- Email: [email protected]
The role of the electrolyte on polyamide NF membranes performances: experimental analysis with NaCl and CaCl2 solutions
Desalination 200 (2006) 135–137
The role of the electrolyte on polyamide NF membranes performances: experimental analysis with NaCl and CaCl2 solutions Carolina Mazzoni, Serena Bandini* Dipartimento di Ingegneria Chimica, Mineraria e delle Tecnologie Ambientali, University of Bologna, Viale Risorgimento 2, I-40136, Bologna, Italy email: [email protected] Received 28 October 2005; accepted 3 March 2006
1. Introduction
2. Experimental methods and results
Nanofiltration (NF) separation efficiency depends on the membrane material as well as on the chemical nature of the process solutions. In the case of electrolyte solutions the membrane performance is greatly affected by the type of the electrolyte used. For single symmetric salts, such as NaCl, with polyamide membranes rejection generally decreases as the concentration increases at constant pH values, whereas rejection goes through a minimum value as feed pH increases [1–4]. In the case of non-symmetric electrolytes, on the contrary, chemical interactions with the membrane can be relevant and different opposite trends are often observed [2,3,5–7]. In this paper, the role of pH and concentration is studied on the ion rejection of Desal-DK polyamide membranes, in the case of aqueous binary mixtures containing NaCl or CaCl2, in order to put in evidence the different role of the electrolyte on the membrane performance.
Experimentation has been performed testing Desal-5 DK membranes (flat thin composite membrane-polyamide selective layer supported on polysulfone, 98% nominal rejection with 1000 ppm MgSO4 at 6.9 bar and 25°C) with binary mixtures in a wide range of composition and pH values in the feed side. Salt rejections were measured as a function of the pressure difference across the membrane for NaCl–water (1–50 mol/m3, pH 3–6.5) and CaCl2–water (1–500 mol/m3, pH 5.3–6.5) solutions at room temperature. From the general point of view, the typical behaviour encountered in these processes is obtained in which • total volume flux increases as the applied pressure increases and decreases as the salt concentration increases; in the geometrical configuration used, the role of concentration polarization in the feed side was remarkable in the case of CaCl2 solutions; • asymptotic rejections are approached at pressure values higher than 20–25 bar.
*Corresponding author.
Presented at EUROMEMBRANE 2006, 24–28 September 2006, Giardini Naxos, Italy. 0011-9164/06/$– See front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2006.03.271
136
C. Mazzoni, S. Bandini / Desalination 200 (2006) 135–137 CaCl2 - water 100
80
80
Ca++ rejection (%)
Na+ rejection (%)
NaCl - water 100
60 40 20
pH = 5.3 pH = 5.8 pH = 6.2
ΔPIN = 25 bar
0
60 40 20
pH = 5.3 pH = 5.8 pH = 6.2
ΔPIN = 25 bar
0 1
10
100
cNaCl (mol
(a)
m–3)
1
10
100
cCaCl2 (mol
(b)
1000
m–3)
Fig. 1. NF of aqueous solutions containing (a) NaCl and (b) CaCl2 at 25°C, through DK membranes. The effect of feed salt concentration on Na+ and Ca2+ rejection, at various feed pH’s.
The role of the electrolyte concentration on the membrane performance is shown in Fig. 1, in which sodium and calcium rejections are reported as a function of the salt concentration at various feed pH values, at 25 bar inlet pressure. Apparently, at a constant pH value in the feed side, NaCl data show the typical trend in which rejection decreases as the salt concentration increases, whereas in the case of CaCl2 solutions calcium rejection goes through a maximum value as the salt concentration increases. Remarkably, also an opposite effect
of the feed pH is obtained on the salt rejection, for the two electrolytes. The trend is quite general and it is observed in the whole range of pressures investigated. 3. Conclusions The behaviours observed in the membrane rejection put in evidence that the chemical nature of the electrolyte can make the difference; in this case calcium interactions with the membrane material are different from those
NaCl - water
30
pH = 5
20
20
–X (mol m–3)
–X (mol m–3)
25
15 pH = 6.5 pH = 6 pH = 5.6 pH = 5
10 5 0
pH = 6.2 pH = 5.8 pH = 5.3 pH = 4.5
0
10
20
30 cNaCl (mol m–3)
40
50
pH = 5.8
pH = 6.5
10 0 –10 –20
–5 (a)
CaCl2 - water
30
–30
60 (b)
0
50
100
150
200
250
300
cCaCl2 (mol m–3)
Fig. 2. NF of aqueous solutions containing (a) NaCl and (b) CaCl2 at 25°C, through Desal-DK membranes. Volume membrane charges calculated through DSPM&DE are reported vs. feed salt concentration, at various feed pH’s. (rP = 0.595 nm, d = 23.73 mm).
