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

Abstract

Experiments of Fe 2+ 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 Fe 2+ 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 ( K GT) for the reaction of 2XNa+Fe 2+=X 2Fe+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. A natural 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 ( E h) 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 d 001-reflections with changing ambient relative humidity was in fairly good agreement with that of a divalent cation adsorbed smectite. Exchange between Fe 2+and Na + at interlayers of smectite could therefore occur in a natural reductive environment.