by an electrokinetic property, such as zeta potential [20]. The zeta potentials of the hydrophobic and the transphilic NOMs were estimated by measuring electrophoretic mobilities in 10 mM NaCI. Fig. 2 presents the effects of NOM concentration on the zeta potential of the hydrophobic and the transphilic NOMs. The zeta potentials decreased linearly with the increasing concentration at 10 mg/L NOM or lower, while the zeta potentials showed a constant value as -25 mV at concentrations higher than
10 mg/L. The zeta potential values as a function of the solution pH were observed at 22 mg/L of the hydrophobic NOM and 18 mg/L of the transphilic NOM, as shown in Fig. 3. The zeta potentials were negative under basic conditions, but became positive with the decreasing pH values. The zeta potentials approached nearly zero at a low pH value, showing the isoelectric point (IEP) at around pH 3.0. Since the pH values of the diluate solution throughout the experiments were varied within 5.5-7.0 in the presence of NOM, the organic matters have negative charges and may foul anion exchange membranes with positively charged functional groups. The acidity is related to the charge density and corresponding functional groups, indicating the electrostatic (charge) forces between foulants and the membrane surface. The total acidity of the non-isolated NOM was 9.1 meq/g DOC, consisting of 7.4 meq/g DOC (81.2%) ofcarboxylic acid and 1.7 meq/g DOC (18.8%) of phenolic acid. The higher content of the carboxylic acid implies that the organic matters have a somewhat high hydrophilicity. And, the low acidity level indicates that the organic matters present may not strongly bind to the functional groups in the anion exchange membrane, therefore, leading to mainly reversible fouling.
Concentrationof NOM (mg/L as DOC) 0 ..... 5
10
15
20
25
~" -10 -15 o -20 ~1 -25 -30 -35 •
Fig. 2. Zeta potentialof the hydrophobicand the transphilic NOM fractions as a functionof concentration;electrolyte 10 mM NaCI; pH of solution5.8--6.0.
H.-J. Lee et al. /Desalination 151 (2002) 43-52 3.2. Fouling characterization o f the anion exchange membranes Observation of properties for the virgin and fouled membranes is of importance since foulants cause to change the electrochemical and physical properties. In this study, anion exchange membranes were characterized in terms of their electric resistances, exchange capacities and zeta potentials, and the characterized values are listed in Table 1. Of the characterized properties, the electric resistances and exchange capacities of anion exchange membranes are thought to be the determining factors in predicting the performances of the ED processes. The electric resistances of all anion exchange membranes increased after the fouling experiments, implying that NOM did foul all the anion exchange membranes. The exchange capacity of the AMX membrane, which is reinforced for general concentration or desalination [21,22], decreased slightly after ED while the other two membranes increased. The reduced exchange capacity may be due to adsorption and chemical binding of the NOM to the membrane structure. The zeta potential indicates the electrical surface charges interacting with their surroundings, although this potential is somewhat different from an actual surface potential [23]. In this study, the zeta potentials were determined for the virgin and fouled membranes in 0.01 M KC1 solution. As seen in
47
Table 1, the zeta potentials of the AMX and ACM membranes became more positive after the fouling experiments, while that of the AM-1 membrane increased. It is suggested that coverage by the NOM reduced cationic functionality of an anion exchange membrane, thus resulting in the reduced zeta potential as well as the reduced exchange capacity [24-26]. The characterization o f anion exchange membranes revealed that the AMX membrane had the highest fouling potential among the examined membranes. Of the anion exchange membranes, the increased exchange capacity of the AM-1 membrane, even after being fouled, is related to its high water content. It is accepted that the greater water content decreases electric resistance and increases exchange capacity [5]. 3.3. Zeta p o t e n t i a l o f the anion exchange membranes as a function o f the solution p H The dependence of the zeta potential of the membranes on the solution pH provides information about the role of the acidic and basic strengths of the compounds adsorbed on the membrane surface. Also, it gives further understanding on specific adsorption of the ions on the membrane surface, depending on the surface properties of the polymer used in the formation of the membrane, the type and kind of electrolyte and the adsorption
Table 1 Characteristics of the virgin and the fouled membranes Membrane
Fouling status
AMX
Virgin Fouled Virgin Fouled Virgin Fouled
AM- 1 ACM "[221
Electric resistance, f~cm2 Ref' Meas. 2.5-3.5 2.6 3.0 1.3-2.0 1.4 1.9 4.0-5.0 5.1 5.5
Exchange capacity, meq/g-dried membrane Rel~ Meas. 1.5-1.8 3.3 -2.9 1.8-2.2 5.3 -5.5 1.~1.7 4.8 -5.1
Zeta potential, mV, meas., pH 6.045.3 -6.1 4.3 -2.8 -3.7 4.2 -3.5
48
H.-J. Lee et al. /Desalination 151 (2002) 43-52
behavior of ions [27-29]. It is well known that the adsorption tendency on the membrane surface depends on the properties of the membrane materials and status (such as virgin and fouled) [30,31]. Anion exchange membranes with tertiary or quarternary amine groups are positively charged over a wide pH range [21 ]. The specific adsorption of counterions on the anion exchange membrane surface would vary according to the solution pH. Under conditions where the pH of the bulk solution (KCI) is higher than the IEP, the concentration of the adsorbed anions, mainly CI-and additional OH-, increases near the shear plane, resulting in negative zeta potentials. The influence of adsorbed CI- on the surface decreased with the increasing concentration of proton in the bulk solution, thus resulting in positive zeta potentials when the pH of the bulk solution (KCI) is lower than the IEP [11,24]. Fig. 4 presents the pH dependence of the zeta potential in 0.01 M KCI, showing the isoelectric point (IEP) as 5.3-5.5 for the AMX membrane. It was found that the IEP of the fouled AMX membrane did not change in Fig. 4, implying that NOM did not cause severe fouling on the membrane surface. The result is probably due to a low charge density of NOM, thus occurring weak adsorptive forces between the organics and the membrane surface. When the pH of the solution was higher than the IEP, the similar zeta potential
E c:
0
,
,
,
pH
,
8. Virgin AMX --0~ Fouled AMX
~
~ ~
Fig. 4. pH effects on the zeta potentials of the virgin and fouled AMX membranes;electrolyte 10 mM KC1.
