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12 The Effect of Halogens on the Performance and Durability of Reverse-Osmosis Membranes

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 2, 2015 | http://pubs.acs.org Publication Date: May 21, 1981 | doi: 10.1021/bk-1981-0153.ch012

JULIUS GLATER, JOSEPH W. McCUTCHAN, SCOTT B. McCRAY, and MICHAEL R. ZACHARIAH Chemical, Nuclear, and Thermal Engineering Department, School of Engineering and Applied Science, University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90024 The rapid expansion of reverse osmosis technology during the past two decades has resulted in the development of a variety of new membranes. Unique polymer systems and fabrication methods have led to the production of membranes with significantly improved performance and reliability. In spite of these developments little is known about chemical sensitivity or life expectancy of reverse osmosis membranes used in desalting applications. Manufacturers are consequently reluctant to guarantee their products for long runs especially in unique chemical environments. Commercial reverse osmosis units employ two basic membrane designs, homogeneous films and thin film composite membranes. The chemical systems involve cellulose acetate and a variety of linear or cross linked aromatic polymers. The functional groups principally consist of amides, ureas, and ethers. Each membrane type is characterized- by specific chemical and physical properties. Little is presently known about chemical interactions between the membrane polymer and pretreatment chemicals dissolved in make-up water. Chemical agents are used in water treatment for disinfection, oxygen scavenging, scale control, etc. When added alone or in combination with other chemicals, these agents may influence the performance of reverse osmosis membranes. The response of membranes to changing chemical environments has been discussed to some extent in the literature but few definitive studies have appeared. Chlorine is the oldest and most widespread method of water disinfection. In reverse osmosis systems, chlorine may be added to feedwater for control of micro-organisms and, in addition, to prevent membrane fouling by microbiological growth. According to Vos et al. [1,2], chlorine will attack cellulose diacetate membranes at concentrations above 50 ppm. Membranes were found to show a sharp increase in salt permeability and a decrease in strength after one week of continuous exposure. Under milder conditions (10 ppm chlorine for 15 days) no detectable change in performance was observed. Spatz and Friedlander [3] have also found cellulose acetate membranes to be resistant to chlorine when exposed to 1.5 ppm for three weeks. 0097-6156/81/0153-0171$05.00/0 © 1981 American Chemical Society

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


172

SYNTHETIC

MEMBRANES:

