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Rejection of ions by NF membranes for binary electrolyte solutions of NaCl, NaNO3, CaCl2 and Ca(NO3)2
Desalination 191 (2006) 303–308
Rejection of ions by NF membranes for binary electrolyte solutions of NaCl, NaNO3, CaCl2 and Ca(NO3)2 Meng Sua, Da-Xin Wanga, Xiao-Lin Wanga*, Masaaki Andob, Takuji Shintanic a
Department of Chemical Engineering, Tsinghua University, Beijing 100084, P R China Tel. & Fax: +86 (10) 6279-4741; email: [email protected] b Membrane Division, Nitto Denko Corporation, 61-7 Azasasadani, Yamadera, Kusatsu, Shiga 525-0042, Japan c Core Technology Center, Nitto Denko Corporation, 1-1-2 Shimohozumi, Ibaraki, Osaka 567-8680, Japan Received 15 March 2005; accepted 1 June 2005
Abstract Permeation experiments of binary electrolyte solutions of NaCl, NaNO3, CaCl2 and Ca(NO3)2 through three kinds of NF membranes (ESNA 1-LF, ESNA 1 and LES 90) were carried out to investigate the effects of the ionic species and their interactions on the rejections of the NF membranes. The experimental results indicate that the observed rejections of Ca2+ by NF membranes all slightly increased with the growth of the concentration of Na+ and the observed rejection of Na+ decreased with the growth of the concentration of Ca2+. The repulsion force of the membrane negative charges on the cation with lower charge density are weaker, thus it presents higher permeability and lower rejection. The observed rejections of Cl! and NO3! by NF membranes were almost constant and hardly changed with the equivalent fractions of Cl! and NO3! in binary electrolyte solutions. For charged species, the electrostatic mechanisms play a more important role in NF separation. Keywords: Nanofiltration; Membrane separation; Binary electrolyte solution
1. Introduction The nanofiltration (NF) membrane is a new type of membrane developed in the 1990s, which has two features in its separation processes, that is, intermediate molecular weight cut-offs *Corresponding author.
(MWCO) ranging from 200 to 2000 Daltons between reverse osmosis (RO) membranes and ultrafiltration (UF) membranes, and the rejection of salts caused by the charge effect due to the materials [1]. The evaluation method of the pore structure and the volume charge density of NF membranes were established on the basis of the
Presented at the International Congress on Membranes and Membrane Processes (ICOM), Seoul, Korea, 21–26 August 2005. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.
doi:10.1016/j.desal.2005.06.041
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M. Su et al. / Desalination 191 (2006) 303–308
SHP model and the TMS model [1]. Models such as electrostatic and steric-hindrance (ES) [2] and Donnan-steric pore (DSPM) [3] were established to predict the permeability of charged solutes through NF membranes. The separation performance of NF membranes on single electrolyte solutions has been widely investigated [4–6]. The rejection of most single electrolytes by NF membranes decreases with the growth of electrolyte concentration. The electrostatic effect is the major factor at low electrolyte concentrations which influences the separation performance of the NF membrane. Also, the steric-hindrance effect is the major factor at high electrolyte concentrations [6]. However, the rejection of the electrolyte containing Mg2+ or Ca2+ by negatively charged NF membranes slightly increases with the growth of electrolyte concentration, and the reason is not yet clear. One explanation regards it as the specific adsorption of Mg2+ or Ca2+ on the surface of NF membranes, which may yield the reverse of membrane charges [5]. More attention has been paid to the separation performance of NF membranes for mixed electrolyte solutions such as three kinds of ions, including NO3! (or SO42-), Cl! and Na+ [5,7,8], or Na+, Ca2+ (or Mg2+) and Cl! [9], and more than three kinds of ions [10]. However, knowledge is still lacking of separation performance on mixed electrolytes by NF membranes. In mixed electrolyte solutions, the rejection of ions by NF membranes is always different from that in a single electrolyte solution as a result of the interaction among ions. Further investigation on mixed electrolyte solutions is necessary in order to establish a more suitable method to simulate the separation performance of NF membranes for their applications to water purification and treatment processes. Permeation experiments of binary electrolyte solutions of NaCl, NaNO3, CaCl2 and Ca(NO3)2 through three kinds of NF membranes (ESNA 1LF, ESNA 1 and LES 90) will be carried out to investigate the effects of the ionic species and
