Reverse osmosis rejection of organic solute from aqueous solution by using cellulose acetate phthalate-polyvinylpyrolidone membrane

NOTES Reverse Osmosis Rejection of Organic Solute from Aqueous Solution by Using Cellulose Acetate Phthalate-Polyvinylpyrolidone Membrane Cellulose acetate phthalate forms a homogeneous interpolymer complex with water-soluble polyvinylpyrolidone. Reverse osmosis properties of membranes prepared from this complex were investigated by using aqueous organic solution. Regular solution theory was applied to the prediction of solute rejection, assuming that the rejection is primarily determined by the distribution of solute between the membrane and the aqueous solution. It indicates that this treatment can serve as a basis for predicting solute rejection in reverse osmosis, if the membrane parameters are estimated reasonably. tern. About 300 ml of feed solution was used each time. The feed solution was kept well stirred by means of a magnetic stirrer fitted in the cell about 1 cm above the membrane surface. Tests with aqueous solution containing a single organic solute were performed. The solute concentration in the feed solution was 0.02 0.001 mole/liter. Hitachi (Ltd) 163 model gas chromatograph was used to determine the concentration of the organic solute in the feed and product solutions. The chromatographic column used was a 2 m long 3/5 mm in diamter stainless-steel tubing packed with 10% squalene on 6 0 - 80 mesh Chromosorb. The column was operated at 120°C. The solute rejection R~ is defined as:

INTRODUCTION Polyvinylpyrolidone (PVP) is a linear water-soluble polymer that can be combined with polyacrylic acid or polymethacrylic acid to form insoluble hydrophilic materials (1). Such blends are sometimes referred to as interpolymer complexes. The hydrogen bond is primarily involved in this complexation. The blending of polymers represents an approach to the problem of modification of properties of polymeric system. Cellulose acetate phthalate forms homogeneous blends with PVP. PVP is not leached out from the blend. Reverse osmosis properties of membrane prepared from this material were investigated. Sourirajan et al. (2) developed the correlation between solute rejection and the chemical nature of the solute. Namely, reverse osmosis data are correlated and discussed with the modified Small number and Taft and Hammett numbers. However, at present there is no adequate method by which the rejection can be estimated. A relation between solute rejection and solubility parameter has been derived from the regular solution theory.

Rs = 1 -

[1]

(Ci)p/(Ci) m

where (C0m and (COp are the solute concentration in the feed and the product solution, respectively. RESULTS AND DISCUSSION According to the solution-diffusion theory, reverse osmosis rejection characteristics of a membrane are determined by the relative permeabilities of solute and solvent, The permeability is the product of dis-

EXPERIMENTAL Cellulose acetate phthalate (CAP, acetyl content 20%, phthalyl content 34%) and polyvinylpyrolidone (PVP, molecular weight 20,000) were commercially obtained from Wak6 Pure Chemicals Co. CAP and PVP in 1:1 mole ratio were dissolved in DMF. This polymer solution was cast on a glass plate and DMF was slowly evaporated at 50°C. The membrane was annealed in water at 80°C for 10 rain. Then, the membrane was preconditioned to give stabilized pure water permeability by operating 80 atm for 3 hr before a test. The experiments were carried out at 50 atm and 25 --_ 0. I°C, using the reverse osmosis apparatus shown in Fig. 1. This consists of a stainless-steel cell with an effective membrane area of 18 cm ~ and a pressure regulator and an external nitrogen cylinder. Compressed nitrogen gas was used to pressurize the sys-

e~ ,'C,

-],n .....

5arnpte inlet

- ~ - - Per meat e

Magnetic stirrer

FIG. 1. Apparatus of reverse osmosis. 561 0021-9797/80/040561-03 $02.00/0

Journal o f Colloid and Interface Science, Vol. 74, No. 2, April 1980

Copyright © 1980 by Academic Press, Inc. All rights of reproduction in any form reserved,

562

NOTES

tribution coefficient and diffusion coefficient. In separation of solute by reverse osmosis, generally, the nonelectrolytes are not highly rejected, whereas most of the electrolytes are highly rejected. Solutes which are strongly sorbed by the membrane exhibit poor reverse osmosis rejection (3). These findings suggest relationships between the reverse osmosis rejection and the distribution coefficient. Thus the data on cellulose acetate mem!~ranes suggest that reverse osmosis rejection differences among various solutes depend more critically on the solute distribution coefficient than upon solute diffusion coefficient. Shor et al. (4) represent the following equation,