C. Mazzoni, S. Bandini / Desalination 200 (2006) 135–137
existing with sodium, and might not be only related to the different ionic strengths in the mixtures. In other words, specific adsorption of ions can occur on the membrane surface and it can greatly affect the corresponding membrane charge values. Volume membrane charge values corresponding to each operative conditions investigated were finally calculated through the Donnan Steric Pore Model and Dielectric Exclusion (DSPM&DE) [7]. Elaborations of experimental rejection vs. flux data have been performed through the “integral” version of the DSPM&DE to obtain average pore radius (rP), effective thickness (d) and membrane charge (–X) values. Results are reported in Fig. 2. Apparently, in the case of NaCl–water solutions, the membrane charge is negative, its absolute value increases as the electrolyte concentration increases and, correspondingly, it increases as the feed pH increases. In the case of CaCl2–water solutions the effect of concentration
137
is remarkably different. At low concentrations the membrane charge is negative, it switches to positive values as the concentration increases and finally decreases till to negative values at higher concentrations. Points of zero charge greatly depend on the salt concentration. References [1] [2] [3] [4] [5] [6] [7]
S. Bandini, J. Drei and D. Vezzani, J. Membr. Sci., 264 (2005) 65–74. Y. Xu and R.E. Lebrun, J. Membr. Sci., 158 (1999) 93–104. S. Szoke, G. Patzay and L. Weiser, Desalination, 151 (2002) 123–129. J. Quin, M.H. Oo, H. Lee and B. Coniglio, J. Membr. Sci., 232 (2004) 153–159. G. Hagmeyer and R. Gimbel, Desalination, 117 (1998) 247–256. W.R. Bowen and J.S. Welfoot, Chem. Eng. Sci., 57 (2002) 1121–1137. S. Bandini and D. Vezzani, Chem. Eng. Sci., 58 (2003) 3303–3326.
The role of the electrolyte on polyamide NF membranes performances: experimental analysis with NaCl and CaCl2 solutions Carolina Mazzoni, Serena Bandini* Dipartimento di Ingegneria Chimica, Mineraria e delle Tecnologie Ambientali, University of Bologna, Viale Risorgimento 2, I-40136, Bologna, Italy email: [email protected] Received 28 October 2005; accepted 3 March 2006
1. Introduction
2. Experimental methods and results
Nanofiltration (NF) separation efficiency depends on the membrane material as well as on the chemical nature of the process solutions. In the case of electrolyte solutions the membrane performance is greatly affected by the type of the electrolyte used. For single symmetric salts, such as NaCl, with polyamide membranes rejection generally decreases as the concentration increases at constant pH values, whereas rejection goes through a minimum value as feed pH increases [1–4]. In the case of non-symmetric electrolytes, on the contrary, chemical interactions with the membrane can be relevant and different opposite trends are often observed [2,3,5–7]. In this paper, the role of pH and concentration is studied on the ion rejection of Desal-DK polyamide membranes, in the case of aqueous binary mixtures containing NaCl or CaCl2, in order to put in evidence the different role of the electrolyte on the membrane performance.
Experimentation has been performed testing Desal-5 DK membranes (flat thin composite membrane-polyamide selective layer supported on polysulfone, 98% nominal rejection with 1000 ppm MgSO4 at 6.9 bar and 25°C) with binary mixtures in a wide range of composition and pH values in the feed side. Salt rejections were measured as a function of the pressure difference across the membrane for NaCl–water (1–50 mol/m3, pH 3–6.5) and CaCl2–water (1–500 mol/m3, pH 5.3–6.5) solutions at room temperature. From the general point of view, the typical behaviour encountered in these processes is obtained in which • total volume flux increases as the applied pressure increases and decreases as the salt concentration increases; in the geometrical configuration used, the role of concentration polarization in the feed side was remarkable in the case of CaCl2 solutions; • asymptotic rejections are approached at pressure values higher than 20–25 bar.
*Corresponding author.
