Serpentinization and H2 production during an iron-clay interaction experiment at 90C under low CO2 pressure

Abstract Interactions between steel canisters and clay-rich material in deep geological repositories for high-level nuclear waste were studied by reacting metallic iron and a claystone from the Callovo-Oxfordian formation of the Paris Basin (COx). The experiment ran at 90 °C for 14 weeks in the presence of water and under 20 mbar of initial CO2 partial pressure (p(CO2)). At the end of the experiment, the pressure in the autoclave reached 50 bar due to H2 production. Water was almost entirely consumed and poorly crystallized 7 A-greenalite-type clays were formed at the expense of illite and mixed-layered illite-smectite minerals (I/S). The dissolution of quartz and pyrite and the crystallization of pyrrhotite were also observed. Considering the results of Mossbauer and Fourier Transformed Infrared (FTIR) spectroscopies, iron carbonate and and/or ferrous hydroxycarbonate were suspected to precipitate but in very small amounts. A follow-up of the mineralogical evolution by XRD (X-ray diffraction), SEM (Scanning Electron Microscopy) and TEM (Transmission Electron Microscopy) shows that serpentinization of clays (formation of greenalite) is accompanied by a significant consumption of water and the production of H2 gas due to the oxidation and dissolution of metallic iron. Experimental results are in good agreement with geochemical modelling with the PHREEQC code. The kinetics of the iron corrosion was evaluated from H2 production and shows that diffusion processes are the rate limiting step.