Coordination-driven interfacial sulfite activation on iron single-atom catalysts and oxysulfur species evolution.

Transition metal-mediated activation of sulfite (S(IV)) has garnered substantial interest in advanced oxidation processes (AOPs) for water remediation. Nevertheless, an incomplete understanding of the interfacial activation mechanism limits further improvement and regulation of treatment performance. Herein, a single-atom iron catalyst (Fe1CN) with a well-defined Fe coordination environment is employed as a model platform to investigate coordination-mediated S(IV) activation. The Fe1CN/S(IV) system achieves rapid acetaminophen (ACT) removal, with a reaction rate higher than those of selected benchmark iron-based catalysts under the tested conditions. Mechanistic investigations indicate that oxysulfur radical chemistry, particularly SO4•-- and SO5•--related pathways, plays an important role in ACT degradation. Combined in situ spectroscopic analyses and theoretical calculations support preferential interfacial coordination of S(IV) with Fe single-atom sites, promoting inner-sphere electron transfer and subsequent oxysulfur species evolution. Fe sites with an average oxidation state of approximately + 2.4 are associated with Fe(II)- and Fe(III)-involved activation pathways that contribute to sustained S(IV) conversion. The Fe1CN/S(IV) system also maintains effective performance in representative water matrices and during 24 h continuous-flow operation. These findings provide mechanistic insights into coordination-regulated S(IV) activation at Fe single-atom sites and offer valuable guidance for efficient S(IV)-based water treatment.