DESALINATION

Limited t e s t i n g on c h l o r i n e s e n s i t i v i t y o f poly(ether/amide) and poly(ether/urea) t h i n f i l m composite membranes have been r e ported by F l u i d Systems D i v i s i o n o f UOP [4]. Polyfether/amide) membrane (PA-300) exposed to 1 ppm c h l o r i n e i n feedwater f o r 24 hours showed a s i g n i f i c a n t d e c l i n e i n s a l t r e j e c t i o n . Additional experiments at F l u i d Systems were d i r e c t e d toward improvement o f membrane r e s i s t a n c e to c h l o r i n e . D i f f e r e n t amide polymers and f a b r i c a t i o n techniques were attempted but these v a r i a t i o n s had l i t t l e e f f e c t on c h l o r i n e r e s i s t a n c e [5]. Chlorine s e n s i t i v i t y o f polyamide membranes was a l s o demonstrated by Spatz and F r i e d lander [3], I t i s g e n e r a l l y concluded that polyamide type membranes d e t e r i o r a t e r a p i d l y when exposed to low c h l o r i n e concentrat i o n s i n water s o l u t i o n . C h l o r i n e d i o x i d e has been used as a water d i s i n f e c t a n t , showi n g fewer undesirable s i d e e f f e c t s than c h l o r i n e [6]. This agent was shown by Vos et a l . [1] to be unreactive toward c e l l u l o s e acetate membranes. The c o m p a t i b i l i t y o f c h l o r i n e dioxide with other membrane types has not been studied. Iodine has had l i m i t e d a p p l i c a t i o n f o r d i s i n f e c t i o n o f swimming pools [7] and small p u b l i c water supplies [8]. One a p p l i c a t i o n i n a reverse osmosis system has a l s o been reported by Turby and Watkins [9]. Advantages o f i o d i n e are greater s t a b i l i t y than c h l o r i n e , lower r e s i d u a l requirement, and diminished chemical r e a c t i v i t y toward d i s s o l v e d organic compounds. Bromine i s another candidate f o r water d i s i n f e c t i o n . This element i s very c o r r o s i v e and r e q u i r e s s p e c i a l techniques f o r handling, however, a bromine d e r i v a t i v e , BrCl i s much l e s s corr o s i v e and i s known to be a more e f f e c t i v e b a c t e r i c i d e [10]. Motivation f o r t h i s research arose from the present i n t e r e s t i n membrane response to changing chemical environments. This i n t e r e s t i s shared by membrane manufacturers as w e l l as operators o f reverse osmosis p l a n t s . Although some r e s u l t s o f c h l o r i n e membrane i n t e r a c t i o n have been published, few o f these studies are d e f i n i t i v e i n terms o f experimental c o n d i t i o n s . Bromine, iodine,and c h l o r i n e dioxide were s e l e c t e d f o r i n v e s t i g a t i o n s i n c e these agents are being considered i n c e r t a i n feedwater d i s i n f e c t i o n a p p l i c a t i o n s . A search o f the l i t e r a t u r e revealed an absence o f c o n t r o l l e d experimental studies i n v o l v i n g exposure o f membranes to halogen agents other than c h l o r i n e . Experimental Procedures A l l membrane exposures were c a r r i e d out by soak t e s t i n g under e q u i l i b r i u m conditions at f i x e d concentrations and constant pH. Pretreatment chemicals were added to b u f f e r s o l u t i o n s at pH 3.0, 5.8 and 8.6. These b u f f e r s , r e p r e s e n t i n g an a r b i t r a r y pH range were prepared according to d i r e c t i o n s given by P e r r i n and Dempsey * BrCl i s p r e s e n t l y being t e s t e d but r e s u l t s are not included i n t h i s paper.



12.

GLATER ET AL.

Effect of

Halogens

on

RO

Membranes

173

[11]. The two lower pH l e v e l s were phosphate b u f f e r s whereas the 8.6 b u f f e r c o n s i s t e d of a b o r i c a c i d , sodium borate system. The b u f f e r s showed no r e a c t i o n with pretreatment chemicals used, and had s u f f i c i e n t b u f f e r c a p a c i t y to maintain constant pH during experiments o f long d u r a t i o n . Soak t e s t s with c h l o r i n e were c a r r i e d out i n heavy walled pyrex t e s t chambers o f approximately 28 l i t e r c a p a c i t y . The contents were s t i r r e d m a g n e t i c a l l y by s i x synchronized s t i r r i n g bars. Membrane samples were hung from the l u c i t e l i d by pyrex hooks. A l l membranes s t u d i e d i n t h i s work were f l a t sheets. Samples of approximately eight square inches were r o l l e d i n t o c y l i n d e r s of 1 inch diameter and 2.5 inches long. D e t a i l s o f t e s t chambers and gas i n j e c t i o n equipment are shown i n Figure 1. C h l o r i n e was i n j e c t e d p e r i o d i c a l l y from a c y l i n d e r c o n t a i n ing 5% CI2 gas i n dry n i t r o g e n . The gas mixture was sparged i n t o the system through two 30 mm f r i t t e r g l a s s d i s c s of medium porosity. C h l o r i n e d i s s i p a t i o n r a t e s were found to be slow and c h l o r i n e l e v e l s could be maintained reasonably constant (±5%) by i n j e c t i n g f r e s h gas at about 12 hour i n t e r v a l s . Flow r a t e s and i n j e c t i o n times were e s t a b l i s h e d by a n a l y s i s o f chamber contents. Experiments with bromine, i o d i n e , and c h l o r i n e d i o x i d e were conducted by hanging membranes i n b u f f e r s o l u t i o n s contained i n three l i t e r j a r s . The s o l u t i o n s were s t i r r e d m a g n e t i c a l l y and the j a r s t i g h t l y stoppered and wrapped with aluminum f o i l to prevent chemical l o s s by v o l a t i l i t y and/or photodecomposition. Halogen l e v e l s were checked p e r i o d i c a l l y by chemical a n a l y s i s and augmented, as needed, by a d d i t i o n of small volumes o f concentrated stock s o l u t i o n s . S o l u t i o n s o f these chemicals are q u i t e s t a b l e when t i g h t l y stoppered, r e f r i g e r a t e d , and stored i n the dark. Bromine and i o d i n e s o l u t i o n s were made up d i r e c t l y from reagent grade chemicals. C h l o r i n e d i o x i d e was prepared from sodium c h l o r i t e and h y d r o c h l o r i c a c i d according to d i r e c t i o n s provided by Rio Linda Chemical Company [12]. Two c h l o r i n e l e v e l s were a r b i t r a r i l y s e l e c t e d f o r t h i s work. A low l e v e l o f 3 ppm represents an average c h l o r i n e r e s i d u a l app l i e d i n water d i s i n f e c t i o n p r a c t i c e . A high l e v e l o f 30 ppm, r e p r e s e n t i n g a t e n f o l d i n c r e a s e , provides extreme c o n d i t i o n s f o r a c c e l e r a t e d t e s t i n g . Concentrations o f the other halogen agents were adjusted so as to correspond to e i t h e r c h l o r i n e l e v e l on a molar b a s i s . For example 3 ppm c h l o r i n e i s approximately equival e n t to 7 ppm Br2, 11 ppm I and 3 ppm C I O 2 . No attempt was made to c o n t r o l s o l u t i o n temperature which ranges about 22 ±1°C i n our a i r conditioned laboratory. During a l l membrane exposures, concentrations o f halogens and c h l o r i n e d i o x i d e were p e r i o d i c a l l y monitored by "wet chemical methods". Halogens were determined by the DPD c o l o r i m e t r i c method described i n references [13] and [14]. C h l o r i n e d i o x i d e at r e a sonably high concentrations (>10 ppm) can be determined by d i r e c t c o l o r i m e t r y [15]. The i n t r i n s i c green c o l o r appears to obey Beer's law. At lower c o n c e n t r a t i o n l e v e l s , t h i s chemical i s determined by the DPD method. 2


