Environmentally persistent free radicals in biochar derived from Laminaria japonica grown in different habitats

Biomass feedstock plays an important role in the formation of environmentally persistent free radicals (EPFRs) in biochars; however, no investigations have been done on the effects of different plant habitats on the formation of EPFRs, especially for algae. Therefore, Laminaria japonica, a large-scale artificially planted marine economic macroalgae, was collected from six different coastal mariculture zones in tropical, subtropical and north-temperate zones from south to north in China. Biochars were obtained at the pyrolysis temperatures of 200−700 °C, and EPFR signs were recorded by an X-band electron paramagnetic resonance (EPR) spectrometer. Temperatures of 300−500 °C were the appropriate for higher levels of spin concentrations, and lower (200 °C) and higher (600−700 °C) temperatures decreased the biochar-bound EPFRs. The types of EPFRs generally belonged to oxygen-centered radicals for biochars produced at 200 and 300 °C, a mixture of carbon- and oxygen-centered radicals for 400 °C biochar, and carbon-centered radicals for 500−700 °C biochars. Different habitats (growth environments) of L. japonica influenced the levels rather than the types of the resulting biochar-bound EPFRs. Pearson correlation analyses showed that biochar-bound EPFRs were significantly positively correlated with the contents of C (p < 0.01) and weakly correlated with the contents of H, N, O and transition elements (Cu, Fe, Mn, Co, Cr, Ni, and Zn) (p > 0.05). Overall, algae biomass-based biochars have the similar levels and types of EPFRs as lignocellulosic-biomass-based biochars, and the different growing habitats of L. japonica grown influence the formation and characteristics of EPFRs in the resulting biochars.