values were observed between the virgin and the fouled AMX membranes. At a lower pH value than IEP, however, the zeta potentials of the fouled membrane decreased (or became more negative), indicating that NOM may adsorb easier [24,26]. 3.4. Electrodialysis performances o f solutions containing N O M
The performances of ED with foulants were compared in terms of the conductivity removal rate in the diluate solutions and changes of the cell resistances. In Fig. 5, the three different anion exchange membranes showed the similar conductivity removal rates. Also, little difference in the resistances of the three anion exchange membranes was observed in Fig. 6. It is suggested that the NOM accumulated on the membrane surface, but loosely packed layer of NOM gave little effect on the eleetromigration of NaCl and the resistance [11]. However, loss in the performance during ED for membranes was seen due to NOM fouling, as shown in Table 2. For the fouling experiments in the presence of NOM, the AMX membrane showed the largest decrease in the current efficiency and the flux of NaCl. The most significant fouling for the AMX membrane is agreeable with the fouling potential characterized in Table 1. The ED performances of all the three membranes recovered after the membranes were cleaned with cleaning solutions (base and deionized water), indicating that the membranes were fouled reversibly. The amount of NOM transported into the concentrate solution in the cell of the AMXmembrane was shown in Fig. 7. The amount of NOM transported into the concentrate compartment increased until 180 min, and then remained at a nearly constant level by the end of the experiment. Fig. 8 shows the amount of organics deposited on the AMX membrane surface during ED in the presence of NOM, estimated from NOM remained in the compartments. While organic matters did not deposit for the initial operational time of 400 min, the deposited amount increased notably when
49
H.-J. Lee et al./ Desalination 151 (2002) 43-52 12-
6-
CD E
0 t~ ~,
AMX AM-1 ACM
4.
8 A&O0
~°°~12o O
2
8 2 200
400
600
200
800
Fig. 5. Conductivity removal rates in the diluate with the different cell set-ups; current density 6 mA/cm2; common cation exchange membrane CMX.
400
600
800
Time (min)
Time (min)
Fig. 6. Time course of cell resistances for the different cell set-ups; current density 6 mA/cm2; common cation exchange membrane CMX.
Table 2 Electrodialysis performances in terms of current efficiency and flux of NaCl Anion exchange membrane in the cell
Diluate solution, 5 L, in experiments
Current efficiency of NaC1, %
Flux of NaCI, mol/m 2 h
AMX
Desalting-NaC1a Fouling-NOM b Desalting-NaCl c Desalting-NaCl Fouling-NOM Desalting-NaCl Desalting-NaCl Fouling-NOM Desalting-NaC1
95 84 93 98 89 98 96 87 97
2.10 1.85 2.06 2.17 1.96 2.11 2.17 2.00 2.16
AM- 1
ACM
aDesalting-NaCl: Diluate solution containing 0.1 M NaCI, before fouling experiment bFouling-NOM: Diluate solution containing 0.1 M NaCI with 110 mg/L NOM CDesalting-NaCl: Diluate solution containing 0.1 M NaCI, after fouling experiment NaCI in diluate compartment was nearly depleted [9]. It seems that, at the early stage, the salt ions transported through the ion exchange membranes, and the organic matters o f N O M migrated at low electric mobilities, thus resulting in adsorption on the membrane surface. Table 3 presents the distribution of N O M after ED expressed as DOC concentration. The recovered N O M for the cell of each membrane was obtained
through analysis of the drained diluate and concentrate solutions at the end o f the experiments. The amount of reversible fouling was determined using the concentration o f organic matter in the cleaning solutions listed in the third row of Table 3, and that of irreversible fouling in the fourth row was estimated from the mass balance of NOM. The cell of the AMX membrane had the greatest tendency o f the irreversible fouling, which was
H.-J. Lee et al. / Desalination 151 (2002) 43-52
50 6.
0.20 -
A ..d
/
E 50.15 ¢: 4.
._C 3'
~'~ 0.10-
0
Q
.~ 2.
