New composite charged reverse osmosis membrane

Desulination,68 (1988) 10 9 -119 Elsevier Science Publishers B .V., Amsterdam - Printed in The Netherlands

109

New Composite Charged Reverse Osmosis Membrane*

K. IKEDA, T . NAKANO, H . ITO, T. KUBOTA and S. YAMAMOTO Nitto Electric Industrial Co., Ltd. Ibaraki, Osaka (Japan)

SUMMARY

The performance of membranes of the NTR-7400 series is controlled by the ion exchange capacity of the skin layer, and by the method of making the membrane. Typically the thickness of the skin layer is about 0 .3 )um as measured by transmission electron microscopy . These membranes have very high water permeabilities and are resistant to chemical attack . In particular, the membranes are stable to 10,000 ppm chlorine for 1 month . The membranes exhibit a stable performance over a wide pH range, from pH 1 to pH 13 even at 80 'C for 1 month . The membranes selectivity properties are rather unusual . Salts containing monovalent cations have higher rejections than salts containing divalent cations. This is the opposite of most normal reverse osmosis membranes . Salt rejection decreases rapidly with increasing salt concentration, but flux is independent of salt concentration . On the other hand, the rejection of neutral solutes is constant over a wide range of concentration. The rejection of amphoteric electrolytes, such as amino acids, depends on pH and increases rapidly at a pH higher than the isoelectric point of the amino acids . The rejection of negatively charged solutes such as ATP is high. Spiral wound modules incorporating these membranes have been used to decolor soy sauce solutions . Tubular modules incorporating these membranes have been used effectively in the treatment of pulp and paper waste waters . The membranes are particularly resistant to fouling by anionic species . Keywords : aulphonated polyether sulphone, composite charged membrane, chlorine resistance, fouling resistance, amphoteric electrolytes .

"Paper presented at the 5th Symposium on Synthetic Membranes in Science and Industry, Tubingen, F .R.G ., September 2-5,1986 .

0011-9164/88/803 .50

C9 1988 Elsevier Science Publishers &V.

110 INTRODUCTION

Composite membranes made by interfacial polymerization are among the most commercially successful reverse osmosis membranes . Examples are NTR-7250 (Nitto Electric Industrial Co ., Ltd., Osaka, Japan) [I], FT-30 (Film Tech Corp, Minneapolis, MN) [2 ] and PEC 1000 (Toray, Inc ., Japan) [ .These 3 ] membranes are characterized by high water flux and salt rejections, but have one serious drawback : poor chlorine resistance. For example, the NTR-7250, one of the more stable interfacial composite membranes, is stable only up to I ppm chlorine. Many of the other membranes are destroyed after exposure to solutions containing as little as i ppm chlorine for a few hours and almost none of these membranes can withstand chlorine concentration continuously. We have developed a new series of charged thin film composite membranes (NTR-7400 series) . These membranes exhibit high water flux and are resistant to chlorine, high and low pH and high temperature . The new membranes have been successfully manufactured in continuous flat sheet and tubular form. RESULTS AND DISCUSSIONS

These new composite membranes are made by coating an ultrafiltration support membrane with a thin permselective skin layer. The skin layer is made from sulphonated polyether sulphone, the support layer is made from neutral polysulphone . Typically the thickness of the skin layer is about 0 .3 'Um . A transmission electron micrograph of a cross-section of one of these membranes is shown in Fig. 1. The materials of this membrane are stable for chemicals, so the composite membrane is also stable . The performance of the membrane is partly controlled by the ion exchange capacity of the skin layer . This is shown in Fig. 2 where the membrane flux and rejection are plotted against ion exchange capacity (IEC) . In general, membrane flux increases and rejection falls as the ion exchange capacity increases . For comparison, sulphonated polyether sulphone asymmetric membranes were also prepared and tested . The method of making these membranes was according to the method of Koyama and Nishimura [4] . In the case of the asymmetric membrane, the rejection is the same, but the flux is about one tenth of composite membranes made from the same materials . A second parameter controlling membrane performance is the method of fabricating the membrane-membrane thickness, density, etc . This is shown in Fig. 3, which shows the flux and rejection of a series of membranes made from the same ion exchange capacity polymer, but under slightly different conditions . Currently, two grades of these membranes are available, one with an

