The escalating challenges of plastic waste accumulation and antibiotic contamination in aquatic environments demand innovative solutions. This study presents a sustainable strategy for upcycling waste polypropylene (PP) into an iron-functionalized catalyst (PP-SO3Fe) via a facile, liquid-assisted mechanochemical method. Under optimized conditions (PP : Fe(III) mass ratio of 5 : 1, 400 rpm, 6 h), the synthesized catalyst effectively activates peroxymonosulfate (PMS), achieving rapid 98.7% degradation of tetracycline (TC) within 7 min. Systematic mechanistic investigations, including radical quenching tests and electron paramagnetic resonance analysis, verified that the degradation is governed by non-radical pathways, primarily involving singlet oxygen (1O2) and high-valent iron–oxo species (FeIVO). The system exhibits favorable stability with 92.2% efficiency retention after 10 cycles and maintains superior performance across a wide pH range and in complex water matrices containing various interfering ions. Notably, HCO3− and NH4+ ions were found to positively enhance the degradation process. This work presents a sustainable strategy for plastic waste valorization while providing an effective solution for antibiotic removal in practical water treatment applications.