0.05 -
2 I.--
"---1--7,
0.00 ~. 200
400
600
200
800
400
600
800
Time (min)
Time (min)
Fig. 7. Time course for transportationof organic matterinto the concentrate; cell structure CMX and AMX; current density 6 mA/cm 2.
Fig. 8. Amount of NOM deposited according to time; cell structure CMX and AMX; current density 6 mA/cm 2.
Table 3 Distribution of organic matters after ED experiments
of 1,460, showing a little difference from the average molecular weight in the feed solution of 1,570, as presented in Fig. 9. The average molecular weight in the concentrate solution was 395, implying that primarily low molecular weight compounds permeated through the membranes. In the ED of the solution containing NOM in the cell comprising the CMX and AMX membranes, the amount of organics in the diluate and concentrate solutions were analyzed and the results are listed in Table 4. A larger percentage of hydrophilic NOM trans-
Cell configuration
AMX,
AM-l,
ACM,
%
%
%
90.9
91.8
92.5
Transported into the concentrate solution
3.3
5.1
3.6
Recovered by cleaning
3.3
3.2
3.0
Irreversible bound on membrane*
2.5
0.2
0.9
Recovered in the diluate solution
*Estimated from the mass balance
0.35 • 0.30
predicted from the characterizations o f the anion exchange membranes in Table 1. O f the anion exchange membranes, the least amount o f irreversible fouling was found in the cell of the AM-1 membrane, which might be due to the highest water content [5]. However, the difference in reversible fouling for each membrane was not
significant as indicated in the third row of Table 3. Comparing the molecular weight distributions of NOM in the diluate and concentrate compartments after ED, the organic matter present in the diluate solution had an average molecular weight
• /
Concentrate after electrodlalysis
~
0.25,
~ 0.15' O.lO, 0.05
'\\
\.~x~~
'\X\
1000
Diluate after electrodlalysis
\"~.
2000
3000
4000
5000
6000
7000
Molecular Weight
Fig. 9. Changes in the molecular weight distributions of non-isolated NOM in the diluate and concentrate solutions.
H.-J. Lee et al. / Desalination 151 (2002) 43-52
51
Table 4 NOM concentrationin the solutionsand depositedNOM on the surface Diluate, feed Non-isolated NOM Hydrophobic NOM Transphilic NOM Hydrophilic NOM
110.4 64.8 24.1 22.3
After ED experiment Diluate Concentrate,mg/L as DOC 100.4 2.7 58.2 0.6 22.6 0.4 19.7 1.7
Deposit*, mg/L as DOC
6.0 1.1 0.9
*Estimated from the mass balance
ported through the AMX membrane into the concentrate compartment than the hydrophobic and the transphilic NOM fractions. The high transport of the hydrophilic NOM is due to low molecular weight as seen in Fig. 1. Also, it was found that the hydrophobic NOM with high molecular weight (Fig. 1) showed the greatest portion of the total amount of deposited NOM on the membrane surface, indicating that it mainly fouled the anion exchange membrane. In the ED experiments of solution containing NOM, it was observed that the organic matter with low molecular weights penetrated through the anion exchange membranes, while the organics of high molecular weights fouled the membranes via deposition on the membrane surface [9,32]. In addition, it was founded that fouling of anion exchange membranes also showed the dependence on the constituents (the hydrophobic and the hydrophilic) of NOM. Therefore, it is suggested for the fouling mechanism of NOM that the hydrophilic organic matter, having a low molecular weight, transported through the anion exchange membranes, while the hydrophobic organic matter, with its high molecular weight and highly negative charge, accumulated on the surface of the anion exchange membranes. Fig. 10 illustrates the fouling mechanism of NOM related to molecular weights and the constituents, explaining that fouling mainly occurs due to the deposition of the hydrophobic organic matter on the anion exchange membranes during ED in the presence of NOM.
Transport through membrane (Hydrophilic NOM with low molecular weight)
i .... !!iili Anion exchange membrane Deposition on the surface (Hydrophobic NOM with high molecular weight)
Fig. 10. Suggestedfoulingmechanismin electrodialysisof solutioncontainingNOM withrespectto molecularweights and the constituents;here Cb is the foulant concentration profile.
4. Conclusions
In this study, fouling potentials were evaluated with the properties of NOM and anion exchange membranes and compared to ED experiments. Natural organic matters were characterized in terms of molecular weight distribution, acidity and electrokinetic property. The results implied that NOM contained a somewhat low potential to cause fouling of anion exchange membranes. Characterizations of the virgin and fouled anion exchange membranes showed that the AMX membrane, known as a reinforced membrane, exhibited the highest fouling tendency.
52
1-1.-,I.. Lee et al. / Desalination 151 (2002) 43-52
The ED experiments showed similar performances between the cells of three anion exchange membranes in terms of changes in the conductivity of the diluate and cell resistance. It was found that the AMX membrane cell showed the largest decrease in ED performances in terms of the current efficiency and the flux of NaC! among four cell configurations, which corresponds to the characterization results. Investigation of the NOM's behavior in fouling experiments revealed that fouling mainly depends on the hydrophobicity as well as the molecular weight distribution.
[ 13] [ 14]
[15] [16] [17]
Acknowledgements
[18]
This work was supported by grant No. (19991-307-005-3) from the Interdisciplinary Research Program of KOSEF (Korea Science and Engineering Foundation).