their interactions on the rejections of the NF membranes. 2. Experimental 2.1. Membranes and electrolytes The NF membranes used in this work were ESNA 1-LF, ESNA 1 and LES 90, manufactured by Nitto Denko Corporation. They are made of aromatic polyamide and negatively charged, providing salt rejection from 50% to 90% under ultra-low-pressure operations. The binary electrolyte solutions prepared in which both the species of cations and those of anions are not more than two, were divided into three types: (1) two cations (Na+, Ca2+) and one anion (NO3!) from the two electrolytes (NaNO3 and Ca(NO3)2), (2) two anions (Cl!, NO3!) and one cation (Na+) from the two electrolytes (NaCl and NaNO3), (3) two cations (Na+, Ca2+) and two anions (Cl!, NO3!) from more than two of the four electrolytes (NaCl, NaNO3, CaCl2 and Ca(NO3)2). The binary electrolyte solutions were prepared in different total equivalent concentrations ranging from 10 mN to 40 mN and different equivalent fractions of ions (xb,ion). The concentrations of cations were measured with a Zeeman atomic absorption spectrophotometer (Hitachi Z-5000), and those of anions were measured with a high-performance liquid ion chromatograph (Shimadzu). 2.2. Permeation experiments The experimental apparatus was essentially the same as shown in a previous paper [11]. The area of the membrane used was 35.3 cm2. All experiments were carried out at 25 ± 0.1EC and pH of 6.4–6.6, which is the pH of the deionized water used in preparing all the samples. The feed crossflow rate was 5.7 ± 0.1 L/min. The applied pressure was in the range of 0.8–2.0 MPa. Both the retentate and permeate were recycled back to the feed tank to keep constant concentration.
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305
The observed rejections of ions by the NF membranes (Robs, ion) were investigated in the three types of binary electrolyte solutions.
Here Cb,ion and Cp,ion are the concentrations of ions in the feed and permeate, respectively. For a single electrolyte solution the observed rejection of the cation or anion by NF membranes is the same as that of the electrolyte. For binary electrolyte solutions the observed rejection of an ion is different from that in single electrolyte solution and depends on the existence of the other ions (cations or anions). 3. Results and discussion 3.1. Membrane evaluation The pure water permeabilities of ESNA 1-LF, ESNA 1 and LES 90 membranes were measured to be 1.25, 1.18 and 1.98 × 10!5 m.s!1.MPa!1, respectively; and the isoelectric points of these membranes were determined to be about pH 4.5. The average pore radii of the three NF membranes were estimated to be 0.30, 0.30 and 0.26 nm, respectively, on the basis of the Spiegler–Kedem equation and the SHP model [1]. 3.2. Binary electrolyte solutions of two cations (Na+, Ca2+) and one anion (NO3!) Binary electrolyte solutions of two cations (Na+, Ca2+) and one anion (NO3!) were prepared from NaNO3 and Ca(NO3)2. In this case, the observed rejection of total electrolytes is the same as that of NO3!. Fig. 1 shows the observed rejection of NO3! by the three NF membranes as a function of the equivalent fraction of Ca2+. The concentration of NO3! was kept at 20 mN (this meant the total concentration of the two electrolytes was held constant). As shown in
Fig. 1. Observed rejection of NO3! by ESNA 1-LF (#), ESNA 1 (!) and LES 90 (•) membranes as a function of the equivalent fraction of Ca2+ in the binary electrolyte solutions of NaNO3 and Ca(NO3)2 at an applied pressure of 1.0 MPa. Concentration of NO3! was 20 mN.
