Biomass from activated sludge reactors operating at different dilution rates (0.09, 0,16, and 0.24/h) was used to examine the effect of sludge age on Ni2+ removal characteristics, running batch adsorption tests. The kinetic studies have revealed that sludge grown at all dilution rates, exhibits both active and passive uptake of Ni2+. Although percentages of active and passive uptake changed with the age of activated sludge biomass, passive uptake was the dominant mechanism in Ni2+ sorption. The data obtained were interpreted in the form of adsorption isotherms. At all dilution rates, linear adsorption isotherms were obtained., indicating the presence of equilibrium between biomass and the free nickel species. (C) 2002 Elsevier Science Ltd. All rights reserved.