Surface-modified magnetic Ni2.33Fe alloy nanoparticles were prepared using a hydrothermal method. Thermogravimetric analysis (TG) and Fourier transform infrared spectroscopy (FTIR) tests demonstrated that the surface was successfully modified by sodium citrate. The surface-modified particles can be used for removing nickel ions from aqueous solution. The adsorption kinetics studies were performed and the pseudo-second-order kinetic model successfully described the kinetic data. The diffusion model indicated the adsorption was regulated by both surface and intraparticle diffusion processes. The Freundlich and Langmuir adsorption models were adopted for the mathematical description of adsorption equilibrium, and it was found that the experimental data fitted very well to the Freundlich model.