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The role of pH and concentration on the ion rejection in polyamide nanofiltration membranes
Journal of Membrane Science 264 (2005) 65–74
The role of pH and concentration on the ion rejection in polyamide nanofiltration membranes Serena Bandini ∗ , Jennifer Drei, Daniele Vezzani Dipartimento di Ingegneria Chimica, Mineraria e delle Tecnologie Ambientali, University of Bologna, Viale Risorgimento 2, I-40136, Bologna, Italy Received in revised form 20 December 2004; accepted 10 March 2005 Available online 23 May 2005
Abstract The role of pH and concentration is studied on the ion rejection of Desal5-DK polyamide NF membranes. Various samples are used taken from two different batches (DK99 and DK02). Extensive experimentation at room temperature on HCl–water (0.001–1 mol/m3 ) as well as on NaCl–water (1–50 mol/m3 ) solutions is presented to characterize the membrane as a function of electrolyte concentration, at feed pH values in the range from 3 to 6.5. The applied pressure in the feed side is varied from 3 to 30 bar. The effect of feed pH is studied both on salt rejection and on total volume flux, and general trends are obtained. DK02 membranes are less pH sensitive than DK99’s. Na+ and H+ rejections are greatly affected by pH. Na+ rejection goes through a minimum value as the feed acidity increases, and correspondingly H+ rejection goes through a maximum value. Minima are approximately located at pH values in the range between 4.5 and 5. Volumetric membrane charge values corresponding to each operative condition investigated are calculated through the Donnan Steric Pore Model and Dielectric Exclusion. Membrane charge is obtained as an increasing function both with pH and with electrolyte concentration in the feed side. A Langmuir-type behaviour is apparent at constant pH values; amphoteric behaviour is re-confirmed. The points of zero charge approximately correspond to the pH values at which NaCl rejections approach their minimum values. © 2005 Elsevier B.V. All rights reserved. Keywords: Nanofiltration; pH; Electrolytes; Amphoteric behaviour; Membrane charge
1. Introduction Separation efficiency of the nanofiltration process basically depends on the very peculiar characteristics of the membranes used as well as on the chemical nature of the solutions treated. In the case of electrolyte solutions, the separation mechanism is remarkably related to steric and electrostatic partitioning effects between the membrane and the external solutions. One of the main features determining separation performances is the membrane charge, partially induced by the electrolyte solutions kept in contact with the membrane itself. ∗
Corresponding author. Tel.: +39 0512093138; fax: +39 051581200. E-mail addresses: [email protected], [email protected] (S. Bandini). 0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.03.054
Ionic rejection measurements, both in binary and in multicomponent systems, put in evidence that process performance is greatly influenced by electrolyte concentration and by acidic characteristics existing in feed solutions [1–18]. In particular, it was observed that dominant salt rejections follow different trends depending upon the electrolyte type. In the case of symmetric salts, such as NaCl, rejection generally decreases as the concentration increases at constant pH values, whereas rejection goes through a minimum value as feed pH increases [6,10–12,17,18]. In the case of non-symmetric electrolytes, on the contrary, chemical interactions with the membrane are relevant and different opposite trends are often observed [5,9,10,12,14]. That general behaviour is basically obtained also in the case of inorganic membranes [15]. The role of pH is also important in determining the transport of organic acids; in the case of lactic acid, for instance, the lac-
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tate rejection increases with pH [19] and shows a maximum value at neutral pH [20]. On the other hand, electrokinetic measurements of streaming potential put in evidence that the membrane surface is endowed with charges which are strictly dependent on feed pH, as well as on type and concentration of electrolytes; amphoteric behaviour is also frequently observed for a wide class of polymeric membranes [5,6,16,21–25]. From a theoretical point of view, it is recognized that the electrostatic phenomena giving rise to ion partitioning are the result of the combined effects of Donnan equilibrium and dielectric exclusion, whereas ionic transport across the membrane can be described through the extended Nernst–Planck equation, appropriately modified to take into account of the hindered transport of ions through narrow pores [8,9,14,26–29]. In these models, a porous vision of the membrane is proposed, which is characterized by means of various adjustable parameters. In the most general cases, the membrane parameters are three, the average pore radius, the volume charge density and the effective membrane thickness. In [8,9,14] the volume membrane charge is expressed as a function of the equivalents concentration existing at the feed/membrane interface, and it is remarkably independent of the electrolyte type; it is calculated by fitting the experimental rejections of single electrolytes through the model equations. This charge model does not contain the pH effect, and proper charge parameters are to be calculated every time the pH is varied. In this work, the role of pH and concentration is studied on the ion rejection of Desal-DK polyamide membranes. Various samples were used taken from two different batches. An extensive detailed experimental investigation is reported in the case of NaCl–water as well as of HCl–water mixtures in a wide range of electrolyte compositions. The effect of feed pH is studied both on salt rejection and on total volume flux, and general trends are obtained. Experimental data available are successively elaborated through the Donnan Steric Pore Model and Dielectric Exclusion (DSPM&DE) [14] to evaluate the adjustable parameters of the membranes tested; in particular, volumetric membrane charge values corresponding to each operative condition investigated are calculated. The results obtained assume a twofold relevance. On one side they contribute to the validation of the DSPM&DE model as a general model; on the other side, they allow us to draw in a clear way the typical behaviour of the membrane charge as a function of pH and concentration in the feed side, in a highly representative case such as NaCl–water solutions.
2. Experimental materials and methods An experimental investigation was performed testing NF membranes in the case of pure water, of NaCl–water as well as of HCl–water mixtures, in a wide range of compositions and pH. The feed pH effect was studied both on salt rejection and
on total volume flux; pH measurements were also performed in the permeate side and H+ rejections were calculated. Desal-5 DK membranes were used, manufactured by Desal Inc. (Vista, CA, USA); DK membranes are polymeric flat thin film composite membranes, in which a polyamide selective layer is supported on a polysulfone layer. Various samples taken from two different batches were tested; in the following, membranes from the first batch delivered in 1999 will be abbreviated as DK99, whereas samples from the second batch delivered in 2002 will be indicated as DK02. For both of them, manufacturer gives the same nominal characterization: 98% nominal rejection, measured at 1000 ppm MgSO4 at 25 ◦ C and 6.9 bar. Experiments were performed with the same apparatus and following the same procedure widely described in a recent paper [14]. Membranes were located in a radial flow circular cell, in which the useful membrane area was 39 cm2 . The membrane apparatus was arranged in a quite typical bench scale plant, working in total recirculation mode; recirculation rate was kept at 700 dm3 h−1 to guarantee a quite negligible concentration polarization. Experiments were performed at 25 ◦ C, in a wide range of feed compositions, from 1 to 50 mol/m3 ; in DK99 measurements, the applied pressure in the feed side was varied from 3 to 25 bar, whereas it was varied from 3 to 30 bar in DK02 tests. Solutions were prepared from reagent-grade chemicals in deionized water, obtained through ionic exchange resins followed by an RO step. The feed pH was adjusted in the range from 3 to 6.5 by HCl or by NaOH. NaCl concentrations in the feed and permeate side were measured by a conductimeter in the experiments at pH higher than 4.5, whereas at pH lower than 4.5 sodium concentration was determined through the high performance liquid chromatography (HPLC) technique, with cationic column followed by electric conductivity detector. Experimental results are reported as a function of the pressure difference across the membrane, indicated as P, which was calculated as the arithmetic mean of the driving forces existing at the inlet and outlet sections of the membrane module. Alternatively, total volume flux data are reported also versus the effective pressure difference, P − π, in which π is the osmotic pressure difference across the membrane, calculated in correspondence with the concentration values in the feed and permeate bulks.
