Mobility of organic compounds in a soft clay-rich rock (Tégulines clay, France).

The migration of organic compounds in soils is a major concern in several environmental issues. Contaminants display distinct behaviours as regards to their specific affinities towards soils constituents. The retention mechanism of hydrophobic compounds by natural organic matter is well known. The retention of ionizable compounds is mainly related to oxides and clay minerals, even if less documented in reductive media. In this work, we investigated the migration of organic compounds in a soft clay-rich sedimentary rock (Tégulines clay, France). The aim was to determine the relative contributions of natural sorbents on retention, and eventual correlations with solutes properties. Both hydrophobic compounds (toluene, benzene, naphthalene) and hydrophilic species (adipate, oxalate, ortho-phthalate, benzoate) were investigated, using batch and diffusion experiments. The retention of neutral aromatic compounds correlates with their lipophilicity (log P), confirming that absorption mechanism prevails, despite a low content of natural organic matter (≤0.5%). A low retention of ionizable compounds was quantified on Tégulines clay. The eventual discrepancies between data acquired on crushed rock and solid samples are discussed. Low effective diffusion coefficients are quantified. These values hint on the relative contributions of steric and electrostatic exclusion, despite a large pore size in such "soft" clay-rock. Overall, the dataset illustrates a general scheme for assessing the migration over a wide variety of organic compounds. This approach may be useful for predictive modelling of the fate of organic compounds in environmental media.