[membrane science and technology] ion exchange membranes - fundamentals and applications volume 12...

Membrane Deterioration

Chapter 14

14.1. MEMBRANE PROPERTY CHANGE WITH ELAPSED TIME

14.1.1 Membrane Characteristic Stability Against Various Agents

The economic viability of electrodialysis plants depends on the life timeof the membrane. In order to make the stability of the membrane clear, Kneifeland Hattenbach (1980) evaluated the membrane property change with elapsedtime under several chemical environments. The ion exchange membranes werecommercially produced mainly by companies in Japan and USA as follows:

DO

(1)

I: 10.

Tokuyama Soda Co. Ltd. (Japan): anion exchange membrane AF-4T/AFS-4T/AV-4T/AC-158 and cation exchange membrane C66-5T/CH-45T/CH-2T/CL-25T.

(2)
Asahi Glass Co. Ltd. (Japan): anion exchange membrane AMV/ASV andcation exchange membrane CMV.
(3)
Asahi Chemical Ind. Co. Ltd. (Japan): anion exchange membrane A-111and cation exchange membrane K-101.
(4)
Ionac Chemical Company (USA): anion exchange membrane MA 3148/MA 3475 R/IM 12 and cation exchange membrane MC 3470.
(5)
Ionics Inc. (USA): anion exchange membrane 111BZL183/103PZL183/103QZL183 and cation exchange membrane 61 AZL 183/61 CZL 183.
(6)
Forschungsinstitut Berghof GmbH (Israeli): anion exchange membranePEN 4/NPES A3 and cation exchange membrane NPES C2.
(7)
American Machine and Foundry (USA): cation exchange membraneAMF-C100/AMF-C311.
(8)
Rhone-Poulenc Chemie GmbH (Germany): anion exchange membraneARP and cation exchange membrane CRP.
The properties to be found in the literature and reference sheets of themembrane producers are not comparable with each other since measurementsare often conducted under different methods and conditions. To obtain com-parable values, the values of exchange capacity, water content, membrane re-sistance, thickness and permselectivity were determined and compiled underuniform conditions in Table 14.1 for the cation exchange membranes and inTable 14.2 for the anion exchange membranes. Here, the permselectivity P isgiven by P ¼ ðt tÞ=ð1 tÞ; where t and t are transport number of counter-ionsin the membrane and the solution, respectively, and t was measured from mem-brane potentials. The results indicated in the tables are average values from tests

1016/S0927-5193(07)12014-3

dx.doi.org/10.1016/S0927-5193(07)12014-3.3d

Table 14.1 Properties of cation exchange membranes

Membrane ExchangeCapacity(eq kg1)

WaterContent(%)

ElectricResistance(1N NaCl,251C, O cm2)

Thickness(mm)

Permselectivity Transport Number

1.0/0.5NKCl

0.1/0.05NKCl

1.0/0.5NKCl

0.1/0.05N KCl

MC 3470 1.5 35 6–10 0.6 0.68 0.94 0.83 0.97MC 3142 1.3 25 20–30 0.22 0.65 0.93 0.82 0.97CMV 2.4 25 2.9 0.15 0.95 0.98 0.98 0.99K 101 1.4 24 2.1 0.24 0.91 0.98 0.95 0.99CL-25T 2.0 31 2.9 0.18 0.81 0.97 0.90 0.98C66-5T 3.3 34 1.5 0.16 0.89 0.97 0.94 0.98CH-45T 2.3 27 2.1 0.17 0.92 0.96CH-2T 2.4 32 2.2 0.17 0.86 0.97 0.93 0.9861 AZL 183 2.0 40 4.5 0.6 0.62 0.93 0.81 0.9761 CZL 183 2.6 34 7.2 0.7 0.81 0.96 0.95 0.98AMF-C311 0.6 16 0.3 0.84 0.98 0.92 0.99AMF-C100 1.1 22 4.8 0.22 0.77 0.98 0.88 0.99NPES C/2 1.9 15–20 3–7 0.4 0.91 0.96CRP 2.6 40 6.3 0.6 0.65 0.82

Source: Kneifel and Hattenbach, 1980.

