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Influence of hydrophobicity on retention in nanofiltration of aqueous solutions containing organic compounds
Journal of Membrane Science 252 (2005) 195–203
Influence of hydrophobicity on retention in nanofiltration of aqueous solutions containing organic compounds L. Braekena,∗ , R. Ramaekersb,1 , Y. Zhanga , G. Maesb,1 , B. Van der Bruggena , C. Vandecasteelea a
Laboratory for Applied Physical Chemistry and Environmental Technology, Department of Chemical Engineering, Faculty of Applied Sciences, Katholieke Universiteit Leuven, W. de Croylaan 46, 3001 Heverlee, Belgium b Laboratory of Physical and Analytical Chemistry, Department of Chemistry, Faculty of Science, Katholieke Universiteit Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium Received 14 September 2004; accepted 18 December 2004 Available online 18 January 2005
Abstract This paper investigates the influence of molecular size and hydrophobicity of dissolved organic compounds in aqueous solution on retention. In a first part, the influence of hydrophobicity was investigated for organic molecules with a similar molecular weight below the molecular weight cut-off (MWCO) of the membrane and a different hydrophobicity, expressed by the logarithm of the octanol–water partition coefficient (log P). A good correlation was found between the hydrophobicity and the retention: molecules with a high log P (hydrophobic) generally had a low retention, while molecules with a negative log P (hydrophilic) showed a high retention. This could be explained by hydration of the molecules. In a second part, the correlation between the retention of molecules with a molecular weight above the MWCO of the membrane and their hydrophobicity was studied. It was found that a high hydrophobicity lowers the retention, but the influence of hydrophobicity decreases if the molecular size (compared to the MWCO of the membrane) increases. Finally, the influence of adsorption on retention was investigated. An increase in permeate concentration and thus a decrease of the corresponding retention, as a function of time, was observed for hydrophobic compounds, until saturation of the membrane was reached (steady-state retention). © 2004 Elsevier B.V. All rights reserved. Keywords: Nanofiltration; Retention; Hydrophobicity; Molecular size
1. Introduction Nanofiltration has proven to be a very effective method for the removal of a wide variety of organic compounds from aqueous solution e.g. pesticides [1], dyes [2–4], endocrine disruptors [5–7], NOM [8], etc. both in water and waste water treatment. Different mechanisms have been reported to influence the retention of organic compounds. Molecular size was reported by several authors [9–11] to be the most important factor determining retention in nanofiltration. Besides the molecular weight, several other parameters have been used ∗ 1
Corresponding author. Tel.: +32 16 32 23 41; fax: +32 16 32 29 91. E-mail address: [email protected] (L. Braeken). Tel.: +32 16 32 74 65; fax: +32 16 32 79 92.
0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.12.017
as a measure for the molecular size. The Stokes diameter rS [9,10] relates the molecular size ds to the diffusivity (DS ) of a compound in water as expressed by Eq. (1) ds = 2rs
with rs =
kT 6πηDS
(1)
However, the diffusivities DS that allow to estimate the Stokes radius are not available for all organic compounds and the Stokes diameter is calculated assuming spherically shaped molecules and thus not taking molecular structure into account. To represent the molecular shape more quantitatively, simulations of the molecular structure by energetic optimisation methods were introduced. The effective diameter [9] is obtained from molecular structure and shape using the
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mean height in projection of the smallest cylinder around the molecule. The molecular length [10,11] was defined as the distance between the two most distant atoms in a molecule (taking into account their van der Waals radii). The molecular width [10,11] is calculated as half the square root of the surface area of a rectangle, enclosing the projection of the molecule on a plane perpendicular to the axis of the molecule i.e. the straight line connecting the two most distant atoms. The molecular size is not the only parameter influencing the retention of organic compounds. Van der Bruggen et al. [9] mentioned the influence of the dipole moment of organic compounds on rejection. Agenson et al. [12] combined molecular width and molecular length with log P to predict the retention of an organic compound using a multi-linear regression analysis. Kiso et al. [13] concluded that pesticides with a high hydrophobicity, as expressed by their octanol–water partition coefficient, have a high retention. However, Kimura et al. [14] pointed out that adsorption of hydrophobic compounds in the membrane pores could lead to an overestimation of the retention at low feed concentrations (100 ppb). Previous research [15] also showed that the logarithm of the octanol–water partition coefficient correlates well with adsorption on the membrane for molecules with a comparable molecular weight below the molecular weight cut-off (MWCO) of the membranes, indicating that hydrophobicity of the compounds influences the evolution of the permeate concentration in time. Other parameters, which could reflect the hydrophobicity of a molecule, such as the water solubility and the dipole moment, turned out to be less useful to describe adsorption. The water solubility is infinite for certain compounds whereas the dipole moments usually have been determined in other solvents. These dipole moments differ significantly from those in water and are therefore not representative. Adsorption on the membrane could be influenced by hydrogen bonding, which was shown by Nghiem et al. [16] for the adsorption of estrone on nanofiltration membranes. Finally, it was reported that adsorption might influence the permeate concentration of certain trace organic compounds, with a molecular weight significantly higher than the MWCO of the used membranes [17]. The aim of this research is to study the relation between the hydrophobicity of organic compounds, expressed by log P, and their retention (steady-state) in nanofiltration. This will be investigated for molecules with a comparable molecular weight below the MWCO of the used membranes and also for molecules with a molecular weight above the MWCO. It is expected that the influence of hydrophobicity is smaller as the molecular size of an organic compound increases compared to the MWCO of the membrane. Secondly, the evolution of the permeate concentration in time and its correlation with adsorption is studied. It is assumed that the permeate concentration of hydrophobic compounds, which adsorb strongly on the membrane surface [15], will increase until saturation of
the membranes is reached, resulting in a steady state permeate concentration [14].
2. Materials and methods 2.1. Compounds In the first series of experiments, nine water-soluble compounds (Fig. 1a, Table 1 (top half)) were chosen with a similar molecular weight between 146.16 and 153.14, in order to exclude the influence of molecular size. Evidently, molecules with a comparable molecular weight can still have different
Fig. 1. (a) Molecular structure of compounds with a molecular weight of 150 ± 5. (b) Molecular structure of estrone, estradiol and salicine.
