To effectively enhance the resource utilization of biomass waste and realize waste recycling, a novel acid-modified magnetic biomass spent tea leaves (NiFe2O4/STLs) was prepared via the pyrolysis and co-precipitation methods and employed to eliminate bivalent mercury (Hg(II)) from water bodies. The as-prepared NiFe2O4/STLs exhibited strong magnetic property of 35.36 emu g−1 and could be easily separated from water by applying a magnetic field. The adsorption of Hg(II) over NiFe2O4/STLs was achieved under the optimal conditions of pH = 8 and T = 30 °C, and the maximum adsorption capacity towards Hg(II) was 204.42 mg g−1 according to the response surface methodology optimization. The removal of Hg(II) over NiFe2O4/STLs primarily involved ion exchange, electrostatic attraction and chelation, following the pseudo-second-order kinetic and Langmuir models, and attributed to an exothermic reaction. In addition, the magnetic biomass NiFe2O4/STLs exhibited good regeneration capability and chemical and mechanical stability. The application results revealed that inorganic salt ions, the nitrogen fertilizer urea, humus and other impurities in four different actual water bodies (tap water, river water, lake water and the effluent of sewage treatment plant) had a little impact on the adsorption of Hg(II) over NiFe2O4/STLs. The as-obtained biomass adsorbent of NiFe2O4/STLs has good application value for the removal of the toxic element Hg(II) from water bodies. [ABSTRACT FROM AUTHOR]