Regulating nitrogen speciation via temperature/Fe2O3 synergy in freshwater sludge for enhanced peracetic acid (PAA) activation and emerging contaminants removal

Abstract

Rational regulation of transition metal sites and surface N species in catalysts is crucial for boosting peracetic acid (PAA) activation and its application in degrading emerging contaminants in water. This study presents a simple strategy for regulating surface Fe(II/III) sites and N speciation in freshwater sludge-derived biochar (FBC) by modulating pyrolysis temperature. As the temperature increased from 300 °C to 900 °C, Fe(III)-driven conversion of pyridinic N to graphitic N increased significantly (6.07 % to 45.00 %), concurrently elevating Fe(II) content. Both synergistically enhanced PAA activation. FBC700 outperformed FBC900 in PAA activation, attributed to its significantly higher specific surface area (65.93 m²/g vs. 22.03 m²/g) despite comparable graphitic N (43.19 % vs. 45.00 %) and Fe(II) content (15.86 % vs. 16.68 %). The dynamic evolution of reactive oxygen species was elucidated: Initial PAA adsorption on FBC700 and activation by electron-rich functional groups and ≡Fe(II) generated CH₃C(O)O^• and ^•OH, attacking PAA. Simultaneously, ≡Fe(III) reduction by PAA produced CH₃C(O)OO^•, while CH₃C(O)O^• decomposed into CH₃^• or reacted to form CH₃C(O)OO^•, ultimately yielding O₂^•−. In addition, the direct electron transfer pathway also contributes to BPA degradation by generated PAA* on FBC700 surface. FBC700/PAA shows great practical potential, evidenced by wide adaptability in water bodies and excellent regeneration. This study provides a novel and facile method for regulating active sites for PAA activation and reutilizing bulk freshwater sludge.