pH-resilient Cu/Si₃N₄ catalyst for efficient heterogeneous Fenton mineralization of methylene blue

Heterogeneous Fenton oxidation is an effective strategy for the removal of organic contaminants from water; however, its practical application is often hindered by narrow pH operating windows and catalyst instability. In this work, metal-supported silicon nitride catalysts (M/Si₃N₄, M = Cu, Fe, Co, Zn) were investigated for the mineralization of Methylene Blue (MB) over a wide pH range (3-8). A novel descriptor, the Heterogeneous Fenton Potential (HFP), is proposed as a simple tool for the preliminary screening of catalytic systems, enabling a qualitative evaluation of their ability to generate reactive oxidizing species from H₂O₂. Among the investigated catalysts, Cu/Si₃N₄ showed superior and pH-resilient performance, achieving nearly complete MB degradation within 5 min and up to 75% Total Organic Carbon (TOC) removal at near-neutral pH (5-6). In contrast, Fe-, Co-, and Zn-based systems exhibited a significant decrease in activity at pH ≥ 6. The enhanced performance of Cu/Si₃N₄ is attributed to the high dispersion of Cu species on the Si₃N₄ support, which facilitates efficient H₂O₂ activation and rapid generation of reactive radicals. These results highlight Cu/Si₃N₄ as a promising catalyst for water treatment applications, combining wide pH operability, high mineralization efficiency, and potential reusability. The proposed approach provides a promising pathway to mitigate the intrinsic pH limitations of conventional Fenton processes, supporting their further development for wastewater treatment.