Behavior of tributyl phosphate in clay rocks: Sorption, diffusion and the effect on radionuclides retention

The migration of organic compounds in soils and sediments is of interest in many research fields: carbon cycling, environmental monitoring, contamination and waste management, etc. In the context of radioactive waste disposal, many studies quantify the migration parameters of organic co-contaminants released by waste packages and engineered barriers. The retention mechanisms and partial exclusion from porous media are key factors in understanding their diffusive behavior. This study focuses on the behavior of tributyl phosphate (TBP), in two media: deep Callovo-Oxfordian clay rock and sub-surface Tégulines clay rock. The retention of TBP and of its degradation product, dibutyl phosphate (DBP), were quantified and compared to that of ethylenediaminetetraacetic acid (EDTA). The potential effect of TBP on radionuclides retention was also assessed. Tributyl phosphate displays no significant effect at near-neutral pH on radionuclide retention: 63 Ni(II), 152 Eu(III), 232 Th(IV). In addition to retention, the diffusion parameters of TBP were quantified for the first time in natural clay-rich media. Still, discrepancies between batch data and through-diffusion modelling show the need to consider specific kinetics for TBP absorption into natural organic matter. Diffusion experiments provide insights on the partial exclusion of TBP from both the COx and Tégulines clay rock's porosity. These data are relevant to quantify size exclusion, which remains whereas anion exclusion seems negligible in low density sub-surface rocks.