Impact of Organic Matter on Iron(II)-Catalyzed Mineral Transformations in Ferrihydrite-Organic Matter Coprecipitates.
Impact of Organic Matter on Iron(II)-Catalyzed Mineral Transformations in Ferrihydrite-Organic Matter Coprecipitates.
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DOI：
10.1021/acs.est.8b03206
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发表时间：
2018-10
影响因子：
11.4
通讯作者：
Laurel K. ThomasArrigo;J. Byrne;A. Kappler;R. Kretzschmar
中科院分区：
文献类型：
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作者：
Laurel K. ThomasArrigo;J. Byrne;A. Kappler;R. Kretzschmar
低结晶的铁（III）（氧合）氧化物如水合铁在土壤和沉积物中大量存在，并且通常以矿物-有机团聚体的形式与有机质（OM）相关联。在缺氧条件下，水相铁（II）和水合铁之间的相互作用导致二次矿物结晶的形成，如绢云母、针铁矿或磁铁矿。然而，铁（II）催化的矿物转化受水合铁伴生OM影响的程度尚不清楚。因此，我们在pH为7的条件下，用0.5-5.0 mM同位素标记的57Fe（II）与水合铁-PGA共沉淀（PGA =聚半乳糖醛酸，C:Fe摩尔比= 0-2.5）和天然富铁有机絮凝体（C:Fe摩尔比= 2.2）进行了5周的反应。结合稳定的Fe同位素示踪剂、PONKCS方法在x射线衍射（XRD）图Rietveld拟合中的新应用以及57Fe Mössbauer光谱，我们试图追踪Fe矿物学的时间演变，并阐明吸附的57Fe（II）的命运。在低C:Fe摩尔比（0-0.05）下，表面吸附的57Fe（II）快速氧化产生富含57Fe的结晶矿物，并在几天内几乎完成矿物转化。随着OM含量的增加，加入的57Fe（II）水溶液与富有机质固体中的Fe之间仍发生原子交换；然而，XRD分析表明，晶体矿物的析出受到了强烈的抑制。对于高om含量的材料（C:Fe≥1.2），Mössbauer光谱显示在最终的Fe（II）反应样品中高达39%的蛭石。由于x射线衍射（XRD）未检测到鳞片石，我们认为Mössbauer-detected鳞片石由具有鳞片石样局部结构的纳米级团簇组成，与天然絮凝体中的鳞片石相似。总的来说，我们的研究结果表明，铁水合物- om共沉淀物的C含量强烈地影响了与水Fe（II）反应过程中Fe矿物转化和铁原子交换的程度和途径。
Poorly crystalline Fe(III) (oxyhydr)oxides like ferrihydrite are abundant in soils and sediments and are often associated with organic matter (OM) in the form of mineral-organic aggregates. Under anoxic conditions, interactions between aqueous Fe(II) and ferrihydrite lead to the formation of crystalline secondary minerals, like lepidocrocite, goethite, or magnetite. However, the extent to which Fe(II)-catalyzed mineral transformations are influenced by ferrihydrite-associated OM is not well understood. We therefore reacted ferrihydrite-PGA coprecipitates (PGA = polygalacturonic acid, C:Fe molar ratios = 0-2.5) and natural Fe-rich organic flocs (C:Fe molar ratio = 2.2) with 0.5-5.0 mM isotopically labeled 57Fe(II) at pH 7 for 5 weeks. Relying on the combination of stable Fe isotope tracers, a novel application of the PONKCS method to Rietveld fitting of X-ray diffraction (XRD) patterns, and 57Fe Mössbauer spectroscopy, we sought to follow the temporal evolution in Fe mineralogy and elucidate the fate of adsorbed 57Fe(II). At low C:Fe molar ratios (0-0.05), rapid oxidation of surface-adsorbed 57Fe(II) resulted in 57Fe-enriched crystalline minerals and nearly complete mineral transformation within days. With increasing OM content, the atom exchange between the added aqueous 57Fe(II) and Fe in the organic-rich solids still occurred; however, XRD analysis showed that crystalline mineral precipitation was strongly inhibited. For high OM-content materials (C:Fe ≥ 1.2), Mössbauer spectroscopy revealed up to 39% lepidocrocite in the final Fe(II)-reacted samples. Because lepidocrocite was not detectable by XRD, we suggest that the Mössbauer-detected lepidocrocite consisted of nanosized clusters with lepidocrocite-like local structure, similar to the lepidocrocite found in natural flocs. Collectively, our results demonstrate that the C content of ferrihydrite-OM coprecipitates strongly impacts the degree and pathways of Fe mineral transformations and iron atom exchange during reactions with aqueous Fe(II).