Properties of Dynamic Polyblend Membranes in the Hyperfiltration of

4 Properties of Dynamic Polyblend Membranes in the Hyperfiltration of Electrolyte Solutions

Downloaded by UNIV OF GUELPH LIBRARY on July 2, 2012 | http://pubs.acs.org Publication Date: January 1, 1985 | doi: 10.1021/bk-1985-0281.ch004

H. G. SPENCER, H. C. RYCHLICKI, and K. S. MENON Department of Chemistry, Clemson University, Clemson, SC 29631 Dynamic polyblend membranes prepared by deposition of a weak acid and a weak base polyelectrolyte pair on a zirconium hydrous oxide ultrafilter were used in hyperfiltration experiments with feed solutions containing simple electrolytes. Ion exclusion model parameters were determined. Effects of ionic surfactants on membrane permeability and electrolyte rejection were also evaluated. The membranes exhibited hyperfiltration properties characteristic of charged gel membranes possessing fixed-charge concentrations and signs dependent on pH. Dynamic polyblend membranes on stainless steel were introduced to expand the family of dynamic membranes to include a membrane providing a high rejection of simple sugars a t high temperatures (1_). Fructose rejection by zirconium hydrous oxide-polyacrylate membranes on stainless steel rarely exceeds OA However, fructose rejection greater tha 0.95 is readily obtained with polyblend membranes. These two types of dynamnic membranes also differ significantly in their rejection of simple electrolytes, especially in the dependence of rejection on pH, type of electrolyte, and concentration of electrolyte. This paper presents the hyperfiltration characteristics of a representative polyblend membrane and uses the results t o obtain membrane parameters. Polyblend Membranes Dynamic polyblend membranes have been prepared by the sequential deposition of pairs of miscible polymers in a mutual solvent on zirconium hydrous oxide membranes supported on porous stainless steel tubes (1). The membranes described in this paper were prepared by deposition of a weak acid polyelectrolyte and a weak base polyelectrolyte to form the blend. The dependence of the concentration and sign of the fixed charge in the layer of the weak acid-weak base polyelectrolyte blend is expected to vary in a predictable manner as the pH of the feed solution is changed. At low pH the polybase will have a full positive charge while the polyacid will be only partially ionized and the membrane is expected to exhibit the electrolyte rejection characteristics of a porous positively charged membrane. At high pH the polybase will not be fully charged and the 0097-6156/85/0281-0047S06.00/0 © 1985 American Chemical Society

In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

48

REVERSE OSMOSIS AND ULTRAFILTRATION

polyacid will be completely ionized to produce a porous negatively charged membrane. The isoelectric point (iep) will occur at an intermediate pH. This sensitivity to pH, when weak acid and base groups occur in the membrane, provides an opportunity to investigate the dependence of hyperfiltration properties of hydrophilic membranes on the net fixed charge concentration and sign.


Ion Exclusion Models Glueckauf (2) has modified the Donnon equilibrium expression for a single electrolyte to account for the micro-homogeneity of the distribution of free fixed charges. For electrolyte A Y , where A is the counter ion and Y the co-ion with respect to the net free^ fixed charge in the rejection layer of the membrane containing singly charged fixed groups, the expression is (2 -

4) where m and m are the molalites of the electrolyte in the rejection layer and adjacent feed solution respectively; M is the molality of the free fixed charges; b is an empirical constant of the system interpreted to be related to the micro-homogeneity of the distribution of fixed charges; n is the sum of a and y; and_G is the ratio of the mean ionic activity coefficient in the rejection layer, y + , to the mean ionic activity coefficient in the feed, y + , Le.,

By relating the apparent passage of the electrolyte, s, to the electrolyte concentrations in hyperfiltration (5),

and assuming the coupling constant for the transport of the solvent and solute, B, is unity, Equation 1 becomes

In a more common, and more theoretically satisfying, treatment of the electrolyte coupling constant, B, the coupling constant for each of the ions of the electrolyte is assumed to be unity (5, 6). Although the values of M are affected by the choice of estimating B, the order of the values with respect to measurements for series of electrolytes with one membrane or an electrolyte with a series of membranes is not altered (6). The simpler estimation of B=l is used in this discussion. Three models can be represented by Equation 4 and limiting cases of it. The ideal model is obtained by assuming G = 1, b = 1. A non-ideal model is obtained by assuming b = 1 and evaluating G in Equation 2 by assuming the activity coefficient dependence on ionic strength, L is the same in the feed solution and the_ reject ion layer; where I = j m.z. in the feed solution and T = Sm + M in the rejection layer, with 5 = 1 for N a N 0 3 and S = 3 for Na^O^, etc. (7). The inhomogeneity model for ion exclusion, Equation 1, was introduced "to retain G = 1 and yet account for the enhanced penetration of electrolyte at low concentration (2 - {*). The


4.

SPENCER ET AL.

