Abiotic manganese oxide formation induced by light and copper-driven Fenton chemistry

Redox-active transition metals such as manganese (Mn) and copper (Cu) frequently coexist in surface water systems, where their interactions can significantly influence contaminant fate and water quality. It is widely known that the oxidation of Mn²⁺(aq) to Mn(IV) oxide solids is driven mainly by biotic processes in which Cu²⁺ serves as an inhibitor of Mn oxidation. However, Cu²⁺ can also trigger Fenton reactions to produce reactive oxygen species (ROS) capable of promoting the abiotic Mn oxidation (i.e., without the involvement of biological organisms), through an alternative natural pathway that remains overlooked in water. In this study, we found that the abiotic oxidation of Mn²⁺ can be facilitated by the co-existence of sunlight and Cu²⁺. By contrasting Cu²⁺ with other divalent cations, such as Ca²⁺, Mg²⁺, and Zn²⁺, this investigation highlights the distinct role of Cu²⁺ in promoting Mn oxidation. Moreover, the study elucidated how light induces ROS to trigger Cu-driven photo-Fenton reactions and identified the roles of ROS in Mn oxidation. Chemical-Fenton reactions (i.e., the interaction between Cu and H₂O₂ in the absence of light) were also examined for their contribution to the Cu-driven Mn photo-oxidation, supporting the involvement of Fenton chemistry in Mn oxidation. By elucidating the novel photochemical mechanism of Mn oxide formation, our findings advance the understanding of heavy metal interactions in aqueous environments and highlight the intricate roles of Cu²⁺ in affecting elemental cycling and the evaluation of heavy metal contamination in water resources. This discovery also underscores the critical contribution of co-existing Fenton-active metals in forming Mn oxide solids.