Zirconium local environment in simplified nuclear glasses altered in basic, neutral or acidic conditions: Evidence of a double-layered gel

Abstract

Borosilicate glasses containing 1 to 8 mol% ZrO2 were leached at pH 9, 7 and 1 until complete alteration. The Zr coordination in the bulk and at the gel surface is quantified using Zr L2- XANES spectra in fluorescence and total electron yield mode, respectively. This provides evidence of a single layer at pH 9 whereas a second alteration layer develops at the gel surface at pH 7 and at pH 1. The surface layer, a witness of the first stages of glass alteration, exhibits larger structural variations than the bulk of the gel. This is related with the alteration conditions, from incipient alteration of pristine glasses to chemical diffusion controlled by the gels formed at a later stage. In all gels investigated, a majority of Zr occurs in [6]Zr sites. However, at neutral and acidic pH, the gels contain a high fraction of 7- and 8-coordinated Zr ([7]Zr and [8]Zr, respectively) sites, and only 6-coordinated ([6]Zr) sites at pH 9. The Zr local environment in the gels, determined by Zr K-edge EXAFS, is similar to that in the pristine glasses, with some additional contribution of Zr second neighbors. Both Zr-site geometry and medium-range structure of the alteration gel bring direct evidence that, under the experimental conditions used, glass dissolution is driven by an in situ hydrolysis/condensation mechanism. This allows the gel structure to mimic that of the pristine glass, including the [6]Zr site distortion or the linkage to the silicate gel framework. The pH has a greater influence on the structure of gels than does their Zr content. The structural evolution around Zr is mainly governed by the pH during alteration, while the [6]Zr site symmetry is controlled by the Zr content of the gel. Certain gels contain more octahedral sites than expected from the concentration of charge compensating cations (Ca, Na), a situation similar to that observed in SiO2ZrO2 xerogels.