Americium is a highly radioactive actinide element found in used nuclear fuel. Its adsorption on aluminum (hydr)oxide minerals is important to study for at least two reasons: (i) aluminum (hydr)oxide minerals are ubiquitous in the subsurface environment and (ii) bentonite clays, which are proposed engineered barriers for the geologic disposal of used nuclear fuel, have the same ≡AlOH sites as aluminum (hydr)oxide minerals. Surface complexation modeling is widely used to interpret the adsorption behavior of heavy metals on mineral surfaces. While americium sorption is understudied, multiple adsorption studies for europium, a chemical analog, are available. In this study we compiled data describing Eu(III) adsorption on three aluminum (hydr)oxide minerals—corundum (α-Al2O3), γ-alumina (γ-Al2O3) and gibbsite (γ-Al(OH)3)—and developed surface complexation models for Eu(III) adsorption on these minerals by employing diffuse double layer (DDL) and charge distribution multisite complexation (CD-MUSIC) electrostatic frameworks. We also developed surface complexation models for Am(III) adsorption on corundum (α-Al2O3) and γ-alumina (γ-Al2O3) by employing a limited number of Am(III) adsorption data sourced from literature. For corundum and γ-alumina, two different adsorbed Eu(III) species, one each for strong and weak sites, were found to be important regardless of which electrostatic framework was used. The formation constant of the weak site species was almost 10,000 times weaker than the formation constant for the corresponding strong site species. For gibbsite, two different adsorbed Eu(III) species formed on the single available site type and were important for the DDL model, whereas the best-fit CD-MUSIC model for Eu(III)-gibbsite system required only one Eu(III) surface species. The Am(III)-corundum model based on the CD-MUSIC framework had the same set of surface species as the Eu(III)-corundum model. However, the log K values of the surface reactions were different. The best-fit Am(III)-corundum model based on the DDL framework had only one site type. Both the CD-MUSIC and the DDL model developed for Am(III)-γ-alumina system only comprised of one site type and the formation constant of the corresponding surface species was ~ 500 times stronger and ~ 700 times weaker than the corresponding Eu(III) species on the weak and the strong sites, respectively. The CD-MUSIC model for corundum and both the DDL and the CD-MUSIC models for γ-alumina predicted the Am(III) adsorption data very well, whereas the DDL model for corundum overpredicted the Am(III) adsorption data. The root mean square of errors of the DDL and CD-MUSIC models developed in this study were smaller than those of two previously-published models describing Am(III)-γ-alumina system, indicating the better predictive capacity of our models. Overall, our results suggest that using Eu(III) as an analog for Am(III) is practical approach for predicting Am(III) adsorption onto well-characterized minerals.

中文翻译：

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Eu(III) 和 Am(III) 在铝（氢氧化）氧化物矿物上的吸附：表面络合模型


镅是在废核燃料中发现的一种高放射性锕系元素。它对铝（氢）氧化物矿物的吸附对于研究很重要，至少有两个原因：（i）铝（氢）氧化物矿物在地下环境中普遍存在，以及（ii）膨润土，它是地质处置的工程屏障。使用过的核燃料中，具有与（氢）氧化铝矿物相同的 ≡AlOH 位点。表面络合模型广泛用于解释重金属在矿物表面的吸附行为。虽然镅的吸附作用尚未得到充分研究，但对铕（一种化学类似物）的吸附研究已有多项。在本研究中，我们收集了描述 Eu(III) 在三种铝（氢）氧化物矿物（刚玉 (α-Al2O3)、γ-氧化铝 (γ-Al2O3) 和三水铝石 (γ-Al(OH)3)）上吸附的数据，并开发了通过采用扩散双层 (DDL) 和电荷分布多位点络合 (CD-MUSIC) 静电框架，建立了 Eu(III) 在这些矿物上吸附的表面络合模型。我们还利用文献中有限数量的 Am(III) 吸附数据，开发了刚玉 (α-Al2O3) 和 γ-氧化铝 (γ-Al2O3) 上 Am(III) 吸附的表面络合模型。对于刚玉和γ-氧化铝，两种不同的吸附Eu(III)物质（一种用于强位点，一种用于弱位点）被发现是重要的，无论使用哪种静电框架。弱位点物质的形成常数几乎比相应强位点物质的形成常数弱10,000倍。 对于三水铝石，在单一可用位点类型上形成两种不同的吸附 Eu(III) 物种，这对于 DDL 模型很重要，而 Eu(III)-三水铝石系统的最佳拟合 CD-MUSIC 模型仅需要一种 Eu(III)表面物种。基于 CD-MUSIC 框架的 Am(III)-刚玉模型具有与 Eu(III)-刚玉模型相同的表面物种集。然而，表面反应的 log K 值不同。基于 DDL 框架的最佳拟合 Am(III)-刚玉模型只有一种位点类型。针对 Am(III)-γ-氧化铝系统开发的 CD-MUSIC 和 DDL 模型均仅包含一种位点类型，并且相应表面物质的形成常数比相应的 Eu( III) 分别在弱位点和强位点上的物种。刚玉的 CD-MUSIC 模型以及 γ-氧化铝的 DDL 和 CD-MUSIC 模型都很好地预测了 Am(III) 吸附数据，而刚玉的 DDL 模型则高估了 Am(III) 吸附数据。本研究中开发的 DDL 和 CD-MUSIC 模型的误差均方根小于之前发布的两个描述 Am(III)-γ-氧化铝系统的模型，表明我们的模型具有更好的预测能力。总体而言，我们的结果表明，使用 Eu(III) 作为 Am(III) 的类似物是预测 Am(III) 在特征良好的矿物上吸附的实用方法。