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13 Dependence of Dynamic Membrane Performance on Formation Materials and Procedures

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H. GARTH SPENCER Department of Chemistry, Clemson University, Clemson, SC 29631

Dynamic membranes originated in the research at the Oak Ridge National Laboratory in the 1960's. Development has produced commercial ultrafiltration and hyperfiltration membranes for industrial separation applications. Research continues in several laboratories to improve the selectivity and productivity of the membranes and to tailor them for specific applications. The development of dynamic membranes and current research is reviewed briefly. Research on polyelectrolyte blend membranes is described in detail as a representative method for tailoring dynamic membranes. Dynamic membranes are formed on microporous supports under appropr i a t e pressure and cross-flow conditions by deposition of solute components contained i n a feed s o l u t i o n . The formation steps are: (a) s e l e c t i o n and conditioning of a porous support; (b) deposition of a f i l t e r a i d , when needed; (c) deposition of a c o l l o i d to form the u l t r a f i l t e r ; (d) deposition of one or more polymers to produce the h y p e r f i l t e r ; and (e) post-formation treatments to enhance selected properties. The properties of dynamic membranes can be influenced at each step i n the formation by a l t e r i n g the materials and procedures. Hence, dynamic membranes are expecially suited for t a i l o r i n g to optimize a membrane's performance i n a s p e c i f i c a p p l i c a t i o n , and a variety of experimental and commercial membranes have been formed. In many cases i t i s possible to remove the membrane by chemical means, recondition the porous support, and reform either the same type or a d i f f e r e n t type of dynamic membrane at the a p p l i c a t i o n s i t e . This feature gives each module a long operating l i f e . Successful applications of dynamic membranes i n a number of i n d u s t r i a l separation processes, membrane s t a b i l i t y at high temperature and over a broad pH range, and membrane reformation c a p a b i l i t y on durable substrates have attracted a s i g n i f i c a n t research and development e f f o r t . Much of the research has been directed toward 0097-6156/85/0269-0295$06.00/0 © 1985 American Chemical Society Lloyd; Materials Science of Synthetic Membranes ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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production of predictable, reproducible, and highly stable membranes; attainment of low cost per unit of feed processed; and expansion of the family of dynamic membranes to improve performance and meet additional application needs. This paper reviews some recent developments i n dynamic membrane research, describes properties and applications of commercial membranes, and reports properties of dynamic polyblend membranes.

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Formation of Dynamic Membranes The poineering research and subsequent development of useful dynamic membranes was accomplished by Johnson and co-workers at the Oak Ridge National Laboratory. This very extensive research has been reported i n a series of reports and i n numerous publications and patents. Papers of special interest are: the detailed report of the i n i t i a l process f o r forming dynamic membranes with a t t r a c t i v e h y p e r f i l t r a t i o n properties by Marcinkowsky, et a l . (1), an early review of the research properties by Johnson (2), and a subsequent review of h y p e r f i l t r a t i o n models and the development of h y p e r f i l t r a tion membranes by Dresner and Johnson (3). These reviews c i t e the major references related to the formation, theory, properties, and applications of dynamic membranes. Two useful membranes developed by the group at the Oak Ridge National Laboratory have dominated the application of dynamic membranes: the hydrous zirconium oxide u l t r a f i l t e r and the hydrous zirconium oxide-poly(acrylic acid) h y p e r f i l t e r . The technology of formation and u t i l i z a t i o n of zirconium oxide-poly(acrylic acid) dynamic membranes has been described i n d e t a i l by Thomas (4). The e f f e c t s of molecular weight of the p o l y ( a c r y l i c a c i d ) , pore d i a meter of the porous support, formation cross-flow v e l o c i t y , format i o n pressure, and pH of p o l y ( a c r y l i c acid) solution during i n i t i a l deposition of the polyacid on the h y p e r f i l t r a t i o n performance are described and discussed. Commercial

