Efficient sulfamethoxazole degradation via the Fe(III)-cMnO₂/peroxymonosulfate system: Key role of Fe(III) in colloidal stability and peroxymonosulfate activation

Colloidal manganese dioxide (cMnO₂) is an effective activator of peroxymonosulfate (PMS) for degrading organic micropollutants. However, its performance is often limited by the instability of manganese valence states during PMS activation. To address this, we introduced Fe(III) into the cMnO₂/PMS system, developing a novel and highly efficient Fe(III)-cMnO₂/PMS advanced oxidation process. This system demonstrates exceptional catalytic activity, achieving 90.44 % degradation of sulfamethoxazole (SMX) within 30 mins with a rate constant 2.74 times higher than that of the cMnO₂/PMS system, alongside significant mineralization. Its performance remains robust (>80 % efficiency) across a wide pH range (3.0–7.0). Mechanistic studies reveal that Fe(III) plays a dual synergistic role: it participates in a coupled Mn(IV)/Mn(III)/Mn(II) and Fe(III)/Fe(II) redox cycle that continuously regenerates reactive species (primarily •OH and •SO₄⁻, with contributions from ¹O₂ and high-valent iron), while simultaneously stabilizing the cMnO₂ colloid against deactivation. The system exhibits remarkable resistance to common water matrix components and maintains high efficiency in various real water samples. This work presents a simple yet effective strategy to enhance PMS-based oxidation, offering strong potential for the practical treatment of refractory organic pollutants in complex aquatic environments.