Cu(II) adsorption by the biochars prepared from three crop straws at 400 degrees C was investigated under acidic conditions. The adsorption increased with increase in pH, i.e. pH from 3.5 to 6.0. The adsorption capacity followed the order: peanut straw char > soybean straw char > canola straw char, while the desorption of pre-adsorbed Cu(II) had a reverse trend. The more negative surface charge on biochars from canola straw led to more electrostatic adsorption of Cu(II) compared to the other two biochars. Fourier transform mid-infrared photoacoustic spectroscopy data revealed that adsorption of Cu(II) caused an apparent shift of the vibrational bands assigned to the carboxyl and phenolic hydroxyl groups along with less negative zeta potential of the biochars that adsorbed Cu(II), which suggested that the Cu(II) was adsorbed specifically through the formation of surface complexes. The Langmuir equation was used to predict adsorption capacity of Cu(II) by the biochars, with maximum predicted adsorption in the ranges 0.58-1.40 and 0.48-0.79 mol/kg at pH 5.0 and 4.5, respectively. The adsorption of Cu(II) by these biochars was greater than that by a commercial activated carbon in the pH ranged from 3.5 to 5.0 with a maximum adsorption of 0.18 mol/kg at pH 5.0. (C) 2011 Elsevier B.V. All rights reserved.