174

SYNTHETIC

M E M B R A N E S : DESALINATION


GAS

EXPOSURE CHAMBER LID

THERMOMETER CHECK VALVE

TUBULAR MEMBRANE SAMPLE 5% C U

T E F L O N C O A T E D M A G N E T I C STIRRING BARS CHLORINE EXPOSURE CHAMBER Figure 1.

Experimental apparatus for soak tests with chlorine



12.

GLATER E TA L .

Effect of Halogens

on RO

175

Membranes

Assessment o f membrane damage was based on performance t e s t i n g before and a f t e r chemical exposure. T e s t i n g was conducted i n a small f l a t p l a t e reverse osmosis u n i t designed t o accommodate membrane d i s c s o f 45 mm diameter. Feed s o l u t i o n r e s e r v o i r temperature was maintained at 25 ± 1°C and the b r i n e was continuously r e c i r c u l a t e d through a f i l t e r at the r a t e o f 600 mL/min. Concent r a t i o n p o l a r i z a t i o n i s considered n e g l i g i b l e i n t h i s c e l l under these c o n d i t i o n s . Membranes were pre-compacted a t 800 p s i g f o r approximately one hour, the pressure then being reduced t o 600 p s i g f o r c o l l e c t i o n o f performance data. At the beginning o f t h i s work, c o n t r o l samples were soaked i n b u f f e r s o l u t i o n s f o r times corresponding to membrane exposure. I t was subsequently found that c o n t r o l samp l e s were u n a f f e c t e d by b u f f e r s o l u t i o n alone and t h i s procedure was discontinued. Feed s o l u t i o n used i n a l l experiments contained sodium c h l o r ide a t a concentration l e v e l o f 5,000 ppm. Membrane s a l t r e j e c t i o n i s evaluated from conductance measurements o f product water and expressed as percent r e j e c t i o n , %R, o r d e s a l i n a t i o n r a t i o , D . These u n i t s are defined by the f o l l o w i n g equations i n which Cp and Cf are sodium c h l o r i d e concentrations i n feed and product respectively. Note that D i s very s e n s i t i v e t o c o n c e n t r a t i o n changes and expands r a p i d l y as 100% r e j e c t i o n i s approached. r

r

C -C f

%R

2

-V" - x 100

=

L

(1)

f C

D

f r " (T P

(

2

)