Fig. 1, the observed rejection of NO3! did change somewhat with the equivalent fraction of Ca2+ in this case. The observed rejections of single NaNO3 and Ca(NO3)2 were similar at a 20 mN concentration as shown in Fig. 1 and the effect of equivalent fraction of Ca2+ on the rejection of NO3! was not obvious, so further investigation needs to be carried out under other different concentrations. Fig. 2 shows the observed rejection of Ca2+ by the three NF membranes as a function of the concentration of Na+ from 0 to 20 mN at an applied pressure of 1.0 MPa, while the concentration of Ca2+ was kept at 20 mN (this meant the total concentration of the two electrolytes ranged from 20 to 40 mN). This indicates that the observed rejection of Ca2+ slightly increased with the growth of the concentration of Na+. Fig. 3 shows the observed rejection of Na+ by the ESNA 1-LF membrane as a function of permeation flux in the solutions. The concentration of Na+ was kept at 10 mN and the concentration of Ca2+ was 0 and 20 mN, respectively. Since the three NF membranes showed similar behavior in this situation, only the experimental results of the
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Fig. 2. Observed rejection of Ca2+ by ESNA 1-LF (#), ESNA 1 (!) and LES 90 (•) membranes as a function of the concentration of Na+ in the binary electrolyte solutions of NaNO3 and Ca(NO3)2 at an applied pressure of 1.0 MPa. Concentration of Ca2+ was 20 mN.
Fig. 3. Observed rejection of Na+ by the ESNA 1-LF membrane as a function of permeation flux in the binary electrolyte solutions of NaNO3 and Ca(NO3)2. Concentration of Na+ was 10 mN and concentrations of Ca2+ were 0 (#) and 20 (!) mN, respectively.
ESNA 1-LF membrane are presented here. As shown in Fig. 3, the observed rejection of Na+ was higher than 0.8 in single NaNO3 solution and dropped to 0.6 with the addition of Ca2+.
Fig. 4. Observed rejection of Cl! (!), NO3! (#) and Na+ (•) by the ESNA 1-LF membrane as a function of the equivalent fraction of Cl! in the binary electrolyte solutions of NaCl and NaNO3 at an applied pressure of 1.0 MPa. The concentration of Na+ was 20 mN.
Figs. 2 and 3 indicate that in the binary electrolyte solutions of two cations (Na+, Ca2+) and one anion (NO3!), the observed rejection of Ca2+, slightly increase, but the observed rejection of Na+ decreases compared to the corresponding single electrolyte solutions. The reason for this phenomenon is that the repulsion forces of the negative charges of the membrane on the cation with lower charge density (Na+) are weaker, thus Na+ presents higher permeability and lower rejection than Ca2+. When binary electrolyte solutions contain both Na+ and Ca2+, Na+ will permeate preferentially, resulting in the increase of the observed rejection of Ca2+ and the reduction of the observed rejection of Na+. The total concentration of the two electrolytes was not held constant in Figs. 2 and 3, so the effect of the concentration of NO3! on the observed rejection of Ca2+ and Na+ cannot be neglected. Both the total electrolytes concentration and the equivalent fraction of ions are important parameters affecting rejections of ions by NF membrane for mixed electrolyte solutions.
