Resource utilization of tannery sludge to prepare biochar as persulfate activators for highly efficient degradation of tetracycline

•A low-cost carbon-based catalyst (TSBC) was prepared by utilizing tannery sludge.•TSBC showed an excellent performance on persulfate activation to remove tetracycline.•SO4•- and •OH were the main reactive species for TC degradation in TSBC/PS system.•TSBC with transition metals, functional groups and defective edges is beneficial.•The used TSBC could be restored as the new after heat treatment.