Modulating the redox cycles of homogenous Fe^(III)/PMS system through constructing electron rich thiomolybdate centres in confined layered double hydroxides

The slow transformation of Fe³⁺ back to Fe²⁺ restrict the practical application of Fe-based catalytic treatment of organic pollutants. Herein, we report a new findings of MgAl-MoS₄ to accelerate the redox cycle of Fe³⁺/Fe²⁺ in Fe³⁺/PMS system. Based on the degradation of 4-chlorophenol (4-CP) profile, the rate constant of Fe³⁺/MgAl-MoS₄/PMS system was 40 to 42 folds higher than MgAl-CO₃/PMS, MgAl-MoS₄/PMS and Fe³⁺/PMS respectively. Moreover, Fe³⁺/MgAl-MoS₄/PMS system was superior both in the degradation efficiency of pollutants as well as in the stoichiometric efficiency of oxidants from the recently reported Fe³⁺/WS₂/PMS, Fe³⁺/MoS₂/PMS or other well-known nano-catalysts systems. This co-catalytic effect of MgAl-MoS₄ on the studied redox metals falls in the order of Fe³⁺ > V⁵⁺ > Cu²⁺ > Mn²⁺~Ag⁺. Furthermore, the Fe³⁺/PMS/MgAl-MoS₄ system shows smooth degradation over a wide pH (3.0–7.0) and complete stabilities in the recycled studies. Additionally, the presence of excessive amounts of inorganic anions or organic matters also did not influence the degradation profile. In mechanism studies, both the unsaturated S²⁻ and Mo⁴⁺ of MgAl-MoS₄ were disclosed to fuel electron continuously during the reduction of Fe³⁺ ions, and thus accelerate the rate limiting step (Fe³⁺/Fe²⁺). Additionally, the layered structure, memory effect and suitable surface charge of LDH material also concentrates the reactants molecules and hence, the boosted effect of MgAl-MoS₄ was associated with the better adsorption of Fe³⁺ ions, the faster PMS decomposition, and the acceleration of Fe³⁺/Fe²⁺ redox cycle. This work indicates a breakthrough in the field of classical homogenous Fe/PMS system, offers the very first report on the role of Mo⁶⁺ and S²⁻ to modulate the redox behaviour of homogenous Fe ions in persulfate based advanced oxidation processes.