Breaking kinetic limitations in Fenton-like processes: Freezing- enhanced activation of peracetic acid by ultra-low Mn(II) dosage for micropollutants degradation.

Peracetic acid (PAA)-based Fenton-like systems have attracted considerable attention as promising advanced oxidation processes (AOPs) for water purification. However, the homogeneous activation of PAA by Mn(II), a widely employed transition metal, remains intrinsically inefficient under conventional conditions. In this study, we propose a novel cryogenic strategy that enables highly efficient activation of PAA by Mn(II) even at ultralow dosages. Notably, at neutral pH, the frozen PAA/Mn(II) system exhibited a 146.4-fold enhancement in the pseudo-first-order rate constant for sulfamethoxazole (SMX) degradation relative to that under ambient conditions. Mechanistic investigations revealed that high-valent manganese species predominated as the reactive intermediates under freezing conditions. Furthermore, theoretical calculations demonstrated that the ice-water interface promotes the localized enrichment of reactants while substantially reducing the critical activation energy barriers, thereby synergistically accelerating the Fenton-like reaction kinetics. A first-principles-based kinetic model quantitatively captured the temporal evolution of manganese species, identifying Mn(V) as the dominant reactive species in the frozen system. In addition, bicarbonate ions were found to further enhance SMX removal, and the frozen system exhibited robustness in complex aqueous matrices. Collectively, this work presents a compelling strategy for boosting PAA-based Fenton-like reactivity and establishes a new conceptual framework for water remediation under cold and freezing conditions.