The approximate site energy distributions were estimated to describe the adsorption of nickel ions on the heterogeneous surfaces of barley straw adsorbents based on the Langmuir-Freundlich model. The average site energy and standard deviation of the site energy distributions of the adsorbents were determined to analyze the interaction between the adsorbents and adsorbate, and the adsorption site heterogeneity. With higher site energy, the pretreated barley straw (PBS) has higher nickel adsorption capacity (59.5 mg/g) than most of the reported adsorbents. The bonding of nickel on PBS was investigated by X-ray absorption spectroscopies. X-ray absorption near edge structure results indicated that the nickel adsorbed on PBS remained as Ni(II). Extended X-ray absorption fine structure data indicated that the adsorbed Ni was bonded to 6 oxygen atoms of carboxyl groups and/or water molecules. The results and methodology in this work are transferable to investigate other adsorption systems for separation applications.