Abstract Iron (Fe) oxides are important host phases for antimony (Sb), a toxic metalloid of environmental concern. In wetland soils and sediments, poorly ordered Fe oxides such as ferrihydrite may undergo reductive dissolution and mineralogical transformation upon reaction with dissolved sulfide (S(-II)). The consequences of these processes for the mobility of associated Sb have not been investigated to date. Here, we allowed Sb(V)-bearing ferrihydrite (molar ratio of Fe:Sb = 400) to react with varying S(-II) concentrations (Fe(III):S(-II) = 0.2, 0.5, and 1) at pH 6 and 8 over 32 days. Changes in speciation and concentration of Fe, S and Sb in the aqueous, colloidal and solid phase were examined through a combination of aqueous-phase analyses, X-ray diffraction and synchrotron X-ray absorption spectroscopy. Addition of S(-II) caused rapid reduction of Fe(III), thereby producing elemental S and Fe(II). X-ray diffractometry and Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy revealed that S(-II) addition resulted in the precipitation of Fe(II) sulfides (mackinawite (FeS) and pyrite (FeS2)) and formation of secondary Fe(III) oxides (goethite (FeOOH) and hematite (Fe2O3)). The formation of mackinawite and pyrite was further confirmed by S K-edge X-ray absorption near-edge structure (XANES) spectroscopy, and was found to occur to the greatest extent in the high-sulfide treatments. The initial reductive dissolution of ferrihydrite was paralleled by a fast increase in dissolved Sb concentrations, with ∼25% of total Sb being released in the high-sulfide treatments. The Sb release was followed by Sb immobilization within ∼1–7 days. Since ion-chromatography ICP-MS revealed antimonate (Sb(OH)6-) as the primary Sb aqueous phase species throughout the experiment, with only negligible concentrations of antimonite (Sb(OH)3) and only very minor amounts ( 3 kDa) size fraction at pH 8 under medium and high S(-II) conditions, while no colloidal Sb was found in other treatments. Together, these results show that Fe oxide sulfidization can have opposing effects on Sb mobility. On the one hand, the initial sulfide-promoted Fe oxide dissolution triggers Sb release into the aqueous phase. On the other hand, Sb can subsequently be immobilized via sorption to secondary Fe oxides and newly-formed Fe sulfides during the later stages of sulfidization. Sulfidization reactions, and the complex opposing impacts on Sb mobility, should therefore be considered for the risk assessment and derivation of adequate management strategies at Sb-impacted sites which experience sulfidic conditions.

中文翻译：

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硫化物系统中锑的迁移率：与硫化物诱导的氧化铁转化的耦合

摘要 铁 (Fe) 氧化物是锑 (Sb) 的重要宿主相，锑 (Sb) 是一种具有环境问题的有毒准金属。在湿地土壤和沉积物中，有序性差的 Fe 氧化物如水铁矿在与溶解的硫化物 (S(-II)) 反应时可能会发生还原溶解和矿物学转变。迄今为止，尚未研究这些过程对相关 Sb 迁移率的影响。在这里，我们允许含 Sb(V) 的水铁矿（Fe:Sb 的摩尔比 = 400）与不同的 S(-II) 浓度（Fe(III):S(-II) = 0.2、0.5 和 1）反应在 32 天的 pH 值 6 和 8。水相、胶体和固相中 Fe、S 和 Sb 的形态和浓度变化通过水相分析、X 射线衍射和同步加速器 X 射线吸收光谱的组合进行检查。S(-II) 的添加导致 Fe(III) 的快速还原，从而产生元素 S 和 Fe(II)。X 射线衍射法和 Fe K 边扩展 X 射线吸收精细结构 (EXAFS) 光谱表明，S(-II) 的添加导致 Fe(II) 硫化物（麦金纳矿 (FeS) 和黄铁矿 (FeS2)）和形成二次 Fe(III) 氧化物（针铁矿 (FeOOH) 和赤铁矿 (Fe2O3)）。通过SK边缘X射线吸收近边缘结构（XANES）光谱进一步证实了mackinawite和黄铁矿的形成，并且发现在高硫化物处理中最大程度地发生。水铁矿的初始还原溶解伴随着溶解的 Sb 浓度的快速增加，在高硫化物处理中释放出约 25% 的 Sb。Sb 释放后在约 1-7 天内固定 Sb。由于离子色谱 ICP-MS 在整个实验过程中显示锑酸盐 (Sb(OH)6-) 作为主要的 Sb 水相物质，只有可忽略不计的锑酸盐 (Sb(OH)3) 浓度和非常少量 (3 kDa)在中等和高 S(-II) 条件下，pH 8 下的尺寸分数，而在其他处理中未发现胶体 Sb。总之，这些结果表明 Fe 氧化物硫化可以对 Sb 迁移率产生相反的影响。一方面，最初的硫化物促进氧化铁溶解触发 Sb 释放到水相中。另一方面，在硫化的后期阶段，Sb 可以通过吸附到二次 Fe 氧化物和新形成的 Fe 硫化物上而被固定。硫化反应，以及对 Sb 迁移率的复杂相反影响，