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Table 1 Compounds, with their molecular weight, the equivalent molecular diameter and the logarithm of the octanol–water partition coefficient Component
Molecular weight
Effective diameter (nm)
log P
pKa
Adipic acid Benzilidene acetone 3,5-Dihydroxybenzoic acid 3,4-Methylnitrophenol Mandelic acid Tartaric acid Triethylene glycol Xylitol Xylose Estradiol Estrone Salicine
146.14 146.19 154.12 153.14 152.5 150.09 150.18 152.15 150.13 272.39 270.37 286.28
1.03 0.99 0.91 0.82 0.97 0.72 1.09 0.81 0.69
0.23 2.04 0.91 2.45 0.57 −1.00 −1.75 −2.56 −1.98 3.94 3.43 −1.41
4.43 *** 4.04 *** 3.38 4.94 *** *** ***
*** = not applicable.
(effective) molecular sizes. To account for the differences in molecular size, the effective diameter of these compounds was calculated, based on the molecular structure, according to the following procedure: 1. The distance between the two most distant atoms was defined as the height of the cylinder (=H) and the straight line connecting these atoms was defined as the axis of the smallest cylinder enclosing the molecule (Fig. 2a). 2. The molecule was projected on a plane perpendicular to the axis and a circle with minimum area was determined around the projected molecule. The diameter (=D) of this circle is used as the diameter of the cylinder (Fig. 2b). 3. The axis of the cylinder was supposed to make an angle β with the membrane surface (Fig. 2c), so the height in projection (H ) was obtained using Eq. (2) H = H cos β + D sin β
(2)
4. The mean height in projection or effective diameter (Deff ) was obtained assuming that the probability distribution of β (=P(β)) is equal to cos β (Eq. (3)) and as a result, the effective diameter was calculated according to Eq. (4) π/2 Deff = (H cos β + D sin β)P(β) dβ (3) 0
Deff =
π 1 H+ D 4 2
(4)
The molecular structure was obtained using an energetic optimisation procedure (Hyperchem and Gaussian). Hyperchem simulates the molecular shape and structure of the compound by molecular mechanics (MM2). The geometries in Gaussian were calculated with the density functional theory (DFT), using the hybrid of Becke’s nonlocal three parameter exchange and correlation functional with the Lee–Yang–Parr correlation function (B3LYP). For the molecular orbital expansion the standard 6-31++G** basis set was employed in all the calculations [18–21]. Both programs simulate the molecular structure of a compound in vacuum, but Gaussian makes it possible to simulate the formation of hydrogen bonds between water molecules
Fig. 2. (a) Smallest cylinder enclosing a molecule of adipic acid; the two most distant atoms are indicated in black. (b) Alignment of adipic acid with the axis of the cylinder. The outer molecules are indicated in black, determining the cylinders diameter. (c) Representation of the molecule as a cylinder making an angle β with the membrane.
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and an organic compound. A water molecule is added to the organic compound and after energetic optimisation, a rather crude criterium was used i.e. the distance between the two parts of the bond (O and H) should be less than about 0.2 nm before the formation of a hydrogen bond was accepted. Depending on the functional groups of a compound, different water molecules can be added to a single compound and an effective diameter of the hydrated molecule (organic compound and bonded water molecules) diameter could be calculated using the previously mentioned procedure. The hydrophobicity of the compounds, expressed as log P, ranged from −2.56 to 2.45. The partition coefficient is the ratio between the concentration of a compound in octanol and the concentration in water and is thus an indication for the hydrophobicity of a compound. The different values of log P were calculated based on a group contribution method [22] and correspond well with experimentally measured values of log P as reported in literature. pKa values were also presented in Table 1 (top half) for the acidic organic compounds because dissociation of the organic acids into ions might also introduce Donnan exclusion as a mechanism of retention [1,9]. However, pKa values are relatively small and the influence of dissociation into ions at pH = 3 was assumed negligible. In the second part of the research, three organic molecules, with a comparable molecular weight, above the MWCO of the membranes, were chosen: estrone and estradiol, two important endocrine disrupters, and salicine. Estrone and estradiol are hydrophobic compounds with a comparable molecular structure. Salicine has a comparable molecular weight, but is more hydrophilic (Fig. 1b, Table 1 (bottom half)). 2.2. Membranes and filtration experiments Three different membranes were used: UTC-20 (Toray Ind. Inc., Shiga, Japan), Desal HL-51 (Osmonics SA, Vista, CA, USA) and NF210 (Dow-Filmtec, Edina, MN, USA). UTC-20 is a polypiperazineamide based membrane with a molecular weight cut-off (MWCO) around 180. Desal-HL51 is a thin film composite membrane with a MWCO of 150–300 and NF210 is a crosslinked polyamide membrane with a MWCO of 180, as indicated by the manufacturers. Nanofiltration experiments were performed in a crossflow equipment on laboratory scale [15]. Flat sheet membranes with an effective membrane area of 0.0059 m2 were used. Permeate and retentate were recycled to the feed solution to obtain a constant feed concentration. The retention of a dissolved organic compound was calculated according to Eq. (5) R=1−
cP cF
(5)
Before the experiments, the membranes were washed with distilled water to remove the protective coat and pressurized during 1 h at 20 bar to avoid compaction. A new membrane was used for each experiment. In all experiments, the temper-
ature of the water was 25 ◦ C and the transmembrane pressure was held at 8 bar. The aqueous feed solution in the first set of experiments contained only one organic compound with a concentration of 2 mmol L−1 . In the second set of experiments, the feed contained 2 mg L−1 of the organic compound due to the low water solubility of estrone and estradiol. These two compounds were first dissolved in a small amount of acetonitrile (<1 ml L−1 ). Salicine has a higher solubility and no acetonitrile was required. However, to compare these three compounds, the same procedure and concentration was used. All experiments were carried out twice and the average of both experiments was taken on the condition that the difference between both was less than 3%. 2.3. Analysis Samples containing a single compound with an aromatic structure (benzilidene acetone, 3,5-dihydroxybenzoic acid, 3,4-methylnitrofenol, mandelic acid and salicine) in distilled water were analysed by measuring UV absorption with a Shimazu UV 1601 double beam spectrophotometer. Table 2 gives the used wavelengths. The determination of xylose is based upon a condensation reaction with phenol in an acid environment, yielding a yellow-orange colour [23]. The wavelength of detection was 485 nm. Compounds with an aliphatic structure and few double bonds, could not be determined using UV-spectrophotometry. Triethyleneglycol was determined by GC. A HP 5890 Series II equipped with a wide boar liquid phase DB-1 (dimethyl polysiloxane) column with a film thickness of 5 m was used. A Waters TM 600S controller equipped with a Waters TM 626 pump was used to determine adipic acid, tartaric acid and xylitol by HPLC using a Shodex RS pack KC-811 column with a length of 300 mm and an inner diameter of 8 mm. Because the concentrations of estrone and estradiol were below the detection limit of UV-spectrophotometry, estrone and estradiol were also measured by HPLC after solid phase extraction (SPE). The extraction was performed on a HyperSEP C18 (Thermo Hypersil-Keystone) column with a bed size of 1 g and a column volume of 6 ml. The column was conditioned using 6 ml of methanol and 12 ml of Milli-Q water before the experiment. Afterwards, 500 ml of a sample was brought on the column at a speed of 10 ml min−1 . After washing (6 ml Milli-Q water) and drying, the sample was eluted from the column using 5 ml acetonitrile. The acetonitrile was evaporated and the residue was solved in 1 ml acetonitrile. Hundred microlitres of an estrone solution (0.150 g estrone/L acetonitrile) was Table 2 Wavelengths, used in UV-spectrophotometry Component
