Efficient electrochemical generation of active chlorine to mediate urea and ammonia oxidation in a hierarchically porous-Ru/RuO2-based flow reactor

The electrochemical chlorination of urea to CO2 and N2 end-products, via active-chlorine-mediated oxidation under nearly neutral conditions, is an effective treatment for medium-concentrated urea-containing wastewater. Herein, we design a novel flow reactor integrated with three-dimensional hierarchically porous Ru/RuO2 architectures anchored on a Ti mesh. The hierarchically macroporous electrode can create sufficient exposure of catalytically active sites and facilitate the microscopic mass transport and diffusion inside the active layer, thereby contributing to the increased removal efficiency of urea-N and ammonia-N. The combined results of electrochemical measurements, UV-visible spectrometry and in situ Raman spectrometry, show that the OCl- species produced by chlorine evolution reaction (CER) are the main active constituents for removing urea-N. Theoretical calculations reveal thLTWAat the Ru/RuO2 possesses a moderate Cl binding strength, lower theoretical overpotentials of CER and a higher conductivity, compared with pure RuO2. On this basis, we assemble a circular flow reactor with the hierarchically porous electrodes in a two-electrode system to obtain an enhanced microfluidic process, which during 9 days of uninterrupted operation, at a high electrolysis current of 500 mA, achieve a total nitrogen removal of 92.6% and an energy consumption of 7.94 kWh kg-1 N, demonstrating the promising application of the novel process.