This investigation emphasized the optimum preparation conditions for the chemical modification of eucalyptus sawdust biochar as a potential biosorbent to promote the removal of nitroimidazoles from aqueous solutions. By ANOVA for central composite design of response surface methodology, we found the activation temperature and impregnation ratio of 85% H3PO4 to sawdust as the significant factors for maximizing the adsorption capacity of metronidazole. Optimum activation temperature, impregnation ratio, and activation time were 500 degrees C, 0.62, and 90 min, respectively. The Brunauer-Emmett-Teller surface area and total pore volume increased from 32.80 m(2) g(-1) to 882.04 m(2) g(-1) and 0.01829 cm(3) g(-1) to 0.4316 cm(3) g(-1) after H3PO4 activation, respectively. The removal efficiencies using biochar prepared at optimum conditions were 97.1% and 96.4% for metronidazole and dimetridazole at 20 mg L-1 concentration and 1.0 g L-1 biochar dosage within 2 h. The adsorption process was described well by pseudo-second-order model, and solute transfer was controlled by both boundary-layer and intraparticle diffusion, Thermodynamic study revealed that the adsorption process was spontaneous and exothermic. The Freundlich model presented better a fit for the adsorption equilibrium. The maximum adsorption capacities for metronidazole and dimetridazole were 167.5 and 200 mg g(-1), respectively. (C) 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.