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Table 1 Observed rejection of Na+, Ca2+, Cl! and NO3! by the ESNA 1-LF membrane in the binary electrolyte solutions of NaCl, NaNO3, CaCl2 and Ca(NO3)2 at an applied pressure of 1.0 MPa (total concentration was 20 mN)
0.2 0.2 0.5 0.5 0.8
0.8 0.8 0.5 0.5 0.2
0.2 0.5 0.5 0.8 0.8
0.8 0.5 0.5 0.2 0.2
3.3. Binary electrolyte solutions of two anions (Cl!, NO3!) and one cation (Na+) Binary electrolyte solutions of two anions (Cl!, NO3!) and one cation (Na+) were prepared from NaCl and NaNO3. In this case, the observed rejection of total electrolytes is the same as that of Na+. Fig. 4 shows the observed rejections of Na+, Cl! and NO3! by ESNA 1-LF membrane as a function of the equivalent fraction of Cl!. The concentration of Na+ was kept at 20 mN (the total concentration of the two electrolytes was held constant). Since three membranes showed similar behavior in this process, only the experimental results of the ESNA 1-LF membrane are presented. As shown in Fig. 4, the observed rejections of Cl! and NO3! by the ESNA 1-LF membrane were almost constant. The equivalent fraction of Cl! has little effect on the observed rejection of NF membrane. The reason for this phenomenon is the similar density charges of Cl! and NO3!. When the two anions occur in one solution, their interaction is weak. The observed rejection of Na+ at different equivalent fractions of Cl! was similar with the linear summation of the observed rejections of single NaCl and NaNO3 as shown in Fig. 4. 3.4. Binary electrolyte solutions of two cations (Na+, Ca2+) and two anions (Cl!, NO3!) Binary electrolyte solutions of two cations
0.568 0.560 0.708 0.754 0.805
0.867 0.932 0.917 0.926 0.915
0.874 0.872 0.872 0.863 0.860
0.789 0.749 0.778 0.749 0.741
(Na+, Ca2+) and two anions (Cl!, NO3!) were prepared from more than two of the four electrolytes (NaCl, NaNO3, CaCl2 and Ca(NO3)2). In this case, the observed rejections of total electrolytes are the same as those of total cations or total anions. Table 1 shows the observed rejections of Na+, Ca2+, Cl! and NO3! by the ESNA 1-LF membrane at an applied pressure of 1.0 MPa. The total concentration was kept at 20 mN. The experimental results of only the ESNA 1-LF membrane are presented since the three membranes showed similar results. As shown in Table 1, the observed rejection of Na+ by NF membrane decreased with the increase of the equivalent fraction of Ca2+, and the observed rejections of Cl! and NO3! were hardly changed with the equivalent fraction of Na+. This indicates that the equivalent fraction of cation (such as Na+) has a strong effect on the rejections of NF membranes. The equivalent fractions of anions (such as Cl!) almost have no effect on the rejections of NF membranes, which seems to be attributed to the similar density charges of Cl! and NO3!. For mixed electrolyte solutions, the observed rejections of total electrolytes were dependent on the total electrolytes concentration and the equivalent fractions of ions. Furthermore, the observed rejection of one cation (or anion) was affected by the proportion of other cations (or anions). Further investigation is expected in order to support the quantitative simulation of the NF membrane processes.
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4. Conclusions Experimental investigation on the rejections of ions by the NF membranes for the binary electrolyte solutions of NaCl, NaNO3, CaCl2 and Ca(NO3)2 indicates that the observed rejections of Ca2+ by NF membranes all slightly increased with the growth of the concentration of Na+ and the observed rejection of Na+ decreased with the growth of the concentration of Ca2+. The repulsion forces of the membrane negative charges on the cation with lower charge density are weaker, thus it presents higher permeability and lower rejection. The observed rejections of Cl! and NO3! by the NF membranes were almost constant and hardly changed with their equivalent fractions in binary electrolyte solutions. For charged species, electrostatic mechanisms play a more important role in NF separation.
[3]
[4]
[5]
[6]
[7]
Acknowledgements
[8]
This work is supported by the National Basic Research Program of China (2003CB615701) and National Natural Science Foundation of China (20376040).
