Layer-to-tunnel manganese oxides transformation triggered by pyrogenic carbon and trace metals: Key role of reducing and oxidizing components cooperation

Manganese oxide (MnO_x) plays crucial roles in shaping various environmental and geochemistry processes, with their reactivity largely dependent on the structure of MnO_x. Tunnel MnO_x effectively hosts a substantial quantity of soil elements within its tunnel structure, exerting significant control over element turnover and pertinent geochemical processes, while the precise determinants regarding the layer-to-tunnel transformation of MnO_x with electron transfer remain unclear. In this study, we delved into the transformation of layer-structured MnO_x during the interaction with coexisting soil redox components (pyrogenic carbon and Tl with differing redox reactivity). Our findings revealed that the transformation from layer to tunnel structure only occurred in the presence of reductive pyrogenic carbon and oxidative Tl(III) rather than sole reductants/oxidants within a short incubation period of 6 weeks. The macro reducing environment created by the pyrogenic carbon and the micro oxidizing environment related to the Tl(III) chelation was pivotal in the cyclic valence change of Mn, resulting in the generation of Mn(III) and vacancies in the Mn structure, the prerequisite for the layer-to-tunnel transformation. Anchoring of oxidative Tl(III) on the surface or inside the tunnel structure of MnO_x through Tl–O–Mn bonding was the key to building a micro oxidative environment under bulk-reducing conditions. During the transformation, Tl was integrated into the tunnel of high-crystallinity MnO_x, and prolonged incubation resulted in the deeper embedding of Tl and the formation of atomic clusters. The embedding of Tl inside of the tunnel MnO_x led to lower solubility and bioaccessibility, with only 0.05–0.26 mg Kg⁻¹ being extracted with soil organic acids through reductive dissolution and 8.7–8.9 % by in vitro physiologically based extraction test. This study underscores the significant role of electron-donating and electron-accepting components in triggering interconnected geochemical processes with MnO_x, carbon, and trace elements.