The present paper aims to study the effectiveness of an abundant marine waste named Posidonia oceanica (P. oceanica) on ammonium removal from aqueous solutions under different experimental conditions. The results of this study have shown that ammonium biosorption onto P. oceanica fibers increases with initial ammonium concentrations and pH increase. Biosorption isothermal data were well fitted by Langmuir model suggesting a monolayer and uniform adsorption process. The kinetic modeling of the experimental data indicated that they were well described by a pseudo-second-order model predicting a chemisorption process. The scanning electron microscopy (SEM), the energy dispersive spectroscopy (EDS) and the Fourier transform Infra-red spectroscopy (FTIR) analysis before and after biosorption of ammonium onto P. oceanica revealed that the main involved mechanisms are both surface ion exchange with magnesium and calcium cations and surface or intraparticle sharing between ammonium and functional groups, such as O-H, N-H, C=O and C-H stretching, and N-H bending. (C) 2010 Elsevier B.V. All rights reserved.