11 Effects of Hydrolysis on Cellulose Acetate Reverse-Osmosis Transport Coefficients Downloaded by STANFORD UNIV GREEN LIBR on July 1, 2012 | http://pubs.acs.org Publication Date: January 1, 1985 | doi: 10.1021/bk-1985-0281.ch011
S. B. MCCRAY and JULIUS GLATER School of Engineering and Applied Sciences, University of California, Los Angeles, CA 90024
The hydrolysis of cellulose acetate involves the removal of acetyl groups from the cellulose acetate polymer backbone. Since the concentration of acetyl groups in the polymer affects the rate of salt and water transport through the membrane, the performance of the membrane changes upon hydrolysis. Our efforts have shown that the two parameter transport model does not adequately describe the transport of salt and water through hydrolized membranes. Our current work represents the study of the applicability of the tree parameter Kedem-Katchalsky transport model to hydrolyzed cellulose acetate membranes. Data are presented which show that the three parameter model more accurately describes transport through hydrolyzed membranes. This is due to the salt-water coupling term which is neglected in the two parameter model.
It has been known for years that cellulose acetate membranes experience hydrolytic decomposition under adverse feedwater pH conditions. While the optimum pH range for extended membrane life is 4-6 (]_»£>.3)> periodic failure of pH control systems can lead to a decrease in salt rejection due to hydrolysis. A knowledge of the effects of hydrolysis on transport coefficients will allow a prediction of membrane performance with time. Hydrolysis of cellulose acetate results in the removal of acetyl groups from the polymer. Vos et al. (]_) studied the hydrolysis of cellulose acetate membranes between 20~90°C, in a pH range of 2-9. They found that acetyl content decreases with time in a pseudo-first 0097-6156/85/0281-0141S06.00/0 © 1985 American Chemical Society
In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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order reaction. The activation energies are 16.^ and 20.1 kcal/mole for the acid (pH 2.0) and base (pH 9.0) catalyzed reactions, respectively. Vos et al. (£) and Sammon et al. (5) measured changes in membrane performance upon hydrolysis. The salt and water permeability coefficients increased exponentially with time after a short induction period. These were on-line tests under alkaline conditions at constant temperature. McCray (3) used soak tests to evaluate the effects of hydrolysis on membrane performance. He showed the water flux increased exponentially and the salt rejection decreased linearly with time for both acid and base catalyzed reactions. Glater and McCray (6) evaluated changes in the SolutionDiffusion transport parameters upon hydrolysis at pH 8.6. The salt and water permeability coefficients increased exponentially with time. Based on their results, it was suggested that the SolutionDiffusion model is not adequate in describing transport through cellulose acetate membranes because it neglects salt-water coupling. The objective of this study is to evaluate the effects of hydrolysis on the Kedem-Katchalsky transport parameters. Using soak tests, the transport parameters of hydrolyzed membranes were determined from reverse osmosis data. Expressions have been developed which relate changes in transport parameters to hydrolysis time and temperature. Membrane Transport Equations The Kedem-Katchalsky transport model is based on nonequilibrium thermodynamics and requires three parameters to characterize salt and water transport through membranes (4,8):
where J is the volumetric flux; AP is the hydrostatic pressure difference across the membrane; Air is the osmotic pressure difference across the membrane; N is the molar salt flux; and C . is the s s, lm log-mean salt concentration across the membrane. The water permeability coefficient, L , characterizes water flow through the membrane. The salt permeability coefficient, u>, describes the diffusive component of salt flux. The reflection
In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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coefficient, a, is a measure of the salt-water coupling as well as the salt rejection of the membrane. Values of a range from 0 to 1. For 0 * 1 , the membrane is ideal, indicating no salt-water coupling and total salt rejection. For a = 0, the membrane shows no semipermeability and thus, no salt rejection. The Solution-Diffusion model assumes each species must first dissolve into the membrane before diffusing through. All transport is assumed to occur by diffusion. Merten {%19) 8 i v e s t n e equations as follows:
