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

•Laminaria japonica from tropical, subtropical and north-temperate mariculture zones.•Pyrolysis temperatures influence both levels and types of the studied radicals.•Habitats influence levels rather than types of the studied radicals.•Significantly positive correlation between the studied radicals and C contents.•Weak or none correlation between the studied radicals and transition metal elements.