Betts Aaron R, Fischel Matthew H H, Evers Anna, Tappero Ryan, Sparks Donald L
Department of Plant and Soil Science, University of Delaware, 221 Academy St., Newark, DE, 19716, USA.
Sustainable Agricultural Systems Laboratory, USDA-Agricultural Research Service, Beltsville, MD, 20705, USA.
Geochem Trans. 2024 Oct 29;25(1):12. doi: 10.1186/s12932-024-00093-9.
Green rusts (GR) are mixed-valence iron (Fe) hydroxides which form in reducing redox environments like riparian and wetland soils and shallow groundwater. In these environments, silicon (Si) can influence Fe oxides' chemical and physical properties but its role in GR formation and subsequent oxidative transformation have not been studied starting at initial nucleation. Green rust sulfate [GR(SO)] and green rust carbonate [GR(CO)] were both coprecipitated from salts by base titration in increasing % mol Si (0, 1, 10, and 50). The minerals were characterized before and after rapid (24 h) aqueous air-oxidation by x-ray diffraction (XRD), scanning electron microscopy (SEM), Fe extended x-ray absorption fine structure spectroscopy (EXAFS), and N-BET surface area. Results showed that only GR(SO4) or GR(CO3) was formed at every tested Si concentration. Increasing % mol Si caused decreased plate size and increased surface area in GR(CO3) but not GR(SO4). GR plate basal thickness was not changed at any condition indicating a lack of Si interlayering. Air oxidation of GR(SO4) at all % mol Si contents transformed by dissolution and reprecipitation into lepidocrocite and goethite, favoring ferrihydrite with higher % Si content. Air oxidation of GR(CO3) transformed into magnetite and goethite but increasing Si caused GR to oxidize while retaining its hexagonal plate structure via solid-state oxidation. Our results indicate that Si has the potential to cause GR to form in smaller particles and upon air oxidation, Si can either stabilize the plate structure or alter transformation to ferrihydrite.
绿锈（GR）是一种混合价态的氢氧化铁，形成于还原氧化还原环境中，如河岸和湿地土壤以及浅层地下水中。在这些环境中，硅（Si）会影响铁氧化物的化学和物理性质，但其在绿锈形成及后续氧化转化过程中从初始成核开始所起的作用尚未得到研究。通过碱滴定法，从盐中共沉淀出硫酸绿锈[GR(SO)]和碳酸绿锈[GR(CO)]，硅的摩尔百分比逐渐增加（0%、1%、10%和50%）。在快速（24小时）水相空气氧化前后，通过X射线衍射（XRD）、扫描电子显微镜（SEM）、铁扩展X射线吸收精细结构光谱（EXAFS）和N - BET比表面积对这些矿物进行了表征。结果表明，在每个测试的硅浓度下仅形成了GR(SO4)或GR(CO3)。硅摩尔百分比的增加导致GR(CO3)的板尺寸减小和表面积增加，但GR(SO4)没有这种变化。在任何条件下，GR板的基底厚度都没有改变，这表明不存在硅夹层。在所有硅摩尔百分比含量下，GR(SO4)的空气氧化通过溶解和再沉淀转化为纤铁矿和针铁矿，硅含量较高时更有利于形成水铁矿。GR(CO3)的空气氧化转化为磁铁矿和针铁矿，但硅含量增加会导致GR通过固态氧化在保持其六方板结构的同时发生氧化。我们的结果表明，硅有可能使绿锈形成更小的颗粒，并且在空气氧化时，硅既可以稳定板结构，也可以改变向水铁矿的转化。
