A dried Azolla filiculoides (A. filiculoides) biomass was used to remove Basic Orange (BO) dye from aqueous solution using batch experiments. The kinetic results demonstrated that the biosorption of BO onto the dried A. filiculoides was well described by the pseudo-second-order kinetic model. The maximum biosorption capacity for BO was 833.33 mg/g based on the Langmuir equation at a temperature of 303 K, a solution pH of 7.0, a biosorbent dosage of 5 g/L and a contact time of 4 h. From the D-R isotherm equation, mean free energy was calculated to be 8.69 kJ/mol at 303 K, suggesting that the biosorption of BO onto the dried A. filiculoide involved chemisorption. The thermodynamic values, including Delta G, Delta H and Delta S obtained from the calculation, suggest that the biosorption of BO on the dried A. filiculoides biomass is feasible, spontaneous and endothermic under the experimental conditions in this study. Fourier transform infrared (FT-IR) spectroscopy was used to examine functional groups that may participate in the interaction between BO and dried A. filiculoides biomass. Results showed that the amino, carboxyl, and hydroxyl groups may be responsible for the biosorption of BO on the biomass. (C) 2010 Elsevier B.V. All rights reserved.