Insights into the Mechanisms of How Interaction with Oxyanions Alters the δ-MnO₂ Structure to Drive Ammonium Oxidation

Oxyanions have been widely demonstrated to modulate the MnO2 structure features. However, the role of oxyanions at environmentally relevant concentrations in regulating the structure of MnO2 to control the transformation of ammonium (NH4+) has received little attention. Herein, the mechanisms of the δ-MnO2 structural transformation mediated by oxyanions and the effects of this process on NH4+ oxidation are explored. The results indicate that bicarbonate and phosphate exert a synergistic effect on modulating the structure of δ-MnO2. Specifically, bicarbonate mainly facilitates the dissolution and recrystallization of δ-MnO2 through the formation of MnCO3. In contrast, during the recrystallization process, phosphate can preserve the crystal structure of δ-MnO2. Through the processes of dissolution and recrystallization, δ-MnO2 particles with a higher concentration of Mn(III) and surface hydroxyl groups, more active Mn(II/III/IV) cycling, higher specific surface area, and higher zeta potential are generated. These characteristics potentially enhance the oxidation capacity of NH4+. Mn(III) and surface hydroxyl groups are identified as the dominant active species that drive the oxidation of NH4+, and their replenishment is accelerated by the interfacial interaction between bicarbonate and phosphate. These findings have broad implications for unraveling the transformation processes of contaminants in aqueous environments and the design of MnO2-based decontamination systems.