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The origin and elimination of water splitting in ion exchange membranes during water demineralisation by electrodialysis
Desalination. 28 (1979) 4142 @ Elsevier ScientificPublishing Company. Amsterdam - Printed in The Netherlands
THE ORIGIN
AND
ELIMINATION
CHANGE MEMBRANES ELECTRODIALYSIS
OF WATER
DURING
WATER
SP LI’M’ING
IN ION
DEMINERALISATION
EX-
BY
R. SLMO!US Department of Physics, The Sydney 2033 (Australia)
University
of New
South
Wales, P-0-B.
1, Kensington,
(Received January 29,1979)
SUMMARY
Water splitting in anion exchange membranes containlng quaternary ammonium groups is due to the presence of tertiary alkyl amino groups in the surface regions. It may be eliminated by methylation, however it reappears during current flow because quaternary ammonium groups in the surface regions are then converted to the tertiary form. The quaternary ammonium groups are more stable in Negev Institute A than in AMF Al00 membranes. By contrast water splitting is not manifested by cation exchange membranes with sulphonic acid groups if the system is sufficiently clean. It is well known that there is a substantial energy loss during water demineralisation by electrodialysis because of water splitting in the ion exchange membranes. The effect originates near those surfaces of the membranes which are depleted of ions during the passage of electric current [l] . Its origin has remained obscure. In the present communication we report results of an investigation of water splitting for ion exchange membranes in NaCl solutions. The cation exchange membranes contained &phonic acid groups (Negev Institute C and AMF ClOO) and the anion exchange membranes, quaternary ammonium groups (Negev Institute A and AMF AlOO). The concentration of salt varied from 1mM to 0.1 M and the current densities from 0.5 A/m2 (50 M/cm2 ) to 3,000 A/m* (0.3 A/cm2 )_ There were two types of experiment. In one, the cell contained two chambers separated by the membrane. Current was supplied through calomel electrodes to avoid introducing pH changes at the electrodes. In the other, the cell contained eight compartments. The central two were separated by the membrane under study and the others by cation exchange membranes. In these the current was supplied through platinum electrodes and the additional compartments isolated the central ones from the pH changes
42
R. SIMONS
occurring at the electrodes. When splitting occurred, experiments were continued until the concentration of H+ and OH- ions in the solutions next to the central membrane was one tenth that of the salt. Thus in 0.1 M NaCl the final pH’s were 2 and 12. The magnitude of the effect was calculated from the rate of change of these pH’s. Water splitting appeared to be an intrinsic property of the anion exchange membranes. However in the case of the Negev Institute membranes it was only manifested after prolonged applied currents. Reproducible results were obtained for a current density of 300 A/m2 (30mA/cm2) in 0.1 M NaCl only after about 20 hours of current flow. With the AMF membranes the effect was always present though it increased in magnitude during the first 10 minutes of current flow. The water splitting could be mostly (- 90%).-eliminated in the Negev Institute membranes by converting them to the OH- ion form and leaving them overnight in a solution of methyl iodide. However it would again be manifested after prolonged applied currents_ Similarly water splitting in the AMF membranes retuned to its original level following a single methylation. Experiments were also performed on Negev Institute anion exchange membranes containing tertiary alkyl amino groups_ The results differed from those for the strongly basic quaternary ammonium ion exchange membranes in the respect that water splitting was manifested from the outset, without any prior current flow. The effect disappeared when the membranes were converted to the quatemary form. These results indicate that water splitting in the anion exchange membranes is due to tertiary amine groups in the surface region. In the strongly basic membranes these arise from the degradation of quaternary ammonium groups. Contrary to our expectations water splitting was not manifested by cation exchange membranes which were sufficiently clean. An essential step in the cleaning process was the soaking of the membranes in concentrated NaCl solutions for several days. However it did occur if the salt solutions were prepared using tap rather than distilled water. Water splitting at cation exchange membranes during desalting by electrodialysis may therefore be ascribed to dissolved impurities. ACKNOWLEDGMENT The author wishes to acknowledge the help and encouragement given to him during discussions with Professor F. de Korosy of the Negev Institute for Arid Zone Research, Beersheva, Israel. REFERENCE 1. K. S. Spiegler, Ed., Principles of Desalination, Academic 1966, p_ 230.
