Powdered waste sludge (PWS) was used for removal of copper(II) ions from aqueous solution by biosorption. Waste activated sludge obtained from a paint industry wastewater treatment plant was pre-treated with 1% hydrogen peroxide solution dried, ground and sieved to a number of fractions with different particle sizes. Biosorption capacity of pre-treated PWS for Cu(II) ion removal was investigated as function of particle sizes (external surface area), copper(II) and PWS concentrations and also pH using batch biosorption experiments. Biosorbed copper ion concentrations increased with increasing specific external surface area of PWS, initial C2+ concentration and pH, but decreased with increasing PWS concentration. Percent copper removal from the solution also increased with increasing external surface area or decreasing particle size of the adsorbent, PWS concentration and pH, but decreased with increasing initial copper ion concentration. The maximum biosorption capacity was 156 mg Cu2+ g PWS-1 when PWS concentration was 0.25 g l(-1) with an average particle size of 64 mu m. The initial volumetric rate of biosorption also increased with increasing specific external surface area of PWS resulting in nearly 70 mg Cu l(-1) h(-1) at a specific external surface area of 943 cm(2) g(-1) with the smallest particle size (53 mu m) tested. Optimal pH resulting in maximum biosorption was 5 with a minimum zeta potential (-15.6 mV) on the surface of the PWS particles. (c) 2005 Elsevier Ltd. All rights reserved.