[19]
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Desalination 151 (2002) 43-52 www.elsevier.com/Iocate/desal
Characterization of anion exchange membranes with natural organic matter (NOM) during electrodialysis Hong-Joo Lee, Do Hee Kim, Jaeweon Cho, Seung-Hyeon Moon* Department of Environmental Science and Engineering, Kwangju Institute of Science and Technology (K-JIST), 10ryong-dong, Buk-gu, Gwang]u 500-712, Korea Tel. +82 (62) 970-2435; Fax +82 (62) 970-2434; email." [email protected]
Received 16 January 2002; accepted 3 June 2002
Abstract
Natural organic matter (NOM) is thought to be a major source of fouling during membrane filtration of natural waters. The organic matter present in surface waters was characterized in terms of its molecular weight distribution, acidity and electrokinetic properties. The fouling potentials of anion exchange membranes were predicted by the characterization. Changes in the physicochemical properties of anion exchange membranes were also examined during electrodialysis (ED) process of solutions containing NOM. The ED performances were evaluated for the three anion exchange membranes (AMX, AM- 1 and ACM) in the presence of NOM. Fouling phenomena in terms of current efficiencyand NaC1 flux were in good agreement with the fouling potentials predicted by the characterization results. Observations of the molecular weight distribution and the constituents of NOM revealed that the hydrophobic NOM fraction with high molecular weights deposited mainly on the membrane surface, providing fouling effects on the anion exchange membrane. Keywords: Natural organic matter; Electrodialysis; Fouling; Characterization
1. Introduction
Naturally occurring dissolved organic substances are thought to be a major foulant during membrane filtration of natural waters. Natural organic matter (NOM) has a heterogeneous nature and contains aromatic and aliphatic macromolecules. Each fraction o f the hydrophobic, the transphilic and *Corresponding author.
the hydrophilic NOMs is characterized by molecular weights, charge densities and structures. Accordingly adsorption of the NOM is affected by the solution chemistry of natural waters and the pH value. In addition, the molecular weights and the constituents of the NOM particles are also found to strongly influence adsorption to the membrane surface [ 1]. Fouling phenomena caused by NOM in natural waters has raised interest in micro-
0011-9164/02/$- See front matter © 2002 Elsevier Science B.V. All rights reserved PII: S001 1 - 9 1 6 4 ( 0 2 ) 0 0 9 7 1 - 2
44
H.-J. Lee et al. /Desalination 151 (2002) 43-52
filtration, ultrafiltration, nanofiltration and reverse osmosis processes [2]. Fouling is essentially caused by the deposition of foulants on the membrane surface, causing deterioration in the membrane performance through a decline in the flux and an increase in the resistance. Investigation of fouling in pressure-driven membrane processes revealed that adsorption of macromolecules from solution does contribute to membrane fouling as well as pore adsorption/plugging [3]. Recently treatment of surface water using ion exchange membranes has been increasing in water purification such as nitrate removal, demineralization and ultrapure water production [4,5]. Fouling of ion exchange membranes during electrodialysis (ED) occurs due to deposition of foulants on the membrane surface, which is dependent on the electrochemical interaction between the surface and the foulants [6]. Negatively charged organic matter moves toward anion exchange membranes under an electric field, and then they accumulate on the membrane surface. Fouling of ion exchange membranes leads to an unacceptably high resistante or power consumption during ED [7]. Sometimes, fouling causes decrease in the flux of ions and the current efficiency [8]. The extent and the nature of such fouling tendencies in ED are related to the electrochemical and physical properties of the membrane and foulants. Therefore, it is crucial to characterize the foulants and membranes for understanding the fouling phenomena [9,10]. In the previous study [11], it was found that anion exchange membranes had different fouling potentials, which were a determined characterization offoulant (humate) and anion exchange membranes. The fouling potential ofhumate on anion exchange membranes was investigated in ED experiments. Since NOM contains foulants other than humate, it is necessary to study the fouling phenomena with respect to the properties of NOM. In this study the properties of NOM and anion exchange membranes were characterized and the fouling potentials were predicted using the NOM collected in river water. In addition, the fouling phenomena of anion exchange membranes duringED ofsolu-
tions containing NOM were discussed according to the constituents and molecular weights.
