Research Paper

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Econ. Environ. Geol. 2023; 56(5): 603-618

Published online October 30, 2023

https://doi.org/10.9719/EEG.2023.56.5.603

© THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY

Uranium Adsorption Properties and Mechanisms of the WRK Bentonite at Different pH Condition as a Buffer Material in the Deep Geological Repository for the Spent Nuclear Fuel

Yuna Oh1, Daehyun Shin1, Danu Kim1, Soyoung Jeon1, Seon-ok Kim2, Minhee Lee3,*

1Major of Earth and Environmental Sciences, Division of Earth Environmental System Science, Pukyong National University
2Department of Energy Resources Engineering, Pukyong National University
3Major of Environmental Geosciences, Division of Earth Environmental System Science, Pukyong National University

Correspondence to : *heelee@pknu.ac.kr

Received: July 26, 2023; Revised: October 7, 2023; Accepted: October 17, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

This study focused on evaluating the suitability of the WRK (waste repository Korea) bentonite as a buffer material in the SNF (spent nuclear fuel) repository. The U (uranium) adsorption/desorption characteristics and the adsorption mechanisms of the WRK bentonite were presented through various analyses, adsorption/desorption experiments, and kinetic adsorption modeling at various pH conditions. Mineralogical and structural analyses supported that the major mineral of the WRK bentonite is the Ca-montmorillonite having the great possibility for the U adsorption. From results of the U adsorption/desorption experiments (intial U concentration: 1 mg/L) for the WRK bentonite, despite the low ratio of the WRK bentonite/U (2 g/L), high U adsorption efficiency (>74%) and low U desorption rate (<14%) were acquired at pH 5, 6, 10, and 11 in solution, supporting that the WRK bentonite can be used as the buffer material preventing the U migration in the SNF repository. Relatively low U adsorption efficiency (<45%) for the WRK bentonite was acquired at pH 3 and 7 because the U exists as various species in solution depending on pH and thus its U adsorption mechanisms are different due to the U speciation. Based on experimental results and previous studies, the main U adsorption mechanisms of the WRK bentonite were understood in viewpoint of the chemical adsorption. At the acid conditions (<ph 3),="" the="" u="" is="" apt="" to="" adsorb="" as="" forms="" of="" uo22+, mainly due to the ionic bond with Si-O or Al-O(OH) present on the WRK bentonite rather than the ion exchange with Ca2+ among layers of the WRK bentonite, showing the relatively low U adsorption efficiency. At the alkaline conditions (>pH 7), the U could be adsorbed in the form of anionic U-hydroxy complexes (UO2(OH)3-, UO2(OH)42-, (UO2)3(OH)7-, etc.), mainly by bonding with oxygen (O-) from Si-O or Al-O(OH) on the WRK bentonite or by co-precipitation in the form of hydroxide, showing the high U adsorption. At pH 7, the relatively low U adsorption efficiency (42%) was acquired in this study and it was due to the existence of the U-carbonates in solution, having relatively high solubility than other U species. The U adsorption efficiency of the WRK bentonite can be increased by maintaining a neutral or highly alkaline condition because of the formation of U-hydroxyl complexes rather than the uranyl ion (UO22+) in solution, and by restraining the formation of U-carbonate complexes in solution.

Keywords buffer, deep geological repository, SNF, uranium adsorption, WRK bentonite

