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High and low affinity sites of ferrihydrite for metal ion adsorption: Data and modeling of the alkaline-earth ions Be, Mg, Ca, Sr, Ba, and Ra
Geochimica et Cosmochimica Acta ( IF 5 ) Pub Date : 2020-10-01 , DOI: 10.1016/j.gca.2020.07.032
Juan C. Mendez , Tjisse Hiemstra

Abstract The alkaline-earth metal ion series comprises Be, Mg, Ca, Sr, Ba, and Ra. Calcium (Ca) and magnesium (Mg) are the most abundant alkaline-earth metal ions in nature and their interaction with the mineral surfaces of metal (hydr)oxides (e.g. ferrihydrite, Fh) affects the bioavailability, mobility, and geochemical cycling of many relevant ions. The adsorption of Ca2+ and Mg2+ ions to well-characterized freshly precipitated Fh has not been extensively measured yet in systems with a large variation in pH (5–10), ionic strength (0.01–1 M), and ion adsorption (0.002–2 μmol m−2). Nor have such adsorption data been interpreted with a surface complexation model that regards the structure of the adsorbed complexes and state-of-the-art insights into the surface structure of this nanomaterial. The primary adsorption data collected in this study (M2+/Fe) were scaled in a consistent manner to the surface area of Fh derived with a recently developed probe-ion methodology, before these data were interpreted with the charge distribution (CD) model, using MO/DFT/B3LYP/6-31+G** optimized hydrated geometries to obtain independently the CD coefficients. The pH-dependent adsorption behavior of Ca and Mg is rather similar. Both cations (M2+) form predominantly bidentate inner-sphere surface complexes ( (FeOH)2Δz0MΔz1), most possibly as a binuclear double corner (2C) complex according to EXAFS. This binding mechanism explains the relatively high H+/Ca2+ exchange ratio and the related pH-dependency of the Ca2+ adsorption. Modeling of the adsorption data reveals and quantifies the surface site heterogeneity of Fh, distinguishing high and low affinity sites for binding M2+ ions. The surface structure of Fh has been evaluated to rationalize this phenomenon and identify possible surface configurations. The increase in the FeOH-M2+ bond strength may be due to a redistribution of charge within specific sets of Fe1 polyhedra at the Fh surface that have in the underlying solid a set of common oxygen ions with an insufficient charge neutralization. According to our surface structural analysis of 2.2–2.8 nm particles, the FeOH site density involved (∼0.3 ± 0.1 nm−2) aggress with the surface site density of high affinity sites found by Ca2+ ion adsorption modeling (0.30 ± 0.03 nm−2). Extending our data analysis to literature data, comprising the full series of alkaline-earth ions, an increase in adsorption affinity with increase in the ionic radius of these cations was found, i.e. Be2+

中文翻译:

水铁矿对金属离子吸附的高和低亲和力位点:碱土离子 Be、Mg、Ca、Sr、Ba 和 Ra 的数据和建模

摘要 碱土金属离子系列包括Be、Mg、Ca、Sr、Ba和Ra。钙 (Ca) 和镁 (Mg) 是自然界中含量最丰富的碱土金属离子,它们与金属(氢)氧化物(例如水铁矿、Fh)的矿物表面的相互作用会影响许多物质的生物利用度、流动性和地球化学循环。相关离子。在 pH (5-10)、离子强度 (0.01-1 M) 和离子吸附 (0.002-2) 变化较大的系统中,尚未广泛测量 Ca2+ 和 Mg2+ 离子对表征良好的新鲜沉淀 Fh 的吸附μmol·m−2)。这种吸附数据也没有用表面复合模型来解释,该模型考虑了吸附复合物的结构和对这种纳米材料表面结构的最新见解。本研究中收集的主要吸附数据 (M2+/Fe) 以一致的方式缩放到 Fh 的表面积,使用最近开发的探针离子方法得出,在这些数据用电荷分布 (CD) 模型解释之前,使用MO/DFT/B3LYP/6-31+G** 优化了水合几何结构以独立获得 CD 系数。Ca 和 Mg 的 pH 依赖性吸附行为非常相似。根据 EXAFS,两种阳离子 (M2+) 主要形成双齿内球表面复合物 ((FeOH)2Δz0MΔz1),最有可能作为双核双角 (2C) 复合物。这种结合机制解释了相对较高的 H+/Ca2+ 交换比和 Ca2+ 吸附的相关 pH 依赖性。吸附数据的建模揭示并量化了 Fh 的表面位点异质性,区分结合 M2+ 离子的高亲和力和低亲和力位点。已评估 Fh 的表面结构以合理化这种现象并确定可能的表面配置。FeOH-M2+ 键强度的增加可能是由于 Fh 表面的特定 Fe1 多面体组内的电荷重新分布,在下面的固体中具有一组常见的氧离子,但电荷中和不足。根据我们对 2.2-2.8 nm 颗粒的表面结构分析,涉及的 FeOH 位点密度(~0.3 ± 0.1 nm-2)与 Ca2+ 离子吸附模型发现的高亲和力位点的表面位点密度(0.30 ± 0.03 nm-2 )。将我们的数据分析扩展到文献数据,包括全系列的碱土离子,
更新日期:2020-10-01
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