2. Experimentals 2.1. Characterization o f natural organic matter
(NOM) NOM was concentrated from the Nakdong River, located in the southeast of Korea, using a reverse osmosis (Saehan, Korea) and was stored at 4°C. All of the samples were filtered with a 0.45 ~tm cellulose acetate membrane filter. The dissolved organic carbon (DOC) content of NOM was measured with a total organic carbon analyzer (DC-180, Dohrmann, USA). Prefiltered NOM solution was processed through XAD-8 and then XAD-4 resin columns to isolate the hydrophobic, transphilic, and hydrophilic fractions in order to further investigate the constituents ofNOM [ 12]. The organic matters adsorbed to XAD-8 and XAD-4 resins were eluted and collected using a 0.01 N NaOH solution, which corresponds to the hydrophobic and transphilic fractions, respectively. The hydrophilic NOM was obtained from an effluent from the XAD-8/4 resins. The molecular weight distribution of NOM was measured using a size-exclusion chromatography method with a liquid chromatograph (Waters 510, Waters, USA) with a Protein-Pak 125 column (Waters, USA) [13]. Standard solutions with various polystyrene sulfonates (250, 1,800, 4,600 and 8,000 Dalton) were used for the estimation of a molecular weight calibration. The zeta potential values of NOM as a function of pH were determined by electrophoretic mobilities using a commercial electrophoresis measurement apparatus (ELS-8000, Otsuka Electronics, Japan) in 0.01 M KCI [14]. Using an automatic titrator (702 SM Titrion, Metrom, Switzerland), the carboxylic and the phenolic acidities were measured for non-isolated NOM substances. After a sample of 50 mL was acidified to pH 3, a titrant (0.05 N NaOH) was added. The pH value was measured to obtain a titration curve and then carboxylic and phenolic
H.-J. Lee et al. / Desalination 151 (2002) 43-52
acidities were calculated and expressed as meq/g DOC of NOM [11]. 2.2. Characterization o f the anion exchange membranes
Commercial anion exchange membranes, NEOSEPTA ®AMX, AM- 1 and ACM (Tokuyama Corp., Japan), were used in this study. The characterized properties were ion exchange capacity, electrical resistance and the zeta potential. The virgin and fouled anion exchange membranes were presoaked in 0.1 N NaOH solution to exchange the counterion from CI- to OH- [15]. After the surface water drops were removed, the concentration of exchanged OH- was estimated by the amount of 0.01 N HCI consumed by the automatic titrator with an end-point of pH 7. Then, the exchange capacity was expressed as meq/g-dried membrane. Furthermore, the electrical resistance in 0.5 M NaC1 solution at 25°C was measured by an LCZ Meter, NF 2321 (NF Electronic Instruments, Japan) set at 100 KHz and expressed as f~cm2[8]. The streaming potential of the membrane surface was measured using a streaming potential cell, BI-EKA (Brookhaven Instruments Corp., USA), and then the zeta potentials were estimated using the Smoluchowski-Helmholtz equation [ 16,17]. The dependence of the zeta potential on the pH was observed for the anion exchange membranes. Consequently their isoelectric points were determined at the pH where the zeta potential approaches zero, giving a net surface charge of zero. For the streaming potential measurements, a KCI solution was used as a reference electrolyte. 2. 3. ED in the presence o f N O M with different cell set-ups
ED experiments were performed for three anion exchange membranes (AMX, AM-1 and ACM) to observe the fouling phenomena. The two-cell pair unit, consisting of an anion exchange membrane and a common cation exchange membrane (CMX) with an effective area of 100 cm 2 each, was assembled in a TS- 1 ED stack with flow-sheet
45
spacers (Tokuyama Corp., Japan). The initial diluate solution in each experiment was 5.0 L of 0.1 M NaCI containing 110 mg/L of non-isolated NOM as DOC at a solution pH of 5.5-7.0. As an initial concentrate solution, 5 L of 0.05 M NaC1 was used in the ED experiments. The flow rates of the diluate and concentrate were maintained at 0.2-0.3 L/min. For the electrode rinse solution, 800 mL of 3 wt.% Na2SO 4 was used. For the chemical cleaning of fouled membranes, base (1.0 wt.% NaOH) and deionized water were circulated sequentially for 20 min in the concentrate and diluate compazlrnents between the experiments. The ED performances for different cell configurations were mainly compared in terms of the NaCI removal rate in the diluate solutions and the cell resistance changes.
3. Results and discussion 3.1. Characterization o f N O M
The dissolved organic carbon (DOC) of the concentrated NOM was 550 mg/L. The molecular weights of the organic substances in NOM were in the range of 100-3,500 Dalton, having an average molecular weight of 1,570. It was found that the NOM fractions isolated with XAD-8/4 resins were 58% hydrophobic, 22% transphilic and 20% hydrophilic constituents. The average molecular weights of the hydrophobic, the transphilic and the hydrophilic NOM constituents were 1,840, 1,650 and 670, respectively. The molecular weight distributions of the hydrophobic NOM, transphilic, and hydrophilic NOMs are presented in Fig. 1. The hydrophobic and transphilic NOMs contain mostly hydrophobic and hydrophilic acids, respectively, as the representative charge property of NOM, while the hydrophilic NOM is mostly comprised of hydrophilic neutrals such as polysaccharides [ 18,19]. The fouling potentials during membrane processes are related to the interaction between the properties of the membrane surfaces and those of the foulants. Therefore, the fouling potential
46
H.-J. Lee et al. / Desalination 151 (2002) 43-52 0.4
10 ¸ pH
0,2-//
0.3
_~ if:
/
/ -10
/
-20
/~/
0.1
• N
-30 -40
0.0
II'
1000
2000 3000 4000 Molecularweight
5000
6000
-50
Fig. 1. Molecular weight distributions of isolated NOM in the feed solution.
