Enhanced biodegradation of PAHs by biochar and a TiO2@biochar composite under light irradiation: Photocatalytic mechanism, toxicity evaluation and ecological response

•Excessive BC and TiO2@BC inhibited the growth and energy metabolism of strain LH-1.•OH and ·O2– played an important role in bacterial damage.•Appropriate amounts of BC and TiO2@BC enhanced the biodegradation of PAHs.•Appropriate amounts of BC and TiO2@BC increased the microbial diversity in the SBR.•Appropriate amounts of BC and TiO2@BC promoted the C and N cycles in the SBR system.