Display omitted •Hydrothermal treatment created good precursor for production of biochar.•Pore formation and enlarged surface area of biochar were obtained with ZnCl2.•The biochar showed superior tetracycline adsorption capacity (673.0 mg·g−1). In this study, sunflower seed husk biochar prepared by ZnCl2-activated and hydrothermal carbonization (HZSF) was studied for its effectiveness in removing tetracycline (TC) from an aqueous solution. The physical and chemical properties of materials were characterized by different methods of surface analysis. The specific surface area of HZSF is significantly enhanced over 1200 times compared with non-modified biochar (HZSF: 1578.3 m2·g−1, SF-700: 1.3 m2·g−1), which has an enhancement effect on the TC adsorption capacity. The HZSF showed that the Langmuir isotherm and pseudo-second-order kinetic models could properly characterize the adsorption processes. In the Langmuir isotherm model, HZSF exhibited effective adsorption performance with qmax of 673.0 mg·g−1 at 298 K for 24 h. The possible mechanisms for the adsorption process were the monolayer, chemical adsorption, and the participation of strong intermolecular forces. In general, HZSF has the potential to be a useful adsorbent for the elimination of antibiotics from water-based solutions.