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 Na-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 (K-GT) 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. 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 Fe2+ and Na+ at interlayers of smectite could therefore occur in a natural reductive environment. (C) 1999 Elsevier Science B.V. All rights reserved.