The necessity to eliminate nickel (Ni) from wastewater stems from its environmental and health hazards. To enhance the Ni adsorption capacity, this research applied a copper sulfate–ammonia complex (tetraamminecopper (II) sulfate monohydrate, [Cu(NH₃)₄]SO₄·H₂O) as a modifying agent for a Phragmites australis-based activated carbon preparation. The physiochemical properties of powdered activated carbon (PAC) and a modified form ([Cu(NH₃)₄]-PAC) were examined by measuring their surface areas, analyzing their elemental composition, and using Boehm’s titration method. Batch experiments were conducted to investigate the impact of various factors, such as Ni(II) concentration, contact time, pH, and ionic strength, on its substance adsorption capabilities. Additionally, the adsorption mechanisms of Ni(II) onto activated carbon were elucidated via Fourier-transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The findings indicated that modified activated carbon ([Cu(NH₃)₄]-PAC) exhibited a lower surface area and total volume than the original activated carbon (PAC). The modification of PAC enhanced its surface’s relative oxygen and nitrogen content, indicating the incorporation of functional groups containing these elements. Furthermore, the modified activated carbon, [Cu(NH₃)₄]-PAC, exhibited superior adsorption capacity relative to unmodified PAC. Both adsorbents’ adsorption behaviors conformed to the Langmuir model and the pseudo-second-order kinetics model. The Ni(II) removal efficiency of PAC and [Cu(NH₃)₄]-PAC diminished progressively with rising ionic strength. Modified activated carbon [Cu(NH₃)₄]-PAC demonstrated notable pH buffering and adaptability. The adsorption mechanism for Ni(II) on activated carbon involves surface complexation, cation exchange, and electrostatic interaction. This research presents a cost-efficient preparation technique for preparing activated carbon with enhanced Ni(II) removal capabilities from wastewater and elucidates its underlying adsorption mechanisms.