Graphene oxide (GO) was synthesized and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray diffraction (XRD), and Fourier transformed infrared spectra (FT-IR). The adsorption of Co(II) on GO was studied as a function of contact time, pH, ionic strength, humic acid, Co(II) concentration, and temperature. The adsorption of Co(II) is strongly dependent on the pH values and weakly dependent on ionic strength. The adsorption thermodynamics of Co(II) on GO was carried out at 303, 313, and 333 K to obtain the corresponding thermodynamic data (ΔH⁰, ΔS⁰, and ΔG⁰), which suggested that the adsorption of Co(II) on GO is a spontaneous and endothermic process. GO provided a promising candidate for the preconcentration and solidification of Co(II) from large volumes of aqueous solutions.