Applied Clay Science 234 (2023) 106865 Available online 17 February 2023 0169-1317/© 2023 Elsevier B.V. All rights reserved. Research Paper Eu(III) sorption on kaolinite: Experiments and modeling Anna S. Semenkova a , Anna Yu. Romanchuk a, * , Irina F. Seregina a , Ivan Mikheev a , Valentina S. Svitelman b , Stepan N. Kalmykov a a Lomonosov Moscow State University, 119991 Moscow, Russia b Nuclear Safety Institute of Russian Academy of Sciences, 52, 115191 Moscow, Russia A R T I C L E INFO Keywords: Kaolinite Europium Sorption Clay Modeling ABSTRACT The present study was focused on the sorption of Eu(III) onto kaolinite. Two kaolinite samples with different origins and compositions were studied. A high-purity natural sample of kaolinite was used to obtain the sorption behavior: dependence of sorption on the ionic strength, pH, and radionuclide concentrations. It was compared with the data for another kaolinite sample containing admixtures of other clay minerals (illite and smectite). In addition, the effect of cations leaching from clay on the sorption was observed. Collected experimental results allowed to develop of a thermodynamic sorption model of Eu(III) sorption onto kaolinite accounting ion- exchange and surface complexation reaction as well competition with the sorption of Al 3+ . The model was also successfully tested on the available literature data for other kaolinite samples. 1. Introduction Information regarding the sorption and migration behavior of ra- dionuclides on barrier materials, including clays, is required to ensure the safety of nuclear waste repositories. It is also necessary to consider the impact of mineral composition when justifying the long-term eff- ciency of engineering barriers. Kaolinite is an abundant mineral, which is a major component of soils and many natural clays (Wenk et al., 2008; Missana et al., 2014; Tournassat et al., 2015). Moreover, kaolinite-based mixtures was used during the decommissioning of a uranium graphite reactor as an engineered geological barrier (Pavliuk et al., 2018). Thus, to assess the safety of engineered barrier systems in which kaolinite may contain, it is essential to gain an understanding of the sorption proper- ties of kaolinite with respect to a wide range of radionuclides and other pollutants. Kaolinite is a 1:1 clay mineral consisting of alternating lattices of alumino-oxide octahedrons and silica‑oxygen tetrahedrons. Due to a low number of isomorphic substitutions, it has a layer charge of about zero, resulting in a lack of interlayer space available for cation binding (Brady et al., 1998). Consequently, there are different types of sorption sites in kaolinite, as well as for other clay minerals: two types of basal surfaces (i.e., siloxane and gibbsite surfaces) and edge surfaces (i.e., broken surfaces) (Vasconcelos et al., 2007; Ma et al., 2017). Depending on the participating surface site, sorption mechanisms differ: cation exchange occurs on the basal planes, and inner-sphere complexation occurs on the edge surface sites (Bradbury and Baeyens, 2002; Huittinen et al., 2010). Bolland et al. (1976) concluded that most of kaolinite's negative surface charge is pH-independent and is a result of isomorphous sub- stitution. Ferris and Jepson (1975) found that cation uptake by kaolinite depends upon the cation chosen, the electrolyte concentration, and the pH of the solution. Ma and Eggleton (1999) attempted to determine the cation exchange capacity (CEC) and the nature of different types of sorption sites on kaolinite, for which structural approach calculations were used. It was shown that particle diameter and thickness play important roles in the cation exchange behavior of kaolinites. The similarity of ionic radii allows consideration of Eu(III) as an analog of the long-lived and radiotoxic trivalent actinides and lantha- nides that comprise high-level radioactive waste (HLW) (Rabung et al., 2005; Kautenburger and Beck, 2010; Songsheng et al., 2012). According to previous studies, depending on the participating surface site, the sorption of actinides and lanthanides on clays was infuenced by many factors, such as the solid/liquid phase ratio, pH, and ionic strength of the solution (Majdan, 2014), as well as the presence of humic, fulvic (Tan et al., 2008), and polycarbonate (Kimura et al., 1999) acids. The infu- ence of factors such as temperature (Miller et al., 1982; Bauer et al., 2005; Tertre et al., 2006; Kautenburger et al., 2019) and pressure (Miller et al., 1983) seemed to be less signifcant. Several papers were devoted to determining the role of the structure and cation form (interlayer cation) of clay minerals (Maza-Rodriguez et al., 1992; Bradbury and Baeyens, 2002; Bradbury et al., 2005). * Corresponding author. E-mail address: romanchukay@my.msu.ru (A.Yu. Romanchuk). Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay https://doi.org/10.1016/j.clay.2023.106865 Received 1 October 2022; Received in revised form 8 February 2023; Accepted 10 February 2023