Contrasting Roles of Phosphate and Bicarbonate in Regulating δ-MnO 2 Structure and Phenolic Oxidation

Oxyanions are known to influence the structural properties of MnO 2; however, their long-term impacts on MnO 2 structural transformation and the consequent regulation of its oxidative performance toward phenolic contaminants remain poorly understood. Here, we investigate how two representative oxyanions, bicarbonate (HCO 3 – ) and phosphate (HPO 4 2– ), regulate the structural evolution of δ-MnO 2 and its oxidative capacity toward bisphenol A (BPA) and phenol. Three δ-MnO 2 materials exposing dominant (001), (144), and (211) facets were synthesized. Among them, δ-MnO 2 -144 exhibited the highest oxidative activity, attributable to its enriched Mn(III), low average Mn oxidation state, large electrochemically active surface area, and structural stability. Incubation experiments reveal that HPO 4 2– and HCO 3 – exert contrasting regulatory effects on δ-MnO 2 reactivity. HPO 4 2– enhances phenolic oxidation by promoting Mn(III) and lattice oxygen accumulation and sustaining Mn(III)/Mn(IV) redox cycling. HCO 3 – suppresses oxidative reactivity by reducing Mn(III) availability, impeding interfacial electron transfer, and inducing phase transformation toward less reactive manganese minerals. Overall, this study demonstrates that the oxidative performance of δ-MnO 2 is a dynamic property governed by oxyanion-mediated structural and redox evolution. These findings improve our ability to predict the environmental fate of phenolic contaminants and to design MnO 2 -based oxidation processes for water treatment.