[9]
References [1] X.L. Wang, T. Tsuru, M. Togoh, S. Nakao and S. Kimura, Evaluation of pore structure and electrical properties of nanofiltration membranes, J. Chem. Eng. Jpn., 28 (1995) 186. [2] X.L. Wang, T. Tsuru, S. Nakao and S. Kimura, The electrostatic and steric-hindrance model for the
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transport of charged solutes through nanofiltration membranes, J. Membr. Sci., 135 (1997) 19. W.R. Bowen, A.W. Mohammad and N. Hilal, Characterization of nanofiltration membranes for predictive purposes use of salts, uncharged solutes and atomic force microscopy, J. Membr. Sci, 126 (1997) 91. X.L. Wang, W.N. Wang and D.X. Wang, Experimental investigation on separation performance of nanofiltration membranes for inorganic electrolyte solutions, Desalination, 145 (2002) 115. L. Paugam, S. Taha, G. Dorange, and F. Quéméneur, Influence of ionic composition on nitrate retention by nanofiltration, Ind. Chem. Eng. Res. Des., 81 (2003) 1199. D.X. Wang, M. Su, Z.Y. Yu, X.L. Wang, M. Ando and T. Shintani, Separation performance of a nanofiltration membrane influenced by species and concentration of ions, Desalination, 175 (2005) 219. L. Paugam, S. Taha, J. Cabon and G. Dorange, Elimination of nitrate ions in drinking waters by nanofiltration, Desalination, 152 (2002) 271. L. Paugam, S. Taha, G. Dorange, P. Jaouen and F. Quéméneur, Mechanism of nitrate ions transfer in nanofiltration depending on pressure, pH, concentration and medium composition, J. Membr. Sci., 231 (2004) 37. J. Garcia-Aleman and J.M. Dickson, Permeation of mixed-salt solutions with commercial and pore-filled nanofiltration membrane: membrane charge inversion phenomena, J. Membr. Sci., 239 (2004) 163. S. Choi, Z. Yun, S. Hong and K. Ahn, The effect of co-existing ions and surface characteristics of nanomembranes on the removal of nitrate and fluoride, Desalination, 133 (2001) 53. X.L. Wang, A.L. Ying and W.N. Wang, Nanofiltration of L-phenylalanine and L-aspartic acid aqueous solutions, J. Membr. Sci., 196 (2002) 59.
Rejection of ions by NF membranes for binary electrolyte solutions of NaCl, NaNO3, CaCl2 and Ca(NO3)2 Meng Sua, Da-Xin Wanga, Xiao-Lin Wanga*, Masaaki Andob, Takuji Shintanic a
Department of Chemical Engineering, Tsinghua University, Beijing 100084, P R China Tel. & Fax: +86 (10) 6279-4741; email: [email protected] b Membrane Division, Nitto Denko Corporation, 61-7 Azasasadani, Yamadera, Kusatsu, Shiga 525-0042, Japan c Core Technology Center, Nitto Denko Corporation, 1-1-2 Shimohozumi, Ibaraki, Osaka 567-8680, Japan Received 15 March 2005; accepted 1 June 2005
Abstract Permeation experiments of binary electrolyte solutions of NaCl, NaNO3, CaCl2 and Ca(NO3)2 through three kinds of NF membranes (ESNA 1-LF, ESNA 1 and LES 90) were carried out to investigate the effects of the ionic species and their interactions on the rejections of the NF membranes. The experimental results indicate that the observed rejections of Ca2+ by NF membranes all slightly increased with the growth of the concentration of Na+ and the observed rejection of Na+ decreased with the growth of the concentration of Ca2+. The repulsion force of the membrane negative charges on the cation with lower charge density are weaker, thus it presents higher permeability and lower rejection. The observed rejections of Cl! and NO3! by NF membranes were almost constant and hardly changed with the equivalent fractions of Cl! and NO3! in binary electrolyte solutions. For charged species, the electrostatic mechanisms play a more important role in NF separation. Keywords: Nanofiltration; Membrane separation; Binary electrolyte solution
1. Introduction The nanofiltration (NF) membrane is a new type of membrane developed in the 1990s, which has two features in its separation processes, that is, intermediate molecular weight cut-offs *Corresponding author.
(MWCO) ranging from 200 to 2000 Daltons between reverse osmosis (RO) membranes and ultrafiltration (UF) membranes, and the rejection of salts caused by the charge effect due to the materials [1]. The evaluation method of the pore structure and the volume charge density of NF membranes were established on the basis of the
Presented at the International Congress on Membranes and Membrane Processes (ICOM), Seoul, Korea, 21–26 August 2005. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.
