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

Abstract

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.