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Waste water treatment using reverse osmosis: real osmotic pressure and chemical functionality as influencing parameters on the retention of carboxylic acids in multi-component systems
•
,
DESALINATION
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ELSEVIER
Desalination 110 (1997) 213-222
Waste water treatment using reverse osmosis: real osmotic pressure and chemical functionality as influencing parameters on the retention of carboxylic acids in multi-component systems V. T6dtheide*, G. Laufenberg, B. Kunz Department of Food Technology, University of Bonn, ROmerstrasse 164, 53117 Bonn, Germany Tel. +49 228-734453; Fax +49 228-734429 Received 22 January 1997; accepted 1 June 1997
Abstract The influence of chemical functionality in carboxylic acids and osmotic pressure of multi-component solutions on the retention in the reverse osmosis process are discussed. Therefore formic-, acetic-, propionic-, glycolic-, acrylic- and methoxyacetic acid (target substances) were combined with one or two other carboxylic acids (active substances), chosen out of a pool of 16. All investigations were carried out with an aromatic polyamide membrane and the operating conditions were kept constant. Although the combination of all effects is extremely complex, the experiments showed that the influence on the retention of a substance equate as an outcome of molecular mass, acidity, functionality and spatial requirement. The influence of functionality in the active substances could be further divided into an additional carboxylic group, double bond and aromaticity. The calculation of the osmotic pressure demonstrated that there was no observable difference between the real and ideal one when the solution contained the same number of components. Therefore it can be concluded that in this study the osmotic pressure has no influence on the retention of the target substance.
Keywords: Reverse osmosis; Multicomponent systems; Organic acids; Osmotic pressure; Chemical functionality; Selectivity; Intermolecular interactions
1. I n t r o d u c t i o n
The need for clean water increases year by year, and yet water sources are reduced by pollution of our waterways by industrial *Corresponding author.
chemicals. Reverse osmosis (RO) is used for the treatment of waste and raw water since it generally operates at room temperature and it is economical. However, there is a lack of information about the selectivity of single substances in multi-component solutions. A membrane process is controlled both by the
0011-9164/97/$17.00 Copyright © 1997 Published by Elsevier Science B.V. All rights reserved. PH SO011-9164(97)00100-8
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v. T6dtheide et aL / Desalination 110 (1997) 213-222
membrane properties and the operating conditions, e.g. structure of membrane, membrane pore size, permeate flow, hydrostatic pressure, temperature, etc. There is also the influence of the feed solution itself. Thus it is necessary to examine the molecular interactions between the solutes and their influence on the retention of a substance to be separated. It is possible to differentiate between solution-specific parameters such as osmotic pressure, chemical potential, pH value, etc., and substance-specific parameters, i.e., molar mass, hydrogen bonds, functionality, steric hindrance, etc. Although several reviews deal with the influence of these parameters on the retention within binary systems [1-4], few studies have examined the influence on the retention within multi-component systems [5-7] where there is a combination of all parameters. In seeking to determine the selectivity of RO associated with treatment of waste water containing multiple substances, it is necessary to predetermine the extent to which the retention of a number of substances is influenced by other solutes.
It was therefore decided to examine the influence on the retention of six carboxylic acids when combined with other carboxylic acids in binary and ternary solutions. It was shown how the influence on retention can be combined with specific-substance parameters. By calculating the osmotic pressure of select feed solutions, it was possible to establish the link between theoretical models and empirical values. This study also sought to promote the ability to increase the separation capacity of a target substance in combination with other substances.
2. Experimental 2.1. Materials
The experiment was performed using an RO apparatus supplied by Sempas Membrantechnik GmbH. This equipment contains a composite membrane synthesized from an aromatic polyamide (Toray: TR70-2514 F). The spiral wound module has a membrane area of approximately 0.7m 2. This resulted in 99% rejection for 0.026mol'L -l NaC1 at 14.4 bar (Fig. 1).
pH-met©r ~I¢F
f
!Jill[l==
ion ¢xch~g~r
P ~.~..
R.O.module
V3
Fig. 1. Flow chart of reverse osmosis plant (internal recirculation). P high-pressure pump; V 1 by-pass valve; V2 throttle valve; V3 excess pressure valve; M1 feed pressure gauge; M2 transmembranepressure gauge; M3 pump pressure gauge; F1 feed flow gauge; S sample point.
V. T6dtheide et al. / Desalination 110 (1997) 213-222
The RO apparatus was used under the following operating conditions: the hydrostatic pressure was 10 bar, the feed flow was maintained at 2.5 L'min -l, and the temperature was stabilized between 18 to 23 °C. In this series of experiments, the model solution was a composite of water as the solvent and at least one carboxylic acid. If the solution consists of more than one solute, a distinction was made between the target substance to be separated, and the active substance which possibly influences the rejection of the target substance. Model solutions: • one component solution (solvent water plus one carboxylic acid) • binary solution (solvent water plus two carboxylic acids) • ternary solution (solvent water plus three carboxylic acids) All carboxylic acids in binary or ternary solutions were used in a concentration of 10 mmol'L- 1 (Table 1). 2.2. M e t h o d s
Firstly, all one component solutions of the target substances were produced and the retention of the components determined. Next, combinations of the target substances and the active substances (Table 1) were prepared, and their influence on the retention of every target substance was closely examined.
215
The most effective active substances, in terms of retention, were subsequently combined in ternary solutions. After 5 min, five samples were taken from the feed (Cr) and the permeate (Cp) in the RO test rig. By this stage ideal operating parameters had been achieved. The different concentrations of the samples were analyzed by HLPC. The following equation was used to calculate the degree of retention (R): R=I-
C
e[%] Cr
For every model solution, five retention data were obtained and the median was determined. Using the U-test by Wilcoxon, Mann and Whitney, this was assessed to see whether the median of the target substance was significantly higher or lower than that of the same substance in any of the other solutions [8]. The osmotic pressure was calculated using the "modified UNIFAC" (DDBST GmbH) computer program which facilitates the calculation of the real osmotic pressure in comparison to the ideal pressure. In calculating the real osmotic pressure of solutions, the activity coefficients of the solutes are required. The program contains a major data base of more than 500 components, which facilitates the comparison of various solutions by the use of a group contribution method "modified UNIFAC" and the "Poynting Factor", resulting in the calculation of the
Table 1 Organic acids under investigation; target substances are emphasized Monocarboxylic a c i d s
Dicarboxylic acids
Oxocarboxylic acids
Aromatic carboxylic acids
Formic Acetic Propionic Butyric Isobutyric Cyclohexanecarboxylic Acrylic
Oxalic Malonic Glutaric Fumaric
Glycolic Methoxy acetic Glyoxylic
Benzoic Phthalic Trimellitic Trimesinic Vanillinic
216
V. TOdtheide et al. / Desalination 110 (1997) 213-222
osmotic pressure as a function of temperature and concentration [9]. The following equations were used to calculate the real and ideal osmotic pressure:
60" •~
~o.