doi:10.1016/j.desal.2005.06.041
304
M. Su et al. / Desalination 191 (2006) 303–308
SHP model and the TMS model [1]. Models such as electrostatic and steric-hindrance (ES) [2] and Donnan-steric pore (DSPM) [3] were established to predict the permeability of charged solutes through NF membranes. The separation performance of NF membranes on single electrolyte solutions has been widely investigated [4–6]. The rejection of most single electrolytes by NF membranes decreases with the growth of electrolyte concentration. The electrostatic effect is the major factor at low electrolyte concentrations which influences the separation performance of the NF membrane. Also, the steric-hindrance effect is the major factor at high electrolyte concentrations [6]. However, the rejection of the electrolyte containing Mg2+ or Ca2+ by negatively charged NF membranes slightly increases with the growth of electrolyte concentration, and the reason is not yet clear. One explanation regards it as the specific adsorption of Mg2+ or Ca2+ on the surface of NF membranes, which may yield the reverse of membrane charges [5]. More attention has been paid to the separation performance of NF membranes for mixed electrolyte solutions such as three kinds of ions, including NO3! (or SO42-), Cl! and Na+ [5,7,8], or Na+, Ca2+ (or Mg2+) and Cl! [9], and more than three kinds of ions [10]. However, knowledge is still lacking of separation performance on mixed electrolytes by NF membranes. In mixed electrolyte solutions, the rejection of ions by NF membranes is always different from that in a single electrolyte solution as a result of the interaction among ions. Further investigation on mixed electrolyte solutions is necessary in order to establish a more suitable method to simulate the separation performance of NF membranes for their applications to water purification and treatment processes. Permeation experiments of binary electrolyte solutions of NaCl, NaNO3, CaCl2 and Ca(NO3)2 through three kinds of NF membranes (ESNA 1LF, ESNA 1 and LES 90) will be carried out to investigate the effects of the ionic species and
their interactions on the rejections of the NF membranes. 2. Experimental 2.1. Membranes and electrolytes The NF membranes used in this work were ESNA 1-LF, ESNA 1 and LES 90, manufactured by Nitto Denko Corporation. They are made of aromatic polyamide and negatively charged, providing salt rejection from 50% to 90% under ultra-low-pressure operations. The binary electrolyte solutions prepared in which both the species of cations and those of anions are not more than two, were divided into three types: (1) two cations (Na+, Ca2+) and one anion (NO3!) from the two electrolytes (NaNO3 and Ca(NO3)2), (2) two anions (Cl!, NO3!) and one cation (Na+) from the two electrolytes (NaCl and NaNO3), (3) two cations (Na+, Ca2+) and two anions (Cl!, NO3!) from more than two of the four electrolytes (NaCl, NaNO3, CaCl2 and Ca(NO3)2). The binary electrolyte solutions were prepared in different total equivalent concentrations ranging from 10 mN to 40 mN and different equivalent fractions of ions (xb,ion). The concentrations of cations were measured with a Zeeman atomic absorption spectrophotometer (Hitachi Z-5000), and those of anions were measured with a high-performance liquid ion chromatograph (Shimadzu). 2.2. Permeation experiments The experimental apparatus was essentially the same as shown in a previous paper [11]. The area of the membrane used was 35.3 cm2. All experiments were carried out at 25 ± 0.1EC and pH of 6.4–6.6, which is the pH of the deionized water used in preparing all the samples. The feed crossflow rate was 5.7 ± 0.1 L/min. The applied pressure was in the range of 0.8–2.0 MPa. Both the retentate and permeate were recycled back to the feed tank to keep constant concentration.
M. Su et al. / Desalination 191 (2006) 303–308
305
The observed rejections of ions by the NF membranes (Robs, ion) were investigated in the three types of binary electrolyte solutions.
Here Cb,ion and Cp,ion are the concentrations of ions in the feed and permeate, respectively. For a single electrolyte solution the observed rejection of the cation or anion by NF membranes is the same as that of the electrolyte. For binary electrolyte solutions the observed rejection of an ion is different from that in single electrolyte solution and depends on the existence of the other ions (cations or anions). 3. Results and discussion 3.1. Membrane evaluation The pure water permeabilities of ESNA 1-LF, ESNA 1 and LES 90 membranes were measured to be 1.25, 1.18 and 1.98 × 10!5 m.s!1.MPa!1, respectively; and the isoelectric points of these membranes were determined to be about pH 4.5. The average pore radii of the three NF membranes were estimated to be 0.30, 0.30 and 0.26 nm, respectively, on the basis of the Spiegler–Kedem equation and the SHP model [1]. 3.2. Binary electrolyte solutions of two cations (Na+, Ca2+) and one anion (NO3!) Binary electrolyte solutions of two cations (Na+, Ca2+) and one anion (NO3!) were prepared from NaNO3 and Ca(NO3)2. In this case, the observed rejection of total electrolytes is the same as that of NO3!. Fig. 1 shows the observed rejection of NO3! by the three NF membranes as a function of the equivalent fraction of Ca2+. The concentration of NO3! was kept at 20 mN (this meant the total concentration of the two electrolytes was held constant). As shown in
Fig. 1. Observed rejection of NO3! by ESNA 1-LF (#), ESNA 1 (!) and LES 90 (•) membranes as a function of the equivalent fraction of Ca2+ in the binary electrolyte solutions of NaNO3 and Ca(NO3)2 at an applied pressure of 1.0 MPa. Concentration of NO3! was 20 mN.
