Solute Separation and Transport Characteristics Through Polyether

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14 Solute Separation and Transport Characteristics Through Polyether Composite (PEC)-1000 Reverse-Osmosis Membranes Y. NAKAGAWA1, K. EDOGAWA1, M. KURIHARA2, and T. TONOMURA1 1

Technical Development Department, Toray Industries, Inc., 3-Chome, Sonoyama, Otsu, Shiga 520, Japan 2 Pioneering Research and Development Laboratories, Toray Industries, Inc., 3-Chome, Sonoyama, Otsu, Shiga 520, Japan Solute separation and transport characteristics through PEC-1000 composite membrane were compared with those of asymmetric cellulose acetate membrane and polyamide composite membrane using monovalent and divalent ions. Cation rejection becomes worse and anion rejection, on the other hand, becomes higher as feedwater salinity becomes lower in the case of PEC1000 membrane. This membrane was confirmed to be negatively charged by membrane potentials and ion-exchange capacities. Permeate flow rate through this membrane was quantitatively related with the molar ratio of divalent metal ions and monovalent metal ions in the feedwater. "Donnan-membrane effects" illustrates that this membrane may be close to the most perfect semipermeable membrane among the three membranes evaluated. In recent years, reverse osmosis (RO) processes have risen to the stage of practical use even in the field of seawater desalination as well as brackish water desalination because of the advantages in saving energy, low capital and operating cost, quick delivery, easy operation and easy maintenance (_^, 2) , PEC-1000 membrane manufactured by Toray Industries Inc., among the various kinds of RO membranes, is reported to exhibit excellently high RO performance (especially in solute rejection) not only in seawater desalination but also in the field of concentration and recovery of valuable materials (_3 - 1). In the former case, for example, complete single stage seawater RO (SWRO) in the Middle East is suggested and examined at a demonstration and practical plants in Kingdom of Saudi Arabia. Permeate salinity low enough for world health organization (WHO) standard (500 ppm TDS) is obtained even at high water recovery rate such as 35 - 40% (8_, 9^). One of the examples of practical SWRO plants at the Arabian Gulf using PEC-1000 membrane elements is also reported (9). 0097-6156/85/0281-O187$06.00/0 © 1985 American Chemical Society

Sourirajan and Matsuura; Reverse Osmosis and Ultrafiltration ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


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REVERSE OSMOSIS AND ULTRAFILTRATION

This membrane designated PEC-1000 is a composite one, where the active layer is prepared from cross-linked polyether on the microporous polysulfone substrate 03, 10). This membrane is expected to belong to negatively charged membranes from the view points of its preparation method (11) . Solute separation and transport characteristics through this membrane has been investigated and studied. At the same time, other representative RO membranes, that is, cellulose acetate (CA) asymmetric membrane and polyamide (PA) composite membrane have been examined, the former of which may be expected to be almost neutrally charged membrane and the latter of which may be expected to be positively charged one. In this paper, some of the experimental results on the solute separation and transport characteristics through these membranes described above, especially through PEC-1000 membrane are mainly reported. Experimental Membranes selected. Three kinds of RO membranes were selected. Commercially available PEC-1000 membrane, CA (SC-3000) membrane manufactured by Toray Ind., Inc. and a PA composite membrane prepared as described in the reference (12) and presumed to be similar enough to the commercial FT-30 membrane were employed. Analytical Methods. Salt concentration in the feedwater and the permeate was determined by measuring the electro-conductivity, ion concentration of which was measured by means of ion chromatography of a Yokogawa Hokushin Electric Ion chromatographic analyzer, Model IC 100. Electron charge density of the membrane was determined by ion exchange method or by measuring membrane potentials. In the case of ion-exchange method for PEC-1000 membrane, cross-linked polyether resin forming the ultrathin active layer was extracted out from membrane itself as completely as possible with dimethylformamide (DMF). Test Results Effects of feedwater salt concentration. Effects of feedwater salt concentration were examined for three kinds of membranes. Sodium chloride (NaCl) concentration in the feedwater was varied from 100 to 35000 ppm. Figure 1 shows the effects of feedwater salt concentration on the permeate through these three membranes. Here, salt concentration in permeate is computed from the relationship between electro-conductivities and NaCl concentrations, assuming that salt in the permeate is composed of sodium ion (Na + ) and chloride ion (CI") in the same molar ratio. There is a big difference between the behaviors of CA membrane and other two membranes, especially PEC-1000 membrane. In the former case, salinity in the permeate decreases in proportion to that of feedwater salinity, which means that salt rejection remains constant in the range described above. In the case of the PEC-1000 membrane, on the other hand, the salinity in the permeate seems to come to a certain asymptotic value in the lower salinity range of feedwater containing below several thousand ppm of NaCl, which