• 55.7
• 54.0 • 49.1 • 44,4
-~ 40.
* 34.1
~ 3o. ~ 20 + o
II [MPa] -
R. T[K] lnxwater"Ywater (real) 1,'wate r [1/mol]
I
I
I
I
I
20
40
60
80
100
Concentration of acetic acid [ m m o l / L 1
II [MPa] = R" T[K] ~
c~ [mol/l] (ideal)
where II is the osmotic pressure [MPa], R the gas constant, T the temperature [K], Vwater the molar volume of solvent [L/moll, Xwater the mole fraction of solvent, Ywaterthe activity coefficient (given by mod. UNIFAC) and c~ the concentration of component i. Input of temperature and concentration of the solutes into the computer program automatically resulted in the required real and ideal osmotic pressures.
Fig. 2. Acetic acid retention vs. feed concentration. 8O •
71.7
• 68.2
• 66.7
60 • 55.9
,u
• 49.11
-~a 40 c~
~ 2o
0
I ! I I 20 40 60 80 Concentration of gly¢olic acid Immol/L 1
I 100
3. Results
Fig. 3. Glycolic acid retention vs. feed concentration.
3.1. One component solutions
solution and the existence of interactions between the various components [10,11]. Although in some investigations propionic or formic acid was used, the usual target substance was acetic acid. Whereas the aim of this work was to determine if target substance specificity existed, and since other carboxylic acids existed in the solution itself, the alternating behaviour of the retention of glycolic-, acrylic-, and methoxyacetic acid was examined in binary solutions. Drawing on the work of previous investigations which highlighted that only particular carboxylic acids showed a significant influence on the retention of acetic acid, this study focused solely on these carboxylic acids. The results obtained from this series of experiments are shown in Figs. 4-6. The retention of the target substance was variously influenced by the active substance; however, there existed a number of similarities in the behaviour of the active substance.
One component solutions of the target substances were prepared in various concentrations (5, 10, 20, 50 and 100mmol.L -1) as described above. These data were required to determine the extent to which the retention of a substance was influenced by increasing the concentration of the solution. Figs. 2 and 3 illustrate that the degree of retention is insignificantly altered in concentrations of 5-30mmol.L -1. Since this corresponded to the concentrations used in the development of the model solutions, it was clear that changes in the degree of retention achieved in this work were not the result of increasing total concentrations of the target solution.
3.2. Binary solutions Previous publications have examined the connection between the composition of the
T6dtheide et al. / Desalination 110 (1997) 213-222 trlmcilitic acid
~
217
9.~
i
phthalic acid
i
3.1
benzoic acid
~
fumaric acid
0
glutoric acid
~
6.6
4,4
malonic acid -12,5 oxalic acid
-10,6
isobutyric acid
~
butyric acid
9
-----
propienic acid
16.1
~
6.7
acetic acid formic acid
I -15
t -12
I -9
I -6
I -3
0
4.7 I 6
3
I 9
I 12
I 15
! 18
Fig. 4. Influence on the retention of glycolic acid due to a combination of various carboxylic acids (10 mmol.L -]) in binary 0 s o l u t i o n s (Rglycolic acid 71.7 ~ -~0).
A Retention 1%1
~ 4 o o
trimeUitic acid phthalic acid benzoic acid
5.1
fumaric acid glutaric acid malonic acid
-2,2
oxalic acid
-2.1
isobutyric acid
-6.~
butyric acid propionic acid
-1.9 I J
acetic acid
0
formic acid I -15
I
I
-12
-9
t -2.8 i~
-6
I 3
-3
I 6
I 9
I 12
I 15
I 18
Fig. 5. Influence on the retention of acrylic acid due to a combination of various carboxylic acids (10 mmol L -l) in binary s o l u t i o n s (Racrylic acid= 56.3% -~0).
A R e t e n t i o n I%1
-0
trimellitic acid phthalic acid
-6,7
benzoic acid
-3,3
I
fumaric acid glutaric acid
2.6
-4.9
malonic acid
-2.2
oxalic acid isobutyric acid
-6.8
butyric acid
-8.8
propionic acid
-3.9
acetic acid
-2.9
formic acid I -15
I -12
I -9
I -6
i -~
]0 0
I 3
I 6
A Retention [%]
I 9
I 12
I 15
I 18
Fig. 6. Influence on the retention of methoxyacetic acid due to a combination of various carboxylic acids (10 mmol .L-1) in binary solutions (Rme~oxyaccac acid = 71.7% g0).
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V. T6dtheide et al. / Desalination 110 (1997) 213-222
•
In general, aromatic carboxylic acids increase the retention of the target substance. • With rising molar mass, aliphatic monocarboxylic acids exhibit decrease in retention of the target substance (except glycolic acid retention). • The greater the number of carboxylic groups in the active substance, the greater the decrease in the retention of the target substance.
3.3. Ternary solutions The interactions between the substances increase in ternary solutions. The effects exhibited in binary solutions interact with each other in ternary solutions. It is necessary to distinguish such interactions in the following way:
•
• • •
Synergistic effect: In a synergism the combined effect is greater than the greatest single effect. This combined effect may be further divided into additive, collective and multiplicative effects. Additive: the total value is equal to the sum of the single values. Collective: the total value is lower than the sum of the single values. Multiplicative: the total value is significantly higher than the sum of the single values. Compensative effect: the individual values counteract each other. Stationary effect: the total value equates to one of the individual values. Opposite effect: combination of the individual values results in opposing behaviour when compared to the individual values.
