Iron minerals are the most important arsenic host in As-contaminated deltaic sediments. Arsenic release from Fe minerals to groundwater exposes millions of people worldwide to a severe health threat. To understand the coupling of Fe mineralogy with As (im)mobilization dynamics, we analyzed the geochemistry and mineralogy of a 46 m long sediment core drilled into the redox transition zone where a high As Holocene aquifer is juxtaposed to a low As Pleistocene aquifer in the Red River delta, Vietnam. We specifically concentrated on mm- to cm-scale redox interfaces within the sandy aquifer. Various Fe phases, such as Fe- and Mn- bearing carbonates, pyrite, magnetite, hematite and Fe-hydroxides (goethite, lepidocrocite) with distinct As concentrations were identified by a combination of high-resolution microscopic, magnetic and spectroscopic methods. The concentration of As and its redox species in the different Fe-minerals were quantified by microprobe analysis and synchrotron X-ray absorption. We developed a conceptual model integrating Fe-mineral transformations and related As (im)mobilization across the redox interfaces. Accordingly, As is first mobilized via the methanogenic dissolution of Fe(III) (oxyhydr)oxide mineral coatings on sand grains when reducing groundwater from the Holocene aquifer intruded into the Pleistocene sands. This stage is followed by the formation of secondary Fe(II)-containing precipitates (mainly Fe- and Mn-bearing carbonates with relatively low As < 70 μg/g), and minor pyrite (with high As up to 5800 μg/g). Due to small-scale changing redox conditions these Fe(II) minerals dissolve again and the oxidative behavior of residual Fe(III)-phases in contact with the reducing water leads to the formation of abundant Fe(III)/Fe(II) (oxyhydr)oxides especially at the studied redox interfaces. Microcrystalline coatings and cementations of goethite, magnetite and hematite have intermediate to high As sorption capacity (As up to 270 μg/g) creating a key sorbent responsible for As (im)mobilization at interfingering redox fronts. Our observations suggest a dynamic system at the redox interfaces with coupled redox reactions of abiotic and biotic origin on a mm to cm-scale. In a final stage, further reduction creates magnetite with low As sorption capacity as important secondary Fe-mineral remaining in reduced gray Pleistocene aquifer sands while considerable Fe and As is released into the groundwater. The presented redox-dependent sequence of Fe phases at redox interfaces provides new insights of their role in As (im)mobilization in reducing aquifers of south and southeast Asia.

铁矿物质是砷污染的三角洲沉积物中最重要的砷宿主。从铁矿物质释放到地下水中的砷使全世界数以百万计的人面临严重的健康威胁。为了了解Fe矿物学与As（不动）迁移动力学的耦合关系，我们分析了钻入氧化还原过渡带的46 m长沉积物岩心的地球化学和矿物学，在该区域中，高As全新世含水层与低As As更新世含水层并置。越南红河三角洲。我们专门研究了砂质含水层中毫米级到厘米级的氧化还原界面。通过高分辨率的显微，磁性和光谱方法的组合，鉴定出了各种Fe相，例如含Fe和Mn的碳酸盐，黄铁矿，磁铁矿，赤铁矿和Fe的氢氧化物（针铁矿，纤铁矿）。通过微探针分析和同步加速器X射线吸收对不同Fe矿物中As及其氧化还原物质的浓度进行了定量。我们开发了一个概念模型，该模型整合了Fe-矿物质转化和相关的跨氧化还原界面的As（固定）迁移。因此，当减少从全新世含水层侵入更新世砂岩中的地下水时，首先通过砂粒上的Fe（III）（羟基）氧化物矿物覆膜的甲烷化溶解而动员了As。在此阶段之后，形成次要的含Fe（II）的沉淀物（主要是As <70μg/ g相对较低的含Fe和Mn的碳酸盐）和次黄铁矿（As最高可达5800μg/ g） 。由于小范围的氧化还原条件变化，这些Fe（II）矿物再次溶解，与还原水接触的残留Fe（III）相的氧化行为导致形成大量Fe（III）/ Fe（II）（羟基氧化物），尤其是在研究的氧化还原界面处。针铁矿，磁铁矿和赤铁矿的微晶涂层和胶结物具有中等到较高的As吸附能力（高达270μg/ g），从而形成了关键的吸附剂，可在指间氧化还原前沿固定As（不固定）。我们的观察结果表明，氧化还原界面处的动态系统与非生物和生物来源的耦合氧化还原反应之间的距离为毫米到厘米。在最后阶段，进一步还原会产生低As吸附能力的磁铁矿，这是重要的次生Fe矿物残留在还原的灰色更新世含水层砂中，同时大量的Fe和As释放到地下水中。在氧化还原界面上呈现的铁相的依赖于氧化还原的顺序提供了新的见解，说明了它们在减少南亚和东南亚含水层中的砷（不）动员中的作用。