Activating Fenton-like reaction by hydrochars containing persistent free radicals derived from various pomelo peel components

To reveal the influence of the diversity of precursors on the formation of environmental persistent free radicals (EPFRs), pomelo peel (PP) and its physically divided portion, pomelo cuticle (PC), and white fiber (WF) were used as precursors to prepare six hydrochars: PPH-Fe, PCH-Fe, WFH-Fe, PPH, PCH, and WFH with and without Fe(III) addition during hydrothermal carbonization (HTC). PPH-Fe and WFH-Fe had higher EPFRs content (9.11 × 10¹⁸ and 8.25 × 10¹⁸ spins·g⁻¹) compared to PPH and WFH (3.33 × 10¹⁸ and 2.96 × 10¹⁸ spins·g⁻¹), indicating that iron-doping favored EPFRs formation. However, PCH-Fe had lower EPFRs content (2.78 × 10¹⁸ spins·g⁻¹) than PCH (7.95 × 10¹⁸ spins·g⁻¹), possibly due to excessive iron leading to the consumption of the generated EPFRs. For another reason, the required Fe(III) amount for EPFRs formation might vary among different precursors. PC has a lower concentration of phenolic compounds but 68–97% fatty acids, while WF and PP are rich in cellulose and lignin. In the Fenton-like reaction, oxygen-centered radicals of hydrochar played a significant role in activating H₂O₂ and efficiently degrading bisphenol A (BPA). Mechanisms of reactive oxygen species (ROS) generation in hydrochar/H₂O₂ system were proposed. EPFRs on hydrochar activate H₂O₂ via electron transfer, creating ·OH and ¹O₂, leading to BPA degradation. More importantly, the embedded EPFRs on the hydrochar's inner surface contributed to the prolonged Fenton-like reactivity of PPH-Fe stored for 45 days. This study demonstrates that by optimizing precursor selection and iron doping, hydrochars can be engineered to maximize their EPFRs content and reactivity, providing a cost-effective solution for the degradation of hazardous pollutants.

Graphical abstract