Table 2 Influence on the retention of target substances in ternary solutions Combinations of acids with carboxylic acids (10 mmol .L- I)
Synergistic Negative Target substance:
acetic acid
Racea c
Glyoxylic/trimellitic Glyoxylic/cyclohexane carboxylic Glutaric/trimellitic Glutaric/cyclohexane carboxylic Formic/trimellitic Formic/oxalic Formic/cyclohexane carboxylic Target substance:
formic acid
Icid=46°/o=O 25.3 11.7 17.7 0 13.9 0 0
Target substance:
propionic
0 0
4.3 6.3 1.2 4.1 8.6 -10.8 - 4.2 8.8 10.8 -10.8 8.6 - 2.9
acid Rpropionic acid=68•0%=0
Benzoic/phthalic Formic/trimellitic Phthalic/trimellitic Benzoic/trimellitic Formic/phthalic Formic/malonic Acetic/formic Acetic/oxalic Formic/oxalic Formic/benzoic Formic/glutaric glycolic acid
Oxalic/Malonic Butyric/isobutyric Butyric/trimellitic Propionic/trimellitic Propionic/isobutyric Propionic/butyric Propionic/benzoic Acetic/propionic
0
Rformicacid=|0.8%=0
Acetic/trimellitic Acetic/cyclohexane carboxylic Acetic/propionic Pripionic/trimellitic Propionic/phthalic Oxalic/malonic Malonic/trimellitic Oxalic/benzoic Butyric/trimellitic Butyric/phthalic Propionic/malonic Propionic/oxalic
Target substance:
Ternary solutions of all target substances were prepared in order to determine the extent to which the retention of the target substance is influenced by combining the active substances. In this series of experiments, only those substances which had previously shown significant influences on retention in binary solutions were combined• No
A retention (%)
0 10.4 15.8 9.8 0
0 -
8 3.4
6.6 -10.1 -3 4 -
Rglycoli c a c i d = 7 1 . 7 % = 0
-
-
ll.9 8.6 7.6 0 0 8.8 4.3 5.7
219
V. T6dtheide et al. / Desalination 110 (1997) 213-222
Moreover the carboxylic acids are capable of forming hydrogen bonds.
Table 2 (continued) T a r g e t substance: acrylic acid Racrylic acid= 56"3% = 0
Fumaric/isobutyric Fumaric/trimellitic Fumaric/formic Formic/butyric Butyric/formic Butyric/isobutyric
0 6.7
0
3.4. Osmotic p r e s s u r e
0 0
2.6 0 0 4.3
Numerous publications have explained and verified the effect that the osmotic pressure of the feed solution influences the retention of the target substance, such that the higher the osmotic pressure of a solution the lower the retention of the target substance [12-14]. It is well known that
respect was paid to the direction of the influence, i.e., retention enhancing and diminishing influences were combined. Aside from data acquisition, special consideration was given to which of the described effects would prevail. The results of this series of experiments are shown in Table 2. Of particular interest is that retention of the target substance is generally decreased by the various active substances in ternary solutions. The target substance is only increased in active substance combinations which have a high spatial requirement, e.g., trimellitic acid. Additionally, the various molar masses influence the retention of the target substance. The retention of formic acid, which has the lowest mass of all carboxylic acids, was only decreased by a combination of the active substances, whereas combinations of active substances with higher spatial requirements and high molar mass resulted in synergistic effects. In these experiments a multiplicative effect was never observed. Several substances, which had previously increased the retention of the target substance in binary solutions, showed an opposite effect when combined in ternary solutions. Increasing the number of aromatic substances in a solution never resulted in an additive effect, and only collective or compensative effects were observed. This study has clearly demonstrated that the retention of a target substance is significantly influenced by the composition of the feed solution. There are Dipol-Dipol, Coulomb- or steric interactions between carboxylic acids.
i 0,8 0,6. 0,4~ 0,2.
0.1
0.2
0,3
0,4
0,5
Concentration gly¢oli¢ acid [tool/L]
Fig. 7. Real osmotic pressure plotted against concentration.
80
7
6o
"o 49.8 "~, 40
~. ~o '1" 0.05
I 0.1
I 0.15
I 0.2
I 0,25
Osmotic pressure [MPu]
Fig. 8. Glycolic acid: real osmotic pressure vs. retention.
220
V. T6dtheide et al. / Desalination 110 (1997) 213-222
Table 3 Glycolic acid in multi-component solutions - - real osmotic pressure and retention Combinationsof acids
One-componentsolutions Glycolicacid, 10 mmol Glycolicacid, 20 mmol Glycolicacid, 30 mmol Binary solutions (each l0 mmol) Glycolic/malonic Glycolic/oxalic Glycolic/fumaric Glycolic/acetic Glycolic/phthalic Glycolic/propionic Glycolic/benzoic Glycolic/isobutyric Glycolic/trimellitic Glycolic/butyric Ternary solutions (each 10 retool) Glycolic/malonic/oxalic Glycolic/butyric/isobutyric Glycolic/propionic/butyric Glycolic/butyric/trimellitic Glycolic/acetic/propionic Glycolic/propionic/benzoic Glycolic/propionic/isobutyric Glycolic/propionic/trimellitic
Real osmotic Retention pressure of glycolic (MPa) acid (%) 0.0246 0.049 0.074
0.049 0.049 0.049 0.049 0.049 0.049 0.049 0.049 0.049 0.049
0.074 0.074 0.074 0.074 0.074 0.074 0.074 0.074
71.7 66.7 55.9
59.2 61. I 73.4 74.6 74.8 78.3 78.4 80.7 81.2 87.8
59.8 62. 63.1 63.5 66 67.6 67.7 73.6
the physical character of solutions do not respond ideally with increase in concentration. Due to the differing solubilities of carboxylic acids, the solutions investigated in this study exhibit real behaviour. It is thus possible that the influence on the retention of the target substance is due to the differing osmotic pressures in ternary and temary solutions. Both the ideal and the real osmotic pressures were calculated for every single combination in binary, ternary and in ternary solutions. Figs. 7 and 8 and Table 3 relate to the collected data obtained in the investigation of glycolic acid; they illustrate a small portion of the total results obtained in this study.