Wavelength λ (nm)
Benzilidene acetone 3,5-Dihydroxybenzoic acid 3,4-Methylnitrophenol Mandelic acid Salicine
290.6 210.0 210.0 210.0 212.0
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added as the internal standard for estradiol and vice versa. The HPLC-column used was a HyPURITY C18 column (Thermo Hypersil-Keystone) with a length of 150 mm and an inner diameter of 4.6 mm. The recovery of estrone and estradiol using this method was 90% and 92%, respectively. 2.4. Preliminary filtration experiments The retention and the water flux were measured for both membranes during 8 h with pure water and with a solution containing xylose. The retention of xylose increased initially and reached a stable value after 2 h of filtration. No significant changes in retention and water flux were observed during the following 6 h. Therefore, further experiments were limited to 2 h of filtration, except for the experiments with estrone and estradiol: to collect a sufficient amount of sample for the analysis with SPE-HPLC, the experiment was run for 15 h. Four different samples of 500 mL were taken during this period and an average retention was calculated. 3. Results and discussion 3.1. Difference between the molecular weight and the effective diameter To account for differences in molecular shape between molecules of the same molecular weight, the effective diameter was calculated for compounds with a molecular weight of 146–154. However, as appears from Table 1, the differences between the effective diameters are significantly larger. The effective diameter was the smallest for xylose (=0.69 nm) whereas triethylene glycol has the largest effective diameter (=1.09 nm). The difference could be explained by the dif-
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ferent molecular structure (Fig. 1a). Triethylene glycol has a long aliphatic chain whereas the atoms of xylose form a ring. Adipic acid has also a longer aliphatic chain than tartaric acid and xylitol, which is reflected in its effective diameter. Benzilidene acetone, 3,4-methylnitrophenol, 3,5dihydroxybenzoic acid and mandelic acid have an aromatic ring with different functional groups resulting in somewhat different diameters. 3.2. Influence of adsorption on permeate concentration and ‘apparent’ retention To study the influence of adsorption on retention, the permeate concentration was measured in time and the corresponding ‘apparent’ retentions were calculated. Fig. 3 shows the apparent retention as a function of time during the first 2 h of filtration for the hydrophobic compounds 3,4methylnitrophenol, benzilidene acetone, mandelic acid and 3,5-dihydroxybenzoic acid, which have a low steady-state retention (Section 4.1). A breakthrough effect could be noticed: retention decreases strongly during the first 15–30 min and then reaches a stable value. The relatively high feed concentration (2 mmol L−1 or ±300 mg L−1 ) results in a quick saturation of the membrane during the first 10–15 min, obtaining a breakthrough. This breakthrough effect was, as expected, only observed for the most hydrophobic compounds (highest values of log P) because they adsorb on the membrane surface or sorb in the membrane structure. The retention of the hydrophilic compounds (Fig. 4) cannot be influenced by adsorption on the membrane because earlier experiments showed no adsorption for these compounds. Generally there are two patterns: the retention as a function of time remains constant for tartaric acid (Fig. 4a) and adipic acid and increases slightly for
Fig. 3. Retention as a function of time for benzilidene acetone (a), 3,4-methylnitrophenol (b), mandelic acid (c) and 3,5-dihydroxybenzoic acid (d).
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Fig. 4. Retention as a function of time for tartaric acid (a) and xylose (b).
compounds with a strongly negative value of log P during the first 5–10 min reaching a stable value as shown in Fig. 4b for xylose. Similar patterns were obtained for triethylene glycol and xylitol. The increase results from pore blocking: a part of the molecules were stuck in the pores, resulting in a decreased mass transfer. An influence of pore blocking was mentioned before by Knyazkova and Maynarovich [24].
4. Influence of molecular size on retention 4.1. Compounds with molecular weight below the MWCO of the membranes The retentions for the nine compounds with a comparable molecular weight are given in Table 3a for UTC-20 and Desal-51-HL. The retentions of the UTC-20 membrane were comparable to those of the Desal HL-51 membrane, as could be expected based on the MWCO of both membranes. Based on the molecular weight of these compounds (between 146 and 154), similar retentions of all compounds Table 3 (a) Retention after 2 h filtration for UTC-20 and Desal HL-51 and (b) average retention for UTC-20 and NF210 Component
(a) UTC-20 and Desal HL-51 Adipic acid Benzilidene acetone 3,5-Dihydroxybenzoic acid 3,4-Methylnitrophenol Mandelic acid Tartaric acid Triethylene glycol Xylitol Xylose Component
(b) UTC-20 and NF210 Estradiol Estrone Salicine
Retention UTC-20
Desal HL-51
21 16 10 0 23 54 82 91 87
32 7 15 0 30 43 68 82 79
were expected. However, the retention of the compounds was ranged from 0% to 91% and 0% to 82% for respectively UTC-20 and Desal-HL-51 membrane (Table 3). Because the molecular weight is only a rough measure for the size of the molecule, the effective diameter was also used. However, the retention of xylose, with the smallest effective diameter, is among the highest retentions (respectively 87% and 79% for the UTC-20 and the Desal-HL-51) while the retention of benzilidene acetone, with a relatively high effective diameter, is very low for both membranes. This is also illustrated in Fig. 5, where no significant correlation between the retention and the effective diameter was obtained. Therefore, the effective diameter is not sufficient to describe the retention behaviour of compounds with a molecular weight below the specified MWCO of the membrane. This proves that the retention is not a result of a mere sieving effect. 4.2. Compounds with molecular weight above the MWCO of the membrane The retentions of estrone, estradiol and salicine, with a molecular weight above the MWCO of the membranes, were measured for UTC-20, Desal-HL-51 and NF210 and are given in Table 3b. The retention of salicine was above 90% for UTC-20 and NF210. This could be expected because the molecular weight of salicine (286.28) is higher than the MWCO of both membranes. However, the retentions of estrone and estradiol are lower than expected based on the MWCO of the membranes.
