The mechanisms of biosorption of Cd2+, Cu2+ and Ni2+ by aerobic granules were examined based on a series of batch experiments as well as X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses. Results showed that soluble Cd2+. Cu2+ and Ni2+ ions could be removed by aerobic granules through three mechanisms, i.e., ion exchange, binding to extracellualr polymers and chemical precipitation. It was found that the equivalent ratio of the released light metals, such as Ca2+, Mg2+ and K+, to the adsorbed heavy metals was in the range of 0.71-0.82, which indicated that the ion exchange would be the main mechanism involved in biosorption of Cd2+, Cu2+ and Ni2+ by aerobic granules. The analyses by FTIR and XPS further revealed that chemical functional groups (e.g., alcoholic, carboxylate and ether) on aerobic granules would be the active binding sites for biosorption of Cd2+, Cu2+ and Ni2+ by aerobic granules. (c) 2007 Elsevier B.V. All rights reserved.