Table 4 Glycolic acid in multi-componentsolutions - - pH value and retention Combinationsof acids
pH value
Retentionof glycolicacid (%)
One-componentsolution Glycolicacid, 10 mmol
2.85
71.7
Binary solutions (each 10 mmol) Glycolic/malonic Glycolic/oxalic Glycolic/fumaric Glycolic/acetic Glycolic/phthalic Glycolic/propionic Glycolic/benzoic Glycolic/isobutyric Glycolic/trimellitic Glycolic/butyric
2.25 1.9 2.38 2.74 2.28 2.67 3.15 2.79 2.14 2.78
59.2 61.1 73.4 74.6 73.8 78.3 78.4 80.7 81.2 87.8
Fig. 7 shows that there is a linear correlation between the concentration and the real osmotic pressure of glycolic acid solutions when calculated using "modified UNIFAX". Fig. 8 shows that the retention of glycolic acid decreases with increasing osmotic pressure. As previously stated, this is due to the interdependence of concentration and osmotic pressure. However, Table 3 illustrates that the osmotic pressure of the investigated solution depends solely on the total concentration of a solution and not on the various substances contained therein. Hence, the influence on the retention of a target substance in this study is not altered by the changing osmotic pressure but rather by the different composition of the substances. There was no observable difference between the real and ideal osmotic pressure when the solution contained the same number of components. 3.5. Acidity
It is known that the degree of ionization of substances stands in relation to the pH value of the solution. Several publications [15,16] have
V. TOdtheide et al. / Desalination 110 (1997) 213-222
shown the influence of pH value on the retention of carboxylic acids. Therefore the retention of carboxylic acids vs. pH value, which has ranged between 1.8 and 3.2, was determined. Table 4 exhibits these connections by example of glycolic acid in the multicomponent solutions. It is shown that there is only a poor connection between the pH value and the retention of the target substance in this study (plainly recognizable in combinations glycolic acid/phthalic acid due to glycolic acid/malonic acid as well as glycolic acid/acetic acid due to glycolic acid/ butyric acid). 4. Discussion
The combination of all the effects studied is extremely complex, and ultimately it is the sum of the interactions between the membrane, the solvent and the solutes which determines the permeation of a single substance. It was the aim of this study to equate the retention of a substance as an outcome of molar mass, functionality, spatial requirement, acidity and osmotic pressure. 4.1. Molar mass
The retention of a target substance increases with an increase in molar mass. Within an acid group, increasing molar mass results in a corresponding increase in the influencing effect on the retention of a target substance. Monocarboxylic acids impair the degree of retention whereas an aromatic acid enhances the degree of retention. A combination of the two acid groups in ternary solutions results in an increase in the degree of retention. 4.2. Functionality
All the examined substances are carboxylic acids and therefore contain at least one functional group. Substances containing further functional groups exhibited additional results and were classified as follows.
221
4.2.1.Additional carboxylic group
If an additional carboxylic group is inserted into the molecule, the acidity increases and there is an increased number of interactions between functional groups. It was generally found that the aliphatic acids such as oxalic-, malonic- and glutaric acid decrease the retention of a target substance. In the majority of cases aromatic polycarboxylic acids result in an increasing retention. However, the aromatic ring produces reverse results. Combining aliphatic dicarboxylic acids with aliphatic monocarboxylic acids decreases the retention of the target substance, whereas combinations of aliphatic dicarboxylic acids with aromatic carboxylic acids result in an increase in the degree of retention. 4.2.2. Double bond
Fumaric acid, the only active substance with a double bond itself, enhances the degree of retention, whereas when combined with other carboxylic acids impairs the retention of the target substance. The double bond interacts with other functional groups in the target substance as is demonstrated when this is combined with acrylic acid. It is possible for interactions with the double bond to occur due to the differing densities of electrons in the molecules, which results in a reduction of the permeation of acrylic acid. 4.2.3. Aromatieity
In the majority of the cases carboxylic acids with an aromatic ring result in an increase in the retention of the target substance. This is particularly the case with trimellitic acid. Since this contains three carboxylic groups with a resultant greater spatial requirement, it demonstrates the greatest influence on the retention of a target substance. 4.3. Spatial requirement
When comparing butyric- and isobutyric acid the more steric ambitious molecule exhibits a
222
v. TOdtheide et al. / Desalination 110 (1997) 213-222
lesser influence on the retention of the target substance. In competition with the target substance, the greater spatial requirement of isobutyric acid inhibits permeation through the membrane. Aromatic carboxylic acid exhibits a higher spatial requirement than aliphatic acid due to the presence of the aromatic ring. The increasing spatial requirement in the sequence benzoic-, phthalic and trimellitic acid, due to the increasing numbers of carboxylic groups in the aromatic ring, similarly exhibits a greater influence on the retention of the target substance. These effects may be explained in one of two ways: first, since more interactions exist between the aromatic carboxylic acid and the target substance, this results in differing densities of electrons in the molecules or in an increase in hydrogen bonds between the carboxylic groups. In the ideal case, a cluster of target and active substances are created which results in ineffective permeation through the membrane. Second, consideration needs to be given to the permeation of the target substance in competition with other substances. Previous studies have shown that the aromatic ring interacts with the aromatic polyamide membrane and therefore facilitates the permeation of other substances. For this reason it is feasible that, when the target substance competes with aliphatic carboxylic acids, permeation through the membrane is enhanced. 4.4. A c i d i t y
The pH value of the feed solution in this study ranged between 1.8 and 3.2. It was observed that within this pH range acidity exhibited an inconsequential influence on the retention of the target substance. 4.5. O s m o t i c p r e s s u r e
The operational conditions of the RO are determined by the relationship between the osmotic pressure of the feed solution and the
applied hydrostatic pressure. It was observed that there existed a linear correlation between the concentration of a solution and the osmotic pressure. Since the composition of the investigated solutions was not consequential in determining the osmotic pressure, it can be concluded that osmotic pressure is determined solely by the total concentration. Within the concentration parameters used in this experiment, no difference existed between real and ideal osmotic pressure. Hence, it can be concluded that the osmotic pressure has no influence on the retention of the target substance.
References
[1] A. Alsaygh, P. Jennings and M. Bader, J. Environ. Sci. Health, A28(8) (1993) 1669. [2] Y. Fang, S. Sourirajan and T. Matsuura, J. Appl. Polym. Sci., 44 (1992) 1959. [3] Y. Kiso, T. Kitao, K. Jinno and M. Miyagi, J. Membr. Sci., 74 (1992) 95. [4] L. Sun, E.M. Perdue and J.F. Mc Carthy, Wat. Res., 29(6) (1995) 1471. [5] S. Hausmanns, G. Laufenberg and B. Kunz, Desalination, 104 (1996) 95. [6] G. Laufenberg, R. Fischbach and B. Kunz, Posterprint 5, Aachener Membrankolloqium, 1995. [7] B. Ditgens, R. Fischbach, G. Laufenberg and B. Kunz, Chemie Ingenieur und Technik, 68(8) (1996) 965. [8] P. Z6fel (ed.), Statistik in der Praxis, Gustav Fischer Verlag, Stuttgart, 1985. [9] J. Gmehling, J. Li and M. Schiller, Ind. Eng. Chem. Res., 32(1) (1993) 178. [10] G. Laufenberg, S. Hausmanns and B. Kunz, J. Membr. Sci., 110 (1996) 59. [11] G. Laufenberg and B. Kunz, Posterprint Euromembrane '95, Bath, 1995. [12] S.Q. Zhang and A.E. Fouda, Desalination, 80 (1991) 211. [13] J.M. Dickson, J. Spencer and M.L. Costa, Desalination, 89 (1992) 63. [14] R. Rautenbach and A. Gr6schl, Desalination, 90 (1993) 93. [15] H.H.P. Fang and E.S.K. Chian, Environ. Sci. Technol., 10(6) (1976) 364. [16] M.K.H. Liew, S. Tanaka and M. Morita, Desalination, 101 (1995) 269.