Retention UTC-20
NF210
75 ± 9 83 ± 3 >97
85 ± 4 65 ± 3 91 ± 1
Fig. 5. Retention as a function of the effective diameter for UTC-20 and Desal-HL-51.
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Fig. 6. Retention as a function of the logarithm of the octanol–water partition coefficient.
5. Influence of hydrophobicity on retention 5.1. Compounds with molecular weight below the MWCO of the membranes The influence of hydrophobicity can be represented by the logarithm of the octanol–water partition coefficient (log P). In Fig. 6, retention of molecules with a molecular weight between 146 and 154 is given as a function of log P for both membranes. As appears from Fig. 6, a nearly linear correlation between log P and retention exists for both membranes. A compound with a high value of log P (hydrophobic compound) permeates relatively easily through the membranes, while a molecule with a high affinity for the water phase (negative value of log P) will be rejected. The R2 -values for the linear plot were 0.9169 and 0.9659 for UTC-20 and DesalHL-51 membrane, respectively. To explain the correlation between hydrophobicity and retention, the molecular structure of the different compounds should be considered. Molecules with a low (negative) value of log P generally have more OH or O groups, which can form hydrogen bonds with the water molecules. The hydrogen bonds between organic compound and water molecules might also be influenced by water–water bondings. Due to these polar groups, hydrophilic compounds have a higher affinity with the water phase and permeate less in the mem-
brane structure. For instance, xylose has four OH groups, which can form hydrogen bonds with the water molecule, as represented in Fig. 7. When the organic compound with associated water molecules is considered, the effective (hydrated) diameter might change. For instance for xylose, considering one layer of water around the molecule, the effective diameter increases from 0.69 to 1.21 nm. Hydrophobic compounds have less polar groups and are thus less solvated. Because of their smaller size, they can enter more easily in the membrane pores and permeate due to the pressure difference over the membrane. Therefore, a better parameter to describe the retention behaviour of an organic compound might be a hydrated, effective diameter. Further research is required to prove the influence of hydration on the molecular size. The results seem to be in contradiction with the results of Kiso [13], who reported an increase of retention with increasing hydrophobicity. However, as mentioned by Kimura et al. [14], adsorption on the membrane results in an overestimation of retention if it is not determined after saturation of the membrane; the permeate concentrations of hydrophobic compounds at low concentration increased significantly during 25 h. Previous research at our laboratory showed a good correlation between the hydrophobicity of a compound and its adsorption behaviour [15], indicating that adsorption would especially influence the retention of hydrophobic compounds. This was discussed in Section 3.2.
Fig. 7. Molecular structure as simulated with DFT for xylose (a) and xylose with four bonded water molecules (b).
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5.2. Compounds with molecular weight above the MWCO of the membrane The retentions of estradiol and estrone (Table 3b) are lower than expected, based on the MWCO of UTC-20 and NF210 and also lower than the retention of salicine for both membranes. If the hydrophobicity of these compounds is considered, it is obvious that there is an influence of the hydrophobicity on the retention. Estrone and estradiol have comparable log P values whereas salicine has a much lower log P and more polar groups (Fig. 1), resulting in more possibilities to form hydrogen bonds with the water molecules and thus resulting in a larger hydrated molecule. The retention of salicine is therefore higher than that of estrone and estradiol meaning that hydrophobicity influences the retention of compounds with a molecular weight above the MWCO. Although, molecular size also plays an important role. Retentions of estrone and estradiol with a molecular weight of approximately 270 are significantly higher than those of 3,4-methylnitrophenol or benzilidene acetone, with a molecular weight of approximately 150 and also a high value of log P (Table 1), indicating that an increase in molecular size (above the MWCO of the membrane) will result in a higher retention. However, the differences in retention between estrone, estradiol and salicine are less than these between 3,4-methylnitrophenol, benzilidene acetone, etc. and xylose (compounds with a molecular weight below the MWCO, Table 3a), meaning that the influence of hydrophobicity on retention decreases as the molecular size, compared to the MWCO, increases. Therefore, it can be concluded that both molecular size and hydrophobicity influence the retention of a dissolved organic compound. The larger the molecule, the less effect hydrophobicity has on its retention. The influence of hydration could explain the fact that the retentions of estrone and estradiol are smaller than the retention expected based on the MWCO. Generally, the MWCO is determined using polar molecules such as polyethylene glycols or saccharides, which are hydrophilic and usually more efficiently hydrated. Due to the absence of hydration, the retentions of hydrophobic compounds will be lower, especially when the molecular weight is not substantially higher than the MWCO.
6. Conclusions Hydrophobicity and molecular size play an important role in the retention of dissolved organic compounds. Hydrophobicity is the most important parameter determining the retention for molecules with a molecular weight below the MWCO of a membrane. This was explained by a more effective solvation of the molecule, resulting in a larger size. However, the influence of hydrophobicity becomes smaller as the molecular size of the organic compound, compared to the MWCO of the membrane, increases. Finally, the influence of adsorption on retention was shown for hydrophobic compounds.
After saturation of the membrane, a breakthrough in permeate concentration was observed, resulting in a lower retention at steady state.
Acknowledgements Leen Braeken is grateful to the IWT (Vlaams Instituut voor de bevordering van Wetenschappelijk-Technologisch onderzoek in de industrie) for a PhD-grant. FWO Vlaanderen (Fund for Scientific Research) is acknowledged for a postdoctoral grant for Riet Ramaekers.
