Plasma- and chemical-grafted amidoxime/carbon nanofiber hybrids (p-AO/CNFs and c-AO/CNFs) were utilized to remove U-238(VI) and Am-241(III) from aqueous solutions, seawater, and groundwater. Characteristic results indicated more nitrogen containing groups in p-AO/CNFs compared to c-AO/CNFs. The maximum adsorption capacities of p-AO/CNFs at pH 3.5 and T = 293 K (588.24 mg of U-238(VI)/g and 40.79 mg of Am-241(III)/g from aqueous solutions, respectively) were significantly higher than those of c-AO/CNFs (263.18 and 22.77 mg/g for U-238(VI) and Am-241(III), respectively), which indicated that plasma-grafting was a highly effective, low-cost, and environmentally friendly method. Adsorption of U-238(VI) on AO/CNFs from aqueous solutions was significantly higher than that of U-238(VI) from seawater and groundwater; moreover, AO/CNFs displayed the highest effective selectivity for U-238(VI) compared to the other radionuclides. Adsorption of U-238(VI) onto AO/CNFs created inner-sphere complexes (e.g., U-C shells) as shown by X-ray absorption fine structure analysis, which was supported by surface complexation modeling. Three inner-sphere complexes gave excellent fits to pH-edge and isothermal adsorption of U-238(VI) on the AO/CNFs. These observations are crucial for the utilization of plasma-grafted, AO-based composites in the preconcentration and immobilization of lanthanides and actinides in environmental remediation.