where AC is the salt concentration difference across the membrane; s and A and B are the water and salt permeability coefficients, respectively. The Solution-Diffusion model neglects salt-water coupling. Determination of Transport Parameters Since the transport parameters are being used to describe transport through reverse osmosis membranes, it was desired to measure them from reverse osmosis data. The salt rejection, r, is defined as:
where C and C are the salt concentrations of the product stream s,p s,m and at the membrane surface, respectively. Using the transport equations given above, a relationship between salt rejection and volumetric flux can be derived from which the transport parameters can be determined. Pusch (JJ_) gives the following equations for determining the Kedem-Katchalsky transport parameters:
where r 00 = a; TT is the osmotic pressure of the solution at the memm brane surface; ujf = wir /C ; and r is the asymptotic salt rejection (the salt rejection at infinite volumetric flow through the
In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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membrane). The asymptotic salt rejection is assumed to be equal to the reflection coefficient. The slope of the line formed from 1/r versus 1/J is u'/r , while the intercept is r^ = a. The water permeability coefficient is determined from a plot of J versus (AP - r o w ) , m The solution-diffusion transport parameters were determined from the following equations:
The salt and water permeabilities are determined from plots of 1/r versus 1/J and J versus (AP - rir ), respectively. Experimental The cellulose acetate membranes used in this study were provided by Hydranautics Water Systems of San Diego, California. The membrane was made of refined cellulose acetate. The membrane is thin (3.6 mils = 0.0093 cm wet thickness) and is cast on a paper backing for structural support which was not removed. Untreated, the membrane gave the following transport parameters: o L P a) A B
= = = =
0.9836 -5 1.85x10 -5 1.41x10 -5 1.85x10 1.92x10~5
3 2 (cm /cm -sec-atm) 3 2 (crrr/cm -sec) 3 2 (cm /cm -sec-atm) (cm3/cm2~sec)
2 Approximately 150 cm of the membrane were placed into one liter jars and tightly stoppered. The jars contained a pH 8.6 boric acid-sodium hydroxide buffer with a salt concentration of 0.05 molar. The jars were placed into constant temperature baths maintained at the desired temperature to ±0.5°C. All membrane samples were washed in distilled water and then cut into a 45 cm diameter disc prior to being placed into the radial flow flat plate reverse osmosis test cell described elsewhere (_3). The feedwater concentration was constant at 5000 ppm NaCl (0.086 molar) in distilled water at 25 ± 0.5°C. The membranes were pressures ized at 5.52x10J kPa (800 psig) for 1.5 hours prior to collecting data. The feed and product water concentrations, and volumetric flux
In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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were then determined using methods previously described (_3). The pressure was then reduced to the desired level and the parameters measured again. The transport parameters of the membranes were determined using this data and equations 5-8. The salt rejection was corrected for concentration polarization using the formulae given by McCray (12).
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Results Changes in Kedem-Katchalsky Transport Parameters. Figure 1 shows the change in the water permeability coefficient, L as a function of time at various hydrolysis temperatures. Since L increases exponentially with time, the following equation can be used to describe the changes in L upon hydrolysis
where
L = initial or baseline value of L yr = rate constant describing the changes p in L . k Lp the lines found in Figure 1 are values of kP , and are The slopes of Lp given in Table I. Table I. Rate Constants for Changes in the Kedem-Katchalsky Transport Parameters Upon Hydrolysis at pH 8.6 T°C
k TD x10 6 (s~ 1 ) LP
k x2106(s""1) a)
k x10 6 (s~ 1 ) a
64.7 60.4 54.5 50.0 45.2 E (teal/mole) Ea x10^ (J/mole)
7.25 3.89 2.03 1.50 0.72 23.3 97.1
33.5 19.1 14.3 8.44 3.61 22.7 95.0
1.69 1.05 0.61 0.36 0.16 2.9 104.3
Figures 2 and 3 show the value of oo and o versus time at various hydrolysis temperatures. These transport parameters also show a first order dependence with time. The equations used to describe these changes are
In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
Downloaded by STANFORD UNIV GREEN LIBR on July 1, 2012 | http://pubs.acs.org Publication Date: January 1, 1985 | doi: 10.1021/bk-1985-0281.ch011
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Figure 1. Logarithm Lp/Lp 0 versus time for pH 8.6 h y d r o l y s i s of $-Hydranautics membranes a t various t e m p e r a t u r e s . Key: O, 64.7 °C; • , 60.4 °C; A, 54.5 °C; V, 50.0 °C; and O, 45.2 °C.
Figure 2. Logarithm U>/
