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Abstract 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

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. Article Highlights: Characteristics and mechanisms for the generation and consumption of EPFRs were proposed. To favor g3-type EPFRs formation, the quantity of Fe(III) and aromatic compounds of the biomass should be matched. The g3-type EPFRs on hydrochars played a major role in the Fenton-like reaction. The external EPFRs of hydrochars were consumed easily while the internal EPFRs persisted during long-term storage. [ABSTRACT FROM AUTHOR]