Adsorption of polar organic molecules on sediments: Case-study on Callovian-Oxfordian claystone.

The release and transport of anthropogenic organic matter through the geosphere is often an environmental criterion of safety. Sedimentary rocks are widely studied in this context as geological barriers for waste management. It is the case of Callovian-Oxfordian claystone (COx), for which several studies report adsorption of anthropogenic organic molecules. In this study, we evaluated and reviewed adsorption data of polar organic molecules on COx claystone. Experiments were performed on raw claystone, decarbonated and clay fractions. Adsorption isotherms were measured with adsorbates of various polarities: adipate, benzoate, ortho-phthalate, succinate, gluconate, oxalate, EDTA, citrate. A significant adsorption was observed for multidentate polycarboxylic acids as evidenced with phthalate, succinate, oxalate, gluconate, EDTA and citrate (R = 1.53, 3.52, 8.4, 8.8, 12.4, 54.7 L kg respectively). Multiple linear regression were performed as a statistical analysis to determine the predictors from these adsorption data. A linear correlation between adsorption data (R) and dipole moment (μ) of adsorbates was evidenced (R = 0.91). Molecules with a high dipole moment, μ(D) > 2.5, displayed a significant adsorption, R≫1 L kg. A qualitative correlation can be easily estimated using the water/octanol partition coefficient, P, of adsorbates (R = 0.77). In this case, two opposite trends were distinguished for polar and apolar molecules. The use of organic carbon content in sediments is relevant for predicting adsorption of apolar compounds, log (P)>+1. The oxides/clays contents may be relevant regarding polar molecules, log (P)<-1. The proposed scheme offers a general methodology for investigation of geo-barriers towards heterogeneous organic plumes.