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suggests that salt rejection becomes worse as feedwater salinity becomes lower. The behavior of the PA composite membrane seems to be in the middle of these two membranes. Ion transport characteristics. Variations of ion concentrations in the permeate were examined with these three kind of membranes in relation to feedwater salt concentrations using NaCl solution of 100 to 10000 ppm under such evaluation conditions as 30 Kg/cm 2 , pH 6.5 and 25 °C as those in Figure 1. Figure 2 indicates the ion transport characteristics of Na + and Cl~ through these membranes. In the case of PEC-1000 membrane, CI" concentration becomes lower in proportion as feedwater salinity becomes lower. On the other hand, Na + concentration seems to reach a certain asymptotic value, that is 7 x 10" mol/liter, even when feedwater salinity becomes lower in this range of salt concentration. This tendency occurs at the feedwater salt concentrations below several thousand ppm or 1 0 - 1 mol/liter. PA composite membrane exhibited the opposite tendency to that of PEC-1000 membrane, that is, CI" concentration is apt to become difficult to be rejected and Na + is more preferably rejected when feedwater salt concentration is lower than or 10" 2 mol/liter. CA membrane was not affected by feedwater salinity in the range of 100 to 10000 ppm or 10~ 3 to 10"* mol/liter of NaCl solution and there is little difference between the variation behaviors of Na + and CI"" concentrations in the permeate. The reason of the behavior differences between these three kinds of membranes in Figure 1 is explained from the concentration behaviors of Na + and Cl~ ions in the permeate in this figure. Salinity in the permeate through PEC-1000 membrane in the feedwater salinity below 10" 2 mol/liter of NaCl solution is mainly composed of a certain asymptotic concentration of Na + and little amount of Cl"~, which produces weak alkaline solution. On the other hand, permeate through PA composite membrane is mainly composed of a certain asymptotic concentration of CI" instead of Na + in the feedwater salinity below 10" mol/liter NaCl solution, which makes the permeate rather acidic. In the case of CA membrane, there is no such asymptotic ion concentration above 10" 3 mol/liter NaCl solution of feedwater, which shows almost neutral pH value. Electron charge density of PEC-1000 membrane. The PEC-1000 membrane, or more correctly the active layer of this membrane is presumed to involve anionic groups, such as sulfonic acid, as shown in the literature (11) . Elementary analysis of the model resin, which were produced in the similar reaction condition as in the membrane fabrication and washed as completely as possible with deionized water, suggests that the active layer of the membrane possesses 0.7 meq/g of negatively charged electron density (See Table I ) . The membrane active layer, which was produced by extracting out the polysulfone substrate with DMF and washed as completely as possible also with deionized water, was soaked in two kinds of lithium chloride (LiCl) solutions (1 N and 1/100 N) and protons in the anionic groups were exchanged by Li + . It was confirmed that active layer of the membrane possess 0.6 meq/g of negatively charged electron density from this experiment (See Table I ) . In order to determine the electron charge densities of these membranes from another kind of analytical approach, membrane



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Figure 1. Effects of feedwater salt concentration on the permeate through membranes.

Figure 2. Ion (Na+ & CI ) transport characteristics through membranes in relation to the feedwater salt concentration.


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NAKAGAWA ET AL. Table I .

Solute Separation Through PEC-1000 Membranes

Electron-Charge Density of PEC-1000 Membrane From elementary a n a l y s i s


Electron-charge density (meq/g) *)

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From ^ ion-exchange method

0.7

0.6

LiCl (IN, 1/100N) solution was used.

potentials through these three kind of membranes were measured by using two different concentration of potassium chloride (KC1) solutions according to the literature (13) (See Figure 3 ) . The ratio of KC1 concentrations in both solutions through the membranes is kept 4 and the concentration in the higher one is varied 10""1* to 0.5 mol/liter, variation behaviors in the membrane potentials for these membranes are shown in Figure 4. It is also suggested that PEC-1000 membrane is negatively charged and PA composite membrane possesses a slight positive charge from this figure. Effects of divalent cations on RO performance of PEC-1000 membrane. Table II shows typical rejections of representative ions involved in the seawater as feedwater when RO performance of PEC-1000 membrane element was evaluated under such operating conditions at 4.9% seawater, 63 Kg/cm 2 , 35°C, pH 6.5 and 25% water recovery. Rejections of divalent cations, such as calcium ion (Ca 2+ ) or magnesium ion ( M g 2 + ) , are nearly ten times higher than those of monovalent cations, such as Na + or K + . Similar results are obtained between divalent anions, such as sulfate ion (SO4. ™ ) , and monovalent anions, such as CI - .

Table II.

