Abstract
The autor has previously reported that when sea water is concentrated by ion exchange membrane method, there is a probability for calcium carbonate to form scale without occurrence of water decomposition due to abnormal behavior of carbonates (CO2+HCO3-+CO3--) in migration.
In this report, the author made a study on the behavior of carbonates by examining the relations between their behavior and the water decomposition and changes in the current efficiency of hydrogen carbonate ion transport through anion exchange membrane with varied current density. The results were as follows:
1. In electrodialysis of mixed solution of NaCl and NaHCO3, the permeability of carbonates indicated a sudden increase at high current density prior to the decomposition of water. This seemed to show occurrence of decomposition of HCO3-→H+CO3-- (decomposition of carbonates) similar to the decomposition of water.
2. In electrodialysis of NaHCO3 solution, there were two stages of variation in the current efficiency to HCO3 transport at 0.8-0.9 and 0.4-0.5. This seemed to be evidence the occurrence of the decomposition of carbonates mentioned above.
3. In the experiment of 2, the limiting current density for the decomposition of carbonates (ICO2-lim) seemed to be proportional to the concentration of solution at desalting compartment as in the case of water decomposition.
