Retention mechanisms and long-term stability of selenium incorporated in pyrite within deep subsurface sedimentary formations in the Horonobe area, Hokkaido, Japan

Pyrite formation could potentially sequester selenium (Se) released from high-level radioactive waste (HLW). To evaluate the incorporation of Se into pyrite for radionuclide migration analyses, it is necessary to clarify the mechanisms underpinning naturally occurring Se retention, and its long-term stability in deep subsurface environments. In this study, Se partitioning and chemical speciation in deep sedimentary formations in the Horonobe area (Hokkaido, Japan) were investigated using sequential extraction and synchrotron-based micro-X-ray fluorescence and absorption spectroscopy (µ-XRF-XAS). Rock and groundwater samples were also collected from multiple depths of the Koetoi and Wakkanai Formations. Selenium concentrations were relatively homogeneous with depth in both formations, and Se was predominantly associated with the sulfide and selenide fractions identified by sequential extraction. The absence of systematic depth-dependent variation indicates that Se has not undergone significant secondary redistribution under reducing subsurface conditions. Micro-XRF mapping showed that Se was spatially associated with Fe- and S-enriched zones identified as framboidal pyrite aggregates. Selenium K-edge XANES spectra confirmed that Se occurred in reduced oxidation states (−I or 0). Se K-edge EXAFS analysis further demonstrated that Se is structurally incorporated into the pyrite lattice through substitution for sulfur, rather than forming a discrete Fe–Se phase. Groundwater chemistry indicated saturation or near-saturation with respect to pyrite, suggesting that Se-bearing pyrite could have remained stable. These results provide field-based evidence for the retention mechanisms and long-term stability of selenium incorporated in pyrite in deep subsurface sedimentary formations, supporting its role as a robust immobilization host in deep geological disposal environments.