Effects of elevated temperature on Wyoming bentonite and its implications for sorption of radioactive strontium

Gianni F. Vettese; Xiaodong Li; Afrida Fairuz; Taavi Vierinen; Lilia Sirén; Damien Prieur; Nina Huittinen; Kathleen A. Law; René Bes; Mika Niskanen; Noora Pakkanen; Markus Olin; Marja Siitari-Kauppi

doi:10.1016/j.clay.2025.107865

Effects of elevated temperature on Wyoming bentonite and its implications for sorption of radioactive strontium

Gianni F. Vettese^*, Xiaodong Li, Afrida Fairuz, Taavi Vierinen, Lilia Sirén, Damien Prieur, Nina Huittinen, Kathleen A. Law, René Bes, Mika Niskanen, Noora Pakkanen, Markus Olin, Marja Siitari-Kauppi

^*Corresponding author for this work

BA45 Nuclear energy

Research output: Contribution to journal › Article › Scientific › peer-review

Abstract

Bentonite is a key barrier material in deep geological facilities for spent nuclear fuel, where it may be exposed to temperatures >100 °C because of radiogenic heating. Understanding how prolonged heating affects its physicochemical properties and radionuclide retention capacity is critical for ensuring long-term repository safety. This study investigated the impacts of dry heating (unconfined, evaporation allowed) at 150 °C for 36 months on the mineralogical and geochemical stability of a Wyoming bentonite, chosen for the Finnish geological disposal facility, and its subsequent sorption behavior with ⁹⁰Sr. Although the bentonite mineralogy remained mostly stable, combined X-ray diffraction, cation exchange capacity, titrations, demonstrated partial Na to Ca exchange within montmorillonite's interlayer following heating, and specific surface area analyses revealed a > 50 % reduction in specific surface area (30 to 14 m²/g). Colloid stability tests revealed that bentonite colloids did not form under repository-relevant saline conditions (I = 0.2 M), and heat treatment did not significantly impact colloid formation in reduced ionic-strength systems. Sorption isotherms demonstrated that Sr sorption was strongly pH-dependent, increasing from ∼40 % at pH 8 to ∼90 % at pH 13, and heating slightly enhanced Sr retention at pH 8. Two-site protolysis non-electrostatic surface complexation and cation exchange modelling suggested that in unheated bentonite, Sr was bound via a combination of surface complexation (∼30 %) and cation exchange (∼70 %) at pH 8, while at pH 13, sorption shifted towards surface complexation. However, after heating, cation exchange dominated at both pH values. Extended X-ray absorption fine structure analysis confirmed outer-sphere Sr sorption at pH 8 and increasing inner-sphere complexation at pH 13. Together, these findings highlight that despite minor physicochemical alterations, bentonite retained its Sr sorption potential, supporting its continued suitability as a buffer material in the ONKALO® repository.

Original language	English
Article number	107865
Journal	Applied Clay Science
Volume	274
DOIs	https://doi.org/10.1016/j.clay.2025.107865
Publication status	Published - 15 Sept 2025
MoE publication type	A1 Journal article-refereed

Funding

This research was mainly funded as part of the EURAD project: WP7 HITEC, which received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 847953, and from Posiva Oy through research cooperation. Part of this research was also funded by SAFER2028, as part of the ABCRad project. We acknowledge the European Synchrotron Radiation Facility (ESRF) for provision of synchrotron radiation facilities under proposal number A-20-1-867. During synchrotron experiments travel and sustenance for TV were funded by the DENSE mobility funding under the SAFER2028 programme. Open access funding provided by University of Helsinki. We thank HelLabs for use of their XRD, and this is HelLabs publication #0024.

Keywords

Bentonite buffer
Elevated temperature
Physicochemical alteration
Radio‑strontium sorption

UN SDGs

This output contributes to the following UN Sustainable Development Goals (SDGs)

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Vettese, G. F., Li, X., Fairuz, A., Vierinen, T., Sirén, L., Prieur, D., Huittinen, N., Law, K. A., Bes, R., Niskanen, M., Pakkanen, N., Olin, M., & Siitari-Kauppi, M. (2025). Effects of elevated temperature on Wyoming bentonite and its implications for sorption of radioactive strontium. Applied Clay Science, 274, Article 107865. https://doi.org/10.1016/j.clay.2025.107865

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abstract = "Bentonite is a key barrier material in deep geological facilities for spent nuclear fuel, where it may be exposed to temperatures >100 °C because of radiogenic heating. Understanding how prolonged heating affects its physicochemical properties and radionuclide retention capacity is critical for ensuring long-term repository safety. This study investigated the impacts of dry heating (unconfined, evaporation allowed) at 150 °C for 36 months on the mineralogical and geochemical stability of a Wyoming bentonite, chosen for the Finnish geological disposal facility, and its subsequent sorption behavior with 90Sr. Although the bentonite mineralogy remained mostly stable, combined X-ray diffraction, cation exchange capacity, titrations, demonstrated partial Na to Ca exchange within montmorillonite's interlayer following heating, and specific surface area analyses revealed a > 50 % reduction in specific surface area (30 to 14 m2/g). Colloid stability tests revealed that bentonite colloids did not form under repository-relevant saline conditions (I = 0.2 M), and heat treatment did not significantly impact colloid formation in reduced ionic-strength systems. Sorption isotherms demonstrated that Sr sorption was strongly pH-dependent, increasing from ∼40 % at pH 8 to ∼90 % at pH 13, and heating slightly enhanced Sr retention at pH 8. Two-site protolysis non-electrostatic surface complexation and cation exchange modelling suggested that in unheated bentonite, Sr was bound via a combination of surface complexation (∼30 %) and cation exchange (∼70 %) at pH 8, while at pH 13, sorption shifted towards surface complexation. However, after heating, cation exchange dominated at both pH values. Extended X-ray absorption fine structure analysis confirmed outer-sphere Sr sorption at pH 8 and increasing inner-sphere complexation at pH 13. Together, these findings highlight that despite minor physicochemical alterations, bentonite retained its Sr sorption potential, supporting its continued suitability as a buffer material in the ONKALO{\textregistered} repository.",

