Thermodynamic sorption modeling can enhance confidence in assessing and demonstrating the radionuclide sorption phenomena onto various mineral adsorbents. In this work, Ca-montmorillonite was successfully purified from Bentonil-WRK bentonite by performing the sequential physical and chemical treatments, and its geochemical properties were characterized using X-ray diffraction, Brunauer-Emmett-Teller analysis, cesium-saturation method, and controlled continuous acidbase titration. Further, batch experiments were conducted to evaluate the adsorption properties of Cs(I) and Sr(II) onto the homoionic Ca-montmorillonite under ambient conditions, and the diffuse double layer model-based inverse analysis of sorption data was performed to establish the relevant surface reaction models and obtain corresponding thermodynamic constants. Two types of surface reactions were identified as responsible for the sorption of Cs(I) and Sr(II) onto Ca-montmorillonite: cation exchange at interlayer site and complexation with edge silanol functionality. The thermodynamic sorption modeling provides acceptable representations of the experimental data, and the species distributions calculated using the resulting reaction constants accounts for the predominance of cation exchange mechanism of Cs(I) and Sr(II) under the ambient aqueous conditions. The surface complexation of cationic fission products with silanol group slightly facilitates their sorption at pH > 8.

The permanent disposal of discharged spent nuclear fuel (SNF) and contaminated radioactive waste generated from the subsequent chemical treatments of SNF has become a serious pending issue in many countries that operate the nuclear power plants. Among the diverse engineering solutions proposed for the disposal of high-level radioactive waste (HLW), deep geological disposal (DGD) has been considered as the most proven and safe option to prevent any significant release of radionuclides into the biosphere and to predictably ensure the long-term performance of disposal system. The DGD system consists of multiple structural components; the bentonite clay-based buffer and tunnel backfills are designed to perform the primary hydrogeochemical functions of 1) inhibiting the ingress of groundwater and reactive substances that could compromise the integrity of canister and 2) retarding the migration of released radionuclides into biosphere by providing the sufficient chemisorption sites. Montmorillonite, which is a 2:1 phyllosilicate mineral belonging to smectite group, constitutes the majority of bentonite, and it mainly predominate the swelling and chemisorption capacities of the clay material. Thus, it is essentially required to thoroughly understand the chemical interactions of major radionuclides and other important substances with montmorillonite in advance to accurately evaluate the long-term retention performance of engineered barriers and to reduce the uncertainties in the safety assessment of a deep geological repository (DGR) ultimately. Thus far, sorption of dissolved species onto mineral adsorbents has been generally described and quantified using the simple sorption-desorption distribution coefficient (Kd) concept; since any specific reaction mechanisms are not considered and reflected in the Kd concept, an empirical Kd value is intrinsically dependent on the aqueous conditions under which it was measured. In this framework, substantial scientific efforts have been made to develop a robust basis for geochemically parametrizing the sorption phenomena more reliably, and the application of thermodynamic sorption modeling (TSM), which is based on the chemical principle of mass action laws, has been studied with the aim of improving overall confidence in the description of radionuclide migration under a wide range of aquatic conditions. The disposal performance demonstration R&D division of KAERI introduced a new reference Ca-bentonite clay called Bentonil-WRK (Clariant Korea) for HLW disposal research in 2021 as the domestic Ca-bentonite sources have being depleted. We successfully separated and purified Ca-montmorillonite from the Bentonil-WRK clay, and its geochemical characteristics were meticulously studied by means of XRD, BET, CEC, FT-IR analyses and controlled acid-base titration. In this work, chemical sorption behaviors of aqueous iodide and benzoate, which are a major fission product in HLW and a model ligand of complex natural organic matters present in the deep geological environment, onto the purified Camontmorillonite were assessed under ambient conditions of S/L = 5 g/L, I = 0.01 M CaCl2, pH = 4- 9, pCO2 = 10-3.4 atm, and T = 25°C. Further, their unique adsorption envelopes and corresponding thermodynamic reaction constants refined from the diffuse double layer model (DDLM)-based inverse modeling of experimental sorption data were discussed.