사용후핵연료 심지층 처분장의 완충재 소재인 WRK 벤토나이트의 pH 차이에 따른 우라늄 흡착 특성과 기작

오유나1 · 신대현1 · 김단우1 · 전소영1 · 김선옥2 · 이민희3,*

1부경대학교 지구환경시스템과학부 지구환경과학전공
2부경대학교 에너지자원공학과
3부경대학교 지구환경시스템과학부 환경지질과학전공

요 약

사용후핵연료(Spent nuclear fuel; SNF) 심지층 처분장의 완충재 소재로서 WRK (waste repository Korea) 벤토나이트가 적합한지를 평가하기 위하여, 대표적인 방사성 핵종인 U (uranium)에 대한 WRK 벤토나이트의 흡/탈착 특성과 흡착 기작을 규명하는 다양한 분석, 흡/탈착 실내 실험, 동역학 흡착 모델링을 다양한 pH 조건에서 수행하였다. 다양한 특성 분석 결과, 주성분은 Ca-몬모릴로나이트이며, U 흡착 능력이 뛰어난 광물학적·구조적 특징들을 가지고 있었다. WRK 벤토나이트의 U 흡착 효율 및 탈착율을 규명하기 위한 흡/탈착 실험 결과, pH 5, 6, 10, 11 조건에서 WRK 벤토나이트와 U 오염수(1 mg/L)가 낮은 비율(2 g/L)로 혼합되었음에도 불구하고 높은 U 흡착 효율(>74%)과 낮은 U 탈착율(<14%)을 보였으며, 이는 WRK 벤토나이트가 SNF 처분장에서 U 거동을 제한하는 완충재 소재로서 적절하게 사용될 수 있음을 의미한다. pH 3과 7 조건에서는 상대적으로 낮은 U 흡착 효율(<45%)이 나타났으며, 이는 U가 용액의 pH 조건에 따라 다양한 형태로 존재하며, 존재 형태에 따라 상이한 U 흡착 기작을 가지기 때문으로 판단된다.
본 연구 실험 결과와 선행연구를 바탕으로 WRK 벤토나이트의 주요 화학적 U 흡착 기작을 pH 범위에 따라 용액 내 U의 존재 형태에 근거하여 설명하였다. pH 3 이하에서 주로 UO22+ 형태로 존재하는 U는 벤토나이트 표면의 Si-O 또는 Al-O(OH)와의 정전기적 인력(예: 이온 결합)에 의해 흡착되기 때문에 pH가 감소할수록 음전하 표면이 약해지는 WRK 벤토나이트 특성에 의해 비교적 낮은 U 흡착 효율이 나타났다. pH 7 이상의 알칼리성 조건에서 U는 음이온 U-수산화 복합체(UO2(OH)3-, UO2(OH)42-, (UO2)3(OH)7- 등)로 존재하며 비교적 높은 흡착 효율이 나타내는데, 이들은 벤토나이트에 포함된 Si-O 또는 Al-O(OH)의 산소원자를 공유하거나 리간드 교환에 의해 새로운 U-복합체가 형성되어 흡착되거나 수산화물 형태의 공침(co-precipitation)에 의해 벤토나이트에 고정되기 때문이다. pH 7의 중성 조건에서는 pH 5와 6보다 오히려 낮은 U 흡착 효율(42%)이 나타났는데, 이러한 결과는 용액 내 존재하는 탄산염(carbonate)에 의해 U가 U-수산화 복합체보다 용해도가 높은 U-탄산염 복합체로 존재하는 경우 가능하다. 연구 결과 pH를 약산성 또는 염기성 조건으로 유지하거나 용액 내 존재하는 탄산염을 제한함으로써 WRK 벤토나이트의 U 흡착 효율을 높일 수 있는 것으로 나타났다.

주요어 WRK 벤토나이트, 사용후핵연료, 심지층 처분, 완충재, 우라늄 흡착

Article

Research Paper

Econ. Environ. Geol. 2023; 56(5): 603-618

Published online October 30, 2023 https://doi.org/10.9719/EEG.2023.56.5.603

Copyright © THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY.

Uranium Adsorption Properties and Mechanisms of the WRK Bentonite at Different pH Condition as a Buffer Material in the Deep Geological Repository for the Spent Nuclear Fuel

Yuna Oh1, Daehyun Shin1, Danu Kim1, Soyoung Jeon1, Seon-ok Kim2, Minhee Lee3,*

1Major of Earth and Environmental Sciences, Division of Earth Environmental System Science, Pukyong National University
2Department of Energy Resources Engineering, Pukyong National University
3Major of Environmental Geosciences, Division of Earth Environmental System Science, Pukyong National University

Correspondence to:*heelee@pknu.ac.kr

Received: July 26, 2023; Revised: October 7, 2023; Accepted: October 17, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