R T In ai = RT[ln ~bi + qSj(1 - Vi/V~)] -~ V i ( ~ ( 8 i - 8j) 2

where 4) is the volume fraction of solute, V is the molar volume, and 8 is the solubility parameter. Suffixes i a n d j denote solute and solvent (or membrane), respectively. Here, the standard state can be chosen as pure solute in both phases. At equilibrium, therefore,_the activity of solute in each phase must be equal. Consequently; the distribution coefficient can be given by the ratio of activity coefficients in each phase. The following expressions can be made, assuming that the solute can be regarded as dilute.

[2]

Rs = 1 - f i g

where K is distribution coefficient, and/3 is the coefficient relating to coupling between solvent flow and solute flow. It is difficult to establish the value of/3 with this membrane. However, the degree of reverse osmosis rejection of organic solute is almost independent of applied pressure. With increasing pressure, Rs approaches a value less than unity. Therefore, /3 = 1 may be a good approximation, and we shall assume this to be the case. According to regular solution theory, activity in each phase is given by Eq. [3] when the molar volumes of solute and solvent are widely different (5).

~ w ~ ~lbm = 1,

(~i)m/((~i)w ~ (Ci)rn/(Ci) w.

[4]

Combinations of Eqs. [1], [2], [3], and [4] give Eq. [5]. In (1 - R s ) =

V i t ~ T (Sw - 8m)(8w + ~m -- 2~i)

q-

gm

Vw

[51

I f ~m > 8w, Eq. [5] predicts that rejection increases as molar volume V~ increases. Then, it shows that rejection decreases as the values of 8 of the membrane and the solute approach one another. The solubility

TABLE I Separation of Organic Solute from Aqueous Solution Using Cellulose Acetate Phthalate-Polyvinylpyrolidone Membrane a

Molar volume (cm z)

Solute

Rejection R, (%)

8)

n-Propyl alcohol n-Butyl alcohol iso-Butyl alcohol sec-Butyl alcohol n-Amyl alcohol iso-Amyl alcohol sec-Amyl alcohol 2-Methyl- 1-butyl alcohol t e r t - A m y l alcohol

26.1 26.6 32.8 40.3 30.9 39.5 44.0 42.8 66.0

11.9 11.4 11.1 10.9 10.9 10.7 10.5 10.6 9.9

Benzene Toluene Ethyl benzene o-Xylene m-Xylene p-Xylene

95.9 98~1 99.0 99,3 98.3 98.9

9.2 8.96 8.8 9.00 8.86 8 .'/9

88.9 105.7 122.4 120.5 122.3 123.3

iso-Butyl formate n-Amyl acetate n-Butyl acetate sec-Butyl acetate Ethyl-n-butyrate

84.2 9t.9 74.5 77.5 74.0

8.47 7.37 8.10 8.10 8.25

116.1 148.9 131.6 133.3 132.2

74.8 91.5 92.0 92.3 108.8 , t08.3 109.4 108.1 109.5

a 50 atm, 25 ± 0.1°C; pure water permeability: 0.0296 (cm2/cm2'hr). Journal of Colloid and Interface Science, Vol. 74, No. 2, April 1980

[31

Membrane parameter A (cal/cm3),

B (cal/cm3)'~, C (tool/era~)

A = -28.6 B = 1.6 C = 2.26 × 10-z

A = -38.2 B = 1.6 C = 3.97 x 10-~

A = -38.2 B = 1.6 C = 3.07 × 10-2

NOTES

563 1,0

parameters are calculated as the square root of the cohesive energy density. Usually they are tabulated (6). There is no appropriate method of calculation of ~rn and Vm (membrane parameters). Accordingly, we used them for convenience as adjustable parameters. Therefore, Eq. [5] is rearranged into Eq. [6] In (1 - R~) = Vi[(A + Bri)/RT + C]

012

0.5

[6] I

o10 09

a = 62w - 6~, B = -2(6w - 6m), C = 1 / V m - 1/Vw.

