MnO 2 Structural Polymorph-Mediated Interaction with Dissolved Organic Matter: Underlying Protection and Transformation Mechanisms

MnO 2 plays an important role in protecting/transforming dissolved organic matter (DOM). However, the diversity of MnO 2 solid-phase speciation challenges the analysis of DOM–mineral interactions. Herein, the adsorption, protection, and transformation of DOM by MnO 2 polymorphs were investigated. Our findings showed that MnO 2 with a large specific surface area adsorbs more DOM; however, the protection of DOM is primarily related to the phase structure of MnO 2 . Compared to the tunnel-structured α- and β-MnO 2, the physical entrapment within the layers provided by δ-MnO 2 is more conducive to maintaining the stability of DOM. Additionally, δ- and α-MnO 2 can generate a large amount of reactive oxygen species (ROS), such as hydrogen peroxide, hydroxyl radical, and superoxide, facilitating the transformation of DOM into low-molecular-weight ( m / z 100–333) lignin or inorganic carbon. By contrast, β-MnO 2 exhibited negligible ROS production, instead oxidizing DOM through Mn 4+ redox to generate midmolecular-weight ( m / z 334–566) lignin, proteins, and lipids, and high-molecular-weight ( m / z 567–800) lignin and lipids. Our results demonstrate that the MnO 2 crystal structure exerts a dominant regulatory influence on DOM protection, whereas the abundance of Mn 4+ and ROS determines the transformation of DOM. These findings provide critical insights into the understanding of DOM fate in the environment.