Formation mechanism of persistent free radicals during pyrolysis of Fenton-conditioned sewage sludge: Influence of NOM and iron

•Fenton conditioning facilitated the formation of persistent free radicals (PFRs).•Achieved the yield of PFRs with the total number spins of 9.533×1019 spins/g DS.•Pyrolysis cleavage of the EPS fractions produced carbon-centered PFRs.•Maillard reaction provided abundant precursors for the formation of PFRs.•Phenoxyl or semiquinone-type PFRs were formed by iron ions with aromatic protein. The present study provided an innovative insight into the formation mechanism of persistent free radicals (PFRs) during the pyrolysis of Fenton-conditioned sludge. Fenton conditioners simultaneously improve the dewatering performance of sewage sludge and catalyze the pyrolysis of sewage sludge for the formation of PFRs. In this process, PFRs with a total number of spins of 9.533×1019 spins/g DS could be generated by pyrolysis of Fenton-conditioned sludge at 400°C. The direct thermal decomposition of natural organic matter (NOM) fractions contributed to the formation of carbon-centered radicals, while the Maillard reaction produced phenols precursors. Additionally, the reaction between aromatic proteins and iron played a crucial role in the formation of phenoxyl or semiquinone-type radicals. Kinetics analysis using discrete distributed activation energy model (DAEM) demonstrated that the average activation energy for pyrolysis was reduced from 178.28 kJ/mol for raw sludge to 164.53 KJ/mol for Fenton conditioned sludge. The reaction factor (fi) indicated that the primary reaction in Fenton-conditioned sludge comprised of 27 parallel first-order reactions, resulting from pyrolysis cleavage of the NOM fractions, the Maillard reaction, and iron catalysis. These findings are significant for understanding the formation process of PFRs from NOM in Fenton-conditioned sludge and provide valuable insight for controlling PFRs formation in practical applications. The proposed pathways for the dissociation of EPS to generate PFRs. (T• Transient radical, PO• Oxygen-centered persistent free radicals, PC• Carbon-centered persistent free radicals, PCO• Carbon-centered neighboring oxygen atoms persistent free radicals, MRP Maillard reaction products) [Display omitted]