Predicting the uptake of Cs, Co, Ni, Eu, Th and U on argillaceous rocks using sorption models for illite

Abstract

Reliable predictions of radiocontaminant migration are a requirement for the establishment of radioactive waste repositories. Parametrization of the necessary sorption models seems to be, however, extremely challenging given the multi-mineralic composition of the lithosphere. In this study it is shown for two argillaceous rocks-Boda and Opalinus Clay relevant for the Hungarian and Swiss repository concepts, respectively-that this task can be substantially simplified by taking into account only the most sorptive mineral fraction, namely the 2:1 clay minerals illite and illite/smectite mixed layers. Two different models were required to blind predict the sorption isotherms of Cs, Co, Ni, Eu, Th and UO 2 measured on the two clay rock samples in a synthetic porewater. Cs sorption was modelled with the generalised Cs (GCs) sorption model and the sorption of the other cations with the 2 site protolysis non electrostatic surface complexation and cation exchange (2SPNE SC/CE) model. The 2SPNE SC/CE model for illite was extended with surface complexation reactions on weak sites for Co, Ni, Eu, UO 2 and on strong sites for Eu-carbonato complexes. Complementary to the sorption measurements and modelling, Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy was used to probe the retention mechanism of Ni on illite, Boda and Opalinus Clay at higher loadings. The reliable blind predictions of the selected metal cations, which are representative for monovalent alkaline metals, divalent transition metals, lanthanides, and trivalent, tetravalent and hexavalent actinides, confirms the applicability of this simplified bottom up approach, and, renders the underlying sorption models particularly useful to predict sorption for the wide range of cations to be considered in the safety analysis of radioactive waste repositories in clayrich environments.

Exploring the Sorption Mechanism of Ni(II) on Illite: Batch Sorption, Modelling, EXAFS and Extraction Investigations

“…Generally, the migration and retention of Ni(II) in contaminated soil and groundwater is largely controlled by its sorption and desorption behaviors at the solid/water interface. Previous studies have demonstrated that Ni(II) strongly and selectively interacted with phyllosilicate minerals that were widely distributed in the soil and sediments, such as montmorillonite 6 – 15 , kaolinite 16 , 17 , bentonite 18 , 19 , attapulgite 1 , 20 , diatomite 21 , and illite 2 , 22 , 23 . These studies showed that environmental factors such as pH, ionic strength, and temperature can affect the sorption/desorption behaviors of Ni(II) to a large extent at both the micro- and macro-scales.…”

Combined X‐ray microanalytical study of the Nd uptake capability of argillaceous rocks

“…Hereinafter, only samples treated with NaCl solution will be discussed, because this electrolyte ensured a sufficient loading of the element of interest to match the sensitivity of micro‐XRF while avoiding precipitation in the solution itself . The concentrations of the key element to be applied for the sorption experiments based on the sorption isotherms were measured and modelled for Eu(III) on illite, taking into account the clay content of the BCF samples (around 40%) as well as the solid‐to‐liquid ratio . The applied Nd(III) initial concentrations were 3.6 × 10 −6 –3.6 × 10 −5 M, resulting in equilibrium concentrations of 10 −6 –10 −5 M …”

“…The concentrations of the key element to be applied for the sorption experiments based on the sorption isotherms were measured and modelled for Eu(III) on illite, taking into account the clay content of the BCF samples (around 40%) as well as the solid‐to‐liquid ratio . The applied Nd(III) initial concentrations were 3.6 × 10 −6 –3.6 × 10 −5 M, resulting in equilibrium concentrations of 10 −6 –10 −5 M …”

“…It is known that argillaceous rocks have several suitable properties of which the most advantageous are the homogeneity, the chemical buffering capacity, the low groundwater flow, the self‐healing capability by crack swelling and the radionuclide retardation related to sorption . The main goal of these experiments is to determine the partition coefficient (R d ) between the mobile phase (dissolved in water) and the sedentary (solid) phase . These bulk experiments are necessary but inadequate to describe the sorption process in detail; thus, sorption experiments involving thin sections and microscopic studies are needed.…”

Surrogate Model for Multi-Component Diffusion of Uranium through Opalinus Clay on the Host Rock Scale

“…The varying mineralogy of the Opalinus Clay is implemented in the simulations with a bottom-up approach [27,28]. The amount of sorption of contaminants in heterogeneous formations can be determined by the additive sorption on individual minerals.…”