Reaction of smectites with iron in aerobic conditions at 75 °C

Abstract

The stability of six dioctahedral smectites with different crystal chemistry was studied in the presence of iron to simulate the possible reactions of clay minerals in contact with Fe canister in a nuclear waste repository. The batch experiments were performed at 75°C for 35days in air. The reaction products were examined by XRD, QXRD, FTIR, BWA analyses and UV–Vis spectroscopy. The reaction of bentonites with metal iron led to the consumption of iron and the formation of magnetite and 7-Å phyllosilicate. The original smectites were partially transformed from Al-rich to Fe-bearing. Appearance of the dioctahedral–trioctahedral domains in the octahedral sheets of smectites resulted in the partial destabilization and/or partial dissolution of the smectite structure indicated by the increase of the layer charge, splitting of the smectite particles and partial disruption of the smectite layers. Benefits of NIR spectroscopy were found in identification of FeMgOH and FeFeOH groups, which were not recognized in the MIR region of reacted iron–bentonite mixtures due to overlapping with other absorption bands. The comparison of the present study with the paper of Osacký et al. (2010) showed that smectites reacted differently with iron in air and nitrogen atmosphere. In the aerobic conditions more iron was consumed and less amount of magnetite was formed. The distinct experimental conditions (aerobic versus anaerobic) had the greatest effect on the layer charge and thickness of the smectite particles. Less pronounced changes of the layer charge and thickness of the smectite particles were observed for the iron–bentonite mixtures reacted in the aerobic conditions. It indicated that smectite destabilization was inhibited in the aerobic conditions.