Press, New York,
N.Y.,
THE ORIGIN
AND
ELIMINATION
CHANGE MEMBRANES ELECTRODIALYSIS
OF WATER
DURING
WATER
SP LI’M’ING
IN ION
DEMINERALISATION
EX-
BY
R. SLMO!US Department of Physics, The Sydney 2033 (Australia)
University
of New
South
Wales, P-0-B.
1, Kensington,
(Received January 29,1979)
SUMMARY
Water splitting in anion exchange membranes containlng quaternary ammonium groups is due to the presence of tertiary alkyl amino groups in the surface regions. It may be eliminated by methylation, however it reappears during current flow because quaternary ammonium groups in the surface regions are then converted to the tertiary form. The quaternary ammonium groups are more stable in Negev Institute A than in AMF Al00 membranes. By contrast water splitting is not manifested by cation exchange membranes with sulphonic acid groups if the system is sufficiently clean. It is well known that there is a substantial energy loss during water demineralisation by electrodialysis because of water splitting in the ion exchange membranes. The effect originates near those surfaces of the membranes which are depleted of ions during the passage of electric current [l] . Its origin has remained obscure. In the present communication we report results of an investigation of water splitting for ion exchange membranes in NaCl solutions. The cation exchange membranes contained &phonic acid groups (Negev Institute C and AMF ClOO) and the anion exchange membranes, quaternary ammonium groups (Negev Institute A and AMF AlOO). The concentration of salt varied from 1mM to 0.1 M and the current densities from 0.5 A/m2 (50 M/cm2 ) to 3,000 A/m* (0.3 A/cm2 )_ There were two types of experiment. In one, the cell contained two chambers separated by the membrane. Current was supplied through calomel electrodes to avoid introducing pH changes at the electrodes. In the other, the cell contained eight compartments. The central two were separated by the membrane under study and the others by cation exchange membranes. In these the current was supplied through platinum electrodes and the additional compartments isolated the central ones from the pH changes
42
R. SIMONS
occurring at the electrodes. When splitting occurred, experiments were continued until the concentration of H+ and OH- ions in the solutions next to the central membrane was one tenth that of the salt. Thus in 0.1 M NaCl the final pH’s were 2 and 12. The magnitude of the effect was calculated from the rate of change of these pH’s. Water splitting appeared to be an intrinsic property of the anion exchange membranes. However in the case of the Negev Institute membranes it was only manifested after prolonged applied currents. Reproducible results were obtained for a current density of 300 A/m2 (30mA/cm2) in 0.1 M NaCl only after about 20 hours of current flow. With the AMF membranes the effect was always present though it increased in magnitude during the first 10 minutes of current flow. The water splitting could be mostly (- 90%).-eliminated in the Negev Institute membranes by converting them to the OH- ion form and leaving them overnight in a solution of methyl iodide. However it would again be manifested after prolonged applied currents_ Similarly water splitting in the AMF membranes retuned to its original level following a single methylation. Experiments were also performed on Negev Institute anion exchange membranes containing tertiary alkyl amino groups_ The results differed from those for the strongly basic quaternary ammonium ion exchange membranes in the respect that water splitting was manifested from the outset, without any prior current flow. The effect disappeared when the membranes were converted to the quatemary form. These results indicate that water splitting in the anion exchange membranes is due to tertiary amine groups in the surface region. In the strongly basic membranes these arise from the degradation of quaternary ammonium groups. Contrary to our expectations water splitting was not manifested by cation exchange membranes which were sufficiently clean. An essential step in the cleaning process was the soaking of the membranes in concentrated NaCl solutions for several days. However it did occur if the salt solutions were prepared using tap rather than distilled water. Water splitting at cation exchange membranes during desalting by electrodialysis may therefore be ascribed to dissolved impurities. ACKNOWLEDGMENT The author wishes to acknowledge the help and encouragement given to him during discussions with Professor F. de Korosy of the Negev Institute for Arid Zone Research, Beersheva, Israel. REFERENCE 1. K. S. Spiegler, Ed., Principles of Desalination, Academic 1966, p_ 230.
Press, New York,
N.Y.,