Mg(H₂O)₆²⁺ as an Activator of PMS: A Transition-Metal-Free Advanced Oxidation Process in Neutral Solutions

The development of sustainable and energy-efficient advanced oxidation processes (AOPs) is significant for the effective removal of persistent organic contaminants from aquatic environments, particularly under neutral pH conditions where many conventional catalytic systems exhibit poor performance. Traditional Fenton and Fenton-like AOPs generally rely on transition-metal ions or metal complexes and require strongly acidic conditions for efficient activation of oxidants. These limitations often lead to secondary pollution through metal sludge formation, catalyst instability, and increased operational costs, thereby restricting their large-scale environmental applications. In this study, we report for the first time that Mg(H₂O)₆²⁺, a non-redox-active, earth-abundant, low-toxicity, and environmentally benign cation, can effectively activate peroxymonosulfate (PMS) to establish a truly transition-metal-free oxidation system. The Mg²⁺/PMS process demonstrates remarkable catalytic activity under near-neutral pH conditions and follows first-order reaction kinetics with respect to both Mg(H₂O)₆²⁺ and PMS concentrations. Rapid and efficient degradation was achieved for a wide range of organic pollutants, including rhodamine B (RhB), acetaminophen (ACP), and phenol (Ph), highlighting the broad applicability of the system. Detailed mechanistic investigations, supported by density functional theory (DFT) calculations, reveal that Mg(H₂O)₆²⁺ acts as a strong Lewis acid that facilitates PMS activation through both radical and non-radical pathways, despite the absence of conventional redox-active transition metals. The generated reactive oxygen species significantly enhance pollutant oxidation and mineralization efficiency. This unprecedented demonstration of Mg²⁺-activated PMS introduces a green, scalable, and sustainable oxidation strategy with substantial potential for future wastewater treatment, environmental remediation, and the development of next-generation metal-free catalytic AOP technologies.