Eluto-frontal chromatography to simulate chemical weathering of COx by low-molecular-weight organic compounds and early pedogenesis processes
Résumé
Excavation of geological materials by civil engineering operations generates fresh parent rocks that are transferred from the deep layers to the surface in a very short time. Thus they are exposed to pedogenetic factors. Prediction of the characteristics and functioning (e.g. plant support and water filter) and impacts of the newly formed soils requires the knowledge of the pedogenetic processes affecting the parent materials (i.e. weathering, transport and neoformation). Reactive transport is here used to assess weathering processes of geological materials produced from the excavation of an underground laboratory in the Callovo-Oxfordian (COx) layer, subsequently disposed of on the soil surface and covered with plants. The COx layer (c. 150-100 million years) is mainly composed of quartz, calcite, dolomite and argillite (complex mixtures of phyllosilicates). COx samples collected between −456.6 and −482.5 m were submitted to eluto-frontal chromatography experiments in the presence of simple organic molecules exhibiting a range of chemical reactivity (i.e. acetate, oxalate, histidine and EDTA). Breakthrough curves of compounds contained in the COx material and mathematical modelling allowed the identification of the major chemical reactions taking place during weathering. Results showed that acidic and complexing organic compounds induced the partial solubilization of minerals and in some cases secondary precipitations. The early pedogenesis processes induced a strong release of minerals (Na, Ca, Mg) and transformation of clay minerals. Further evolution of COx material is dependent on the presence of carbonates, leading to carbonated soils such as Calcosols. Reactive transport experiments showed that early pedogenetic processes are of most importance as they produced strong changes in the parent material composition and released significant amounts of solutes into the soil solution.