The catalytic effect of AQDS as an electron shuttle on Mn(II) oxidation to birnessite on ferrihydrite at circumneutral pH

Birnessite (δ-MnO ₂ ) is the most common manganese (Mn) oxide mineral in soils, sediments, and ocean manganese nodules, and it significantly affects the speciation and mobility of trace metals and organic pollutants. Abiotic oxidation of Mn(II) by dissolved O ₂ is an important birnessite formation pathway, however, it has rarely been reported at neutral pH due to its very slow oxidation kinetics. Anthraquinone-2, 6-disulfonate (AQDS) is an important electron shuttle in biotic systems and might induce birnessite formation by promoting abiotic Mn(II) oxidation. Herein, the effects of AQDS concentration and types of mineral surfaces on 24 mM Mn(II) oxidation were explored at pH 7.0 using macroscopic and spectroscopic analyses. In the absence of AQDS, birnessite cannot form through the abiotic oxidation of 24 mM Mn(II) unless pH ≥ 8.5. In contrast, birnessite rapidly forms at pH 7.0 in the presence of AQDS, which acts as a catalyst. The catalytic effect of AQDS first increases and then decreases with increasing concentration, and as a “shuttle” the concentration almost remains constant during Mn(II) oxidation. Additionally, in the absence of ferrihydrite or in the presence of montmorillonite, which is an analogous insulating mineral, AQDS shows a weak catalytic effect on Mn(II) oxidation; thus, no birnessite forms. The mechanisms of Mn(II) oxidation promoted by AQDS and ferrihydrite can be described as AQDS acting as an electronic carrier with semiconductor ferrihydrite as a specific channel facilitating electron transfer between Mn(II) and O ₂ on its surface. This study provides new evidence that AQDS can transfer electrons in abiotic systems as in biotic systems, leading to efficient Mn(II) oxidation and birnessite formation through an abiotic pathway at circumneutral pH in various geological settings.