Hematite and iron carbonate precipitation-coexistence at the iron–montmorillonite–salt solution–CO2 interfaces under high gas pressure at 150 °C

The hydrothermal reactivity of swelling clays has relevant implications on the geological storage of radioactive waste and greenhouse gases because the clay geo-materials have been proposed as engineered or natural barriers due to their low permeability in confined systems and their high capacity to sequester ions. In the present study, the iron–montmorillonite–salt solution–CO2 interactions were investigated under high gas pressure (200 bar) at 150 °C.Various chemical processes were characterized at the solid–fluid interfaces such as the dissolution of montmorillonite fine particles and oxidative-dissolution of elemental iron. The ionic supersaturation of solution and possibly the surface complexation in the system produced the precipitation of hematite nanoparticles (< 200 nm) after 15 days of solid–fluid contact. The hematite nanoparticles dispersed and/or coagulated on the clay matrix caused a stable red coloration of the montmorillonite composite. We assume that initial dissolved oxygen was progressively consumed in this closed-stirred system favouring the presence of divalent iron (in-situ change of redox conditions) and then leading the surface precipitation of iron carbonate nanocrystals (< 500 nm) after 60 days of solid–fluid contact. Thus, an atypical mineral coexistence of hematite–iron carbonate was observed in our system. A qualitative comparison with the blank experiment, i.e. at the same P–T conditions, but without CO2 injection, suggested that the carbon dioxide increased the hydrothermal reactivity of montmorillonite because the hematite and iron carbonate formation were not observed after the same reaction time.