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

Oxyanions are known to influence the structural properties of MnO2; however, their long-term impacts on MnO2 structural transformation and the consequent regulation of its oxidative performance toward phenolic contaminants remain poorly understood. Here, we investigate how two representative oxyanions, bicarbonate (HCO3–) and phosphate (HPO42–), regulate the structural evolution of δ-MnO2 and its oxidative capacity toward bisphenol A (BPA) and phenol. Three δ-MnO2 materials exposing dominant (001), (144), and (211) facets were synthesized. Among them, δ-MnO2-144 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 HPO42– and HCO3– exert contrasting regulatory effects on δ-MnO2 reactivity. HPO42– enhances phenolic oxidation by promoting Mn(III) and lattice oxygen accumulation and sustaining Mn(III)/Mn(IV) redox cycling. HCO3– 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-MnO2 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 MnO2-based oxidation processes for water treatment.