Formation of persistent free radicals in sludge biochar by hydrothermal carbonization

The uses of biochar as soil fertilizer can offset global warming and reduce dependence on limited mineral resources in the future circular economy, yet may contain contaminants that ultimately enter food chain. In particular, persistent free radicals are emerging previously detected but underlying mechanisms radical formation not established. Here we studied generation during hydrothermal carbonization waste sludge at 160–220 ºC for 0.5–2 h with solid weight ratios 10%w–40%w using electron paramagnetic resonance Fourier transform infrared spectrometry. Results reveal concentration increases temperature, reaction time, ratio biochars, reaching a content 47.2 × 1015 spins/g 220 ºC, 2 heating, 40%w ratio. Moreover, low temperature about 160 favors production oxygen-centered radicals, whereas higher produces carbon-centered radicals. Our findings imply ecotoxicity should be assessed prior applications to prevent adverse health effects.