Wine processing waste sludge (WPWS) has been shown to be an effective sorbent for sorption of some heavy metals (i.e., lead and chromium), but the sorption mechanism of heavy metal by WPWS has remained obscure. The objective of this study was to explore the sorption mechanism of WPWS for heavy metals using nickel (Ni) as the sorbate. The WPWS has been characterized with wet chemistry, infrared (IR), X-ray diffraction, scanning electron microscopy (SEM), carbon13 magic-angle nuclear magnetic resonance (C-13 NMR), and energy-dispersive spectrometry chemical analyses. The sludge contained high organic carbon (40.5%), nitrogen (23.4%), and cation-exchange capacity (1218 cmol(c) kg(-1)).SEM investigation of WPWS showed porous-like activated carbon. IR analysis of WPWS revealed that R-NH2 and R-COOH were the major functional groups. The proportion of organic functional groups in WPWS quantified by 13 C NMR analysis was in the following order: alkyl-C > carboxyl-C > N-alkyl-C > aromatic-C > O-alkyl-C > acetal-C > phenolic-C. The parameters affecting the sorption of Ni were the pH, initial concentration of Ni, particle size of WPWS, and reaction temperature. The WPWS sorption isotherms of Ni are only well described by the Langmuir sorption isotherm. A pseudo-second-order sorption kinetic model describes successfully the kinetics of sorption of Ni onto WPWS at different operation parameters (i.e., pH, initial Ni concentration, and particle size). Under the steady-state reaction conditions, the Gibb free energy (DeltaGdegrees) ranges from -18.969 to -23.616 kJ mol(-1), and DeltaHdegrees and DeltaSdegrees are 3.366 kJ mol(-1) and 6.056 J mol(-1) K-1, respectively, indicating that higher temperature favors spontaneous reaction for Ni sorption by WPWS. According to the thermodynamic sorption parameters under steady-state conditions, this sorption is a spontaneous and endothermic reaction. The sorption mechanisms include physical adsorption and chemical complexation. Amino and carboxyl groups are the prominent functional groups interacting with Ni. The sorption capacity is influenced by several parameters such as the pH, initial Ni concentration, temperature, and particle size of WPWS.