Typical Water Quality and Performance of PEC-1000 Membrane Element in SWRO

Constituent TDS (ppm) C a 2 + (mg/1) M g 2 + (mg/1) Na + (mg/1) K+ (mg/1) S0^2"(mg/1) CI" (mg/1)

Feedwater 49,000

520 1,740 14,800

568 3,670 26,300

Permeate

206 0.3 1.6 75 3.7 3.4 109

Rejection (%)** 99.64 99.95 99.92 99.57 99.44 99.92 99.64

* Operating conditions: 63 Kg/cm 2 , 35°C, 25% water recovery ** Calculated at the base of average seawater ionic concentration RO performance may be analytically expressed by A and B values based on the solution-diffusion theory (14).



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REVERSE OSMOSIS A N D ULTRAFILTRATION

Figure 3. Apparatus for the measurements of membrane potential: (^,02, KCl solution; S, aqueous solution saturated with KCl; SB, agar-bridge saturated with KCl; V, potentiometer; M, membrane; and E, electrodemeter.

Figure 4. Variation curves in membrane potentials in relation to concentration of lower KCl solution (C2).


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where,


Jv: Js:

volume flux solute flux

Figure 5 shows the relationships between molar ratio of divalent cations such as C a 2 + or M g 2 + to Na + as monovalent ion and relative A value for the membranes in the combination system composed of 1.0 % by weight of NaCl solution and some calcium chloride (CaCl2) or magnesium chloride (MgCl2). Here, relative A value means the ratio of A value to the initial A value (Ao) at the evaluation conditions in the absence of divalent cations. In regard to both of CA membrane and PA composite membrane, little variation in relative A value was observed. On the other hand, there is a decrease in relative A value in the case of PEC-1000 membrane when divalent cations, such as Ca 2 + or M g 2 + , were added to the NaCl solution, degrees of which were determined quantitatively by the molar ratio of divalent cations to monovalent ion. This tendency was not observed even in the case of the PEC-1000 membrane when S0i»2~ as divalent anion was dosed as Na2S0it salt. These results suggest that there is some interaction between the functional sulfonic acid groups of PEC-1000 membrane and divalent metal ions, such as C a 2 + or

Mg 2+ . Relationships between the molar ratio of M g 2 + to Na + and Na + rejections for such three membranes is exhibited in Figure 6, where MgCl2 was dosed to NaCl solution in the range of molar ratio of 0 to 3.0 under the initial evaluation conditions of 0.13 % NaCl, 30 Kg/cm2, 25°C and pH 6.5. PEC-1000 membrane exhibits highest ion rejections among these three kinds of membranes and CA membrane exists in the lowest position in rejection. The variation behaviors of ion rejections, however, seems to be similar in all membranes and Na + rejection becomes worse as molar ratio of M g 2 + to Na + becomes higher on every kind of membrane, degrees of which are different each other, though. The variation behaviors of each ion rejection, for example, is illustrated in Figure 7 in the case of PEC-1000 membrane at a similar operating conditions as those in Figure 6 except that the total feedwater salinity is 0.03 mol/liter. Discussion Characteristics of charged membrane. When solid material such as membrane or resin which are negatively charged, for example, are contacted with saline water, there exists theoretical relationships exhibited in the following equations as the result of "Donnan equilibrium" under atmospheric condition (14).



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Figure 5. Relationships between molar ratio of divalent cation (Mg 2+ or Ca 2 + ) to monovalent cation (Na+) and relative A values.

Figure 6. Relationships between molar ratio of divalent cation (Mg 2+ ) to monovalent cation (Na+) and Na + rejection.


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where, X~: A^: B-p Co: Y:

fixed charge density anion concentration in the solid phase cation concentration in the solid phase salt concentration in the liquid phase activity coefficient

When membranes are negatively charged, distinctive characteristics of charged membranes come into appearance in the salinity range of X~ » Co because it approaches to asymptotic values of A£ -* 0 and Bi + X" according to equations (3) and (4) (See Figure 8 ) . There exists quite opposite relationship for positively charged membranes. These relationships, however, are strictly achieved only when equilibrium exists in the system without hydrodynamic forces, such as an applied pressure in RC evaluation system. In spite of the existence of such restriction mentioned above, similar relationships are obtained in our experiment as already shown in Figure 2 even in the system where hydrodynamic force exists; in RO evaluation system. Therefore, from these ion separation and transport characteristics through these three membranes described in Figure 2 and Figure 8, schematic transport characteristics through these three kinds of representative RO membranes may be illustrated in Figure 9. Donnan-membrane effects. In the hyperfiltration or RO which hydrodynamic forces exist through membrane, it is reported that the presence of a membrane-impermeable ion in the pressurized solution can markedly affect the transport characteristics of co-ions through the membranes and this effect, at the same time, is general for any kind of membrane (16). Lonsdale et al. show a following equation from theoretical point of view, when the impermeable ion, for example, is anionic ion such as citrate and permeable ion is Cl~ (16).

where, S_: S0:

rejection for membrane-permeable anion salt rejection in the absence of membrane-impermeable anion B: solute permeability coefficient in the absence of membrane-impermeable anion B*: solute permeability coefficient in the presence of membrane-impermeable anion Ci 1 : concentration of membrane-impermeable anion in the feedwater C_ f : concentration of membrane-permeable anion in the feedwater Equation 5 leads to following relationships (See Figure 10).



