Mechanistic elucidation and tuning pathway of environmentally persistent free radicals during sewage sludge pyrolysis

Pyrolysis is a promising technology for the treatment of vast amounts of sewage sludge due to its lower emission of secondary pollutants compared with that of combustion and landfilling. However, the generation of environmentally persistent free radicals (EPFRs, emerging pollutants) due to the high temperature and oxygen deficiency of the pyrolytic atmosphere is a difficult issue. In this study, we systematically investigated the pyrolytic panorama and correlated the pyrolytic parameters to EPFR formation using electron spin resonance (ESR) spectrometry and in situ thermogravimetry–Fourier transform infrared–mass spectrometry (TG–FTIR–MS). The results showed that the EPFR concentration in biochar increased from 8.2×1018 spins g−1 at 300 °C to 16.3 ×1018 spins g−1 at 400 °C but decreased to 0.6 ×1018 spins g−1 at 600 °C, indicating that the majority of EPFRs decomposed at 600 °C during pyrolysis. Notably, the N species and ash in the sludge were also found to reduce the formation of EPFRs during sludge pyrolysis. Aromatic compounds, including carboxyl, hydroxyl, and aldehyde groups, are the main precursors of EPFRs based on the correlation of EPFR concentrations and intermediates determined via TG–FTIR–MS. The formation pathway of EPFRs in sludge char involves three processes, namely, the adsorption of aromatic compounds, elimination of water, and electron transfer from aromatic compounds to char. The proposed formation mechanism of EFPRs during sludge pyrolysis not only broadens the understanding of the pollutant formation during sludge pyrolysis but also provides a strategy for managing emerging pollutants during pyrolysis.