Environ Sci Technol. 2015 Apr 7;49(7):4310-6. doi: 10.1021/acs.est.5b00022. Epub 2015 Mar 19.
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.
我们采用 Mn-54 放射性示踪剂来描述在缺氧条件下 pH 值为 7.5 时，水溶液中的 Mn(II) 和水钠锰矿（δ-Mn(IV)O2）之间 Mn 原子交换的程度和动力学。固液相间 Mn 原子的交换非常迅速，在 2-4 天内达到动态平衡。我们提出，在反应的初始阶段，Mn 原子的交换通过连续的歧化-复分解反应发生，其中吸附的 Mn(II) 向晶格 Mn(IV) 的界面电子转移生成不稳定的 Mn(III)阳离子，这些阳离子迅速歧化，重新形成水溶液中的 Mn(II)和固相 Mn(IV)。在 Mn(III)OOH 相成核后，通过水溶液中的 Mn(II)向固相 Mn(III)的电子转移，可能会发生额外的交换。我们的结果为在这种底物与溶解的 Mn(II)接触时，水钠锰矿表面快速且广泛地生成瞬态 Mn(III)物种提供了证据。我们进一步表明，在我们的实验中，HEPES 缓冲液是水钠锰矿结构中晶格 Mn(IV)的还原剂。这里提出的方法和结果介绍了 Mn-54 示踪剂的应用，作为一种简便的工具来进一步研究在一系列环境相关地球化学条件下，不稳定的 Mn(III)表面物种的形成动力学及其对 Mn-氧化物结构和反应性的影响。