3. Results In Fig. 1 volume flux is reported as a function of the pressure difference across various DK99 samples, for the case of HCl–water solutions. Since HCl is present at very low concentrations, these data can be also considered as pure water flux data as a function of pH. In Fig. 1(a) a very good reproducibility can be observed in membrane performances: the typical behaviour encountered in these processes is obtained
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and then increases to 89% at pH 3. The same trend is observed for all the compositions investigated; (v) total volume flux linearly increases as the effective applied pressure increases and it is not greatly affected by the pH value. On the contrary, a slight feed composition effect occurs on membrane permeability, as it can be observed from the corresponding values of the slopes reported in Figs. 3(b) and 4(b) in comparison with the water membrane permeability reported in Fig. 1(a). That quite typical behaviour had been already observed by various authors, for Desal-DK membranes [6,9] as well as for different membranes [4,5,10,12,16], and it has been ultimately reported also by [18], although each author puts in evidence only some of the features above described. The trend observed for DK99 membranes has been reconfirmed also by the experimental results obtained with DK02 membranes, which are reported in Fig. 5. Apparently, the feed pH affects separation performances in a different way: at feed pH values in the range from 5.3 to 6.2, DK02 are less pH sensitive than DK99, although for both the membranes NaCl rejections are very close at pH 5.8 (in this case, permeate concentrations measured with DK99 are 15–20% lower than DK02’s). Fig. 1. NF of aqueous solutions containing HCl, through DK99 membrane, at 25 ◦ C: total volume flux vs. the pressure difference across the membrane. (a) Water membrane permeability; (b) pH effect.
in which the volume flux linearly depends on the driving force (average water permeability is also reported as the slope of the straight line fitting the experimental results). In Fig. 1(b) no meaningful dependence of water flux is observed as a function of the feed pH. The case of NaCl–water solutions is considered in Figs. 2–5 in which the role of concentration and pH is reported on salt rejection and on total volume flux. The behaviour observed can be summarized as follows: - At a constant pH value in the feed solution (Fig. 2): (i) NaCl rejection increases as the feed salt composition decreases; (ii) asymptotic rejections are approached at driving pressures higher than 20 bar; (iii) total volume flux linearly increases as the applied pressure increases and decreases as the salt concentration increases; this behaviour is obviously due to the increasing effect of the osmotic pressure difference across the membrane on the effective pressure difference, as the salt concentration increases. - At a constant salt composition in the feed solution (Figs. 3 and 4): (iv) NaCl rejection goes through a minimum value as the feed acidity increases. In the case of 5 mol/m3 NaCl–water solutions (Fig. 3(a)), asymptotic Na+ rejection decreases from 87% at pH 6 to 61% at pH 4.5
Fig. 2. NF of aqueous solutions containing NaCl, through DK99 membrane, at 25 ◦ C and 5.8 feed pH. The effect of salt concentration in the feed on (a) sodium rejection and (b) total volume flux, vs. the pressure difference across the membrane.
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On the other hand, the behaviour observed in Fig. 6(a) is widely recognized as quite typical and it is mainly due to a change in the membrane charge as a function of pH. Based on zeta-potential measurements, the membrane charge is also observed to become more negative as the pH increases, at a constant electrolyte concentration. Various authors [8,9,28] widely documented that higher membrane charge values, both in the negative and in the positive range, can explain higher rejection values at a constant salt concentration. As a consequence, in the case of NaCl–water mixtures, at constant electrolyte concentration, we can expect that negative membrane charge increases as pH increases, and that the macroscopic effect is a higher rejection of chloride ion. The behaviour is exactly reversed in the case of positively charged membranes: as pH decreases, positive membrane charge increases and the macroscopic effect is a higher sodium rejection. A minimum value of co-ion exclusion (chloride in the negative charge case, sodium in the positive charge case) is thus achieved in the case of neutral membrane, which approximately should correspond to the case of zero net charge membrane, at which charge separation effects vanish. In the specific cases reported in Fig. 6(a), it can be observed that the minimum rejection of DK99 membranes is located
Fig. 3. NF of aqueous solutions containing 5 mol/m3 NaCl, through DK99 membrane at 25 ◦ C: the effect of feed pH. (a) Sodium rejection vs. the applied pressure; (b) total volume flux vs. the effective pressure difference.
Differences are better remarked in Fig. 6(a), in which Na+ asymptotic rejections are compared: pH values at which the minima exist are not coincident, as well as the corresponding minimum rejection values. However, in the pH range investigated, it is re-confirmed that DK02 rejections are less pH sensitive than DK99’s. The same behaviour was also obtained for all the pressures investigated. The trend observed at point (i) is qualitatively and quantitatively explained by several authors. Zeta-potential measurements [25] widely documented that, in the pH range in which the membrane charge is negative, its absolute value increases as the electrolyte concentration increases, at a constant pH value. All the models which assume a behaviour of that kind for the membrane charge, such as Donnan Steric Pore Model [8,9,26,27] as well as Donnan Steric Pore Model and Dielectric Exclusion [14], or previous models [28], are able to explain qualitatively the reduction of NaCl rejection with the salt concentration. Generally speaking, it can be observed that, as the electrolyte concentration increases, not only the membrane charge becomes more negative, but also the membrane potential follows the same trend; as a consequence the Donnan effect becomes more favourable to counter-ion partitioning inside the membrane and salt rejection decreases.
Fig. 4. NF of aqueous solutions containing 20 mol/m3 NaCl, through DK99 membrane at 25 ◦ C: the effect of feed pH. (a) Sodium rejection vs. the applied pressure; (b) total volume flux vs. the effective pressure difference.
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Fig. 6. NF of aqueous solutions containing NaCl, through DK membranes, at 25 ◦ C: (a) sodium and (b) proton rejections vs. the feed pH at P = 25 bar applied at the inlet section. (Open symbols, +) DK99; (closed symbols, , ) DK02.
Fig. 5. Feed pH effect in NF of aqueous solutions containing NaCl, through DK membranes at 25 ◦ C: sodium rejection is reported vs. total volume flux at different feed salt concentrations. DK99: P = 3–25 bar; DK02: P = 3–30 bar.
at a pH value close to 4.5 and it is concentration independent, whereas in the case of DK02 membranes it is located at a pH value close to 5, with the exception of the 1 mol/m3 case for which minimum rejection is measured closed to 4.5. The opposite effect is observed on H+ rejection, in agreement with what measured also by other authors [13,18]. H+ rejection goes through a maximum value, which approximately corresponds to pH values close to 4.5, both in the case of DK99 membranes and in the case of DK02 membranes. The trend is observed for NaCl–HCl–water solutions
(Fig. 6(b)), as well as for HCl–water solutions (Fig. 7). In particular, in those cases in which the membrane charge is negative, H+ rejection increases as the H+ concentration increases, and correspondingly the H+ concentration in the permeate decreases. From a qualitative point of view, in the case of negatively charged membranes, H+ is greatly attracted towards the permeate, and very low negative rejections are measured; in the case of positively charged membranes, chloride easily passes across the membrane (at pH 3 chloride rejection is lower than sodium rejection) and H+ permeation is more favoured than Na+ , thus giving high Na+ rejections in correspondence with low H+ rejections. As a conclusion, the trend observed in Fig. 6(a) is suggestive of a qualitatively similar behaviour of the membranes investigated, although different rejections values clearly show that different values of the membrane charge should exist, in correspondence with the same bulk electrolyte concentration. On the other hand, the trend observed in Fig. 6(b) puts in evidence that proton interactions with the membrane material are quite similar.
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the simplified approach called procedure B is used, as well as the “integral” version of DSPM&DE. Procedure B is based on the total co-ion exclusion assumption, which leads to the development of simple analytical relationships to describe rejection of single symmetric salts as a function of membrane parameters, both in the case in which data are available as a function of P and in asymptotic conditions. The “integral” version is obtained by integration of the basic transport equations under the assumption of a constant potential gradient through the membrane; it can be used with confidence in the case of multicomponent solutions and in all cases in which no ion is a trace component. The corresponding equations are extensively reported in [14]. For DK99 and DK02 membranes, the following calculations were performed: Fig. 7. NF of aqueous solutions containing HCl, through DK99 membrane, at 25 ◦ C: proton rejection vs. the feed pH, at various pressure differences applied in the inlet section.
