Protonated biomass of the seaweed Sargassum muticum was investigated for its ability to remove cadmium(II) from aqueous solutions. In this work, a nonideal, semiempirical, thermodynamically consistent (NICCA) isotherm was proposed to fit the experimental ion binding data obtained in NaNO3 0.05 mol L-1. This model describes the competition between protons and metal ions satisfactorily. Moreover, it reflects the complexity of the macromolecular systems that take part in biosorption considering the heterogeneity of the sorbent. It was demonstrated in this work that the NICCA isotherm constitutes a great improvement with respect to a simpler Langmuir competitive equation, which was not able to describe all the experimental data satisfactorily. Potentiometric acid-base titrations in the absence of cadmium were made to estimate the maximum amount of acid functional groups (2.61 mmol g(-1)) and the conditional proton binding parameters, log (K) over tilde (H) (3.8) and m(H) (0.54). The values of the binding parameters for the cadmium ion were chosen to provide the best simultaneous description of the isotherm at pH 4.5, as well as the dependence of cadmium adsorption on pH. Values of log (K) over tilde (Cd) (3.1), n(Cd) (1.8), and p (0.19) in the case of the NICCA isotherm or log K-Cd (2.94-3.4) for Langmuir competitive models were obtained. Kinetic experiments were performed at two different pH values (3.0 and 4.5), establishing the time dependence that represents the sorption of cadmium with a pseudo-second-order kinetic model. It was observed that 4 h is enough to ensure that the equilibrium uptake was reached. (c) 2005 Elsevier Inc. All rights reserved.