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AU - Vettese, Gianni F.

AU - Li, Xiaodong

AU - Fairuz, Afrida

AU - Vierinen, Taavi

AU - Sirén, Lilia

AU - Prieur, Damien

AU - Huittinen, Nina

AU - Law, Kathleen A.

AU - Bes, René

AU - Niskanen, Mika

AU - Pakkanen, Noora

AU - Olin, Markus

AU - Siitari-Kauppi, Marja

PY - 2025/9/15

Y1 - 2025/9/15

N2 - Bentonite is a key barrier material in deep geological facilities for spent nuclear fuel, where it may be exposed to temperatures >100 °C because of radiogenic heating. Understanding how prolonged heating affects its physicochemical properties and radionuclide retention capacity is critical for ensuring long-term repository safety. This study investigated the impacts of dry heating (unconfined, evaporation allowed) at 150 °C for 36 months on the mineralogical and geochemical stability of a Wyoming bentonite, chosen for the Finnish geological disposal facility, and its subsequent sorption behavior with 90Sr. Although the bentonite mineralogy remained mostly stable, combined X-ray diffraction, cation exchange capacity, titrations, demonstrated partial Na to Ca exchange within montmorillonite's interlayer following heating, and specific surface area analyses revealed a > 50 % reduction in specific surface area (30 to 14 m2/g). Colloid stability tests revealed that bentonite colloids did not form under repository-relevant saline conditions (I = 0.2 M), and heat treatment did not significantly impact colloid formation in reduced ionic-strength systems. Sorption isotherms demonstrated that Sr sorption was strongly pH-dependent, increasing from ∼40 % at pH 8 to ∼90 % at pH 13, and heating slightly enhanced Sr retention at pH 8. Two-site protolysis non-electrostatic surface complexation and cation exchange modelling suggested that in unheated bentonite, Sr was bound via a combination of surface complexation (∼30 %) and cation exchange (∼70 %) at pH 8, while at pH 13, sorption shifted towards surface complexation. However, after heating, cation exchange dominated at both pH values. Extended X-ray absorption fine structure analysis confirmed outer-sphere Sr sorption at pH 8 and increasing inner-sphere complexation at pH 13. Together, these findings highlight that despite minor physicochemical alterations, bentonite retained its Sr sorption potential, supporting its continued suitability as a buffer material in the ONKALO® repository.

AB - Bentonite is a key barrier material in deep geological facilities for spent nuclear fuel, where it may be exposed to temperatures >100 °C because of radiogenic heating. Understanding how prolonged heating affects its physicochemical properties and radionuclide retention capacity is critical for ensuring long-term repository safety. This study investigated the impacts of dry heating (unconfined, evaporation allowed) at 150 °C for 36 months on the mineralogical and geochemical stability of a Wyoming bentonite, chosen for the Finnish geological disposal facility, and its subsequent sorption behavior with 90Sr. Although the bentonite mineralogy remained mostly stable, combined X-ray diffraction, cation exchange capacity, titrations, demonstrated partial Na to Ca exchange within montmorillonite's interlayer following heating, and specific surface area analyses revealed a > 50 % reduction in specific surface area (30 to 14 m2/g). Colloid stability tests revealed that bentonite colloids did not form under repository-relevant saline conditions (I = 0.2 M), and heat treatment did not significantly impact colloid formation in reduced ionic-strength systems. Sorption isotherms demonstrated that Sr sorption was strongly pH-dependent, increasing from ∼40 % at pH 8 to ∼90 % at pH 13, and heating slightly enhanced Sr retention at pH 8. Two-site protolysis non-electrostatic surface complexation and cation exchange modelling suggested that in unheated bentonite, Sr was bound via a combination of surface complexation (∼30 %) and cation exchange (∼70 %) at pH 8, while at pH 13, sorption shifted towards surface complexation. However, after heating, cation exchange dominated at both pH values. Extended X-ray absorption fine structure analysis confirmed outer-sphere Sr sorption at pH 8 and increasing inner-sphere complexation at pH 13. Together, these findings highlight that despite minor physicochemical alterations, bentonite retained its Sr sorption potential, supporting its continued suitability as a buffer material in the ONKALO® repository.

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KW - Elevated temperature

KW - Physicochemical alteration

KW - Radio‑strontium sorption

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Effects of elevated temperature on Wyoming bentonite and its implications for sorption of radioactive strontium

Abstract

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Keywords

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