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Figure 7. Relationships between mole fraction of MgCl2 and each ion rejection in the case of PEC-1000 membrane.

Figure 8. Concentration of counter-ions B£ and of co-ions Ai in the fixed charge phase as against the electrolyte concentration C of the outer aqueous phase.



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Figure 9.


197

Permeation scheme through membranes.

Figure 10. The rejection of co-ion as a function of the initial rejection and the ratio of membrane-impermeable ion to co-ion.


198


as as as


and

as

Figure 10 shows that rejection for membrane-permeable anion (S_) becomes worse as salt rejection in the absence of membrane impermeable anion (So) becomes lower at any concentration ratio of CiVC-*. Though the value of rejection itself is not strictly independent on membrane materials (16), membranes which show higher SQ value may be considered to be more perfect semipermeable membrane than those which show lower So value from this theory. PEC-1000 membrane may be considered to be the most perfect semipermeable membrane among these three kinds of membranes evaluated in this paper from the comparison of Figure 6 and Figure 10 because M g 2 + is considered to be almost impermeable cation and Na + is permeable as shown in Figure 7. The perfectneps of PEC-1000 membrane is also supported by the phenomenological transport study by Pusch et al. (ljO). Conclusions Results in ion separation and transport characteristics through the membrane suggest that PEC-1000 membrane is negatively charged and it shows quite different behaviors from those of other representative RO membranes as CA membrane and PA composite membrane, which may be suggested to be neutrally charged and slightly positively charged, respectively. PEC-1000 membrane is also confirmed to be negatively charged by ion-exchange capacities and membrane potential data. Water permeability (A value) of the membrane becomes lower in the presence of divalent metal ions, degrees of which are quantatively determined by the molar ratio to monovalent metal ions contained in the feedwater. This membrane may be considered to be the most perfect semipermeable membrane among the membranes, reported in this paper from the ion rejection behaviors derived from "Donnan-membrane effects11 through membranes. Literature Cited 1. 2. 3. 4. 5.

6.

Satone, H. Proc. 9th Annual Conference of NWSIA. Vol.1, Session II, Washington, D.C., 1981. Brandt, D.C.; Battey, R.F. Proc. 10th Annual Conference of WSIA. Vol.I, Session II, Honolulu, Hawaii, July 25-29, 1982. Kurihara, M.; Kanamaru, N.; Harumiya, N.; Yoshimura, K.; Hagiwara, S. Desalination 1980, 32, 13. Kurihara, M.; Harumiya, N.; Kanamaru, N.; Tonomura, T.; Nakasatomi, M. Desalination 1981, 38, 449. Kurihara, M.; Nakagawa, Y.; Takeuchi, H.; Kanamaru, N.; Tonomura , T. Proc. 10th Annual Conference of WSIA. Vol. II, Session VII. Honolulu, Hawaii, July 2 5 - 2 9 , 1982. Kurihara, M. Membrane, 1983, 8, 97.


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8.


9.

10. 11. 12. 13. 14. 15. 16.

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Nakagawa, Y. Workshop on Membrane Separation Technology: Application to Foods and Biochemical Processes, National Taiwan Univ., March 1 7 - 1 8 , 1983. Kurihara, M.; Nakagawa, Y.; Takeuchi, H.; Kanamaru, N.; Tonomura, T. Desalination 1983, 46, 101. Doelle, R.A.; Kallenberg, K.H.; Heyden, W. Poster Presentations of First World Congress on Desalination and water Reuse, Florence, Italy, May 2 3 - 2 7 , 1983. Chen, J.Y.; Kurihara, M; Pusch W. Desalination 1983, 46, 379. Kurihara, M; Watanabe, T.; Inoue, T.; U.S. Patent 4 366 062, 1982. Cadotte, J.E., U.S. Patent 4 277 344, 1981. Yamabe, T. et al. Nihon Kagaku Kaishi 1975, 10, 1713. Lonsdale, H.K. Desalination 1973, 13, 317. Hanai, T. "Maku to Ion": Kagaku Doujin, Kyoto, 1980, P171. Lonsdale, H.K.; Pusch, W.; Walch, A.; J.C.S. Faraday Trans I 1975, 71, 501.

RECEIVED March 13, 1985


Solute Separation and Transport Characteristics Through Polyether

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