Assessing the effect on the generation of environmentally persistent free radicals in hydrothermal carbonization of sewage sludge

Environmentally persistent free radicals (EPFRs) have attracted increasing research interest in recent years. Herein, the generation of EPFRs during the hydrothermal carbonization of sewage sludge (SS) was studied. First, the surface morphology, functional groups, constituent elements and free radicals were characterized for a holistic description of the raw SS and the selected hydrochar obtained from hydrothermal carbonization of SS (SHC). Then, the impact of hydrothermal temperature, residence time and initial pH on the formation of EPFRs was explored in detail through the investigation of g-factors and intensities of EPFRs identified in SHC. The results have shown that the formation of EPFRs was affected by the factors mentioned above, in which the impact of temperature is the greatest. Two types of EPFRs were spotted in the hydrochar, oxygen-centered (O-centered) and carbon-centered (C-centered) EPFRs, which were caught in 120–150 °C and 260–280 °C, respectively. Moreover, the intensities of Electron Paramagnetic Resonance (EPR) signals enhanced with increasing hydrothermal temperature. Whereas, residence time and initial pH only affected the amount of EPFRs in a manner. Additionally, the half-life of the O-centered EPFRs and the C-centered EPFRs was determined as long as 160.45 days and 401.10 days, respectively, indicating that EPFRs are stable in a long time.