Accelerated CrO electroreduction with NO/NO electron shuttle and in cathode microchannels.

The primary limitation of CrO electroreduction arises from the electrostatic repulsion between the cathodic electric double layer and anionic species. To overcome this limitation, we developed a strategy that involved two steps: transfering electrons from the cathode surface to the bulk solution via the NO/NO redox cycle, and shortening the diffusion distance of anions to the cathodic surface by means of cathodic microchannels whose average radius is only 10 μm. When CrO solution containing NO flowed through the cathodic microchannels, the CrO removal rate reached 18.9 mg·L·h, and the volumetric mass transfer efficiency was 0.35. The value was 7.1 and 8.8 times higher, respectively, than that achieved using flat electrodes without electron shuttles. The contribution of microchannels and electron shuttles to CrO removal was 32 % and 68 %, respectively. This electron shuttling in microchannels was effective across a wide pH range and remained stable during 70 h of continuous operation. The resulting Cr was subsequently converted into Cr(OH) precipitate by reacting with OH- generated from water electrolysis over the cathode. These results demonstrate a novel approach for CrO removal and Cr(OH) recovery. SYNOPSIS: This study presents a new strategy using electron shuttles for the efficient treatment and resource recovery of the challenging CrO.