111

Fig. 1. Scanning electron micrograph of the NTR-7400 membrane . Skin layer, sulphonated polyether sulphone ; support layer, polysuiphone . 100

a

200

150

W

50

100

rn x

50

0 5

1 .5 1 IBC (meq/g)

20

Fig . 2 . Flux and rejection characteristics of various composite membranes in which the ion exchange cepncity of the membrane material is varied from 0.5-2 .0 meq/g. Test solution is 0.5% NaC1 solution, 25'C, 5 MPa.



112 100

c o 50

UU N n d

x

Fig. 3 . Flux and rejection for of a series of composite membranes using ion exchange material having an ion exchange capacity of 1 meq/g .

100

N'2- -O

-` o

(10000 ppm)

0-

I

80

?,

60

~

40

y

` Cellulose ,Acetates (100 ppm) i 1

0 .5% NaCl 2 MPa 25°C

ar C H

20

aq .

I 0 0

I 4 10 20 Elapsed Time (d)

I

30

Final Rejection initial Rejection x 100

Fig. 4, The chlorine resistance of NTR-7410 and cellulose acetate membranes after exposure to 10000 and 100 ppm chlorine, respectively .

NaCI rejection of 10% (NTR-7410), the other with a rejection of 50% (NTR-7450) . For solution decoloring applications, low rejection membranes were developed first . Development of higher rejection membranes is underway. Fig . 4 shows the chlorine resistance of the NTR-7410 memrane and cellulose acetate membrane . Samples of NTR-7410 membranes were immersed in a solution containing 10,000 ppm chlorine at room temperature and were tested periodically for loss by rejection . Cellulose acetate samples were tested with 100 ppm chlorine solutions at the same time for comparison . The NTR-7410 membrane shows excellent chlorine resistance at all practical levels of active chlorine and the chlorine resistance of this membrane is



113

S

r '

NTR-7410 (90°C, 1 Month) 1

Cellulose Acetate

4 l

(25°C, 1 Month)

S

20

0 .5% NaC1 aq_ 2 MPa 25°C

1

6

s

10

12

14

p H w Final Rejection x 100 Initial Rejection

Fig. 5. The effect of pH on the rejection properties of NTR-7410 and cellulose acetate membranes .

higher than the chlorine resistance of the cellulose acetate membrane . The NTR-7450 membrane showed a similar high chlorine resistance . Fig, 5 shows the effect of pH and high temperature on the membrane performance . Again, cellulose acetate membranes were used as a comparison . As shown, the NTR-7410 membranes were completely unaffected after storage for one month at 80°C in test solutions ranging from pH 1 to pH 13 . Cellulose acetate, on the other hand, cannot withstand temperatures of 80° C at any pH and, at 25 °C, cellulose acetate membranes are unstable below pH 4 and above pH S . The solute rejection performances for NTR-7410, NTR-7450 and cellulose acetate membranes are shown in Table I for a variety of solutes . NTR-7400 series rejection of the species depends on the charge of the inorganic ion . As shown in Table I, salts containing monovalent cations have higher rejections than salts containing divalent cations . This is opposite to most normal reverse osmosis membranes such as cellulose acetate membranes and is related to the negative charge of the sulphonated polyether sulphone layer . For the same reasons, the rejection of the negatively charged organic solutes is higher than for neutral organic solutes . We also show pure water fluxes for each membrane in Table I . As shown in Fig. 6, the salt rejection decreases rapidly with increasing salt concentration, but flux is almost independent of salt concentration . On the other hand, the rejection of neutral solutes is constant over a wide range of concentrations, but the flux decreases with increasing sucrose concentration . Fig. 7 shows the reverse osmosis performance of NTR-7450 and cellulose acetate membrane at low solute concentrations . The salt rejection of NTR-7450

114 TABLE I Rejection (%) of various salutes by RO membranes Conditions: 0 .5%, 25'C, 1 MPa, * 1 mM. Solute

(M .W.)

