Tailoring peroxyacetic acid(PAA) activation by sewage sludge derived atomic-Fe clusters/Fe-N₄ catalyst via thermally...

Fe nanoclusters/FeN₄ units embedded in graphitized carbon derived from biomass are highly efficient catalysts. However, simple physical mixing of precursors during pyrolysis tends to cause Fe to agglomerate into large nanoparticles. In this study, we introduce a novel peroxyacetic acid (PAA) conditioning strategy to transform sewage sludge (SS) into an enhanced Fe single-atom catalyst. This strategy modulates the evolution of Fe active sites by promoting the formation of adjacent Fe atomic clusters through thermal treatment. During sludge conditioning, PAA/Fe²⁺ triggers the dissolution and breakdown of SS, exposing nitrogen (N) and oxygen (O) atoms that bind with iron, thereby creating Fe immobilization sites. Characterization results show that conditioning promotes the formation of highly dispersed, few-atom Fe clusters/Fe-N₄ sites (FeN₄-Fe_NCP@SBC) at elevated temperatures, with Fe content exceeding 2.34 %. In contrast, untreated samples easily form Fe nanoparticles. The FeN₄-Fe_NCP@SBC can be used as superior Fenton-like catalyst in PAA-triggered antibiotic degradation. Singlet oxygen (¹O₂) plays a dominant role in degradation, as demonstrated by scavenging and ESR analysis. O₂ and HO are identified as important intermediates in the generation of ¹O₂ and are recognized as key species in FeN₄-Fe_NCP@SBC-initiated PAA activation. The atomic-Fe cluster induced shift of the Fe center from low-spin (t_2g⁶ e_g⁰) to medium-spin (t_2g⁵ e_g¹) facilitates partial occupation of the dz² orbital, forming a σ* bond with OH. This promotes H being lost from OH to form O, and subsequent direct desorption of O can generate ¹O₂. The study provides a method to create SS catalysts with single atoms and Fe clusters for PAA and antibiotic degradation.