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Transport properties of ion-exchange membrane systems in LysHCl solutions
Desalination 200 (2006) 149–151
Transport properties of ion-exchange membrane systems in LysHCl solutions Natalia Pismenskayaa*, Kristina Igritskayaa, Elena Belovaa, Victor Nikonenkoa, Gerald Pourcellyb a
Membrane Institute, Kuban State University, Stavropolskaya Str., 149, 350040 Krasnodar, Russia email: [email protected] b Institut Européen des Membranes, Université Montpellier II, CC 047, Place Eugène Bataillon, 34095 Montpellier Cédex 5, France Received 20 October 2005; accepted 1 March 2006
Electromembrane methods of amino acids purification and separation are very attractive. They do not need any additional chemicals, minimize the desired product loss and are energy efficient and manufacturable. Detailed study of amino acid transfer mechanisms inside membranes and at their surfaces provides a basis to improve these methods. For these purposes, concentration dependencies of cation-exchange (CMX, MK-40) and anion-exchange (AMX, MA-41) membranes electrical conductivity in 0.025–1 M LysHCl and NaCl solutions as well as current–voltage curves (CVC) and chronopotentiograms (ChP) of CMX membrane in 0.01 M solutions are obtained. It is shown that in the case of CMX and MK-40 cation-exchange membranes (Fig. 1a) the replacement of NaCl to LysHCl decreases values of the membranes electrical conductivity (k) more than 10 times. CMX and MK-40 membranes electrical conductivity grows with dilution of LysHCl solution at C < 0.1 M, whereas in the case of NaCl it keeps decreasing. This behavior of membranes is *Corresponding author.
caused by two reasons: (1) the value of LysH+ counter-ion diffusion coefficient is less than that of Na+ (in solution: DNa+ = 1.33 ´ 10–5 cm2/s, DLysH+ = 0.667 ´ 10–5 cm2/s); (2) when diluting external solution, the cation-exchange membrane internal solution is acidified due to the Donnan exclusion of OH– ions; as a consequence, an increase in LysH22+ fraction in this solution occurs as a result of LysHCl hydrolysis. The Donnan exclusion of OH– ions is proved by the increase in pH during equilibration of these membranes in LysHCl solution if they were initially equilibrated with NaCl solution. In the case of anion-exchange membranes (Fig. 1b), their electrical conductivity is mainly determined by the counter-ion (Cl–); the type of co-ion (LysH+ or Na+) influences weakly. This influence is also weak as for the value of the volume fraction ( f2) of the intergel spaces filling with the equilibrium electroneutral solution; f2 being obtained using the microheterogeneous model. For a homogeneous AMX membrane f2 is equal to 0.11 in NaCl solutions and to 0.07 in LysHCl ones. Some decrease in the AMX intergel phase fraction may be caused by a reduction
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.2006.03.277
150
N. Pismenskaya et al. / Desalination 200 (2006) 149–151
10
10
0.8
8
8
CMX NaCl MK-40 NaCl CMX LysHCl MK-40 LysHCl
4
0.4
κ, mS/cm–1
6
κ, mS/cm–1
κ, mS/cm–1
0.6
0.2
(a)
0.4
0.6
0.8
1.0
1.2
C, M
AMX NaCl AMX LysHCl MA-41 NaCl MA-41 LysHCl
0
0 0.2
4
2
2
0 0.0
6
0.0
0.2
0.4
0.6
(b)
0.8
1.0
1.2
C, M
Fig. 1. Concentration dependencies of cation-exchange (a) and anion-exchange (b) membranes electrical conductivity in NaCl and LysHCl solutions.
value calculated by the convection–diffusion model (Fig. 2). At the same time, while the slope of the initial part of the CVC for CMX/ LysHCl system is lower than that for CMX/ NaCl, the overlimiting current growth is observed under significantly less potential drops (Fig. 2). Such trend of the curve indicates more 3.5
C = 0.01 M, V = 0.056 cm/s, h = 0.54 cm
3.0
CMX NaCl CMX LysHCl
2.5
i,
mA/cm2
of the membrane swelling in LysH+ form, because of the hydrophobicity of this ion. The increase in f2 of membrane with a better electrical conductivity than equilibrated diluted solution leads to negligible k growth, independently on electrolyte type. In the case of heterogeneous MA-41 membrane, f2 is equal to 0.23 (NaCl) and 0.22 (LysHCl). The large volume of internal electroneutral solution (about two times higher than in the homogeneous AMX) is the reason of the fact that the specific conductivity of MA-41 decreases, when replacing a more mobile Na+ for a less mobile LysH+ ion, stronger in comparison with the case of AMX membrane. The ability of LysH+ ion to hydrolyze with the generation of H+ and OH– ions and, depending on solution pH, to transform into zwitterion Lys± (pH > 7.6) or two-charge Lys2+ ion (pH < 5.7) is apparently the main reason of the difference in electrochemical characteristics of CMX/NaCl and CMX/LysHCl membrane systems. Thus, the CVC shape of CMX membrane in NaCl solutions is close to the “classical” one; the limiting current found by the crossing of CVC tangents coincides with the
2.0 1.5 1.0 ilim Na+,
0.5 ilim LysH+ 0.0 0
2
4
6
Δϕ, V
Fig. 2. Current–voltage curves of CMX membrane in NaCl and LysHCl solutions.
