A Mn-54 Radiotracer Study of Mn Isotope Solid–Liquid Exchange during Reductive Transformation of Vernadite (δ-MnO₂) by Aqueous Mn(II)

We employed Mn-54 radiotracers to characterize the extent and dynamics of Mn atom exchange between aqueous Mn­(II) and vernadite (δ-Mn­(IV)­O₂) at pH 7.5 under anoxic conditions. Exchange of Mn atoms between the solid and liquid phase is rapid, reaching dynamic equilibrium in 2–4 days. We propose that during the initial stages of reaction, Mn atom exchange occurs through consecutive comproportionation-disproportionation reactions where interfacial electron transfer from adsorbed Mn­(II) to lattice Mn­(IV) generates labile Mn­(III) cations that rapidly disproportionate to reform aqueous Mn­(II) and solid-phase Mn­(IV). Following nucleation of Mn­(III)­OOH phases, additional exchange likely occurs through electron transfer from aqueous Mn­(II) to solid-phase Mn­(III). Our results provide evidence for the fast and extensive production of transient Mn­(III) species at the vernadite surface upon contact of this substrate with dissolved Mn­(II). We further show that HEPES buffer is a reductant of lattice Mn­(IV) in the vernadite structure in our experiments. The methods and results presented here introduce application of Mn-54 tracers as a facile tool to further investigate the formation kinetics of labile Mn­(III) surface species and their impacts on Mn-oxide structure and reactivity over a range of environmentally relevant geochemical conditions.