,
DESALINATION
%,%
ELSEVIER
Desalination 110 (1997) 213-222
Waste water treatment using reverse osmosis: real osmotic pressure and chemical functionality as influencing parameters on the retention of carboxylic acids in multi-component systems V. T6dtheide*, G. Laufenberg, B. Kunz Department of Food Technology, University of Bonn, ROmerstrasse 164, 53117 Bonn, Germany Tel. +49 228-734453; Fax +49 228-734429 Received 22 January 1997; accepted 1 June 1997
Abstract The influence of chemical functionality in carboxylic acids and osmotic pressure of multi-component solutions on the retention in the reverse osmosis process are discussed. Therefore formic-, acetic-, propionic-, glycolic-, acrylic- and methoxyacetic acid (target substances) were combined with one or two other carboxylic acids (active substances), chosen out of a pool of 16. All investigations were carried out with an aromatic polyamide membrane and the operating conditions were kept constant. Although the combination of all effects is extremely complex, the experiments showed that the influence on the retention of a substance equate as an outcome of molecular mass, acidity, functionality and spatial requirement. The influence of functionality in the active substances could be further divided into an additional carboxylic group, double bond and aromaticity. The calculation of the osmotic pressure demonstrated that there was no observable difference between the real and ideal one when the solution contained the same number of components. Therefore it can be concluded that in this study the osmotic pressure has no influence on the retention of the target substance.
Keywords: Reverse osmosis; Multicomponent systems; Organic acids; Osmotic pressure; Chemical functionality; Selectivity; Intermolecular interactions
1. I n t r o d u c t i o n
The need for clean water increases year by year, and yet water sources are reduced by pollution of our waterways by industrial *Corresponding author.
chemicals. Reverse osmosis (RO) is used for the treatment of waste and raw water since it generally operates at room temperature and it is economical. However, there is a lack of information about the selectivity of single substances in multi-component solutions. A membrane process is controlled both by the
0011-9164/97/$17.00 Copyright © 1997 Published by Elsevier Science B.V. All rights reserved. PH SO011-9164(97)00100-8
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v. T6dtheide et aL / Desalination 110 (1997) 213-222
membrane properties and the operating conditions, e.g. structure of membrane, membrane pore size, permeate flow, hydrostatic pressure, temperature, etc. There is also the influence of the feed solution itself. Thus it is necessary to examine the molecular interactions between the solutes and their influence on the retention of a substance to be separated. It is possible to differentiate between solution-specific parameters such as osmotic pressure, chemical potential, pH value, etc., and substance-specific parameters, i.e., molar mass, hydrogen bonds, functionality, steric hindrance, etc. Although several reviews deal with the influence of these parameters on the retention within binary systems [1-4], few studies have examined the influence on the retention within multi-component systems [5-7] where there is a combination of all parameters. In seeking to determine the selectivity of RO associated with treatment of waste water containing multiple substances, it is necessary to predetermine the extent to which the retention of a number of substances is influenced by other solutes.
It was therefore decided to examine the influence on the retention of six carboxylic acids when combined with other carboxylic acids in binary and ternary solutions. It was shown how the influence on retention can be combined with specific-substance parameters. By calculating the osmotic pressure of select feed solutions, it was possible to establish the link between theoretical models and empirical values. This study also sought to promote the ability to increase the separation capacity of a target substance in combination with other substances.
2. Experimental 2.1. Materials
The experiment was performed using an RO apparatus supplied by Sempas Membrantechnik GmbH. This equipment contains a composite membrane synthesized from an aromatic polyamide (Toray: TR70-2514 F). The spiral wound module has a membrane area of approximately 0.7m 2. This resulted in 99% rejection for 0.026mol'L -l NaC1 at 14.4 bar (Fig. 1).
pH-met©r ~I¢F
f
!Jill[l==
ion ¢xch~g~r
P ~.~..
R.O.module
V3
Fig. 1. Flow chart of reverse osmosis plant (internal recirculation). P high-pressure pump; V 1 by-pass valve; V2 throttle valve; V3 excess pressure valve; M1 feed pressure gauge; M2 transmembranepressure gauge; M3 pump pressure gauge; F1 feed flow gauge; S sample point.
V. T6dtheide et al. / Desalination 110 (1997) 213-222
The RO apparatus was used under the following operating conditions: the hydrostatic pressure was 10 bar, the feed flow was maintained at 2.5 L'min -l, and the temperature was stabilized between 18 to 23 °C. In this series of experiments, the model solution was a composite of water as the solvent and at least one carboxylic acid. If the solution consists of more than one solute, a distinction was made between the target substance to be separated, and the active substance which possibly influences the rejection of the target substance. Model solutions: • one component solution (solvent water plus one carboxylic acid) • binary solution (solvent water plus two carboxylic acids) • ternary solution (solvent water plus three carboxylic acids) All carboxylic acids in binary or ternary solutions were used in a concentration of 10 mmol'L- 1 (Table 1). 2.2. M e t h o d s
Firstly, all one component solutions of the target substances were produced and the retention of the components determined. Next, combinations of the target substances and the active substances (Table 1) were prepared, and their influence on the retention of every target substance was closely examined.
215
The most effective active substances, in terms of retention, were subsequently combined in ternary solutions. After 5 min, five samples were taken from the feed (Cr) and the permeate (Cp) in the RO test rig. By this stage ideal operating parameters had been achieved. The different concentrations of the samples were analyzed by HLPC. The following equation was used to calculate the degree of retention (R): R=I-
C
e[%] Cr
For every model solution, five retention data were obtained and the median was determined. Using the U-test by Wilcoxon, Mann and Whitney, this was assessed to see whether the median of the target substance was significantly higher or lower than that of the same substance in any of the other solutions [8]. The osmotic pressure was calculated using the "modified UNIFAC" (DDBST GmbH) computer program which facilitates the calculation of the real osmotic pressure in comparison to the ideal pressure. In calculating the real osmotic pressure of solutions, the activity coefficients of the solutes are required. The program contains a major data base of more than 500 components, which facilitates the comparison of various solutions by the use of a group contribution method "modified UNIFAC" and the "Poynting Factor", resulting in the calculation of the
Table 1 Organic acids under investigation; target substances are emphasized Monocarboxylic a c i d s
Dicarboxylic acids
Oxocarboxylic acids
Aromatic carboxylic acids
Formic Acetic Propionic Butyric Isobutyric Cyclohexanecarboxylic Acrylic
Oxalic Malonic Glutaric Fumaric
Glycolic Methoxy acetic Glyoxylic
Benzoic Phthalic Trimellitic Trimesinic Vanillinic
216
V. TOdtheide et al. / Desalination 110 (1997) 213-222
osmotic pressure as a function of temperature and concentration [9]. The following equations were used to calculate the real and ideal osmotic pressure:
60" •~
~o.
