Sequential extraction and spectroscopic characterisation of organic matter from the Callovo-Oxfordian formation
Résumé
One site under consideration for the disposal of medium and high level, long lived radioactive waste is in the eastern part of the Paris Basin (Meuse/Haute Marne, France), where the National Radioactive Waste Management Agency (ANDRA) operates an underground laboratory. The sedimentary host formation is a 130 m thick, clay-rich Callovo-Oxfordian deposit ca. 350–550 m below ground in the selected area. The major fraction (>95%) of the organic matter (OM) is associated with minerals. In order to better understand the importance of OM on the chemistry of this formation, we have developed a protocol allowing previous termextraction/next termfractionation of organic compounds in the rocks. The protocol is based on classical methods currently used for the isolation of humic substances from soils and for the separation of kerogen in ancient sediments using previous termsequential extractionnext term with solvents and water at different pH value. This allows us to the remove a large part of the minerals and to suspend almost all (>90%) of the OM (associated with residual recalcitrant minerals) in water. The OM can then be analysed via previous termspectroscopicnext term methods, such as mass spectrometry (MS), Fourier transform infra-red spectroscopy (FTIR), or C (1s) near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The molecular composition of the solvent-soluble, directly-extractable part shows the OM to be thermally immature. The solvent-soluble fractions obtained after acid and alkaline treatments are mostly polar in nature and have a high degree of aliphaticity. C(1s) NEXAFS analysis of water soluble organic fractions indicates a progressive increase in aliphaticity and a decrease in carboxylic/carbonyl groups with consecutive previous termextractionnext term steps.