4. Membrane charge calculation All the NF experimental data available for the two series of DK membranes, partially reported in Figs. 2–6, were used to evaluate the adjustable membrane parameters. In particular, the values of the volumetric membrane charge corresponding to each operative conditions were calculated. 4.1. Calculation procedure Calculations were performed through the Donnan Steric Pore Model and Dielectric Exclusion (DSPM&DE), widely discussed in the recent paper [14], which is basically an extension of the DSPM model originally proposed by Bowen et al. [8,26,27]. The DSPM&DE model takes into account of the dielectric exclusion phenomenon as a partitioning mechanism in addition to steric hindrance and to Donnan equilibrium; no further adjustable parameters are introduced in addition to the typical parameters of the DSPM model, such as the average pore radius, the effective membrane thickness and the volumetric membrane charge. The contribution of the image forces is considered dominant with respect to Born dielectric effects. In other words, the difference between the dielectric constant of the aqueous solution inside the pores (εP ) and the corresponding value of the membrane material (εM ) is assumed dominant with respect to the difference between the solvent dielectric properties inside the membrane pores and in the external bulk solutions (εS ). In addition, the properties of the solution inside the membrane pores are also assumed equal to those existing in the external feed solution, that is εP = εS ; εS is assumed close to the pure water value at room temperature and εM is assumed as a typical value for polymeric matrix, thus giving εS/ εM = 80/3 [14]. In [14] authors presented a general assessment for membrane characterization, and various new procedures were introduced for membrane parameters calculation. In this paper,
(i) Average pore radius (rP ), effective membrane thickness (δ) and volumetric membrane charges values (X) were firstly calculated by fitting NaCl rejection data versus total volume flux, for each composition value investigated, at pH 5.8, by using the typical equations of procedure B. (ii) Assuming rP and δ calculated at step (i) as constant values, X values corresponding to the complete pH range investigated were successively calculated by fitting all the remaining sets of NaCl rejection-flux data, making use of procedure B (sets at pH 3 excepted). (iii) X values at pH 3 were calculated by fitting rejection data through the general equations of the DSPM&DE model, in its “integral” version, applied for the case of three ions. In this case procedure B could not be applied, since proton concentration was comparable to sodium and chloride concentrations. (iv) In the cases in which only asymptotic rejections were available, the corresponding analytical relationship, derived from procedure B, was used, assuming as pore radius the rP value calculated at step (i). A graphical representation of the quality of the fitting procedure is reported in Fig. 8, as an example, both for DK99 and for DK02 membranes. 4.2. Results The results obtained from the complete characterization of DK99 and DK02 membranes are reported in Table 1, in which membrane charge values calculated from rejection versus flux data (elaborations at points (i) to (iii)) are also compared with the corresponding values obtained from asymptotic rejection data (elaborations at point (iv)). First of all, it can be observed that pore radius and membrane thickness values are very close for both the membranes. Secondly, for DK02 membranes, charge values calculated from rejection data at P = 25 are systematically lower (2–24%) than those calculated from data in a wide pressure range. Although 25 bar pressure difference is a high value, the results show that the real asymptotic conditions are achieved
S. Bandini et al. / Journal of Membrane Science 264 (2005) 65–74 Table 1 Membrane parameters calculated through the DSPM&DE model. Elaboration of NaCl rejection-flux data DK99 (rP # = 0.57 nm, ␦# = 19.3 m, LP = 6.41 × 10−8 m h−1 Pa−1 ) NaCl
(mol/m3 )
pH 6 4.76 9.86 20.45 20.57 47.13
−Xa
(mol/m3 )
pH 5.8 1.06 2.54 5.13 10.06 20.92 50.82
1.03 2.41 4.16 7.67 14.45 24.29
pH 5.6 4.51 10.65 20.58 39.32
2.74 4.12 9.77 15.27
pH 5.3 5.25 9.46 20.60 38.82
1.55 3.97 8.16 11.80
pH 5 4.66 9.47 20.05
1.29 2.26 3.72
pH 4.5 4.53 9.38
0.58 0.58
pH 3 4.48 9.68 19.43
Table 1 ( Continued ) DK02 (rP # = 0.59 nm, δ# = 23.7 m, LP = 7.07 × 10−8 m h−1 Pa−1 ) NaCl (mol/m3 ) 20.18 50.21
−Xb (mol/m3 )
−Xc (mol/m3 )
10.57 23.29
9.32 22.77
pH 5.6 1.03 5.01 20.15
9.09 14.42 16.72 18.92 29.70
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pH 5.3 1.03 2.53 5.04 20.18
−0.44 2.86 7.45
1.96 2.74 7.79
−0.83 1.5 2.13 6.76
pH 5 1.07 5.03 20.24
0 ± 0.21 2.11 6.22
pH 4.5 1.07 5.05 20.24
0 ± 0.19 0 ± 0.05 3.87
LP = water permeability. a From data in the range P = 3–25 bar. b From data in the range P = 3–30 bar. c From data at P = 25 bar. # From data at pH 5.8.
at slightly higher pressures. Data available at 30 bar and reported in Fig. 5 put in evidence this aspect. However these results clearly indicate that it is possible to get reasonable and meaningful membrane charge values, although underestimated values, basing only on few rejection data measured at conditions close to the asymptotic ones. X values are also plotted in Figs. 9 and 10. Apparently, membrane charge absolute values increase as the feed electrolyte concentration increases and, correspondingly, increase as the feed pH increases. The trend obtained
−2.50 −2.00 −12.70
DK02 (rP # = 0.59 nm, δ# = 23.7 m, LP = 7.07 × 10−8 m h−1 Pa−1 ) NaCl (mol/m3 )
−Xb (mol/m3 )
pH 6.5 5.02 20.24 pH 6.2 2.53 5.03 10.05 20.18 50.24
6.10 13.55 3.59 4.9 6.78 13.43 26.01
pH 6 5.01 20.24 pH 5.8 1.08 2.56 5.03 10.05
−Xc (mol/m3 )
2.73 4.23 5.69 11.59 25.24 3.92 10.28
2.48 4.11 5.8
1.16 2.11 3.17 4.92
Fig. 8. NF of aqueous solutions containing NaCl, through DK membranes, at 25 ◦ C: fitting results through DSPM&DE model-procedure B [14] (dashed lines) of the corresponding experimental data. (Open symbols) DK02; (closed symbols) DK99.
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Fig. 9. NF of aqueous solutions containing NaCl, through DK99 membrane, at 25 ◦ C. Volume membrane charge values fitted through DSPM&DE modelprocedure B [14] (rP = 0.57 nm, δ = 19.3 m) are reported (a) vs. feed equivalents concentration and (b) vs. feed pH.
qualitatively reproduces the behaviour observed for surface membrane charges values calculated from electrokinetics measurements [25]. In the case of DK99 a Langmuir-type behaviour seems to be consistent with the data (Fig. 9(a)); on the contrary, in the case of DK02 a linear trend is observed at high pH values, whereas at low pH’s a Langmuirtype behaviour might better describe the experimental trend (Fig. 10(a)). The membrane charge values calculated for both the membranes clearly confirm what has been put in evidence also by the rejection data reported in Figs. 5 and 6. Although membranes performances are quite comparable at pH 5.8 (very close values of rP , δ and X), in the whole pH range investigated the charge variability range for DK99 is wider than for DK02. In addition, the typical amphoteric behaviour already observed for this kind of membranes is re-confirmed: in both cases a sign change occurs as a function of pH. The pH value corresponding to the zero net charge value is somewhat different for the two membranes: it is located in the pH range from 4 to 5 and approximately corresponds to the pH value at which NaCl rejections approach their minimum values.
Fig. 10. NF of aqueous solutions containing NaCl, through DK02 membrane, at 25 ◦ C. Volume membrane charge values fitted through DSPM&DE model-procedure B [14] (rP = 0.59 nm, δ = 23.7 m) are reported (a) vs. feed equivalents concentration and (b) vs. feed pH.
Finally, it must be observed that a pH effect exists also at very low salt concentrations for which non-zero membrane charges are calculated. Calculations here presented clearly demonstrate a wide validity of the DSPM&DE model. Remarkably, the model allows us to calculate the volumetric membrane charge in a wide range of feed compositions and pH, including its sign change, keeping the other membrane parameters (in particular the pore radius) as constant values and equal to those calculated at a reference pH value, without any variation of the dielectric constants values. Of course, the membrane charges calculated are fitting values and the values obtained depend upon the model used and could be different from the corresponding values calculated from zeta-potential measurements. However, we can use these data with confidence to get information from their relative values. Whatever the real values may be, it is a matter of fact that the volume membrane charge of DK membranes is an increasing function both with pH and with electrolyte concentration in the feed solution, at least in the NaCl–water case. Empirical correlations can be obtained straightforwardly, to be used in the DSPM&DE model for predictive purposes.
