Effect of pyrolysis conditions on environmentally persistent free radicals (EPFRs) in biochar from co-pyrolysis of urea and cellulose

Biochar derived from nitrogen-rich pyrolysis of biomass can be used as a soil conditioner, but it contains a large amount of environmental persistent free radicals (EPFRs). EPFRs are a newly identified environmentally harmful substance, and the detection and research on EPFRs in nitrogen-rich pyrolyzed char is lacking. Biochars prepared from cellulose-urea mixtures at different temperatures, residence times, and urea ratios were analyzed in this study. EPFRs in biochar prepared at 500 °C had the highest spin concentrations. Substituted aromatic compounds were the precursors to the EPFRs. The types of EPFRs in biochars shifted from oxygen-centered at 400 °C to carbon- and oxygen-centered in the 450–600 °C range due to a reduction in oxygen-containing functional groups. Residence time experiments showed that most EPFRs formed in the first 5 min of pyrolysis. C was the main element used for the formation of EPFRs, while N content was negatively correlated with the concentration of EPFRs. Pyrolysis temperature was the key factor determining the types of EPFRs produced, while proportion of urea only affected the concentrations of EPFRs and not type. The results of this study are of great significance for understanding the environmental behavior of common EPFRs in nitrogen-rich biochar.