• 55.7
• 54.0 • 49.1 • 44,4
-~ 40.
* 34.1
~ 3o. ~ 20 + o
II [MPa] -
R. T[K] lnxwater"Ywater (real) 1,'wate r [1/mol]
I
I
I
I
I
20
40
60
80
100
Concentration of acetic acid [ m m o l / L 1
II [MPa] = R" T[K] ~
c~ [mol/l] (ideal)
where II is the osmotic pressure [MPa], R the gas constant, T the temperature [K], Vwater the molar volume of solvent [L/moll, Xwater the mole fraction of solvent, Ywaterthe activity coefficient (given by mod. UNIFAC) and c~ the concentration of component i. Input of temperature and concentration of the solutes into the computer program automatically resulted in the required real and ideal osmotic pressures.
Fig. 2. Acetic acid retention vs. feed concentration. 8O •
71.7
• 68.2
• 66.7
60 • 55.9
,u
• 49.11
-~a 40 c~
~ 2o
0
I ! I I 20 40 60 80 Concentration of gly¢olic acid Immol/L 1
I 100
3. Results
Fig. 3. Glycolic acid retention vs. feed concentration.
3.1. One component solutions
solution and the existence of interactions between the various components [10,11]. Although in some investigations propionic or formic acid was used, the usual target substance was acetic acid. Whereas the aim of this work was to determine if target substance specificity existed, and since other carboxylic acids existed in the solution itself, the alternating behaviour of the retention of glycolic-, acrylic-, and methoxyacetic acid was examined in binary solutions. Drawing on the work of previous investigations which highlighted that only particular carboxylic acids showed a significant influence on the retention of acetic acid, this study focused solely on these carboxylic acids. The results obtained from this series of experiments are shown in Figs. 4-6. The retention of the target substance was variously influenced by the active substance; however, there existed a number of similarities in the behaviour of the active substance.
One component solutions of the target substances were prepared in various concentrations (5, 10, 20, 50 and 100mmol.L -1) as described above. These data were required to determine the extent to which the retention of a substance was influenced by increasing the concentration of the solution. Figs. 2 and 3 illustrate that the degree of retention is insignificantly altered in concentrations of 5-30mmol.L -1. Since this corresponded to the concentrations used in the development of the model solutions, it was clear that changes in the degree of retention achieved in this work were not the result of increasing total concentrations of the target solution.
3.2. Binary solutions Previous publications have examined the connection between the composition of the
T6dtheide et al. / Desalination 110 (1997) 213-222 trlmcilitic acid
~
217
9.~
i
phthalic acid
i
3.1
benzoic acid
~
fumaric acid
0
glutoric acid
~
6.6
4,4
malonic acid -12,5 oxalic acid
-10,6
isobutyric acid
~
butyric acid
9
-----
propienic acid
16.1
~
6.7
acetic acid formic acid
I -15
t -12
I -9
I -6
I -3
0
4.7 I 6
3
I 9
I 12
I 15
! 18
Fig. 4. Influence on the retention of glycolic acid due to a combination of various carboxylic acids (10 mmol.L -]) in binary 0 s o l u t i o n s (Rglycolic acid 71.7 ~ -~0).
A Retention 1%1
~ 4 o o
trimeUitic acid phthalic acid benzoic acid
5.1
fumaric acid glutaric acid malonic acid
-2,2
oxalic acid
-2.1
isobutyric acid
-6.~
butyric acid propionic acid
-1.9 I J
acetic acid
0
formic acid I -15
I
I
-12
-9
t -2.8 i~
-6
I 3
-3
I 6
I 9
I 12
I 15
I 18
Fig. 5. Influence on the retention of acrylic acid due to a combination of various carboxylic acids (10 mmol L -l) in binary s o l u t i o n s (Racrylic acid= 56.3% -~0).
A R e t e n t i o n I%1
-0
trimellitic acid phthalic acid
-6,7
benzoic acid
-3,3
I
fumaric acid glutaric acid
2.6
-4.9
malonic acid
-2.2
oxalic acid isobutyric acid
-6.8
butyric acid
-8.8
propionic acid
-3.9
acetic acid
-2.9
formic acid I -15
I -12
I -9
I -6
i -~
]0 0
I 3
I 6
A Retention [%]
I 9
I 12
I 15
I 18
Fig. 6. Influence on the retention of methoxyacetic acid due to a combination of various carboxylic acids (10 mmol .L-1) in binary solutions (Rme~oxyaccac acid = 71.7% g0).
218
V. T6dtheide et al. / Desalination 110 (1997) 213-222
•
In general, aromatic carboxylic acids increase the retention of the target substance. • With rising molar mass, aliphatic monocarboxylic acids exhibit decrease in retention of the target substance (except glycolic acid retention). • The greater the number of carboxylic groups in the active substance, the greater the decrease in the retention of the target substance.
3.3. Ternary solutions The interactions between the substances increase in ternary solutions. The effects exhibited in binary solutions interact with each other in ternary solutions. It is necessary to distinguish such interactions in the following way:
•
• • •
Synergistic effect: In a synergism the combined effect is greater than the greatest single effect. This combined effect may be further divided into additive, collective and multiplicative effects. Additive: the total value is equal to the sum of the single values. Collective: the total value is lower than the sum of the single values. Multiplicative: the total value is significantly higher than the sum of the single values. Compensative effect: the individual values counteract each other. Stationary effect: the total value equates to one of the individual values. Opposite effect: combination of the individual values results in opposing behaviour when compared to the individual values.