IonExchangeMem

branes:

FundamentalsandApplica

tions

294

Table 14.2 Properties of anion exchange membranes

Membrane ExchangeCapacity(eq kg1)

WaterContent(%)

ElectricResistance(1N NaCl,

251C, O cm2)

Thickness(mm)

Permselectivity Transport Number

1.0/0.5NKCl

0.1/0.05NKCl

1.0/0.5NKCl

0.1/0.05NKCl

MA 3148 0.8 18 12–70 0.20 0.85 0.98 0.92 0.99MA 3475 R 1.4 31 5–13 0.6 0.70 0.94 0.85 0.97IM 12 0.5 22 8 0.18 0.4 0.70 0.71 0.85AMV 1.9 19 2–4.5 0.14 0.92 0.99 0.96 0.99ASV 2.1 24 2.1 0.15 0.91 0.99 0.95 0.99A 111 1.2 31 2–3 0.21 0.45 0.91 0.73 0.95AF-4T 2.0 24 1.7 0.16 0.90 0.98 0.96 0.99AFS-4T 1.8 25 3.0 0.19 0.95 1.00 0.97 1.00AV-4T 1.4 24 2.4 0.15 0.90 0.98 0.95 0.99AVS-4T 1.5 20 5.1 0.17 0.93 0.99 0.97 1.00AC-158 2.5 0.82 0.98 0.91 0.99111 BZL 183 0.5 0.52 0.95 0.77 0.97103 PZL 183 1.2 38 4.9 0.6 0.43 0.90 0.72 0.95103 QZL 219 1.5 30 8.0 0.7 0.70 0.85 0.95 0.98PEN 4 0.1–0.3 31 3–12 0.3 0.5–0.7 0.77NPES A/3 0.2–0.7 27 3–12 0.4 0.4–0.8 0.6–0.9ARP 1.8 34 6.9 0.5 0.79 0.90

Source: Kneifel and Hattenbach, 1980.

Mem

braneDeterio

ratio

n295

Ion Exchange Membranes: Fundamentals and Applications296

of membranes from various membrane deliveries and hence possibly of differentproduction charges.

In this experiment, the membrane samples were subjected to the variousagents at room and elevated temperatures for periods up to five years in thelaboratory, and the membrane characteristics such as electric resistance, perm-selectivity and ion exchange capacity were measured during the test. Fig. 14.1shows the electric resistance for six membrane types after having been subjected

0

0.5

1.0

Xt/X

0

100 500d

AF 4T

1000 15000

0.5

1.0

Xt/X

0

500d

MA 3148

1000 15000

0.5

1.0

Xt/X

0

500d

a)

AMV

1000 1500

0

0.5

1.0

Xt/X

0

500d

CMV

1000 1500

0

0.5

1.0

Xt/X

0

500d

MC 3142

0

0.5

1.0

Xt/X

0

100 500

t

d

C 66 5T

Figure 14.1 Membrane resistance after exposure to various agents at room temperature(Kneifel and Hattenbach, 1980): (K) distilled water, (’) 0.2N NaCl, (J) 0.1N NaCl, (D)0.1N HNO3 and (m) 0.1N NaOH. ‘a)’ indicates membrane destruction.

Membrane Deterioration 297

to different agents at room temperature. The strongest variations for mostmembranes are caused by the effect of a NaOH solution, especially appliedto the heterogeneous membranes MA 3148 and MC 3142 (strong decrease inthe resistance values) and the anion exchange membranes AF-4T (strong in-crease in the resistance values) and AMV (crumbled). Good stability is ex-hibited by the membrane C66-5T. The effect of the oxidizing solution onthe electric resistance at room temperature is shown in Fig. 14.2, indicatingparticularly strong decrease in the resistance value of the membrane type CMV.All membranes with the exception of MC 3470 and MA 3475 R were destroyedat 851C.

The dependence on duration test in 1N NaCl at 851C of resistance isshown in Fig. 14.3, of permselectivity in Fig. 14.4 and of exchange capacity inFig. 14.5. The strong increase in resistance of the membrane types AMV, AV-4T, CMV, AF-4T and CH-45T is striking. The membrane types 103 QZL 219and 61 CZL 183 showed no significant changes. Variations in the permselectivi-ties in trial of up to 100 days were slight with the exception of membranetype MA 3148. Longer duration however showed a marked decrease in valuesin the cases of membrane types AFS-4T, C66-5T, AF-4T, MA 3148, CRP

1.0

0

MC 3470

MA 3475 R C66 - 5T

AF - 4T

AMV

CMV

xt/x0

85 °C

25 °C

K 101

61CZL 183

a)a)

a)

a)

1.0

0

10-2 10-1 100 103102101

d

t

Figure 14.2 Membrane resistance after exposure to an oxidizing agent (Kneifel andHattenbach, 1980). 0.1N K2CrO4+1N HCl+1N NaCl. ‘a)’ indicates membrane de-struction.

x t/x

0x t

/x0

61CZL 183

AV - 4T MA 3148

AMV CMV

K101

2.0

1.0

0

1.0

0

10-1 102 103100 10110-2

d

10-1 102 103100 10110-2

x t/x

0

1.0

0

MC 3470

MA 3475R 103 QZL 219

MC 3142 AF - 4T

C66 - 5T

CH - 45T CRP

ARP1M - 12

d

t

Figure 14.3 Membrane resistance after exposure to 1N NaCl at 851C (Kneifel andHattenbach, 1980).


and ARP. The samples of types AF-4T and MA 3148 crumbled and type CMVshrank strongly. The values of exchange capacity, with one exception (IM 12)changed slightly for trial times up to 100 days. In longer trials the exchangecapacities – with the exception of K 101 – were reduced to less than 60% of theinitial values.

