Interfacial Carbon-Shell Engineering in Polyethylene Terephthalate-Derived FeNi@C Nanostructures for Adsorption-Dominated Crystal Violet Removal and Surface-Mediated Transformation

Polyethylene terephthalate (PET) waste was converted into magnetically recoverable FeNi@C core-shell nanostructures through a molten-salt-assisted route combining hydrogen electrochemical reduction of Fe/Ni oxides with PET-derived carbonization. The synthesis atmosphere was used as an interfacial design parameter to regulate the carbon shell, producing FeNi@C-Air and FeNi@C-Ar with different concentration of accessible interfacial active sites and surface chemistry. Intermittent air exposure generated a more open and porous graphitized carbon shell with greater interfacial active sites around a Ni-rich FeNi 3 core and interfacial Fe 4 C 0.63 phase, leading to superior crystal violet (CV) removal and rapid magnetic separation (M s = 22.14 emu g -1 ). FeNi@C-Air achieved 98.73% CV removal within 120 min for 100 mg L -1 CV, with an experimental capacity of 141.04 mg g -1 . Kinetic analyses indicate a multistep interfacial process involving rapid surface capture, pore diffusion, electrostatic and non-electrostatic interactions, and oxygen-containing/defective carbon sites. LC-MS and TOC analyses confirm adsorption-dominated removal with partial transformation into smaller soluble intermediates rather than complete mineralization. H 2 O 2 addition produced only a modest additional TOC decrease, supporting a limited surface-mediated Fenton-like contribution. In natural Hun River water, FeNi@C-Air achieved 94.90% CV removal by adsorption alone and 98.95% in the presence of H 2 O 2 . The PET-derived carbon shell substantially suppressed Fe and Ni leaching compared with unsupported FeNi, while enabling magnetic recovery and reuse. This work demonstrates a circular and interface-controlled strategy for transforming waste PET into magnetic carbon-based adsorbents with strong dye-removal performance, partial transformation functionality, improved leaching resistance and applicability.