<p>Coal fly ash is generated in large quantities by coal-fired power plants and is commonly disposed of as waste despite containing reactive mineral phases. This study investigates the valorization of coal fly ash into a magnetic Fe₃O₄-decorated composite for Cu(II) removal from aqueous systems, linking waste reutilization with the development of low-cost and magnetically recoverable materials for water treatment. The composite was synthesized via a coprecipitation–thermal treatment route, promoting the deposition of iron oxide on chemically activated fly ash. The physicochemical properties of the resulting material were characterized using X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and N₂ adsorption–desorption (BET) analysis. Cu(II) concentrations before and after adsorption were quantified by atomic absorption spectroscopy (AAS). Batch adsorption experiments were performed under fixed experimental conditions to evaluate the influence of composite composition and contact time on Cu(II) removal. The Fe₃O₄-decorated fly ash composite exhibited enhanced magnetic recoverability and improved Cu(II) removal compared with non-magnetic fly ash, enabling efficient solid–liquid separation. Under the selected experimental conditions, approximately 80% Cu(II) removal was achieved, yielding an adsorption capacity of ~0.32 mg g⁻¹ at an initial Cu(II) concentration of 4 mg L⁻¹ and a contact time of 120 min. Kinetic analysis indicated that Cu(II) uptake followed pseudo-second-order behavior, suggesting chemisorption-controlled adsorption. The developed material is positioned as a low-cost, sustainable, and magnetically recoverable adsorbent for coal fly ash valorization rather than as a high-capacity adsorbent. The findings highlight the role of chemical activation and magnetic modification in tailoring fly ash-based composites and provide a basis for further performance optimization.</p>