GFZ German Research Center for Geosciences, Telegrafenberg, 14473 Potsdam, Germany.
Department of Earth Sciences, Freie Universität Berlin, 12249 Berlin, Germany.
Environ Sci Technol. 2020 Mar 17;54(6):3297-3305. doi: 10.1021/acs.est.9b07092. Epub 2020 Mar 4.
"Green rust" (GR), a redox-active Fe(II)-Fe(III) layered double hydroxide, is a potential environmentally relevant mineral substrate for arsenic (As) sequestration in reduced, subsurface environments. GR phases have high As uptake capacities at circum-neutral pH conditions, but the exact interaction mechanism between the GR phases and As species is still poorly understood. Here, we documented the bonding and interaction mechanisms between GR sulfate and As species [As(III) and As(V)] under anoxic and circum-neutral pH conditions through scanning transmission electron microscopy (STEM) coupled with energy-dispersive X-ray (EDX) spectroscopy and combined it with synchrotron-based X-ray total scattering, pair distribution function (PDF) analysis, and As K-edge X-ray absorption spectroscopy (XAS). Our highly spatially resolved STEM-EDX data revealed that the preferred adsorption sites of both As(III) and As(V) are at GR crystal edges. Combining this data with differential PDF and XAS allowed us to conclude that As adsorption occurs primarily as bidentate binuclear (C) inner-sphere surface complexes. In the As(III)-reacted GR sulfate, no secondary Fe-As phases were observed. However, authigenic parasymplesite (ferrous arsenate nanophase), exhibiting a threadlike morphology, formed in the As(V)-reacted GR sulfate and acts as an additional immobilization pathway for As(V) (∼87% of immobilized As). We demonstrate that only by combining high-resolution STEM imaging and EDX mapping with the bulk (differential) PDF and extended X-ray absorption fine structure (EXAFS) data can one truly determine the de facto As binding nature on GR surfaces. More importantly, these new insights into As-GR interaction mechanisms highlight the impact of GR phases on As sequestration in anoxic subsurface environments.
“绿锈”（GR）是一种具有氧化还原活性的 Fe(II)-Fe(III) 层状双氢氧化物，是还原、地下环境中砷（As）固定的潜在环境相关矿物基质。GR 相在近中性 pH 条件下具有高的 As 摄取能力，但 GR 相与 As 物种之间的确切相互作用机制仍知之甚少。在这里，我们通过扫描透射电子显微镜（STEM）结合能量色散 X 射线（EDX）光谱记录了缺氧和近中性 pH 条件下 GR 硫酸盐与 As 物种[As(III)和 As(V)]之间的键合和相互作用机制，并结合了基于同步加速器的 X 射线总散射、配分函数（PDF）分析和 As K 边 X 射线吸收光谱（XAS）。我们高空间分辨率的 STEM-EDX 数据表明，As(III)和 As(V)的首选吸附位点均在 GR 晶体边缘。将这些数据与差 PDF 和 XAS 结合起来，我们可以得出结论，As 的吸附主要发生在双齿双核（C）内球表面络合物中。在 GR 硫酸盐反应的 As(III)中，没有观察到次生 Fe-As 相。然而，在 GR 硫酸盐反应的 As(V)中形成了自生的似明矾石（亚铁砷酸盐纳米相），呈线状形态，作为 As(V)的另一种固定途径（约 87%的固定 As）。我们证明，只有将高分辨率 STEM 成像和 EDX 映射与体相（差）PDF 和扩展 X 射线吸收精细结构（EXAFS）数据相结合，才能真正确定 GR 表面上实际的 As 结合性质。更重要的是，这些关于 As-GR 相互作用机制的新见解突出了 GR 相在缺氧地下环境中对 As 固定的影响。
