Humic acid changes effect of naturally occurring oxidants on the environmental transformation of molybdenum disulfide...

2H-phase molybdenum disulfide (2H-MoS₂) has been considered to be a chemically stable two-dimensional (2D) nanomaterial. Nonetheless, the persistence of 2H-MoS₂ in the presence of environmental redox-active matrices, such as naturally occurring oxidants (e.g., manganese dioxide (MnO₂)) and natural organic matter (NOM), remains largely unknown. Herein, we examined the interplay between 2H-MoS₂, MnO₂ (a common natural oxidant), and NOM species (i.e., Aldrich humic acid (ALHA) and Suwannee River natural organic matter (SRNOM)). The results show that MnO₂ accelerates the oxidative dissolution of 2H-MoS₂, regardless of the presence of dissolved oxygen. The effect of NOM on the MnO₂-induced fate of 2H-MoS₂ was found to depend on its affinity for 2H-MoS₂ and the functionality of NOM. ALHA preferentially adsorbed on hydrophobic 2H-MoS₂ nanosheets due to the enrichment of reductive polycyclic aromatics and polyphenolic constituents. The preferential ALHA adsorption counteracted the MnO₂-triggered oxidative transformation of 2H-MoS₂, as revealed by the cathodic response of 2H-MoS₂ (i.e., decreased the open circuit potential by 0.0338 V) and the emergence of reductive Mo‒C bonds at 228.8 and 231.9 eV upon the addition of ALHA. This work evaluated the persistence of 2H-MoS₂, illustrating its susceptibility to decomposition by naturally occurring oxidants and the influence of NOM on it. These findings are crucial for revealing the fate and transport of MoS₂ in aquatic environments and provide guidelines for related applications in natural or engineered systems for MoS₂ and potentially other 2D materials.