Product f l u x was determined from measurements o f product volume as a f u n c t i o n o f time. Flux values determined i n mL/hr are converted t o g a l / f t day (GFD) using the f o l l o w i n g equation based on a c i r c u l a r membrane area o f 15.91 cm . 2

2

GFD = (mL/hr) x 0.370

(3)

B a s e l i n e performance data was measured on untreated membranes at 400, 600, and 800 p s i g i n order t o assess the r e l a t i o n s h i p between performance and operating pressure. Chemically exposed membranes, however, were run a t 600 p s i g only and performance compared with b a s e l i n e data a t t h i s s i n g l e pressure. Results and D i s c u s s i o n This study was conducted i n an e f f o r t t o l e a r n more about the i n t e r a c t i o n o f halogens with commercial reverse osmosis membranes under a v a r i e t y o f experimental c o n d i t i o n s . Membranes used i n t h i s work r e p r e s e n t i n g s e v e r a l d i f f e r e n t polymer systems were pro-


SYNTHETIC

176

MEMBRANES:

DESALINATION

vided through the cooperation o f manufacturers l i s t e d i n Table I. Changes i n membrane performance are compared with " b a s e l i n e data" (Table II) using untreated membranes with 5,000 ppm sodium c h l o r i d e feed.


Table I.

Commercial Membranes Studied

UCLA Code

Manufacturer

Mfg. Code

Polymer Type

u-•1

Fluid

TFC-RC-100

Poly(ether/urea) ( t h i n f i l m composite)

C-•2

Environgenics

CA Blend 72°C cure

CA-CTA 50/50 (homogeneous)

V-•1

Hydranautics

Y

Homogeneous CA (coated with v i n y l acetate)

A-•2

Dupont

Aramid B-9

Homogeneous Aromatic Polyamide

X--2

FilmTec

FT-30

Composition unknown ( t h i n f i l m composite)

Systems

I n i t i a l l y a l l membranes were exposed to 3 ppm c h l o r i n e i n b u f f e r s o l u t i o n s a t pH l e v e l s o f 3.0, 5.8, and 8.6 f o r three weeks. Both c e l l u l o s e acetate type membranes C-2 and V - l were unaffected by c h l o r i n e under these c o n d i t i o n s . Continued exposure a t higher c h l o r i n e l e v e l s d i d not a l t e r b a s e l i n e membrane performance. For example, membrane C-2 exposed t o 125 ppm c h l o r i n e f o r 10 days at pH 3 continued to perform a t b a s e l i n e l e v e l s . In subsequent work, c e l l u l o s e acetate membranes were a l s o found to be unresponsive t o bromine, i o d i n e , and c h l o r i n e d i o x i d e . I t can be g e n e r a l l y concluded that c e l l u l o s e acetate type membranes are halogen r e s i s t a n t . By c o n t r a s t , membranes U - l , A-2 and X-2 are a l l c h l o r i n e sens i t i v e , each responding i n a unique manner. U-l i s a t h i n f i l m composite membrane, the a c t i v e l a y e r c o n s i s t i n g o f c r o s s - l i n k e d poly(ether/urea) polymer. A-2 i s a homogeneous aromatic polyamide c o n t a i n i n g c e r t a i n p o l y e l e c t r o l y t e groups. X-2 i s a t h i n f i l m composite membrane o f p r o p r i e t a r y composition. The pH performance p r o f i l e s o f each membrane a f t e r f o r t y hours exposure to 3.0 ppm c h l o r i n e are shown i n Figures 2, 3, and 4. Membrane U - l shows a t y p i c a l performance d e c l i n e with greatest e f f e c t a t pH 3.0. The d e c l i n e i s p r o g r e s s i v e (Figure 5) and r e s u l t s i n n e a r l y complete membrane f a i l u r e a f t e r 64 hours o f exposure. Membrane A-2 appears t o " t i g h t e n up" on c h l o r i n e exposure as measured by product f l u x below b a s e l i n e l e v e l s as shown i n Figure 3. Membrane t i g h t e n i n g continues p r o g r e s s i v e l y up t o a p o i n t a f t e r which i t i s followed by a sharp performance d e c l i n e . This i s


GLATER E T AL.