N. Pismenskaya et al. / Desalination 200 (2006) 149–151 7
C = 0.01 M, V = 0.056 cm/s, h = 0.54 cm
6 5
iτ1/2
4 3 2 1 0 0.5
CMX NaCl, exper CMX NaCl Sand's eqv CMX LysHCl exper CMX LysHCl Sand's eqn
1.5
2.5
3.5
i, mA/cm2
Fig. 3. Dependencies of transition time on current density, represented in Sand’s coordinates.
intensive development of the coupled effects of concentration polarization in the CMX/LysHCl system in comparison with the CMX/NaCl. The generation of H+ and OH– ions at the membrane/ solution interface seems to be more probable among them.
151
The invariability of it 1/2 values presented as a function of i for CMX/NaCl and CMX/ LysHCl systems (Fig. 3) shows that the counterion transfer through the CMX in NaCl and LysHCl solutions runs with diffusion control. Here t is the transition time obtained for a current density i from ChP. However in the case of CMX/LysHCl system, the values of t calculated by Sand’s equation exceed more than 20% the experimental ones, while theoretical and experimental values of t for CMX/NaCl system are practically identical. Probably, the generation of H+ and OH– ions at the CMX surface, caused by ampholyte hydrolysis and accelerated by electric field, leads to alkalization of adjoining solution. As a result, the OH– ions react with the LysH+ ionsproducing zwitterions Lys±, which leads to a decrease in the charge carrier concentration in the depleted diffusion layer. Acknowledgements This study is supported by RFBR 03-0396571 and 06-03-96609 grants.
Transport properties of ion-exchange membrane systems in LysHCl solutions Natalia Pismenskayaa*, Kristina Igritskayaa, Elena Belovaa, Victor Nikonenkoa, Gerald Pourcellyb a
Membrane Institute, Kuban State University, Stavropolskaya Str., 149, 350040 Krasnodar, Russia email: [email protected] b Institut Européen des Membranes, Université Montpellier II, CC 047, Place Eugène Bataillon, 34095 Montpellier Cédex 5, France Received 20 October 2005; accepted 1 March 2006
Electromembrane methods of amino acids purification and separation are very attractive. They do not need any additional chemicals, minimize the desired product loss and are energy efficient and manufacturable. Detailed study of amino acid transfer mechanisms inside membranes and at their surfaces provides a basis to improve these methods. For these purposes, concentration dependencies of cation-exchange (CMX, MK-40) and anion-exchange (AMX, MA-41) membranes electrical conductivity in 0.025–1 M LysHCl and NaCl solutions as well as current–voltage curves (CVC) and chronopotentiograms (ChP) of CMX membrane in 0.01 M solutions are obtained. It is shown that in the case of CMX and MK-40 cation-exchange membranes (Fig. 1a) the replacement of NaCl to LysHCl decreases values of the membranes electrical conductivity (k) more than 10 times. CMX and MK-40 membranes electrical conductivity grows with dilution of LysHCl solution at C < 0.1 M, whereas in the case of NaCl it keeps decreasing. This behavior of membranes is *Corresponding author.
