Dual-state iron hexacyanoferrates as acid resistance catalysts for peroxymonosulfate activation: mechanistic insights into acetaminophen removal

Heterogeneous peroxymonosulfate (PMS) activation is a promising approach for removing persistent organic micropollutants, but metal leaching under acidic conditions remains a significant challenge. In this study, iron hexacyanoferrate (Fe-HCF) with a mixed-valence state (Fe 2+ and Fe 3+ ) was investigated as an acid-resistant and highly effective catalyst for PMS activation. Fe-HCF achieved 100% removal of 100 mg/L acetaminophen (ACP) within 180 min. During the process, Fe dissolution remained below 0.7 mg/L despite the low pH (~ 3), and the crystalline structure was preserved. Notably, Fe-HCF showed superior ACP removal with minimal leaching compared with FeO, Fe 2 O 3 , and Fe 0 . Mechanistic analysis revealed that ACP degradation proceeded via three routes: (1) PMS activation at Fe 3+ sites to produce 1 O 2 , (2) decomposition of H 2 O 2 (produced from PMS and water) at Fe 3+ sites to form ·O 2 − , and (3) PMS activation at Fe 2+ sites to form ·OH and ·SO 4 − radicals. DFT calculations confirmed that Fe 3+ sites activate PMS more effectively, consistent with the experimental dominance of ·O 2 − (47.17%) and 1 O 2 (42.80%). Consequently, the system achieved enhanced TOC removal (~ 45.98%), high PMS utilization (~ 94.20%), and excellent recyclability. Furthermore, Fe-HCF demonstrated stable performance across pH 3–9 and successfully degraded ACP in real wastewater effluent without inhibition.