Thermodynamic Modeling of Selenite Sorption Onto Ca-type Bentonil WRK Montmorillonite

Seonggyu Choi,Bong-Ju Kim,Jae-Kwang Lee,Jang-Soon Kwon 한국방사성폐기물학회 2023 한국방사성폐기물학회 학술논문요약집 Vol.21 No.1

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 Review of the Development of Diffusion Databases for Safety Assessment of Deep Geological Disposal in Various Countries

Seonggyu Choi,Jae-Kwang Lee,Nak-Kyu Kim,Jang-Soon Kwon 한국방사성폐기물학회 2021 한국방사성폐기물학회 학술논문요약집 Vol.2021 No.11

Spectroscopic and Thermochemical Investigation of U(VI)-Aromatic Carboxylate Precipitates in an Aqueous Solution

Seonggyu Choi,Jong-Il Yun 한국방사성폐기물학회 2020 한국방사성폐기물학회 학술논문요약집 Vol.2020 No.07

UV-Vis Absorption Properties of Uranyl-Nicotinic Acid Complexes in Aqueous Medium

Seonggyu Choi,Jong-Il Yun 한국방사성폐기물학회 2017 한국방사성폐기물학회 학술논문요약집 Vol.2017 No.10

Stability constants and spectroscopic properties of thorium(IV)-arsenazo III complexes in perchloric acid

Choi, Seonggyu,Yun, Jong-Il Springer-Verlag 2019 Journal of radioanalytical and nuclear chemistry Vol.319 No.1

Application of Thermodynamic Sorption Modelling: Separation and Characterization of Bentonil-WRK Montmorillonite

Seonggyu Choi,Bong-Ju Kim,Euo Chang Jung,Jae-Kwang Lee,Nak Kyu Kim,Jang-Soon Kwon 한국방사성폐기물학회 2022 한국방사성폐기물학회 학술논문요약집 Vol.20 No.1

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.

Spectrophotometric study of the uranyl monobenzoate complex at moderate ionic strength

Choi, Seonggyu,Yun, Jong-Il Pergamon Press 2019 Polyhedron Vol.161 No.-

<P><B>Abstract</B></P> <P>The protonation of benzoate and the complexation of uranyl monobenzoate in NaClO<SUB>4</SUB> solutions ranging from 0.1 to 0.5 M were quantitatively investigated by means of ultraviolet–visible (UV–Vis) spectrophotometry. The spectrophotometric study revealed that the uranyl monobenzoate species has a significantly enhanced absorption than that of free uranyl, especially in the UV region. The non-linear regression analysis of the UV–Vis absorption data was the first to calculate the protonation constant of benzoate and the stability constant of uranyl monobenzoate at moderate ionic strength. Based on the SIT (specific ion interaction theory), the protonation constants were extrapolated to the zero ionic strength as log β P ° = 4.21 ± 0.01, and the ion interaction coefficient of the benzoate anion with the sodium cation was determined as ε ( <SUP> Ben - </SUP> , <SUP> Na + </SUP> ) = 0.17 ± 0.05 kg·mol<SUP>−1</SUP>. In the same manner, the stability constant of uranyl monobenzoate at zero ionic strength and the ion interaction coefficient of the uranyl monobenzoate cation with perchlorate anion were determined to be log β U ° = 2.91 ± 0.01 and ε ( <SUP> <SUB> UO 2 </SUB> B e n + </SUP> , <SUP> <SUB> ClO 4 </SUB> - </SUP> ) = 0.22 ± 0.07 kg·mol<SUP>−1</SUP>, respectively.</P> <P><B>Graphical abstract</B></P> <P>The stability constants and spectroscopic properties of the uranyl monobenzoate complex in 0.1–0.5 M NaClO<SUB>4</SUB> medium were investigated by UV–Vis spectrophotometry. The ion interaction coefficient of the uranyl monobenzoate cation with perchlorate anion was determined based on the SIT for the first time.</P> <P>[DISPLAY OMISSION]</P>

Thermodynamic Modeling of Calcium-Sodium Exchange Reactions in Bentonil-WRK Montmorillonite

Seonggyu Choi,Hyun-Kyu Lee,Jae-Kwang Lee,Jeong-Yun Jang,Sang-Ho Lee 한국방사성폐기물학회 2024 한국방사성폐기물학회 학술논문요약집 Vol.22 No.1

Thermodynamic Modeling of Iodide and Benzoate Sorption Onto Ca-type Bentonil-WRK Montmorillonite

Seonggyu Choi,Bong-Ju Kim,Ja-Young Goo,Jae-Kwang Lee 한국방사성폐기물학회 2023 한국방사성폐기물학회 학술논문요약집 Vol.21 No.2

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.

우라늄(VI)-천연유기물질 모델리간드 착화합물 형성거동 규명

최승규(Seonggyu Choi),윤종일(Jong-Il Yun) 대한지질학회 2021 대한지질학회 학술대회 Vol.2021 No.10

고준위방사성폐기물 처분 안전성 평가에서 우라늄(U), 플루토늄(Pu) 및 아메리슘(Am) 등 주요 악틴족 핵종들의 화학종분포, 용해도, 수착과 같은 지화학거동을 정확하게 계산하고 예측하기 위해서는 심층환경에서 핵종들이 가질 수 있는 다양한 착화합물 형성반응을 열역학 및 분자구조적인 측면에서 포괄적으로 이해하는 것이 필요하다. 많은 종류의 작용기 중에서 방향족 카복실산(Aromatic carboxylic acid) 구조는 흄산(Humic acid) 및 풀브산(Fulvic acid)과 같은 거대 천연유기물질(Natural Organic Matters)에 풍부하게 존재하고 환경 내 방사성핵종의 이동과 지연을 결정하는 주요 유기리간드 구조이다. 하지만 이러한 천연유기물질의 구조는 매우 복잡하고 또한 생성되는 조건에 따라서도 불균질하기 때문에 핵종과의 결합거동 규명에 어려움이 있다. 이에 복잡한 천연유기물질을 간단한 모델리간드로 나누고 각 구성성분에 대한 적절한 가중치를 부여함으로써 핵종-천연유기물질간의 거시적인 상호작용을 평가하는 연구법이 활용되며 이 때 주요 모델리간드에 대하여 핵종 결합반응의 평형상수를 도출하고 이를 수치해석적으로 매개변수화(Parametrization)하는 과정이 수반된다. 천연유기물질 내 방향족 카복실산 구조에는 다양한 분자단이 위치하고 각각의 분자단은 고유한 유발, 공명 및 입체효과를 가진다. 이에 본 연구에서는 산화조건 하 악틴족 핵종과 방향족 카복실산 구조의 거시적인 결합거동을 현 수준보다 정확하게 평가할 수 있는 결합모델을 구축하고자 UV-Vis 흡수 및 Raman 분광법을 활용하여 다양한 우라늄(VI)-방향족 카복실산 착화합물의 고유한 분광학 및 열역학특성을 도출하였다. 그 결과 방향족 카복실산의 반응도를 조절하는 주요 탄소 및 산소사슬 치환기의 우라늄(VI) 결합에 대한 정량적 효과를 측정하고 하메트 이론의 도입을 통해 선형 열역학 상관관계를 도출하였다. 본 연구결과는 향후 천연유기물질과 6가 악틴족 핵종간의 상호작용 모델링을 분자구조 수준에서 접근할 수 있는 기반자료로써 활용될 것으로 기대된다.