The accumulation of waste plastic has caused serious environmental pollution. A key scientific challenge lies in efficiently converting high‑carbon-content plastics into functional carbon materials while avoiding the use of corrosive reagents and minimizing catalyst waste. In this study, we propose an integrated in-situ catalytic strategy to directly carbonize polypropylene (PP) plastic into FeS@carbon nanoflowers using ferrocene and sulfur as a dual catalyst system. During pyrolysis, these components react to form FeS nanoparticles, which serve not only as a catalytic byproduct but also as a Faradaic material. Embedded within the carbon material, FeS significantly enhances the charge storage capability of the material, thereby improving ion removal performance. This strategy eliminates the need for acid leaching and achieves full utilization of the catalyst, avoiding secondary pollution and material waste. The resulting FeS@carbon nanoflowers exhibit excellent capacitive deionization (CDI) performance. In a 500 mg L−1 NaCl solution at 1.2 V, the electrode reaches a high salt adsorption capacity (SAC) of 64.47 ± 0.97 mg g−1, with fast ion removal kinetics and good cycling stability. Moreover, in-situ electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) measurements clarify the mechanism of Cl− capture in the FeS@carbon nanoflowers. This study demonstrates a sustainable catalytic carbonization pathway where catalyst-derived byproducts are functionally reintegrated, achieving the direct conversion of plastic waste into high-value functional materials for water treatment.