Table 2 Influence on the retention of target substances in ternary solutions Combinations of acids with carboxylic acids (10 mmol .L- I)
Synergistic Negative Target substance:
acetic acid
Racea c
Glyoxylic/trimellitic Glyoxylic/cyclohexane carboxylic Glutaric/trimellitic Glutaric/cyclohexane carboxylic Formic/trimellitic Formic/oxalic Formic/cyclohexane carboxylic Target substance:
formic acid
Icid=46°/o=O 25.3 11.7 17.7 0 13.9 0 0
Target substance:
propionic
0 0
4.3 6.3 1.2 4.1 8.6 -10.8 - 4.2 8.8 10.8 -10.8 8.6 - 2.9
acid Rpropionic acid=68•0%=0
Benzoic/phthalic Formic/trimellitic Phthalic/trimellitic Benzoic/trimellitic Formic/phthalic Formic/malonic Acetic/formic Acetic/oxalic Formic/oxalic Formic/benzoic Formic/glutaric glycolic acid
Oxalic/Malonic Butyric/isobutyric Butyric/trimellitic Propionic/trimellitic Propionic/isobutyric Propionic/butyric Propionic/benzoic Acetic/propionic
0
Rformicacid=|0.8%=0
Acetic/trimellitic Acetic/cyclohexane carboxylic Acetic/propionic Pripionic/trimellitic Propionic/phthalic Oxalic/malonic Malonic/trimellitic Oxalic/benzoic Butyric/trimellitic Butyric/phthalic Propionic/malonic Propionic/oxalic
Target substance:
Ternary solutions of all target substances were prepared in order to determine the extent to which the retention of the target substance is influenced by combining the active substances. In this series of experiments, only those substances which had previously shown significant influences on retention in binary solutions were combined• No
A retention (%)
0 10.4 15.8 9.8 0
0 -
8 3.4
6.6 -10.1 -3 4 -
Rglycoli c a c i d = 7 1 . 7 % = 0
-
-
ll.9 8.6 7.6 0 0 8.8 4.3 5.7
219
V. T6dtheide et al. / Desalination 110 (1997) 213-222
Moreover the carboxylic acids are capable of forming hydrogen bonds.
Table 2 (continued) T a r g e t substance: acrylic acid Racrylic acid= 56"3% = 0
Fumaric/isobutyric Fumaric/trimellitic Fumaric/formic Formic/butyric Butyric/formic Butyric/isobutyric
0 6.7
0
3.4. Osmotic p r e s s u r e
0 0
2.6 0 0 4.3
Numerous publications have explained and verified the effect that the osmotic pressure of the feed solution influences the retention of the target substance, such that the higher the osmotic pressure of a solution the lower the retention of the target substance [12-14]. It is well known that
respect was paid to the direction of the influence, i.e., retention enhancing and diminishing influences were combined. Aside from data acquisition, special consideration was given to which of the described effects would prevail. The results of this series of experiments are shown in Table 2. Of particular interest is that retention of the target substance is generally decreased by the various active substances in ternary solutions. The target substance is only increased in active substance combinations which have a high spatial requirement, e.g., trimellitic acid. Additionally, the various molar masses influence the retention of the target substance. The retention of formic acid, which has the lowest mass of all carboxylic acids, was only decreased by a combination of the active substances, whereas combinations of active substances with higher spatial requirements and high molar mass resulted in synergistic effects. In these experiments a multiplicative effect was never observed. Several substances, which had previously increased the retention of the target substance in binary solutions, showed an opposite effect when combined in ternary solutions. Increasing the number of aromatic substances in a solution never resulted in an additive effect, and only collective or compensative effects were observed. This study has clearly demonstrated that the retention of a target substance is significantly influenced by the composition of the feed solution. There are Dipol-Dipol, Coulomb- or steric interactions between carboxylic acids.
i 0,8 0,6. 0,4~ 0,2.
0.1
0.2
0,3
0,4
0,5
Concentration gly¢oli¢ acid [tool/L]
Fig. 7. Real osmotic pressure plotted against concentration.
80
7
6o
"o 49.8 "~, 40
~. ~o '1" 0.05
I 0.1
I 0.15
I 0.2
I 0,25
Osmotic pressure [MPu]
Fig. 8. Glycolic acid: real osmotic pressure vs. retention.
220
V. T6dtheide et al. / Desalination 110 (1997) 213-222
Table 3 Glycolic acid in multi-component solutions - - real osmotic pressure and retention Combinationsof acids
One-componentsolutions Glycolicacid, 10 mmol Glycolicacid, 20 mmol Glycolicacid, 30 mmol Binary solutions (each l0 mmol) Glycolic/malonic Glycolic/oxalic Glycolic/fumaric Glycolic/acetic Glycolic/phthalic Glycolic/propionic Glycolic/benzoic Glycolic/isobutyric Glycolic/trimellitic Glycolic/butyric Ternary solutions (each 10 retool) Glycolic/malonic/oxalic Glycolic/butyric/isobutyric Glycolic/propionic/butyric Glycolic/butyric/trimellitic Glycolic/acetic/propionic Glycolic/propionic/benzoic Glycolic/propionic/isobutyric Glycolic/propionic/trimellitic
Real osmotic Retention pressure of glycolic (MPa) acid (%) 0.0246 0.049 0.074
0.049 0.049 0.049 0.049 0.049 0.049 0.049 0.049 0.049 0.049
0.074 0.074 0.074 0.074 0.074 0.074 0.074 0.074
71.7 66.7 55.9
59.2 61. I 73.4 74.6 74.8 78.3 78.4 80.7 81.2 87.8
59.8 62. 63.1 63.5 66 67.6 67.7 73.6
the physical character of solutions do not respond ideally with increase in concentration. Due to the differing solubilities of carboxylic acids, the solutions investigated in this study exhibit real behaviour. It is thus possible that the influence on the retention of the target substance is due to the differing osmotic pressures in ternary and temary solutions. Both the ideal and the real osmotic pressures were calculated for every single combination in binary, ternary and in ternary solutions. Figs. 7 and 8 and Table 3 relate to the collected data obtained in the investigation of glycolic acid; they illustrate a small portion of the total results obtained in this study.