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It is self-evident that, to avoid the use of empirical correlations, the crucial point to solve is the understanding of the correct mechanism at the basis of the membrane charge formation.
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Education University and Research (MIUR ex 60%) is acknowledged.
References 5. Conclusions A NF experimental investigation has been performed in the case of NaCl–water as well as of HCl–water mixtures in a wide range of electrolytes compositions. The effect of pH value in the feed solution has been studied both on salt rejection and on total volume flux. Various samples of DesalDK membranes were tested, taken from two different batches. For both of them general trends were obtained: in general, DK02 membranes are less pH sensitive than DK99. The most interesting aspects of the experimental results obtained are represented by the exactly opposite behaviours observed for Na+ and H+ rejections as a function of feed pH. Na+ rejection goes through a minimum value as the feed acidity increases, and, correspondingly, H+ rejection goes through a maximum value. Minima are approximately located at pH values in the range between 4.5 and 5. Also the effect of ionic concentration is different: whereas NaCl rejection decreases as the salt concentration increases, H+ rejection increases as the H+ concentration increases, as far as the membrane is negatively charged. The behaviour suggests that proton interactions with the membrane might be different from those occurring for electrolytes. In other words, solution acidity contributes to the membrane charge formation in a different way with respect to NaCl concentration. Experimental data have been elaborated through the Donnan Steric Pore Model and Dielectric Exclusion to calculate the adjustable membrane parameters. The results obtained contribute to the full validation of the DSPM&DE model as a general model. Above all, in the case of NaCl–water solutions, volume membrane charge is obtained as an increasing function both with pH and with electrolyte concentration in the feed side. A Langmuir-type behaviour is apparent at constant pH; amphoteric behaviour is re-confirmed. The results here obtained might be used to calculate empirical correlations to be introduced as input values in the general equations of the DSPM&DE model to predict membrane performances corresponding to different operative conditions. It is self-evident that, to avoid the use of empirical correlations, the crucial point to solve is the understanding of the correct mechanism at the basis of the membrane charge formation. X values here reported are a valuable starting point for that purpose, which will be pursued in a next paper.
Acknowledgements Grateful thanks are due to Drs. L. Bertocchi and C. Mazzoni for their relevant contribution in performing the experiments. Financial support by Italian Ministry of
[1] T. Tsuru, M. Urairi, S. Nakao, S. Kimura, Reverse osmosis of single and mixed electrolytes with charged membranes: experiments and analysis, J. Chem. Eng. Jpn. 24 (1991) 518–524. [2] P. Lipp, R. Gimbel, F.H. Frimmel, Parameters influencing the rejection properties of FT30 membranes, J. Membr. Sci. 95 (1994) 185–197. [3] M. Nystr¨om, L. Kaipia, S. Luque, Fouling and retention of nanofiltration membranes, J. Membr. Sci. 98 (1995) 249–262. [4] M.S. Hall, V.M. Starov, D.R. Lloyd, Reverse osmosis of multicomponent electrolyte solutions. Part II. Experimental verification, J. Membr. Sci. 128 (1997) 39–53. [5] G. Hagmeyer, R. Gimbel, Modelling the salt rejection of nanofiltration membranes for ternary mixtures and for single salts at different pH values, Desalination 117 (1998) 247–256. [6] G. Hagmeyer, R. Gimbel, Modelling the rejection of nanofiltration membranes using zeta potential measurements, Sep. Purif. Technol. 15 (1999) 19–30. [7] M.D. Afonso, M.N. de Pinho, Transport of MgSO4 , MgCl2 and Na2 SO4 across an amphoteric nanofiltration membrane, J. Membr. Sci. 179 (2000) 137–154. [8] W.R. Bowen, H. Mukhtar, Characterization and prediction of separation performance of nanofiltration membranes, J. Membr. Sci. 112 (1996) 263–274. [9] W.R. Bowen, J.S. Welfoot, Modelling the performance of membrane nanofiltration — critical assessment and model development, Chem. Eng. Sci. 57 (2002) 1121–1137. [10] Y. Xu, R.E. Lebrun, Investigation of the solute separation by charged nanofiltration membrane: effect of pH, ionic strength and solute type, J. Membr. Sci. 158 (1999) 93–104. [11] X. Wang, W. Wang, D. Wang, Experimental investigation on separation performance of nanofiltration membranes for inorganic electrolyte solutions, Desalination 145 (2002) 115–122. [12] S. Szoke, G. Patzay, L. Weiser, Characteristics of thin-film nanofiltration membranes at various pH-values, Desalination 151 (2002) 123–129. [13] J. Tanninen, M. Nystrom, Separation of ions in acidic conditions using NF, Desalination 147 (2002) 295–299. [14] S. Bandini, D. Vezzani, Nanofiltration modeling: the role of dielectric exclusion in membrane characterization, Chem. Eng. Sci. 58 (2003) 3303–3326. [15] C. Labbez, P. Fievet, A. Szymczyk, A. Vidonne, A. Foissy, J. Pagetti, Analysis of the salt retention of a titania membrane using the “DSPM” model: effect of pH, salt concentration and nature, J. Membr. Sci. 208 (2002) 315–329. [16] M. Ernst, A. Bismark, J. Springler, M. Jekel, Zeta-potential and rejection rates of a polyethersulfone nanofiltration membrane in single salt solutions, J. Membr. Sci. 165 (2000) 251–259. [17] L. Paugam, S. Taha, G. Dorange, P. Jaouen, F. Qu´em´eneur, Mechanism of nitrate ions transfer in nanofiltration depending on pressure, pH, concentration and medium composition, J. Membr. Sci. 231 (2004) 37–46. [18] J. Quin, M.H. Oo, H. Lee, B. Coniglio, Effect of feed pH on permeate pH and ion rejection under acidic conditions in NF process, J. Membr. Sci. 232 (2004) 153–159. [19] J.M.K. Timmer, H.C. van der Horst, T. Robbertsen, Transport of lactic acid through reverse osmosis and nanofiltration membranes, J. Membr. Sci. 85 (1993) 205–216. [20] V. Freger, T.C. Arnot, J.A. Howell, Separation of concentrated organic/inorganic salt mixtures by nanofiltration, J. Membr. Sci. 178 (2000) 185–193.
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[21] M.D. Afonso, G. Hagmeyer, R. Gimbel, Streaming potential measurements to assess the variation of nanofiltration membranes surface charge with the concentration of salt solutions, Sep. Purif. Technol. 22–23 (2001) 529–541. [22] M.J. Ariza, J. Benavente, Streaming potential along the surface of polysulfone membranes: a comparative study between two different experimental systems and determination of electrokinetic and adsorption parameters, J. Membr. Sci. 190 (2001) 119– 132. [23] E. Childress, M. Elimelech, Effect of solution chemistry on the surface charge of polymeric reverse osmosis and nanofiltration membranes, J. Membr. Sci. 119 (1996) 253– 268. [24] J.M.M. Peeters, M.H.V. Mulder, H. Strathmann, Streaming potential measurements as a characterization method for nanofiltration membranes, Colloids Surf. A: Physicochem. Eng. Aspects 150 (1999) 247–259.
[25] J.H. Tay, J. Liu, D.D. Sun, Effect of solution physico-chemistry on the charge property of nanofiltration membranes, Water Res. 36 (2002) 585–598. [26] W.R. Bowen, A.W. Mohammad, N. Hilal, Characterization of nanofiltration membranes for predictive purposes — use of salts, uncharged solutes and atomic force microscopy, J. Membr. Sci. 126 (1997) 91–105. [27] W.R. Bowen, A.W. Mohammad, Characterization and prediction of nanofiltration membrane performance — a general assessment, Inst. Chem. Eng. 76A (1998) 885–893. [28] T. Tsuru, S. Nakao, S. Kimura, Calculation of ion rejection by extended Nernst–Planck equation with charged reverse osmosis membranes for single and mixed electrolyte solutions, J. Chem. Eng. Jpn. 24 (1991) 511–517. [29] M.S. Hall, V.M. Starov, D.R. Lloyd, Reverse osmosis of multicomponent electrolyte solutions. Part I. Theoretical development, J. Membr. Sci. 128 (1997) 23–37.
