Azolla rongpong, a fresh water macro alga, was tested for its ability to remove acid dyes (acid red 88 (AR88), acid green 3 (AG3), acid orange 7 (AO7) and acid blue 15 (AB15)) separately from aqueous solution. The sorption isotherms, obtained at different ranges of pH (2-3.5) and temperature (25-35 degrees C), were fitted using Langmuir, Freundlich, Redlich-Peterson and Sips models. The maximum dye uptake of 83.33 mg/g was obtained for AG3 at optimum conditions of pH (2.5) and temperature (30 degrees C) according to Langmuir isotherm model. Various thermodynamic parameters such as Delta G(circle), Delta H-circle and Delta S-circle were calculated indicating that the present system was spontaneous and endothermic process. The pseudo-first and -second order kinetic models were also applied to the experimental kinetic data obtained during biosorption of four acid dyes and high correlation coefficients favor pseudo-second order model for the present systems. Since A. rongpong performed very well in the case of AG3 biosorption compared to other dyes, AG3 was selected as the model dye for examining the potential of A. rongpong in continuous biosorption. A glass column (2 cm i.d. and 35 cm height) was used to conduct continuous experiments. At 25 cm (bed height), 5 mL/min (flow rate) and 100 mg/L (initial dye concentration), A. rongpong exhibited AG3 uptake of 84.87 mg/g. The experimental data were analyzed using Bed Depth Service Time and Thomas models and the model parameters were evaluated. (c) 2006 Elsevier B.V. All rights reserved.