Most experimental data at room temperature and 851C were obtained formembrane types AMV, CMV, AF-4T, C66-5T and K 101. Results for thesemembranes in long-term exposure tests in salt solution at room temperatureindicated good stability. At 851C the membranes AMV, CMV and AF-4Tshowed low, type C66-5T fair and K 101 good stability, respectively.

**

**

Xt/X

0X

t/X0

1.0

0.5

1.0

0.5

10-2 10-1 100 101 102 103

d

10-2 10-1 100 101 102 103

d

103 QZL 219

61CZL 183

MC 3142

MA 3475R

CMV

AMVMC 3470

K 101

AF - 4TC66 - 5T

MA 3148

AFS - 4T

a)

a)

a)

Figure 14.4 Permselectivity after exposure to 1N NaCl at 851C (Kneifel and Hattenbach,1980). ‘a)’ indicates membrane destruction.

100 101 102 103

d

AMV

IM 12

AFS - 4T

AF - 4T0.2

1.0

Xt/X

0X

t/X0

0.2

1.0

K101

CMV

C66 - 5T

a)

t

Figure 14.5 Exchange capacity after exposure to 1N NaCl at 851C (Kneifel andHattenbach, 1980).


14.1.2 Performance Change of Ion Exchange Membranes in Long-Term

Seawater Electrodialysis

Long-term seawater electrodialysis was carried out using a small-scaleelectrodialyzer (effective membrane area: 2 dm2) integrated with commercially


available homogeneous ion exchange membranes and heterogeneous mem-branes produced in the laboratory (Hanzawa et al., 1966). The types of themembranes (cation exchange membrane/anion exchange membrane) applied inthis experiment were:

(1)
Tokuyama Soda Co.: Neocepta CL-25T (St DVB PVC)/AVS-4T(St DVB VP PVC), including PVC fiber reinforcement.
(2)
Asahi Chemical Ind. Co.: Aciplex CK-2 (St DVB)/CA-3 (St DVB), in-cluding no reinforcement.
(3)
Asahi Glass Co.: Selemion CMV (St BD)/AST (St BD), including glassfiber reinforcement.
(4)
Japan Monopoly Corp.: Amberlite XE-69 PVC/Amberlite XE-119 PVC;heterogeneous membranes produced from strong acidic cation exchangeresin Amberlite XE-69 and strong basic anion exchange resin XE-119,with PVC powder to form sheets including no reinforcement.
The membranes described above are produced from the following mate-rials: St, styrene; DVB, divinylbenzene; PVC, polyvinyl chloride; VP, vinylpyri-dine; BD, butadiene.

Sand filtered seawater was supplied into the electrodialyzer and electrodi-alyzed at current density of 1A dm2 for periods of up to four years. Electro-lyte concentration in a concentrating cell C00 (Baume’s solution density Be0),volume flux across a membrane pair q, current efficiency Z and feeding seawatertemperature T are plotted against running time t. The plots are shown in Fig. 14.6(Neocepta CL-25T/AVS-4T), Fig. 14.7 (Aciplex CK-2/CA-3), Fig. 14.8 (SelemionCMV/AST) and Fig. 14.9 (Amberlite XE-69 PVC/XE-119 PVC), showingperiodical changes of C00 and q caused by the seasoning change of seawatertemperature T. After t ¼ 25,000 h, chlorine gas was unfortunately contaminatedinto the feeding seawater. Due to this trouble, Z is recognized to be decreasedduring this period. Except the data affected by this trouble, the performance ofthe homogeneous membranes (Figs. 14.6–14.8) was quite stable. However, C00, qand Z of heterogeneous membrane (Fig. 14.9) are decreased with time. In parallelwith the experiment mentioned above, long-term seawater electrodialysis wasperformed setting the current density at 2–4A dm2. However, the performanceand durability were not evaluated because the membranes were destroyed dueto the precipitation of CaCO3 and CaSO4 on the membrane surfaces.

14.2. SURFACE FOULING

14.2.1 Mechanism of Surface Fouling

Surface fouling is usually caused by deposits of macromolecules or col-loidal matter from the feed solution. Grossman and Sonin (1973) discussed the

t (h)0 5000 10000 15000 20000 25000 30000

15°C20°C

25°C

25°C20°C15°C

21

20

19

18

17

18

16

14

90

80

70

30

20

10

T (

°C)

(%

)q

(ml/h

)C

'' (°

Be'

)

t

Figure 14.6 Seawater electrodialysis test for a long term (Neocepta CL-25T/AVS-4T)(Hanzawa et al., 1966).


mechanism of the surface fouling as follows by assuming that the membranesare covered with thin surface films of the deposited material.

