Bioavailability assessment of hexachlorobenzene in soil as affected by wheat straw biochar

► Effects of biochar on sorption and bioavailability of HCB were investigated. ► Sorption of HCB by biochar was 42 times higher than soil. ► Biochar reduced the dissipation, volatilization and bioavailability of HCB in soil. ► HPCD extraction correlated better with earthworm bioassay than butanol extraction. ► Biochar could be used in immobilizing POPs in contaminated sites.