This study focused on evaluating the suitability of the WRK (waste repository Korea) bentonite as a buffer material in the SNF (spent nuclear fuel) repository. The U (uranium) adsorption/desorption characteristics and the adsorption mechanisms of the WRK bentonite were presented through various analyses, adsorption/desorption experiments, and kinetic adsorption modeling at various pH conditions. Mineralogical and structural analyses supported that the major mineral of the WRK bentonite is the Ca-montmorillonite having the great possibility for the U adsorption. From results of the U adsorption/desorption experiments (intial U concentration: 1 mg/L) for the WRK bentonite, despite the low ratio of the WRK bentonite/U (2 g/L), high U adsorption efficiency (>74%) and low U desorption rate (<14%) were acquired at pH 5, 6, 10, and 11 in solution, supporting that the WRK bentonite can be used as the buffer material preventing the U migration in the SNF repository. Relatively low U adsorption efficiency (<45%) for the WRK bentonite was acquired at pH 3 and 7 because the U exists as various species in solution depending on pH and thus its U adsorption mechanisms are different due to the U speciation. Based on experimental results and previous studies, the main U adsorption mechanisms of the WRK bentonite were understood in viewpoint of the chemical adsorption. At the acid conditions (22+, mainly due to the ionic bond with Si-O or Al-O(OH) present on the WRK bentonite rather than the ion exchange with Ca2+ among layers of the WRK bentonite, showing the relatively low U adsorption efficiency. At the alkaline conditions (>pH 7), the U could be adsorbed in the form of anionic U-hydroxy complexes (UO2(OH)3-, UO2(OH)42-, (UO2)3(OH)7-, etc.), mainly by bonding with oxygen (O-) from Si-O or Al-O(OH) on the WRK bentonite or by co-precipitation in the form of hydroxide, showing the high U adsorption. At pH 7, the relatively low U adsorption efficiency (42%) was acquired in this study and it was due to the existence of the U-carbonates in solution, having relatively high solubility than other U species. The U adsorption efficiency of the WRK bentonite can be increased by maintaining a neutral or highly alkaline condition because of the formation of U-hydroxyl complexes rather than the uranyl ion (UO22+) in solution, and by restraining the formation of U-carbonate complexes in solution.

Keywords buffer, deep geological repository, SNF, uranium adsorption, WRK bentonite

사용후핵연료 심지층 처분장의 완충재 소재인 WRK 벤토나이트의 pH 차이에 따른 우라늄 흡착 특성과 기작

오유나1 · 신대현1 · 김단우1 · 전소영1 · 김선옥2 · 이민희3,*

1부경대학교 지구환경시스템과학부 지구환경과학전공
2부경대학교 에너지자원공학과
3부경대학교 지구환경시스템과학부 환경지질과학전공

Received: July 26, 2023; Revised: October 7, 2023; Accepted: October 17, 2023

요 약

사용후핵연료(Spent nuclear fuel; SNF) 심지층 처분장의 완충재 소재로서 WRK (waste repository Korea) 벤토나이트가 적합한지를 평가하기 위하여, 대표적인 방사성 핵종인 U (uranium)에 대한 WRK 벤토나이트의 흡/탈착 특성과 흡착 기작을 규명하는 다양한 분석, 흡/탈착 실내 실험, 동역학 흡착 모델링을 다양한 pH 조건에서 수행하였다. 다양한 특성 분석 결과, 주성분은 Ca-몬모릴로나이트이며, U 흡착 능력이 뛰어난 광물학적·구조적 특징들을 가지고 있었다. WRK 벤토나이트의 U 흡착 효율 및 탈착율을 규명하기 위한 흡/탈착 실험 결과, pH 5, 6, 10, 11 조건에서 WRK 벤토나이트와 U 오염수(1 mg/L)가 낮은 비율(2 g/L)로 혼합되었음에도 불구하고 높은 U 흡착 효율(>74%)과 낮은 U 탈착율(<14%)을 보였으며, 이는 WRK 벤토나이트가 SNF 처분장에서 U 거동을 제한하는 완충재 소재로서 적절하게 사용될 수 있음을 의미한다. pH 3과 7 조건에서는 상대적으로 낮은 U 흡착 효율(<45%)이 나타났으며, 이는 U가 용액의 pH 조건에 따라 다양한 형태로 존재하며, 존재 형태에 따라 상이한 U 흡착 기작을 가지기 때문으로 판단된다.
본 연구 실험 결과와 선행연구를 바탕으로 WRK 벤토나이트의 주요 화학적 U 흡착 기작을 pH 범위에 따라 용액 내 U의 존재 형태에 근거하여 설명하였다. pH 3 이하에서 주로 UO22+ 형태로 존재하는 U는 벤토나이트 표면의 Si-O 또는 Al-O(OH)와의 정전기적 인력(예: 이온 결합)에 의해 흡착되기 때문에 pH가 감소할수록 음전하 표면이 약해지는 WRK 벤토나이트 특성에 의해 비교적 낮은 U 흡착 효율이 나타났다. pH 7 이상의 알칼리성 조건에서 U는 음이온 U-수산화 복합체(UO2(OH)3-, UO2(OH)42-, (UO2)3(OH)7- 등)로 존재하며 비교적 높은 흡착 효율이 나타내는데, 이들은 벤토나이트에 포함된 Si-O 또는 Al-O(OH)의 산소원자를 공유하거나 리간드 교환에 의해 새로운 U-복합체가 형성되어 흡착되거나 수산화물 형태의 공침(co-precipitation)에 의해 벤토나이트에 고정되기 때문이다. pH 7의 중성 조건에서는 pH 5와 6보다 오히려 낮은 U 흡착 효율(42%)이 나타났는데, 이러한 결과는 용액 내 존재하는 탄산염(carbonate)에 의해 U가 U-수산화 복합체보다 용해도가 높은 U-탄산염 복합체로 존재하는 경우 가능하다. 연구 결과 pH를 약산성 또는 염기성 조건으로 유지하거나 용액 내 존재하는 탄산염을 제한함으로써 WRK 벤토나이트의 U 흡착 효율을 높일 수 있는 것으로 나타났다.

