Dissolved Oxygen Diffusion and Consumption in Tuffaceous Sandstone: A Luminescence-Based Oxygen Measurement Approach for Intact Rocks

Dissolved oxygen (DO) remaining in radioactive waste repositories after backfilling is expected to be consumed by Fe(II)-bearing geological media, limiting redox-sensitive radionuclide migration into groundwater. However, DO consumption rates have been primarily evaluated using crushed samples, with limited research on intact rock samples comprising Fe(II)-bearing primary and clay minerals. To address this limitation, this study developed a simple experimental apparatus using a non-consumptive luminescence-based oxygen sensor that enables high-precision DO measurements with minimal artifacts. For validation, DO diffusion and consumption experiments were conducted using Kimachi tuffaceous sandstone (Japan) (porosity, 21.2%; smectite content, 29 wt.%) to evaluate the swelling effect of smectite. In this laboratory-scale study, the newly obtained rate constant was (6.55 ± 7.52) × 10−3 L mol−1 s−1, which was comparable to previously reported values of (1.57 ± 4.05) × 10−3 L mol−1 s−1. This result provides empirical support for the previously proposed mechanism that smectite swelling, which could potentially alter DO consumption, is constrained by the rigid rock structure, thereby maintaining a consumption rate similar to that of other Fe(II)-bearing geological media. Furthermore, by integrating previous findings indicating that effective DO diffusion coefficients (3.2 × 10−12 to 2.3 × 10−9 m2 s−1) depend on geological media porosity (21.2%–81.5%), these empirical laboratory parameters can be used to model and predict the transition from oxic to anoxic conditions in full-scale geological repository systems.