54Mn Radiotracers Demonstrate Continuous Dissolution and Reprecipitation of Vernadite (δ-MnO2) during Interaction with Aqueous Mn(II)

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• 作者：Evert J. Elzinga
• 刊名：Environmental Science & Technology
• 出版年：2016
• 出版时间：August 16, 2016
• 年：2016
• 卷：50
• 期：16
• 页码：8670-8677
• 全文大小：416K
• 年卷期：0
• ISSN：1520-5851

文摘

⁵⁴Mn radiotracers were used to assess Mn atom exchange between aqueous Mn(II) and vernadite (δ-MnO₂) at pH 5.0. Continuous solid–liquid redistribution of ⁵⁴Mn atoms occurred, and systems are near isotopic equilibrium after reaction for 3 months. Despite this extensive exchange, X-ray diffraction and X-ray absorption spectroscopy data showed no major changes in vernadite bulk mineralogy. These results demonstrate that the vernadite–Mn(II) interface is dynamic, with the substrate undergoing continuous dissolution and reprecipitation mediated by aqueous Mn(II) without observable impacts on its mineralogy. Interfacial redox reactions between adsorbed Mn(II) and solid-phase Mn(IV) and Mn(III) are proposed as the main drivers of this process. Interaction between aqueous Mn(II) and structural Mn(III) likely involves interfacial electron transfer coupled with Mn atom exchange. The exchange of aqueous Mn(II) and solid-phase Mn(IV) is more complex and is proposed to result from coupled interfacial comproportionation–disproportionation reactions, where electron transfer from adsorbed Mn(II) to lattice Mn(IV) produces transient Mn(III) species that disproportionate to regenerate aqueous Mn(II) and structural Mn(IV). These findings provide further evidence of the importance of Mn(II)_(aq)–MnO_2(s) interactions and the attendant production of transient Mn(III) intermediates to the geochemical functioning of phyllomanganates in environments undergoing Mn redox cycling.