Molecular-scale insights into differences in the adsorption of cesium and selenium on biogenic and abiogenic ferrihydrite
Molecular-scale insights into differences in the adsorption of cesium and selenium on biogenic and abiogenic ferrihydrite
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DOI：
10.1016/j.gca.2019.02.001
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发表时间：
2019-04
影响因子：
5
通讯作者：
S. Kikuchi;T. Kashiwabara;T. Shibuya;Y. Takahashi
中科院分区：
文献类型：
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作者：
S. Kikuchi;T. Kashiwabara;T. Shibuya;Y. Takahashi
羟基氧化铁（例如水铁矿、针铁矿）的吸附反应对自然环境中微量元素的地球化学循环具有相当大的影响。然而，人们越来越认识到，目前主要来自非生物产生的羟基氧化铁的知识不一定适用于在地下水排放区和热液喷口等自然环境中普遍存在的生物羟基氧化铁（BIOS）。为了了解 BIOS 吸附的差异，我们研究了从温泉收集的 BIOS（由 2 线水铁矿组成）和非生物水铁矿上的 Cs(I)、Se(IV) 和 Se(VI) 吸附，并结合 zeta 电位测量以及 Cs 和 Se K 边扩展 X 射线吸收精细结构 (EXAFS) 分析。 BIOS 在较宽的 pH 范围 (pH>6) 和初始 Cs(I) 浓度下显示出更大的 Cs(I) 吸附。 EXAFS 分析表明 Cs(I) 在 BIOS 上形成了外球复合体。 BIOS 的等电点 (pHiep) (pHiep=4.1) 显着低于非生物水铁矿的等电点 (pHiep=8.7)，表明带负电的 BIOS 比带正电的非生物水铁矿在静电上更有利于阳离子 Cs(I) 的吸附。因此，我们得出结论，BIOS 上对 Cs(I) 的吸附量大于非生物水铁矿上的吸附量主要是由静电效应引起的，可能是由于有机和无机杂质（例如秸秆、胞外聚合物和 Si）在水铁矿上的吸附和/或共沉淀所致。相比之下，阴离子 Se(IV) 和 Se(VI) 在 BIOS 上的吸附低于在非生物水铁矿上的吸附，这可以通过静电排斥来解释。特别是，在酸性至中性 pH 条件下，Se(VI) 吸附的减少比 Se(IV) 吸附的减少大约显着 7-80 倍。 EXAFS 分析表明，Se(IV) 在 BIOS 和非生物水铁矿上形成内球复合物，而 Se(VI) 形成外球复合物。因此，内球 Se(IV) 络合物的形成似乎抑制了 BIOS 上的静电排斥，因为它们对化学键的亲和力更强。结果表明，BIOS 的吸附化学不同于非生物水铁矿的吸附化学，可以通过静电效应和化学效应来解释。这些发现将有助于更好、更系统地了解各种微量元素的吸附行为以及 BIOS 在自然环境中的作用。
Adsorption reactions on iron oxyhydroxides (e.g., ferrihydrite, goethite) have considerable impacts on the geochemical cycles of trace elements in natural environments. However, there is a growing recognition that the current state of knowledge that was primarily derived from abiotically produced iron oxyhydroxides is not necessarily applicable to biogenic iron oxyhydroxides (BIOS) that are ubiquitous in natural environments such as groundwater discharge areas and hydrothermal vents. To understand the difference in adsorption on BIOS, we studied Cs(I), Se(IV), and Se(VI) adsorption on both BIOS (consisting of 2-line ferrihydrite) collected from a hot spring and abiogenic ferrihydrite combined with zeta-potential measurements and Cs and Se K-edge extended X-ray absorption fine structure (EXAFS) analyses. The BIOS showed larger adsorption of Cs(I) under a wide pH range (pH > 6) and initial Cs(I) concentrations. The EXAFS analyses revealed that Cs(I) formed an outer-sphere complex on BIOS. The isoelectric point (pHiep) of BIOS (pHiep= 4.1) was notably lower than that of abiogenic ferrihydrite (pHiep= 8.7), suggesting that the negatively charged BIOS are more electrostatically favorable for the adsorption of cationic Cs(I) than the positively charged abiogenic ferrihydrite. Therefore, we concluded the larger adsorption of Cs(I) on BIOS than on abiogenic ferrihydrite is mainly caused by the electrostatic effect, probably resulting from the adsorption and/or coprecipitation of organic and inorganic impurities (e.g., stalks, extracellular polymeric substances, and Si) on the ferrihydrite. In contrast, anionic Se(IV) and Se(VI) adsorption on BIOS was lower than that on abiogenic ferrihydrite, which can be explained by electrostatic repulsion. In particular, the decrease of Se(VI) adsorption was approximately 7–80 times more significant than that of Se(IV) at acidic to neutral pH. The EXAFS analysis showed that Se(IV) formed an inner-sphere complex, whereas Se(VI) formed an outer-sphere complex on both BIOS and abiogenic ferrihydrite. Therefore, the formation of inner-sphere Se(IV) complexes seems to suppress electrostatic repulsion on BIOS because of their stronger affinity for chemical bonding. The results revealed that the adsorption chemistry of BIOS is different from that of abiogenic ferrihydrite and can be explained by both electrostatic and chemical effects. The findings will contribute to a better and more systematic understanding of the adsorption behavior of various trace elements and the role of BIOS in natural environments.