Surface interaction-driven Fe self-cycling in MoO2-cocatalyzed Fenton-like system for efficient organic wastewater treatment

MoO 2 has been extensively recognized as an efficient cocatalyst in Fe-based Fenton-like systems. However, current understanding has predominantly limited its functional role to that of a reducing agent. This study elucidates a previously unexplored mechanism involving surface interaction-mediated self-sustained Fe redox cycling in MoO 2 -cocatalytic Fe(III)/peroxymonosulfate (PMS) system. Experimental findings demonstrate the formation of multiple reactive species (i.e., free radicals and high-valent iron-oxo species (Fe IV =O), leading to differential degradation efficiencies for various pollutants (i.e., higher than 96.5 % for sulfamethoxazole, phenol, benzoic acid, atrazine, and carbamazepine, and 28.9 % for p -nitrobenzoic acid). Notably, the observed PMS decomposition efficiency exceeds the stoichiometric reduction capacity of MoO 2 , thereby challenging the conventional perception of its sole function as a sacrificial electron donor. Kinetic analyses further reveal that Fe(II), generated via the reduction of Fe(III) by MoO 2 , accounts for less than 27.8 % of the total PMS decomposition. Instead, the surface-adsorbed Fe(III) (Fe(III) sur ) on MoO 2 is identified as the dominant active site for PMS activation, preferentially yielding Fe IV O as the key reactive intermediate. Based on these observations, we propose that MoO 2 not only serves as a reducing agent but, more importantly, might play as a ‘solid ligand’ that regulates the reaction activity and pathway of Fe(III) with PMS. This work provides critical insights into the design of self-sustaining Fenton-like systems for wastewater treatment.