Table 4 Glycolic acid in multi-componentsolutions - - pH value and retention Combinationsof acids
pH value
Retentionof glycolicacid (%)
One-componentsolution Glycolicacid, 10 mmol
2.85
71.7
Binary solutions (each 10 mmol) Glycolic/malonic Glycolic/oxalic Glycolic/fumaric Glycolic/acetic Glycolic/phthalic Glycolic/propionic Glycolic/benzoic Glycolic/isobutyric Glycolic/trimellitic Glycolic/butyric
2.25 1.9 2.38 2.74 2.28 2.67 3.15 2.79 2.14 2.78
59.2 61.1 73.4 74.6 73.8 78.3 78.4 80.7 81.2 87.8
Fig. 7 shows that there is a linear correlation between the concentration and the real osmotic pressure of glycolic acid solutions when calculated using "modified UNIFAX". Fig. 8 shows that the retention of glycolic acid decreases with increasing osmotic pressure. As previously stated, this is due to the interdependence of concentration and osmotic pressure. However, Table 3 illustrates that the osmotic pressure of the investigated solution depends solely on the total concentration of a solution and not on the various substances contained therein. Hence, the influence on the retention of a target substance in this study is not altered by the changing osmotic pressure but rather by the different composition of the substances. There was no observable difference between the real and ideal osmotic pressure when the solution contained the same number of components. 3.5. Acidity
It is known that the degree of ionization of substances stands in relation to the pH value of the solution. Several publications [15,16] have
V. TOdtheide et al. / Desalination 110 (1997) 213-222
shown the influence of pH value on the retention of carboxylic acids. Therefore the retention of carboxylic acids vs. pH value, which has ranged between 1.8 and 3.2, was determined. Table 4 exhibits these connections by example of glycolic acid in the multicomponent solutions. It is shown that there is only a poor connection between the pH value and the retention of the target substance in this study (plainly recognizable in combinations glycolic acid/phthalic acid due to glycolic acid/malonic acid as well as glycolic acid/acetic acid due to glycolic acid/ butyric acid). 4. Discussion
The combination of all the effects studied is extremely complex, and ultimately it is the sum of the interactions between the membrane, the solvent and the solutes which determines the permeation of a single substance. It was the aim of this study to equate the retention of a substance as an outcome of molar mass, functionality, spatial requirement, acidity and osmotic pressure. 4.1. Molar mass
The retention of a target substance increases with an increase in molar mass. Within an acid group, increasing molar mass results in a corresponding increase in the influencing effect on the retention of a target substance. Monocarboxylic acids impair the degree of retention whereas an aromatic acid enhances the degree of retention. A combination of the two acid groups in ternary solutions results in an increase in the degree of retention. 4.2. Functionality
All the examined substances are carboxylic acids and therefore contain at least one functional group. Substances containing further functional groups exhibited additional results and were classified as follows.
221
4.2.1.Additional carboxylic group
If an additional carboxylic group is inserted into the molecule, the acidity increases and there is an increased number of interactions between functional groups. It was generally found that the aliphatic acids such as oxalic-, malonic- and glutaric acid decrease the retention of a target substance. In the majority of cases aromatic polycarboxylic acids result in an increasing retention. However, the aromatic ring produces reverse results. Combining aliphatic dicarboxylic acids with aliphatic monocarboxylic acids decreases the retention of the target substance, whereas combinations of aliphatic dicarboxylic acids with aromatic carboxylic acids result in an increase in the degree of retention. 4.2.2. Double bond
Fumaric acid, the only active substance with a double bond itself, enhances the degree of retention, whereas when combined with other carboxylic acids impairs the retention of the target substance. The double bond interacts with other functional groups in the target substance as is demonstrated when this is combined with acrylic acid. It is possible for interactions with the double bond to occur due to the differing densities of electrons in the molecules, which results in a reduction of the permeation of acrylic acid. 4.2.3. Aromatieity
In the majority of the cases carboxylic acids with an aromatic ring result in an increase in the retention of the target substance. This is particularly the case with trimellitic acid. Since this contains three carboxylic groups with a resultant greater spatial requirement, it demonstrates the greatest influence on the retention of a target substance. 4.3. Spatial requirement
When comparing butyric- and isobutyric acid the more steric ambitious molecule exhibits a
222
v. TOdtheide et al. / Desalination 110 (1997) 213-222
lesser influence on the retention of the target substance. In competition with the target substance, the greater spatial requirement of isobutyric acid inhibits permeation through the membrane. Aromatic carboxylic acid exhibits a higher spatial requirement than aliphatic acid due to the presence of the aromatic ring. The increasing spatial requirement in the sequence benzoic-, phthalic and trimellitic acid, due to the increasing numbers of carboxylic groups in the aromatic ring, similarly exhibits a greater influence on the retention of the target substance. These effects may be explained in one of two ways: first, since more interactions exist between the aromatic carboxylic acid and the target substance, this results in differing densities of electrons in the molecules or in an increase in hydrogen bonds between the carboxylic groups. In the ideal case, a cluster of target and active substances are created which results in ineffective permeation through the membrane. Second, consideration needs to be given to the permeation of the target substance in competition with other substances. Previous studies have shown that the aromatic ring interacts with the aromatic polyamide membrane and therefore facilitates the permeation of other substances. For this reason it is feasible that, when the target substance competes with aliphatic carboxylic acids, permeation through the membrane is enhanced. 4.4. A c i d i t y
The pH value of the feed solution in this study ranged between 1.8 and 3.2. It was observed that within this pH range acidity exhibited an inconsequential influence on the retention of the target substance. 4.5. O s m o t i c p r e s s u r e
The operational conditions of the RO are determined by the relationship between the osmotic pressure of the feed solution and the
applied hydrostatic pressure. It was observed that there existed a linear correlation between the concentration of a solution and the osmotic pressure. Since the composition of the investigated solutions was not consequential in determining the osmotic pressure, it can be concluded that osmotic pressure is determined solely by the total concentration. Within the concentration parameters used in this experiment, no difference existed between real and ideal osmotic pressure. Hence, it can be concluded that the osmotic pressure has no influence on the retention of the target substance.
References
[1] A. Alsaygh, P. Jennings and M. Bader, J. Environ. Sci. Health, A28(8) (1993) 1669. [2] Y. Fang, S. Sourirajan and T. Matsuura, J. Appl. Polym. Sci., 44 (1992) 1959. [3] Y. Kiso, T. Kitao, K. Jinno and M. Miyagi, J. Membr. Sci., 74 (1992) 95. [4] L. Sun, E.M. Perdue and J.F. Mc Carthy, Wat. Res., 29(6) (1995) 1471. [5] S. Hausmanns, G. Laufenberg and B. Kunz, Desalination, 104 (1996) 95. [6] G. Laufenberg, R. Fischbach and B. Kunz, Posterprint 5, Aachener Membrankolloqium, 1995. [7] B. Ditgens, R. Fischbach, G. Laufenberg and B. Kunz, Chemie Ingenieur und Technik, 68(8) (1996) 965. [8] P. Z6fel (ed.), Statistik in der Praxis, Gustav Fischer Verlag, Stuttgart, 1985. [9] J. Gmehling, J. Li and M. Schiller, Ind. Eng. Chem. Res., 32(1) (1993) 178. [10] G. Laufenberg, S. Hausmanns and B. Kunz, J. Membr. Sci., 110 (1996) 59. [11] G. Laufenberg and B. Kunz, Posterprint Euromembrane '95, Bath, 1995. [12] S.Q. Zhang and A.E. Fouda, Desalination, 80 (1991) 211. [13] J.M. Dickson, J. Spencer and M.L. Costa, Desalination, 89 (1992) 63. [14] R. Rautenbach and A. Gr6schl, Desalination, 90 (1993) 93. [15] H.H.P. Fang and E.S.K. Chian, Environ. Sci. Technol., 10(6) (1976) 364. [16] M.K.H. Liew, S. Tanaka and M. Morita, Desalination, 101 (1995) 269.