Developments

Commercial dynamic u l t r a f i l t r a t i o n membranes are produced by the Gaston County Dyeing Machine Co. and by CARRE, Inc. The former uses porous carbon tubes and the l a t t e r porous metal tubes as the membrane substrate and containment material. The u l t r a f i l t r a t i o n properties of the CARRE, Inc. ZOSS u l t r a f i l t e r , hydrous zirconium oxide on porous stainless s t e e l tubes, are provided i n Table I as an example of a dynamic u l t r a f i l t r a t i o n membrane. CARRE, Inc. also produces a series of dynamic h y p e r f i l t r a t i o n membranes on porous metal tubes. The major product i s the ZOPA h y p e r f i l t e r : hydrous zirconium oxide-poly(acrylic acid) on porous s t a i n l e s s s t e e l tubes. The h y p e r f i l t r a t i o n properties of the ZOPA membranes are also l i s t e d i n Table I. The most a t t r a c t i v e propert i e s of the ZOPA membrane are d u r a b i l i t y at temperatures of at least 100 C, high membrane permeability, and reformation c a p a b i l i t y . The hydrous zirconium oxide-poly(acrylic acid) membranes provide modest r e j e c t i o n of simple e l e c t r o l y t e s . Although the membrane permeability i s high compared to most cast reverse osmosis /hyper-

Lloyd; Materials Science of Synthetic Membranes ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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Table I. CARRE, Inc. Membrane Specifications

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Ultraf i l t r a t i o n ZOSS

Hyperf i l t r a t i o n ZOPA

Flow geometry

Tubular

Tubular

Membrane support

Stainless s t e e l (316L)

Stainless s t e e l (316L)

Membrane material

Zirconium oxide

Zirconium oxide polyacrylate

Method of Replacement

In place chemical solution

In place chemical solution

Prefiltration requirement

40 mesh screen

40 mesh screen

Pressure l i m i t a t i o n

Greater than 1000 psig

Greater than 1000 psig

Temperature l i m i t a t i o n

Greater than 100°C (212°F)

Greater than 100°C (212°F)

pH range

2-13

4-10

Permeability* with test solution at 100°F (gfd/psi)

0.05 to 0.4

0.05 to 0.07

At 200°F

0.4 to 1.2

0.2 to 0.3

Salt r e j e c t i o n *

5 to 20%

80 to 90%

*Test solution 1000 mg/L of NaN0~ i n water.