Effect of Halogens

on RO

Membranes


100

100

90

- 90

80

- 80

70

60

50 -

3 40

30

20

10

BASELINE

8.6

5.8 pH

Figure 2.

3.0

1

BASELINE

5.8

8.6

3.0

pH

Performance of U-l membrane after 40-h exposure to 3.0 ppm chlorine at various pH levels


178

MEMBRANES:

DESALINATION


SYNTHETIC

Figure 3.

Performance of A-2 membrane after 40-h exopsure to 3.0 ppm chlorine at various pH levels


GLATER E TAL.

Effect of Halogens

on RO

Membranes


12.

Figure 4.

Performance of X-2 membrane after 40-h exposure to 3.0 ppm chlorine at various pH levels


179


Figure 5.

Change in flux and salt rejection of U-l membrane on continued exposure to 3.0 ppm chlorine at various pH levels


12.

GLATER E TA L .

Table I I .


UCLA Code

Effect of Halogens

on RO

Membranes

181

Commercial Membrane Performance Baseline Data

Membrane ID

Operating Pressure (psi)

Product Flux (GFD)

Desal.* Ratio (D )

% Salt Rejection**

r

U-l

F l u i d Systems TFC-RC-100

400 600 800

9.6 13.8 18.4

437 479 415

99.8 99.8 99.8

C-2

Envirogenics CA-CTA Blend 72°C cure

400 600 800

12.8 19.2 25.2

16.2 21.6 21.8

93.8 95.3 95.4

V-l

Hydranautics CA coated with v i n y l acetate

400 600 800

12.6 20.2 26.1

36.0 44.2 40.2

97.2 97.7 97.5

A-2

Dupont B-9

400 600 800

10.3 15.7 21.3

32.5 37.0 31.3

96.9 97.3 96.8

X-2

FilmTec FT-30

400 600 800

16.2 25.8 33.3

59.9 66.9 60.8

98.3 98.5 98.4

* n

feed r _ SS aa ll tt cone, cone, i i nn prod.

** Percent s a l t r e j e c t i o n based on 5,000 ppm NaCl feed s o l u t i o n . shown i n Figure 6 using 30 ppm c h l o r i n e a t pH 3.0 i n order t o amp l i f y this effect. The performance o f membrane X-2 i s s t r o n g l y pH dependent, showing greatest f l u x change a t pH 8.6 and appearing t o t i g h t e n up a t pH 3.0. For some unknown reason, s a l t r e j e c t i o n remains constant and near b a s e l i n e f o r the e n t i r e 88 hour exposure p e r i o d shown i n Figure 7. The next s e t o f experiments were designed t o compare c h l o r i n e with bromine and i o d i n e i n terms o f membrane s e n s i t i v i t y . Experiments with A-2 and X-2 were run f o r f o r t y hours but U - l was exposed f o r only 16 hours because o f r a p i d d e t e r i o r a t i o n on exposure to bromine. Concentrations o f a l l halogens were equivalent t o 3 ppm C I 2 on a molar b a s i s . Performance p r o f i l e s f o r membranes U - l , A-2 and X-2 are shown i n Figures 8, 9, and 10 r e s p e c t i v e l y . Only product f l u x i s reported i n t h i s case s i n c e i t appears to be a more s e n s i t i v e i n d i c a t o r o f performance changes. In an e f f o r t t o b e t t e r i n t e r p r e t these r e s u l t s , a l i t e r a t u r e review on aqueous halogen chemistry was conducted [16,17]. Halogen molecules r e a c t i n water s o l u t i o n t o produce s e v e r a l chemical species as shown i n the f o l l o w i n g equations where X represents CI, Br, o r I.



182

SYNTHETIC MEMBRANES: DESALINATION

BASELINE

16

64

72

112

BASELINE

EXPOSURE TIME. HOURS

Figure 6.