Presented at EUROMEMBRANE 2006, 24–28 September 2006, Giardini Naxos, Italy. 0011-9164/06/$– See front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2006.03.271
136
C. Mazzoni, S. Bandini / Desalination 200 (2006) 135–137 CaCl2 - water 100
80
80
Ca++ rejection (%)
Na+ rejection (%)
NaCl - water 100
60 40 20
pH = 5.3 pH = 5.8 pH = 6.2
ΔPIN = 25 bar
0
60 40 20
pH = 5.3 pH = 5.8 pH = 6.2
ΔPIN = 25 bar
0 1
10
100
cNaCl (mol
(a)
m–3)
1
10
100
cCaCl2 (mol
(b)
1000
m–3)
Fig. 1. NF of aqueous solutions containing (a) NaCl and (b) CaCl2 at 25°C, through DK membranes. The effect of feed salt concentration on Na+ and Ca2+ rejection, at various feed pH’s.
The role of the electrolyte concentration on the membrane performance is shown in Fig. 1, in which sodium and calcium rejections are reported as a function of the salt concentration at various feed pH values, at 25 bar inlet pressure. Apparently, at a constant pH value in the feed side, NaCl data show the typical trend in which rejection decreases as the salt concentration increases, whereas in the case of CaCl2 solutions calcium rejection goes through a maximum value as the salt concentration increases. Remarkably, also an opposite effect
of the feed pH is obtained on the salt rejection, for the two electrolytes. The trend is quite general and it is observed in the whole range of pressures investigated. 3. Conclusions The behaviours observed in the membrane rejection put in evidence that the chemical nature of the electrolyte can make the difference; in this case calcium interactions with the membrane material are different from those
NaCl - water
30
pH = 5
20
20
–X (mol m–3)
–X (mol m–3)
25
15 pH = 6.5 pH = 6 pH = 5.6 pH = 5
10 5 0
pH = 6.2 pH = 5.8 pH = 5.3 pH = 4.5
0
10
20
30 cNaCl (mol m–3)
40
50
pH = 5.8
pH = 6.5
10 0 –10 –20
–5 (a)
CaCl2 - water
30
–30
60 (b)
0
50
100
150
200
250
300
cCaCl2 (mol m–3)
Fig. 2. NF of aqueous solutions containing (a) NaCl and (b) CaCl2 at 25°C, through Desal-DK membranes. Volume membrane charges calculated through DSPM&DE are reported vs. feed salt concentration, at various feed pH’s. (rP = 0.595 nm, d = 23.73 mm).
C. Mazzoni, S. Bandini / Desalination 200 (2006) 135–137
existing with sodium, and might not be only related to the different ionic strengths in the mixtures. In other words, specific adsorption of ions can occur on the membrane surface and it can greatly affect the corresponding membrane charge values. Volume membrane charge values corresponding to each operative conditions investigated were finally calculated through the Donnan Steric Pore Model and Dielectric Exclusion (DSPM&DE) [7]. Elaborations of experimental rejection vs. flux data have been performed through the “integral” version of the DSPM&DE to obtain average pore radius (rP), effective thickness (d) and membrane charge (–X) values. Results are reported in Fig. 2. Apparently, in the case of NaCl–water solutions, the membrane charge is negative, its absolute value increases as the electrolyte concentration increases and, correspondingly, it increases as the feed pH increases. In the case of CaCl2–water solutions the effect of concentration
137
is remarkably different. At low concentrations the membrane charge is negative, it switches to positive values as the concentration increases and finally decreases till to negative values at higher concentrations. Points of zero charge greatly depend on the salt concentration. References [1] [2] [3] [4] [5] [6] [7]
S. Bandini, J. Drei and D. Vezzani, J. Membr. Sci., 264 (2005) 65–74. Y. Xu and R.E. Lebrun, J. Membr. Sci., 158 (1999) 93–104. S. Szoke, G. Patzay and L. Weiser, Desalination, 151 (2002) 123–129. J. Quin, M.H. Oo, H. Lee and B. Coniglio, J. Membr. Sci., 232 (2004) 153–159. G. Hagmeyer and R. Gimbel, Desalination, 117 (1998) 247–256. W.R. Bowen and J.S. Welfoot, Chem. Eng. Sci., 57 (2002) 1121–1137. S. Bandini and D. Vezzani, Chem. Eng. Sci., 58 (2003) 3303–3326.