Lloyd; Materials Science of Synthetic Membranes ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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f i l t r a t i o n membranes, the r e j e c t i o n of simple e l e c t r o l y t e s i s s i g n i f i c a n t l y lower. Hence, ZOPA membranes are not usually suitable for desalination and other applications requiring very high reject i o n of simple e l e c t r o l y t e s . However, they provide very high reject i o n of e l e c t r o l y t e s with larger cations or anions, such as dyes and i o n i c surfactants. Fouling i n the presence of calcium and magnesium ions may also l i m i t t h i s application i n some cases. Modest rejections of low molecular weight molecular solutes are also a t tained with the hydrous zirconium oxide-poly(acrylic acid) membranes. Additional types of h y p e r f i l t r a t i o n membranes produced by CARRE, Inc. include polyblend membranes prepared by the deposition of pairs of polymers that form miscible blends (5). High r e j e c t i o n of molecular solute species i n the molecular weight range above about 80 i s obtainable with these dynamic polyblend membranes. Their properties w i l l be described i n a l a t e r section. The largest ZOSS and ZOPA systems currently i n operation are used i n the t e x t i l e industry f o r recycling wash water i n dyeing processes and f o r renovating caustic solutions (6, 7). Systems are also being used f o r oil-water separation, nuclear industry applications, and food processing. Recent Developments A s i g n i f i c a n t advance i n the preparation of dynamic membranes was r e a l i z e d when the technology f o r depositing the membranes on porous stainless s t e e l tubes was developed by Gaddis and Brandon (8). The replacement of porous carbon and ceramic materials by sintered porous metal as a membrane support has f a c i l i t a t e d manufacture of large single-pass membrane systems, s i g n i f i c a n t l y reduced tube breakage, and permitted high pressure operation. Tanny (9) has prepared u l t r a f i l t e r s by depositing hydrous z i r conium oxide on p l i a b l e porous materials i n a f l u t e d configuration, providing a large membrane area i n a small volume suitable f o r low pressure u l t r a f i l t r a t i o n . Wang (10) has prepared and characterized cross-linked polyv i n y l alcohol) dynamic membranes on porous ceramic tubes. The post-formation cross-linking with a solution containing o x a l i c acid, boric acid, and K C r i S O ^ ^ produced membranes with good s t a b i l i t y i n both a c i d i c and basic solutions. Several research groups have investigated applications and properties of dynamic u l t r a f i l t e r s and h y p e r f l i t e r s . Groves and co-workers (11) have extensively investigated the use of dynamic membranes and others f o r a v a r i e t y of applications i n t e x t i l e processing, such a renovation of dye process wash water, wool scouring, and t e x t i l e s i z e recovery, Trauter and co-workers (12, 13) have also investigated the application of dynamic zirconium oxide u l t r a f i l t e r s f o r t e x t i l e s i z e recovery. Fuis (14), at the Council f o r S c i e n t i f i c and I n d u s t r i a l Research, S.A., i s investigating indust r i a l applications of dynamic u l t r a f i l t e r s and h y p e r f i l t e r s and also the e f f e c t s of a l t e r i n g preparation methods. Gaddis and Spencer (6, 8^ 15) have investigated the use of dynamic h y p e r f i l t r a t i o n membranes f o r r e j e c t i n g pollutants i n t e x t i l e dyeing process wastewater, f o r treatment of dye manufacture wastewater, and for r e c y c l ing shower water i n spacecraft.

Lloyd; Materials Science of Synthetic Membranes ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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Dynamic Polyblend H y p e r f i l t r a t i o n Membranes Several types of dynamic polyblend membranes have been formed on stainless s t e e l supports at CARRE, Inc. Investigations at Clemson University have been concerned with a preliminary determination of some of the properties of these p o t e n t i a l l y useful membranes (5). The formation procedures and h y p e r f i l t r a t i o n properties of one type of polyelectrolyte blend membrane are described here i n d e t a i l . Preliminary work. Extensive l i t e r a t u r e exists on polyelectolytes i n solution and i n cross-linked states as ion-exchange beads and membranes (16). However, research on p o l y e l e c t r o l y t e blend mem­ branes has been l i m i t e d . Michaels and co-workers (17 - 21) devel­ oped homogeneous cast membranes using strong acid-strong base pairs of polyelectrolytes suitable f o r a v a r i e t y of applications but not for desalination. Kaneko and co-workers (22 - 24) prepared homo­ geneous molded membranes from blends of oppositely-charged poly­ electrolytes f o r u l t r a f i l t r a i t o n and d i a l y s i s . Dresner and Johnson ( 3) formed and tested a few polyblend membranes dynamically formed by the simultaneous deposition of pairs of oppositely-charged poly­ e l e c t r o l y t e s on a porous substrate. However, development of these membranes was not pursued. Experimental. The complementary polyelectrolytes used i n the membrane formation were p o l y ( a c r y l i c acid) (Aerysol, Rohm and Haas) and a high molecular weight weak base p o l y e l e c t r o l y t e containing secondary and t e r t i a r y amine groups. A hydrous zirconium oxide membrane on s t a i n l e s s s t e e l (a ZOSS membrane produced by CARRE, Inc.) was used as a substrate. o-*- t Jmodule was 1.27 cm (0.50 in.) i n diameter and 4.09 X 10 nr (0.44 f t ) i n membrane area. The solutes used i n the characterization tests were reagent grade NaN03 and Na2S0^, and food grade fructose and sucrose. The charac­ t e r i z a t i o n solutions were 2 g/L f o r the s a l t s and 2% (w/v) f o r the sugars. The polyelectrolytes were deposited i n sequence from aqueous solutions at room temperature under normal operating cross flow and applied pressure conditions (7). Characterization consisted of determinations of membrane permeability (flux to pressure r a t i o , J/ρ) and solute rejections (r) over a broad range of pH. Experi­ ments were carried out at temperatures between 30 and 70 C, pres­ sures up to 6.9 MPa (1,000 p s i g ) , and cross flow v e l o c i t i e s of 1 to 2 m/s. E l e c t r o l y t e rejections were determined by measuring the conductivity of the feed and permeate solutions. The concentrations of the sugar solutions were measured with a refTactometer. Membrane Properties. The e f f e c t s of each formation step on the h y p e r f i l t r a t i o n properties of a representative p o l y e l e c t r o l y t e blend membrane and the characterization properties of the completed mem­ brane are provided i n Table I I . The membrane permeability (J/p) has been corrected to 50 C, and indicated by (J/p) , using the equation ne

