Due to a high solubilization of organic matter, classical sorption models were not able to correctly describe the equilibrium of heavy metals uptake by activated sludge after disintegration processes (e.g. ultrasonic irradiation). A new semi-mechanistic model deriving from the simple Freundlich equation is developed in this study to describe metal interactions within activated sludge previously subjected to ultrasonic treatment: the equilibrium between both aqueous (complexation with soluble/colloidal organic matter released during the treatment) and particulate (sorption on sludge flocs, precipitation) metal fractions is modeled. The results of biosorption and precipitation tests were described by the new model. This allowed a better understanding of the influence of activated sludge properties (surface functional groups, structure, chemical composition) on the fate of both copper and cadmium in sonicated activated sludge. (C) 2010 Elsevier B.V. All rights reserved.