The safe disposal of high-level radioactive waste (HLW), including the discharged spent nuclear fuel (SNF) and contaminated by-products produced from relevant chemical treatments, has become a serious pending problem for numerous countries that operate the nuclear power plants. The deep geological disposal (DGD) has thus far been considered the most proven and viable solution for isolation of the HLW and preventing any significant release of radionuclides into the biosphere. The DGD system consists of the multiple engineered and natural barrier components. Among them, the montmorillonite-based buffer and tunnel backfills are designed to perform the two major geochemical functions: 1) preventing the ingress of groundwater and any chemicals that compromise the safety of waste canister and 2) retarding the migration of released radionuclides by providing sufficient chemisorption sites. Therefore, it is essential to investigate the sorption mechanism of radionuclides onto montmorillonite and develop a thermodynamic reaction model in advance in order to accurately predict the long-term performance of engineered barriers and to reduce the uncertainties in the safety assessment of a deep geological repository (DGR) ultimately; thus far, sorption of chemical species onto mineral adsorbents has been widely described based on the concept of sorption-desorption distribution coefficient (Kd), the value of which is intrinsically conditional, and active scientific efforts have been made to develop robust thermodynamic sorption models which offer the potential to improve confidence in demonstration of radionuclide migration under a wide range of geochemical conditions. The natural montmorillonites are generally classified into Na-type or Ca-type according to its exchangeable cation, and the Ca-montmorillonite containing clays are being considered as candidate materials for the engineered barriers of DGR in several countries; they generally have advantages of higher thermal conductivity and lower price than the Na-montmorillonite based clays, but their sorption capacities are still comparable. In this framework, we aimed to investigate the chemical interactions of Ca-montmorillonite with selenite [Se(IV)], which is a major oxyanionic species in terms of HLW disposal, and develop a reliable thermodynamic sorption model (TSM). The present work summarizes the characterization of Ca-montmorillonite separated from the newly adopted reference bentonite (Bentonil-WRK) by means of XRD, BET, FTIR, CEC measurement, and acid-base titration. Further, its sorption behaviors with aqueous selenite species under aqueous conditions of S/L = 5 g/L, I = 0.01-0.1 m CaCl2, pH = 4.5-8.5, pCO2 = 10-3.5 atm, and T = 25°C were examined, and the resulting thermodynamic data are discussed as well.

A deep geological disposal system, which consists of the engineered and natural barrier components, is the most proven and widely adopted concept for a permanent disposal of the high level radioactive waste (HLW) thus far. The clay-based engineered barrier is designed to not only absorb mechanical stress caused by the geological activities, but also prevent inflow of groundwater to canister and outflow of radionuclides by providing abundant sorption sites. The principal mineralogical constituent of the clay material is montmorillonite, which is a 2:1 phyllosilicate having two tetrahedral sheets of SiO2 sandwiching an octahedral sheet of Al2O3. The stacking of SiO2 and Al2O3 sheets form the layered structures, and ion-exchange and water uptake reactions occur in the interlayer space. In order to reliably assess the radionuclide retention capacity of engineered barrier under wide geochemical conditions relevant to the geological disposal environments, sorption mechanisms between montmorillonite and radionuclides should be explicitly investigated in advance. Thus far, sorption behavior of mineral adsorbents with radionuclides has been quantified by the sorption-desorption distribution coefficient (Kd), which is simply defined as the ratio of radionuclide concentration in the solid phase to that in the equilibrium solution; the Kd value is conditional, and there have been scientific efforts to develop geochemically robust bases for parameterizing the sorption phenomena more reliably. In this framework, application of thermodynamic sorption model (TSM), which is theoretically based on the concept of widely accepted equilibrium models for aquatic chemistry, offers the potential to improve confidence in demonstration of radionuclide sorption reactions on the mineral adsorbents. Specifically, it is generally regarded in the TSM that coordination of radionuclides on montmorillonite takes place at the surficial aluminol and silanol groups while their ion-exchange reactions occur in the interlayer space also. The effects of electrical charge on the surface reactions are additionally corrected in accordance with the numerous theories of electrochemical interface. The present work provides an overview of the current status of application of TSM for quantifying sorption behaviors of radionuclides on montmorillonite and experimental results for physical separation and characterization of Ca-montmorillonite from the newly adopted reference bentonite (Bentonil- WRK) by means of XRD, BET, FTIR, CEC measurement, and acid-base titration. The determined mineralogical and chemical properties of the montmorillonite obtained will be used as input parameters for further sorption studies of radionuclides with the Bentonil-WRK montmorillonite.