Fig. 1, the observed rejection of NO3! did change somewhat with the equivalent fraction of Ca2+ in this case. The observed rejections of single NaNO3 and Ca(NO3)2 were similar at a 20 mN concentration as shown in Fig. 1 and the effect of equivalent fraction of Ca2+ on the rejection of NO3! was not obvious, so further investigation needs to be carried out under other different concentrations. Fig. 2 shows the observed rejection of Ca2+ by the three NF membranes as a function of the concentration of Na+ from 0 to 20 mN at an applied pressure of 1.0 MPa, while the concentration of Ca2+ was kept at 20 mN (this meant the total concentration of the two electrolytes ranged from 20 to 40 mN). This indicates that the observed rejection of Ca2+ slightly increased with the growth of the concentration of Na+. Fig. 3 shows the observed rejection of Na+ by the ESNA 1-LF membrane as a function of permeation flux in the solutions. The concentration of Na+ was kept at 10 mN and the concentration of Ca2+ was 0 and 20 mN, respectively. Since the three NF membranes showed similar behavior in this situation, only the experimental results of the
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Fig. 2. Observed rejection of Ca2+ by ESNA 1-LF (#), ESNA 1 (!) and LES 90 (•) membranes as a function of the concentration of Na+ in the binary electrolyte solutions of NaNO3 and Ca(NO3)2 at an applied pressure of 1.0 MPa. Concentration of Ca2+ was 20 mN.
Fig. 3. Observed rejection of Na+ by the ESNA 1-LF membrane as a function of permeation flux in the binary electrolyte solutions of NaNO3 and Ca(NO3)2. Concentration of Na+ was 10 mN and concentrations of Ca2+ were 0 (#) and 20 (!) mN, respectively.
ESNA 1-LF membrane are presented here. As shown in Fig. 3, the observed rejection of Na+ was higher than 0.8 in single NaNO3 solution and dropped to 0.6 with the addition of Ca2+.
Fig. 4. Observed rejection of Cl! (!), NO3! (#) and Na+ (•) by the ESNA 1-LF membrane as a function of the equivalent fraction of Cl! in the binary electrolyte solutions of NaCl and NaNO3 at an applied pressure of 1.0 MPa. The concentration of Na+ was 20 mN.
Figs. 2 and 3 indicate that in the binary electrolyte solutions of two cations (Na+, Ca2+) and one anion (NO3!), the observed rejection of Ca2+, slightly increase, but the observed rejection of Na+ decreases compared to the corresponding single electrolyte solutions. The reason for this phenomenon is that the repulsion forces of the negative charges of the membrane on the cation with lower charge density (Na+) are weaker, thus Na+ presents higher permeability and lower rejection than Ca2+. When binary electrolyte solutions contain both Na+ and Ca2+, Na+ will permeate preferentially, resulting in the increase of the observed rejection of Ca2+ and the reduction of the observed rejection of Na+. The total concentration of the two electrolytes was not held constant in Figs. 2 and 3, so the effect of the concentration of NO3! on the observed rejection of Ca2+ and Na+ cannot be neglected. Both the total electrolytes concentration and the equivalent fraction of ions are important parameters affecting rejections of ions by NF membrane for mixed electrolyte solutions.