The role of pH and concentration on the ion rejection in polyamide nanofiltration membranes Serena Bandini ∗ , Jennifer Drei, Daniele Vezzani Dipartimento di Ingegneria Chimica, Mineraria e delle Tecnologie Ambientali, University of Bologna, Viale Risorgimento 2, I-40136, Bologna, Italy Received in revised form 20 December 2004; accepted 10 March 2005 Available online 23 May 2005
Abstract The role of pH and concentration is studied on the ion rejection of Desal5-DK polyamide NF membranes. Various samples are used taken from two different batches (DK99 and DK02). Extensive experimentation at room temperature on HCl–water (0.001–1 mol/m3 ) as well as on NaCl–water (1–50 mol/m3 ) solutions is presented to characterize the membrane as a function of electrolyte concentration, at feed pH values in the range from 3 to 6.5. The applied pressure in the feed side is varied from 3 to 30 bar. The effect of feed pH is studied both on salt rejection and on total volume flux, and general trends are obtained. DK02 membranes are less pH sensitive than DK99’s. Na+ and H+ rejections are greatly affected by pH. Na+ rejection goes through a minimum value as the feed acidity increases, and correspondingly H+ rejection goes through a maximum value. Minima are approximately located at pH values in the range between 4.5 and 5. Volumetric membrane charge values corresponding to each operative condition investigated are calculated through the Donnan Steric Pore Model and Dielectric Exclusion. Membrane charge is obtained as an increasing function both with pH and with electrolyte concentration in the feed side. A Langmuir-type behaviour is apparent at constant pH values; amphoteric behaviour is re-confirmed. The points of zero charge approximately correspond to the pH values at which NaCl rejections approach their minimum values. © 2005 Elsevier B.V. All rights reserved. Keywords: Nanofiltration; pH; Electrolytes; Amphoteric behaviour; Membrane charge
1. Introduction Separation efficiency of the nanofiltration process basically depends on the very peculiar characteristics of the membranes used as well as on the chemical nature of the solutions treated. In the case of electrolyte solutions, the separation mechanism is remarkably related to steric and electrostatic partitioning effects between the membrane and the external solutions. One of the main features determining separation performances is the membrane charge, partially induced by the electrolyte solutions kept in contact with the membrane itself. ∗
Corresponding author. Tel.: +39 0512093138; fax: +39 051581200. E-mail addresses: [email protected], [email protected] (S. Bandini). 0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.03.054
Ionic rejection measurements, both in binary and in multicomponent systems, put in evidence that process performance is greatly influenced by electrolyte concentration and by acidic characteristics existing in feed solutions [1–18]. In particular, it was observed that dominant salt rejections follow different trends depending upon the electrolyte type. In the case of symmetric salts, such as NaCl, rejection generally decreases as the concentration increases at constant pH values, whereas rejection goes through a minimum value as feed pH increases [6,10–12,17,18]. In the case of non-symmetric electrolytes, on the contrary, chemical interactions with the membrane are relevant and different opposite trends are often observed [5,9,10,12,14]. That general behaviour is basically obtained also in the case of inorganic membranes [15]. The role of pH is also important in determining the transport of organic acids; in the case of lactic acid, for instance, the lac-
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tate rejection increases with pH [19] and shows a maximum value at neutral pH [20]. On the other hand, electrokinetic measurements of streaming potential put in evidence that the membrane surface is endowed with charges which are strictly dependent on feed pH, as well as on type and concentration of electrolytes; amphoteric behaviour is also frequently observed for a wide class of polymeric membranes [5,6,16,21–25]. From a theoretical point of view, it is recognized that the electrostatic phenomena giving rise to ion partitioning are the result of the combined effects of Donnan equilibrium and dielectric exclusion, whereas ionic transport across the membrane can be described through the extended Nernst–Planck equation, appropriately modified to take into account of the hindered transport of ions through narrow pores [8,9,14,26–29]. In these models, a porous vision of the membrane is proposed, which is characterized by means of various adjustable parameters. In the most general cases, the membrane parameters are three, the average pore radius, the volume charge density and the effective membrane thickness. In [8,9,14] the volume membrane charge is expressed as a function of the equivalents concentration existing at the feed/membrane interface, and it is remarkably independent of the electrolyte type; it is calculated by fitting the experimental rejections of single electrolytes through the model equations. This charge model does not contain the pH effect, and proper charge parameters are to be calculated every time the pH is varied. In this work, the role of pH and concentration is studied on the ion rejection of Desal-DK polyamide membranes. Various samples were used taken from two different batches. An extensive detailed experimental investigation is reported in the case of NaCl–water as well as of HCl–water mixtures in a wide range of electrolyte compositions. The effect of feed pH is studied both on salt rejection and on total volume flux, and general trends are obtained. Experimental data available are successively elaborated through the Donnan Steric Pore Model and Dielectric Exclusion (DSPM&DE) [14] to evaluate the adjustable parameters of the membranes tested; in particular, volumetric membrane charge values corresponding to each operative condition investigated are calculated. The results obtained assume a twofold relevance. On one side they contribute to the validation of the DSPM&DE model as a general model; on the other side, they allow us to draw in a clear way the typical behaviour of the membrane charge as a function of pH and concentration in the feed side, in a highly representative case such as NaCl–water solutions.
2. Experimental materials and methods An experimental investigation was performed testing NF membranes in the case of pure water, of NaCl–water as well as of HCl–water mixtures, in a wide range of compositions and pH. The feed pH effect was studied both on salt rejection and
on total volume flux; pH measurements were also performed in the permeate side and H+ rejections were calculated. Desal-5 DK membranes were used, manufactured by Desal Inc. (Vista, CA, USA); DK membranes are polymeric flat thin film composite membranes, in which a polyamide selective layer is supported on a polysulfone layer. Various samples taken from two different batches were tested; in the following, membranes from the first batch delivered in 1999 will be abbreviated as DK99, whereas samples from the second batch delivered in 2002 will be indicated as DK02. For both of them, manufacturer gives the same nominal characterization: 98% nominal rejection, measured at 1000 ppm MgSO4 at 25 ◦ C and 6.9 bar. Experiments were performed with the same apparatus and following the same procedure widely described in a recent paper [14]. Membranes were located in a radial flow circular cell, in which the useful membrane area was 39 cm2 . The membrane apparatus was arranged in a quite typical bench scale plant, working in total recirculation mode; recirculation rate was kept at 700 dm3 h−1 to guarantee a quite negligible concentration polarization. Experiments were performed at 25 ◦ C, in a wide range of feed compositions, from 1 to 50 mol/m3 ; in DK99 measurements, the applied pressure in the feed side was varied from 3 to 25 bar, whereas it was varied from 3 to 30 bar in DK02 tests. Solutions were prepared from reagent-grade chemicals in deionized water, obtained through ionic exchange resins followed by an RO step. The feed pH was adjusted in the range from 3 to 6.5 by HCl or by NaOH. NaCl concentrations in the feed and permeate side were measured by a conductimeter in the experiments at pH higher than 4.5, whereas at pH lower than 4.5 sodium concentration was determined through the high performance liquid chromatography (HPLC) technique, with cationic column followed by electric conductivity detector. Experimental results are reported as a function of the pressure difference across the membrane, indicated as P, which was calculated as the arithmetic mean of the driving forces existing at the inlet and outlet sections of the membrane module. Alternatively, total volume flux data are reported also versus the effective pressure difference, P − π, in which π is the osmotic pressure difference across the membrane, calculated in correspondence with the concentration values in the feed and permeate bulks.