49

Dynamic Polyblend Membranes

inhomogeneity model for hyperfiltration preserves this assignment. The resulting operational_equations are presented for three types of electrolytes in Table I, assuming M > aysm, Le., at low electrolyte concentrations. Table L Ion Exclusion Model Equations for Electrolyte Hyperfiltration by Charges Gel Membranes in the Limit of Low Concentration Electrolyte Type


Model

a=y=l

Ideal

sM = m

Non-ideal Inhomogeneity

sRG = m n = m2 b - l sM

a=l, y=2

a=2, y=l 2

2

sM* = (2m)*

3

2

sflBG V = (2m)*

2

3b 1

sM = (2m) 2

sK5 G = (2m)

sffi = (2m) "

s^

= (2M)/2

Using these equations for low concentrations the dependence of s on m for a single electrolyte and the dependence of s on the type of electrolyte at fixed m can be used to determine membrane parameters. The ideal model indicates that slopes of log s vs. log m should be 1, 2 and { for a=y=l; a=2, y=l; a=l, y=2 electrolytes respectively. Although dynamic zirconium hydrous oxide membranes exhibit slopes near the ideal values, the zirconium hydrous oxide-poly aery late membranes and other dynamic hyperfiltration membranes exhibit slopes that are significantly less than ideal (8, 9). TRe non-ideal model has been applied to a zirconium hydrous oxide membrane to account for the concentration dependence of s using different types of electrolytes (7). The value of M obtained using the non-ideal model was about six times as large as M determined by the ideal model and was constant for the various electrolyte types. However, application of this non-ideal model when the slope of log s vs. log m is ^significantly different from the ideal slope does not give constant values of M (6). The inhomogeneity model can account for large deviations in the slope of log s vs. log m and will be applied to the hyperfiltration results obtained with the polyblend ^membranes. It provides two parameters from hyperfiltration experiments, M and b, that are useful in characterizing the membranes. _ The values of the fixed charge concentration index, M , and the fixed charge distribution index, b, change as intuitively expected (6). The fixed charge concentration in a zirconium oxide-polyacrylate membrane at pH near 7 should be much higher than_in a zirconium oxide membrane at the same pH, which is near its iep. M is larger and b is smaller for the polymer-containing membrane. The presence of an organic polyelectrolyte on a zirconium oxide substrate generally reduce b, making the electrolyte rejection less sensitive to concentration, A change in pH from 4 to 7 increases the ionization of the poly(acrylic acid) and should result in a higher fixed charge concentration unless the swelling is too great Hyperfiltration of NaNO- solutions using dynamic (formed-in-place) zirconium oxide-poly (acrylic acid) membranes usually gives a small increase in M and a significant decrease in b when the pH is changed from H o 7 which accounts for its greater rejection of simple electrolytes at concentrations used in the normal test evaluation; 0.01-0.03m.


50


Experimental


Membranes. Three materials were deposited in sequence under hyperfiltration pressure and cross flow conditions; zirconium hydrous oxide, poly(acrylic acid) (Acrysol from Rohm and Haas), and a weak base polymer containing primary amine groups. The porous stainless steel support tube was 0.203 crruin diameter by 30.5 cm in length, providing a membrane area of 0.00195 m . The details of forming zirconium hydrous oxide-polyacrylate have been reported by Thomas (10). The formation of polyblend membranes and the effects of each step on the membrane performance has been described (1_). Hyperfiltration Procedures. A hyperfiltration system provided by CARRE, Inc. was used as a test apparatus (U_). The temperature, pressure, and cross flow velocity could be varied and measured. Experiments were carried out over a broad pH range, at temperatures between 30 and 70°C, under gauge pressures up to 5.5 MPa, and at cross flow velocities up to 18 m/s. Unless otherwise stated the test solution concentrations were 2 g/L for the electrolytes. The electrolyte rejections were determined by measuring the conductivity of the feed and permeate solutions. When an ionic surfactant was present conductivities of the simple electrolytes in the feed were corrected t o remove the contribution of the surfactant. Rejection of surfactant by the membranes was effectively unity. Materials. The simple electrolytes used in membrane formation and in the hyperfiltration tests were reagent grade. Distilled water was used in all formations and experiments The surfactants were used as obtained; sodium dodecyl sulfate (SDS) and polyoxyethylene ( E 2 J dodecyl ether (BRI3-35) from Fisher Scientific Co. and dodecyl trimethyl ammonium chloride (DTAC) from Pfaltz and Bauer, Inc. The CMC of these surfactants are (12): 2A g/L at 50°C for SDS, 0.056 g/L at 50°C for BRIJ-35, and 0.9 g/L a t 30°C for DTAC. Results and Discussion Hyperfiltration Parameters. The effects of applied pressure, cross flow velocity, and temperature on the rejection, r, of NaNO- and volume flux, 3, were investigated using one of the polyblend membranes, membrane A. The gauge pressure was varied from 2.0 to k.2 MPa at pH 2.6 and 6.3 maintaining the temperature, T, at 325 ± 1 K and the cross flow velocity, F, in the range 13 to 17 m/s. The flux increased linearly with gauge pressure, but the intercept on the gauge pressure axis was offset from zero. In all calculations the applied pressure, p, is the gauge pressure corrected for this offset. The offset and slope were identical a t both pH 2.6 and 6.3. The rejection was independent of p at pH 6.3 and increased only slightly with increasing p at pH 2.6. The rejections are not corrected for pressure differences. The cross flow velocity was investigated at pH 6.3, T = 330K, and p = 2.7 MPa over the range 2 to 1^ m/s. Both the flux and rejection of NaNO- were essentially constant; J = bjt ± 0.2 m/s and r = 0.27 ± 0.01. For a cross flow velocity of 2 m/s the Reynolds number is approximately 6000, indicating turbulent flow in all measurements. The effect of temperature was investigated over the range 305 t o 330K at pH 6.3, F = 16 ± 2 m/s, and p = 2.7 MPa. The rejection of NaNOwas independent of the temperature in this range and the dependence of the