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Influence of hydrophobicity on retention in nanofiltration of aqueous solutions containing organic compounds L. Braekena,∗ , R. Ramaekersb,1 , Y. Zhanga , G. Maesb,1 , B. Van der Bruggena , C. Vandecasteelea a
Laboratory for Applied Physical Chemistry and Environmental Technology, Department of Chemical Engineering, Faculty of Applied Sciences, Katholieke Universiteit Leuven, W. de Croylaan 46, 3001 Heverlee, Belgium b Laboratory of Physical and Analytical Chemistry, Department of Chemistry, Faculty of Science, Katholieke Universiteit Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium Received 14 September 2004; accepted 18 December 2004 Available online 18 January 2005
Abstract This paper investigates the influence of molecular size and hydrophobicity of dissolved organic compounds in aqueous solution on retention. In a first part, the influence of hydrophobicity was investigated for organic molecules with a similar molecular weight below the molecular weight cut-off (MWCO) of the membrane and a different hydrophobicity, expressed by the logarithm of the octanol–water partition coefficient (log P). A good correlation was found between the hydrophobicity and the retention: molecules with a high log P (hydrophobic) generally had a low retention, while molecules with a negative log P (hydrophilic) showed a high retention. This could be explained by hydration of the molecules. In a second part, the correlation between the retention of molecules with a molecular weight above the MWCO of the membrane and their hydrophobicity was studied. It was found that a high hydrophobicity lowers the retention, but the influence of hydrophobicity decreases if the molecular size (compared to the MWCO of the membrane) increases. Finally, the influence of adsorption on retention was investigated. An increase in permeate concentration and thus a decrease of the corresponding retention, as a function of time, was observed for hydrophobic compounds, until saturation of the membrane was reached (steady-state retention). © 2004 Elsevier B.V. All rights reserved. Keywords: Nanofiltration; Retention; Hydrophobicity; Molecular size
1. Introduction Nanofiltration has proven to be a very effective method for the removal of a wide variety of organic compounds from aqueous solution e.g. pesticides [1], dyes [2–4], endocrine disruptors [5–7], NOM [8], etc. both in water and waste water treatment. Different mechanisms have been reported to influence the retention of organic compounds. Molecular size was reported by several authors [9–11] to be the most important factor determining retention in nanofiltration. Besides the molecular weight, several other parameters have been used ∗ 1
Corresponding author. Tel.: +32 16 32 23 41; fax: +32 16 32 29 91. E-mail address: [email protected] (L. Braeken). Tel.: +32 16 32 74 65; fax: +32 16 32 79 92.
0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.12.017
as a measure for the molecular size. The Stokes diameter rS [9,10] relates the molecular size ds to the diffusivity (DS ) of a compound in water as expressed by Eq. (1) ds = 2rs
with rs =
kT 6πηDS
(1)
However, the diffusivities DS that allow to estimate the Stokes radius are not available for all organic compounds and the Stokes diameter is calculated assuming spherically shaped molecules and thus not taking molecular structure into account. To represent the molecular shape more quantitatively, simulations of the molecular structure by energetic optimisation methods were introduced. The effective diameter [9] is obtained from molecular structure and shape using the
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mean height in projection of the smallest cylinder around the molecule. The molecular length [10,11] was defined as the distance between the two most distant atoms in a molecule (taking into account their van der Waals radii). The molecular width [10,11] is calculated as half the square root of the surface area of a rectangle, enclosing the projection of the molecule on a plane perpendicular to the axis of the molecule i.e. the straight line connecting the two most distant atoms. The molecular size is not the only parameter influencing the retention of organic compounds. Van der Bruggen et al. [9] mentioned the influence of the dipole moment of organic compounds on rejection. Agenson et al. [12] combined molecular width and molecular length with log P to predict the retention of an organic compound using a multi-linear regression analysis. Kiso et al. [13] concluded that pesticides with a high hydrophobicity, as expressed by their octanol–water partition coefficient, have a high retention. However, Kimura et al. [14] pointed out that adsorption of hydrophobic compounds in the membrane pores could lead to an overestimation of the retention at low feed concentrations (100 ppb). Previous research [15] also showed that the logarithm of the octanol–water partition coefficient correlates well with adsorption on the membrane for molecules with a comparable molecular weight below the molecular weight cut-off (MWCO) of the membranes, indicating that hydrophobicity of the compounds influences the evolution of the permeate concentration in time. Other parameters, which could reflect the hydrophobicity of a molecule, such as the water solubility and the dipole moment, turned out to be less useful to describe adsorption. The water solubility is infinite for certain compounds whereas the dipole moments usually have been determined in other solvents. These dipole moments differ significantly from those in water and are therefore not representative. Adsorption on the membrane could be influenced by hydrogen bonding, which was shown by Nghiem et al. [16] for the adsorption of estrone on nanofiltration membranes. Finally, it was reported that adsorption might influence the permeate concentration of certain trace organic compounds, with a molecular weight significantly higher than the MWCO of the used membranes [17]. The aim of this research is to study the relation between the hydrophobicity of organic compounds, expressed by log P, and their retention (steady-state) in nanofiltration. This will be investigated for molecules with a comparable molecular weight below the MWCO of the used membranes and also for molecules with a molecular weight above the MWCO. It is expected that the influence of hydrophobicity is smaller as the molecular size of an organic compound increases compared to the MWCO of the membrane. Secondly, the evolution of the permeate concentration in time and its correlation with adsorption is studied. It is assumed that the permeate concentration of hydrophobic compounds, which adsorb strongly on the membrane surface [15], will increase until saturation of
the membranes is reached, resulting in a steady state permeate concentration [14].
2. Materials and methods 2.1. Compounds In the first series of experiments, nine water-soluble compounds (Fig. 1a, Table 1 (top half)) were chosen with a similar molecular weight between 146.16 and 153.14, in order to exclude the influence of molecular size. Evidently, molecules with a comparable molecular weight can still have different
Fig. 1. (a) Molecular structure of compounds with a molecular weight of 150 ± 5. (b) Molecular structure of estrone, estradiol and salicine.