3. Results In Fig. 1 volume flux is reported as a function of the pressure difference across various DK99 samples, for the case of HCl–water solutions. Since HCl is present at very low concentrations, these data can be also considered as pure water flux data as a function of pH. In Fig. 1(a) a very good reproducibility can be observed in membrane performances: the typical behaviour encountered in these processes is obtained
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and then increases to 89% at pH 3. The same trend is observed for all the compositions investigated; (v) total volume flux linearly increases as the effective applied pressure increases and it is not greatly affected by the pH value. On the contrary, a slight feed composition effect occurs on membrane permeability, as it can be observed from the corresponding values of the slopes reported in Figs. 3(b) and 4(b) in comparison with the water membrane permeability reported in Fig. 1(a). That quite typical behaviour had been already observed by various authors, for Desal-DK membranes [6,9] as well as for different membranes [4,5,10,12,16], and it has been ultimately reported also by [18], although each author puts in evidence only some of the features above described. The trend observed for DK99 membranes has been reconfirmed also by the experimental results obtained with DK02 membranes, which are reported in Fig. 5. Apparently, the feed pH affects separation performances in a different way: at feed pH values in the range from 5.3 to 6.2, DK02 are less pH sensitive than DK99, although for both the membranes NaCl rejections are very close at pH 5.8 (in this case, permeate concentrations measured with DK99 are 15–20% lower than DK02’s). Fig. 1. NF of aqueous solutions containing HCl, through DK99 membrane, at 25 ◦ C: total volume flux vs. the pressure difference across the membrane. (a) Water membrane permeability; (b) pH effect.
in which the volume flux linearly depends on the driving force (average water permeability is also reported as the slope of the straight line fitting the experimental results). In Fig. 1(b) no meaningful dependence of water flux is observed as a function of the feed pH. The case of NaCl–water solutions is considered in Figs. 2–5 in which the role of concentration and pH is reported on salt rejection and on total volume flux. The behaviour observed can be summarized as follows: - At a constant pH value in the feed solution (Fig. 2): (i) NaCl rejection increases as the feed salt composition decreases; (ii) asymptotic rejections are approached at driving pressures higher than 20 bar; (iii) total volume flux linearly increases as the applied pressure increases and decreases as the salt concentration increases; this behaviour is obviously due to the increasing effect of the osmotic pressure difference across the membrane on the effective pressure difference, as the salt concentration increases. - At a constant salt composition in the feed solution (Figs. 3 and 4): (iv) NaCl rejection goes through a minimum value as the feed acidity increases. In the case of 5 mol/m3 NaCl–water solutions (Fig. 3(a)), asymptotic Na+ rejection decreases from 87% at pH 6 to 61% at pH 4.5
Fig. 2. NF of aqueous solutions containing NaCl, through DK99 membrane, at 25 ◦ C and 5.8 feed pH. The effect of salt concentration in the feed on (a) sodium rejection and (b) total volume flux, vs. the pressure difference across the membrane.
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On the other hand, the behaviour observed in Fig. 6(a) is widely recognized as quite typical and it is mainly due to a change in the membrane charge as a function of pH. Based on zeta-potential measurements, the membrane charge is also observed to become more negative as the pH increases, at a constant electrolyte concentration. Various authors [8,9,28] widely documented that higher membrane charge values, both in the negative and in the positive range, can explain higher rejection values at a constant salt concentration. As a consequence, in the case of NaCl–water mixtures, at constant electrolyte concentration, we can expect that negative membrane charge increases as pH increases, and that the macroscopic effect is a higher rejection of chloride ion. The behaviour is exactly reversed in the case of positively charged membranes: as pH decreases, positive membrane charge increases and the macroscopic effect is a higher sodium rejection. A minimum value of co-ion exclusion (chloride in the negative charge case, sodium in the positive charge case) is thus achieved in the case of neutral membrane, which approximately should correspond to the case of zero net charge membrane, at which charge separation effects vanish. In the specific cases reported in Fig. 6(a), it can be observed that the minimum rejection of DK99 membranes is located
Fig. 3. NF of aqueous solutions containing 5 mol/m3 NaCl, through DK99 membrane at 25 ◦ C: the effect of feed pH. (a) Sodium rejection vs. the applied pressure; (b) total volume flux vs. the effective pressure difference.
Differences are better remarked in Fig. 6(a), in which Na+ asymptotic rejections are compared: pH values at which the minima exist are not coincident, as well as the corresponding minimum rejection values. However, in the pH range investigated, it is re-confirmed that DK02 rejections are less pH sensitive than DK99’s. The same behaviour was also obtained for all the pressures investigated. The trend observed at point (i) is qualitatively and quantitatively explained by several authors. Zeta-potential measurements [25] widely documented that, in the pH range in which the membrane charge is negative, its absolute value increases as the electrolyte concentration increases, at a constant pH value. All the models which assume a behaviour of that kind for the membrane charge, such as Donnan Steric Pore Model [8,9,26,27] as well as Donnan Steric Pore Model and Dielectric Exclusion [14], or previous models [28], are able to explain qualitatively the reduction of NaCl rejection with the salt concentration. Generally speaking, it can be observed that, as the electrolyte concentration increases, not only the membrane charge becomes more negative, but also the membrane potential follows the same trend; as a consequence the Donnan effect becomes more favourable to counter-ion partitioning inside the membrane and salt rejection decreases.
Fig. 4. NF of aqueous solutions containing 20 mol/m3 NaCl, through DK99 membrane at 25 ◦ C: the effect of feed pH. (a) Sodium rejection vs. the applied pressure; (b) total volume flux vs. the effective pressure difference.
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Fig. 6. NF of aqueous solutions containing NaCl, through DK membranes, at 25 ◦ C: (a) sodium and (b) proton rejections vs. the feed pH at P = 25 bar applied at the inlet section. (Open symbols, +) DK99; (closed symbols, , ) DK02.
Fig. 5. Feed pH effect in NF of aqueous solutions containing NaCl, through DK membranes at 25 ◦ C: sodium rejection is reported vs. total volume flux at different feed salt concentrations. DK99: P = 3–25 bar; DK02: P = 3–30 bar.
at a pH value close to 4.5 and it is concentration independent, whereas in the case of DK02 membranes it is located at a pH value close to 5, with the exception of the 1 mol/m3 case for which minimum rejection is measured closed to 4.5. The opposite effect is observed on H+ rejection, in agreement with what measured also by other authors [13,18]. H+ rejection goes through a maximum value, which approximately corresponds to pH values close to 4.5, both in the case of DK99 membranes and in the case of DK02 membranes. The trend is observed for NaCl–HCl–water solutions
(Fig. 6(b)), as well as for HCl–water solutions (Fig. 7). In particular, in those cases in which the membrane charge is negative, H+ rejection increases as the H+ concentration increases, and correspondingly the H+ concentration in the permeate decreases. From a qualitative point of view, in the case of negatively charged membranes, H+ is greatly attracted towards the permeate, and very low negative rejections are measured; in the case of positively charged membranes, chloride easily passes across the membrane (at pH 3 chloride rejection is lower than sodium rejection) and H+ permeation is more favoured than Na+ , thus giving high Na+ rejections in correspondence with low H+ rejections. As a conclusion, the trend observed in Fig. 6(a) is suggestive of a qualitatively similar behaviour of the membranes investigated, although different rejections values clearly show that different values of the membrane charge should exist, in correspondence with the same bulk electrolyte concentration. On the other hand, the trend observed in Fig. 6(b) puts in evidence that proton interactions with the membrane material are quite similar.
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the simplified approach called procedure B is used, as well as the “integral” version of DSPM&DE. Procedure B is based on the total co-ion exclusion assumption, which leads to the development of simple analytical relationships to describe rejection of single symmetric salts as a function of membrane parameters, both in the case in which data are available as a function of P and in asymptotic conditions. The “integral” version is obtained by integration of the basic transport equations under the assumption of a constant potential gradient through the membrane; it can be used with confidence in the case of multicomponent solutions and in all cases in which no ion is a trace component. The corresponding equations are extensively reported in [14]. For DK99 and DK02 membranes, the following calculations were performed: Fig. 7. NF of aqueous solutions containing HCl, through DK99 membrane, at 25 ◦ C: proton rejection vs. the feed pH, at various pressure differences applied in the inlet section.
