Impact of Organic Matter on Iron(II)-Catalyzed Mineral Transformations in Ferrihydrite–Organic Matter Coprecipitates,Environmental Science & Technology

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Impact of Organic Matter on Iron(II)-Catalyzed Mineral Transformations in Ferrihydrite–Organic Matter Coprecipitates
Environmental Science & Technology ( IF 10.8 ) Pub Date : 2018-10-16 , DOI: 10.1021/acs.est.8b03206
Laurel K. ThomasArrigo ₁ , James M. Byrne ₂ , Andreas Kappler ₂ , Ruben Kretzschmar ₁

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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).

中文翻译：

有机物对铁（II）催化的水铁矿-有机物共沉淀物中矿物转化的影响

结晶度较弱的Fe（III）（羟基）氧化物如亚铁酸盐在土壤和沉积物中含量很高，并且经常以矿物-有机聚集体的形式与有机物（OM）结合。在缺氧条件下，Fe（II）水溶液与亚铁酸盐之间的相互作用导致形成结晶性次生矿物，如纤铁矿，针铁矿或磁铁矿。但是，人们还不太了解Fe（II）催化的矿物转化受亚铁水合物相关OM的影响。因此，我们使水铁矿-PGA共沉淀物（PGA =聚半乳糖醛酸，C：Fe摩尔比= 0–2.5）和天然富铁有机絮凝物（C：Fe摩尔比= 2.2）与0.5–5.0 mM同位素标记⁵⁷Fe（II）在pH 7下持续5周。依靠稳定的Fe同位素示踪剂的组合，PONKCS方法在X射线衍射（XRD）图的Rietveld拟合和⁵⁷ FeMössbauer光谱学上的新应用，我们试图追踪Fe矿物学的时间演变并阐明其命运吸附⁵⁷ Fe（II）。在低的C：Fe摩尔比（0-0.05）下，表面吸附的⁵⁷ Fe（II）的快速氧化导致⁵⁷ Fe富集的结晶矿物，并在几天内几乎完成了矿物转化。随着OM含量的增加，添加的水溶液⁵⁷之间的原子交换富含有机物的固体中仍存在Fe（II）和Fe；然而，XRD分析表明，晶体矿物沉淀被强烈抑制。对于高OM含量的材料（C：Fe≥1.2），穆斯堡尔光谱法显示最终Fe（II）反应的样品中高达39％的纤铁矿。由于XRD无法检测到纤铁矿，因此我们建议由Mössbauer检测到的纤铁矿由具有类似于纤铁矿的局部结构的纳米簇组成，类似于在天然絮凝物中发现的纤铁矿。总的来说，我们的结果表明，水铁矿-OM中的C含量会在与Fe（II）水溶液反应期间强烈影响Fe矿物转化和铁原子交换的程度和途径。

更新日期：2018-10-16

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