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Table 1 Compounds, with their molecular weight, the equivalent molecular diameter and the logarithm of the octanol–water partition coefficient Component
Molecular weight
Effective diameter (nm)
log P
pKa
Adipic acid Benzilidene acetone 3,5-Dihydroxybenzoic acid 3,4-Methylnitrophenol Mandelic acid Tartaric acid Triethylene glycol Xylitol Xylose Estradiol Estrone Salicine
146.14 146.19 154.12 153.14 152.5 150.09 150.18 152.15 150.13 272.39 270.37 286.28
1.03 0.99 0.91 0.82 0.97 0.72 1.09 0.81 0.69
0.23 2.04 0.91 2.45 0.57 −1.00 −1.75 −2.56 −1.98 3.94 3.43 −1.41
4.43 *** 4.04 *** 3.38 4.94 *** *** ***
*** = not applicable.
(effective) molecular sizes. To account for the differences in molecular size, the effective diameter of these compounds was calculated, based on the molecular structure, according to the following procedure: 1. The distance between the two most distant atoms was defined as the height of the cylinder (=H) and the straight line connecting these atoms was defined as the axis of the smallest cylinder enclosing the molecule (Fig. 2a). 2. The molecule was projected on a plane perpendicular to the axis and a circle with minimum area was determined around the projected molecule. The diameter (=D) of this circle is used as the diameter of the cylinder (Fig. 2b). 3. The axis of the cylinder was supposed to make an angle β with the membrane surface (Fig. 2c), so the height in projection (H ) was obtained using Eq. (2) H = H cos β + D sin β
(2)
4. The mean height in projection or effective diameter (Deff ) was obtained assuming that the probability distribution of β (=P(β)) is equal to cos β (Eq. (3)) and as a result, the effective diameter was calculated according to Eq. (4) π/2 Deff = (H cos β + D sin β)P(β) dβ (3) 0
Deff =
π 1 H+ D 4 2
(4)
The molecular structure was obtained using an energetic optimisation procedure (Hyperchem and Gaussian). Hyperchem simulates the molecular shape and structure of the compound by molecular mechanics (MM2). The geometries in Gaussian were calculated with the density functional theory (DFT), using the hybrid of Becke’s nonlocal three parameter exchange and correlation functional with the Lee–Yang–Parr correlation function (B3LYP). For the molecular orbital expansion the standard 6-31++G** basis set was employed in all the calculations [18–21]. Both programs simulate the molecular structure of a compound in vacuum, but Gaussian makes it possible to simulate the formation of hydrogen bonds between water molecules
Fig. 2. (a) Smallest cylinder enclosing a molecule of adipic acid; the two most distant atoms are indicated in black. (b) Alignment of adipic acid with the axis of the cylinder. The outer molecules are indicated in black, determining the cylinders diameter. (c) Representation of the molecule as a cylinder making an angle β with the membrane.
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and an organic compound. A water molecule is added to the organic compound and after energetic optimisation, a rather crude criterium was used i.e. the distance between the two parts of the bond (O and H) should be less than about 0.2 nm before the formation of a hydrogen bond was accepted. Depending on the functional groups of a compound, different water molecules can be added to a single compound and an effective diameter of the hydrated molecule (organic compound and bonded water molecules) diameter could be calculated using the previously mentioned procedure. The hydrophobicity of the compounds, expressed as log P, ranged from −2.56 to 2.45. The partition coefficient is the ratio between the concentration of a compound in octanol and the concentration in water and is thus an indication for the hydrophobicity of a compound. The different values of log P were calculated based on a group contribution method [22] and correspond well with experimentally measured values of log P as reported in literature. pKa values were also presented in Table 1 (top half) for the acidic organic compounds because dissociation of the organic acids into ions might also introduce Donnan exclusion as a mechanism of retention [1,9]. However, pKa values are relatively small and the influence of dissociation into ions at pH = 3 was assumed negligible. In the second part of the research, three organic molecules, with a comparable molecular weight, above the MWCO of the membranes, were chosen: estrone and estradiol, two important endocrine disrupters, and salicine. Estrone and estradiol are hydrophobic compounds with a comparable molecular structure. Salicine has a comparable molecular weight, but is more hydrophilic (Fig. 1b, Table 1 (bottom half)). 2.2. Membranes and filtration experiments Three different membranes were used: UTC-20 (Toray Ind. Inc., Shiga, Japan), Desal HL-51 (Osmonics SA, Vista, CA, USA) and NF210 (Dow-Filmtec, Edina, MN, USA). UTC-20 is a polypiperazineamide based membrane with a molecular weight cut-off (MWCO) around 180. Desal-HL51 is a thin film composite membrane with a MWCO of 150–300 and NF210 is a crosslinked polyamide membrane with a MWCO of 180, as indicated by the manufacturers. Nanofiltration experiments were performed in a crossflow equipment on laboratory scale [15]. Flat sheet membranes with an effective membrane area of 0.0059 m2 were used. Permeate and retentate were recycled to the feed solution to obtain a constant feed concentration. The retention of a dissolved organic compound was calculated according to Eq. (5) R=1−
cP cF
(5)
Before the experiments, the membranes were washed with distilled water to remove the protective coat and pressurized during 1 h at 20 bar to avoid compaction. A new membrane was used for each experiment. In all experiments, the temper-
ature of the water was 25 ◦ C and the transmembrane pressure was held at 8 bar. The aqueous feed solution in the first set of experiments contained only one organic compound with a concentration of 2 mmol L−1 . In the second set of experiments, the feed contained 2 mg L−1 of the organic compound due to the low water solubility of estrone and estradiol. These two compounds were first dissolved in a small amount of acetonitrile (<1 ml L−1 ). Salicine has a higher solubility and no acetonitrile was required. However, to compare these three compounds, the same procedure and concentration was used. All experiments were carried out twice and the average of both experiments was taken on the condition that the difference between both was less than 3%. 2.3. Analysis Samples containing a single compound with an aromatic structure (benzilidene acetone, 3,5-dihydroxybenzoic acid, 3,4-methylnitrofenol, mandelic acid and salicine) in distilled water were analysed by measuring UV absorption with a Shimazu UV 1601 double beam spectrophotometer. Table 2 gives the used wavelengths. The determination of xylose is based upon a condensation reaction with phenol in an acid environment, yielding a yellow-orange colour [23]. The wavelength of detection was 485 nm. Compounds with an aliphatic structure and few double bonds, could not be determined using UV-spectrophotometry. Triethyleneglycol was determined by GC. A HP 5890 Series II equipped with a wide boar liquid phase DB-1 (dimethyl polysiloxane) column with a film thickness of 5 m was used. A Waters TM 600S controller equipped with a Waters TM 626 pump was used to determine adipic acid, tartaric acid and xylitol by HPLC using a Shodex RS pack KC-811 column with a length of 300 mm and an inner diameter of 8 mm. Because the concentrations of estrone and estradiol were below the detection limit of UV-spectrophotometry, estrone and estradiol were also measured by HPLC after solid phase extraction (SPE). The extraction was performed on a HyperSEP C18 (Thermo Hypersil-Keystone) column with a bed size of 1 g and a column volume of 6 ml. The column was conditioned using 6 ml of methanol and 12 ml of Milli-Q water before the experiment. Afterwards, 500 ml of a sample was brought on the column at a speed of 10 ml min−1 . After washing (6 ml Milli-Q water) and drying, the sample was eluted from the column using 5 ml acetonitrile. The acetonitrile was evaporated and the residue was solved in 1 ml acetonitrile. Hundred microlitres of an estrone solution (0.150 g estrone/L acetonitrile) was Table 2 Wavelengths, used in UV-spectrophotometry Component
