Shrinking Core model, widely accepted in the solid-fluid non-catalytic reaction system was applied to analyze the adsorption of metal ions like Zn(II), Cr(VI), Cd(II) and Pb(II) from the aqueous solutions onto the clarified sludge (CLS). The diffusivities of different metal ions in clarified sludge were determined by global optimization. The depth of penetration was also estimated for different initial concentrations. The experimental concentration profiles at various initial concentrations for all these four different metal ions matched well with the model predictions. © 2009 Springer Science+Business Media, LLC.