Reinoso-Maset Estela, Ly Jacques
DEN-Service d'Étude du Comportement des Radionucléides (SECR), CEA, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France.
DEN-Service d'Étude du Comportement des Radionucléides (SECR), CEA, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France.
J Environ Radioact. 2016 Jun;157:136-48. doi: 10.1016/j.jenvrad.2016.03.014. Epub 2016 Apr 12.
Uranium and the long-lived decay product radium-226 are abundantly present in mine wastes produced during uranium extraction activities. In the case of release to the surrounding environment, these radionuclides are at trace level compared to groundwater solutes, and the presence, content and properties of clay minerals in the host environment influence the extent of radionuclide sorption and, in turn, migration. Since clays are known to have the distinctive property of retaining ions, the aim of this work was to study the sorption of trace U(VI) and Ra(II) on a common phyllosilicate mineral, kaolinite, in the presence of excess K, a common groundwater cation, in order to obtain a thermodynamic database that describes the ion exchange equilibria occurring at the mineral-solution interface. Following a detailed experimental protocol using chemical and radiochemical analytical techniques, batch experiments over a wide pH range (from 2 to 11) and fixed concentration (ca. 10(-9) M), and additional adsorption isotherms at two different solution pH (6.2 and 10.4) over a concentration range (10(-10) to 10(-4) M) were carried out to measure the distribution coefficient (Kd) of U(VI) and Ra(II) sorption on kaolinite. The experimental sorption data was processed according to a general multisite sorbent/multispecies sorbate ion exchange model, which allowed deducing the charge of adsorbed species and the stoichiometry of the associated adsorption equilibria on kaolinite's surface sites. Aqueous speciation calculations predicted Ra(II) as Ra(2+) over the working pH range, and its adsorption curves and isotherms were explained using three sorption sites. Adsorption of U(VI) occurred on four sorption sites and was governed by its solution speciation, with positively charged hydroxylated (UO2(2+) and UO2(OH)(+)) and silicate (UO2(H3SiO4)(+)) species being adsorbed between pH 2 and 6, whereas its negatively charged forms (UO2(OH)3(-) and UO2(OH)4(2-)) dominated U(VI) sorption at pH > 7. Nonlinear fitting of the experimental data using the ion exchange model provided the associated equilibrium constants as corrected selectivity coefficients.
铀以及长寿命衰变产物镭 - 226大量存在于铀提取活动产生的矿山废料中。如果释放到周围环境中，与地下水中的溶质相比，这些放射性核素处于痕量水平，并且宿主环境中粘土矿物的存在、含量和性质会影响放射性核素的吸附程度，进而影响其迁移。由于已知粘土具有保留离子的独特性质，这项工作的目的是研究在常见的地下水阳离子钾过量存在的情况下，痕量U(VI)和Ra(II)在一种常见的层状硅酸盐矿物高岭土上的吸附情况，以便获得一个描述矿物 - 溶液界面发生的离子交换平衡的热力学数据库。按照使用化学和放射化学分析技术的详细实验方案，在较宽的pH范围（从2到11）和固定浓度（约10⁻⁹ M）下进行批量实验，并在两个不同的溶液pH（6.2和10.4）下在浓度范围（10⁻¹⁰到10⁻⁴ M）内进行额外吸附等温线实验，以测量U(VI)和Ra(II)在高岭土上吸附的分配系数（Kd）。根据一个通用的多位点吸附剂/多物种吸附质离子交换模型对实验吸附数据进行处理，该模型可以推断吸附物种的电荷以及高岭土表面位点上相关吸附平衡的化学计量关系。水相形态计算预测在工作pH范围内Ra(II)为Ra²⁺，并使用三个吸附位点解释了其吸附曲线和等温线。U(VI)的吸附发生在四个吸附位点上，并受其溶液形态控制，带正电荷的羟基化（UO₂²⁺和UO₂(OH)⁺）和硅酸盐（UO₂(H₃SiO₄)⁺）物种在pH 2到6之间被吸附，而其带负电荷的形式（UO₂(OH)₃⁻和UO₂(OH)₄²⁻）在pH > 7时主导U(VI)的吸附。使用离子交换模型对实验数据进行非线性拟合，得到了作为校正选择性系数的相关平衡常数。
