Study of uranium(VI) and radium(II) sorption at trace level on kaolinite using a multisite ion exchange model

Abstract

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.

Application of the multi-site ion exchanger model to the sorption of Sr and Cs on natural clayey sandstone

“…Clay minerals are known to support various sorption site types, such as planar sites associated to the permanent negative charge resulting from isomorphous substitution of lattice cations, and surface hydroxyl-groups able to protonate or deprotonate depending on pH. In the present ionexchange model, clay minerals are defined as an exchanger with different negatively charged sorption sites, named X i , contributing to the intrinsic surface reactivity (Reinoso and Ly, 2016). Considering the ion-exchange between a cation M m+ and H + taken as reference, the reaction associated to sorption site X i is expressed by the following equilibrium (Eq.1):…”

Factors Controlling Natural Radionuclides Migration in Sedimentary Rock Samples, Gabal Um‐Hamd, Southwestern Sinai, Egypt

“…In Figure , the high values of ( 226 Ra/ 230 Th) and ( 226 Ra/ 238 U) activity ratios suggest a reduced mobility of 226 Ra compared to 230 Th and 238 U in the study soils, which also suggests that the clay fraction is involved in the retention of 226 Ra in these soils. Sorption of this radionuclide on clay minerals was widely studied through experiments …”

Industrial Deployment of Reactive Transport Simulation: An Application to Uranium In situ Recovery

“…Through its specifi c activity and its relatively long half-life (1620 y), 226 Ra must be considered as ultra-trace element with typical concentrations in the range of ppb to ppt. Therefore, its mobility in natural context is principally governed by sorption at the surface of ancillary minerals, natural organic matter and coprecipitation in SO 4 -bearing minerals (see among others references dedicated to the mobility of 226 Ra in mining context: Lestini et al 2013;Sajih et al 2014;Reinoso-Maset and Ly 2016;Robin et al 2017;Bordelet et al 2018, Lestini et al 2019. No proper Ra mineral phase has ever been found in natural environments.…”