Wavelength λ (nm)
Benzilidene acetone 3,5-Dihydroxybenzoic acid 3,4-Methylnitrophenol Mandelic acid Salicine
290.6 210.0 210.0 210.0 212.0
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added as the internal standard for estradiol and vice versa. The HPLC-column used was a HyPURITY C18 column (Thermo Hypersil-Keystone) with a length of 150 mm and an inner diameter of 4.6 mm. The recovery of estrone and estradiol using this method was 90% and 92%, respectively. 2.4. Preliminary filtration experiments The retention and the water flux were measured for both membranes during 8 h with pure water and with a solution containing xylose. The retention of xylose increased initially and reached a stable value after 2 h of filtration. No significant changes in retention and water flux were observed during the following 6 h. Therefore, further experiments were limited to 2 h of filtration, except for the experiments with estrone and estradiol: to collect a sufficient amount of sample for the analysis with SPE-HPLC, the experiment was run for 15 h. Four different samples of 500 mL were taken during this period and an average retention was calculated. 3. Results and discussion 3.1. Difference between the molecular weight and the effective diameter To account for differences in molecular shape between molecules of the same molecular weight, the effective diameter was calculated for compounds with a molecular weight of 146–154. However, as appears from Table 1, the differences between the effective diameters are significantly larger. The effective diameter was the smallest for xylose (=0.69 nm) whereas triethylene glycol has the largest effective diameter (=1.09 nm). The difference could be explained by the dif-
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ferent molecular structure (Fig. 1a). Triethylene glycol has a long aliphatic chain whereas the atoms of xylose form a ring. Adipic acid has also a longer aliphatic chain than tartaric acid and xylitol, which is reflected in its effective diameter. Benzilidene acetone, 3,4-methylnitrophenol, 3,5dihydroxybenzoic acid and mandelic acid have an aromatic ring with different functional groups resulting in somewhat different diameters. 3.2. Influence of adsorption on permeate concentration and ‘apparent’ retention To study the influence of adsorption on retention, the permeate concentration was measured in time and the corresponding ‘apparent’ retentions were calculated. Fig. 3 shows the apparent retention as a function of time during the first 2 h of filtration for the hydrophobic compounds 3,4methylnitrophenol, benzilidene acetone, mandelic acid and 3,5-dihydroxybenzoic acid, which have a low steady-state retention (Section 4.1). A breakthrough effect could be noticed: retention decreases strongly during the first 15–30 min and then reaches a stable value. The relatively high feed concentration (2 mmol L−1 or ±300 mg L−1 ) results in a quick saturation of the membrane during the first 10–15 min, obtaining a breakthrough. This breakthrough effect was, as expected, only observed for the most hydrophobic compounds (highest values of log P) because they adsorb on the membrane surface or sorb in the membrane structure. The retention of the hydrophilic compounds (Fig. 4) cannot be influenced by adsorption on the membrane because earlier experiments showed no adsorption for these compounds. Generally there are two patterns: the retention as a function of time remains constant for tartaric acid (Fig. 4a) and adipic acid and increases slightly for
Fig. 3. Retention as a function of time for benzilidene acetone (a), 3,4-methylnitrophenol (b), mandelic acid (c) and 3,5-dihydroxybenzoic acid (d).
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Fig. 4. Retention as a function of time for tartaric acid (a) and xylose (b).
compounds with a strongly negative value of log P during the first 5–10 min reaching a stable value as shown in Fig. 4b for xylose. Similar patterns were obtained for triethylene glycol and xylitol. The increase results from pore blocking: a part of the molecules were stuck in the pores, resulting in a decreased mass transfer. An influence of pore blocking was mentioned before by Knyazkova and Maynarovich [24].
4. Influence of molecular size on retention 4.1. Compounds with molecular weight below the MWCO of the membranes The retentions for the nine compounds with a comparable molecular weight are given in Table 3a for UTC-20 and Desal-51-HL. The retentions of the UTC-20 membrane were comparable to those of the Desal HL-51 membrane, as could be expected based on the MWCO of both membranes. Based on the molecular weight of these compounds (between 146 and 154), similar retentions of all compounds Table 3 (a) Retention after 2 h filtration for UTC-20 and Desal HL-51 and (b) average retention for UTC-20 and NF210 Component
(a) UTC-20 and Desal HL-51 Adipic acid Benzilidene acetone 3,5-Dihydroxybenzoic acid 3,4-Methylnitrophenol Mandelic acid Tartaric acid Triethylene glycol Xylitol Xylose Component
(b) UTC-20 and NF210 Estradiol Estrone Salicine
Retention UTC-20
Desal HL-51
21 16 10 0 23 54 82 91 87
32 7 15 0 30 43 68 82 79
were expected. However, the retention of the compounds was ranged from 0% to 91% and 0% to 82% for respectively UTC-20 and Desal-HL-51 membrane (Table 3). Because the molecular weight is only a rough measure for the size of the molecule, the effective diameter was also used. However, the retention of xylose, with the smallest effective diameter, is among the highest retentions (respectively 87% and 79% for the UTC-20 and the Desal-HL-51) while the retention of benzilidene acetone, with a relatively high effective diameter, is very low for both membranes. This is also illustrated in Fig. 5, where no significant correlation between the retention and the effective diameter was obtained. Therefore, the effective diameter is not sufficient to describe the retention behaviour of compounds with a molecular weight below the specified MWCO of the membrane. This proves that the retention is not a result of a mere sieving effect. 4.2. Compounds with molecular weight above the MWCO of the membrane The retentions of estrone, estradiol and salicine, with a molecular weight above the MWCO of the membranes, were measured for UTC-20, Desal-HL-51 and NF210 and are given in Table 3b. The retention of salicine was above 90% for UTC-20 and NF210. This could be expected because the molecular weight of salicine (286.28) is higher than the MWCO of both membranes. However, the retentions of estrone and estradiol are lower than expected based on the MWCO of the membranes.
