Schaefer Michael V, Handler Robert M, Scherer Michelle M
Environmental Sciences, University of California, Riverside, CA, 92521, USA.
Sustainable Futures Institute, Michigan Technological University, Houghton, MI, 49931, USA.
Geochem Trans. 2017 Dec 5;18(1):7. doi: 10.1186/s12932-017-0045-0.
Iron (Fe) and manganese (Mn) are the two most common redox-active elements in the Earth's crust and are well known to influence mineral formation and dissolution, trace metal sequestration, and contaminant transformations in soils and sediments. Here, we characterized the reaction of aqueous Fe(II) with pyrolusite (β-MnO) using electron microscopy, X-ray diffraction, aqueous Fe and Mn analyses, and Fe Mössbauer spectroscopy. We reacted pyrolusite solids repeatedly with 3 mM Fe(II) at pH 7.5 to evaluate whether electron transfer occurs and to track the evolving reactivity of the Mn/Fe solids. We used Fe isotopes (56 and 57) in conjunction with Fe Mössbauer spectroscopy to isolate oxidation of Fe(II) by Fe(III) precipitates or pyrolusite. Using these complementary techniques, we determined that Fe(II) is initially oxidized by pyrolusite and that lepidocrocite is the dominant Fe oxidation product. Additional Fe(II) exposures result in an increasing proportion of magnetite on the pyrolusite surface. Over a series of nine 3 mM Fe(II) additions, Fe(II) continued to be oxidized by the Mn/Fe particles suggesting that Mn/Fe phases are not fully passivated and remain redox active even after extensive surface coverage by Fe(III) oxides. Interestingly, the initial Fe(III) oxide precipitates became further reduced as Fe(II) was added and additional Mn was released into solution suggesting that both the Fe oxide coating and underlying Mn phase continue to participate in redox reactions when freshly exposed to Fe(II). Our findings indicate that Fe and Mn chemistry is influenced by sustained reactions of Fe(II) with Mn/Fe oxides.
铁（Fe）和锰（Mn）是地壳中最常见的两种具有氧化还原活性的元素，众所周知，它们会影响土壤和沉积物中的矿物形成与溶解、痕量金属螯合以及污染物转化。在此，我们利用电子显微镜、X射线衍射、水溶液中铁和锰的分析以及铁穆斯堡尔谱对水溶液中的亚铁离子（Fe(II)）与软锰矿（β-MnO）的反应进行了表征。我们在pH值为7.5的条件下，将软锰矿固体与3 mM的Fe(II)反复反应，以评估电子转移是否发生，并追踪锰/铁固体不断变化的反应活性。我们结合使用铁同位素（56和57）以及铁穆斯堡尔谱，以区分Fe(III)沉淀物或软锰矿对Fe(II)的氧化作用。通过这些互补技术，我们确定Fe(II)最初被软锰矿氧化，且纤铁矿是主要的铁氧化产物。额外的Fe(II)暴露导致软锰矿表面磁铁矿的比例增加。在连续九次添加3 mM Fe(II)的过程中，Fe(II)持续被锰/铁颗粒氧化，这表明锰/铁相并未完全钝化，即使在被Fe(III)氧化物广泛覆盖表面后仍保持氧化还原活性。有趣的是，随着Fe(II)的添加，最初的Fe(III)氧化物沉淀物进一步被还原，并且有额外的锰释放到溶液中，这表明当新暴露于Fe(II)时，Fe氧化物涂层和下层的锰相都继续参与氧化还原反应。我们的研究结果表明，铁和锰的化学性质受到Fe(II)与锰/铁氧化物持续反应的影响。
