The interacting mechanisms of metallic cations (Zn2+ and Co2+) to active chemical groups oil the extracellular polymeric Substances (EPS) of the aerobic granules, including loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS), were examined by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy. For Zn2+ and Co2+, LB-EPS showed stronger binding properties than TB-EPS and the process of them was described well by the Langmuir isotherm. Compared to the single-metal system, binary-metal addition induced competitive binding between the Zn2+ and Co2+ with reduction of the maximal binding capacity for both EPS. The main chemical groups involved in the interactions between contaminants were apparently alcohol, carboxyl and amino, These groups were part of the EPS structural polymers, namely, polysaccharides, proteins, and hydrocarbon-like products. When biosorption and flocculation Occurred at the same time, the LB-EPS were used not only as chelate sorbents but also as flocculants to further enhance their sorption capacity. (C) 2009 Elsevier Inc. All rights reserved.