Waste sludge samples obtained from a paint industry wastewater C treatment plant was dried, ground and pre-treated with 1% H2O2 to improve the biosorption capacity. The powdered waste sludge (PWS) was sieved to different mesh sizes and used for biosorption of zinc(II) ions from aqueous solution. Kinetics of zinc biosorption onto PWS was investigated by using the PWS samples with particle size of 64 mu m. The pseudo-first and -second order rate expressions were used to correlate the experimental data. The kinetic constants were determined for both models and the second order rate expression was found to be more suitable. Three different biosorption isotherms were used to correlate the equilibrium biosorption data and the isotherm constants were determined. The Langmuir isotherm was found to fit the experimental data better than the other isotherms tested. Maximum biosorption capacity was found to be 82 mg Zn g PAS(-1). High biosorption capacity and low saturation constant of pre-treated PWS used for zinc ion biosorption makes the adsorbent more attractive as compared to other alternatives. (c) 2005 Elsevier Inc. All rights reserved.