U D U

a r

2

(J/p)

0

= (J/p)expj2500(l/T - 1/323)] .

The deposition of the p o l y ( a c r y l i c acid) converts the ZOSS u l t r a f i l t e r to a h y p e r f i l t r a t i o n membrane exhibiting a simple e l e c t r o l y t e

Lloyd; Materials Science of Synthetic Membranes ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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r e j e c t i o n of greater than 0.85. Subsequent deposition of the polybase t y p i c a l l y reduces the r e j e c t i o n of simple e l e c t r o l y t e s , espe­ c i a l l y i n the pH range 6 to 8, and reduces the membrane permeability s i g n i f i c a n t l y . The extent of the reduction of the membrane perme­ a b i l i t y can be controlled within l i m i t s by a l t e r i n g the deposition procedure. The type of polybase, concentration and pH of deposition a f f e c t the membrane permeability (5). Assuming that the e l e c t r o l y t e r e j e c t i o n by the membrane i s determined primarily by e l e c t r o l y t e exclusion, rules f o r t h i s phenomemon can be invoked to determine the sign of the net fixed charge. For a p o s i t i v e net fixed charge, the r e j e c t i o n of NaNO^ should exceed Na2S04, while the opposite order should occur when the net fixed charge i s negative. In terms of t h i s model, the mem­ brane described i n Table I I has a p o s i t i v e net fixed charge at pH 4, where only a small f r a c t i o n of the p o l y ( a c r y l i c acid) i s d i s s o c i a ­ ted. At pH 7, where both the weak base and the weak acid polyelec­ t r o l y t e s are primarily i n t h e i r ionic forms, the rejections of Na2S0^ and NaN03 are not s i g n i f i c a n t l y d i f f e r e n t . A study of r e ­ j e c t i o n of e l e c t r o l y t e s of d i f f e r e n t charge type as a function of pH provides information about the r e l a t i v e amounts of the two poly­ e l e c t r o l y t e s i n the active region of the membrane. The model shows that the minimum i n the curve of the rejection of NaNO^ i s an i n d i ­ cator of the i s o e l e c t r i c point of the membrane. See Table III f o r comparison of the sign of the fixed charge as a function of pH f o r a ZOPA membrane, a polyblend membrane with an excess of weak-acid p o l y e l e c t r o l y t e and a polyblend membrane with an excess of weakbase p o l y e l e c t r o l y t e . Table I I .

H y p e r f i l t r a t i o n Properties of a Dynamic Polyelectrolyte Membrane and i t s Precursors

Membrane Form

Solute

Temp. (°C)

ZOSS Polyacid added Polybase added

NaN0~ NaNO^ Fructose Fructose NaN0 NaN0 Sucrose Sucrose Na S04 Na S04

38 34 68 69 71 72 65 70 71 72

3

3

2

2

pH

Rej.