Fig. 14.10 shows the salt concentration distribution in a desalting and aconcentrating cell (thickness: a). The linear concentration distribution in thediffusion layers is an approximation which results from the Nernst diffusionmodel for the convective–diffusion process. In the Nernst diffusion model, the saltions are assumed to arrive at the membrane surface by diffusion across hypo-thetical diffusion layers of thickness d. Using Eqs. (11.1) and (11.2), one obtainsthe salt concentration C0 at the desalting surface of the membrane as follows:

C0 ¼ C t t

FDid (14.1)

where i is current density, C the salt concentration in the desalting cell outside thediffusion layer,D the salt diffusion coefficient in the solution, t and t the transportnumbers of counter-ions in the membrane and the solution, respectively, and F

t (h)0 5000 10000 15000 20000 25000 30000

15°C20°C

25°C

25°C20°C15°C

14

13

12

11

30

28

26

24

90

80

70

30

20

10

T (

°C)

(%

)q

(ml/h

)C

'' (°

Be'

)t

Figure 14.7 Seawater electrodialysis test for a long term (Aciplex CK-2/CA-3) (Ha-nzawa et al., 1966).


the Faraday constant. Here, it should be noticed that the phenomenologicalmeaning of d is exhibited by the salt concentration gradient at the membrane/solution interface (x ¼ 0) as follows (cf. Section 11.1, Eq. (11.2)):

d ¼C C0

ðdC=dxÞx¼0

(14.2)

The limiting current density ilim is introduced by substituting C0 ¼ 0 into Eq.(14.1) as follows, which was already expressed in Eq. (11.4):

ilim ¼FDC

ðt tÞd(14.3)

When a surface film is present on the membrane as shown in Fig. 14.11, thematerial balance in the diffusion layer is expressed based on the Nernst diffusionmodel introduced from Eq. (14.1) as

C C ¼t t

FDid (14.4)

15°C20°C

25°C

25°C20°C15°C

14

13

12

11

30

28

26

24

90

80

70

30

20

10

t (h)0 5000 10000 15000 20000 25000 30000

T (

°C)

(%

)q

(ml/h

)C

'' (°

Be'

)t

Figure 14.8 Seawater electrodialysis test for a long term (Selemion CMV/AST) (Ha-nzawa et al., 1966).


The material balance in the film is expressed by

C C0 ¼t t

FDfiD (14.5)

where C is salt concentration at the interface between the diffusion layer and thefilm, D the film thickness, Df the salt diffusion coefficient in the film and t thetransport number in the film. In Eq. (14.4), materials composing the surface filmare assumed to be uncharged (neutral), so t in the film (in Eq. (14.5)) is approxi-mated by t in the solution (in Eq. (14.4)). At limiting current density, substitutingi ¼ ilim and C0 ¼ 0 into Eqs. (14.4) and (14.5) leads to

C C ¼t t

FDilimd (14.6)

C ¼t t

FDilimD (14.7)

15°C

20°C25°C

25°C20°C

15°C

C''

(°B

e' )q

(ml/h

)(%

)

15

14

13

12

11

22

20

18

70

60

50

30

20

10

t (h)0 5000 10000 15000 20000 25000 30000

T (

°C)

t

Figure 14.9 Seawater electrodialysis test for a long term (Amberlite XE-69 PVC/XE-119 PVC) (Hanzawa et al., 1966).


Canceling C from Eqs. (14.6) and (14.7), the limiting current density of themembrane having the film is introduced as follows:

ilim ¼FDC

ðt tÞd1

1þ fp

(14.8)

Here, fp is the fouling parameter defined by Eq. (14.9), which is dimensionless andindicates the effect of the film on the limiting current density.

fp ¼DDDfd

(14.9)

It is clear from Eqs. (14.8) and (14.9) that a fouling film depresses the limitingcurrent density only if the quantity fp is comparable with unity or larger, thatis, when the film thickness is comparable with the Nernst diffusion layer thick-ness and/or the film diffusion coefficient is small compared with the diffusion

a

i

C'

x x

Concentratingcell

Anion exchange membrane

Desaltingcell

Cation exchange membrane

Concentratingcell

C0 C0

Figure 14.10 Salt concentration distribution in electrodialysis channels according toNernst diffusion model (Grossman and Sonin, 1973).


coefficient in the solution. fp represents the ratio of the ohmic resistance ofthe film to the ohmic resistance of the fluid within the Nernst diffusion layer.Since in many practical cases the diffusion layer is quite thin (cf. Table 11.1)compared with the thickness of the desalting cell (a in Fig. 14.10), fp can beof order unity even if the ohmic resistance of the fouling layer is quite smallcompared with that of the desalting cell. In other words, the film may bringabout a significant reduction in the limiting current density, and yet contributenegligibly to the ohmic resistance of the system as a whole. However, if Dis comparable with the thickness of the desalting cell, or if Df is very small, afilm capable of depressing the limiting current would also increase the resistanceof the system.