4.

SPENCER ET AL.


51

membrane permeability, J/p, on T for all electrolyte solutions was described by (11)

The membrane permeabilities are adjusted t o 323K using Equation 5. adjustment of rejection for temperature is necessary.

No


Effect of pH. The effects of pH on the passage, s, where

and on membrane permeability are indicated for four test polyblend membranes in Figures 1 and 2. The passages exhibit maxima for the three membranes at different values of pH; 5 for A, 8 for B, and 7 for C. The membrane permeabilities of membranes A and B also exhibit maxima at pH 5 for A and 7 for B. The membrane permeability appears to increase at low pH for membrane C, similar to zirconium hydrous oxide-polyacrylate membranes. The effect of pH on the rejection of Na^SO. and membrane permeability was also determined using membrane A, The results with Na-SO. are compared with results with NaNO~ in Figure 3. The electrolyte exclusion models predict that for dilute solutions the passage for Na^SO. will exceed NaNO- when the net free fixed charge in the rejection layer i? positive and the order will be reversed when the membrane charge is negative. The model is consistent with a positive membrane charge at low pH and a negative charge at high pH with the crossover point indicative of the isoelectric point, iep, at pH 4- to 5. This ionic character of a weak acid-weak base polyelectrolyte pair is expected. The location of the iep at pH 4 to 5 and the smaller passage at pH 10 than a t low pH are interpreted by the model to indicate an excess of the polyacid over the polybase in the rejection layer of the membrane. The pKa's of the polyelectrolytes are approximately k and 10. The maximum in s appears to occur at a higher pH than the iep defined above. Effect of Electrolyte Concentration. Membrane A, with iep near pH 5f was also used to investigate the effect of NaNO- concentration on its passage and on the membrane permeability. The experiments were carried out at pH 7.0 and 10.2 with T = 329K, p = 2.7 MPa, and F between 12 and 16 m/s. The dependence of the passage on concentration is presented in Figure 4 as log s vs. log m. This dependence is linear and described by

at pH 7.0 and

at pH = 10.2. Applying the inhomogeneous ion exclusion model M = l.Onn and b = 0.55 a t pH 7.0 and M = 0.8ni and b = 0.66 a t pH 10.2. The model indexes imply a higher fixed charge concentration and a lower fixed charge distribution homogeneity at the pH nearer the iep, i.e., at pH 7. This effect of pH on the fixed charge concentration is unexpected unless the swelling at the higher pH is much greater than at pH 7; probably the result of fewer



52


Figure 1. Passage of NaNO- vs. pH for polyblend membranes: O, A; A , B; V , C; and D , D.

Figure 2. Membrane permeability vs. pH for polyblend membranes: O, A; A , B; V > C; and • , D.



4. SPENCER ET AL.


53

Figure 3. Electrolyte passage and membrane permeability vs. pH: O, NaNOand A , Na2SO^.

Figure 4. Logarithm of passage vs. logarithm of electrolyte concentration in the feed.



54


Figure 5. NaNO- passage and membrane permeability vs. pH for polyblend membrane D: O, NaNO- only; • , NaNO- + BRIJ-35; V , NaNO- + DTAC; and A • NaNCU + SDS.

Figure 6.

NaNO- passage and membrane permeability vs. pH for polyblend membrane C: O, NaNQ3 only and A , NaN0 3 + SDS.


4.

SPENCER ET AL.


55

ionic cross links at pH 10.2 where the membrane is less stoichiometric with respect to polyelectrolyte charges. Also, the analysis could be modified somewhat by the possibility of significant hydroxide ion transport at the higher pH, which is not accounted for in the present model (

Properties of Dynamic Polyblend Membranes in the Hyperfiltration of

Recommend Documents