Insight of electron transfer for chlorine mediated ammonia oxidation in a three-dimension (3D) electrode system

Chlorine-Mediate Electrochemical Advanced Oxidation (Cl-EAO) technology emerges as a highly promising method for ammonia removal. However, the electron transfer mechanism behind the Cl-EAO process in three-dimension (3D) electrode systems remains unclear. This study revealed a two-stage linear degradation trend for ammonia under low chlorine-nitrogen mass ratio (Cl/N) conditions. This segmented degradation was attributed to changes in the electron transfer process. Ammonia and intermediates serve as electron donors on the granular activated carbon (GAC) surface before and after the “inflection point”, respectively. It has been observed that Cl/N demonstrates a volcanic relationship with the number of electron transfers, leading to the development of a universal volcanic model. Using this model, the optimal electron transfer moment (Cl/N = 10) and electron transfer occupancies for ammonia oxidation reaction (0.94), oxygen evolution reaction (0.02) and chlorine evolution reaction (0.04) in the 3D electrode system were obtained. Additionally, quenching experiments showed that the chlorine radicals (70 %) and active chlorine (56 %) were the main contributors to 2D and 3D electrode systems, respectively. Based on the important results found in this work, we elucidated the electron transfer mechanism behind the ammonia oxidation on anode and GAC: chlorine radical-mediated three-electron transfer and single-electron transfer breakpoint chlorination mechanism, respectively.