Eu(III) sorption on kaolinite: Experiments and modeling

Highlights

• A robust model describing different experimental data on sorption of Eu³⁺ onto kaolinite was made.

• Competition of Al³⁺ with Eu³⁺ towards ion exchange onto kaolinite was founded and modeled.

• The dependence of sorption on edge site with the specific surface area was approved.

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 Al³⁺. The model was also successfully tested on the available literature data for other kaolinite samples.

Introduction

Information regarding the sorption and migration behavior of radionuclides 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 efficiency 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 properties 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 substitution. 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 lanthanides 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 influenced 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 influence 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 significant. 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).

A few works were devoted to actinide and lanthanide sorption on kaolinite. Am(III) and Eu(III) demonstrated the clear pH dependence of sorption on KGa-1b, St. Austell, Maoming, and Iwamoto kaolinite powder mineral samples (Samadfam et al., 2000; Coppin et al., 2002; Tertre et al., 2006; Buda et al., 2008; Huittinen et al., 2010; Ho et al., 2011; Ma et al., 2017). In addition, pH-independent sorption was observed in the works (Kang and Hahn, 2004; Kautenburger and Beck, 2010; Křepelová et al., 2011). It was shown that, due to the different properties of the kaolinite samples studied, different sorption results are sometimes obtained (Ma et al., 2017). Sorption of Eu(III) and Cm(III) on synthetic and natural kaolinite (St. Austell, Great Britain) were studied during sorption experiments and by the time-resolved laser fluorescence spectroscopy (TRLFS) method (Huittinen et al., 2010). Natural kaolinite showed higher sorption of Eu(III) compared to the synthetic sample. The authors assumed that its more negative surface charge promotes higher sorption onto natural kaolinite. Thermodynamic descriptions of Am(III)/Eu(III) sorption onto kaolinite were provided in only four papers (Tertre et al., 2006; Olin et al., 2007; Marsac et al., 2015; Ma et al., 2017). Although all of them used the concept of describing the interaction of Eu by ion exchange and complexation with edge sites, the values of the constants differ significantly from paper to paper. In all cases, the model was applied to only one experimental dataset. In the work of Ma et al. (2017), the authors tried to make models for different kaolinite samples, but the constants' values also differed from sample to sample.

This study aimed to create a union conceptual model of Eu(III) sorption onto kaolinite regardless of origin. For this purpose, a highly pure natural sample of kaolinite was used for sorption experiments. Sorption experiments were performed under various pH values, ionic strengths, and Eu(III) concentrations. Together with sorption, the composition of the solutions was analyzed and accounted for. Furthermore, thermodynamic modeling of the experimental data, together with literature data for other kaolinites, was carried out. Finally, the influence of different characteristics of the kaolinites, such as surface area, ion-exchange capacity, and the presence of accessory minerals, on Eu(III) sorption was accounted for to create a generalized conceptual model of interaction.