주요어 WRK 벤토나이트, 사용후핵연료, 심지층 처분, 완충재, 우라늄 흡착

    Fig 1.

    Figure 1.XRD pattern of the WRK bentonite.
    Economic and Environmental Geology 2023; 56: 603-618https://doi.org/10.9719/EEG.2023.56.5.603

    Fig 2.

    Figure 2.Zeta potential graph of the WRK bentonite at the pH range of 1-11.
    Economic and Environmental Geology 2023; 56: 603-618https://doi.org/10.9719/EEG.2023.56.5.603

    Fig 3.

    Figure 3.SEM/TEM photomicrographs of the WRK bentonite (a, b: SEM image; c: TEM image).
    Economic and Environmental Geology 2023; 56: 603-618https://doi.org/10.9719/EEG.2023.56.5.603

    Fig 4.

    Figure 4.FTIR spectrum of the WRK bentonite.
    Economic and Environmental Geology 2023; 56: 603-618https://doi.org/10.9719/EEG.2023.56.5.603

    Fig 5.

    Figure 5.U adsorption efficiency on the WRK bentonite at various pH conditions (a: pH 5, 6, 10, and 11; b: pH 3 and 7).
    Economic and Environmental Geology 2023; 56: 603-618https://doi.org/10.9719/EEG.2023.56.5.603

    Fig 6.

    Figure 6.U desorption rate on the WRK bentonite at various pH conditions (a: pH 5; b: pH 6; c: pH 10; d: pH 11).
    Economic and Environmental Geology 2023; 56: 603-618https://doi.org/10.9719/EEG.2023.56.5.603

    Fig 7.

    Figure 7.XRD patterns of the WRK bentonite before and after the U adsorption.
    Economic and Environmental Geology 2023; 56: 603-618https://doi.org/10.9719/EEG.2023.56.5.603

    Fig 8.

    Figure 8.SEM photomicrographs and results of EDS analysis of the WRK bentonite before and after the U adsorption.
    Economic and Environmental Geology 2023; 56: 603-618https://doi.org/10.9719/EEG.2023.56.5.603

    Fig 9.

    Figure 9.Pseudo 2nd-order model (t/qt vs. t) of the WRK bentonite at 3, 5, 6, 7, 10, and 11.
    Economic and Environmental Geology 2023; 56: 603-618https://doi.org/10.9719/EEG.2023.56.5.603

    Fig 10.

    Figure 10.XRD patterns and basal spacing (d(001)) of the WRK bentonite before and after the U adsorption at pH 3 and 5.
    Economic and Environmental Geology 2023; 56: 603-618https://doi.org/10.9719/EEG.2023.56.5.603

    Fig 11.