(J/p) (gfd/psi)

3.9 6.9 3.8 7.1 7.2 3.8 3.8 6.9 7.0 3.9

0.05 0.86 0.95 0.95 0.80 0.83 0.97 0.97 0.85 0.66

0.42 0.18 0.04 0.02 0.02 0.03 0.04 0.02 0.02 0.03

ρ = 5.5 MPa (800 psi) (Reproduced with permission from Ref. .5. Copyright 1984, Elsevier) Attainment of high sugar r e j e c t i o n at high temperature was the motivaiton for investigating polyblend membranes. I t i s not d i f ­ f i c u l t to obtain fructose rejections greater than 0.95 with the

Lloyd; Materials Science of Synthetic Membranes ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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polyblend membranes, while fructose rejections greater than 0.4 are rarely obtained with ZOPA membranes. The penalty for t h i s improved r e j e c t i o n of small hydrophilic non-electrolytes, represented by fructose and sucrose, i s a reduction i n both the membrane permea b i l i t y and the r e j e c t i o n of simple e l e c t r o l y t e s near the i s o e l e c t r i c point of the membrane. The membrane permeability for those membranes with fructose rejection greater than 0.95 i s i n the range 0.02 to 0.05 gfd/psi. The passage (s = 1 - r) of fructose i s d i r e c t l y related to membrane permeability i n polyelectrolyte blend membranes, with s = 1.7 ( J / P ) Q (5), and not s p e c i f i c a l l y related to any single a l t e r a t i o n i n the formation procedures or materials. The passage of sucrose i s about one-third the passage of fructose. The r e j e c t i o n of NaNO^ has been determined as a function of the feed solution concentration ( c ) . Graphs of l o g s vs. log c are l i n e a r for ZOSS, ZOPA, and polyelectrolyte blend membranes. The slopes are near unity for ZOSS membranes (2), i n the range 0.3 0.5 for ZOPA membranes (_2), and 0.1 to 0.3 for p o l y e l e c t r o l y t e blend membranes. The t h e o r e t i c a l value of the slope for an ion Table I I I .

Dependence of the Sign of the Fixed Charge of a ZOPA and Dynamic Polyblend Membrane on pH

Membrane

pH

r(NaN0 )

r(Na S0 )

Sign of Fixed Charge

3

2

4

ZOPA (Weak acid Polyelectrolyte)

4.0 7.0

0.60 0.68

0.59 0.88

* -

Polyblend (excess weakacid polyelectrolyte)

3.8 6.0 8.0

0.80 0.68 0.62

0.65 0.82 0.87

+ -

Polyblend (excess weak-base polyelectrolyte)

3.9 7.2

0.82 0.82

0.55 0.77

+ +

*Inconclusive difference i n rejections. become p o s i t i v e at low pH.

ZOPA membranes normally

exclusion mechanism with the r a t i o of mean i o n i c a c t i v i t y c o e f f i c ients for the e l e c t r o l y t e i n the membrane and i n the feed solution equal to one i s unity (2). Reduction of the slope i s realized by the conversion of a ZOSS u l t r a f i l t e r to a ZOPA h y p e r f i l t e r and again by conversion to the p o l y e l e c t r o l y t e blend membrane. The polyelectrolyte membranes exhibit h y p e r f i l t r a t i o n propert i e s d i s t i n c t l y d i f f e r e n t from ZOPA membranes and should f i n d use i n d i f f e r e n t applications. They r e t a i n t h e i r h y p e r f i l t r a t i o n propert i e s at temperatures as high as 100 C and they appear to be stable i n many application environments.

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Concluding Remarks Dynamic membranes formed on porous metal tubes are commercially available and are being used in several industrial separation pro­ cesses. The in situ formation procedures coupled with a permanent porous support provide the capability of reformation and tailoring of the membranes to obtain attractive performance characteristics in a variety of challengine operating environments; including high temperature and extremes in pH. The general procedure of forming polyblend membranes is but one example the tailoring process. There is potential for greater use of dynamic membranes in selected indus­ t r i a l environments. The major technical challenge is to extend the variety of formation procedures and materials to obtain improved membrane characteristics for selected applications. The number of laboratories involved in the research on dynamic membranes has i n ­ creased significantly during the past ten years steming from the pioneering work at the Oak Ridge National Laboratory. Acknowledgment The support of CARRE, Inc. for the research on polyblend membranes is greatly appreciated.

