(687f) Removal and Recycle of Boric Acid from Waste Water with Electrodalysis

Yurina Nakamura, Atsushi Iizuka, Tadashi Shoji, Miyuki Noguchi, Akihiro Yamasaki

The multi-stage electrodialysis method was applied to waste water containing boric acid for recycling boric acid as solid. The electrodialysis unit was composed of two cells formed by an anion exchange membrane (AEM) sandwiched by two bipolar membranes (BPM), and five or ten units was stacked to form the electrodialysis equipment. The cell located at the higher potential side was called the concentration cell, and that at the lower potential side was called the feed cell. The sodium borate solution of which the initial concentration was set 100 ppm was circulated in a batch style through each cell for 60 or 120 min. The flow rate through each cell was changed in the range of 0.1 to 1.0 L/min, and the volume of the circulation solution through the feed cell to that through the concentration cell (the volume ratio hereafter) was changed in the range of 2 to 8. The overall electric potential applied in the electrodialysis equipment was constant at 12.5 V. The time changes of boron in the cells were monitored with ICP-AES.

The concentration of boron in the feed side decreased and that in the concentration cell increased with time, and almost leveled off at the later stage of the electrodialysis. This result indicates that the boric acid was transported through the AEM driven by the electric potential applied along the cells. The boric ions B(OH)₄^-, which is the dominant form of boric acid at pH higher than 9.14, in the feed cell would be transported to the concentration cell through the AEM, and the boron concentration decreased with time. The removal anions in the feed cell were compensated by hydroxyl ions supplied from the BPM. In the concentration cell, the boric ions would be coupled with protons supplied from the BPM to form non-valent boric acid, and the boron concentration increased with time. The boron concentration in the concentration cell at the later stage of the electrodialysis depended on the volume ratio; the higher volume ratio resulted in the higher boron concentration. When the ratio was 4, the boron concentration in the concentration cell increased at about 400 ppm at 60 min of the operation. The concentrated solution was then introduced to the second stage of the electrodialysis. As a result, the boron concentration in the concentration cell was 1500 ppm, while that in the feed cell was 286 ppm at 60 min. The solution in the concentration cell was further introduced to the third electrodialysis unit, and after 60 min, the boron concentration of the concentration solution was 4000 ppm, and at the fourth stage the boron concentration in the concentration cell was about 10000 ppm, which is close to the solubility of boric acid. The solid boric acid would be easily recovered from the concentrated solution, of which the concentration is close to the solubility. The sequences of the boron concentrations in the feed and concentration cell depended on the flow rate as well as the volume ratio, and the number of the stages required to be concentrated to the concentration close to the solubility of boric acid depended on both of them. On the other hand, at the initial stage of the electrodialysis, the boron concentration was decreased below the Japanese standard of the waste water for boron at 10 ppm. The present method can be applied to the treatment of waste water containing boric acid to achieve simultaneous removal and recycle of boric acid and their salts.