4. Membrane charge calculation All the NF experimental data available for the two series of DK membranes, partially reported in Figs. 2–6, were used to evaluate the adjustable membrane parameters. In particular, the values of the volumetric membrane charge corresponding to each operative conditions were calculated. 4.1. Calculation procedure Calculations were performed through the Donnan Steric Pore Model and Dielectric Exclusion (DSPM&DE), widely discussed in the recent paper [14], which is basically an extension of the DSPM model originally proposed by Bowen et al. [8,26,27]. The DSPM&DE model takes into account of the dielectric exclusion phenomenon as a partitioning mechanism in addition to steric hindrance and to Donnan equilibrium; no further adjustable parameters are introduced in addition to the typical parameters of the DSPM model, such as the average pore radius, the effective membrane thickness and the volumetric membrane charge. The contribution of the image forces is considered dominant with respect to Born dielectric effects. In other words, the difference between the dielectric constant of the aqueous solution inside the pores (εP ) and the corresponding value of the membrane material (εM ) is assumed dominant with respect to the difference between the solvent dielectric properties inside the membrane pores and in the external bulk solutions (εS ). In addition, the properties of the solution inside the membrane pores are also assumed equal to those existing in the external feed solution, that is εP = εS ; εS is assumed close to the pure water value at room temperature and εM is assumed as a typical value for polymeric matrix, thus giving εS/ εM = 80/3 [14]. In [14] authors presented a general assessment for membrane characterization, and various new procedures were introduced for membrane parameters calculation. In this paper,
(i) Average pore radius (rP ), effective membrane thickness (δ) and volumetric membrane charges values (X) were firstly calculated by fitting NaCl rejection data versus total volume flux, for each composition value investigated, at pH 5.8, by using the typical equations of procedure B. (ii) Assuming rP and δ calculated at step (i) as constant values, X values corresponding to the complete pH range investigated were successively calculated by fitting all the remaining sets of NaCl rejection-flux data, making use of procedure B (sets at pH 3 excepted). (iii) X values at pH 3 were calculated by fitting rejection data through the general equations of the DSPM&DE model, in its “integral” version, applied for the case of three ions. In this case procedure B could not be applied, since proton concentration was comparable to sodium and chloride concentrations. (iv) In the cases in which only asymptotic rejections were available, the corresponding analytical relationship, derived from procedure B, was used, assuming as pore radius the rP value calculated at step (i). A graphical representation of the quality of the fitting procedure is reported in Fig. 8, as an example, both for DK99 and for DK02 membranes. 4.2. Results The results obtained from the complete characterization of DK99 and DK02 membranes are reported in Table 1, in which membrane charge values calculated from rejection versus flux data (elaborations at points (i) to (iii)) are also compared with the corresponding values obtained from asymptotic rejection data (elaborations at point (iv)). First of all, it can be observed that pore radius and membrane thickness values are very close for both the membranes. Secondly, for DK02 membranes, charge values calculated from rejection data at P = 25 are systematically lower (2–24%) than those calculated from data in a wide pressure range. Although 25 bar pressure difference is a high value, the results show that the real asymptotic conditions are achieved
S. Bandini et al. / Journal of Membrane Science 264 (2005) 65–74 Table 1 Membrane parameters calculated through the DSPM&DE model. Elaboration of NaCl rejection-flux data DK99 (rP # = 0.57 nm, ␦# = 19.3 m, LP = 6.41 × 10−8 m h−1 Pa−1 ) NaCl
(mol/m3 )
pH 6 4.76 9.86 20.45 20.57 47.13
−Xa
(mol/m3 )
pH 5.8 1.06 2.54 5.13 10.06 20.92 50.82
1.03 2.41 4.16 7.67 14.45 24.29
pH 5.6 4.51 10.65 20.58 39.32
2.74 4.12 9.77 15.27
pH 5.3 5.25 9.46 20.60 38.82
1.55 3.97 8.16 11.80
pH 5 4.66 9.47 20.05
1.29 2.26 3.72
pH 4.5 4.53 9.38
0.58 0.58
pH 3 4.48 9.68 19.43
Table 1 ( Continued ) DK02 (rP # = 0.59 nm, δ# = 23.7 m, LP = 7.07 × 10−8 m h−1 Pa−1 ) NaCl (mol/m3 ) 20.18 50.21
−Xb (mol/m3 )
−Xc (mol/m3 )
10.57 23.29
9.32 22.77
pH 5.6 1.03 5.01 20.15
9.09 14.42 16.72 18.92 29.70
71
pH 5.3 1.03 2.53 5.04 20.18
−0.44 2.86 7.45
1.96 2.74 7.79
−0.83 1.5 2.13 6.76
pH 5 1.07 5.03 20.24
0 ± 0.21 2.11 6.22
pH 4.5 1.07 5.05 20.24
0 ± 0.19 0 ± 0.05 3.87
LP = water permeability. a From data in the range P = 3–25 bar. b From data in the range P = 3–30 bar. c From data at P = 25 bar. # From data at pH 5.8.
at slightly higher pressures. Data available at 30 bar and reported in Fig. 5 put in evidence this aspect. However these results clearly indicate that it is possible to get reasonable and meaningful membrane charge values, although underestimated values, basing only on few rejection data measured at conditions close to the asymptotic ones. X values are also plotted in Figs. 9 and 10. Apparently, membrane charge absolute values increase as the feed electrolyte concentration increases and, correspondingly, increase as the feed pH increases. The trend obtained
−2.50 −2.00 −12.70
DK02 (rP # = 0.59 nm, δ# = 23.7 m, LP = 7.07 × 10−8 m h−1 Pa−1 ) NaCl (mol/m3 )
−Xb (mol/m3 )
pH 6.5 5.02 20.24 pH 6.2 2.53 5.03 10.05 20.18 50.24
6.10 13.55 3.59 4.9 6.78 13.43 26.01
pH 6 5.01 20.24 pH 5.8 1.08 2.56 5.03 10.05
−Xc (mol/m3 )
2.73 4.23 5.69 11.59 25.24 3.92 10.28
2.48 4.11 5.8
1.16 2.11 3.17 4.92
Fig. 8. NF of aqueous solutions containing NaCl, through DK membranes, at 25 ◦ C: fitting results through DSPM&DE model-procedure B [14] (dashed lines) of the corresponding experimental data. (Open symbols) DK02; (closed symbols) DK99.
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Fig. 9. NF of aqueous solutions containing NaCl, through DK99 membrane, at 25 ◦ C. Volume membrane charge values fitted through DSPM&DE modelprocedure B [14] (rP = 0.57 nm, δ = 19.3 m) are reported (a) vs. feed equivalents concentration and (b) vs. feed pH.
qualitatively reproduces the behaviour observed for surface membrane charges values calculated from electrokinetics measurements [25]. In the case of DK99 a Langmuir-type behaviour seems to be consistent with the data (Fig. 9(a)); on the contrary, in the case of DK02 a linear trend is observed at high pH values, whereas at low pH’s a Langmuirtype behaviour might better describe the experimental trend (Fig. 10(a)). The membrane charge values calculated for both the membranes clearly confirm what has been put in evidence also by the rejection data reported in Figs. 5 and 6. Although membranes performances are quite comparable at pH 5.8 (very close values of rP , δ and X), in the whole pH range investigated the charge variability range for DK99 is wider than for DK02. In addition, the typical amphoteric behaviour already observed for this kind of membranes is re-confirmed: in both cases a sign change occurs as a function of pH. The pH value corresponding to the zero net charge value is somewhat different for the two membranes: it is located in the pH range from 4 to 5 and approximately corresponds to the pH value at which NaCl rejections approach their minimum values.
Fig. 10. NF of aqueous solutions containing NaCl, through DK02 membrane, at 25 ◦ C. Volume membrane charge values fitted through DSPM&DE model-procedure B [14] (rP = 0.59 nm, δ = 23.7 m) are reported (a) vs. feed equivalents concentration and (b) vs. feed pH.
Finally, it must be observed that a pH effect exists also at very low salt concentrations for which non-zero membrane charges are calculated. Calculations here presented clearly demonstrate a wide validity of the DSPM&DE model. Remarkably, the model allows us to calculate the volumetric membrane charge in a wide range of feed compositions and pH, including its sign change, keeping the other membrane parameters (in particular the pore radius) as constant values and equal to those calculated at a reference pH value, without any variation of the dielectric constants values. Of course, the membrane charges calculated are fitting values and the values obtained depend upon the model used and could be different from the corresponding values calculated from zeta-potential measurements. However, we can use these data with confidence to get information from their relative values. Whatever the real values may be, it is a matter of fact that the volume membrane charge of DK membranes is an increasing function both with pH and with electrolyte concentration in the feed solution, at least in the NaCl–water case. Empirical correlations can be obtained straightforwardly, to be used in the DSPM&DE model for predictive purposes.