caused by two reasons: (1) the value of LysH+ counter-ion diffusion coefficient is less than that of Na+ (in solution: DNa+ = 1.33 ´ 10–5 cm2/s, DLysH+ = 0.667 ´ 10–5 cm2/s); (2) when diluting external solution, the cation-exchange membrane internal solution is acidified due to the Donnan exclusion of OH– ions; as a consequence, an increase in LysH22+ fraction in this solution occurs as a result of LysHCl hydrolysis. The Donnan exclusion of OH– ions is proved by the increase in pH during equilibration of these membranes in LysHCl solution if they were initially equilibrated with NaCl solution. In the case of anion-exchange membranes (Fig. 1b), their electrical conductivity is mainly determined by the counter-ion (Cl–); the type of co-ion (LysH+ or Na+) influences weakly. This influence is also weak as for the value of the volume fraction ( f2) of the intergel spaces filling with the equilibrium electroneutral solution; f2 being obtained using the microheterogeneous model. For a homogeneous AMX membrane f2 is equal to 0.11 in NaCl solutions and to 0.07 in LysHCl ones. Some decrease in the AMX intergel phase fraction may be caused by a reduction
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.2006.03.277
150
N. Pismenskaya et al. / Desalination 200 (2006) 149–151
10
10
0.8
8
8
CMX NaCl MK-40 NaCl CMX LysHCl MK-40 LysHCl
4
0.4
κ, mS/cm–1
6
κ, mS/cm–1
κ, mS/cm–1
0.6
0.2
(a)
0.4
0.6
0.8
1.0
1.2
C, M
AMX NaCl AMX LysHCl MA-41 NaCl MA-41 LysHCl
0
0 0.2
4
2
2
0 0.0
6
0.0
0.2
0.4
0.6
(b)
0.8
1.0
1.2
C, M
Fig. 1. Concentration dependencies of cation-exchange (a) and anion-exchange (b) membranes electrical conductivity in NaCl and LysHCl solutions.
value calculated by the convection–diffusion model (Fig. 2). At the same time, while the slope of the initial part of the CVC for CMX/ LysHCl system is lower than that for CMX/ NaCl, the overlimiting current growth is observed under significantly less potential drops (Fig. 2). Such trend of the curve indicates more 3.5
C = 0.01 M, V = 0.056 cm/s, h = 0.54 cm
3.0
CMX NaCl CMX LysHCl
2.5
i,
mA/cm2
of the membrane swelling in LysH+ form, because of the hydrophobicity of this ion. The increase in f2 of membrane with a better electrical conductivity than equilibrated diluted solution leads to negligible k growth, independently on electrolyte type. In the case of heterogeneous MA-41 membrane, f2 is equal to 0.23 (NaCl) and 0.22 (LysHCl). The large volume of internal electroneutral solution (about two times higher than in the homogeneous AMX) is the reason of the fact that the specific conductivity of MA-41 decreases, when replacing a more mobile Na+ for a less mobile LysH+ ion, stronger in comparison with the case of AMX membrane. The ability of LysH+ ion to hydrolyze with the generation of H+ and OH– ions and, depending on solution pH, to transform into zwitterion Lys± (pH > 7.6) or two-charge Lys2+ ion (pH < 5.7) is apparently the main reason of the difference in electrochemical characteristics of CMX/NaCl and CMX/LysHCl membrane systems. Thus, the CVC shape of CMX membrane in NaCl solutions is close to the “classical” one; the limiting current found by the crossing of CVC tangents coincides with the
2.0 1.5 1.0 ilim Na+,
0.5 ilim LysH+ 0.0 0
2
4
6
Δϕ, V
Fig. 2. Current–voltage curves of CMX membrane in NaCl and LysHCl solutions.
N. Pismenskaya et al. / Desalination 200 (2006) 149–151 7
C = 0.01 M, V = 0.056 cm/s, h = 0.54 cm
6 5
iτ1/2
4 3 2 1 0 0.5
CMX NaCl, exper CMX NaCl Sand's eqv CMX LysHCl exper CMX LysHCl Sand's eqn
1.5
2.5
3.5
i, mA/cm2
Fig. 3. Dependencies of transition time on current density, represented in Sand’s coordinates.
intensive development of the coupled effects of concentration polarization in the CMX/LysHCl system in comparison with the CMX/NaCl. The generation of H+ and OH– ions at the membrane/ solution interface seems to be more probable among them.
151
The invariability of it 1/2 values presented as a function of i for CMX/NaCl and CMX/ LysHCl systems (Fig. 3) shows that the counterion transfer through the CMX in NaCl and LysHCl solutions runs with diffusion control. Here t is the transition time obtained for a current density i from ChP. However in the case of CMX/LysHCl system, the values of t calculated by Sand’s equation exceed more than 20% the experimental ones, while theoretical and experimental values of t for CMX/NaCl system are practically identical. Probably, the generation of H+ and OH– ions at the CMX surface, caused by ampholyte hydrolysis and accelerated by electric field, leads to alkalization of adjoining solution. As a result, the OH– ions react with the LysH+ ionsproducing zwitterions Lys±, which leads to a decrease in the charge carrier concentration in the depleted diffusion layer. Acknowledgements This study is supported by RFBR 03-0396571 and 06-03-96609 grants.