Fe(II)–Na ion exchange at interlayers of smectite: adsorption–desorption experiments and a natural analogue

Experiments of Fe2+ adsorption–desorption to Na-smectite were carried out to understand the effects of the corrosion products of a carbon-steel container on the alteration of bentonite as a buffer material in the environment of an underground repository for radioactive waste. Refined smectite reacted with Fe2+ in NaCl solutions under conditions that proportions of the liquid/solid (L/S=solution/smectite) ranged from 50 to 500 ml/g, and the ionic strengths (I) were 0.001 and 0.05. Ion selectivity coefficients (KGT) for the reaction of 2XNa+Fe2+=X2Fe+2Na+ were obtained: 0.06±0.00 when I=0.001, L/S=200 ml/g, and 1.85±0.29 when I=0.05, L/S=50 ml/g. ral analogue of the alteration between a carbon steel container and bentonite, where an iron-support has been kept in contact with Na-type bentonite for about 2000 days at a gallery in a bentonite mine, was also studied. It was found that the color of the bentonite had changed from pale gray to green near the iron support. The redox potential (Eh) could be measured directly for the ‘green bentonite’ and was found to be from −8 to +46 mV vs. SHE. Powder XRD analyses for the ‘green bentonite’ showed that a variation of d001-reflections with changing ambient relative humidity was in fairly good agreement with that of a divalent cation adsorbed smectite. ge between Fe2+and Na+ at interlayers of smectite could therefore occur in a natural reductive environment.