14.2.2 Formation of Films on the Membrane Surface

Surface fouling discussed above is caused by deposition of suspendedmatters in a feeding solution on the membrane surfaces to form films. Smallparticles suspended in a feeding solution are usually removed using sand filtra-tion, membrane filtration, coagulation sedimentation filtration, etc. However,

Surface film

i

C'

Desalting cell

Diffusionlayer

Cationexchangemembrane

Diffusion layer

Concentratingcell

C0

C*

Figure 14.11 Salt concentration distribution with surface films on a cation exchangemembrane (Grossman and Sonin, 1973).


extremely small particles pass through the filter, enter into an electrodialyzerand deposit on the membranes. Constituents of the substances attached to themembrane surface are shown in Table 8.8. Attention should be given to the factthat microorganisms passing through the filter enter into an electrodialyzer andbreed at the membrane surface. Ohwada et al. (1981) measured viable bacteriacount, chlorophyll a and organic components in the substances attached to themembranes integrated into the electrodialyzer operating in salt manufacturingplants (Table 14.3).

Table 14.3 Deposition of substances on ion exchange membranes and microorganismsin the deposit (Ohwada et al., 1981).


14.2.3 Removal of Films on the Membrane Surface

The deposit is removed by (1) stack disassembling (cf. Section 1.5.3 inApplications), (2) periodic current reversal (cf. Section 2.6 in Applications) and(3) washing with a chemical reagent. We discuss (3) in this section.

Urabe and Doi (1987) washed the membranes with a mixed solution ofalkalis, mineral salts and organic solvents. In this research, cation exchangemembrane CL-25T and anion exchange membrane ACH-45T were integratedinto an electrodialyzer (effective membrane area: 0.25m2, number of membranepairs: 500). Seawater was supplied and desalinated passing an electric current.In the course of the electrodialysis operation, cell voltage was increased from0.7 to 1.2V due to the precipitation of fine substances to the membrane surfaces.So, the 25–301C washing solution mixed with tap water (500 l) and methanol(500 l) dissolving NaCl (90 kg) and NaOH (40 kg) was circulated through thedesalting and concentrating cells for 3 h. Then, the cell voltage was restoredto original 0.7V without the decrease of the current efficiency, permselectivityand strength of the membranes. In the washing operation, NaCl, NaOH andmethanol are estimated to dissolve the substances precipitated in the membraneinto the external solution.

Yamashita (1976) (Tokuyama Soda Co.) washed the membranes withammonia water dissolving citric acid and EDTA. In this study, filtrated indus-trial waste water was supplied to an electrodialyzer (effective membrane area:5 dm2, number of membrane pairs: 20) maintaining the solution velocity andstatic head loss, respectively, at 40 l min1 and 154mmHg, respectively. After166m3 of the solution was electrodialyzed, it became difficult to operate theapparatus because the static head is increased to 452mmHg due to the adhesionof suspended substances in the feeding solution to flow paths in the electro-dialyzer. So, a 301C aqueous solution dissolving 2% citric acid and 1% EDTAwas supplied to the electrodialyzer at the flow velocity of 40 l min1, keeping pHat 4.5 by adding ammonia water. The static head was decreased to 162mmHgafter 45min, and then after operating further 15min, the electrodialyzer was


disassembled, and it was found that the attached substances disappeared.Finally the electrodialyzer was assembled again and the operation was pro-ceeded supplying the solution at the static head of 162mmHg. In this exper-iment, complex forming agents such as citric acid and EDTA dissolved thesubstances attached to the membrane at a reasonable pH value.

Ueno et al. (1980) supplied seawater to an electrodialyzer incorporatedwith Selemion CMV and Selemion ASV. Static pressure at the inlet of theapparatus was 0.6 kg cm2 G1 at first. In the course of operation, the pressurewas gradually increased due to the attachment of slime to the membrane sur-faces in desalting cells, and eventually it became impossible to operate becausethe pressure attained the limiting value of 1.2 kg cm2 G1. In order to avoid thetrouble, 900ml of an aqueous solution dissolving 0.5% Neoplex Paste (KaoAtlas Co., anion-surfactant, effective component: sodium dodecyl benzene sul-fate) and 0.5% hydrazine was supplied into 1m3 of water in a washing line, andwas circulated through the circulating tank and the electrodialyzer for 30min.Next, adding 25.7 kg of 35% hydrogen peroxide into the circulating tank, thewashing solution was circulated through the electrodialyzer for 3 h keeping thetemperature at 25–301C. Consequently, the pressure was decreased to 0.6 kgcm2 G1 and it became possible to operate the electrodialyzer again. After theexperiment, the apparatus was disassembled and it was confirmed that the slimewas removed perfectly. Electric resistance of the ion exchange membranes wasnot altered by the washing.