NaC1 Na2 SO 4 MgC1 o MgS0 4 Sucrose Dyestuff ATP* Aspartic acid pH 2 pH 12

( 58) (142) ( 95) (120) (342) (300) (507) (133)

Water flux

(m'/m'd) (gfd) (MPa) (psi)

Pressure

NTR-7410

NTR-7450

15 55 4 9 5 98 99

51 92 13 32 36 100 100

0 80

20 98

12 .0 292 1 .0 143

2.2 55 1 .0 143

Cellulose

Acetates

11 33 20 41 35

54 96 80 92 85

-

-

2 .0 49 1 .0 143

1.0 24 1 .4 200

100 Sucrose

0

50 a4

u CG

a

0

0

0 150 6 4 h

x

? PiIa P a-~

10O

~'

-n--

1~'1Pa

50,

a 0 0-51vipa a 0 0 s le 15 0 5 10 15 Concentration (3) Concentration (%1 Fig . 6. The reverse osmosis properties of NTIt-7450 membranes with NaCl and sucrose solutions .





115 Cellulose Acetate 100

NTR-7450 -100

z W

n• a

9i

50 2~TPa 25'C 0

C

y_ -

p -q

$- ~

• •

0

1,1hull

Yv

~

50UIt

a KCI

CA

~~~a NdCl

„ P" -B_ 3

LiCI

vf1J

~`

100 1000 10000 Concentration (ppm)

11uY Y

IIIOfII 1

1

10000 100 1000 Concentration (ppm)

1

Fig . : . The effectof salt concentration on the flux and rejection of NTR-7450 and cellulose acetate membranes, TAY LE I I Typical amino acids L-Asparticacid (Asp) Chemical structure

L-Isoleucine (Ile)

L-Ornithine (OM)

CH, H2N-CH, I I I CH2 CH, CH, I I I ,C-CH, H 2 N-CH COOH

COON

HE N-CH

H 2 N-CH

I COOH Molecular weight Isoelectric point

133 2.8

131 5.9

I COON 132 9 .7

increases with decreasing salt concentration, and the flux depends on the kind of salt . The reason for the flux difference is not completely clear, but is probably related to the degree of hydration of the membrane and the effect of salt concentration on this hydration . We are continuing experiments on this effect. Tests with amino acid solute solutions are informative . These solutes are



116

100

C

50 0

v m a

0

0

4

g

12

pH Fig . 8 .'1'he effect of the pH of amino acid feed solution on the rejection of NTR-7410 membranes .

pH < Isoelectric Point

COOH H 3 N-i,$ I

o

pH '> Isoelectric Point

COO H 7 N-3I

~ Q

M's Fig. 9 . Schematic illustration of the selectivity characteristics of sulphonated polysulphone composite membranes.

amphoteric and the charge sign of the amino acid depends on pH . The amino acids tested are shown in Table II . As indicated, the molecular weights of these solutes were almost the same, but their isoelectric points were different . As shown in Fig . 8, the rejection of amino acid increases rapidly at any pH higher than the isoelectric point of the amino acids . A possible explanation of the



117

N

N OH OH OH

N

HO-P-0-P-0-P-0 - CH2 0

0 HO 0H

Fig, 10, Chemical structure of ATP . 100

0

200

400 Molecular weight

600

Fig. 11 . The effect of solute molecular weight on the rejection of NPR-7410 membranes .

selectivity characteristics of these membranes is illustrated in Fig. 9 . The positively charged amino acids dissolve and diffuse in the skin layer, but the negatively charged acids are repulsed by the negatively charged skin layer . It is important to test a strongly negatively charged solute such as adenosine triphosphate (ATP) . To understand the effect of the negative charged membrane, Fig . 10 shows the structure of ATP . The negatively charged density increases with increasing numbers of phosphate groups . Fig . 11 shows the effect of solute molecular size and charge on membrane performance . The neutral solutes have low rejections and the negatively charged solutes have high rejections . In a real situation, of course, the membranes would be used with solute mixtures . Fig. 12 shows the separation of mixed organic and salt solutions . As shown, the membrane is able to concentrate the organic compounds while si-