Fig. 3. Zetapotentialsofthe hydrophobicandthe transphilic NOM as a functionofpH; electrolyte 10 mM NaCI.
of the membrane may be determined by an electrokinetic property, such as zeta potential [20]. The zeta potentials of the hydrophobic and the transphilic NOMs were estimated by measuring electrophoretic mobilities in 10 mM NaCI. Fig. 2 presents the effects of NOM concentration on the zeta potential of the hydrophobic and the transphilic NOMs. The zeta potentials decreased linearly with the increasing concentration at 10 mg/L NOM or lower, while the zeta potentials showed a constant value as -25 mV at concentrations higher than
10 mg/L. The zeta potential values as a function of the solution pH were observed at 22 mg/L of the hydrophobic NOM and 18 mg/L of the transphilic NOM, as shown in Fig. 3. The zeta potentials were negative under basic conditions, but became positive with the decreasing pH values. The zeta potentials approached nearly zero at a low pH value, showing the isoelectric point (IEP) at around pH 3.0. Since the pH values of the diluate solution throughout the experiments were varied within 5.5-7.0 in the presence of NOM, the organic matters have negative charges and may foul anion exchange membranes with positively charged functional groups. The acidity is related to the charge density and corresponding functional groups, indicating the electrostatic (charge) forces between foulants and the membrane surface. The total acidity of the non-isolated NOM was 9.1 meq/g DOC, consisting of 7.4 meq/g DOC (81.2%) ofcarboxylic acid and 1.7 meq/g DOC (18.8%) of phenolic acid. The higher content of the carboxylic acid implies that the organic matters have a somewhat high hydrophilicity. And, the low acidity level indicates that the organic matters present may not strongly bind to the functional groups in the anion exchange membrane, therefore, leading to mainly reversible fouling.
Concentrationof NOM (mg/L as DOC) 0 ..... 5
10
15
20
25
~" -10 -15 o -20 ~1 -25 -30 -35 •
Fig. 2. Zeta potentialof the hydrophobicand the transphilic NOM fractions as a functionof concentration;electrolyte 10 mM NaCI; pH of solution5.8--6.0.
H.-J. Lee et al. /Desalination 151 (2002) 43-52 3.2. Fouling characterization o f the anion exchange membranes Observation of properties for the virgin and fouled membranes is of importance since foulants cause to change the electrochemical and physical properties. In this study, anion exchange membranes were characterized in terms of their electric resistances, exchange capacities and zeta potentials, and the characterized values are listed in Table 1. Of the characterized properties, the electric resistances and exchange capacities of anion exchange membranes are thought to be the determining factors in predicting the performances of the ED processes. The electric resistances of all anion exchange membranes increased after the fouling experiments, implying that NOM did foul all the anion exchange membranes. The exchange capacity of the AMX membrane, which is reinforced for general concentration or desalination [21,22], decreased slightly after ED while the other two membranes increased. The reduced exchange capacity may be due to adsorption and chemical binding of the NOM to the membrane structure. The zeta potential indicates the electrical surface charges interacting with their surroundings, although this potential is somewhat different from an actual surface potential [23]. In this study, the zeta potentials were determined for the virgin and fouled membranes in 0.01 M KC1 solution. As seen in
47
Table 1, the zeta potentials of the AMX and ACM membranes became more positive after the fouling experiments, while that of the AM-1 membrane increased. It is suggested that coverage by the NOM reduced cationic functionality of an anion exchange membrane, thus resulting in the reduced zeta potential as well as the reduced exchange capacity [24-26]. The characterization o f anion exchange membranes revealed that the AMX membrane had the highest fouling potential among the examined membranes. Of the anion exchange membranes, the increased exchange capacity of the AM-1 membrane, even after being fouled, is related to its high water content. It is accepted that the greater water content decreases electric resistance and increases exchange capacity [5]. 3.3. Zeta p o t e n t i a l o f the anion exchange membranes as a function o f the solution p H The dependence of the zeta potential of the membranes on the solution pH provides information about the role of the acidic and basic strengths of the compounds adsorbed on the membrane surface. Also, it gives further understanding on specific adsorption of the ions on the membrane surface, depending on the surface properties of the polymer used in the formation of the membrane, the type and kind of electrolyte and the adsorption
Table 1 Characteristics of the virgin and the fouled membranes Membrane
Fouling status
AMX
Virgin Fouled Virgin Fouled Virgin Fouled
AM- 1 ACM "[221
Electric resistance, f~cm2 Ref' Meas. 2.5-3.5 2.6 3.0 1.3-2.0 1.4 1.9 4.0-5.0 5.1 5.5
Exchange capacity, meq/g-dried membrane Rel~ Meas. 1.5-1.8 3.3 -2.9 1.8-2.2 5.3 -5.5 1.~1.7 4.8 -5.1
Zeta potential, mV, meas., pH 6.045.3 -6.1 4.3 -2.8 -3.7 4.2 -3.5
48
H.-J. Lee et al. /Desalination 151 (2002) 43-52
behavior of ions [27-29]. It is well known that the adsorption tendency on the membrane surface depends on the properties of the membrane materials and status (such as virgin and fouled) [30,31]. Anion exchange membranes with tertiary or quarternary amine groups are positively charged over a wide pH range [21 ]. The specific adsorption of counterions on the anion exchange membrane surface would vary according to the solution pH. Under conditions where the pH of the bulk solution (KCI) is higher than the IEP, the concentration of the adsorbed anions, mainly CI-and additional OH-, increases near the shear plane, resulting in negative zeta potentials. The influence of adsorbed CI- on the surface decreased with the increasing concentration of proton in the bulk solution, thus resulting in positive zeta potentials when the pH of the bulk solution (KCI) is lower than the IEP [11,24]. Fig. 4 presents the pH dependence of the zeta potential in 0.01 M KCI, showing the isoelectric point (IEP) as 5.3-5.5 for the AMX membrane. It was found that the IEP of the fouled AMX membrane did not change in Fig. 4, implying that NOM did not cause severe fouling on the membrane surface. The result is probably due to a low charge density of NOM, thus occurring weak adsorptive forces between the organics and the membrane surface. When the pH of the solution was higher than the IEP, the similar zeta potential
E c:
0
,
,
,
pH
,
8. Virgin AMX --0~ Fouled AMX
~
~ ~
Fig. 4. pH effects on the zeta potentials of the virgin and fouled AMX membranes;electrolyte 10 mM KC1.
