Effect of controlled temperature and biomass addition on the formed environmental persistent free radicals (EPFRs) in sewage sludge-based biochar from pyrolysis treatment

Sludge-based biochar derived from pyrolysis treatment is often utilized into soil agricultural fertilizer and soil amendment. While recent studies found that a new class of emerging environmental pollutant, environmental persistent free radicals (EPFRs) could be produced in combustion-based particles and airborne particulate matter. This study was aimed to determine if similar radicals were present in biochar derived from the pyrolyzed organic solid waste. The formation of EPFRs in sludge-based biochar during the pyrolysis treatment were in detailed characterized. EPFRs in biochar derived from sewage sludge pyrolysis at 200–600 ℃ and with various biomass addition were identified by using Electron paramagnetic resonance spectrometer. Three types of EPFRs were mainly spotted in the obtained biochar, including oxygen-centered, carbon-centered and F-centered radicals. Results revealed that the spin concentration of EPFRs decreased with temperature, and an increasing pyrolysis temperature could inhibit the formation of EPFRs. Lower pyrolysis temperature (＜400 ℃) favored the production of carbon-centered radicals, whereas higher temperature (＞400 ℃) mainly produced oxygen-centered radicals. Adding biomass also significantly affected the spin concentration and types of EPFRs. The spin concentration of EPFRs reached the highest value when adding 50% of biomass. The g-values of EPFRs in biochar were all less than 2.0030, indicating that carbon-centered and F-centered free radicals had occupied the dominance. In addition, the determination on the half-lives of EPFRs indicated that carbon-centered EPFRs were almost transformed to oxygen-centered EPFRs with storage time increased. The oxygen-centered free radicals became the dominant type of EPFRs after 100 days and even 200 days storage. The obtained findings imply the ecotoxicity of sludge-based biochar and give in-depth insight on the formation EPFRs, thus guiding the innocuous utilization of sludge.