Reaction of smectites with iron in a nitrogen atmosphere at 75 °C

The stability of seven dioctahedral smectites coming from bentonites of different origin was studied in the presence of iron to simulate the possible reactions between clay minerals and Fe canisters in a nuclear waste repository. The batch experiments were performed at 75 °C for 35 days in a nitrogen atmosphere. The reaction products were examined by XRD, FTIR, HRTEM, BWA analysis and UV–Vis spectroscopy. Magnetite and 7-Å clay minerals (berthierine-like) were the main reaction products besides iron-smectite reaction. Bentonites reacted differently with iron. The quantitative XRD revealed that consumption of iron and formation of magnetite was significantly higher for Fe-rich smectites. Fe-rich smectites were the most affected by the reaction. This indicated that the octahedral iron ions may be largely responsible for smectite decomposition indicated by the decrease of the smectite content, partial dehydroxylation of its structure and alteration of octahedral and tetrahedral sheets. The layer charge and thickness of the smectites decreased during the reaction. Arrangement of the smectite layers was apparently changed, and rather large particles were split into thinner particles or even layers.

Research highlights
► The mineralogical composition of the bentonites changed significantly.
► Partial destabilization of the smectite structure was observed.
► Fe-rich smectites were the most affected by the reaction with iron.