Electron transfer-constrained dehalogenation of halogenated pollutants by a sulfur second-shell coordinated Fe site under directed persulfate activation.

Halogenated organic pollutants (HOPs) represent a class of persistent and toxic contaminants whose robust C-F/C-Cl bonds pose a significant challenge to complete mineralization. Here, we engineered a sulfur second-shell coordinated Fe single-atom catalyst supported on nitrogen-doped carbon that selectively steered nonradical pathways during persulfate oxidation. The distinct electronic interplay between electron-rich Fe and electron-deficient S sites customized singlet oxygen (1O2) generation from peroxymonosulfate and electron transfer processes (ETP) from peroxydisulfate, via precise modulation of oxygen-binding configurations and electron-transfer numbers. This catalytic platform delivered efficient degradation (∼100%) and dehalogenation (80-100%) of representative HOPs, including traditional chlorophenol compounds and emerging fluoroquinolone antibiotics. Combined nuclear magnetic resonance and mass spectrometry analyses revealed synergistic dechlorination of p-chlorophenol by 1O2 and ETP. 1O2 first oxidized the piperazine ring, followed by ETP-driven defluorination of ciprofloxacin. Theoretical calculations confirmed ETP-induced elongation of C-F/C-Cl bonds via optimal binding modes between Fe sites and halogen- or oxygen-containing functional groups of substrates. The implemented continuous-flow device demonstrated efficient HOPs removal, anti-interference ability at high COD (∼1700 mg L-1), and stable operation over 13.3 days (∼960 batch cycles) with acceptable Fe loss (below 3%). This study established a customizable nonradical dehalogenation framework and developed advanced oxidation technologies for next-generation wastewater treatment.