Nitrogen-doped biochar-supported single-atom iron catalyst for peroxymonosulfate activation: Unveiling a dual...

Resource utilization of biochar for advanced oxidation processes is promising yet mechanistically unclear. Herein, a single-atom iron catalyst supported on nitrogen-doped biochar (SA Fe-N-BC) was synthesized to activate peroxymonosulfate (PMS) for efficient degradation of indole (IND), a recalcitrant nitrogenous pollutant. Optimized via central composite design-response surface methodology (0.170 g L^-1 of SA Fe-N-BC, 0.250 g L^-1 of PMS, and pH 5.27), the SA Fe-N-BC/PMS system achieved 100% IND removal within 10 min, exhibiting a high apparent rate constant (0.812 min^-1) - a 20.7-fold enhancement over the unmodified system. The system exhibited robust performance over a broad pH range (3.00-11.00), in complex water matrices, and maintained 91.5% efficiency after 5 cycles. It also efficiently degraded diverse pollutants. Mechanistic studies, including quenching, electron paramagnetic resonance (EPR) characterization, and probe experiments, revealed a dual nonradical pathway dominated by singlet oxygen (¹O₂, 47.0% contribution) and high-valent iron-oxo species (Fe(IV)=O, 42.5% contribution). X-ray photoelectron spectroscopy (XPS) characterization indicated that the carbonyl functional group and Fe(III)-N₄ were the key active sites. Additionally, Electrochemical analysis provided solid evidence for the catalyst-mediated electron transfer processes. This work advances agricultural waste valorization and provides a mechanistic foundation for designing stable and high-efficiency environmental remediation materials.