Chitosan (CHI) is a biopolymer that can be used on complexation and adsorption of heavy metals in water. Chitosan can be chemically functionalized to modulate the pH range of solubility and favoring the complexation and adsorption processes with metal ions. Thus, in this study, it was investigated the synthesis and characterization of carboxymethyl-chitosan (CMC) as well as its application for the complexation and adsorption of Cd(II) and Cr(VI) ions at different pH conditions and compared to pristine chitosan. The properties of the synthesized derivative were extensively characterized by potentiometric titration, Fourier transform infrared spectroscopy (FIR) and ultraviolet-visible (UV-vis) spectroscopy. The complexation and adsorption behaviors of CHI and CMC were assessed using atomic absorption spectrometer (AAS) and zeta potential analysis. The results demonstrated that O-carboxymethylation of chitosan has occurred with a degree of functionalization higher than 50% leading to the formation of CMC soluble in alkaline medium. In addition, the effective incorporation of carboxylic groups in the chitosan chain (CMC) has significantly altered the complexation and adsorption responses towards heavy metal cations (Cd2+) and anions (chromates) as compared to CHI. Therefore, these systems offer an attractive alternative as biosorbents for the removal of heavy metal pollutants from the wastewater. (C) 2015 Elsevier BV. All rights reserved.