14.3. ORGANIC FOULING

14.3.1 Organic Fouling Phenomena

Organic fouling of ion exchange membranes is one of the major pro-blems in electrodialysis. It is caused by the precipitation of colloids on themembranes and because most of the colloids present in natural water are nega-tively charged, it is almost always the anion exchange membranes which areaffected. Korngold et al. (1970) examined experimentally the process of organicfouling using multicompartment electrodialysis cells and humate as the foulingagents as follows.

Fouling is defined as the voltage rise across anion exchange membranesdue to fouling agents. The apparatus used for the experiments was a multi-compartment cell series with forced circulation. Two measuring electrodes wereinserted in each cell member so that they were near the membrane surfacesadjoining the members. The membranes were clamped tightly between two cellmembers.

Fig. 14.12 shows the fouling curves of AMF anionic membranes in 0.1NKCl solutions containing 1.0 and 0.1% humate, respectively. Reversal of thecurrent immediately reduced the voltage on the membrane to its original values,

40

35

30

25

20

15

10

5

00 1 2 3 4 5

Cur

rent

orig

inal

sig

n

Cur

rent

rev

ersa

l

1.0% humate

0.1% humate

Mec

hani

cal c

lean

ing

of th

e su

rfac

e

Cur

rent

rev

ersa

lC

urre

nt o

rigin

al s

ign

t (h)

V (

Vol

t)

Figure 14.12 Fouling of AMF A-63 membranes in a mechanically stirred apparatus byNa-humate containing 0.1N KCl solutions. i ¼ 19mA/cm2; V ¼ voltage across themembrane (Korngold et al., 1970).


but its reversal to its original direction raised it again almost instantaneouslyas high as it had been before reversal. Mechanical cleaning of the membranesurface also helped for only a very short time. There was, of course, much morehumic acid precipitated in the 1% solution. Fig. 14.13 shows the influenceof current density on the organic fouling. The abscissa is not the time but theamount of electricity passed across a set of eight consecutive AMF A-63 mem-branes in the multicell apparatus. Fouling is given on the ordinate as the averagevoltage drop across these membranes. Fig. 14.14 shows the result of experimentsin which the fouling has been performed in solutions of increasing KClconcentration, keeping all other parameters unchanged. It is clear that foulingincreases quickly with decreasing KCl concentration.

It was found in the following experiment that fouling acidifies the solutionon the side of the anionic membrane on which humic acid precipitates andmakes the solution on its other side alkaline. In this experiment using a sixcompartment cell, an AMF anionic membrane was put between cells 3 and 4with voltage measuring electrodes near its surface (Fig. 14.15). Both cells wereseparated by cationic AMF membranes from their adjoining compartmentswhich contained 0.1N NaCl to insulate them from the cells with the currentelectrodes. HCl was added to the cathode cell (pH2) and NaOH to the anodecell (pH11). The electrolyte in the measuring cells 3 (desalting cell) and 4(concentrating cell) was 0.1N in each cell in the control experiment, and cell 3

30

20

10

0

t (h)0.1 1 10 100

0.04 N KCI

0.03 N KCI

0.1 N KCI

V (

volt)

Figure 14.14 Influence of KCl concentration on fouling (Korngold et al., 1970). AMFA-63 membranes. 1000 ppm Na-humate. 10mA cm2. Flow velocity 1.8 cm s1. pH 8.4.

V (

volt)

10

5

0

Coulombs10 100 1000 5000

5 mA/cm210 mA/cm220 mA/cm2

Figure 14.13 Influence of current density on fouling (Korngold et al., 1970). AMF A-63membranes. 0.04N KCl containing 1000 ppm Na-humate. Flow velocity 1.8 cm s1. pH8.4.


HCl NaOH

Cathode Anode

0.1NNaCl

0.1NNaCl

0.1NNaCl

0.1NNaCl

0.1NNaCl

0.1NNaCl

0.06%sodiumhumate

Cell 1 A Cell 2 K Cell 3 A* Cell 4 K Cell 5 A Cell 6

desalt. concent.