16

64

72

112

EXPOSURE TIME, HOURS

Change in flux and salt rejection of A-2 membrane on continued exposure to 30 ppm chlorine at pH 3.0

EXPOSURE TIME, HOURS

Figure 7.

Change in flux and salt rejection of X-2 membrane on continued exposure to 30 ppm chlorine at various pH levels


12.

GLATER

F TA L .

Effect of Halogens on RO

183

Membranes

60

5 Q 40

5 Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 2, 2015 | http://pubs.acs.org Publication Date: May 21, 1981 | doi: 10.1021/bk-1981-0153.ch012

X 30

O 20

1 1 I UJ BASE- CU Br LINE

V

y

2

BASE- C l Br LINE pH 5.8 2

2

l

BASE- CU LINE

Br

2

l

2

2

pH 8.6

pH 3.0

Figure 8.

Relative influence of halogens on the performance of U-l membrane after 16 hour exposure at various pH levels

Figure 9.

Relative influence of halogens on the performance of A-2 membrane after 40-h exposure at various pH levels



184

SYNTHETIC

Figure 10.

MEMBRANES:

DESALINATION

Relative influence of halogens on the performance of X-2 membrane after 40-h exposure at various pH levels


12.

GLATER

E T AL.

Effect of Halogens

X

2

=

H

2° ^

H

0

X +

HOX^ H

H

+

+

on RO

+

x

Membranes

185

4

"

( )

+ OX"

(5)

In b a s i c s o l u t i o n , the f o l l o w i n g r e a c t i o n s may a l s o occur. X

2

+ 20H"

X" + OX" + H 0

(6)

2


30X" ^ 2X" + X0~

(7)

Reaction 7 i s i n s i g n i f i c a n t f o r c h l o r i n e a t room temperature but takes p l a c e t o a considerable extent f o r bromine and i s n e a r l y complete f o r i o d i n e . The d i s t r i b u t i o n o f halogen species i n aqueous s o l u t i o n depends on pH and e q u i l i b r i u m constants f o r the above r e a c t i o n s . Table I I I l i s t s the d i s t r i b u t i o n o f species f o r each halogen a t the three pH l e v e l s reported i n t h i s paper. D e r i v a t i o n o f equat i o n s f o r c a l c u l a t i o n o f halogen species c o n c e n t r a t i o n are presented i n reference [18]. Table I I I .

F r a c t i o n a l D i s t r i b u t i o n o f Halogen Species i n Aqueous S o l u t i o n as a Function o f pH Mole Percent HOX

PH x

2

ox"

Chlorine

3.0 5.8 8.6

0.01 0.0 0.0

99.99 97.72 6.36

0.0 2.28 93.64

Bromine

3.0 5.8 8.6

69.63 1.17

30.37 98.71 Mostly as BrO^

0.0 0.12

Iodine

3.0 5.8 8.6

99.75 93.60

0.25 6.40 A l l as 10^

Based on a t o t a l halogen c o n c e n t r a t i o n o f 4.23xl0 as X . This i s equivalent to 3.0 ppm C I 2 .

0.0 0.0

_i>

molar

2

Chemical attack on membrane U - l i s revealed by i n c r e a s i n g product f l u x which i s e v i d e n t l y r e l a t e d t o breaking chemical bonds w i t h i n the polymer. Membranes A-2 and X-2 respond t o chemical a t tack by decreased product f l u x which probably r e s u l t s from halogen a d d i t i o n to these polymers. On t h i s b a s i s , the order o f halogen a c t i v i t y below pH 5.8 i s Br2 > Cl2-> 1 2 - With membrane U - l a t pH 8.6 the order changes t o C l > B r > I « One may conclude that 2

2

2


186

SYNTHETIC

MEMBRANES:

DESALINATION

chemical attack o f a l l halogens i s greater with decreasing pH and that X and HOX are the most chemically a c t i v e species. Note that these species do not e x i s t with bromine and i o d i n e at pH 8.6. Performance data on membrane X-2 a t pH 8.6 i s d i f f i c u l t t o i n t e r p r e t but may be due t o a change i n the mechanism o f chemical attack as pH i n c r e a s e s . A l l three membranes are responsive t o attack by halogens. Chemical i n t e r a c t i o n e v i d e n t l y proceeds by more than one r e a c t i o n mechanism. A p o s s i b l e explanation involves halogen a d d i t i o n as evidenced by membrane t i g h t e n i n g . A second process may r e s u l t i n chemical bond cleavage which u l t i m a t e l y causes membrane f a i l u r e . Halogen attack on membrane U - l i s probably dominated by bond c l e a vage which i s enhanced as pH decreases. Membranes A-2 and X-2 e v i d e n t l y respond according to more complicated chemical models. The observed membrane t i g h t e n i n g may r e s u l t from halogen a d d i t i o n . Subsequent performance d e c l i n e i s probably r e l a t e d to bond cleavage. With membrane X-2, i t appears that both the extent and mechanism o f halogen attack are s t r o n g l y pH dependent. The l a s t s e r i e s o f experiments were conducted with c h l o r i n e dioxide. This i n t e r e s t i n g chemical i s a stronger o x i d i z i n g agent than c h l o r i n e and i s reported t o attack organic compounds by o x i dation without halogen a d d i t i o n [19]. As with halogens, c e l l u l o s e acetate membranes were found t o be unresponsive t o c h l o r i n e dioxide on long exposure over the usual pH range. This i a a perplexi n g observation s i n c e these same membrane types are s e v e r e l y damaged by ozone [18] and both O 3 and C I O 2 have n e a r l y the same o x i dation p o t e n t i a l i n a c i d s o l u t i o n . By c o n t r a s t , membrane U - l was so severely damaged by c h l o r i n e dioxide that r e p r o d u c i b l e experimental data could not be c o l l e c t e d . The response o f membranes A-2 and X-2 exposed t o 30.0 ppm C I O 2 f o r 40 hours i s i l l u s t r a t e d i n Figures 11 and 12. Both membranes show only s l i g h t response at pH 3.0 and 5.8 but are s e v e r e l y damaged a t pH 8.6. The chemistry o f c h l o r i n e dioxide i n aqueous s o l u t i o n i s e v i d e n t l y very pH dependent. One a d d i t i o n a l experiment was conducted i n an e f f o r t to shed more l i g h t on the mechanism o f membrane damage. Samples o f membrane A-2 were examined by the B e i l s t e i n t e s t [20] f o l l o w i n g exposure. This s e n s i t i v e q u a l i t a t i v e t e s t f o r organo-halogen spec i e s w i l l i n d i c a t e the i n c o r p o r a t i o n o f halogen i n t o the membrane. B e i l s t e i n t e s t s were p o s i t i v e f o l l o w i n g membrane exposure t o c h l o r i n e bromine o r i o d i n e and negative f o l l o w i n g exposure t o c h l o r i n e dioxide.


2

Conclusions The i n t e r a c t i o n o f halogens and c h l o r i n e d i o x i d e with reverse osmosis membranes i s dependent on the membrane polymer, the s o l u t i o n pH, and the halogen i n v o l v e d . C e l l u l o s e acetate was unresponsive to halogen agents under experimental conditions described


GLATER E T A L .

Effect of Halogens

on RO

Membranes

1


163

BASELINE

8.6

5.8 pH

Figure 11.

3.0

BASELINE

8.6

5.8

3.0

pH

Performance of A-2 membrane after 40-h exposure to 30 ppm chlorine dioxide at various pH levels



188 SYNTHETIC MEMBRANES: DESALINATION


12.

GLATER E T AL.

Effect of Halogens

on RO

Membranes

189

i n t h i s paper. A l l other membranes t e s t e d are halogen s e n s i t i v e , each responding i n a c h a r a c t e r i s t i c manner. The most aggressive chemical i s c h l o r i n e d i o x i d e , producing severe membrane damage a t high pH, but being reasonably i n e r t t o ward membranes A-2 and X-2 a t low pH. In general, the other h a l o gens can be arranged i n order o f r e a c t i v i t y as Br2 > CI2 > I2 low pH and C l > B r > I a t high pH. The mechanism o f membrane attack probably i n v o l v e s s e v e r a l processes such as halogen s u b s t i t u t i o n , halogen a d d i t i o n , and various bond cleavage r e a c t i o n s . The dominant mechanism i s r e l a t e d to pH which i n turn determines the d i s t r i b u t i o n o f halogen species i n s o l u t i o n . Membranes exposed to halogens are found t o form organo-halogen bonds i n the polymer s t r u c t u r e . Exposure to c h l o r i n e d i o x i d e , however, i n v o l v e s no uptake o f c h l o r i n e atoms i n s p i t e o f severe membrane damage. Further work should be d i r e c t e d a t b e t t e r understanding the nature o f halogen-membrane i n t e r a c t i o n . These e f f o r t s may provide g u i d e l i n e s f o r development o f more halogen r e s i s t a n t membranes. a


