Effects of pH and metal ions on oxytetracycline sorption to maize-straw-derived biochar

► Oxytetracycline (OTC) sorbed to biochar according to pseudo-second-order kinetics. ► OTC sorption increased with increasing pH but decreased at high pH values. ► Zwitterions are the most sorbed oxytetracycline species on biochar. ► Cu2+ strongly enhances the sorption of OTC to biochar through metal bridging. ► π-π interactions and metal bridging were the main OTC-biochar sorption mechanisms.