Water Treatment Using Boron-Doped Diamond Powder-Packed Electrolysis Flow Cell
Authors: Nomura Rina , Akashi Megumi , Matsumoto Satoshi , Kondo Takeshi 
Published: December 2024

Water treatment by electrolysis can decompose persistent organic compounds, such as low-molecular-weight organic compounds in urine, which cannot be decomposed by microbial water treatments, into H2O and CO2. Anodic oxidation is being considered for use as one of the processes in the water reclamation system required for manned space exploration. Boron-doped diamond (BDD) electrodes can generate efficiently OH radical, which is a strong active oxidizing species, when high potential is applied in aqueous solution. In addition, BDD electrodes are known to be useful as electrode materials for electrolytic water treatment because of their physical and chemical stability. However, BDD electrodes are usually thin films, so their shape, size, and the configuration of electrolytic cells are restricted. Boron-doped diamond powder (BDDP)-packed electrolysis flow cell has been reported to be useful for water treatment with high efficiency because of the large electrode surface area. In this study, the BDDP-packed electrolytic flow cell with a closed system was developed for improvement of the decomposition efficiency.

BDDP was prepared by using commercially available diamond powder with a particle size of 40-60 μm as a substrate material and depositing a BDD layer on its surface by microwave plasma CVD. The BDDP-packed electrolytic flow cell is as shown in Figure 1a. A plastic cylinder with an inner diameter of 1 cm was bonded to a glass filter (particle holding capacity: 20-25 μm, diameter: 10 mm), and a Pt wire was placed as a current collector above the filter. 0.8 g of BDDP was packed into the filter without a binder so that it was in full contact with the current collector. Electrolysis of 50 mL of 0.1 M Na2SO4 solution containing 50 μM methylene blue (MB) was performed for 60 min at a constant voltage of 5 V. The MB concentration after electrolysis was estimated by UV-vis absorption spectroscopy. In addition, the chemical oxygen demand (COD) of the electrolyte was measured before and after electrolysis.

The result of MB constant voltage electrolysis with the BDDP-packed electrolysis flow cell is shown in Fig. 1b. MB was not sufficiently decomposed at a flow rate of 2.0 mL/min. The reason for this is thought to be that the bubbles generated on the BDDP surface caused insufficient contact between the BDDP. When the flow rate was increased to 20.0 mL/min to flush out all bubbles, about 90% MB degradation was achieved in 15 min and about 99% in 30 min. The higher the flow rate, the faster the MB concentration decreased. These results suggest that the closed BDDP-packed electrolysis flow cell allows bubbles generated on the electrode surface to be quickly flushed out with the treated water. The COD of the electrolyte before and after the electrolysis was measured and it ranged from 42.2 to 21.9. The decrease in COD suggests that part of MB was completely converted to CO2. In the future, we intend to quantitatively clarify the urine treatment capacity of the cell and apply it to a water recovery system in space.

Figure 1