A: anion exchange membrane desalt.: desalting cell

A*: test anion exchange membrane

K: cation exchange membrane,

concent.: concentrating cell

+ H+OH−

Figure 14.15 Six compartment cell for measuring pH changes caused by organic fouling(Korngold et al., 1970).


received an amount of 0.06% sodium humate in the main experiment. A currentof 60mA (i ¼ 19mA cm2) was passed through the system in both experimentsand the voltage across the membrane and the pH in both measuring compart-ments was measured as a function of time. NaCl was added every 10min to cell 3to compensate for what was being lost by electrodialysis. The pH in cells 2 and 5was determined every 10min to ensure that they did not change and they iso-lated the experimental cells from the electrode compartments. Cells 3 and 4were stirred mechanically at constant speed. Table 14.4 shows the results. It isclear that fouling causes changes of pH on both sides of the membrane. Theresults mentioned above indicate that fouling by anionic colloids of insolublecolloid acids is auto-catalytic process, triggered by polarization on the anionicmembrane and increasing the polarization on it so as to increase fouling morerapidly after an incubation period. The underlying chemical process must be theprecipitation of humic acid on the membrane. The layer of this ‘‘cation-active’’colloid on the anion-active membrane gives rise to a composite, ‘‘sandwich’’membrane which becomes depleted of salt ions from both sides because itreceives the current in this ‘‘closing’’ direction. Thus, an increasing part ofcurrent conduction is taken over by the ions of water, making the humate side(desalting side) acidic and the other side (concentrating side) alkaline. The H+

ions entering the humate solution auto-catalytically increase the precipitationof humic acid so that fouling increases rapidly once it has started.

Table 14.4 Acid is generated on the cathode side of an anion exchange membrane andalkali is generated on its anode side

Time (min.) Control: No Humate With Humate

V pH in cell 3(desalt.)

pH in cell 4(concent.)

V pH in cell 3(desalt.)

pH in cell 4(concent.)

0 2.2 5.5 5.5 2.5 6.6 5.55 2.2 5.4 4.5 5.717 2.3 5.2 7.5 5.5 6.030 2.3 5.1 8.2 5.5 6.040 2.3 5.0 5.6 8.7 5.5 6.063 2.3 4.8 9.0 4.2 6.580 2.4 4.5 10.5 3.4 7.095 2.5 4.5 10.7 3.2 7.0110 2.5 4.5 6.3 11.8 3.0 10.9130 2.5 4.6 12.5 2.9150 2.5 4.6 6.3 12.2 2.8 11.1165 2.5 4.5 14.0 2.7

Note: desalt., desalting cell; concent., concentrating cell. 600 ppm humate in 0.1 NNaCl 19 mA/cm2 (mechanical stirring).Source: Korngold et al., 1970.


14.3.2 Anti-Organic Fouling Membranes

14.3.2.1 Macroreticular Anion Exchange Membrane

In order to prevent organic fouling, Kusumoto et al. (1976) developed thefollowing macroreticular (macroporous) anion exchange membrane. Namely, asyrupy monomer solution was prepared by mixing styrene-divinylbenzene withpolybutadiene rubber (viscosity increasing agent) and t-amylalcohol (macro-porous structure forming agent). The macroreticular membrane was obtainedby coating the mixed solution to a polypropyrene reinforcement and poly-merizing under pressure. In this investigation, the membranes were synthesizedby changing the ratio of t-amylalcohol as shown in Table 14.5.

Fig. 14.16 gives the influence of amylalcohol ratio to electric resistance,transport number and diffusion constant of the membranes synthesized above,showing that the macroporous structure develops with the increase of t-amylalcohol content. The anti-organic fouling performance was measured usinga two-cell electrodialysis unit filling with a 0.05N NaCl+100 ppm dodecyl-benzene sulfonate solution in the cathode chamber (desalting side) and a 0.05NNaCl solution in the anode chamber (concentrating side). Passing an electriccurrent of 2mA cm2, electric resistance change due to the organic fouling of themembrane was evaluated as shown in Fig. 14.17 by measuring the potentialdifference Vm across the membrane. The result shows that the electric resistanceincrease due to the organic fouling is suppressed with the development of macro-porous structure.

6

4

2

0

RA (

Ω c

m2 )

8

6

4

2

0

D (

cm2 h

−1)

0.2 0.4 0.6 0.8 1.0(t-AmOH)/(St + DVB) (wt. Ratio)

0.9

0.8

0.7

t (−)

Figure 14.16 Electrochemical properties of a macroreticular anion exchange membrane(Kusumoto et al., 1976). RA: electric resistance (251C, 0.5N NaCl), t: transport number(251C, 0.5N/2.5N NaCl) and D: NaCl diffusion constant (251C).

Table 14.5 Composition of monomer mixed solutiona

No. Stb DVBc PBd t-AmOHe

1 0.7 0.3 0.1 0.42 0.7 0.3 0.1 0.53 0.7 0.3 0.1 0.84 0.7 0.3 0.1 1.0

Source: Kusumoto et al., 1976.aWeight.bStyrene.cDivinylbenzene.dPolybutadiene.et-Amylalcohol.


Vm

(vo

lts)

2

1

00 2 4 6 8 10

Time (h)

No.4No.3

No.2

No.1

Figure 14.17 Anti-organic fouling of a macroreticular anion exchange membrane(Kusumoto et al., 1976).


