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

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.