Membrane parameters A, B, and C were determined so that the plot of In (1 - R~) against the right-hand side of Eq. [6] has the slope of unity within a homologous series. Experimental points were approximately on unit slope. Rejection data and membrane parameters are represented in Table I. In a homologous series, generally, rejection increases as the number of carbon atoms, or the molecular weight increases. It may be seen from the Table I that the more hydrophilic the solute, the poorer the rejection is likely to be. The chemical nature of the membrane surface must be ultimately such that it has a preferential sorption for water and/or preferential repulsion for the solute. If the membrane has a preferential sorption for organic solute, rejection must be poorer. However, steric configuration and branching play an important role in determining rejection. Increasing rejection with increased branching is illustrated by the isomers of amyl alcohol. Geometrically large, branched molecules do not diffuse as readily as do linear molecules of the same molecular weight because they present a larger cross section to the membranes. This explains the increase in rejection wlth increased branching. The active layer of cellulose acetate membrane has been shown to be continuous, that is, lacking in distinct pores but having alternate amorphous and crystalline region. There may be small gaps or spaces in the amorphous region. The solute may be distributed in the spaces of the amorphous region~ Because of the hydrophilic nature of cellulose, water may be specifically absorbed in this region. The values of 6m and Vm obtained from membrane parameters are close to those of water rather than those of cellulose acetate. This may indicate that the solute is distributed between this region and the feed solution. For dissociated solutes, rejection increases with increasing dissociation of solute. An ionized solute is generally more soluble in the aqueous phase and more immiscible in the membrane phase than an unionized solute. In such a case, the dissociation constant Ka is introduced into the treatment of the distribution coefficient in a similar manner as dealt in solvent extraction (8). By means of this approach, the following proportional relation can be expected to hold for the solute of high Ka: In (1 - R~) ~ pKa.

08

[7]

[8]

The order of solute rejection in reverse osmosis corresponds to the order of solute dissociation in the feed

7006

05 04 Q3 02

P Ka

FIG. 2. Correlation of In (1 - R,) against pKa for monocarboxyfic acid, (2) phenyl acetic acid, (3) benzoic acid, (4) formic acid, (5) m-nitrobenzoic acid, (6) o-chlorobenzoic acid, (7) chloroacetic acid, (8) cyanoacetic acid, (9)o-nitrobenzoic acid, (10) dichloroacetic acid. (12) trifluoroacetic acid. solution. The experimental results confirmed the above proportionality as shown in Fig. 2. By using Eq. [8], rejection data for dissociated solute are correlated with Taft and Hammett numbers through Ka. REFERENCES 1. Bailey, F. E., Lundberg, R. D., and Collard, R. W., J. Polym. Sci. A2, 845 (1964); Osada, Y., and Sato, M., Polym. LetL Ed. 14, 129 (1976). 2. Matsuura, T., Dickson, J. M., and Sourirajan, S., Ind. Eng. Chem. Process Des. Dev. 15, 149 (1976). 3. Heyde, M. E., Peters, C. R., and Anderson, J. E., J. Colloid Interface Sci. 50, 476 (1975). 4. Shor, A. J., Kraus, K. A., Smith, W. T., and Johnson, J. S., J. Phys. Chem. 72, 2200 (1968); Johnson, J. S., Dresner, L., and Kraus, K. A., in "Principles of Desalination" (K. S. Spiegler, Ed.), p. 373. Academic Press, New York, 1966. 5. Hildebrand, J. H., Prausnitz, J. M., and Scott, R,~L., "Regular and Related Solutions," p. 166. Van Nostrand Reinhold, New York, 1970. 6. Barton, A. F. M., Chem. Rev. 75, 731 (1975). 7. Kurokawa, Y., Tsuchiya, T., and Yui, N., Desalination 29, 233 (1979). 8. Dyrssen, D., Liljenzin, J. O., and Rydberg, J., "Solvent Extraction Chemistry," p. 517. NorthHolland, Amsterdam, 1976. YOICHI KUROKAWA KAORU UENO N o m o YUI Department o f Applied Science Faculty o f Engineering Trhoku University Sendai 980, Japan Received February 14, 1979; accepted October 22, 1979 Journal of Colloid and Interface Science, Vol. 74, No. 2, April 1980