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It is self-evident that, to avoid the use of empirical correlations, the crucial point to solve is the understanding of the correct mechanism at the basis of the membrane charge formation.
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Education University and Research (MIUR ex 60%) is acknowledged.
References 5. Conclusions A NF experimental investigation has been performed in the case of NaCl–water as well as of HCl–water mixtures in a wide range of electrolytes compositions. The effect of pH value in the feed solution has been studied both on salt rejection and on total volume flux. Various samples of DesalDK membranes were tested, taken from two different batches. For both of them general trends were obtained: in general, DK02 membranes are less pH sensitive than DK99. The most interesting aspects of the experimental results obtained are represented by the exactly opposite behaviours observed for Na+ and H+ rejections as a function of feed pH. Na+ rejection goes through a minimum value as the feed acidity increases, and, correspondingly, H+ rejection goes through a maximum value. Minima are approximately located at pH values in the range between 4.5 and 5. Also the effect of ionic concentration is different: whereas NaCl rejection decreases as the salt concentration increases, H+ rejection increases as the H+ concentration increases, as far as the membrane is negatively charged. The behaviour suggests that proton interactions with the membrane might be different from those occurring for electrolytes. In other words, solution acidity contributes to the membrane charge formation in a different way with respect to NaCl concentration. Experimental data have been elaborated through the Donnan Steric Pore Model and Dielectric Exclusion to calculate the adjustable membrane parameters. The results obtained contribute to the full validation of the DSPM&DE model as a general model. Above all, in the case of NaCl–water solutions, volume membrane charge is obtained as an increasing function both with pH and with electrolyte concentration in the feed side. A Langmuir-type behaviour is apparent at constant pH; amphoteric behaviour is re-confirmed. The results here obtained might be used to calculate empirical correlations to be introduced as input values in the general equations of the DSPM&DE model to predict membrane performances corresponding to different operative conditions. It is self-evident that, to avoid the use of empirical correlations, the crucial point to solve is the understanding of the correct mechanism at the basis of the membrane charge formation. X values here reported are a valuable starting point for that purpose, which will be pursued in a next paper.
Acknowledgements Grateful thanks are due to Drs. L. Bertocchi and C. Mazzoni for their relevant contribution in performing the experiments. Financial support by Italian Ministry of
[1] T. Tsuru, M. Urairi, S. Nakao, S. Kimura, Reverse osmosis of single and mixed electrolytes with charged membranes: experiments and analysis, J. Chem. Eng. Jpn. 24 (1991) 518–524. [2] P. Lipp, R. Gimbel, F.H. Frimmel, Parameters influencing the rejection properties of FT30 membranes, J. Membr. Sci. 95 (1994) 185–197. [3] M. Nystr¨om, L. Kaipia, S. Luque, Fouling and retention of nanofiltration membranes, J. Membr. Sci. 98 (1995) 249–262. [4] M.S. Hall, V.M. Starov, D.R. Lloyd, Reverse osmosis of multicomponent electrolyte solutions. Part II. Experimental verification, J. Membr. Sci. 128 (1997) 39–53. [5] G. Hagmeyer, R. Gimbel, Modelling the salt rejection of nanofiltration membranes for ternary mixtures and for single salts at different pH values, Desalination 117 (1998) 247–256. [6] G. Hagmeyer, R. Gimbel, Modelling the rejection of nanofiltration membranes using zeta potential measurements, Sep. Purif. Technol. 15 (1999) 19–30. [7] M.D. Afonso, M.N. de Pinho, Transport of MgSO4 , MgCl2 and Na2 SO4 across an amphoteric nanofiltration membrane, J. Membr. Sci. 179 (2000) 137–154. [8] W.R. Bowen, H. Mukhtar, Characterization and prediction of separation performance of nanofiltration membranes, J. Membr. Sci. 112 (1996) 263–274. [9] W.R. Bowen, J.S. Welfoot, Modelling the performance of membrane nanofiltration — critical assessment and model development, Chem. Eng. Sci. 57 (2002) 1121–1137. [10] Y. Xu, R.E. Lebrun, Investigation of the solute separation by charged nanofiltration membrane: effect of pH, ionic strength and solute type, J. Membr. Sci. 158 (1999) 93–104. [11] X. Wang, W. Wang, D. Wang, Experimental investigation on separation performance of nanofiltration membranes for inorganic electrolyte solutions, Desalination 145 (2002) 115–122. [12] S. Szoke, G. Patzay, L. Weiser, Characteristics of thin-film nanofiltration membranes at various pH-values, Desalination 151 (2002) 123–129. [13] J. Tanninen, M. Nystrom, Separation of ions in acidic conditions using NF, Desalination 147 (2002) 295–299. [14] S. Bandini, D. Vezzani, Nanofiltration modeling: the role of dielectric exclusion in membrane characterization, Chem. Eng. Sci. 58 (2003) 3303–3326. [15] C. Labbez, P. Fievet, A. Szymczyk, A. Vidonne, A. Foissy, J. Pagetti, Analysis of the salt retention of a titania membrane using the “DSPM” model: effect of pH, salt concentration and nature, J. Membr. Sci. 208 (2002) 315–329. [16] M. Ernst, A. Bismark, J. Springler, M. Jekel, Zeta-potential and rejection rates of a polyethersulfone nanofiltration membrane in single salt solutions, J. Membr. Sci. 165 (2000) 251–259. [17] L. Paugam, S. Taha, G. Dorange, P. Jaouen, F. Qu´em´eneur, Mechanism of nitrate ions transfer in nanofiltration depending on pressure, pH, concentration and medium composition, J. Membr. Sci. 231 (2004) 37–46. [18] J. Quin, M.H. Oo, H. Lee, B. Coniglio, Effect of feed pH on permeate pH and ion rejection under acidic conditions in NF process, J. Membr. Sci. 232 (2004) 153–159. [19] J.M.K. Timmer, H.C. van der Horst, T. Robbertsen, Transport of lactic acid through reverse osmosis and nanofiltration membranes, J. Membr. Sci. 85 (1993) 205–216. [20] V. Freger, T.C. Arnot, J.A. Howell, Separation of concentrated organic/inorganic salt mixtures by nanofiltration, J. Membr. Sci. 178 (2000) 185–193.
74
S. Bandini et al. / Journal of Membrane Science 264 (2005) 65–74
[21] M.D. Afonso, G. Hagmeyer, R. Gimbel, Streaming potential measurements to assess the variation of nanofiltration membranes surface charge with the concentration of salt solutions, Sep. Purif. Technol. 22–23 (2001) 529–541. [22] M.J. Ariza, J. Benavente, Streaming potential along the surface of polysulfone membranes: a comparative study between two different experimental systems and determination of electrokinetic and adsorption parameters, J. Membr. Sci. 190 (2001) 119– 132. [23] E. Childress, M. Elimelech, Effect of solution chemistry on the surface charge of polymeric reverse osmosis and nanofiltration membranes, J. Membr. Sci. 119 (1996) 253– 268. [24] J.M.M. Peeters, M.H.V. Mulder, H. Strathmann, Streaming potential measurements as a characterization method for nanofiltration membranes, Colloids Surf. A: Physicochem. Eng. Aspects 150 (1999) 247–259.
[25] J.H. Tay, J. Liu, D.D. Sun, Effect of solution physico-chemistry on the charge property of nanofiltration membranes, Water Res. 36 (2002) 585–598. [26] W.R. Bowen, A.W. Mohammad, N. Hilal, Characterization of nanofiltration membranes for predictive purposes — use of salts, uncharged solutes and atomic force microscopy, J. Membr. Sci. 126 (1997) 91–105. [27] W.R. Bowen, A.W. Mohammad, Characterization and prediction of nanofiltration membrane performance — a general assessment, Inst. Chem. Eng. 76A (1998) 885–893. [28] T. Tsuru, S. Nakao, S. Kimura, Calculation of ion rejection by extended Nernst–Planck equation with charged reverse osmosis membranes for single and mixed electrolyte solutions, J. Chem. Eng. Jpn. 24 (1991) 511–517. [29] M.S. Hall, V.M. Starov, D.R. Lloyd, Reverse osmosis of multicomponent electrolyte solutions. Part I. Theoretical development, J. Membr. Sci. 128 (1997) 23–37.