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

MnO₂ plays an important role in protecting/transforming dissolved organic matter (DOM). However, the diversity of MnO₂ solid-phase speciation challenges the analysis of DOM-mineral interactions. Herein, the adsorption, protection, and transformation of DOM by MnO₂ polymorphs were investigated. Our findings showed that MnO₂ with a large specific surface area adsorbs more DOM; however, the protection of DOM is primarily related to the phase structure of MnO₂. Compared to the tunnel-structured α- and β-MnO₂, the physical entrapment within the layers provided by δ-MnO₂ is more conducive to maintaining the stability of DOM. Additionally, δ- and α-MnO₂ 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 (<i>m</i>/<i>z</i> 100-333) lignin or inorganic carbon. By contrast, β-MnO₂ exhibited negligible ROS production, instead oxidizing DOM through Mn⁴⁺ redox to generate midmolecular-weight (<i>m</i>/<i>z</i> 334-566) lignin, proteins, and lipids, and high-molecular-weight (<i>m</i>/<i>z</i> 567-800) lignin and lipids. Our results demonstrate that the MnO₂ crystal structure exerts a dominant regulatory influence on DOM protection, whereas the abundance of Mn⁴⁺ and ROS determines the transformation of DOM. These findings provide critical insights into the understanding of DOM fate in the environment.