    Figure 11.pH changes of the supernatants before and after the U adsorption at pH 3, 5, and 6.
    Economic and Environmental Geology 2023; 56: 603-618https://doi.org/10.9719/EEG.2023.56.5.603

    Fig 12.

    Figure 12.U adsorption efficiency of the WRK bentonite at pH 3, 5, and 6.
    Economic and Environmental Geology 2023; 56: 603-618https://doi.org/10.9719/EEG.2023.56.5.603

    Fig 13.

    Figure 13.U adsorption mechanisms of the WRK bentonite in presence of carbonate (modified from Wentong and Bingkun, 1990).
    Economic and Environmental Geology 2023; 56: 603-618https://doi.org/10.9719/EEG.2023.56.5.603

    Fig 14.

    Figure 14.U adsorption mechanisms of the WRK bentonite in absence of carbonate (modified from Wentong and Bingkun, 1990).
    Economic and Environmental Geology 2023; 56: 603-618https://doi.org/10.9719/EEG.2023.56.5.603

    Principle components of the WRK bentonite with others from XRF analysis.


    Component (wt. %)WRKbentoniteKJ-Ⅱ bentoniteMX-80 bentoniteGMZ bentonite
    SiO267.2458.8157.8474.04
    Al2O318.9315.1714.0116.92
    MgO4.182.702.932.96
    CaO3.945.722.312.58
    Fe2O33.475.284.922.17
    K2O0.991.270.340.5
    Na2O0.571.063.660.5
    TiO20.520.670.630.09
    P2O50.080.210.060.03
    MnO0.090.130.070.05
    SrO0.03--0.12
    ZrO20.02---
    Bentonite typeCa-bentoniteCa-bentoniteCa-bentoniteNa-bentonite
    ReferenceThis studyYoo et al., 2016Lee et al., 2021Lee et al., 2018


    Comparison of specific surface area and CEC value for the WRK bentonite with others.


    Bentonite typeSpecific surface areaCECReference
    WRK55.9 m2/g78.11 meq/100 gThis study
    KJ-Ⅱ61 m2/g64.7 meq/100 gYoo et al., 2016
    MX-8021 m2/g75 meq/100 gHu et al., 2009; Chaparro et al., 2016
    FEBEX33 m2/g94 meq/100 gMayordomo et al., 2016; Mota-Heredia et al., 2023
    GMZ25 m2/g77.3 meq/100 gLi et al., 2016; He et al., 2019
    Kunige-V119.2 m2/g73.2 meq/100 gKomine, 2004; Sasagawa et al., 2018


    Principle components of the WRK bentonite before and after the U adsorption from XRF analysis.


    ComponentWRK bentonite (wt. %)
    Before U adsorptionAfter U adsorption
    SiO267.2467.67
    Al2O318.9318.38
    MgO4.183.97
    CaO3.944.19
    Fe2O33.472.98
    K2O0.991.00
    Na2O0.570.95
    TiO20.520.57
    P2O50.080.16
    MnO0.090.09
    SrO0.03-
    ZrO20.02-
    U3O8-0.04


    Parameter values of two adsorption kinetic models for the WRK bentonite at 3, 5, 6, 7, 10, and 11.


    pseudo 1st-order modelpseudo 2nd-order model
    pHqe, expqe, calk1R2qe, calk2R2
    30.2520.002-0.0250.57430.244-19.5830.9999
    50.4130.0100.0340.62370.41048.1590.9999
    60.3830.002-0.0690.80380.357-9.4690.9997
    70.1980.004-0.0730.37640.19318.8470.9845
    100.4060.0510.1090.92790.40623.4380.9999
    110.3790.2050.3360.93980.3833.8130.9965

    *qe, exp: experimental adsorption capacity in equilibrium; qe, cal: calculated adsorption capacity in equilibrium.



    Aqueous U speciation at the pH range of 3-6 (Brix et al., 2021).


    pHAqueous U species
    Uranyl cation(Cation) U-hydroxyl complex ions
    3UO22+-
    5UO22+UO2OH+, (UO2)2(OH)22+
    6-(UO2)3(OH)5+, (UO2)4(OH)7+

    KSEEG
    Dec 29, 2023 Vol.56 No.6, pp. 629~909

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