Biochars produced by pyrolysis of hardwood at 450 degrees C (HW450) and corn straw at 600 degrees C (CS600) were characterized and investigated as adsorbents for the removal of Cu(II) and Zn(II) from aqueous solution. The adsorption data were well described by a Langmuir isotherm, with maximum Cu(II) and Zn(II) adsorption capacities of 12.52 and 11.0 mg/g for CS600, 6.79 and 4.54 mg/g for HW450, respectively. Thermodynamic analysis suggested that the adsorption was an endothermic process and did not occur spontaneously. Although Cu(II) adsorption was only marginally affected by Zn(II). Cu(II) competed with Zn(II) for binding sites at Cu(II) and Zn(II) concentrations >= 1.0 mM. Results from this study indicated that plant-residue or agricultural waste derived biochar can act as effective surface sorbent, but their ability to treat mixed waste streams needs to be carefully evaluated on an individual basis. (C) 2011 Elsevier Ltd. All rights reserved.