Hodgdon and Sudbury (1971) applied for a patent of ‘‘Ion exchangemembranes having a macroporous surface area’’, in which the membranes werefabricated with a polymeric macroporous surface lessening the tendency to foulwhen employed in the electrodialysis of solutions containing fouling constituent.

14.3.2.2 Formation of Thin Cation Exchange Layer on the Anion Exchange

Membrane

Another anti-organic fouling membrane was developed by sulfonating thesurface of an anion exchange membrane and forming thin cation exchange layeron the anion exchange membrane (Kusumoto and Mizutani, 1975). The syn-thetic process starts from sulfuric acid treatment of styrene-divinylbenzene basemembrane and the modified membrane is obtained via chloromethylation andamination (Fig. 14.18). Table 14.6 shows the electric resistance RA and transportnumber t to be not influenced by sulfuric acid treatment time t. Fig. 14.19 givesthe influence of t to the potential difference Vm in the electrodialysis experimentcaused by the organic fouling due to dodecyl-sulfonic acid showing that the anti-organic fouling performance of membrane no. 4 (t ¼ 3.0 h) and no. 5 (t ¼ 5.0 h)

Table 14.6 Sulfuric acid treatment and membrane characteristics

No. ta (h) A Sb (meq per Gram Dry Membrane) RAc (O cm2) td

1 0 – 2.2 0.912 0.5 Not detected 2.3 0.913 1.25 Trace 2.3 0.914 3.0 0.01 2.3 0.915 5.0 0.08 2.4 0.91

Source: Kusumoto and Mizutani, 1975.aTreating time in 98% sulfuric acid.bSulfonic acid concentration.cElectric resistance: 0.5N NaCl, 251C.dTransport number: 0.5N NaCl/2.5N NaCl, 251C.

Modified anion exchange membrane

Styrene.divynylbenzene base membrane

Sulfuric acid treatment

ChloromethylationCH2OCH2Cl 1 part, CCl4 2 parts, SnCl4

25 °C, 4h

Washing and drying Washing by 80 % H2SO4, 40 % H2SO4, H2O

Amination15 % trimethyl amineroom temperature, 6 h

Figure 14.18 Synthetic process of a modified anion exchange membrane (Kusumotoand Mizutani, 1975).


Vm

(vo

lts)

10

1

0.1

Time (h)0 1 2 6

No.4,5

No.1No.2 No.3

Figure 14.19 Anti-organic fouling of a modified anion exchange membrane (Kusumotoand Mizutani, 1975).


is excellent. The anti-organic fouling mechanism of this membrane is similar tothe divalent ion permeability decreasing mechanism across the ion exchangemembrane discussed in Sections 3.6 and 3.7.

REFERENCES

Grossman, G., Sonin, A. A., 1973, Membrane fouling in electrodialysis: A model andexperiments, Desalination, 12, 107–125.

Hanzawa, N., Yuyama, T., Suzuki, K., Nakayama, M., 1966, Studies on durability ofion exchange membrane (IV): Long term electrodialytic concentration test, ScientificPapers of the Odawara Salt Experiment Station, Japan Monopoly Corporation,Odawara, Japan, No. 11, pp. 1–13.

Hodgdon, R. B., Sudbury, Jr., 1971, Ion exchange membranes having a macroporoussurface area, UA Patent, 3,749,655.

Kneifel, K., Hattenbach, K., 1980, Properties and long-term behavior of ion exchangemembranes, Desalination, 34, 77–95.

Korngold, E., Korosy, F. D. E., Rahav, R., Taboch, M. F., 1970, Fouling of anion-selective membranes in electrodialysis, Desalination, 8, 195–220.

Kusumoto, K., Mizutani, Y., 1975, New anion-exchange membrane resistant to organicfouling, Desalination, 17, 121–130.


Kusumoto, K., Ihara, H., Mizutani, Y., 1976, Preparation of macroreticular anionexchange membrane and its behavior relating to organic fouling, J. Appl. Polym. Sci.,20, 3207–3213.

Ohwada, K., Shimizu, U., Taga, N., 1981, Microorganism and organic matter depositedon the ion exchange membrane, Bull. Sea Water Sci. Jpn., 34(6), 367–372.

Ueno, K., Ozawa, T., Ooki, H., Ishida, T., Nakajima, K., Sudo, T., 1980, Washingmethod of ion-exchange membranes, JP Patent, S55-33662.

Urabe, S., Doi, K., 1987, Washing method of ion-exchange membranes, JP Patent,S62-52624.

Yamashita, I., 1976, Removing method of fouling substances in an electrodialyzer, JPPatent, S51-131477.

[membrane science and technology] ion exchange membranes - fundamentals and applications volume 12...

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