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307
Table 1 Observed rejection of Na+, Ca2+, Cl! and NO3! by the ESNA 1-LF membrane in the binary electrolyte solutions of NaCl, NaNO3, CaCl2 and Ca(NO3)2 at an applied pressure of 1.0 MPa (total concentration was 20 mN)
0.2 0.2 0.5 0.5 0.8
0.8 0.8 0.5 0.5 0.2
0.2 0.5 0.5 0.8 0.8
0.8 0.5 0.5 0.2 0.2
3.3. Binary electrolyte solutions of two anions (Cl!, NO3!) and one cation (Na+) Binary electrolyte solutions of two anions (Cl!, NO3!) and one cation (Na+) were prepared from NaCl and NaNO3. In this case, the observed rejection of total electrolytes is the same as that of Na+. Fig. 4 shows the observed rejections of Na+, Cl! and NO3! by ESNA 1-LF membrane as a function of the equivalent fraction of Cl!. The concentration of Na+ was kept at 20 mN (the total concentration of the two electrolytes was held constant). Since three membranes showed similar behavior in this process, only the experimental results of the ESNA 1-LF membrane are presented. As shown in Fig. 4, the observed rejections of Cl! and NO3! by the ESNA 1-LF membrane were almost constant. The equivalent fraction of Cl! has little effect on the observed rejection of NF membrane. The reason for this phenomenon is the similar density charges of Cl! and NO3!. When the two anions occur in one solution, their interaction is weak. The observed rejection of Na+ at different equivalent fractions of Cl! was similar with the linear summation of the observed rejections of single NaCl and NaNO3 as shown in Fig. 4. 3.4. Binary electrolyte solutions of two cations (Na+, Ca2+) and two anions (Cl!, NO3!) Binary electrolyte solutions of two cations
0.568 0.560 0.708 0.754 0.805
0.867 0.932 0.917 0.926 0.915
0.874 0.872 0.872 0.863 0.860
0.789 0.749 0.778 0.749 0.741
(Na+, Ca2+) and two anions (Cl!, NO3!) were prepared from more than two of the four electrolytes (NaCl, NaNO3, CaCl2 and Ca(NO3)2). In this case, the observed rejections of total electrolytes are the same as those of total cations or total anions. Table 1 shows the observed rejections of Na+, Ca2+, Cl! and NO3! by the ESNA 1-LF membrane at an applied pressure of 1.0 MPa. The total concentration was kept at 20 mN. The experimental results of only the ESNA 1-LF membrane are presented since the three membranes showed similar results. As shown in Table 1, the observed rejection of Na+ by NF membrane decreased with the increase of the equivalent fraction of Ca2+, and the observed rejections of Cl! and NO3! were hardly changed with the equivalent fraction of Na+. This indicates that the equivalent fraction of cation (such as Na+) has a strong effect on the rejections of NF membranes. The equivalent fractions of anions (such as Cl!) almost have no effect on the rejections of NF membranes, which seems to be attributed to the similar density charges of Cl! and NO3!. For mixed electrolyte solutions, the observed rejections of total electrolytes were dependent on the total electrolytes concentration and the equivalent fractions of ions. Furthermore, the observed rejection of one cation (or anion) was affected by the proportion of other cations (or anions). Further investigation is expected in order to support the quantitative simulation of the NF membrane processes.
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4. Conclusions Experimental investigation on the rejections of ions by the NF membranes for the binary electrolyte solutions of NaCl, NaNO3, CaCl2 and Ca(NO3)2 indicates that the observed rejections of Ca2+ by NF membranes all slightly increased with the growth of the concentration of Na+ and the observed rejection of Na+ decreased with the growth of the concentration of Ca2+. The repulsion forces of the membrane negative charges on the cation with lower charge density are weaker, thus it presents higher permeability and lower rejection. The observed rejections of Cl! and NO3! by the NF membranes were almost constant and hardly changed with their equivalent fractions in binary electrolyte solutions. For charged species, electrostatic mechanisms play a more important role in NF separation.
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Acknowledgements
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This work is supported by the National Basic Research Program of China (2003CB615701) and National Natural Science Foundation of China (20376040).
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