Literature Cited 1.

Marcinkowsky, A. E.; Kraus, Κ. Α.; Phillips, H. O.: Johnson, J. S., Jr.; Shor, A. J. J. Am. Chem. Soc. 1966, 88, 5744. 2. Johnson, J. S., Jr. In "Reverse Osmosis Membrane Research"; Lonsdale, Η. K.; Pondall, Η. Ε., Eds.; Plenum: New York, 1972; pp. 379-404. 3. Dresner, L.; Johnson, J. S., Jr. In "Principles of Desalina­ tion"; Spiegler, K. S., Ed.; Academic: New York, 1980; Part B, Second Ed., pp. 401-450. 4. Thomas, D. G. In "Reverse Osmosis and Synthetic Membranes"; Sourirajan, S., Ed.; National Research Council Canada: Ottawa, 1977; pp. 295-312. 5. Spencer, H. G.; Todd, D. K.; McClellan, D. B. Desalination 1984, 49, 193. 6. Brandon, C. Α.; Jernigan, D. Α.; Gaddis, J. L.; Spencer, H. G. Desalination 1981, 39, 301. 7. Brandon, C. Α.; Gaddis, J. L.; Spencer, H. G. In "Synthetic Membranes"; Turbak, A. F., Ed.; ACS Symposium Series No. 154, American Chemical Society: Washington, DC, 1981; Vol. II, pp. 435-453. 8. "Dynamic Hyperfiltration Membrane for High Temperature Space­ craft Wash Water Recycle," National Aeronautics and Space Agency, 1978. 9. Tanny, G. "Recent Progress in the Theory and Applications of Dynamically Fromed Membranes," presented at the 5th Seminar on Membrane Separation Technology, Clemson University, Clemson, SC, 1980. 10. Wang, Y.; et al. Desalination 1983, 46, 335.

Lloyd; Materials Science of Synthetic Membranes ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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Buckley, C. Α.; Townsend, R. B.; Groves, G. R. Water Sci. Technol. 1982, 14, 705, and unpublished data. Trauter, J.; Egbers, G. TPI, Text. Prax. Int. 1983, 38, 599. Trauter, J. TPI, Text. Prax. Int. 1983, 38, 690. Fuls, P. F., personal communication. Gaddis, J. L.; Spencer, H. G. Symp. Proc: Text. Industry Technol. 1978, pp. 115-123. Helfferich, F. "Ion Exchange"; McGraw-Hill: New York, 1962. Michaels, A. S.; Miekka, R. G. J. Phys. Chem. 1961, 65, 1765. Michaels, A. S.; Mir, L.; Schneider, N. S. J. Phys. Chem. 1965, 69, 1447. Michaels, A. S.; Falkenstein, G. L.; Schneider, N. S. J. Phys. Chem. 1965, 69, 1465. Michaels, A. S. Ind. Eng. Chem. 1965, 57, 32. Michaels, A. S. U. S. Patents 3 419 430, 1968; 3 419 431, 1968; 3 467 604, 1969. Kaneko, S.; Tagawa, K.; Negoro, J.; Miwa, M.; Tsuchida, E. Japanese Patent 74 10 232, 1974; Chem. Abstr. 1974, 81, 14466. Kaneko, S.; Tagawa, K.; Miwa, M.; Negoro, J.; Tsuchida, E. Japanese Patent 74 10 233, 1974; Chem. Abstr. 1974, 81, 14467. Miwa, M.; Tagawa, K.; Kaneko, S.; Negoro, J.; Tsuchida, E. Japanese Patent 74 15 735, 1974; Chem. Abstr. 1974, 81, 137 322.

Received September 6, 1984

Lloyd; Materials Science of Synthetic Membranes ACS Symposium Series; American Chemical Society: Washington, DC, 1985.