118

100

P

Dye tuff (M .14 .300) 50 ppm

PM Bacitracine (M .W .1411) 500ppm~

c n 50

NTR-7410 1 MPa 25°C

v a a a

-0 'NaCl

0

I 0 .1

I 1 .0 NaCl Concentration

10,0

(s)

Fig. 12 . The rejection of organic compounds and NaCl from mixed solutions with NTR-7410 membranes . TABLE III Decoloring of soy sauce with NTR-7410 Spiral wound module Feed

Permeate

14 .1 2 .3 1 .20

13 .2 2 .0 0 .25

Solutes

NaCl Total nitrogen Color

(%) (%)

Performance

Color removal Flux

(%) (m 3/d/gpd)

80 1 .1/263

Test conditions

Pressure Flow rate Temperature

(MPa/psi) (1/min) (`C/`F)

0.85J121 16 40/104

multaneously desalting the solution . All of these results are consistent with a Donnan exclusion contribution to the salt rejection mechanism. To show the practical utility of these membranes, a number of industrially important feed solutions have been tested . For example, soy sauce solution was tested with 2" NTR-7410 spiral wound module . The purpose of this experiment was to decolor the soy sauce . The components of soy sauce are salt, amino acids and coloring matter . The coloring matter has to be rejected, but the salt and the amino acids have to permeate the membranes . Table III shows the experimental data obtained The membrane shows high flux and high removal of coloring matter in spite of high salt concentration . The fouling resistance of the membranes was also good . This is related to the charge of the NTR-7410

119

membrane_ An NTR-7410 membrane and a normal polysulphone OF membrane were immersed in soy sauce for 4 months . Before immersion, both membranes were white, but after 4 months the negative charged NTR-7410 membrane was white, but the neutral polysulphone membrane had changed to dark brown. NTR-7410 tubular modules were used to treat pulp waste waters . These waters contain coloring matter such as lignin sulphonates and the purpose of ultrafiltration is to remove coloring matter . Polysulphone OF tubular modules were used as a control . The feed temperature was 60`C and the test was continued for 6 months . Both modules had COD rejections of about 90%, but the NTR-7410 flux is about 3 times as high as the flux of the polysulphone membranes . This high flux is achieved because negatively charged lignin sulphonates do not foul the NTR-7410 membrane . CONCLUSION The NTR-7400 series composite charged RO membranes have high water flux and are resistant to chlorine, high and low pH, high temperature and anionic solute fouling . The membranes unusual selectivity properties are due to the charged nature of the permselective layer. The NTR-7400 series membranes should find applications in decoloring of solutions with high salt concentrations, concentration and purification of food and fermented products, concentration of coenzymes, pulp industrial wastewater treatment and desalination of water with a low salt concentration .

REFERENCES 1 Y . Kamiyama, N . Yoshioka, K. Mataui and K . Nakagome, Desalination, 51 (1984) 79 . 2 J .E. Cadotte, R.J . Peterson, R .E . Larson and E .E. Erickson, Desalination, 32 (1980) 25 . 3 M. Kurihara, N. Kanamaru, N. Herumiya, K. Yoshimura and S . Hagiwara, Desalination, 32 (1980) 13 . 4 K . Koyama and N . Nishimura, Membrane, 6 (1980) 189.