values were observed between the virgin and the fouled AMX membranes. At a lower pH value than IEP, however, the zeta potentials of the fouled membrane decreased (or became more negative), indicating that NOM may adsorb easier [24,26]. 3.4. Electrodialysis performances o f solutions containing N O M
The performances of ED with foulants were compared in terms of the conductivity removal rate in the diluate solutions and changes of the cell resistances. In Fig. 5, the three different anion exchange membranes showed the similar conductivity removal rates. Also, little difference in the resistances of the three anion exchange membranes was observed in Fig. 6. It is suggested that the NOM accumulated on the membrane surface, but loosely packed layer of NOM gave little effect on the eleetromigration of NaCl and the resistance [11]. However, loss in the performance during ED for membranes was seen due to NOM fouling, as shown in Table 2. For the fouling experiments in the presence of NOM, the AMX membrane showed the largest decrease in the current efficiency and the flux of NaCl. The most significant fouling for the AMX membrane is agreeable with the fouling potential characterized in Table 1. The ED performances of all the three membranes recovered after the membranes were cleaned with cleaning solutions (base and deionized water), indicating that the membranes were fouled reversibly. The amount of NOM transported into the concentrate solution in the cell of the AMXmembrane was shown in Fig. 7. The amount of NOM transported into the concentrate compartment increased until 180 min, and then remained at a nearly constant level by the end of the experiment. Fig. 8 shows the amount of organics deposited on the AMX membrane surface during ED in the presence of NOM, estimated from NOM remained in the compartments. While organic matters did not deposit for the initial operational time of 400 min, the deposited amount increased notably when
49
H.-J. Lee et al./ Desalination 151 (2002) 43-52 12-
6-
CD E
0 t~ ~,
AMX AM-1 ACM
4.
8 A&O0
~°°~12o O
2
8 2 200
400
600
200
800
Fig. 5. Conductivity removal rates in the diluate with the different cell set-ups; current density 6 mA/cm2; common cation exchange membrane CMX.
400
600
800
Time (min)
Time (min)
Fig. 6. Time course of cell resistances for the different cell set-ups; current density 6 mA/cm2; common cation exchange membrane CMX.
Table 2 Electrodialysis performances in terms of current efficiency and flux of NaCl Anion exchange membrane in the cell
Diluate solution, 5 L, in experiments
Current efficiency of NaC1, %
Flux of NaCI, mol/m 2 h
AMX
Desalting-NaC1a Fouling-NOM b Desalting-NaCl c Desalting-NaCl Fouling-NOM Desalting-NaCl Desalting-NaCl Fouling-NOM Desalting-NaC1
95 84 93 98 89 98 96 87 97
2.10 1.85 2.06 2.17 1.96 2.11 2.17 2.00 2.16
AM- 1
ACM
aDesalting-NaCl: Diluate solution containing 0.1 M NaCI, before fouling experiment bFouling-NOM: Diluate solution containing 0.1 M NaCI with 110 mg/L NOM CDesalting-NaCl: Diluate solution containing 0.1 M NaCI, after fouling experiment NaCI in diluate compartment was nearly depleted [9]. It seems that, at the early stage, the salt ions transported through the ion exchange membranes, and the organic matters o f N O M migrated at low electric mobilities, thus resulting in adsorption on the membrane surface. Table 3 presents the distribution of N O M after ED expressed as DOC concentration. The recovered N O M for the cell of each membrane was obtained
through analysis of the drained diluate and concentrate solutions at the end o f the experiments. The amount of reversible fouling was determined using the concentration o f organic matter in the cleaning solutions listed in the third row of Table 3, and that of irreversible fouling in the fourth row was estimated from the mass balance of NOM. The cell of the AMX membrane had the greatest tendency o f the irreversible fouling, which was
H.-J. Lee et al. / Desalination 151 (2002) 43-52
50 6.
0.20 -
A ..d
/
E 50.15 ¢: 4.
._C 3'
~'~ 0.10-
0
Q
.~ 2.
0.05 -
2 I.--
"---1--7,
0.00 ~. 200
400
600
200
800
400
600
800
Time (min)
Time (min)
Fig. 7. Time course for transportationof organic matterinto the concentrate; cell structure CMX and AMX; current density 6 mA/cm 2.
