Construction of microchannel charcoal cathodes with spatial-constraint capability for enhancing reduction of NO3− in high-salinity water

Abstract

Electrochemistry is a potential technology to remove nitrate from high-salinity water. Due to electrostatic repulsion of cathodes to NO3− anions, the adsorption of NO3− on cathode surface will be interfered, thus limiting the reduction reaction at cathodes. To restrain the electrostatic repulsion, an alkali-etched microchannel charcoal (AEMC) was fabricated as a cathode. Its channel radius was <10 μm but the channel length was more than 4 cm. The function of alkali etching was to create the oxygen-containing groups as activesite. During reaction, NO3− anions can flow through cathodic microchannel with high aspect ratio and be reduced by electrons at active sites throughout microchannel surface. For this kind of spatial-constraint cathode, because the diffusion length was constrained in micrometer scale, diffusion driven by concentration gradients will counteracting electrostatic repulsion. Compared to electrodes without spatial constraint, AEMC displayed 1.6 times of nitrate reduction ratio (from 43.9 % to 70.8 %) and 5.2 times of Faraday efficiency (from 11.6 % to 60.4 %). This AEMC cathode also displayed low energy consumption (0.18 kWh/g (NO3−-N)) and a good nitrogen formation rate (100 %) if chloride was abundant. The highest NO3−-N removal capability reached 31.3 mg/(L h cm2), which was higher than those of most reported metal electrodes. These results demonstrated a novel approach to improve nitrate reduction reaction employing a spatial-constraint cathode.