A new type of the bipolar membrane electrodialysis (BPED) for the simultaneous removal and recovery of boron in the waste water was developed. Two-cell type electrodialysis unit composed of an anion exchange membrane (AEM) sandwiched by two bipolar membranes (BPM) was used, and the semi-batch type operation with circulating solutions through the cells, namely, the recovery cell (cathode side) and the concentration cell (anode side). The volume ratio of the solutions fed to the cell was set 4:1 (recovery cell : concentration cell). The boric ions in the recovery cell were transported through the AEM to the concentration cell, and the boron concentration in the concentration cell should be concentrated to 5 times of the initial concentration for the ideal case. The concentrated solution was then fed to the second-stage BPED system to concentrate the solution, and the obtained concentrated solution was fed to the third-stage BPED. The operation was continued step-by-step until the boron concentration was high enough to recover boric acid easily. The experimental results with a stack of five cells showed the concentration ratio in the concentration cell was lower than 5, while the concentration of the recovery cell was lower than 8 mg-B/L, which was below the regulatory value in the waste water in Japan. After three-stages of the BPED processing, the boron concentration in the concentration cell reached close to the solubility of boric acid, c.a., 10,000 mg/L, with which boric acid can be recovered as a recycled resource for boron. We also investigated the effect of coexisting ions such as sulfuric ions were investigated. No selectivity of boric acid ions were observed for the anion exchange membranes used. However, two-step separation process of normal ED followed by BPED enabled the separation of sulfuric ions from boric ions.

Development of a novel separation and recycle process of boron in waste water with bipolar membrane electrodialysis

255th National Meeting (2018)

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