Freezing-Induced Manganese Vacancies in Birnessite Boost Bisphenol A Degradation

Birnessite (δ-MnO2) is a redox-active mineral that mediates the abiotic oxidation of organic contaminants, yet how freezing alters its structure and reactivity remains unclear. Here, we show that freezing induces the formation of Mn vacancies (VMn) in δ-MnO2 and markedly enhances its oxidative reactivity. δ-MnO2 after freezing exhibited a 9.5-fold higher oxidation rate toward bisphenol A (BPA) than pristine δ-MnO2, and the reactivity increased further with freeze-thaw cycles. During freezing, δ-MnO2 particles concentrate in liquid-like regions between ice crystals, creating conditions favorable for defect formation. In situ XAS reveals that freezing increases the average Mn oxidation state while decreasing local Mn coordination, with ex situ XAS, XPS, Raman, and EPR analyses further supporting VMn formation. Accordingly, VMn formation decreases the proportion of Mn(III) and increases that of Mn(IV), which strengthens Mn-O bonding and enhances the oxidative capacity of δ-MnO2. Electrochemical analysis and theoretical calculations further show that VMn tunes the electronic structure of δ-MnO2 and promotes interfacial electron transfer. These findings offer new insight into freezing as a previously overlooked driver of manganese oxide reactivity, with implications for contaminant transformation and natural self-purification in cold regions.