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Graphical abstract Abstract As promising sorbent, nanofibers with high specific surface area and adsorption site loading show excellent adsorption capacity. Polyacrylonitrile (PAN) is an industrial by-product with abundant nitrile groups (C N). However, raw polyacrylonitrile shows almost no binding ability toward metal ions. Herein, aminated polyacrylonitrile (APAN) nanofibers are synthesized by electrospinning followed by grafting branched polyethyleneimine (bPEI). The microstructures of these nanofibers show a uniform fiber diameter of 595 nm and a high surface area of 2.81 m2 g−1. In addition, the > N / NH group loading is as high as 5.7 wt.%. Fourier transform infrared spectroscopy and elemental analysis results confirm the formation of covalent bonds (C(O) NH) between PAN and bPEI, resulting in a stable structure and hence excellent reproducibility. And the original adsorption capacity can be maintained by 61.6% after 5 cycles. The influence of solution pH, adsorption isotherm, initial concentration, and contact time on Cu2+ adsorption is systematically investigated. The maximum adsorption capacity of APAN nanofibers is 149.8 mg g-1, which is difficult to achieve by most low density adsorbents. Furthermore, sorption-isotherm experiments reveal that the adsorption behavior obeys Langmuir model, and the adsorption kinetic agrees second-order dynamics model. Cu2+ ions are chelated with > N / NH groups in five-membered rings. Therefore, these APAN nanofibers open up delighted opportunities for the efficient removal of Cu2+ from industrial wastewater. [ABSTRACT FROM AUTHOR]