Quantification of the formed environmentally persistent free radicals (EPFRs) in the pyrolyzed sewage sludges from municipal and industrial wastewater treatment

Nitrogen-rich pyrolysis of sewage sludge has a potential benefit being one way of resource utilization. But a new environmental pollutant, environmentally persistent free radicals (EPFRs), can be formed in sludge-based pyrolysis biochar. In this study, EPFRs formed in different kinds of sludge pyrolysis were investigated, and the impact of the controlled pyrolysis conditions on EPFRs was deeply analyzed, including the changing temperatures, residence times, and oxygen content. Results reveal that the F-centered, carbon-centered, as well as oxygen-centered radicals were spotted in the resultant biochar, which was dependent on pyrolysis conditions. All the biochar derived from papermaking sludge pyrolysis contribute a higher spin concentration of EPFRs, compared to other sludge-based biochar. The pyrolysis temperature of 600 °C resulted in the highest spin concentration of 6.5159×1016 spins/g in Municipal sludge. The residence time research showed that most EPFRs were generated in the first hour of pyrolysis treatment. In addition, the increase of a limited oxygen content could promote EPFRs formation in the resulted biochar. The findings concentration on EPFRs formation provide valuable insights for estimating EPFRs generation and associated risk during pyrolyzed sewage sludges, providing an opportunity to control EPFRs production.