Valorization of copper smelting slag into Fe3O4@zeolite for peroxymonosulfate activation and methylene blue degradation: A combined performance, pathway mechanism and life cycle assessment

The massive accumulation of copper smelting slag (CSS) poses severe environmental and ecological concerns, while efficient peroxymonosulfate (PMS) activators for wastewater remediation remain costly and susceptible to metal leaching. Herein, a “waste-to-resource” strategy was developed to convert industrial CSS into Fe3O4@zeolite composite catalyst via a facile alkali fusion-hydrothermal method. The catalytic performance was systematically evaluated under varying parameters. The catalytic performance was systematically evaluated for the degradation of methylene blue (MB) under varying operational parameters. Furthermore, radical quenching experiments, LC-MS analysis, and a prospective “cradle-to-treatment” life cycle assessment (LCA) were integrated to elucidate the degradation mechanism, scalability, and environmental sustainability of the evaluated system. In the optimal Fe3O4@zeolite/PMS system (3.0 g/L catalyst, 1.0 mmol/L PMS, initial pH 8.0, 25 °C), 98.5% MB degradation was achieved within 60 min, accompanied by 65.6% total organic carbon (TOC) removal, indicating substantial mineralization. The catalyst maintained >90% efficiency across a broad pH range (pH 6.0–14.0) and exhibited robust tolerance to common anions. Reusability tests confirmed >90% activity retention over five consecutive runs with negligible metal leaching. Radical quenching experiments and EPR spectroscopy revealed a dual-pathway oxidation mechanism involving both radicals (SO4●-, •OH, O2●-) and singlet oxygen (1O2), with 1O2-mediated non-radical oxidation serving as the dominant pathway. LC-MS identified nine degradation intermediates, allowing the delineation of a four-stage mineralization pathway involving N-demethylation, ring cleavage, and final oxidation to inorganic species. Prospective LCA demonstrated that transitioning from laboratory to industrial synthesis reduces the overall environmental burden by >98.5%, shifting the primary hotspot to alkaline reagents (NaOH). Crucially, utilizing zero-burden CSS avoids the severe ecological penalties of conventional silicon/aluminum precursors, while 5-cycle magnetic recovery curtails the wastewater treatment environmental impact to 25%–33% of the single-use baseline.