Impact of alkali on the passivation of silicate glass

Marie Collin,Thibault Charpentier,Maxime Fournier,Stéphane Gin,Mélanie Moskura

doi:10.1038/s41529-018-0036-3

Abstract

Amorphous silica-rich surface layers, also called gels, can passivate silicate glass and minerals depending on environmental conditions. However, several uncertainties remain on the mechanisms controlling the formation of these layers. In this paper, the influence of exogenous ions supplied by solutions is studied, both on the formation and on the properties of the gel formed on international simple glass (ISG). ISG was altered at 90 °C, pH90 °C 7, in silica-saturated solutions containing various alkaline cations separately (Li+, Na+, K+, and Cs+). The alteration kinetics observed with Li and Na in the solution is similar to that observed with no ions, while K and Cs in the solution tend to decrease glass alteration. Furthermore, for K or Cs ions, the kinetics decreases as the ionic strength of the solution increases. The passivation layer formed in these solutions shows a selectivity toward cations following the series K > Cs > Na >> Li. These alkalis replace Ca from pristine glass in the altered structures, leading to differences in [AlO4]− units charge compensation. Importantly, exchange between Ca and alkali also affects the total quantity of water inside each gel and this effect is well correlated with the observed drop in glass alteration.

Highlights

The current French approach for high-level waste storage is by incorporating radionuclides inside a borosilicate glass matrix.Subsequently, these glass canisters will be stored in a deep geological formation.[1]
international simple glass (ISG) glass powders of 20–40 μm in particle size were altered at 90 ° C, pH 90 °C 7 in Si-saturated conditions with alkali introduced separately in the solution (LiCl, KCl and CsCl with concentrations ranging from 0 to 70 mmol L−1 and NaCl with a concentration of 20 mmol L−1: as Na is present in pristine glass, only one concentration was studied to compare with other alkali)
The glass alteration kinetics appears to leave this used as glass alteration tracer—displays different behaviors

Summary

INTRODUCTION

The current French approach for high-level waste storage is by incorporating radionuclides inside a borosilicate glass matrix. Glass alteration mechanisms are still heavily debated inside the scientific community, as no consensus has been reached yet for the way the passivation layer forms on the glass surface.[6,7,8,9] To better understand glass alteration at high reaction progress, Gin, et al.[10] and Collin, et al.[11] performed studies in Si-saturated conditions and slightly alkaline pH conditions, and characterized the passivating material. The drop in alteration rate could be a consequence of the gel selectivity for larger cations that could reduce water accessibility to the pristine material This raises interesting question regarding the upgrade of nuclear waste safe storage

RESULTS

DISCUSSION

METHODS