Fig. 8. Amount of NOM deposited according to time; cell structure CMX and AMX; current density 6 mA/cm 2.
Table 3 Distribution of organic matters after ED experiments
of 1,460, showing a little difference from the average molecular weight in the feed solution of 1,570, as presented in Fig. 9. The average molecular weight in the concentrate solution was 395, implying that primarily low molecular weight compounds permeated through the membranes. In the ED of the solution containing NOM in the cell comprising the CMX and AMX membranes, the amount of organics in the diluate and concentrate solutions were analyzed and the results are listed in Table 4. A larger percentage of hydrophilic NOM trans-
Cell configuration
AMX,
AM-l,
ACM,
%
%
%
90.9
91.8
92.5
Transported into the concentrate solution
3.3
5.1
3.6
Recovered by cleaning
3.3
3.2
3.0
Irreversible bound on membrane*
2.5
0.2
0.9
Recovered in the diluate solution
*Estimated from the mass balance
0.35 • 0.30
predicted from the characterizations o f the anion exchange membranes in Table 1. O f the anion exchange membranes, the least amount o f irreversible fouling was found in the cell of the AM-1 membrane, which might be due to the highest water content [5]. However, the difference in reversible fouling for each membrane was not
significant as indicated in the third row of Table 3. Comparing the molecular weight distributions of NOM in the diluate and concentrate compartments after ED, the organic matter present in the diluate solution had an average molecular weight
• /
Concentrate after electrodlalysis
~
0.25,
~ 0.15' O.lO, 0.05
'\\
\.~x~~
'\X\
1000
Diluate after electrodlalysis
\"~.
2000
3000
4000
5000
6000
7000
Molecular Weight
Fig. 9. Changes in the molecular weight distributions of non-isolated NOM in the diluate and concentrate solutions.
H.-J. Lee et al. / Desalination 151 (2002) 43-52
51
Table 4 NOM concentrationin the solutionsand depositedNOM on the surface Diluate, feed Non-isolated NOM Hydrophobic NOM Transphilic NOM Hydrophilic NOM
110.4 64.8 24.1 22.3
After ED experiment Diluate Concentrate,mg/L as DOC 100.4 2.7 58.2 0.6 22.6 0.4 19.7 1.7
Deposit*, mg/L as DOC
6.0 1.1 0.9
*Estimated from the mass balance
ported through the AMX membrane into the concentrate compartment than the hydrophobic and the transphilic NOM fractions. The high transport of the hydrophilic NOM is due to low molecular weight as seen in Fig. 1. Also, it was found that the hydrophobic NOM with high molecular weight (Fig. 1) showed the greatest portion of the total amount of deposited NOM on the membrane surface, indicating that it mainly fouled the anion exchange membrane. In the ED experiments of solution containing NOM, it was observed that the organic matter with low molecular weights penetrated through the anion exchange membranes, while the organics of high molecular weights fouled the membranes via deposition on the membrane surface [9,32]. In addition, it was founded that fouling of anion exchange membranes also showed the dependence on the constituents (the hydrophobic and the hydrophilic) of NOM. Therefore, it is suggested for the fouling mechanism of NOM that the hydrophilic organic matter, having a low molecular weight, transported through the anion exchange membranes, while the hydrophobic organic matter, with its high molecular weight and highly negative charge, accumulated on the surface of the anion exchange membranes. Fig. 10 illustrates the fouling mechanism of NOM related to molecular weights and the constituents, explaining that fouling mainly occurs due to the deposition of the hydrophobic organic matter on the anion exchange membranes during ED in the presence of NOM.
Transport through membrane (Hydrophilic NOM with low molecular weight)
i .... !!iili Anion exchange membrane Deposition on the surface (Hydrophobic NOM with high molecular weight)
Fig. 10. Suggestedfoulingmechanismin electrodialysisof solutioncontainingNOM withrespectto molecularweights and the constituents;here Cb is the foulant concentration profile.
4. Conclusions
In this study, fouling potentials were evaluated with the properties of NOM and anion exchange membranes and compared to ED experiments. Natural organic matters were characterized in terms of molecular weight distribution, acidity and electrokinetic property. The results implied that NOM contained a somewhat low potential to cause fouling of anion exchange membranes. Characterizations of the virgin and fouled anion exchange membranes showed that the AMX membrane, known as a reinforced membrane, exhibited the highest fouling tendency.
52
1-1.-,I.. Lee et al. / Desalination 151 (2002) 43-52
The ED experiments showed similar performances between the cells of three anion exchange membranes in terms of changes in the conductivity of the diluate and cell resistance. It was found that the AMX membrane cell showed the largest decrease in ED performances in terms of the current efficiency and the flux of NaC! among four cell configurations, which corresponds to the characterization results. Investigation of the NOM's behavior in fouling experiments revealed that fouling mainly depends on the hydrophobicity as well as the molecular weight distribution.
[ 13] [ 14]
[15] [16] [17]
Acknowledgements
[18]
This work was supported by grant No. (19991-307-005-3) from the Interdisciplinary Research Program of KOSEF (Korea Science and Engineering Foundation).
[19]
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