2

2

t

2

Acknowledgments The authors wish to acknowledge the support o f t h i s r e s e a r c h provided by the O f f i c e o f Water Research and Technology, U.S. Department o f the I n t e r i o r , Washington, D.C, under Grant No. 14-340001-7810. P a r t i a l support was a l s o provided by the State o f C a l i f o r n i a S a l i n e Water Research Funds administered by the Water Resources Center a t the U n i v e r s i t y o f C a l i f o r n i a , Davis, C a l i f o r n i a . We a l s o express our thanks to the f i v e membrane manufacturers f o r t h e i r s p l e n d i d cooperation i n p r o v i d i n g samples f o r t h i s study. Literature Cited

1. 2. 3. 4. 5. 6. 7. 8. 9.

Vos, K. D., et a l . , Desalination 5, 157 (1968). Saline Water Conversion Report, pg. 232, U.S. Department of the Interior, Office of Saline Water, 1966. Spatz, D. D., Friedlander, R. H., "Chemical Stability of SEPA Membranes for RO/UF", report from Osmonics, Inc., Hopkins, Minnesota, 1977. Progress Report by Fluid Systems Division of U.O.P. on Contract No. 14-30-0001-3303, to the Office of Water Research and Technology, U.S. Department of the Interior, July 1975. Progress Report by Fluid Systems Division of U.O.P. on Contract No. 14-34-0001-6516, to the Office of Water Research and Technology, U.S. Department of the Interior, March 1976. Special Report from the International Ozone Institute, Environ. Sci. Technol. 11, 26 (1977). Black, A. P., et a l . , Am. J . Public Health 49, 1060 (1959). Black, A. P., et a l . , JAWWA, pp. 1401-1421, November (1965). Turby, R. L . , Watkins, F . , Proc. 7th Ann. Conf., National Water Supply Improvement Assoc., New Orleans, LA, September 1979.


190

10. 11. 12. 13.


14. 15. 16. 17. 18.

19. 20.

SYNTHETIC

MEMBRANES:

DESALINATION

Mills, J . F . , Schneider, J . A., Ind. Eng. Chem. Prod. Res. Develop. 12, 160 (1973) . Perrin, D. D., Dempsey, B., "Buffers for pH and Metal Ion Control", John Wiley and Sons, New York, 1974. Private Communication with Bruce Hicks, Rio Linda Chemical Co., Rio Linda, California, 1979. "Colorimetric Procedures and Chemical Lists for Water and Wastewater Analysis", Hatch Chemical Co., Ames, Iowa, 1971. "Standard Methods for the Examination of Water and Wastewater", 14th ed., American Public Health Assoc., Inc., New York, 1975. Gordon, G., et al., "The Chemistry of Chlorine Dioxide", Prog, in Inorg. Chem., Wiley-Interscience 15, 201 (1972). White, G. C., "Handbook of Chlorination", Van Nostrand Reinhold Co., New York, 1972. Hoehn, R. C., JAWWA 68, 302 (1976). McCutchan, J . W., Glater, J., Final Report to the Membrane Process Division, Office of Water Research and Technology, U.S. Department of the Interior, under Grant No. 14-34-00017810, January 1980. Ward, W. J., Proc. 36th Int. Water Conf., Pittsburgh, .PA, November 4-6, 1975. Shriner, R. L . , et a l . , "The Systematic Identification of Organic Compounds", John Wiley and Sons, New York, 1964.

RECEIVED

December 4, 1980.


Synthetic Membranes - ACS Publications - American Chemical Society

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