O-Carboxymethyl chitosan (O-CMC), a nontoxic and biodegradable derivative of the natural polysaccharide-chitosan, was recently found to be a viable alternative for the toxic depressants used in the flotation separation of chalcopyrite and molybdenite. In this work, the adsorption characteristics of O-CMC on molybdenite/chalcopyrite surfaces and the associated interaction mechanisms were investigated by electrokinetic study, infrared spectroscopy, Atomic force microscopy (AFM) imaging, X-ray photoelectron spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The experimental results demonstrated that O-CMC adsorbed on both mineral surfaces. However, the interactions between O-CMC and chalcopyrite are mostly weak physical interactions such as electrostatic interaction, and most of the adsorbed O-CMC molecules can be removed mechanically (e.g., washing) or displaced by xanthate. In comparison, the adsorption of O-CMC on molybdenite is dictated by hydrophobic interaction and electrostatic interaction, and is barely affected by rinsing or xanthate addition. As a result, the strong adsorption of O-CMC on molybdenite over the chalcopyrite lead to the depression of molybdenite and selective separation of two minerals in flotation. In addition, infrared spectroscopy and XPS revealed that no strong chemical interactions were involved during the adsorption O-CMC on molybdenite and chalcopyrite surfaces. (C) 2019 Elsevier Inc. All rights reserved.