Vertical Distribution of Ammonium Ion Concentration to the Air-Cathode of a Single-Chamber Microbial Fuel Cell

Abstract

A microbial fuel cell (MFC) is a promising technology for wastewater treatment that can achieve removal of organic matter from wastewater and electricity generation simultaneously. In addition, it is known that not only organic matter but also ammonia in wastewater can be removed by single chamber microbial fuel cell (SCMFC). However, little is known of this ammonia removal mechanism. Some researchers reported that reduction of ammonium ion concentration occurred through biological nitrification / denitrification process by biofilm on the surface of an air-cathode in the SCMFC. Meanwhile, there is another hypothesis that ammonia volatilization occurs through the air-cathode as a result of decrease in the degree of ammonia dissociation and promotion of conversion to ammonia molecule, induced by local pH increase near the air-cathode. However, no direct evidence about the ammonia volatilization has not been reported. Therefore, in this study, SCMFC with a new (biofilm-less) air-cathode was operated and vertical distribution of ammonium ion concentration to the air-cathode was measured. If ammonia volatilization occurs, a sharp decrease in the concentration must be observed near the air-cathode. In our previous study, the vertical pH distribution to the air-cathode was measured with an ion selective microelectrode. Therefore, we applied this method to a micro-ammonium ion electrode. The concentration of ammonium ion at 10 mm from the air-cathode was 2 mM and there was a trend to decrease gradually as it approached to the air-cathode. Especially, the concentration of ammonium ion decreased sharply at approximately 0.6 mm from the air-cathode, and it became less than one-tenth within the distance of 0.5 mm. In our previous study under almost the same operational condition, the maximum pH was observed on the surface of the air-cathode, and it was about 9.3. Since the fraction of ammonium ion dissociation under this pH is about 50 % at most, the sharp decrease in the ammonium ion concentration cannot be explained only by this reason. Hence, something that caused the decrease in ammonium ion concentration must have happened near the air-cathode. The effect of nitrification can be denied because the biofilms were not formed on the surface of the air-cathode. From these above reasons, it is natural to think that ammonia volatilization through the air-cathode occurred in this study. Acknowledgments This research was financially supported by Grant-in-Aid for Japan Society for the Promotion of Science Fellows (Project No. 15J0490902). Figure 1