Retention UTC-20
NF210
75 ± 9 83 ± 3 >97
85 ± 4 65 ± 3 91 ± 1
Fig. 5. Retention as a function of the effective diameter for UTC-20 and Desal-HL-51.
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Fig. 6. Retention as a function of the logarithm of the octanol–water partition coefficient.
5. Influence of hydrophobicity on retention 5.1. Compounds with molecular weight below the MWCO of the membranes The influence of hydrophobicity can be represented by the logarithm of the octanol–water partition coefficient (log P). In Fig. 6, retention of molecules with a molecular weight between 146 and 154 is given as a function of log P for both membranes. As appears from Fig. 6, a nearly linear correlation between log P and retention exists for both membranes. A compound with a high value of log P (hydrophobic compound) permeates relatively easily through the membranes, while a molecule with a high affinity for the water phase (negative value of log P) will be rejected. The R2 -values for the linear plot were 0.9169 and 0.9659 for UTC-20 and DesalHL-51 membrane, respectively. To explain the correlation between hydrophobicity and retention, the molecular structure of the different compounds should be considered. Molecules with a low (negative) value of log P generally have more OH or O groups, which can form hydrogen bonds with the water molecules. The hydrogen bonds between organic compound and water molecules might also be influenced by water–water bondings. Due to these polar groups, hydrophilic compounds have a higher affinity with the water phase and permeate less in the mem-
brane structure. For instance, xylose has four OH groups, which can form hydrogen bonds with the water molecule, as represented in Fig. 7. When the organic compound with associated water molecules is considered, the effective (hydrated) diameter might change. For instance for xylose, considering one layer of water around the molecule, the effective diameter increases from 0.69 to 1.21 nm. Hydrophobic compounds have less polar groups and are thus less solvated. Because of their smaller size, they can enter more easily in the membrane pores and permeate due to the pressure difference over the membrane. Therefore, a better parameter to describe the retention behaviour of an organic compound might be a hydrated, effective diameter. Further research is required to prove the influence of hydration on the molecular size. The results seem to be in contradiction with the results of Kiso [13], who reported an increase of retention with increasing hydrophobicity. However, as mentioned by Kimura et al. [14], adsorption on the membrane results in an overestimation of retention if it is not determined after saturation of the membrane; the permeate concentrations of hydrophobic compounds at low concentration increased significantly during 25 h. Previous research at our laboratory showed a good correlation between the hydrophobicity of a compound and its adsorption behaviour [15], indicating that adsorption would especially influence the retention of hydrophobic compounds. This was discussed in Section 3.2.
Fig. 7. Molecular structure as simulated with DFT for xylose (a) and xylose with four bonded water molecules (b).
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5.2. Compounds with molecular weight above the MWCO of the membrane The retentions of estradiol and estrone (Table 3b) are lower than expected, based on the MWCO of UTC-20 and NF210 and also lower than the retention of salicine for both membranes. If the hydrophobicity of these compounds is considered, it is obvious that there is an influence of the hydrophobicity on the retention. Estrone and estradiol have comparable log P values whereas salicine has a much lower log P and more polar groups (Fig. 1), resulting in more possibilities to form hydrogen bonds with the water molecules and thus resulting in a larger hydrated molecule. The retention of salicine is therefore higher than that of estrone and estradiol meaning that hydrophobicity influences the retention of compounds with a molecular weight above the MWCO. Although, molecular size also plays an important role. Retentions of estrone and estradiol with a molecular weight of approximately 270 are significantly higher than those of 3,4-methylnitrophenol or benzilidene acetone, with a molecular weight of approximately 150 and also a high value of log P (Table 1), indicating that an increase in molecular size (above the MWCO of the membrane) will result in a higher retention. However, the differences in retention between estrone, estradiol and salicine are less than these between 3,4-methylnitrophenol, benzilidene acetone, etc. and xylose (compounds with a molecular weight below the MWCO, Table 3a), meaning that the influence of hydrophobicity on retention decreases as the molecular size, compared to the MWCO, increases. Therefore, it can be concluded that both molecular size and hydrophobicity influence the retention of a dissolved organic compound. The larger the molecule, the less effect hydrophobicity has on its retention. The influence of hydration could explain the fact that the retentions of estrone and estradiol are smaller than the retention expected based on the MWCO. Generally, the MWCO is determined using polar molecules such as polyethylene glycols or saccharides, which are hydrophilic and usually more efficiently hydrated. Due to the absence of hydration, the retentions of hydrophobic compounds will be lower, especially when the molecular weight is not substantially higher than the MWCO.
6. Conclusions Hydrophobicity and molecular size play an important role in the retention of dissolved organic compounds. Hydrophobicity is the most important parameter determining the retention for molecules with a molecular weight below the MWCO of a membrane. This was explained by a more effective solvation of the molecule, resulting in a larger size. However, the influence of hydrophobicity becomes smaller as the molecular size of the organic compound, compared to the MWCO of the membrane, increases. Finally, the influence of adsorption on retention was shown for hydrophobic compounds.
After saturation of the membrane, a breakthrough in permeate concentration was observed, resulting in a lower retention at steady state.
Acknowledgements Leen Braeken is grateful to the IWT (Vlaams Instituut voor de bevordering van Wetenschappelijk-Technologisch onderzoek in de industrie) for a PhD-grant. FWO Vlaanderen (Fund for Scientific Research) is acknowledged for a postdoctoral grant for Riet Ramaekers.
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