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

Manganese oxide (MnOx) plays crucial roles in shaping various environmental and geochemistry processes, with their reactivity largely dependent on the structure of MnOx. Tunnel MnOx 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 MnOx with electron transfer remain unclear. In this study, we delved into the transformation of layer-structured MnOx 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 MnOx 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 MnOx, and prolonged incubation resulted in the deeper embedding of Tl and the formation of atomic clusters. The embedding of Tl inside of the tunnel MnOx 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 MnOx, carbon, and trace elements.

氧化锰（MnOx）在各种环境和地球化学过程中起着至关重要的作用，其反应活性在很大程度上取决于MnOx的结构。隧道状MnOx能在其隧道结构中有效地容纳大量土壤元素，对元素周转和相关地球化学过程具有重要的控制作用，然而关于伴随电子转移的MnOx从层状到隧道状转变的确切决定因素仍不清楚。在本研究中，我们深入研究了层状结构的MnOx在与共存的土壤氧化还原组分（具有不同氧化还原活性的热解碳和铊）相互作用过程中的转变。我们的研究结果表明，在6周的短培养期内，从层状到隧道状结构的转变仅在还原性热解碳和氧化性Tl(III)同时存在时发生，而不是在仅有单一还原剂/氧化剂的情况下发生。热解碳所创造的宏观还原环境以及与Tl(III)螯合相关的微观氧化环境对锰的循环价态变化至关重要，导致了Mn(III)的产生以及Mn结构中的空位，这是从层状到隧道状转变的前提条件。通过Tl - O - Mn键将氧化性Tl(III)锚定在MnOx的表面或隧道结构内部是在整体还原条件下构建微观氧化环境的关键。在转变过程中，铊被整合到高结晶度MnOx的隧道中，并且延长培养时间会导致铊更深地嵌入以及原子簇的形成。铊嵌入隧道状MnOx内部导致其溶解度和生物可利用性降低，通过还原溶解用土壤有机酸仅能提取出0.05 - 0.26 mg Kg⁻¹，通过体外基于生理学的提取试验仅能提取出8.7 - 8.9%。本研究强调了电子供体和电子受体组分在触发与MnOx、碳和微量元素相关的相互关联的地球化学过程中的重要作用。