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+/MgAlMoS4/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+similar to 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.

Automated summary

This study demonstrates that MgAl-MoS4 can accelerate the redox cycle of Fe3+/Fe2+, enhancing the degradation efficiency of organic pollutants. The Fe3+/MgAl-MoS4/PMS system shows higher rate constant and stoichiometric efficiency compared to other systems. The co-catalytic effect of MgAl-MoS4 is superior in various metal ions, pH ranges, environmental conditions, and impurities, showing stable performance.